CardiologyTier 2Topic

Electrocardiography

Role

The 12-lead ECG records the heart's electrical activity from the body surface - the single most important and immediately available cardiac investigation
Performed in essentially every acute presentation with a cardiac or unexplained systemic component

Leads and territories

Leads	Territory	Coronary artery
II, III, aVF	Inferior	Right coronary artery (usually)
V1-V4	Anterior/septal	Left anterior descending
I, aVL, V5-V6	Lateral	Circumflex
V1-V2 reciprocal / posterior leads	Posterior	Right coronary/circumflex

Systematic interpretation

Systematic interpretation: rate, rhythm, axis, intervals (PR, QRS, QT), then segment/morphology (ST changes, T wave changes, Q waves)
Reciprocal ST depression supports (not just coincidentally accompanies) an ST-elevation MI diagnosis

Acting on the ECG

Directly triggers time-critical management decisions (STEMI -> immediate reperfusion pathway)
Serial ECGs are used when the initial ECG is non-diagnostic but suspicion remains high - dynamic changes over time support the diagnosis

Associations

Acute coronary syndrome, arrhythmia diagnosis, electrolyte abnormality (hyperkalaemia, hypokalaemia patterns), drug toxicity (QT prolongation, digoxin effect)

Limitations

A single normal ECG does not exclude significant coronary disease or intermittent arrhythmia - serial or ambulatory monitoring extends its diagnostic yield

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