Investigations

Perform

• a blood count ^b  
• urea, electrolytes and creatinine ^d
• glucose ^a  
• lipid levels ^a  
• serial cardiac enzymes, ideally CK-MB ^c

Less helpful alternatives include

• troponin T  ^{a b c}  or troponin I ^c  
• myoglobin ^c  
• serial creatine kinase (CK) ^{b c},  aspartate transaminase (AST) ^a,  lactate dehydrogenase (LDH) ^a taken over 24 hours ^a  

• a 12-lead ECG ^{a  b} - read it carefully! ^a  
  Look for features suggestive of cardiac ischaemia
  • any ST elevation particularly if in 2 or more leads or not known to be old  ^{a b}
  • any ST depression particularly if not known to be old ^b
  • any Q waves particularly if in 2 leads or more or not known to be old ^{a b}  
  • any T wave inversion particularly if not known to be old ^b
  • any conduction defect particularly if not known to be old ^b
    
  followed by serial ECGs ^b   
  

If there is a left bundle branch block, look for ^{a b} 

• ST-elevation of 1 mm concordant with the QRS complex
• ST-elevation of 5 mm discordant with QRS complex
• ST-depression of 1 mm in leads V1, V2 or V3

• a chest X-ray ^a

Use the following clinical prediction rule to rank your patient for risk of a myocardial infarction ^a

If clinical finding not present, go onto next question a			probability of MI
ST elevation or Q waves in 2 or more leads, not known to be old			75%
			.
chest pain began 48 hours ago or more	=>	ST-T changes of ischaemia or strain, not known to be old	22%
		no changes or old	1.6%
previous history of angina or MI	=>	ST- T changes of ischaemia or strain, not known to be old	26%
		longest pain episode < 1 hour	4.0%
		pain worse than prior angina or the same as a prior MI	11%
.		pain not as bad	1.0%
pain radiates to neck, left shoulder or left arm	=>	aged < 40	2.4%
.		chest pain reproduced by palpation	1.3%
		pain radiates to back, abdomen or legs	7.7%
.		chest pain stabbing	1.8%
		chest pain not stabbing	17%
-
ST- T changes of ischaemia or strain, not known to be old			26%
no changes or old			2.0%

Patients at low-risk for a myocardial infarction can be assessed in a rapid evaluation unit ^b by: 

• CK-MB at 0, 4, 8, 12 hours
• serial 12-lead ECGs
• clinical assessment at 0, 6, 12 hours
• exercise ECG: if all the above negative

Use the following clinical prediction rule to help determine admission to coronary care units ^a 
Look for the following risk factors:

• pain worse than prior angina or the same as the pain associated with a prior myocardial infarction
• systolic blood pressure < 110 mmHg
• crackles above the bases bilaterally
• ST elevation or Q waves, not known to be old, in two or more leads
• ST segment or T wave changes, not known to be old, indicative of myocardial ischaemia.

Risk of major complications

Group a	risk of major complication at 4 days
suspected MI on ECG or suspected ischaemia on ECG and 2 or more risk factors	high
suspected ischaemia on ECG and 1 or less risk factor	moderate
one risk factor with no MI or ischaemia on ECG	low
no risk factors	very low