Cardiac Cycle

The cardiac cycle describes 1 complete contraction and relaxation of all 4 chambers of the heart during a standard heartbeat. The cardiac cycle includes 7 phases, which together describe the cycle of ventricular filling, isovolumetric contraction, ventricular ejection, and isovolumetric relaxation. The cycle is frequently represented in a graph known as a pressure-volume loop, which shows how intraventricular volumes and pressures are related to one another throughout the cardiac cycle.

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Definition and Anatomy Review

Definitions

The cardiac cycle describes 1 complete contraction and relaxation of all 4 chambers of the heart during a standard heartbeat. The 7 phases of the cardiac cycle all occur in < 1 second.

  • Systole: cardiac muscle contraction
  • Diastole: cardiac muscle relaxation
Ventricular systole and diastole, and the coinciding phases of the cardiac cycle

Ventricular systole and diastole, and the coinciding phases of the cardiac cycle

Image by Lecturio.

Anatomy review

Blood sequentially flows through the heart in 1 direction through the following structures: 

  • Deoxygenated blood enters the heart via the superior vena cava (SVC) and the inferior vena cava (IVC) → 
  • Right atrium (RA) → tricuspid valve → right ventricle (RV) → pulmonary valve → 
  • Pulmonary trunk → pulmonary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs (blood is oxygenated) → pulmonary veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins → 
  • Left atrium (LA) → mitral valve → left ventricle (LV) → aortic valve → aorta → 
  • Systemic arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries (blood is deoxygenated) → veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins → SVC/IVC → back to the heart
General structure and flow of blood through the heart

The general structure and flow of blood through the heart:
Blue areas represent deoxygenated blood, which passes through the right side of the heart. Red areas represent oxygenated blood, which passes through the left side of the heart. The right side of the heart pumps the deoxygenated blood to the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs. The left side of the heart pumps blood out to the systemic arterial system.

Image by Lecturio.

Phases of the Cardiac Cycle

Phases 6, 7, and 1: ventricular filling

Ventricular filling occurs in 3 parts:

  • Ventricles are in diastole (relaxed).
  • Pressure in the ventricles is lower than the pressure in the atria.
  • = Atrioventricular (AV) valves (tricuspid and mitral valves) are open.
  • Phase 6:
    • Blood rapidly fills the ventricles.
    • Result of the high-pressure gradient between the atria and ventricles
  • Phase 7:
    • Marked by slower passive filling
    • P wave of the 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. Normal Electrocardiogram (ECG) occurs marking atrial depolarization.
    • 90% of the ventricular volume is obtained through passive filling in phases 6 and 7.
  • Phase 1:
    • Atrial systole (i.e., atrial contraction) occurs.
    • Supplies the remaining 10% of the ventricular volume at rest (40% during exercise)
  • End-diastolic volume (EDV): 
    • The volume of blood in a single ventricle at the end of ventricular filling
    • Approximately 130 mL in each ventricle

Phase 2: isovolumetric contraction

Isovolumetric means “same volume”. Phase 2 represents ventricular contraction while both the AV valves and the semilunar valves are closed, which = no change in ventricular volume.

  • Ventricles depolarize: = QRS complex 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. Normal Electrocardiogram (ECG)
  • Ventricular systole (i.e., contraction) begins.
  • Ventricular pressure rises sharply:
    • Ventricular pressure > atrial pressure
    • Tricuspid and mitral valves close as blood surges back against the valves (heart sound S1).
  • Semilunar valves (aortic and pulmonic valves) remain closed because:
    • Aortic pressure > LV pressure 
    • Pulmonary trunk pressure > RV pressure
  • = No blood is ejected during phase 2 of ventricular contraction.
Diagram of the cardiac cycle highlighting phase 2

Diagram of the cardiac cycle highlighting phase 2, isovolumetric contraction:
Left ventricular volume (blue) remains the same, but the left ventricular pressure (red) sharply increases, occurring immediately following left ventricular depolarization (seen as the QRS complex on the 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. Normal Electrocardiogram (ECG) (yellow line)).

Image by Lecturio.

Phases 3 and 4: ventricular ejection

  • Blood is ejected from the ventricle during ventricular contraction (i.e., systole):
    • Phase 3: rapid ejection
    • Phase 4: slower ejection
  • Begins when the semilunar valves open, which occurs when:
    • LV pressure > aortic pressure
    • RV pressure > pulmonary trunk pressure
  • Changes in ventricular pressure:
    • Initially, a sharp increase continues in the 1st half of phase 3.
    • Decrease as blood is ejected during the 2nd half of phase 3 and continues through phase 4
  • End-systolic volume (ESV): 
    • Residual volume left in the ventricles at the end of systole
    • Approximately 60 mL
  • Stroke volume: 
    • The amount of fluid ejected during systole
    • Stroke volume = EDV ‒ ESV
  • Ejection fraction (EF): 
    • The percentage of the EDV ejected during ventricular systole
    • EF = stroke volume / EDV
    • Average EF: approximately 55%–60% 
  • Ventricular repolarization occurs during phases 3 and 4: = T wave 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. Normal Electrocardiogram (ECG)
Cardiac cycle highlighting phases 3 and 4

