Normal Electrocardiogram (ECG)

An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Electrocardiograms are simple, inexpensive, noninvasive, and readily obtained. 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. In the healthy state, an ECG records predictable, reproducible waves and complexes, which correspond to electromechanically coupled physiologic events in the heart. Under pathologic conditions, the ECG can detect arrhythmias, ischemia, inflammation, and more.

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Overview

In 1902 the electrocardiogram (ECG) was invented by Willem Einthoven, a Dutch physician. Einthoven received the 1924 Nobel Prize in Physiology or Medicine for the invention.

Terminology

Electrocardiogram is abbreviated/referred to as: 

  • ECG: American spelling 
  • EKG: European spelling
  • The terms may be used interchangeably.
  • Clinically referred to as a 12-lead ECG

ECG electrodes and leads

  • Conductive electrodes are affixed to the skin with adhesive backing.
  • 10 electrodes are required to produce a 12-lead ECG:
    • 1 electrode is affixed to each limb:
      • Historically, a resting ECG was affixed to the distal limb.
      • In the clinical setting, a resting ECG is affixed to the thorax near the corresponding limb.
    • 6 electrodes are placed on the precordium:
      • V1: 4th intercostal space (ICS), RIGHT margin of the sternum
      • V2: 4th ICS, LEFT margin of the sternum
      • V4: 5th ICS, midclavicular line
      • V3: midway between V2 and V4
      • V5: 5th ICS, anterior axillary line 
      • V6: 5th ICS, midaxillary line 
  • 12 lead is created by ECG machine/software voltages transmitted from electrodes:
    • 3 bipolar limb leads (recording obtained from pairs of limb electrodes):
      • I: right arm (-) and left arm (+) electrodes
      • II: right arm (-) and left leg (+) electrodes
      • III: left arm (-) and left leg (+) electrodes
      • Einthoven triangle: A schematic triangle made from the 3 electrodes involved in creating leads I, II, and III.
    • 3 augmented unipolar limb leads (recording obtained from a limb electrode and the central terminal):
      • Augmented Vector Right (aVR): right arm (+) electrode and the central terminal (-)
      • Augmented Vector Left (aVL): left-arm (+) electrode and the central terminal (-)
      • Augmented Vector Foot (aVF): left leg electrode (+) and the central terminal (-)
    • 6 precordial leads (recording obtained from the corresponding chest electrode and the central terminal):
      • V1, V2: septal leads
      • V3, V4: anterior leads
      • V5, V6: lateral leads

ECG tracing

  • Organized as a graph in boxes:
    • Small box = 1 x 1 mm
    • Large box = 5 x 5 mm (5 small boxes)
  • X and Y axes:
    • X axis = time in seconds
    • Y axis = voltage in mV
  • X-axis utility:
    • ECG tracing speed = 25 mm/second (25 small boxes/second or 5 large boxes/second) 
    • 1 small box = 0.04 second
    • 1 large box = 0.2 second
    • Allows for heart rate calculation and rhythm determination:
      • Bradycardia vs. tachycardia
      • Regular vs. irregular
    • Allows for measurement of clinically relevant intervals and durations
  • Y-axis utility:
    • 1 small box = 0.1 mV
    • Allows for determination of voltage amplitude of ECG waveforms
    • Amplitude correlates with electromechanically coupled events in the cardiac cycle:
      • No deflection = no cardiac conduction or contraction (e.g., isoelectric baseline)
      • Small deflection = low voltage associated with thin myocardium (atria) and modest contraction (e.g., P wave)
      • Large deflection = high voltage associated with thick myocardium (ventricle) and forceful contraction (e.g., QRS complex)
Measuring time and voltage with ECG graph paper

ECG voltage pulse and size of squares

Image: “Measuring time and voltage with ECG graph paper” by Markus Kuhn. License: Public Domain

Components

A normal ECG tracing will have several predictable and reproducible components corresponding to electromechanical events in the cardiac cycle. 

Electrical impulse of heart contraction

  • Isoelectric baseline:
    • Flat tracing free of positive or negative deflections in between waves and/or complexes
    • Represents periods of electrical inactivity in the cardiac cycle
  • Waves:
    • P wave:
      • Represents atrial depolarization 
      • Positive deflection in inferior/lateral leads
    • T wave:
      • Represents ventricular repolarization
      • Positive deflection
  • Intervals:
    • PR interval:
      • From the beginning of the P wave to the initial defection of the QRS complex
      • Represents the time needed for the electrical impulse to travel from the sinoatrial (SA) node to the atrioventricular (AV) node
    • QT interval:
      • From the beginning of the QRS complex to the end of the T wave
      • Represents ventricular depolarization, contraction, and repolarization
    • RR interval:
      • The time between 2, successive QRS complexes
      • Used to calculate heart rate
  • Segments:
    • PQ segment: isoelectric segment between the P wave and initial deflection of the QRS complex
    • ST segment: isoelectric segment between the S wave and the initial deflection of the T wave
    • TP segment: isoelectric baseline between the T wave and the initial deflection of the P wave
  • QRS complex: 
    • Represents ventricular depolarization
    • Composed of 3 waves:
      • Q wave: negative deflection
      • R wave: positive deflection
      • S wave: negative deflection
  • Depending on the ECG lead monitored:
    • QS wave: Positive deflection may not be apparent (i.e. no R wave).
    • RS wave: Negative deflection may not be apparent (i.e. no Q wave).
    • QRS complex: Collective ventricular depolarization regardless of the presence or absence of all components.
Parts of ECG waveforms and intervals

