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Cardiac Physiology

A complex system of coordinated electrical circuitry within the heart governs cardiac Cardiac Total Anomalous Pulmonary Venous Return (TAPVR) muscle activity. The heart generates its own electrical impulses within its pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias cells. The signal then travels through specialized myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology, which act as electrical wiring, distributing the signal throughout the heart. Once the signal “leaves” the specialized conduction system, it passes to each myocyte through channels Channels The Cell: Cell Membrane called gap junctions Gap Junctions Connections between cells which allow passage of small molecules and electric current. Gap junctions were first described anatomically as regions of close apposition between cells with a narrow (1-2 nm) gap between cell membranes. The variety in the properties of gap junctions is reflected in the number of connexins, the family of proteins which form the junctions. The Cell: Cell Junctions (which connect myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology to each other) and causes them to contract. An electrical impulse is created by the opening and closing of ion channels Channels The Cell: Cell Membrane, allowing the flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure of charged particles across the myocardial cell membrane Cell Membrane A cell membrane (also known as the plasma membrane or plasmalemma) is a biological membrane that separates the cell contents from the outside environment. A cell membrane is composed of a phospholipid bilayer and proteins that function to protect cellular DNA and mediate the exchange of ions and molecules. The Cell: Cell Membrane. The flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure of charged particles changes the voltage across the membrane and opens up additional voltage-gated channels Channels The Cell: Cell Membrane, allowing the signal to propagate throughout the heart.

Last updated: 3 Dec, 2021

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Electrical Conduction in the Heart

Anatomy of the cardiac Cardiac Total Anomalous Pulmonary Venous Return (TAPVR) conduction system

Sinoatrial (SA) node:

  • A compact region of pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias cells (modified myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology) that serves as the primary pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias of the heart
  • Located within the subepicardial tissue at the junction of the right atrium and superior vena cava Superior vena cava The venous trunk which returns blood from the head, neck, upper extremities and chest. Mediastinum and Great Vessels: Anatomy
  • Depolarizes regularly:
  • Signal originates in the SA node → atrial myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology → atrioventricular (AV) node

The AV node:

  • A compact region of pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias cells that receives input from the SA node and propagates it toward the ventricles
  • Located near the intersection of the interventricular septum Interventricular Septum Ventricular Septal Defect (VSD) and the septal leaflet of the tricuspid valve Tricuspid valve The valve consisting of three cusps situated between the right atrium and right ventricle of the heart. Heart: Anatomy
  • Characterized by slow conduction and a long refractory period:
    • Causes a delay in propagation of electrical signal to the ventricles
    • Allows ventricles to fill with blood from the atrial contraction before the ventricles contract
  • If the SA node fails, the AV node has its own autorhythmicity and can serve as the primary pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias with a slower rhythm (HR: 40–60/min).

Bundle of His Bundle of His Small band of specialized cardiac muscle fibers that originates in the atrioventricular node and extends into the membranous part of the interventricular septum. The bundle of his, consisting of the left and the right bundle branches, conducts the electrical impulses to the heart ventricles in generation of myocardial contraction. Heart: Anatomy and Purkinje fibers Purkinje fibers Modified cardiac muscle fibers composing the terminal portion of the heart conduction system. Heart: Anatomy:

Cardiac conduction system and intrinsic rhythms

Cardiac Cardiac Total Anomalous Pulmonary Venous Return (TAPVR) conduction system and intrinsic rhythms:
Location of pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias cells within the conduction system of the heart and their corresponding intrinsic rhythms

Image by Lecturio.

Nonpacemaker myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology:

  • Cardiac Cardiac Total Anomalous Pulmonary Venous Return (TAPVR) muscle cells that are not part of the SA or AV nodes
  • Contract when they receive an electrical signal
  • Connected to each other via gap junctions Gap Junctions Connections between cells which allow passage of small molecules and electric current. Gap junctions were first described anatomically as regions of close apposition between cells with a narrow (1-2 nm) gap between cell membranes. The variety in the properties of gap junctions is reflected in the number of connexins, the family of proteins which form the junctions. The Cell: Cell Junctions → capable of conducting electrical signals from 1 cell to the next

Summary of the electrical pathway

  • Clinically, the electrophysiological activity of the heart can be monitored using ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG)
  • The diagram below summarizes the formation and propagation of the electrical wave.
Diagram outlining the electrical pathway of the heart

Diagram outlining the electrical pathway of the heart
AV: atrioventricular

Image by Lecturio.

Conduction times

Action potentials travel at different speeds through different tissues and segments of the conduction system.

