Deceived by the Fick principle: blood flow distribution in heart failure

The Fick principle states that oxygen uptake (V̇O2) is cardiac output (Qc) * arterial-venous O2 content difference [ΔC(a-v)O2]. Blood flow distribution is hidden in Fick principle, and its relevance during exercise in heart failure (HF) is undefined. To highlight the role of blood flow distribution, we evaluated peak exercise V̇O2, Qc, and ΔC(a-v)O2, before and after HF therapeutic interventions.

Symptom-limited cardiopulmonary exercise tests with Qc measurement (inert gas rebreathing) was performed in 234 HF patients before and 6 months after successful exercise training, cardiac resynchronization therapy, or percutaneous edge-to-edge mitral valve repair. Considering all tests (n = 468), a direct correlation between peakV̇O2 and peakQc (R2 = 0.47) and workload (R2 = 0.70) was observed. Patients were grouped according to treatment efficacy in Group 1 (peakV̇O2 increase >10%, n = 93), Group 2 (peakV̇O2 change between 0 and 10%, n = 60), and Group 3 (reduction in peakV̇O2, n = 81). Post-treatment peakV̇O2 changes poorly correlated with peakQc and peakΔC(a-v)O2 changes. Differently, post-procedure peakQc vs. peakΔC(a-v)O2 changes showed a close negative correlation (R2 = 0.46), becoming stronger grouping patients according to peakV̇O2 improvement (R2 = 0.64, 0.79, and 0.58 in Groups 1, 2, and 3, respectively). In 76% of patients, peakQc and ΔC(a-v)O2 changes diverged regardless of treatment.

The bulk of these data suggests that blood flow distribution plays a pivotal role on peakV̇O2 determination regardless of HF treatment strategies. Accordingly, for assessing HF treatment efficacy on exercise performance, the sole peakV̇O2 may be deceptive and the combination of V̇O2, Qc and ΔC(a-v)O2, must be considered.