title: Differential effect of head-up tilt on cardiovagal and sympathetic baroreflex sensitivity in humans.
abstract: 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.
authors: Neurovascular Research Laboratory, School of Kinesiology, The University of Western Ontario, London, Ontario, Canada N6A 3K7.
authors: D D
authors: DD
authors: O'Leary
authors: D S
authors: DS
authors: Kimmerly
authors: A D
authors: AD
authors: Cechetto
authors: J K
authors: JK
authors: Shoemaker
mesh_terms: 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*