Why ECG Interpretation Matters

The electrocardiogram (ECG or EKG) is one of the most widely used diagnostic tools in medicine. It records the electrical activity of the heart over time and can reveal a remarkable array of conditions — from arrhythmias and conduction blocks to signs of myocardial infarction and electrolyte disturbances. For any clinician or healthcare student, developing a systematic approach to ECG reading is an essential skill.

Understanding the Basics: What the ECG Measures

A standard 12-lead ECG captures electrical impulses from 12 different angles (leads), providing a three-dimensional view of heart activity. The key waveform components are:

  • P wave: Represents atrial depolarization (the electrical activation of the atria).
  • PR interval: The time from the start of atrial depolarization to the start of ventricular depolarization; reflects conduction through the AV node.
  • QRS complex: Represents ventricular depolarization — the most prominent feature of a normal ECG.
  • ST segment: Reflects the period between ventricular depolarization and repolarization; critical for detecting ischemia or infarction.
  • T wave: Represents ventricular repolarization.
  • QT interval: Measures the total time for ventricular depolarization and repolarization; prolongation can indicate arrhythmia risk.

The Systematic Approach: 8 Steps

A structured approach ensures no finding is missed. Use this checklist every time:

Step 1: Confirm the Patient and Technical Quality

Verify the patient's name, age, date, and clinical context. Check for adequate gain, correct paper speed (25 mm/s is standard), and absence of artefact or poor electrode contact.

Step 2: Assess the Rate

The easiest method: divide 300 by the number of large squares between two consecutive R waves. Normal heart rate is 60–100 bpm. Below 60 is bradycardia; above 100 is tachycardia.

Step 3: Determine the Rhythm

Is the rhythm regular or irregular? Use the R-R interval to assess regularity. Ask: Is every QRS preceded by a P wave? Is the P-wave morphology consistent? A regular rhythm with consistent P waves typically indicates sinus rhythm.

Step 4: Evaluate the P Wave

Normal P waves are upright in leads I and II, inverted in aVR. Duration should be <120 ms. Broad, notched P waves may suggest left atrial enlargement; tall, peaked P waves may indicate right atrial enlargement.

Step 5: Measure the PR Interval

Normal range: 120–200 ms (3–5 small squares). A prolonged PR interval (>200 ms) suggests first-degree AV block. A shortened PR interval may indicate a pre-excitation syndrome such as Wolff-Parkinson-White.

Step 6: Analyze the QRS Complex

Normal QRS duration is <120 ms. A wide QRS (>120 ms) suggests bundle branch block, ventricular pacing, or a ventricular ectopic rhythm. Look for Q waves (normal septal Q waves are small; pathological Q waves are >40 ms wide or >25% of the R wave height) and assess R-wave progression across the chest leads.

Step 7: Examine the ST Segment

The ST segment should be isoelectric (flat, at the same level as the baseline). ST elevation in two or more contiguous leads is a hallmark of acute STEMI until proven otherwise and is a medical emergency. ST depression may indicate ischaemia or subendocardial injury. Saddle-shaped ST elevation across multiple leads may suggest pericarditis.

Step 8: Assess the T Waves and QT Interval

T waves are normally upright in most leads and inverted in aVR. T-wave inversion may reflect ischaemia, ventricular hypertrophy, or bundle branch block. The corrected QT interval (QTc) should be <440 ms in men and <460 ms in women; prolongation raises risk of torsades de pointes.

Common ECG Findings and Their Significance

Finding Possible Cause
ST elevation (≥1 mm in 2+ leads) STEMI, Prinzmetal angina, pericarditis
Wide QRS (>120 ms) Bundle branch block, ventricular rhythm
Irregular rhythm, no P waves Atrial fibrillation
Prolonged QTc Electrolyte imbalance, medications, congenital syndromes
Tall, peaked T waves Hyperkalemia, early STEMI

Practice Makes Proficient

ECG interpretation is a skill that improves significantly with repetition. Reading real-world ECGs — ideally in a supervised clinical setting with feedback — accelerates competency faster than any textbook alone. Use this systematic framework every time, and over time it will become second nature.