  • Atrial myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology: approximately 0.5‒1 m/sec
  • AV node: approximately 0.05 m/sec (slowest)
  • Bundle of His Bundle of His Small band of specialized cardiac muscle fibers that originates in the atrioventricular node and extends into the membranous part of the interventricular septum. The bundle of his, consisting of the left and the right bundle branches, conducts the electrical impulses to the heart ventricles in generation of myocardial contraction. Heart: Anatomy and the left and right bundle branches: approximately 2 m/sec
  • Purkinje fibers Purkinje fibers Modified cardiac muscle fibers composing the terminal portion of the heart conduction system. Heart: Anatomy: approximately 4 m/sec (fastest)
  • Ventricular myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology: approximately 0.5 m/sec
Cardiac conduction system and conduction times of respective segments

Cardiac Cardiac Total Anomalous Pulmonary Venous Return (TAPVR) conduction system and conduction times of respective segments
SA: sinoatrial
AV: atrioventricular
RV: right ventricle
LV: left ventricle
RA: right atrium
LA: left atrium

Image by Lecturio.

Electrophysiology of Nonpacemaker Myocytes

Background

  • Electrical potential:
    • Difference in the concentration of charged particles between 1 point and another (in physiology, usually across a cell membrane Cell Membrane A cell membrane (also known as the plasma membrane or plasmalemma) is a biological membrane that separates the cell contents from the outside environment. A cell membrane is composed of a phospholipid bilayer and proteins that function to protect cellular DNA and mediate the exchange of ions and molecules. The Cell: Cell Membrane)
    • A form of potential energy
    • Capable of producing an electrical current
    • Unit: volts
  • Electrical current:
    • The flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure of charged particles from 1 point to another (in physiology, usually across a cell membrane Cell Membrane A cell membrane (also known as the plasma membrane or plasmalemma) is a biological membrane that separates the cell contents from the outside environment. A cell membrane is composed of a phospholipid bilayer and proteins that function to protect cellular DNA and mediate the exchange of ions and molecules. The Cell: Cell Membrane)
    • Written as “I” (e.g., flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure of K+ ions: IK+)
  • Polarization:
    • Electrical potential exists across a membrane.
    • In cardiac Cardiac Total Anomalous Pulmonary Venous Return (TAPVR) physiology:
      • Hyperpolarized state: The cell is in a more negative state (i.e., a larger concentration gradient is present).
      • Depolarized state: The cell is in a less negative/slightly positive state (i.e., a smaller concentration gradient is present).

Resting membrane potential Resting membrane potential Membrane Potential (RMP)

Nonpacemaker cardiac Cardiac Total Anomalous Pulmonary Venous Return (TAPVR) myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology depolarize only when they receive an electrical stimulus. When nonpacemaker cardiac Cardiac Total Anomalous Pulmonary Venous Return (TAPVR) myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology are not stimulated, they exist in a resting state and have an RMP.

  • RMP: the electrical potential across the myocyte membrane Myocyte Membrane Ion Channel Myopathy in its resting state
  • Created by membrane permeability and the concentration differences of several key ions:
    • K+
    • Na+
    • Calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes ( Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+)
    • Cl
  • Can be calculated using the complex Goldman-Hodgkin-Katz equation
  • Na+/K+ ATPase pump Pump ACES and RUSH: Resuscitation Ultrasound Protocols:
    • Establishes the major concentration differentials of K+ and Na+ across the membrane
    • Pumps 3 Na+ out of the cell in exchange for bringing 2 K+ into the cell
    • → 3 positive charges move out and only 2 positive charges move in
    • → inside of the cell becomes more negative
  • Voltage-gated K+ channels Channels The Cell: Cell Membrane: open at RMP, allowing for a slow trickle of K+ out of the cell
  • At rest, myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology are in a hyperpolarized state:
    • RMP = –90 mV
    • Depicted by an isoelectric (flat) line on graphs showing how the membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential changes over time
Ion conductances at resting potential

Ion conductances at resting potential:
At the hyperpolarized resting potential, voltage-gated K+ channels Channels The Cell: Cell Membrane are the only channels Channels The Cell: Cell Membrane that are open; thus, K+ is the primary contributor to the resting membrane potential Resting membrane potential Membrane Potential of cells.

Image by Lecturio.

Action potentials

  • Action potential Action Potential Abrupt changes in the membrane potential that sweep along the cell membrane of excitable cells in response to excitation stimuli. Membrane Potential:
    • An electrical stimulus leads to the opening of voltage-gated ion channels Channels The Cell: Cell Membrane, allowing ions to flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure into and out of the cell down their concentration gradients (i.e., ion currents).
    • While the current is flowing, the membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential actively changes → action potential Action Potential Abrupt changes in the membrane potential that sweep along the cell membrane of excitable cells in response to excitation stimuli. Membrane Potential
    • An action potential Action Potential Abrupt changes in the membrane potential that sweep along the cell membrane of excitable cells in response to excitation stimuli. Membrane Potential can be divided into phases 0‒4 (usually described as beginning with 4).
    • Often depicted as a graph, showing the change in membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential over time
  • Phase 4 (resting potential):
    • RMP = ‒90 mV
    • Depicted by an isoelectric line
  • Phase 0 (rapid depolarization Depolarization Membrane Potential):
    • Induced by the voltage change from the action potential Action Potential Abrupt changes in the membrane potential that sweep along the cell membrane of excitable cells in response to excitation stimuli. Membrane Potential generated by pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias cells
    • Voltage-gated Na+ channels Channels The Cell: Cell Membrane are activated:
      • Cause a rapid influx of Na+
      • The cell becomes less negative.
  • Phases 1–3 ( repolarization Repolarization Membrane Potential):
Table: Ion channels Channels The Cell: Cell Membrane and their activity during nonpacemaker action potentials
Channel Phase 4 Phase 0 Phase 1 Phase 1 Skin: Structure and Functions Phase 2 Phase 2 Skin: Structure and Functions Phase 3 Phase 3 Skin: Structure and Functions
Voltage-gated Na+ channels Channels The Cell: Cell Membrane Active Deactivating
Fast K+ channels Channels The Cell: Cell Membrane Active
L-type Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ channels Channels The Cell: Cell Membrane Activating Active Deactivating
Delayed K+ channels Channels The Cell: Cell Membrane Active Active

