How can a high functional capacity be reconciled with a low resting Ejection Fraction (EF) and severely impaired Stroke Volume Index (SVI)?

Reconciling High Functional Capacity with Low SVI and Normal EF

A stroke volume index of 23.69 ml/m² is severely reduced and represents paradoxical low-flow severe aortic stenosis with preserved ejection fraction—a condition where the left ventricle is small, hypertrophied, and restrictive, generating normal EF (55-60%) through minimal volume changes but profoundly limiting forward cardiac output. 1, 2

Understanding the Hemodynamic Paradox

The patient likely compensated for the severely low SVI through marked cardioacceleration (elevated heart rate) to maintain adequate cardiac output during exercise. 3

• In heart failure and restrictive physiology, patients achieve cardiac output primarily by increasing heart rate rather than stroke volume, as the hypertrophied ventricle cannot augment preload or improve systolic emptying 3
• The small, thick-walled left ventricle typical of paradoxical low-flow AS operates near maximal volume even at rest, exhausting preload reserve and preventing stroke volume augmentation during exercise 3
• Cardiac output = heart rate × stroke volume; therefore, a patient with SVI of 23.69 ml/m² could theoretically maintain adequate output if heart rate increased sufficiently (e.g., from 70 to 120+ bpm during exercise) 3

The Ejection Fraction Misconception

Ejection fraction of 55-60% does not indicate normal cardiac function in this context—it reflects a small ventricle ejecting a small volume efficiently, not adequate forward flow. 3, 1

• The European Society of Cardiology emphasizes that EF can remain normal despite severely impaired systolic function when the ventricle is small and hypertrophied 3
• In paradoxical low-flow AS, the left ventricle is typically small with thick walls, diastolic dysfunction, and restrictive physiology despite normal EF 1
• A severely reduced SVI (<30 ml/m²) carries independent prognostic significance with significantly reduced 5-year survival (adjusted HR 1.60,95% CI 1.17-2.18), regardless of preserved EF 4, 5

Critical Diagnostic Considerations

This hemodynamic profile (SVI 23.69 ml/m², EF 55-60%, low gradients) mandates immediate confirmation of aortic stenosis severity using aortic valve calcium scoring by CT. 3, 1

• The American College of Cardiology defines paradoxical low-flow AS as LVEF ≥50%, SVI <35 ml/m², AVA <1.0 cm², indexed AVA <0.6 cm²/m², and mean gradient <40 mmHg 1, 2
• Gradients may appear "only moderate" (30-40 mmHg) because low flow across the valve generates lower pressure gradients even when stenosis is anatomically severe 1
• Common pitfall: dismissing low gradients as indicating "moderate" stenosis when in low-flow states, gradients severely underestimate anatomic severity 1

Confirmatory Testing Algorithm

1. Obtain aortic valve calcium score by CT (Class IIa recommendation): men ≥3000 Agatston units or women ≥1600 units confirms severe AS 1
2. Calculate dimensionless index (ratio of LVOT velocity to aortic velocity), which is less affected by flow state 1
3. Consider dobutamine stress echocardiography to differentiate true severe AS from pseudo-stenosis, though often not feasible due to restrictive physiology in paradoxical low-flow AS 3, 1
4. Measure alternative AVA using 3D TEE or cardiac CT to verify LVOT diameter, as 2D echo frequently underestimates this in small hypertrophied ventricles 1

Prognostic Implications of SVI 23.69 ml/m²

This SVI is below the critical mortality threshold of 30 ml/m² and indicates very high risk even if asymptomatic. 1, 4, 5

• Each 5 ml/m² reduction in SVI below normal values is associated with a 20% increase in adjusted mortality risk 5
• In patients with preserved EF, the mortality threshold is at SVI <30 ml/m²; 1- and 3-year survival is significantly worse for SVI <30 ml/m² relative to SVI >35 ml/m² (HR 1.80,95% CI 1.32-2.47 and HR 1.38,95% CI 1.12-1.70) 4
• Markedly elevated BNP without other explanation supports consideration for intervention 1

Management Recommendations

If calcium score confirms severe AS, the patient requires close surveillance every 3-6 months with serial echocardiography and exercise testing for early symptom detection, as deterioration can be rapid. 3, 1

• The European Society of Cardiology and American College of Cardiology recommend conservative management with close surveillance for asymptomatic Stage C4 patients 1
• Intervention (Class IIa) should be considered only after careful confirmation that AS is severe and if symptoms develop 1
• Critical caveat: if dobutamine testing shows lack of flow reserve, this indicates very poor prognosis with either medical or surgical therapy and requires heart team discussion regarding transcatheter vs. surgical options 1

Why Performance Appeared "Good"

The patient's apparently good exercise performance likely reflects compensatory tachycardia masking the severely compromised stroke volume, not true cardiovascular reserve. 3

• Patients with heart failure achieve cardiac output primarily through cardioacceleration, with heart rate reserve being the primary mechanism to augment output 3
• The slope of heart rate increase during exercise is nearly identical between patients with heart failure and healthy individuals, but patients with heart failure start from an elevated baseline and reach lower maximal values 3
• This compensation is unsustainable and does not indicate adequate cardiovascular function—it represents a failing adaptive mechanism that will eventually decompensate 3, 1