Baroreflex responses to limb venous distension in humans

Abstract

The venous distension reflex (VDR) is a pressor response evoked by peripheral venous distension and accompanied by increased muscle sympathetic nerve activity (MSNA). The effects of venous distension on the baroreflex, an important modulator of blood pressure (BP), have not been examined. The purpose of this study was to examine the effect of the VDR on baroreflex sensitivity (BRS). We hypothesized that the VDR will increase the sympathetic BRS (SBRS). Beat-by-beat heart rate (HR), BP, and MSNA were recorded in 16 female and 19 male young healthy subjects. To induce venous distension, normal saline equivalent to 5% of the forearm volume was infused into the veins of the occluded forearm. SBRS was assessed from the relationship between diastolic BP and MSNA during spontaneous BP variations. Cardiovagal BRS (CBRS) was assessed with the sequence technique. Venous distension evoked significant increases in BP and MSNA. Compared with baseline, during the maximal VDR response period, SBRS was significantly increased (-3.1 ± 1.5 to -4.5 ± 1.6 bursts·100 heartbeats^-1·mmHg^-1, P < 0.01) and CBRS was significantly decreased (16.6 ± 5.4 to 13.8 ± 6.1 ms·mmHg^-1, P < 0.01). No sex differences were observed in the effect of the VDR on SBRS or CBRS. These results indicate that in addition to its pressor effect, the VDR altered both SBRS and CBRS. We speculate that these changes in baroreflex function contribute to the modulation of MSNA and BP during limb venous distension.

Keywords: arterial baroreflex; autonomic nervous system; muscle sympathetic nerve activity; venous distension reflex.

Figure 1.

A: schematic diagram of the experimental protocol. B: representative recordings of muscle sympathetic nerve activity (MSNA), blood pressure (BP), and beat-by-beat heart rate (HR). Time scales of the bottom panel are different from the top panel. Post-Inf, post-infusion; Pre-Inf, pre-infusion; VDR, maximal response period to venous distension; WE, wrist to elbow occlusion.

Figure 2.

Representative curves of baroreflex control of MSNA burst incidence during baseline and the venous distension period in one subject. The slope of the linear regression between MSNA and diastolic blood pressure in the two conditions (baseline and VDR) was calculated to determine baroreflex sensitivity. VDR increased both MSNA and DBP, as indicated by a right and upward shift in the operating point. It is also clear that the slope relating the change in MSNA relative to the change in DBP was more negative during VDR. At baseline, ES was negative, whereas it was positive during VDR period. DBP, diastolic blood pressure; ES, error signal; MSNA, muscle sympathetic nerve activity; T50, the diastolic blood pressure corresponding to burst incidence = 50 on the curve; VDR, venous distension reflex.

Figure 3.

Absolute values of cardiovascular variables and muscle sympathetic nerve activity (MSNA) during venous distension in male and female subjects. A: mean arterial pressure (MAP). B: heart rate (HR). C: MSNA burst frequency (BI). D: total MSNA. White bars indicate average values in males and gray bars indicate average values in females. Small circles represent individual values of male subjects. Small triangles represent individual values of female subjects. P values for intervention, sex, and the interaction were from the linear mixed-effects model. Age and BMI were applied as covariates in the linear mixed-effects model. Post hoc: *P < 0.05 compared with baseline; †P < 0.05 compared with Pre-Inf. There was no significant difference in these variables between male and female groups. Base, baseline; BMI, body mass index; Pre-Inf, pre-infusion; VDR, maximal response period.

Figure 4.

Baroreflex sensitivity during venous distension in male and female subjects. A: cardiovagal baroreflex sensitivity (CBRS). B: sensitivity of baroreflex control of MSNA burst incidence (SBRS-BI). C: sensitivity of baroreflex control of MSNA total activity (SBRS-total MSNA). D: sensitivity of baroreflex control of MSNA burst area (SBRS-burst area). White bars indicate average values in males and gray bars indicate average values in females. Small circles represent individual values of male subjects. Small triangles represent individual values of female subjects. P values for intervention, sex, and the interaction were from the linear mixed-effects model. Age and BMI were applied as covariates in the linear mixed-effects model. Post hoc: *P < 0.05 compared with baseline; †P < 0.05 compared with Pre-Inf. ‡P < 0.05 compared with male group in the same period. Base, baseline; BMI, body mass index; MSNA, muscle sympathetic nerve activity; Pre-Inf, pre-infusion; VDR, maximal response period.