Myocardial infarction

Overview

•Central crushing chest pain - severe, radiating to left arm/jaw/back; not relieved by GTN

•Diaphoresis, nausea and vomiting - particularly with inferior MI (vagal activation)

•Atypical presentations - epigastric pain, fatigue, syncope, or silent MI - more common in diabetics and elderly

⚠️
A normal resting 12-lead ECG does NOT exclude ACS - up to 30% of patients with ACS have a normal or non-diagnostic ECG at presentation. Serial ECGs and troponin are essential.

Investigations

🥇 First-line

•12-lead ECG - repeat every 15-30 minutes if non-diagnostic

•High-sensitivity cardiac troponin (hs-cTn) - rises within 1 hour; serial measurements at 0 and 1-3 hours; rise and/or fall with at least one value above 99th percentile = MI

🏆 Gold standard

•Coronary angiography - defines anatomy, confirms culprit lesion, guides revascularisation

Management

•Immediate (all ACS): aspirin 300 mg PO loading dose - give as soon as possible

•Sublingual glyceryl trinitrate (GTN) - avoid if systolic BP <90 mmHg or right ventricular MI (preload-dependent)

•IV morphine (or diamorphine) for ongoing pain - give with antiemetic (e.g. metoclopramide)

•Oxygen - only if SpO₂ <94%; hyperoxia is harmful and must be avoided in non-hypoxic patients

Complications

Mechanical complications post-MI
ComplicationTimingKey features
LV free wall rupture3-14 daysSudden collapse, cardiac tamponade (Beck's triad: raised JVP, muffled sounds, hypotension). No new murmur.
Papillary muscle rupture (acute MR)DaysAcute pulmonary oedema, early-to-mid systolic murmur at apex; inferoposterior MI
VSDWithin first weekAcute biventricular failure, loud pansystolic murmur at left sternal edge
LV aneurysmWeeks-monthsPersistent ST elevation, heart failure, arrhythmias, mural thrombus (embolic risk)
Dressler syndrome2-10 weeksPleuritic chest pain, fever, pericardial/pleural effusion; treat with high-dose aspirin or NSAIDs

Complication	Timing	Key features
LV free wall rupture	3-14 days	Sudden collapse, cardiac tamponade (Beck's triad: raised JVP, muffled sounds, hypotension). No new murmur.
Papillary muscle rupture (acute MR)	Days	Acute pulmonary oedema, early-to-mid systolic murmur at apex; inferoposterior MI
VSD	Within first week	Acute biventricular failure, loud pansystolic murmur at left sternal edge
LV aneurysm	Weeks-months	Persistent ST elevation, heart failure, arrhythmias, mural thrombus (embolic risk)
Dressler syndrome	2-10 weeks	Pleuritic chest pain, fever, pericardial/pleural effusion; treat with high-dose aspirin or NSAIDs

💡
Inferior MI causes ischaemia of the AV node (RCA supplies SA and AV nodes in most people) → bradyarrhythmias and AV block, typically responsive to atropine or temporary pacing.

ECG Localisation

ECG territory and culprit vessel
TerritoryLeadsCulprit artery
AnteriorV2-V4 (ST elevation)LAD
InferiorII, III, aVF (ST elevation)RCA (usually)
LateralI, aVL, V5-V6LCx
PosteriorTall R waves + ST depression V1-V3 (reciprocal); confirm with V7-V9RCA/LCx

Territory	Leads	Culprit artery
Anterior	V2-V4 (ST elevation)	LAD
Inferior	II, III, aVF (ST elevation)	RCA (usually)
Lateral	I, aVL, V5-V6	LCx
Posterior	Tall R waves + ST depression V1-V3 (reciprocal); confirm with V7-V9	RCA/LCx

•STEMI criteria: ST elevation ≥1 mm in two contiguous limb leads, or ≥2 mm in two contiguous precordial leads

•New LBBB in context of chest pain - treat as STEMI equivalent

•Pathological Q wave (old definition): ≥0.04 s duration AND amplitude ≥25% of R wave in that lead - indicates prior infarction

🎯
Anterior STEMI = ST elevation in V2-V4 (LAD territory). Inferior Q waves (e.g. lead III ≥0.04 s, depth ≥25% R wave) indicate a previous inferior MI - these are distinct findings on the same ECG.