OBJECTIVE To assess whether the use of inverted lead aVR (−aVR) would improve the classification of acute inferior or lateral myocardial infarction presenting with ST elevation.

DESIGN Observational study. The presence of ⩾ 1 mm ST elevation in lead−aVR (derived by manual assessment of ST depression in conventional lead aVR) was determined by a single investigator, blinded to patient outcome.

PATIENTS 173 consecutive patients with chest pain for ⩽ 12 hours and ST elevation of ⩾ 1 mm in inferior leads (II, III, aVF) or lateral leads (I, aVL, V5, V6), excluding those with anterolateral ST elevation.

MAIN OUTCOME MEASURE Incidence of ST elevation in lead −aVR in patients with inferior or lateral ST elevation, or both.

RESULTS ST elevation in lead−aVR was present in 25 of 136 patients (18%) with inferior but no lateral ST elevation (indicating greater superior involvement) and in three of 11 patients (27%) with lateral but no inferior ST elevation (indicating greater inferior involvement). ST elevation in lead −aVR bridged the gap between inferior and lateral ST elevation in 15 of 25 (60%) patients with inferior and lateral chest lead (V5/V6) ST elevation, and in all patients with inferior and lateral limb lead (I/aVL) ST elevation. The presence of ST elevation in lead −aVR was associated with a larger infarct size as defined by median peak creatine kinase on serial sampling: 1780 v987  mmol/l; p = 0.021.

CONCLUSIONS Use of lead−aVR improves the ECG classification of acute inferior or lateral acute myocardial infarction and thus may be useful as part of the routine 12 lead ECG assessment of such patients.