Differential effect of head-up tilt on cardiovagal and sympathetic baroreflex sensitivity in humans.
The purpose of this study was to determine the effect of baroreceptor unloading on the sensitivity of the cardiovagal and sympathetic arms of the baroreflex during upright posture. Beat-by-beat R-R interval, arterial blood pressure and cardiac output (Doppler ultrasound), as well as muscle sympathetic nerve activity (MSNA) were recorded during periods in supine (Supine) and 60 deg head-up tilt (HUT) positions (n = 8 volunteers). Cardiovagal baroreflex sensitivity (BRS) was measured by the spontaneous sequence analysis method using systolic blood pressure and R-R interval, while sympathetic BRS was determined using the slope of the linear relationship between decreasing segments of diastolic blood pressure (DBP) and corresponding increases in MSNA. On changing to HUT, mean R-R interval and cardiac output decreased, while mean measures of MSNA, DBP and total peripheral resistance increased (P < 0.05). Cardiovagal BRS decreased from Supine to 60 deg HUT (19 +/- 2 ms mmHg(-1) versus 7.6 +/- 1.2 ms mmHg(-1); P < 0.01). In contrast, sympathetic BRS increased from -6.1 +/- 1.4 a.u. mmHg(-1) in Supine to -14 +/- 2 a.u. mmHg(-1) in HUT (P < 0.01). Thus, HUT produced differential effects on cardiac versus sympathetic BRS. The data suggest that dynamic baroreflex-mediated cardiovascular control is dominated by sympathetic control during baroreceptor unloading.
O'Leary
D D
DD
Neurovascular Research Laboratory, School of Kinesiology, The University of Western Ontario, London, Ontario, Canada N6A 3K7.
Kimmerly
D S
DS
Cechetto
A D
AD
Shoemaker
J K
JK
D000328:Adult; D017704:Baroreflex / Q000502:physiology*; D001794:Blood Pressure / Q000502:physiology; D005260:Female; D006257:Head / Q000502:physiology*; D006321:Heart / Q000294:innervation* / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology; D006487:Hemostasis / Q000502:physiology; D006801:Humans; D008297:Male; D010543:Peroneal Nerve / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013223:Statistics as Topic; D016683:Supine Position / Q000502:physiology; D013564:Sympathetic Nervous System / Q000502:physiology; D014630:Vagus Nerve / Q000502:physiology*