Cardiorespiratory interactions during fixed-pace resistive breathing

We tested the hypothesis that the arterial baroreflex was important in the origin of respiratory sinus arrhythmia (RSA) under conditions of normal and resistive breathing. That is, mechanical effects of breathing [indicated by instantaneous lung volume (ILV)] would directly influence left ventricular stroke volume (LVSV), which in turn would influence systolic arterial blood pressure (SABP), causing variation in R-R interval through the baroreflex. Eight healthy young subjects (four men and four women) were monitored in the supine position while breathing with a fixed frequency (0.2 Hz) and tidal volume for 15 min through each of three resistances (R0, R1, and R2) producing inspiratory (-) and expiratory (+) pressures of +/- 1.6, +/- 5.4, and +/- 16.6 cmH2O, respectively. LVSV was estimated by stroke distance [(SDist); by Doppler ultrasound]. There were no differences across R0, R1, and R2 for the mean values of R-R interval, SDist, or SABP. Cross-spectral analysis showed that, at R0, each value of R-R interval, SDist, and SABP lagged ILV by approximately 80 degrees. At R1 and R2, phase was reduced from ILV to SDist and R-R interval, and the transfer magnitude for SDist (R2 only), SABP, and R-R interval increased. The transfer magnitude from SDist to SABP significantly increased as a function of resistance breathing, whereas that from SABP to R-R interval significantly decreased. There were no changes in phase relationships from SDist to SABP to R-R interval. Thus the magnitude of RSA (ILV to R-R interval) was increased, but the transfer through the arterial baroreflex (SABP to R-R interval) was reduced. Although factors other than the arterial baroreflex are probably involved in the genesis of RSA, the constant phase relationship across the levels of breathing resistance among SDist, SABP, and R-R interval suggests an important functional link caused by mechanical effects of breathing.