Central sleep apnea (CSA) is thought to arise as a consequence of chronic heart failure. We have attempted to determine the relationship between the severity of CSA and the respiratory gas indexes during cardiopulmonary exercise testing (CPX), indexes well-known to reflect the severity of heart failure. Twenty consecutive cardiac patients (59.0 ± 15.3 years) with CSA underwent CPX. End-tidal PCO₂(PETCO₂) was measured at rest and at peak exercise as a substitute for PaCO₂, along with the peak oxygen uptake (VO₂) and the ratio of the increase in ventilation to the increase in CO₂output (VE/VCO₂ slope). Peak VO₂, % peak VO₂, and the VE/VCO₂ slope of the subjects were 15.5 ± 5.8 mL/min/kg, 52.8 ± 16.7%, and 37.9 ± 12.5, respectively, showing moderate to severely decreased exercise capacity. While PETCO₂ at both rest and peak exercise significantly correlated with peak VO₂ (r = 0.63 and r = 0.51, respectively) and the VE/VCO₂ slope (r = -0.77 and r = -0.91, respectively), none of these 3 parameters correlated with the apnea-hypopnea index. The apnea-hypopnea index in the subjects with lower resting PETCO₂ was not notably different from that in the subjects with relatively high PETCO₂.
Although the severity of CSA is assumed to correlate with the severity of heart failure, and a lowering of PaCO₂ during wakefulness is considered to be one of the mechanisms behind CSA, the severity of CSA does not correlate with the respiratory gas indexes of CPX or the level of PETCO₂ in cardiac patients with moderate to severely decreased exercise capacity.