Sympathetic responses during saline infusion into the veins of an occluded limb

Abstract

Animal studies have shown that the increased intravenous pressure stimulates the group III and IV muscle afferent fibres, and in turn induce cardiovascular responses. However, this pathway of autonomic regulation has not been examined in humans. The aim of this study was to examine the hypothesis that infusion of saline into the venous circulation of an arterially occluded vascular bed evokes sympathetic activation in healthy individuals. Blood pressure, heart rate, and muscle sympathetic nerve activity (MSNA) responses were assessed in 19 young healthy subjects during local infusion of 40 ml saline into a forearm vein in the circulatory arrested condition. From baseline (11.8 +/- 1.2 bursts min(-1)), MSNA increased significantly during the saline infusion (22.5 +/- 2.6 bursts min(-1), P < 0.001). Blood pressure also increased significantly during the saline infusion. Three control trials were performed during separate visits. The results from the control trial show that the observed MSNA and blood pressure responses were not due to muscle ischaemia. The present data show that saline infusion into the venous circulation of an arterially occluded vascular bed induces sympathetic activation and an increase in blood pressure. We speculate that the infusion under such conditions stimulates the afferent endings near the vessels, and evokes the sympathetic activation.

Figure 1. Representative tracing of heart rate (HR), muscle sympathetic nerve activity (MSNA) and arterial blood pressure (BP) obtained from a Finapres during Bier block procedure

Wrap: the arm was elevated and wrapped with a tight elastic bandage from the wrist to the elbow. The pneumatic cuff on the upper arm was then inflated to 250 mmHg. Infusion: 40 ml saline was infused into the occluded arm via the venous catheter. Circulatory occlusion was then maintained for 20 min before the cuff was deflated.

Figure 2. Mean arterial blood pressure (MAP), heart rate (HR) and MSNA measured as burst rate (right upper panel) and total activity (right lower panel) during the Bier block procedure

B: baseline (6 min). Wrap: the first min of wrapping. Infu: the last min of the saline infusion. P-0: the first 30 s of the post-infusion. P-5: the 5th to 6th min from the end of the infusion (2 min). R: recovery (6 min). Subject number = 19. Data are means ±s.e.m.

Figure 4. Mean arterial blood pressure (MAP), heart rate (HR) and MSNA measured as burst rate (right upper panel) and total activity (right lower panel) during the Bier block procedure in the second visit

Pre-I: pre-infusion, a 3 min period just before the onset of the saline infusion. Subject number = 7.

Figure 3. Representative tracing of heart rate (HR), MSNA and arterial blood pressure (BP) obtained from the same subject as in Fig. 1 during the second visit

Pre-Infusion: 3 min period just before the onset of the saline infusion. The interval between the wrap and the infusion during the second visit was much longer than during the first visit. From the onset of the occlusion, the Pre-Infusion period was at the approximate same time when the infusion was performed in the first visit. After the end of the infusion, circulatory occlusion was maintained for 10 min before the cuff was deflated.

Figure 5. Representative tracing of heart rate (HR), MSNA and arterial blood pressure (BP) obtained from another subject during the saline infusion under the circulatory occlusion condition

The exsanguination procedure (i.e. wrapping forearm, etc.) was not performed before upper arm cuff inflation in this control trial.