Propagation of depolarization Depolarization Membrane Potential

Propagation refers to how electrical signals spread to every myocyte in the heart.

  • Myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology are connected to each other via gap junctions Gap Junctions Connections between cells which allow passage of small molecules and electric current. Gap junctions were first described anatomically as regions of close apposition between cells with a narrow (1-2 nm) gap between cell membranes. The variety in the properties of gap junctions is reflected in the number of connexins, the family of proteins which form the junctions. The Cell: Cell Junctions.
  • Gap junctions Gap Junctions Connections between cells which allow passage of small molecules and electric current. Gap junctions were first described anatomically as regions of close apposition between cells with a narrow (1-2 nm) gap between cell membranes. The variety in the properties of gap junctions is reflected in the number of connexins, the family of proteins which form the junctions. The Cell: Cell Junctions:
  • Action potentials (i.e., the flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure of ions) pass through gap junctions Gap Junctions Connections between cells which allow passage of small molecules and electric current. Gap junctions were first described anatomically as regions of close apposition between cells with a narrow (1-2 nm) gap between cell membranes. The variety in the properties of gap junctions is reflected in the number of connexins, the family of proteins which form the junctions. The Cell: Cell Junctions: propagation of action potential Action Potential Abrupt changes in the membrane potential that sweep along the cell membrane of excitable cells in response to excitation stimuli. Membrane Potential to the next myocyte
  • Myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology have an absolute refractory period Absolute refractory period Membrane Potential of approximately 250 msec (compared with 1‒2 msec of skeletal muscle).

Electrophysiology of Pacemaker Cells

Action potentials

Pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias cells located in the SA and AV nodes undergo continuous changes in action potential Action Potential Abrupt changes in the membrane potential that sweep along the cell membrane of excitable cells in response to excitation stimuli. Membrane Potential; thus, they do not have a true resting potential.

  • Phase 4 ( pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias potential):
    • Slow spontaneous depolarization Depolarization Membrane Potential during diastole Diastole Post-systolic relaxation of the heart, especially the heart ventricles. Cardiac Cycle (relaxation of the heart muscle) from approximately ‒60 mV up to its threshold Threshold Minimum voltage necessary to generate an action potential (an all-or-none response) Skeletal Muscle Contraction potential of ‒40 mV
    • Mediated primarily by the “funny current (If) through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels Channels The Cell: Cell Membrane:
      • An inward current of Na+
      • An outward current of K+
    • Mediated partly by:
      • An inward current of Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ through transient or T-type (transient) Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ channels Channels The Cell: Cell Membrane
      • An outward current of K+ through delayed K+ channels Channels The Cell: Cell Membrane
  • Phase 0 ( depolarization Depolarization Membrane Potential):
  • Phase 3 Phase 3 Skin: Structure and Functions ( repolarization Repolarization Membrane Potential):
    • Voltage-gated L-type Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ channels Channels The Cell: Cell Membrane are inactivated → Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ influx stops
    • Delayed voltage-gated K+ channels Channels The Cell: Cell Membrane open → efflux of K+ membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential becomes more negative again
Phases of a cardiac pacemaker action potential

Phases of a cardiac Cardiac Total Anomalous Pulmonary Venous Return (TAPVR) pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias action potential Action Potential Abrupt changes in the membrane potential that sweep along the cell membrane of excitable cells in response to excitation stimuli. Membrane Potential:
Phases 4, 0, 3, and 4 occur in sequence. Colored lines depict the duration of respective currents.
If: “funny” current
I Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)(T): transient, short-acting calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes ( Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+) current
I Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)(L): long-lasting Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ current
IK: K+ current

Image by Lecturio.
Table: Ion channels Channels The Cell: Cell Membrane and their activity during pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias action potentials
Channel type Phase 4 Phase 0 Phase 3 Phase 3 Skin: Structure and Functions
HCN channel Active*
Transient or T-type Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ channels Channels The Cell: Cell Membrane Active Inactivated
L-type Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ channels Channels The Cell: Cell Membrane Active Inactivated
Delayed K+ channels Channels The Cell: Cell Membrane Active Active
*Primary current responsible for the membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential during the phase
HCN: hyperpolarization-activated cyclic nucleotide-gated
Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+: calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes ions

Comparison of pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias and nonpacemaker action potentials

Compared with nonpacemaker action potentials, pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias action potentials have the following characteristics:

Pacemaker (green) and nonpacemaker (red) action potentials

Pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias (green) and nonpacemaker (red) action potentials:
Nonpacemaker action potentials begin with quick depolarization Depolarization Membrane Potential followed by slow repolarization Repolarization Membrane Potential, whereas pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias action potentials have a longer depolarization Depolarization Membrane Potential phase. Nonpacemaker action potentials also start from an isoelectric (flat) line, whereas pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias action potentials have none because of their constant oscillation between repolarization Repolarization Membrane Potential and depolarization Depolarization Membrane Potential.

