16. ELECTROCARDIOGRAM (ECG)

Electrocardiogram (ECG)

Introduction:

As action potentials propagate through the heart, they generate electrical currents that can be detected at the surface of the body.

  • An electrocardiogram, abbreviated either ECG or EKG (from the German word Elektrokardiogram), is a recording of these electrical signals.
  • The ECG is a composite record of action potentials produced by all of the heart muscle fibers during each heartbeat. The instrument used to record the changes is an electrocardiograph.
  • In clinical practice, electrodes are positioned on the arms and legs (limb leads) and at six positions on the chest (chest leads) to record the ECG.
  • The electrocardiograph amplifies the heart’s electrical signals and produces 12 different tracings from different combinations of limb and chest leads.
  • Each limb and chest electrode records slightly different electrical activity because of the difference in its position relative to the heart.
  • By comparing these records with one another and with normal records, it is possible to determine:
    1. If the conducting pathway is abnormal,
    2. If the heart is enlarged,
    3. If certain regions of the heart are damaged, and
    4. The cause of chest pain.

ECG Waves

In a typical record, three clearly recognizable waves appear with each heartbeat:

P Wave

The first, called the P wave, is a small upward deflection on the ECG. The P wave represents atrial depolarization, which spreads from the SA node through contractile fibers in both atria.

QRS Complex

The second wave, called the QRS complex, begins as a downward deflection, continues as a large, upright, triangular wave, and ends as a downward wave. The QRS complex represents rapid ventricular depolarization, as the action potential spreads through ventricular contractile fibers.

T Wave

The third wave is a dome-shaped upward deflection called the T wave. It indicates ventricular repolarization and occurs just as the ventricles are starting to relax. The T wave is smaller and wider than the QRS complex because repolarization occurs more slowly than depolarization.

Clinical Significance of ECG

  • Larger P waves indicate enlargement of an atrium; an enlarged Q wave may indicate a myocardial infarction; and an enlarged R wave generally indicates enlarged ventricles.
  • The T wave is flatter than normal when the heart muscle is receiving insufficient oxygen—as in coronary artery disease. The T wave may be elevated in hyperkalemia (high blood K+ level).
  • Analysis of an ECG also involves measuring the time spans between waves, called intervals or segments:
    • P–Q Interval: Time from beginning of P wave to beginning of QRS complex. Indicates conduction time from atrial to ventricular excitation.
    • S–T Segment: Begins at end of S wave and ends at start of T wave. Represents time when ventricular fibers are depolarized. Elevated in acute myocardial infarction; depressed in low oxygen supply.
    • Q–T Interval: Time from start of QRS to end of T wave. Represents ventricular depolarization to repolarization. Can be lengthened by myocardial damage or ischemia.

Stress Testing and Holter Monitoring

Sometimes it is helpful to evaluate the heart’s response to physical exercise (stress testing). Narrowed coronary arteries may carry adequate oxygenated blood at rest but fail during exercise, showing changes on the ECG.

Continuous ambulatory electrocardiography (Holter monitoring) records the ECG for 24 hours to detect brief or unpredictable abnormal rhythms and inadequate blood flow. Electrodes on the chest are connected to a battery-operated monitor, storing heart activity data for later analysis.

Correlation of ECG Waves with Cardiac Cycle

The atria and ventricles depolarize and contract at different times. Systole is the contraction phase; diastole is relaxation. Timing at a heart rate of 75 bpm is as follows:

  1. Action potential arises in SA node, propagates through atrial muscle to AV node (~0.03 sec). P wave appears.
  2. Atria contract (atrial systole). AV node slows conduction (~0.1 sec delay), allowing ventricles to fill.
  3. Action potential rapidly propagates via AV bundle, bundle branches, Purkinje fibers. Depolarization produces QRS complex. Atrial repolarization occurs but is masked by QRS.
  4. Ventricular contraction (ventricular systole) begins, continuing during S–T segment, ejecting blood toward semilunar valves.
  5. Ventricular repolarization begins at apex, spreads through ventricles, producing T wave (~0.4 sec after P wave).
  6. Ventricular relaxation (ventricular diastole) completes (~0.6 sec). Next cycle begins with P wave at 0.8 sec.

Events in the heart occur in cycles that repeat for life, allowing alternation of blood filling and ejection into aorta and pulmonary trunk.

Detailed Notes:

For PDF style full-color notes, open the complete study material below:

PATH: PHARMD/ PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ HUMAN ANATOMY AND PHYSIOLOGY/ ELECTROCARDIOGRAM (ECG).

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