Interpreting ECG

Systematic ECG interpretation requires assessing rate, rhythm, conduction, and morphology. The ECG records the average of all cardiac action potentials as field potentials on the body surface at one point in time (12-lead) or continuously (rhythm strip). It records electrical events, not mechanical ones.

Heart rate calculation methods: (1) 10× method — count R waves in 6 seconds and multiply by 10; quickest for irregular rhythms but least accurate. (2) Sequence (square counting) method — identify an R wave on a thick line; assign successive thick lines 300, 150, 100, 75, 60, 50; stop at the next R wave; good for regular rhythms. (3) 300 method — divide 300 by the number of large boxes between R waves. (4) 1500 method — divide 1500 by the number of small boxes; most accurate.

Normal durations: PR interval 0.12-0.20 s (3-5 small squares); prolonged PR indicates first-degree AV block or increased vagal tone; shortened PR suggests a junctional rhythm or pre-excitation. QRS complex 0.06-0.10 s; widened QRS indicates bundle branch block, electrolyte imbalance, or ventricular pacemaker. QT interval 0.36-0.44 s (varies with HR, age, sex); prolonged QT extends the vulnerable period and increases arrhythmia risk.

Rhythm assessment: normal sinus rhythm requires a P wave before every QRS, constant PR interval, and regular R-R interval. Sinus arrhythmia — variation in R-R with respiration (inspiration ↑HR, expiration ↓HR via vagal tone) — is a normal variant. Complete heart block shows independent P and QRS rates (atrial rate ~80/min, ventricular ~40/min) with no fixed PR relationship.

The isoelectric lead — where QRS positive and negative deflections are equal — reflects the heart''s electrical axis perpendicular to that lead. Shift in the isoelectric lead (e.g. towards right-sided leads) suggests right axis deviation, which can indicate right ventricular hypertrophy or a tall/slender body habitus. Larger than normal QRS amplitude indicates increased ventricular mass (hypertrophy from aortic stenosis, heart failure, CAD). Smaller amplitude suggests increased impedance between heart and electrode (effusion, obesity, lung disease).

Rhythm strip monitoring uses 3-5 electrodes and is configured for 1-2 leads (commonly II, V1, V6) for continuous rate and rhythm monitoring. A 12-lead ECG adds ischaemia and axis assessment.