Image by Lecturio.

Regulation of the HR

Heart rate, chronotropy, and dromotropy

Chronotropy refers to the modulation of HR at the level of the pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias cells. The SA node rate is primarily controlled by the ANS ANS The ans is a component of the peripheral nervous system that uses both afferent (sensory) and efferent (effector) neurons, which control the functioning of the internal organs and involuntary processes via connections with the CNS. The ans consists of the sympathetic and parasympathetic nervous systems. Autonomic Nervous System: Anatomy (sympathetic and parasympathetic nerves).

  • Normal resting HR: 60–100/min
  • Tachycardia Tachycardia Abnormally rapid heartbeat, usually with a heart rate above 100 beats per minute for adults. Tachycardia accompanied by disturbance in the cardiac depolarization (cardiac arrhythmia) is called tachyarrhythmia. Sepsis in Children: HR > 100/min
  • Bradycardia Bradycardia Bradyarrhythmia is a rhythm in which the heart rate is less than 60/min. Bradyarrhythmia can be physiologic, without symptoms or hemodynamic change. Pathologic bradyarrhythmia results in reduced cardiac output and hemodynamic instability causing syncope, dizziness, or dyspnea. Bradyarrhythmias: HR < 60/min
  • Negative chronotropy:
    • Slowing down of the HR
    • Mediated by parasympathetic/vagal activation:
      • By acetylcholine Acetylcholine A neurotransmitter found at neuromuscular junctions, autonomic ganglia, parasympathetic effector junctions, a subset of sympathetic effector junctions, and at many sites in the central nervous system. Receptors and Neurotransmitters of the CNS
      • At the muscarinic (M2) receptors Receptors Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors
  • Positive chronotropy:
    • Increase in the HR
    • Mediated by sympathetic activation:
      • By norepinephrine Norepinephrine Precursor of epinephrine that is secreted by the adrenal medulla and is a widespread central and autonomic neurotransmitter. Norepinephrine is the principal transmitter of most postganglionic sympathetic fibers, and of the diffuse projection system in the brain that arises from the locus ceruleus. Receptors and Neurotransmitters of the CNS
      • At the β1-adrenergic receptors Receptors Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors
  • There is a constant, low level of vagal tone slightly suppressing the intrinsic rate of the SA node.

Dromotropy is the modulation of conduction velocity through the AV node (also controlled by the ANS ANS The ans is a component of the peripheral nervous system that uses both afferent (sensory) and efferent (effector) neurons, which control the functioning of the internal organs and involuntary processes via connections with the CNS. The ans consists of the sympathetic and parasympathetic nervous systems. Autonomic Nervous System: Anatomy):

  • Sympathetic: speeds up conduction through the AV node
  • Parasympathetic: slows down conduction through the AV node
Autonomic control of the heart rate at the sa node

Autonomic control of the HR at the SA node:
The sympathetic nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. Nervous System: Anatomy, Structure, and Classification increases the HR (positive chronotropy) by acting on the β1-adrenergic receptors Receptors Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors of the SA node. The parasympathetic nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. Nervous System: Anatomy, Structure, and Classification decreases the HR (negative chronotropy) via the vagus by acting on the muscarinic (M2) receptors Receptors Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors in the SA node.

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Parasympathetic control of HR

Cholinergic nerves release Release Release of a virus from the host cell following virus assembly and maturation. Egress can occur by host cell lysis, exocytosis, or budding through the plasma membrane. Virology acetylcholine Acetylcholine A neurotransmitter found at neuromuscular junctions, autonomic ganglia, parasympathetic effector junctions, a subset of sympathetic effector junctions, and at many sites in the central nervous system. Receptors and Neurotransmitters of the CNS, which brings about 2 primary changes within myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology:

  • Decreases cAMP levels, which in turn:
    • Slows down depolarization by ↓ the “funny” current:
      • → ↓ Na+ flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure into the cell during phase 4
      • → Longer time for the membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential to reach its threshold Threshold Minimum voltage necessary to generate an action potential (an all-or-none response) Skeletal Muscle Contraction (phase 4 line is flatter)
    • Phosphorylation Phosphorylation The introduction of a phosphoryl group into a compound through the formation of an ester bond between the compound and a phosphorus moiety. Post-translational Protein Processing of the Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ channel → ↓ Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ influx → moving the threshold Threshold Minimum voltage necessary to generate an action potential (an all-or-none response) Skeletal Muscle Contraction potential farther from the current membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential
  • Opens more K+ channels Channels The Cell: Cell Membrane → ↑ K+ efflux:
    • Makes the cell more negative
    • → Longer time to reach the threshold Threshold Minimum voltage necessary to generate an action potential (an all-or-none response) Skeletal Muscle Contraction potential
Parasympathetic control of the hr via the av node

