Medicine
Electrocardiogram Interpretation for Cardiac Arrhythmias
Quick fact
Atrial fibrillation, one of the most common arrhythmias, often shows up on an ECG as an irregularly irregular rhythm with no clear P waves — yet many people with it feel completely fine.
Why this is interesting
Your heart beats around 100,000 times a day. How can a 10-second strip of squiggles tell a doctor whether that beat is safe or deadly?
Read the full explanation
Understanding Electrocardiogram Interpretation for Cardiac Arrhythmias
Think of the heart as a house with the ECG as an electrical wiring diagram. Each heartbeat is controlled by a tiny electrical signal that spreads through the heart muscle. The ECG translates that signal into a visible trace. A normal beat produces a series of waves: the P wave is the electrical impulse causing the atria (upper chambers) to contract; the QRS complex is the larger signal that makes the ventricles (lower chambers) pump; the T wave represents the ventricles resetting. To interpret arrhythmias, you look for three things: rate, rhythm, and the shapes of these waves. First, estimate the heart rate by counting large squares between QRS complexes. Then check whether the rhythm is regular or irregular. Then ask: Is there one P wave before every QRS? Is the PR interval constant and short? Is the QRS narrow or wide? A normal ECG shows a regular rhythm, one P wave per QRS, and a PR interval of 0.12–0.20 seconds. If P waves are missing and the rhythm is irregular, think atrial fibrillation. If the QRS is wide and fast, think ventricular tachycardia. If P waves occur but no QRS follows, think heart block.
A deeper explanation
The ECG works because body fluids conduct electricity, so electrodes placed on the skin record voltage changes as the heart muscle depolarizes and repolarizes. Each wave reflects a specific ion movement across cardiac cell membranes: P wave from atrial depolarization, QRS from rapid ventricular depolarization, T wave from ventricular repolarization. Arrhythmias arise when the heart's electrical system misfires — either because an abnormal pacemaker site generates extra impulses, or because conduction through the heart is blocked or slowed. The ECG reveals these disturbances because timing and shape of the waves change predictably. For example, a narrow QRS complex suggests the impulse is travelling down the normal His-Purkinje pathway, while a wide, bizarre QRS suggests the ventricles are being activated abnormally, as in ventricular tachycardia. A PR interval that lengthens then drops a QRS is classic for second-degree heart block. Understanding these patterns matters because an ECG is often the first test performed and the difference between a stable rhythm and a lethal one can be decided within seconds. In emergencies, interpreting the ECG correctly guides whether a patient needs pacing, defibrillation, or simply monitoring.