The role of the right ventricle during hypobaric hypoxic exercise: Insights from patients after the Fontan operation

Objectives: The principal objective of this study was to examine the importance of the fight ventricle for maximal systemic oxygen transport during exercise at high altitude by studying patients after the Fortan operation. Background: High-altitude-induced hypoxia causes a reduction in maximal oxygen uptake. Normal right ventricular pump function may be critical to sustain cardiac output in the face of hypoxic pulmonary vasoconstriction. We hypothesized that patients after the Fontan operation, who lack a functional subpulmonary ventricle, would have a limited exercise capacity at altitude, with an inability to increase cardiac output. Methods: We measured oxygen uptake (V̇O₂, Douglas bag), cardiac output (Q̇c, C₂H₂ rebreathing), heart rate (HR) (ECG), blood pressure (BP) (cuff), and O₂ Sat (pulse oximetry) in 11 patients aged 14.5 ± 5.2 yr (mean ± SD) at 4.7 ± 1.6 yr after surgery. Data were obtained at rest, at three submaximal steady state workrates, and at peak exercise on a cycle ergometer. All tests were performed at sea level (SL) and at simulated altitude (ALT) of 3048 m (10,000 ft, 522 torr) in a hypobaric chamber. Results: At SL, resting O₂ sat was 92.6 ± 4%. At ALT, O₂ sat decreased to 88.2 ± 4.6% (P < 0.05) at rest and decreased further to 80 ± 6.3% (P < 0.05) with peak exercise. At SL, V̇O₂ increased from 5.1 ± 0.9 mL · kg^-1 · min^-1 at rest to 23.5 ± 5.3 mL · kg^-1 · min^-1 at peak exercise and CI (Q̇c · m^-2) increased from 3.3 ± 0.7 L · m^-2 to 6.2 ± 1.2 L · m^-2.V̇O₂ peak, 17.8 ± 4 mL · kg^-1 · min^-1 (P < 0.05), and CI peak, 5.0 ± 1.5 L · m^-2 (P < 0.05), were both decreased at ALT. Remarkably, the relationship between Q̇c and V̇O₂ was normal during submaximal exercise at both SL and ALT. However at ALT, stroke volume index (SVI, SV · m^-2) decreased from 37.7 ± 8.6 mL · min^-1 · m² at rest, to 31.3 ± 8.6 mL · min^-1 · m² at peak exercise (P < 0.05), whereas it did not fall during sea level exercise. Conclusions: During submaximal exercise at altitude, right ventricular contractile function is not necessary to increase cardiac output appropriately for oxygen uptake. However, normal right ventricular pump function may be necessary to achieve maximal cardiac output during exercise with acute high altitude exposure.