🔬 12-Lead ECG

The 12-lead Electrocardiogram (ECG) is a non-invasive test that records the electrical activity of the heart over a period of time. By placing electrodes on the limbs and chest, it provides a 'map' of cardiac conduction from 12 different angles. It is the gold standard for diagnosing acute myocardial infarction, arrhythmias, and conduction delays.

Chest pain or suspected acute coronary syndrome (ACS). Palpitations, syncope, or unexplained dizziness. Shortness of breath or suspected pulmonary embolism (S1Q3T3). Pre-operative assessment for major surgery. Monitoring of patients on medications known to prolong the QT interval. Assessment of electrolyte disturbances, particularly hyperkalaemia. Evaluation of hypertension and suspected left ventricular hypertrophy.

Ten electrodes are placed on the patient: four on the limbs and six across the chest (V1-V6). V1-V2 are placed in the 4th intercostal space at the sternal borders; V4 in the 5th intercostal space mid-clavicular line; V3 between V2 and V4; V5 and V6 in the anterior and mid-axillary lines respectively. The machine records the electrical activity of the heart from 12 different perspectives (leads). The patient must remain still and relaxed to minimise muscle artefact.

Normal sinus rhythm (rate 60–100 bpm) with a consistent P-wave before every QRS complex. Normal cardiac axis (-30 to +90 degrees). PR interval between 120-200ms. QRS duration less than 120ms. Normal ST-segments (iso-electric) and T-waves (upright in most leads except aVR and V1). No evidence of hypertrophy or pathological Q-waves. Appropriate R-wave progression across precordial leads.

Interpretation should be systematic: check patient details and calibration (25mm/s, 10mm/mV). Determine the rate (300/large squares) and rhythm (regularity, P-wave association). Assess the axis (leads I and II). Examine the P-wave, PR interval (0.12-0.20s), QRS duration (<0.12s), and ST-segments/T-waves in anatomical groups (distinguishing between anterior, lateral, and inferior leads). Finally, calculate the corrected QT interval (QTc). Always compare with previous tracings if available.

ST-segment elevation indicates acute myocardial infarction (STEMI), while ST-depression or T-wave inversion may suggest ischaemia or NSTEMI. Pathological Q-waves indicate previous infarction. Arrhythmias include atrial fibrillation (absent P-waves, irregular rhythm), SVT, or ventricular tachycardia. Bundle branch blocks (LBBB or RBBB) alter the QRS morphology. Prolonged QT intervals increase the risk of torsades de pointes. Potentially life-threatening signs like epsilon waves (ARVC) or Brugada pattern are also sought.

The 12-lead ECG is the primary diagnostic tool for suspected ACS and cardiac arrhythmias. It allows for the rapid identification of patients requiring immediate reperfusion therapy (PCI or thrombolysis). It is also used to monitor the effects of medications, electrolyte imbalances (particularly potassium and calcium), and to screen for inherited cardiac conditions in patients with syncope or a family history of sudden death. High diagnostic yield in chest pain and palpitations.

Incorrect electrode placement (e.g., lead reversal) which can mimic infarct patterns or axis deviations. Misinterpreting normal variants (e.g., J-point elevation in young athletes or 'High Take-Off') as acute ischaemia. Over-reliance on automated machine interpretations, which are frequently inaccurate. Failing to repeat the ECG if the patient's symptoms change or if the initial tracing is equivocal.

A single ECG is a snapshot and may be normal even in the presence of significant pathology (e.g., intermittent arrhythmias or early-stage MI). It has limited sensitivity for certain conditions like pulmonary embolism or stable angina. The quality of the tracing can be affected by patient movement, tremors (Parkinson's), or poor electrode contact. It does not provide information on the mechanical function of the heart, which requires echocardiography.

Students must master the recognition of STEMI patterns and common arrhythmias like Atrial Fibrillation. Recognition of 'LBBB as a STEMI equivalent' in the context of new-onset chest pain is a common exam theme. Understand the significance of a prolonged PR interval (heart block) and its clinical grades. Always relate ECG findings to the patient's clinical presentation.