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.