Does the severity of central sleep apnea correlate with respiratory gas indexes during cardiopulmonary exercise testing?

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 PCO(2)(PETCO(2)) was measured at rest and at peak exercise as a substitute for PaCO(2), along with the peak oxygen uptake (V(.)O(2)) and the ratio of the increase in ventilation to the increase in CO(2)output (V(.)E/V(.)CO(2) slope). Peak VO(2), % peak VO(2), and the VE/V(.)CO(2) 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 PETCO(2) at both rest and peak exercise significantly correlated with peak VO(2) (r = 0.63 and r = 0.51, respectively) and the VE/V(.)CO(2) 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 PETCO(2) was not notably different from that in the subjects with relatively high PETCO(2). Although the severity of CSA is assumed to correlate with the severity of heart failure, and a lowering of PaCO(2) 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 PETCO(2) in cardiac patients with moderate to severely decreased exercise capacity.