What is the likely cause of a patient's dyspnea on exertion, given normal white blood cell (WBC) count, normal hemoglobin (HgB) level, normal pulmonary function tests (PFTs) with normal diffusing capacity of the lungs for carbon monoxide (DLCO), and a pending cardiac evaluation due to elevated troponin levels?

Cardiac Etiology is the Most Likely Cause of This Patient's Dyspnea on Exertion

Given the normal pulmonary workup (normal PFTs, normal DLCO, normal 6-minute walk test, and only mild hyperinflation on CT) combined with elevated troponin requiring cardiac catheterization, this patient's dyspnea on exertion is most likely cardiac in origin and warrants completion of the pending cardiac evaluation before attributing symptoms to any other cause. 1, 2

Critical Diagnostic Reasoning

Why Pulmonary Causes Are Effectively Ruled Out

• Normal pulmonary function tests with normal DLCO argue strongly against significant pulmonary parenchymal disease as a cause of exertional dyspnea 3, 1
• Normal right ventricular systolic pressure (14 mmHg) on echocardiography excludes pulmonary hypertension as a contributing factor 3
• Mild bilateral hyperinflation alone, without obstruction on PFTs, is insufficient to cause significant exertional dyspnea 3
• Normal 6-minute walk test performance further supports that pulmonary mechanics are not limiting exercise capacity 3, 1

Why Cardiac Etiology Must Be Pursued

Elevated troponin in the absence of overt ischemic disease requires systematic evaluation for multiple cardiac causes beyond acute coronary syndrome 3. The differential diagnosis includes:

• Myocardial ischemia from coronary artery disease - the most common cardiac cause of dyspnea when myocardial oxygen supply does not meet demand, which is why cardiac catheterization is appropriately pending 3, 2
• Heart failure with preserved ejection fraction (HFpEF) - can cause dyspnea primarily with minimal activity despite normal lung capacity and is particularly challenging to diagnose 1, 2
• Cardiomyopathies (hypertrophic, dilated, or restrictive) - can present with dyspnea and elevated troponin 3, 4
• Valvular heart disease - aortic stenosis or mitral valve disease can cause elevated left atrial pressures and exertional symptoms 2, 4
• Cardiac arrhythmias - supraventricular tachycardia, atrial fibrillation, or complete heart block can manifest as dyspnea on exertion 3, 4

Understanding the Elevated Troponin

Troponin elevation has nearly absolute myocardial tissue specificity but does not indicate the mechanism of myocardial injury 3. In this patient without overt acute coronary syndrome presentation, the elevated troponin could represent:

• Chronic myocardial injury from heart failure (acute or chronic) 3
• Demand ischemia from undiagnosed coronary disease 3, 5
• Myocarditis or pericarditis 3
• Pulmonary embolism (though normal RVSP makes this less likely) 3, 6
• Renal dysfunction (though hemoglobin is normal, suggesting adequate renal function) 3

The key diagnostic principle is that a rising and/or falling pattern of troponin distinguishes acute myocardial infarction from chronic elevation 3. Serial troponin measurements should have been obtained 6-9 hours apart to establish this pattern 3.

Essential Next Steps in Cardiac Evaluation

Immediate Diagnostic Testing Required

• Cardiac catheterization (already appropriately scheduled) to evaluate for obstructive coronary artery disease 3, 2
• B-type natriuretic peptide (BNP) or NT-proBNP measurement - BNP <100 pg/mL or NT-proBNP <125 pg/mL effectively excludes heart failure 1, 2, 4
• Review of echocardiogram for diastolic dysfunction parameters - assess left ventricular systolic and diastolic function, left atrial size, E/e' ratio, and valvular abnormalities 2, 4

If Initial Cardiac Workup is Non-Diagnostic

Cardiopulmonary exercise testing (CPET) is the definitive noninvasive tool to distinguish cardiac, pulmonary, vascular, and deconditioning causes when diagnosis remains elusive after initial evaluation 1, 2, 4. CPET parameters to evaluate include:

• VE/VCO2 slope - assesses ventilatory inefficiency 2
• O2 pulse trajectory - evaluates for myocardial ischemia or reduced stroke volume 3, 2
• Peak VO2 - determines aerobic capacity and functional limitation 3, 1
• VE/MVV ratio - identifies ventilatory mechanical limitation 3, 2

Common Diagnostic Pitfalls to Avoid

History and physical examination alone have high specificity (96%) but low sensitivity (59%) for cardiac causes, and should not be relied upon to exclude cardiac disease 3, 2. Specific pitfalls include:

• Assuming normal hemoglobin excludes anemia as a contributor - while 12.2 g/dL is technically "normal," it may be insufficient for this patient's baseline and could contribute to exertional symptoms 1
• Overlooking heart failure with preserved ejection fraction - many patients with HFpEF present without peripheral edema or obvious signs of congestion 2
• Misinterpreting "normal" echocardiogram - subtle diastolic dysfunction or valvular abnormalities may be present but not prominently reported 2, 4
• Attributing symptoms to deconditioning prematurely - deconditioning is a diagnosis of exclusion after cardiac and pulmonary causes are definitively ruled out 1, 2

Clinical Algorithm for This Patient

1. Complete pending cardiac catheterization to evaluate coronary anatomy 3, 2
2. Obtain BNP/NT-proBNP level if not already done 2, 4
3. Review echocardiogram specifically for diastolic dysfunction, valvular disease, and structural abnormalities 2, 4
4. If cardiac catheterization shows no obstructive disease and BNP is normal, consider:
   • Stress echocardiography to evaluate for inducible ischemia or diastolic dysfunction with exercise 3, 2
   • CPET to definitively characterize the physiologic limitation 1, 2, 4
5. If all cardiac testing is normal, then reconsider deconditioning or subtle pulmonary vascular disease not detected by resting studies 1, 2

The patient should continue cardiac evaluation as planned, and no alternative diagnosis should be presumed until the cardiac workup is complete 1, 2, 4.