Key Notes on ECG Interpretation

Intended Learning Outcomes

  • Explain the ECG and its importance in a clinical setting.
  • Measurement of electrical activity and ECG leads.
  • Identify the components of the PQRST sequence: waves, intervals, and segments.
  • Identify features and variations of normal sinus rhythm.

ECG Basics

  • Electrocardiogram (ECG):
    • Action potentials in cardiac muscle cells generate electrical voltages.
    • ECG lead represents the heart's electrical activity.
    • Measures voltage changes between two points on the body, recording changes over milliseconds.
    • Comprises 10 electrodes: 4 on limbs (1 neutral) and 6 on the chest, connected to the ECG machine (9 active electrodes give 12 views).

Electrical Pathways of the Heart

  • Key Components:
    • Sinoatrial (SA) Node
    • Atrioventricular (AV) Node
    • Bundle of His
    • Right and Left Bundle Branches
    • Purkinje Fibres
  • Conducting system coordinates heart contractions.

Limb Leads

  • Record electrical activity in the frontal (vertical) plane:
    • Lead I: Right arm to left arm.
    • Lead II: Right arm to left leg.
    • Lead III: Left arm to left leg.

Augmented Limb Leads

  • View additional electrical activity in the frontal plane:
    • aVR: Right arm to central electrode.
    • aVL: Left arm to central electrode.
    • aVF: Left foot to central electrode.

Chest Leads (Precordial Leads)

  • Record voltage signals in a horizontal plane:
    • V1: 4th intercostal space, right margin of the sternum.
    • V2: 4th intercostal space, left margin of the sternum.
    • V3: Midway between V2 and V4.
    • V4: 5th intercostal space, mid-clavicular line.
    • V5: 5th intercostal space, anterior axillary line.
    • V6: 5th intercostal space, mid-axillary line.

ECG Measurement

  • Timescale (x-axis):
    • 1 large square = 0.2 sec (200 ms).
    • 1 small square = 0.04 sec (40 ms).
  • Amplitude (y-axis):
    • 1 small square = 0.1 mV.

ECG Waves and Segments

  • P Wave:
    • Represents atrial depolarization.
    • Normal shape: smooth, rounded, upright in leads I and II.
  • QRS Complex:
    • Represents ventricular depolarization, composed of:
    • Q wave: initial ventricular depolarization.
    • R wave: main mass of ventricles.
    • S wave: last to depolarize.
  • T Wave:
    • Represents ventricular repolarization; oriented in same direction as preceding QRS.
  • Segments connect waves and baseline signals should be isoelectric.
  • Intervals include waveform and segments:
    • PR Interval: from start of P wave to start of QRS complex (120-200 ms).
    • QRS Interval: from start of Q wave to end of S wave (should be < 120 ms).
    • QT Interval: from start of QRS to end of T wave (should be < 440 ms in men, < 460 ms in women).

Normal Sinus Rhythm (NSR)

  • Characterized by:
    • Regular rhythm with < 0.12 second variation.
    • Heart rate of 60-100 bpm.
    • Each QRS complex preceded by normal P wave (1:1 ratio).
    • Consistent PR interval (120-200 ms).
    • QRS complexes < 120 ms wide.
    • T wave upright in all leads except aVR.

Abnormal Rhythms

  • Sinus Tachycardia:
    • Resting heart rate > 100 bpm.
    • Common causes: exercise, stress, caffeine, hyperthyroidism.
  • Sinus Bradycardia:
    • Resting heart rate < 60 bpm; often benign in athletes.
    • Risks: SA node dysfunction, medication effects, metabolic disorders.

Summary of Rhythm

  • P Wave: Atrial depolarization.
  • QRS Complex: Ventricular depolarization.
  • T Wave: Ventricular repolarization.
  • PR Interval: Delay at AV node.
  • ST Segment: Beginning of ventricular repolarization, should be flat.