Diagram of the cardiac cycle highlighting phases 3 and 4, ventricular ejection:
As the left ventricle remains contracted, the ventricular pressure (red line) peaks and begins to decrease as blood is ejected out of the ventricle. The phase coincides with the ST segment of the electrocardiogram Electrocardiogram 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. Normal Electrocardiogram (ECG) followed by the T wave of ventricular repolarization (yellow line).

Image by Lecturio.

Phase 5: isovolumetric relaxation

  • Beginning of ventricular diastole
  • Aortic and pulmonary valves close due to decreasing intraventricular pressures.
  • Intraventricular pressure sharply decreases as the ventricular muscles relax and the cavity expands.
  • T wave resolution 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. Normal Electrocardiogram (ECG)
Diagram of the cardiac cycle highlighting phase 5

Diagram of the cardiac cycle highlighting phase 5, isovolumetric relaxation:
As the left ventricle begins isovolumetric relaxation, the ventricular pressure sharply drops (red line), but the volume does not change (blue line). The aortic valve closes in response to the pressure differential between the aorta and the left ventricle. Aortic pressure is shown by the top dotted line. The ventricle finishes repolarizing as shown on the electrocardiogram Electrocardiogram 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. Normal Electrocardiogram (ECG) (yellow line).

Image by Lecturio.

Atrial pressures

  • Atrial systole: contraction of the atria (phase 1)
  • Atrial diastole: relaxation of the atria (phases 2–7)
  • Atrial pressure waveforms:
    • A wave: represents atrial contraction (i.e., atrial systole (also known as the “atrial kick”))
    • C wave: reverberation of pressure into the atrium during ventricular contraction
    • V wave: 
      • During phases 3–5, the atria fill and the AV valves are closed.
      • The V wave marks the sudden drop in pressure as the AV valves open at the beginning of ventricular filling.
Diagram of the cardiac cycle highlighting the phases of atrial diastole

Diagram of the cardiac cycle highlighting the phases of atrial diastole:
Atrial pressure is shown by the lower dotted line. The A wave indicates the increase in atrial pressure during atrial contraction. As the ventricles contract against the closed atrioventricular (AV) valves, the increase in pressure reverberates through the atria as seen in the C wave. The C wave coincides with the isovolumetric contraction of the left ventricle. Atrial relaxation begins during ventricular systole. With the AV valves closed, the atria are passively filled, increasing the atrial pressure. The V wave signifies the sudden fall in atrial pressure when the AV valves open to begin phase 6 (passive ventricular filling).

Image by Lecturio.

Normal Pressures in the Cardiac Chambers and Great Vessels

The normal pressures found in the heart chambers and great vessels during ventricular systole and diastole are shown in the table and image below:

  • Differences in pressure between 1 chamber and another cause:
    • Valves to open or close
    • Blood to move forward in the circuit
  • Low-pressure areas: right side of the heart
  • High-pressure areas: left side of the heart
Table: Normal pressures in the heart chambers and great vessels
Location Pressure during ventricular systole (mm Hg) Pressure during ventricular diastole (mm Hg)
Right atrium 0–4 0–4
Right ventricle 25 4
Pulmonary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries 25 10
Left atrium 8–10 8–10
Left ventricle 120 10
Aorta 120 80
Pressures (in mm hg) within the chambers of the heart and great vessels

Pressures (in mm Hg) within the chambers of the heart and great vessels:
The numerator represents the highest pressure achieved during systole and the denominator represents the lowest pressure achieved during diastole. Pressure differentials directly cause valve opening and blood movement.
A: aorta
RA: right atrium
PT: pulmonary trunk
LA: left atrium
RV: right ventricle
LV: left ventricle
* denotes end-diastolic pressure

Image by Lecturio.

