Investigations

• ECG, followed by serial ECGs, looking for ^b

• acute injury: ST elevation in ≥ 1 mm in 2 contiguous limb leads, or ≥ 2 mm in 2 contiguous precordial leads,
• acute ischaemia: ST depression ≥1 mm in 2 contiguous leads or symmetrical T wave inversion of >≥3 mm in 2 contiguous leads,
• in patients with BBB: ST depression or elevation ≥ 1 mm towards deflection of main QRS deflection in 2 contiguous leads, or ≥ 7 mm away from QRS deflection and > 50% amplitude of T wave in 2 contiguous leads

Abnormalities on serial ECGs help diagnose a myocardial infarction or unstable angina

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability	LR- (95% CI)	Post-test Probability
chest pain (pre-test probability: 52%) b	myocardial infarction or unstable angina (ECG at 24 hours, cardiac enzymes)	initial ECG	9.5 (5.6 to 16)	91%	0.75 (0.70 to 0.79)	44%
		serial ECG	55 (18 to 170)	98%	0.66 (0.62 to 0.70)	41%

Note:

• Few patients with a normal ECG go onto to have life-threatening complications (1.3%: 95% CI: 0.0% to 3.0%) ^c