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 breathi ng [indicated by instantaneous lung volume (ILV)] would directly influ ence left ventricular stroke volume (LVSV), which in turn would influe nce systolic arterial blood pressure (SABP), causing variation in R-R interval through the baroreflex. Eight healthy young subjects (four me n and four women) were monitored in the supine position while breathin g with a fixed frequency (0.2 Hz) and tidal volume for 15 min through each of three resistances (R(0), R(1), and R(2)) producing inspiratory (-) and expiratory (+) pressures of +/-1.6, + 5.4, and +/- 16.6 cmH(2 )O, respectively. LVSV was estimated by stroke distance [(SDist); by D oppler ultrasound]. There were no differences across R(0), R(1), and R (2) for the mean values of R-R interval, SDist, or SABP. Cross-spectra l analysis showed that, at R(0), each value of R-R interval, SDist, an d SABP lagged ILV by similar to 80 degrees. At R(1) and R(2), phase wa s reduced from ILV to SDist and R-R interval, and the transfer magnitu de for SDist (R(2) only), SABP, and R-R interval increased. The transf er magnitude from SDist to SABP significantly increased as a function of resistance breathing, whereas that from SABP to R-R interval signif icantly decreased. There were no changes in phase relationships from S Dist to SABP to R-R interval. Thus the magnitude of RSA (ILV to R-R in terval) was increased, but the transfer through the arterial barorefle x (SABP to R-R interval) was reduced. Although factors other than the arterial baroreflex are probably involved in the genesis of RSA, the c onstant phase relationship across the levels of breathing resistance a mong SDist, SABP, and R-R interval suggests an important functional li nk caused by mechanical effects of breathing.