Parts of ECG waveforms and intervals

Image by Lecturio. License: CC BY-NC-SA 4.0

Correlation to electromechanical coupling

  • The cardiac electrical cycle begins spontaneously in the SA node of the right atrium: 
    • ECG: termination of TP segment, onset of the P wave 
    • Mechanical: atria infused with blood from passive venous filling:
      • Vena cava: fills the right atrium
      • Pulmonary veins: fill the left atrium
  • Electrical impulse of depolarization spreads throughout the atria via the internodal pathways and arrives at the AV node located in the AV septum:
    • ECG: completion of the P wave:
      • The atria repolarize electrically during the ventricular portions of the cardiac electrical cycle.
      • Atrial repolarization is obscured by the QRS complex on ECG tracing.
    • Mechanical: atrial contraction, ventricular relaxation:
      • Tricuspid/mitral valves open
      • Pulmonic/aortic valves close
      • Ventricles fill with blood
  • Electrical activity is slowed considerably by specialized conductive cells in the central portions of the AV node: 
    • ECG: PQ segment (isoelectric baseline)
    • Mechanical: ventricles fill with blood from atrial contraction, atria relax and passively fill with blood
  • Electrical activity resumes as the cardiac impulse arrives at the rapidly conducting pathways in the interventricular septum (atrioventricular bundle or His bundle):
    • ECG: onset of Q wave
    • Mechanical: ventricular septum contracts, passive atrial filling continues:
      • Tricuspid/mitral valves shut, chordae tendineae taut
      • Papillary muscles isometrically contract to maintain the integrity of tricuspid/mitral apparatus
  • Electrical activity spreads toward the apex of the heart along the right and left bundle branches traversing the thickest portions of the myocardium:
    • ECG: R wave
    • Mechanical: continuation of ventricular free wall contraction, passive atrial filling:
      • Tricuspid/mitral valves “balloon” back into the atria
      • Pulmonic/aortic valves open
  • Electrical activity terminates in the Purkinje fibers, penetrating the deepest portions of the myocardium near the endocardium:
    • ECG: S wave
    • Mechanical: completion of ventricular contraction, the continuation of passive atrial filling 
  • Cardiac electrical activity plateaus briefly:
    • ECG: ST segment
    • Mechanical: ventricles begin to relax, passive atrial filling continues:
      • Tricuspid/mitral valves closed
      • Pulmonic/aortic valves closed
  • Ventricular repolarization:
    • ECG: T wave and TP segment
    • Mechanical: ventricular relaxation, passive atrial filling continues:
      • Pressure gradient exists between filling atria and emptied ventricles
      • Ventricles begin to passively fill
      • Tricuspid/mitral valves partially open
  • Another cardiac cycle begins

Systematic Interpretation

  1. Calibration (voltage and speed): standard:
    • Paper/tracing speed = 25 mm/second
    • 1 mm (horizontal) = 0.04 second
    • 1 mm (vertical) = 0.1 mV
  2. Calculate heart rate:
    • Calculation: divide 300 by the number of large squares between RR intervals
    • Normal heart rate: 60–100/min
  3. Determine rhythm: normal sinus rhythm criteria:
    • Normal P-wave morphology
    • A regular QRS complex follows every P wave
    • Normal, constant PR/RR intervals
  4. Determine timing intervals (PR, QRS, QTC):
    • Manual calculation by measuring horizontal blocks
    • Electronic calculation by machine/software (usually listed in the top-left corner)
    • PR interval 0.12–0.2 second
    • QRS complex < 0.12 second
    • QTC interval 0.30–0.46 second
  5. Determine mean QRS axis:
    • Direction of QRS deflection:
      • Positive: The mean electrical vector travels towards the positive electrode in a given lead.
      • Negative: The mean electrical vector travels away from the positive electrode in a given lead.
    • Normal: -30°–100°
    • Normally positive in lead II and lead aVF
  6. Evaluate P-wave morphology by voltage size and deflection:
    • Normal time < 0.12 second
    • Normally upright in lead II and lead aVF
  7. Evaluate QRS morphology and/or voltage:
    • Normal duration < 0.12 second
    • R wave should transition in amplitude in the precordial leads:
      • Lowest voltage in V1
      • Highest voltage in V6
  8. Evaluate ST-segment and T-wave morphology:
    • ST segment:
      • Flat, isoelectric segment after QRS complex, but before T wave
      • Normally no depression or elevation
    • T wave: normally concordant with QRS complex
  9. Compare with prior tracings if available.
ECG interpretation

Normal ECG: 12-lead tracing with a V1 rhythm strip displayed at the bottom

Image: “ECG interpretation” by Rodhullandemu. License: Public Domain

References

  1. Sattar Y, Chhabra L. Electrocardiogram. [Updated 2020 Nov 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. https://www.ncbi.nlm.nih.gov/books/NBK549803/
  2. Prutkin, J. (2019). ECG tutorial: Basic principles of ECG analysis. Retrieved June 6, 2021, from https://www.uptodate.com/contents/ecg-tutorial-basic-principles-of-ecg-analysis
  3. Prutkin, J. (2019). ECG tutorial: Electrical components of the ECG. Retrieved June 6, 2021, from https://www.uptodate.com/contents/ecg-tutorial-electrical-components-of-the-ecg

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