What is the initial step in ruling out myocardial infarction (MI) in a suspected patient?

Initial Workup for Suspected Myocardial Infarction

The initial step in ruling out myocardial infarction (MI) in a suspected patient is to obtain a 12-lead ECG within 10 minutes of presentation, followed immediately by cardiac troponin measurement. 1

Immediate Assessment Algorithm

1. Obtain 12-lead ECG immediately (target within 10 minutes of arrival)

   • Look for ST-segment elevation or depression, T-wave inversion, new left bundle branch block (LBBB), or pathological Q waves 2
   • Serial ECGs may be necessary if initial ECG is non-diagnostic but clinical suspicion remains high 2
   • Consider continuous ECG monitoring for dynamic changes 2

2. Draw blood for cardiac biomarkers

   • High-sensitivity cardiac troponin (hs-cTn) is the preferred biomarker 2
   • Initial troponin should be measured at presentation 1
   • Serial measurements are crucial to detect a rise and/or fall pattern 2

3. Administer immediate treatment while assessment continues

   • Aspirin 162-325 mg (chewed and swallowed) 1
   • Oxygen if saturation is low
   • Sublingual nitroglycerin for ongoing chest pain
   • Adequate analgesia as needed 1

Troponin Testing Protocols

The timing of repeat troponin measurements depends on the assay used and local protocols:

• Traditional approach: 0 and 3-hour measurements 3
• Accelerated protocols:
  • 0 and 2-hour measurements 4, 3
  • 0 and 1-hour measurements for high-sensitivity assays 5, 3

Interpretation of Results

ECG Findings

• ST-segment elevation in two contiguous leads (≥0.2 mV in men ≥40 years, ≥0.25 mV in men <40 years, ≥0.15 mV in women in leads V2-V3, or ≥0.1 mV in all other leads) indicates STEMI 2
• New horizontal or downsloping ST depression ≥0.05 mV in two contiguous leads and/or T-wave inversion >0.1 mV in two contiguous leads suggests NSTEMI 2

Troponin Results

• MI is diagnosed when there is a rise and/or fall of cardiac troponin with at least one value above the 99th percentile upper reference limit (URL) 2
• This must be accompanied by at least one of the following:
  • Symptoms of ischemia
  • New ECG changes (ST-T changes or new LBBB)
  • Development of pathological Q waves
  • Imaging evidence of new loss of viable myocardium or new wall motion abnormality 2

Common Pitfalls to Avoid

1. Relying solely on initial troponin value

   • A single normal troponin does not rule out MI; serial measurements are essential to detect a rise/fall pattern 2
   • Chronic elevations can occur in renal failure, heart failure, and other conditions 2

2. Misinterpreting ECG findings

   • ST elevation can occur in conditions other than MI (e.g., pericarditis, LV hypertrophy, Brugada syndrome) 2
   • LBBB can mask or mimic ST-segment changes 6

3. Delayed testing

   • Door-to-ECG time should be <10 minutes 1
   • Delays in obtaining ECG or troponin measurements can lead to delayed diagnosis and treatment

4. Overlooking atypical presentations

   • Women, elderly, and diabetic patients may present with atypical symptoms 2
   • Maintain high index of suspicion even with non-classic presentations

Special Considerations

• For patients with LBBB, consider using modified Sgarbossa criteria to identify MI 6
• In patients with renal failure or other conditions with chronic troponin elevation, focus on dynamic changes rather than absolute values 2
• Type 2 MI (supply-demand mismatch without acute atherothrombosis) requires identification and treatment of the underlying cause 7

Next Steps After Initial Assessment

Based on initial ECG and troponin results:

• STEMI: Immediate reperfusion therapy (primary PCI preferred if available within 120 minutes, otherwise fibrinolysis) 1
• Elevated troponin or concerning ECG changes: Admit for further evaluation and treatment
• Normal initial workup but high clinical suspicion: Continue serial ECGs and troponin measurements
• Low-risk patients with normal initial and serial troponins: Consider early discharge with appropriate follow-up

The rapid and accurate diagnosis of MI is critical for improving patient outcomes, with the combination of prompt ECG and serial high-sensitivity troponin measurements forming the cornerstone of the initial evaluation.