Can a type II non‑ST‑segment elevation myocardial infarction occurring during severe influenza B infection cause ST‑segment depression on the electrocardiogram?

Can Type II NSTEMI from Influenza B Cause ST Depression on ECG?

Yes, Type II NSTEMI triggered by influenza B infection can absolutely cause ST-segment depression on the ECG, and this finding is both common and clinically significant.

Understanding Type II NSTEMI in the Context of Influenza

Type II NSTEMI occurs when myocardial oxygen supply-demand mismatch leads to myocardial injury without acute coronary atherothrombosis 1. Influenza B infection creates multiple conditions that precipitate Type II MI, including:

• Tachycardia from fever and systemic inflammatory response 1
• Hypoxemia from respiratory involvement 1
• Increased metabolic demand from the acute infection 1
• Potential hypotension in severe cases 1

The diagnosis requires three components: elevated cardiac troponin above the 99th percentile, clinical evidence of myocardial ischemia, and absence of acute coronary atherothrombosis 1.

ECG Findings in Type II NSTEMI

ST-segment depression is a hallmark ECG finding in NSTEMI regardless of whether it is Type I or Type II 2. Specifically:

• ST-segment depression ≥0.5 mm (0.05 mV) is diagnostic, particularly when present in multiple leads 2
• The magnitude of ST depression correlates with mortality risk and extent of disease 2, 3
• Even minimal ST depression (as little as 0.05 mV) predicts adverse outcomes 4
• The sum of ST-segment depression across all leads is a powerful independent predictor of 30-day mortality 3

Evidence from Influenza Studies

Research specifically examining influenza infections demonstrates that ECG abnormalities are frequently encountered, with one study showing:

• 28% of H1N1 influenza patients exhibited ECG changes on admission 5
• ST-segment depression was observed in 6 of 50 patients (12%) with confirmed H1N1 infection 5
• These ECG changes were transient and reversed during disease regression 5
• The changes did not correlate with alterations in cardiac-specific biochemistry or cardiac ultrasonography 5

Additionally, influenza infection significantly increases acute MI risk, with an adjusted relative incidence of 6.16 (95% CI 4.11-9.24) during the first week after infection 6.

Clinical Implications and Management

The primary focus of treatment for Type II NSTEMI is addressing the underlying precipitating condition 1. In the context of influenza B:

1. Treat the infection aggressively with appropriate antiviral therapy and supportive care 1
2. Correct oxygen supply-demand mismatch by managing fever, tachycardia, hypoxemia, and maintaining hemodynamic stability 1
3. Serial troponin measurements are essential, using high-sensitivity assays at presentation and repeat at 1-3 hours 2
4. Continuous ECG monitoring is recommended, as recurrent ischemic episodes can occur in approximately 27% of patients within the first 24-48 hours 2

Critical Pitfalls to Avoid

Do not assume that mild troponin elevations with ST depression in an influenza patient automatically require coronary angiography 4. The COVID-19 pandemic literature provides relevant guidance:

• Mild troponin elevations (e.g., <2-3 times the upper limit of normal) in older patients with pre-existing cardiac disease do not require work-up for Type I MI unless strongly suggestive clinically by angina chest pain and/or ECG changes 4
• Marked elevations (>5 times the upper limit of normal) may indicate severe respiratory failure, shock, or Type I MI triggered by the infection 4
• Echocardiography should be considered to help diagnose the underlying cause when troponin is markedly elevated but Type I MI is not clinically evident 4

Prognostic Significance

Persistent or dynamic ST-segment depression carries important prognostic information 2:

• One-year mortality is 16.3% with ≥0.5 mm ST-segment deviation compared to 6.8% for isolated T-wave changes 2
• The number of leads showing ST depression and magnitude ≥0.2 mV correlate with increased risk of death and recurrent ischemic events 2
• Serial ECGs at 15-30 minute intervals should be obtained if the initial ECG is non-diagnostic but clinical suspicion remains high 2

In summary, ST-segment depression is an expected and clinically significant finding in Type II NSTEMI from influenza B, reflecting myocardial ischemia from supply-demand mismatch rather than coronary occlusion, and management should focus on treating the underlying infection while monitoring for complications.