Lab: ECG Practical

This practical session develops systematic ECG interpretation skills, connecting electrophysiological principles with clinical trace analysis.

The ECG records the average of all cardiac action potentials as body surface potentials. Electrode placement must be precise: V1 (4th ics right sternal border), V2 (4th ics left sternal border), V3 (midway between V2 and V4), V4 (5th ics midclavicular line), V5 (anterior axillary line, same level as V4), V6 (mid-axillary line, same level as V4). Limb electrodes placed on distal limbs with minimal muscle bulk to reduce artefact. Dots (adhesive electrodes) should be attached to the cable before application to the patient.

Morphological interpretation: Wide P waves indicate slow atrial depolarisation (e.g. left atrial enlargement). Narrow QRS occurs because Purkinje fibres (5.0 m/s) rapidly depolarise the large ventricular mass simultaneously — a fast electrical event producing a tall, narrow complex. Large QRS amplitude = ↑ventricular mass (ventricular hypertrophy from aortic stenosis, heart failure, CAD). The isoelectric lead is the lead where positive and negative QRS deflections are equal, indicating the electrical axis is perpendicular to that lead. A shift in the isoelectric lead toward right-sided chest leads suggests right axis deviation (right ventricular hypertrophy, or tall slender body habitus); shift toward left indicates left axis deviation.

The aVL lead typically shows the largest negative Q wave because it aligns with the axis of interventricular septal depolarisation (left → right, away from aVL). Sinus arrhythmia is a normal variation — R-R interval shortens on inspiration (↓vagal tone → ↑HR) and lengthens on expiration (↑vagal tone → ↓HR). This can be felt at one''s own pulse.

A 12-lead ECG printed at standard speed (25 mm/s) and standard gain (10 mm/mV) should be calibrated before interpretation — the calibration rectangle at the start/end of the strip shows 1 mV = 10 mm (10 small squares). Deviations from standard calibration alter amplitude interpretation. Rhythm strips use 3-5 electrodes and monitor leads II, V1, or V6 continuously for rate and rhythm.