Pressure-Volume Loops

  • A graphic demonstration of how volumes and pressures change in the left ventricle throughout the cardiac cycle:
    • Removes “time” from the cardiac cycle graphs
    • Result: The diagram appears as a loop.
  • X-axis: LV volume
  • Y-axis: LV pressure
  • Point A: mitral valve opens
  • Point B: mitral valve closes
  • Point C: aortic valve opens
  • Point D: aortic valve closes
  • Ventricular filling (phases 6, 7, and 1): 
    • Segment A → B
    • Seen as the “flat-ish” line across the bottom of the graph, moving from left to right
    • Volume increases.
    • Because the mitral valve is open, pressure increases minimally.
  • Isovolumetric contraction (phase 2): 
    • Segment B → C
    • Seen as the vertical line going straight up
    • The mitral valve closes at point B.
    • The aortic valve doesn’t open until point C: = Volume change is not possible with both valves closed.
    • The ventricular contraction causes an increase in LV pressure without changes in volume.
  • Ventricular ejection (phases 3 and 4): 
    • Segment C → D
    • Seen as the curved line along the top, moving from right to left
    • The aortic valve opens allowing blood to leave: = volume falls
    • Ventricles contract: Pressure increases until volume falls, causing pressure to also fall.
  • Isovolumetric relaxation (phase 5): 
    • Segment D → A
    • Seen as the vertical line going straight down
    • The aortic valve closes.
    • The mitral valve does not open until point A: = Volume change is not possible with both valves closed.
    • Ventricular relaxation causes a drop in LV pressure without changes in volume.
Left ventricular pressure-volume loop

Left ventricular pressure-volume loop:
The diagram illustrates the relationship between left intraventricular pressure and volume throughout the cardiac cycle. The segment from point A to point B represents ventricular filling. The mitral valve opens at point A and closes at point B. The segment from point B to point C represents isovolumetric contraction. The aortic valve opens at point C. The curved line from point C to point D represents ventricular ejection. The aortic valve closes at point D. The segment from point D to point A represents isovolumetric relaxation.

Image by Lecturio.

Clinical Relevance

The following are common conditions affecting the cardiac cycle.

  • Heart failure (HF): Congestive heart failure Congestive heart failure Congestive heart failure refers to the inability of the heart to supply the body with normal cardiac output to meet metabolic needs. Echocardiography can confirm the diagnosis and give information about the ejection fraction. Congestive Heart Failure ( CHF CHF Congestive heart failure refers to the inability of the heart to supply the body with normal cardiac output to meet metabolic needs. Echocardiography can confirm the diagnosis and give information about the ejection fraction. Congestive Heart Failure) refers to the inability of the heart to supply the body with the normal cardiac output required to meet metabolic needs. Congestive heart failure Congestive heart failure Congestive heart failure refers to the inability of the heart to supply the body with normal cardiac output to meet metabolic needs. Echocardiography can confirm the diagnosis and give information about the ejection fraction. Congestive Heart Failure may result in chest pain Chest Pain Chest pain is one of the most common and challenging complaints that may present in an inpatient and outpatient setting. The differential diagnosis of chest pain is large and includes cardiac, gastrointestinal, pulmonary, musculoskeletal, and psychiatric etiologies. Chest Pain, exertional 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 episodes of hypotension Hypotension Hypotension is defined as low blood pressure, specifically < 90/60 mm Hg, and is most commonly a physiologic response. Hypotension may be mild, serious, or life threatening, depending on the cause. Hypotension, dizziness, and/or 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. Ejection fraction of the LV is used to clinically categorize HF into either HF with preserved EF (≥ 50%) or HF with reduced EF (≤ 40%). Severity, prognosis, and treatment regimens differ for each category.
  • Cardiomyopathies: a group of myocardial diseases associated with structural changes of the myocardium and impaired systolic and/or diastolic function in the absence of other heart disorders. Cardiomyopathies are classified as dilated, restrictive, hypertrophic, or arrhythmogenic. With abnormal ventricular structure, the pumping action of the ventricles can be severely impaired, resulting in heart failure and/or volume overload. 

References

  1. Mohrman, D. E., & Heller, L. J. (2018). Overview of the cardiovascular system. In Mohrman, D. E., & Heller, L. J. (Eds.), Cardiovascular physiology, (9th ed. pp. 1–22). McGraw-Hill Education. https://www.accessmedicine.mhmedical.com/content.aspx?aid=1153946098
  2. Mohrman, D. E., & Heller, L. J. (2018). Vascular control. In Mohrman, D. E., & Heller, L. J. (Eds.), Cardiovascular physiology, (9th ed. pp. 128–159). McGraw-Hill Education. https://www.accessmedicine.mhmedical.com/content.aspx?aid=1153946722
  3. Mohrman, D. E., & Heller, L. J. (2018). Regulation of arterial pressure. In Mohrman, D. E., & Heller, L. J. (Eds.), Cardiovascular physiology, (9th ed. pp. 175–196). McGraw-Hill Education. https://www.accessmedicine.mhmedical.com/content.aspx?aid=1153946898
  4. Baumann, B. M. (2016). Systemic hypertension. In Tintinalli, J. E., et al. (Ed.), Tintinalli’s emergency medicine: A comprehensive study guide, (8th ed., pp. 399–407). McGraw-Hill Education. https://www.accessmedicine.mhmedical.com/content.aspx?aid=1121496251
  5. Conduction System Tutorial. (n.d.). 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).
  7. Saladin, K. S., Miller, L. (2004). Anatomy and physiology. (3rd ed., pp. 739–740).

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