Parasympathetic control of the HR via the AV node
AV: atrioventricular
AP: action potential Action Potential Abrupt changes in the membrane potential that sweep along the cell membrane of excitable cells in response to excitation stimuli. Membrane Potential
Vm: membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential
HCN: hyperpolarization-activated cyclic nucleotide-gated

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Sympathetic control of HR

Norepinephrine Norepinephrine Precursor of epinephrine that is secreted by the adrenal medulla and is a widespread central and autonomic neurotransmitter. Norepinephrine is the principal transmitter of most postganglionic sympathetic fibers, and of the diffuse projection system in the brain that arises from the locus ceruleus. Receptors and Neurotransmitters of the CNS is released from sympathetic nerves, which binds to β1-adrenergic receptors Receptors Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors in the myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology and causes an intracellular increase in cAMP cAMP An adenine nucleotide containing one phosphate group which is esterified to both the 3′- and 5′-positions of the sugar moiety. It is a second messenger and a key intracellular regulator, functioning as a mediator of activity for a number of hormones, including epinephrine, glucagon, and acth. Phosphodiesterase Inhibitors, thereby increasing the HR via 2 mechanisms:

  • ↑ “Funny” current through the HCN receptors Receptors Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors → ↑ Na+ influx during phase 4 → increases the rate of depolarization Depolarization Membrane Potential (steeper slope of phase 4)
  • Phosphorylation Phosphorylation The introduction of a phosphoryl group into a compound through the formation of an ester bond between the compound and a phosphorus moiety. Post-translational Protein Processing of Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ channels Channels The Cell: Cell Membrane → ↑ Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ influx → lowering of action potential Action Potential Abrupt changes in the membrane potential that sweep along the cell membrane of excitable cells in response to excitation stimuli. Membrane Potential threshold Threshold Minimum voltage necessary to generate an action potential (an all-or-none response) Skeletal Muscle Contraction (moves closer to the membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential)
Sympathetic control of hr via the av node

Sympathetic control of HR via the AV node
AV: atrioventricular
AP: action potential Action Potential Abrupt changes in the membrane potential that sweep along the cell membrane of excitable cells in response to excitation stimuli. Membrane Potential
Vm: membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential
HCN: hyperpolarization-activated cyclic nucleotide-gated

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Other factors influencing pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias activity

