Continuous positive airway pressure normalizes cardiac autonomic and hemodynamic responses to a laboratory stressor in apneic patients

Objectives: We examined the effect of continuous positive airway pressure ( CPAP) treatment for sleep apnea on cardiac contractility, heart rate variab ility, and hemodynamics at rest and in response to a laboratory stressor. Subjects and instrumentation: Forty-one apneic patients were studied on thr ee occasions: before treatment, after 1 full night of CPAP treatment, and a fter 1 week of CPAP treatment. The subjects were randomly assigned to recei ve effective treatment or placebo. Contractility and hemodynamics were dete rmined with impedance cardiography, and parasympathetic activity was assess ed by analysis of heart rate variability. Measures were determined at rest and in response to a stressor. Design and results: For the cardiac sympathetic (contractility) measures (p reejection period, cardiac acceleration index [CAI], and low-frequency/high -frequency ratio) significant interactions were found in the combination tr eatment (CPAP vs placebo) by study day (day 1, day 3, day 11) by test perio d (baseline, preparation, talking) [p < 0.01]. For these measures, there we re no differences between the treatment groups or responses to the stressor on day 1. Levels in placebo-treated subjects did not change or respond on the subsequent study days. In the CPAP-treated subjects, there was a decrea se in these indexes at baseline, which became significantly lower by day 11 (ie, CAI levels were 24 <Omega>/s(2), 22 Omega /s(2), anti 14 Omega /s(2) on day 1, day 3, and day 11, respectively). These measures also became resp onsive to the stressor by showing increased sympathetic activity (CAT level s on day 11 were 14 Omega /s(2) at baseline, 32 Omega /s(2) during speech p reparation, and 36 Omega /s(2) while speaking). The parasympathetic indexes , such as high-frequency power or band of heart rate variability as determi ned by spectral analysis, showed a significant day-by-treatment interaction (p < 0.005), whereas the CPAP-treated group had significantly more parasym pathetic activity after I week of treatment. For the hemodynamic measures ( stroke volume [SV], cardiac output, and systemic vascular resistance [SVR]) , there were significant treatment-by-study day-by-test-period interactions (p < 0.01). SV and cardiac output increased across days, and SVR decreased in the CPAP-treated patients. Conclusions: These results indicate that CPAP normalizes contractility, inc reases cardiac vagal tone, and changes hemodynamic regulation from being re sistance dominated to being cardiac dominated. Thus, after 1 week of treatm ent with CPAP, many of the indicators of poor cardiac functioning in apnea patients are improved.