ecg

Electrocardiogram (ECG) Study Notes

Learning Outcomes

  • To discuss when an ECG is required in maternity care
  • To explain the purpose of an ECG
  • To demonstrate how to perform an ECG

Indications for Midwives Performing an ECG

  • Assess Pre-existing Heart Conditions:
    • Important for identifying underlying cardiac issues before, during, or after pregnancy.
  • Investigate Symptoms:
    • Symptoms may include:
    • Shortness of breath
    • Heart palpitations
    • Chest pain
  • Monitor Heart in High-risk Pregnancies:
    • Essential for pregnancies with identified risks like gestational diabetes or preeclampsia.
  • Assess Chest Trauma:
    • Important post-trauma to check for cardiac effects.
  • Evaluate Electrolyte Imbalances:
    • Can affect heart rhythm and function, requiring monitoring via ECG.

Introduction to ECG

  • Definition and Purpose:
    • An ECG is used to determine the cardiac rhythm and the condition of the conducting tissues.
    • Provides information about:
    • Chamber size
    • Presence of myocardial ischaemia and infarction
    • Effects of drugs on the heart
  • Features of ECG:
    • One of the simplest, fastest procedures to evaluate heart function.
    • The heart produces significant electrical activity that can be measured through ECG.
  • Electrophysiological Representation:
    • Each heartbeat corresponds to a series of electrical activities known as the PQRST complex.

Structure and Function of the Heart

  • Heart Composition:
    • A pump composed of cardiac muscle, also referred to as myocardium.
    • Shape: hollow cone-shaped organ surrounded by a fluid-filled sac called the pericardium for protection and smooth movement.
  • Chambers of the Heart:
    • Divided into four chambers:
    • Two atria: Left atrium (LA) and Right atrium (RA)
    • Two ventricles: Left ventricle (LV) and Right ventricle (RV)
    • The left and right sides of the heart are separated by a structure called the septum.
  • Functionality:
    • Right Side: Pumps blood to the lungs for gaseous exchange (known as pulmonary circulation).
    • Left Side: Pumps oxygenated blood to the rest of the body (known as systemic circulation).
  • Lining of the Heart:
    • The heart is lined by the endocardium covering the chambers and valves.
  • Intrinsic Electrical System:
    • The heart’s pumping is controlled by an intrinsic electrical conducting system.

Electrical Activity in the Heart

  • Fundamental Basis of the ECG:
    • Electrical activation of a heart muscle cell leads to depolarization of its membrane.
    • Depolarization travels along the cell or fiber and is communicated to adjacent cells.
    • Resulting in a moving wave that creates electrical currents detectable by surface electrodes, culminating in an ECG result.

Conducting System of the Heart

  • Main Components:
    • Sinoatrial Node (SA Node):
    • Located in the right atrium and functions as the primary pacemaker of the heart.
    • Generates electrical stimuli at a rate of 60-100 times per minute.
    • Causes atria to contract simultaneously, allowing blood to fill the ventricles prior to their contraction.
    • Atrioventricular Node (AV Node):
    • Located in the wall of the septum, introduces a short delay allowing for atrial contraction and ventricular filling.
    • AV Bundle (Bundle of His):
    • Transmits electrical impulses from the AV node to the ventricles via right and left bundle branches.
    • Purkinje Fibres:
    • Located in the sub-endocardium, spread throughout the ventricle, triggering ventricular contraction.

Pathway of Depolarization

  • Sequence in the Heart:
    1. SA node
    2. Intermodal pathway
    3. AV node
    4. AV bundle
    5. Bundle branches
    6. Purkinje fibers
  • Action Potentials:
    • Electrical changes (action potentials) in the autorhythmic cells invoke mechanical contractions in the contractile cells leading to a heartbeat.

The ECG Waveform

  • Components of the ECG Wave:
    • P Wave:
    • Represents atrial depolarization; indicates that the atria are electrically stimulated to pump blood to ventricles.
    • QRS Complex:
    • Characterized by a short downward section followed by a tall upward section, indicating ventricular depolarization and blood pumping.
    • ST Segment:
    • A flat segment that denotes the period between the end of ventricular contraction and the start of the T wave.
    • T Wave:
    • Final upward curve representing recovery (repolarization) of the ventricles.

Normal ECG Wave Tracing

  • ECG Components:
    • P wave = Atrial depolarization
    • QRS complex = Ventricular depolarization
    • T wave = Ventricular repolarization
  • Intervals:
    • P-R Interval: Duration between the start of atrial depolarization and the start of ventricular depolarization.
    • R-R Interval: Duration between successive R waves, used to measure heart rate.

Measuring and Interpreting ECGs

  • 12 Lead ECG:
    • Most common method; utilizes 10 physical electrodes yielding 12 leads (views) of heart electrical activity.
    • View Types:
    • 6 chest electrodes (V1 - V6)
    • 4 limb electrodes
    • Provides critical perspectives for diagnosing cardiac pathology.
  • Normal ECG Interpretation:
    • A single ECG complex represents one cardiac cycle, enabling assessment of heart rate and rhythm.
    • To determine heart rate: Count the number of large squares between R waves and divide by 300.
    • Pay special attention to changes in the ST segment as they may indicate clinical significance.

Application of ECG Electrodes

  • Electrode Placement:
    • Important for accurate measurement and representation of heart activity.

References

  • Cook, N., & Shepherd, A. (2025). Essentials of anatomy and physiology for nursing practice. Sage.
  • Carpenter, J., & Hunter, L. (2025). The anatomy and physiology textbook for midwives. Routledge.
  • Hampton, J., Adlam, D., & Hampton, J. (2019). 150 ECG cases. Elsevier.
  • Rowlands, A., & Sargent, A. (2019). The ECG Workbook. M&K publishing.
  • Stroobandt, R., Barold, S., & Sinnaeve, A. (2016). ECG from basics to essentials step by step.