  • Thyroid Thyroid The thyroid gland is one of the largest endocrine glands in the human body. The thyroid gland is a highly vascular, brownish-red gland located in the visceral compartment of the anterior region of the neck. Thyroid Gland: Anatomy hormones Hormones Hormones are messenger molecules that are synthesized in one part of the body and move through the bloodstream to exert specific regulatory effects on another part of the body. Hormones play critical roles in coordinating cellular activities throughout the body in response to the constant changes in both the internal and external environments. Hormones: Overview and Types (i.e., T3 T3 A T3 thyroid hormone normally synthesized and secreted by the thyroid gland in much smaller quantities than thyroxine (T4). Most T3 is derived from peripheral monodeiodination of T4 at the 5′ position of the outer ring of the iodothyronine nucleus. The hormone finally delivered and used by the tissues is mainly t3. Thyroid Hormones and T4 T4 The major hormone derived from the thyroid gland. Thyroxine is synthesized via the iodination of tyrosines (monoiodotyrosine) and the coupling of iodotyrosines (diiodotyrosine) in the thyroglobulin. Thyroxine is released from thyroglobulin by proteolysis and secreted into the blood. Thyroxine is peripherally deiodinated to form triiodothyronine which exerts a broad spectrum of stimulatory effects on cell metabolism. Thyroid Hormones) → ↑ HR
  • ↑ Circulating catecholamines Catecholamines A general class of ortho-dihydroxyphenylalkylamines derived from tyrosine. Adrenal Hormones (e.g., epinephrine Epinephrine The active sympathomimetic hormone from the adrenal medulla. It stimulates both the alpha- and beta- adrenergic systems, causes systemic vasoconstriction and gastrointestinal relaxation, stimulates the heart, and dilates bronchi and cerebral vessels. Sympathomimetic Drugs and norepinephrine Norepinephrine Precursor of epinephrine that is secreted by the adrenal medulla and is a widespread central and autonomic neurotransmitter. Norepinephrine is the principal transmitter of most postganglionic sympathetic fibers, and of the diffuse projection system in the brain that arises from the locus ceruleus. Receptors and Neurotransmitters of the CNS from the adrenal medulla Adrenal Medulla The inner portion of the adrenal gland. Derived from ectoderm, adrenal medulla consists mainly of chromaffin cells that produces and stores a number of neurotransmitters, mainly adrenaline (epinephrine) and norepinephrine. The activity of the adrenal medulla is regulated by the sympathetic nervous system. Adrenal Glands: Anatomy) → ↑ HR
  • ↑ K+ → ↓ HR (because K+ affects membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential)
  • Ischemia Ischemia A hypoperfusion of the blood through an organ or tissue caused by a pathologic constriction or obstruction of its blood vessels, or an absence of blood circulation. Ischemic Cell Damage (↓ in O2) → ↓ HR (works through a different K+ channel)
  • Drugs:
    • Antiarrhythmic agents
    • Ca CA Condylomata acuminata are a clinical manifestation of genital HPV infection. Condylomata acuminata are described as raised, pearly, flesh-colored, papular, cauliflower-like lesions seen in the anogenital region that may cause itching, pain, or bleeding. Condylomata Acuminata (Genital Warts)2+ channel blockers
    • β-adrenergic blockers
    • Digoxin Digoxin A cardiotonic glycoside obtained mainly from digitalis lanata; it consists of three sugars and the aglycone digoxigenin. Digoxin has positive inotropic and negative chronotropic activity. It is used to control ventricular rate in atrial fibrillation and in the management of congestive heart failure with atrial fibrillation. Its use in congestive heart failure and sinus rhythm is less certain. The margin between toxic and therapeutic doses is small. Cardiac Glycosides
Table: Major factors influencing the HR
Factor Increased HR (positive chronotropy) Decreased HR (negative chronotropy)
ANS ANS The ans is a component of the peripheral nervous system that uses both afferent (sensory) and efferent (effector) neurons, which control the functioning of the internal organs and involuntary processes via connections with the CNS. The ans consists of the sympathetic and parasympathetic nervous systems. Autonomic Nervous System: Anatomy* Sympathetic nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. Nervous System: Anatomy, Structure, and Classification Parasympathetic nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. Nervous System: Anatomy, Structure, and Classification
Thyroid Thyroid The thyroid gland is one of the largest endocrine glands in the human body. The thyroid gland is a highly vascular, brownish-red gland located in the visceral compartment of the anterior region of the neck. Thyroid Gland: Anatomy hormones Hormones Hormones are messenger molecules that are synthesized in one part of the body and move through the bloodstream to exert specific regulatory effects on another part of the body. Hormones play critical roles in coordinating cellular activities throughout the body in response to the constant changes in both the internal and external environments. Hormones: Overview and Types Hyperthyroidism Hyperthyroidism Hypersecretion of thyroid hormones from the thyroid gland. Elevated levels of thyroid hormones increase basal metabolic rate. Thyrotoxicosis and Hyperthyroidism Hypothyroidism Hypothyroidism Hypothyroidism is a condition characterized by a deficiency of thyroid hormones. Iodine deficiency is the most common cause worldwide, but Hashimoto’s disease (autoimmune thyroiditis) is the leading cause in non-iodine-deficient regions. Hypothyroidism
K+ Hypokalemia Hypokalemia Hypokalemia is defined as plasma potassium (K+) concentration < 3.5 mEq/L. Homeostatic mechanisms maintain plasma concentration between 3.5-5.2 mEq/L despite marked variation in dietary intake. Hypokalemia can be due to renal losses, GI losses, transcellular shifts, or poor dietary intake. Hypokalemia Hyperkalemia Hyperkalemia Hyperkalemia is defined as a serum potassium (K+) concentration >5.2 mEq/L. Homeostatic mechanisms maintain the serum K+ concentration between 3.5 and 5.2 mEq/L, despite marked variation in dietary intake. Hyperkalemia can be due to a variety of causes, which include transcellular shifts, tissue breakdown, inadequate renal excretion, and drugs. Hyperkalemia
Circulating catecholamines Catecholamines A general class of ortho-dihydroxyphenylalkylamines derived from tyrosine. Adrenal Hormones
  • ↑ Serum epinephrine Epinephrine The active sympathomimetic hormone from the adrenal medulla. It stimulates both the alpha- and beta- adrenergic systems, causes systemic vasoconstriction and gastrointestinal relaxation, stimulates the heart, and dilates bronchi and cerebral vessels. Sympathomimetic Drugs
  • ↑ Serum norepinephrine Norepinephrine Precursor of epinephrine that is secreted by the adrenal medulla and is a widespread central and autonomic neurotransmitter. Norepinephrine is the principal transmitter of most postganglionic sympathetic fibers, and of the diffuse projection system in the brain that arises from the locus ceruleus. Receptors and Neurotransmitters of the CNS
Blood flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure/O2 Ischemia Ischemia A hypoperfusion of the blood through an organ or tissue caused by a pathologic constriction or obstruction of its blood vessels, or an absence of blood circulation. Ischemic Cell Damage/ hypoxia Hypoxia Sub-optimal oxygen levels in the ambient air of living organisms. Ischemic Cell Damage

*Most important factor

Clinical Relevance

Atrioventricular node Atrioventricular node A small nodular mass of specialized muscle fibers located in the interatrial septum near the opening of the coronary sinus. It gives rise to the atrioventricular bundle of the conduction system of the heart. Heart: Anatomy blocks

