What is the management of influenza with elevated Creatine Kinase-MB (CK-MB) levels indicating cardiac involvement?

Management of Influenza with Elevated CK-MB

Elevated CK-MB in influenza patients most commonly reflects skeletal muscle injury (myositis) rather than true myocarditis, and cardiac troponins should be measured immediately to differentiate between these conditions. 1, 2

Initial Cardiac Assessment

When CK-MB is elevated in an influenza patient, perform the following workup immediately:

• Obtain cardiac troponin I or T levels - these are far more sensitive and specific for myocardial injury than CK-MB 3
• Perform 12-lead ECG to assess for ST-segment changes, Q waves, low-voltage QRS complexes with electrical alternans, or conduction abnormalities like complete left bundle branch block 4, 5
• Order echocardiography if troponins are elevated or ECG shows abnormalities to assess for wall motion abnormalities and left ventricular function 5, 2
• Check CK-MB index (CK-MB/total CK ratio) - if <6%, skeletal muscle injury is more likely than cardiac involvement 1

Distinguishing Myocarditis from Myositis

The critical distinction: Research demonstrates that only 12% of influenza patients have elevated total CK levels, and when CK-MB is elevated, the CK-MB fraction rarely exceeds 6% 1. In a study of 152 patients with confirmed influenza, cardiac troponins were not elevated in any patient despite some having elevated CK levels, indicating skeletal muscle injury rather than myocarditis 1.

True influenza myocarditis presents with:

• Cardiac symptoms developing 4-7 days after initial influenza symptoms 5
• Progressive dyspnea as the most common cardiac symptom 5
• Elevated cardiac troponins (not just CK-MB) 1, 2
• ECG changes including ST elevation with Q waves or complete left bundle branch block 5
• Global wall motion abnormalities on echocardiography 5

Influenza-related myositis presents with:

• Muscle pain and weakness in extremities 6
• Elevated total CK and CK-MB but normal troponins 1, 6
• CK-MB fraction <6% 1
• Normal ECG or only clinically insignificant abnormalities 2
• Normal echocardiography 2

Management Algorithm

If Troponins Are Normal (Myositis):

• Provide supportive care with hydration and pain management 6
• Initiate oseltamivir 75 mg orally twice daily for 5 days regardless of symptom duration 4, 7
• Monitor symptoms - myositis typically resolves within one week 6
• No cardiac-specific interventions are required 1, 2

If Troponins Are Elevated (Myocarditis):

• Initiate oseltamivir 75 mg orally twice daily for 5 days immediately, regardless of symptom duration 4, 7
• Adjust oseltamivir to 75 mg once daily if creatinine clearance <30 mL/min 4, 7
• Obtain chest radiograph and ECG 3, 4
• Assess for volume depletion and provide IV fluids as indicated 3, 4
• Provide oxygen therapy if hypoxic, targeting PaO2 >8 kPa and SaO2 ≥92% 3, 4
• Monitor vital signs at least twice daily including temperature, respiratory rate, pulse, blood pressure, mental status, and oxygen saturation 3, 4
• Obtain full blood count, urea, creatinine, electrolytes, liver function tests, and arterial blood gases 4

ICU/HDU Transfer Criteria:

Transfer to intensive care if any of the following develop:

• Cardiogenic shock or persistent hypotension 4
• Severe cardiac dysfunction on echocardiography with clinical decompensation 4
• Failure to maintain SpO2 >92% despite FiO2 >60% 8
• Altered mental status or encephalopathy 8
• CURB-65 score of 4 or 5 if concurrent pneumonia is present 4

Common Pitfalls to Avoid

Do not assume elevated CK-MB equals myocarditis - the majority of cases represent skeletal muscle injury 1. Always check troponins before initiating cardiac-specific management 1, 2.

Do not delay antiviral therapy - oseltamivir should be started immediately when cardiac complications are suspected, as cardiac involvement typically develops 4-7 days after initial symptoms 4, 5.

Do not overlook the timing - cardiac symptoms appearing 4-7 days after influenza onset with progressive dyspnea should raise high suspicion for true myocarditis 5.

Do not rely on CK-MB alone - studies show that ambulatory adults with acute influenza frequently have abnormal ECG findings (53% on day 1) that are clinically insignificant and resolve without elevated troponins or echocardiographic changes 2.

Prognosis

In patients with true influenza myocarditis, left ventricular dysfunction typically normalizes in survivors, though fulminant cases can occur 5. In contrast, influenza-related myositis resolves completely within one week with supportive care alone 6.