Study Notes on Electrocardiogram (ECG)

Learning Outcomes Explained

1. Recall the simple anatomy of the heart and its electrical conduction system.

  • Heart Anatomy: The heart's electrical activity originates from special muscle cells in the right atrium, forming the sinoatrial (SA) node. The heart also comprises atria and ventricles. Calcium release from the sarcoplasmic reticulum is crucial for muscle contraction. Neighboring cardiac cells are connected by gap junctions.

  • Electrical Conduction System: This system is a network of specialized heart muscle cells within the heart walls that send electrical signals to initiate muscle contractions. It includes:

    • Sinoatrial (SA) Node: Located in the right atrium, it serves as the heart's pacemaker, generating electrical signals that lead to depolarisation of the atrial muscles.

    • Atrioventricular (AV) Node: After the atria contract, the signal reaches the AV node, which then initiates ventricular depolarisation, resulting in ventricular contraction and simultaneous atrial repolarisation.

    • Action potentials propagate rapidly through this system via gap junctions, ensuring coordinated contraction.

2. Explain how the electrical conduction system coordinates the pumping function of the heart.

  • The heart's function relies on the highly coordinated contractions of its muscular cells, akin to a precisely timed wave.

  • The SA node initiates an electrical signal, causing atrial depolarisation and subsequent contraction, filling the ventricles.

  • This signal then travels to the AV node, which triggers ventricular depolarisation and contraction, effectively pumping blood into the pulmonary and systemic circulation.

  • This sequence ensures that cardiac muscle cells contract at the correct timing for effective blood pumping.

  • The propagation of action potentials through gap junctions facilitates rapid and coordinated contraction across the heart.

  • Cardiac muscle action potentials are prolonged due to slow voltage-gated calcium channels, which allow a sustained influx of calcium ions. This enhances the depolarisation phase and triggers the release of Ca2+^{2+} from the sarcoplasmic reticulum, essential for acto-myosin activity and muscle contraction (excitation-contraction coupling).

3. Describe how a simple ECG is measured and explain the positioning of body electrodes.

  • ECG Measurement: The heart's electrical activity is measured non-invasively by strategically placed electrical probes that record differences in electric potential (voltage) over time. This recording is called an Electrocardiogram (ECG or EKG). It directly captures the heart's electrical potentials without needing a transducer.

  • An ECG measures the overall electrical activity and dipole changes between electrode pairs over time, rather than localized activity. Each pair of electrodes registers only the electrical component aligned with their specific orientation.

  • Electrode Positioning (Simple ECG - Einthoven Triangle): For a simple ECG, common electrode placements on the body form the Einthoven Triangle, utilizing the right arm, left arm, and left leg.

    • Lead I: Differential measurement between the right arm (-) and left arm (+).

    • Lead II: Differential measurement between the right arm (-) and left leg (+).

    • Lead III: Differential measurement between the left arm (-) and left leg (+).

  • For more comprehensive analysis beyond the Einthoven Triangle, additional electrodes are placed on various body areas, including chest leads, to obtain a true three-dimensional view of the heart's electrical activity. A 12-lead ECG, for instance, includes these classical leads alongside additional chest leads for extensive directional coverage.

4. Identify the important characteristics of a normal ECG (P, QRS, and T waves) and relate these to depolarisation, repolarisation, and blood pressure.

  • A healthy heart maintains a steady rhythm with consistent P-P intervals and consistent shapes for the P, QRS, and T waves.

  • P wave: Represents the depolarisation of the atrial muscles, initiated by the SA node's electrical signal.

  • QRS complex: Represents the depolarisation of the ventricles, following the signal reaching the AV node. This leads to the contraction of the ventricles.

  • T wave: Represents the repolarisation of the ventricles, as they reset for the next contraction. During ventricular contraction, the atria simultaneously undergo repolarisation.

  • (Note: The provided information does not directly relate these waves to blood pressure changes. The relationship is primarily between the waves and the electrical events of depolarisation and repolarisation.)

5. List common defects associated with abnormal ECGs.

  • Any absent features or changes in the duration of P, QRS, or T waves can indicate an arrhythmia. Arrhythmias can vary from harmless to life-threatening.

  • Common Heart Rhythms (Abnormalities):

    • Tachycardia: Heart beats faster than the normal resting rate of 60-100 bpm.

    • Bradycardia: Heart beats slower than the normal resting rate of 60-100 bpm.

  • Examples of Arrhythmias:

    • Heart Block:

      • Change observed: Partial absence of QRST waves.

      • Cause: A block in the electric signal conduction from the SA node to the AV node.

      • Possible origins: Myocardial infarction and cardiac infections.

    • Atrial Fibrillation:

      • Change observed: Absence of the P wave and irregular P-P intervals.

      • Cause: A malfunctioning SA node.

      • Common in: The elderly or after cardiac surgery.

    • Ventricular Fibrillation:

      • Change observed: Highly irregular electrical activity with no distinct P, QRS, or T waves.

      • Cause: Chaotic electrical impulses in the ventricles, preventing effective pumping of blood.

      • Consequences: Life-threatening condition leading to cardiac arrest if not treated immediately. It requires immediate defibrillation.