Atrioventricular node Atrioventricular node A small nodular mass of specialized muscle fibers located in the interatrial septum near the opening of the coronary sinus. It gives rise to the atrioventricular bundle of the conduction system of the heart. Heart: Anatomy blocks occur when an anatomical or functional impairment of the conduction system of the heart produces a delay or interruption in the transmission of action potentials from the atria to the ventricles through the AV node. Affected individuals may be asymptomatic or may present with syncope Syncope Syncope is a short-term loss of consciousness and loss of postural stability followed by spontaneous return of consciousness to the previous neurologic baseline without the need for resuscitation. The condition is caused by transient interruption of cerebral blood flow that may be benign or related to a underlying life-threatening condition. Syncope, chest pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways, dyspnea Dyspnea Dyspnea is the subjective sensation of breathing discomfort. Dyspnea is a normal manifestation of heavy physical or psychological exertion, but also may be caused by underlying conditions (both pulmonary and extrapulmonary). Dyspnea, and bradycardia Bradycardia Bradyarrhythmia is a rhythm in which the heart rate is less than 60/min. Bradyarrhythmia can be physiologic, without symptoms or hemodynamic change. Pathologic bradyarrhythmia results in reduced cardiac output and hemodynamic instability causing syncope, dizziness, or dyspnea. Bradyarrhythmias depending on the severity of the block. Diagnosis is established based on ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG), and treatment is based on the type of block and hemodynamic stability of the affected individual.

  • 1st-degree AV block AV block Atrioventricular (AV) block is a bradyarrhythmia caused by delay, or interruption, in the electrical conduction between the atria and the ventricles. Atrioventricular block occurs due to either anatomic or functional impairment, and is classified into 3 types. Atrioventricular block (AV block): delayed conduction through the AV node. Affected individuals have sinus rhythm; however, their overall HR is slower.
  • 2nd-degree AV block AV block Atrioventricular (AV) block is a bradyarrhythmia caused by delay, or interruption, in the electrical conduction between the atria and the ventricles. Atrioventricular block occurs due to either anatomic or functional impairment, and is classified into 3 types. Atrioventricular block (AV block): delayed conduction through the AV node. Some atrial action potentials fail to make it through the AV node, resulting in ventricular bradycardia Bradycardia Bradyarrhythmia is a rhythm in which the heart rate is less than 60/min. Bradyarrhythmia can be physiologic, without symptoms or hemodynamic change. Pathologic bradyarrhythmia results in reduced cardiac output and hemodynamic instability causing syncope, dizziness, or dyspnea. Bradyarrhythmias.
    • Mobitz type I ( Wenckebach Wenckebach Atrioventricular block (AV block)): progressive increase in conduction delay until a signal fails to make it through the AV node altogether, resulting in the signal (and thus a mechanical contraction) being “dropped.”
    • Mobitz type II: there is no progressive increase in delayed conduction; however, conduction through the AV node is intermittent with some “dropped” signals that do not make it through to the ventricles. Dropped signals often occur in a regular Regular Insulin pattern (e.g., 2:1 pattern). Mobitz type II block almost always results from conduction system disease below the level of the AV node.
  • 3rd-degree AV block AV block Atrioventricular (AV) block is a bradyarrhythmia caused by delay, or interruption, in the electrical conduction between the atria and the ventricles. Atrioventricular block occurs due to either anatomic or functional impairment, and is classified into 3 types. Atrioventricular block (AV block): a complete block through the AV node resulting in atrial-ventricular dissociation Dissociation Defense Mechanisms (activation and contraction are independent of each other as no atrial impulses reach the ventricles). Affected individuals have ventricular bradycardia Bradycardia Bradyarrhythmia is a rhythm in which the heart rate is less than 60/min. Bradyarrhythmia can be physiologic, without symptoms or hemodynamic change. Pathologic bradyarrhythmia results in reduced cardiac output and hemodynamic instability causing syncope, dizziness, or dyspnea. Bradyarrhythmias driven by an escape Escape With constant immune mechanisms holding unstable tumor cells in equilibrium, tumor-cell variants may emerge. These cancer cells may express fewer antigens on their surfaces or lose their MHC class I expression.Variants may also protect themselves from T-cell attack via expression of IC molecules on their surfaces, like normal cells.Creation of an immunosuppressive state in the microenvironment is another way to grow without immunologic interference. Cancer Immunotherapy pacemaker Pacemaker A device designed to stimulate, by electric impulses, contraction of the heart muscles. It may be temporary (external) or permanent (internal or internal-external). Bradyarrhythmias distal to the block.

