Interactions between heart rate variability and pulmonary gas exchange efficiency in humans

The respiratory component of heart rate variability (respiratory sinus arrhythmia, RSA) has been associated with improved pulmonary gas exchange efficiency in humans via the apparent clustering and scattering of heart beats in time with the inspiratory and expiratory phases of alveolar ventilation, respectively. However, since human RSA causes only marginal redistribution of heart beats to inspiration, we tested the hypothesis that any association between RSA amplitude and pulmonary gas exchange efficiency may be indirect. In 11 patients with fixed-rate cardiac pacemakers and 10 healthy control subjects, we recorded R-R intervals, respiratory flow, end-tidal gas tension and the ventilatory equivalents for carbon dioxide and oxygen during 'fast' (0.25 Hz) and 'slow' paced breathing (0.10 Hz). Mean heart rate, mean arterial blood pressure, mean arterial pressure fluctuations, tidal volume, end-tidal CO(2), and were similar between pacemaker and control groups in both the fast and slow breathing conditions. Although pacemaker patients had no RSA and slow breathing was associated with a 2.5-fold RSA amplitude increase in control subjects (39 +/- 21 versus 97 +/- 45 ms, P < 0.001), comparable (main effect for breathing frequency, F(1,19) = 76.54, P < 0.001) and reductions (main effect for breathing frequency, F(1,19) = 23.90, P < 0.001) were observed for both cohorts during slow breathing. In addition, the degree of (r = 0.36, P = 0.32) and reductions (r = 0.29, P = 0.43) from fast to slow breathing were not correlated to the degree of associated RSA amplitude enhancements in control subjects. These findings suggest that the association between RSA amplitude and pulmonary gas exchange efficiency during variable-frequency paced breathing observed in prior human work is not contingent on RSA being present. Therefore, whether RSA serves an intrinsic physiological function in optimizing pulmonary gas exchange efficiency in humans requires further experimental validation.