Bundle branch and fascicular blocks Fascicular Blocks Bundle branch and fascicular blocks occur when the normal electrical activity in the His-Purkinje system is interrupted. These blocks can be due to many etiologies that may affect the structure of the heart or the conduction system directly. Bundle Branch and Fascicular Blocks

Bundle branch and fascicular blocks Fascicular Blocks Bundle branch and fascicular blocks occur when the normal electrical activity in the His-Purkinje system is interrupted. These blocks can be due to many etiologies that may affect the structure of the heart or the conduction system directly. Bundle Branch and Fascicular Blocks occur when normal electrical activity in the His-Purkinje system is interrupted. Bundle branch and fascicular blocks Fascicular Blocks Bundle branch and fascicular blocks occur when the normal electrical activity in the His-Purkinje system is interrupted. These blocks can be due to many etiologies that may affect the structure of the heart or the conduction system directly. Bundle Branch and Fascicular Blocks can occur due to many etiologies that may affect Affect The feeling-tone accompaniment of an idea or mental representation. It is the most direct psychic derivative of instinct and the psychic representative of the various bodily changes by means of which instincts manifest themselves. Psychiatric Assessment the structure of the heart or the conduction system directly (e.g., myocardial ischemia Myocardial ischemia A disorder of cardiac function caused by insufficient blood flow to the muscle tissue of the heart. The decreased blood flow may be due to narrowing of the coronary arteries (coronary artery disease), to obstruction by a thrombus (coronary thrombosis), or less commonly, to diffuse narrowing of arterioles and other small vessels within the heart. Coronary Heart Disease, myocarditis Myocarditis Myocarditis is an inflammatory disease of the myocardium, which may occur alone or in association with a systemic process. There are numerous etiologies of myocarditis, but all lead to inflammation and myocyte injury, most often leading to signs and symptoms of heart failure. Myocarditis, cardiomyopathy Cardiomyopathy Cardiomyopathy refers to a group of myocardial diseases associated with structural changes of the heart muscles (myocardium) and impaired systolic and/or diastolic function in the absence of other heart disorders (coronary artery disease, hypertension, valvular disease, and congenital heart disease). Cardiomyopathy: Overview and Types). Although usually asymptomatic, bundle branch and fascicular blocks Fascicular Blocks Bundle branch and fascicular blocks occur when the normal electrical activity in the His-Purkinje system is interrupted. These blocks can be due to many etiologies that may affect the structure of the heart or the conduction system directly. Bundle Branch and Fascicular Blocks may occasionally cause syncope Syncope Syncope is a short-term loss of consciousness and loss of postural stability followed by spontaneous return of consciousness to the previous neurologic baseline without the need for resuscitation. The condition is caused by transient interruption of cerebral blood flow that may be benign or related to a underlying life-threatening condition. Syncope.

  • Bundle branch blocks Bundle Branch Blocks Bundle branch and fascicular blocks occur when the normal electrical activity in the His-Purkinje system is interrupted. These blocks can be due to many etiologies that may affect the structure of the heart or the conduction system directly. Bundle Branch and Fascicular Blocks: a block in the downward progression of electrical impulses through 1 of the bundle branches in the interventricular septum Interventricular Septum Ventricular Septal Defect (VSD). The block forces electrical signals down the other bundle branch such that the ventricle depolarizes 1st. Electrical waves then travel through the myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology directly to cause depolarization Depolarization Membrane Potential on the affected side.
  • Fascicular blocks Fascicular Blocks Bundle branch and fascicular blocks occur when the normal electrical activity in the His-Purkinje system is interrupted. These blocks can be due to many etiologies that may affect the structure of the heart or the conduction system directly. Bundle Branch and Fascicular Blocks: a block in 1 of the more distal Purkinje fibers Purkinje fibers Modified cardiac muscle fibers composing the terminal portion of the heart conduction system. Heart: Anatomy. The affected area will receive electrical signals more slowly from the surrounding myocytes Myocytes Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal, cardiac, and smooth. They are derived from embryonic (precursor) muscle cells called myoblasts. Muscle Tissue: Histology.

Antiarrhythmic medications

The following classes of drugs are used for the treatment of arrhythmias:

References

  1. Mohrman, D.E., Heller, L.J. (2018). Overview of the cardiovascular system. Cardiovascular physiology, 9e. New York, NY: McGraw-Hill Education. https://accessmedicine.mhmedical.com/content.aspx?aid=1153946098
  2. Mohrman, D.E., Heller, L.J. (2018). Vascular control. Cardiovascular physiology, 9e. New York, NY: McGraw-Hill Education. https://accessmedicine.mhmedical.com/content.aspx?aid=1153946722
  3. Mohrman, D.E., Heller, L.J. (2018). Regulation of arterial pressure. Cardiovascular physiology, 9e. New York, NY: McGraw-Hill Education. https://accessmedicine.mhmedical.com/content.aspx?aid=1153946898
  4. Baumann, B.M. (2016). Systemic hypertension. In Tintinalli, J.E., et al. (Eds.), Tintinalli’s emergency medicine: A comprehensive study guide, 8e. New York, NY: McGraw-Hill Education. https://accessmedicine.mhmedical.com/content.aspx?aid=1121496251
  5. University of Minnesota. Conduction System Tutorial. Retrieved June 1, 2021, from http://www.vhlab.umn.edu/atlas/conduction-system-tutorial/cardiac-action-potentials.shtml
  6. Hall, J.E., Guyton, A.C. (2016). The Heart. In Guyton and Hall Textbook of Medical Physiology (13th ed). https://www.elsevier.com/books/guyton-and-hall-textbook-of-medical-physiology/hall/978-1-4557-7005-2

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