Effect of bilateral carotid body resection on cardiac baroreflex control of blood pressure during hypoglycemia

Abstract

Hypoglycemia results in a reduction in cardiac baroreflex sensitivity and a shift in the baroreflex working range to higher heart rates. This effect is mediated, in part, by the carotid chemoreceptors. Therefore, we hypothesized hypoglycemia-mediated changes in baroreflex control of heart rate would be blunted in carotid body-resected patients when compared with healthy controls. Five patients with bilateral carotid body resection for glomus tumors and 10 healthy controls completed a 180-minute hyperinsulinemic, hypoglycemic (≈3.3 mmol/L) clamp. Changes in heart rate, blood pressure, and spontaneous cardiac baroreflex sensitivity were assessed. Baseline baroreflex sensitivity was not different between groups (P>0.05). Hypoglycemia resulted in a reduction in baroreflex sensitivity in both the groups (main effect of time, P<0.01) and responses were lower in resected patients when compared with controls (main effect of group, P<0.05). Hypoglycemia resulted in large reductions in systolic (-17±7 mm Hg) and mean (-14±5 mm Hg) blood pressure in resected patients that were not observed in controls (interaction of group and time, P<0.05). Despite lower blood pressures, increases in heart rate with hypoglycemia were blunted in resected patients (interaction of group and time, P<0.01). Major novel findings from this study demonstrate that intact carotid chemoreceptors are essential for increasing heart rate and maintaining arterial blood pressure during hypoglycemia in humans. These data support a contribution of the carotid chemoreceptors to blood pressure control and highlight the potential widespread effects of carotid body resection in humans.

Keywords: blood pressure; carotid body tumor; chemoreceptor cells; glucose.

Figure 1. Reflex responses to hypoxia

The hypoxic ventilatory response was significantly blunted in resected patients when compared with controls (*p<0.01 vs Control), with no significant difference in the heart rate response (p=0.24).

Figure 2. Baseline baroreflex sensitivity

Baroreflex sensitivity was not impaired at baseline in resected patients (A: ms/mmHg, p=0.28; B: beat/min/mmHg, p=0.48). In addition, blood pressure distribution (%CV of systolic blood pressures) was not different between groups (C: p=0.11). Columns represent group means. Blood pressure distribution is also shown as a frequency distribution curve (D and E), which depicts systolic blood pressure ±25 mmHg from average (x-axis) and the frequency of occurrence of each level of arterial pressure in percent of the total time of occurrence (y-axis) over 5 min.

Figure 3. Changes in heart rate, blood pressure, and baroreflex sensitivity during hypoglycemia

In control subjects, heart rate increased during hypoglycemia (A: ^†p<0.01 vs baseline) and blood pressure was maintained at baseline levels (B: p>0.05). Hypoglycemia-mediated reductions in blood pressure were observed in carotid body resected patients (B: ^†p<0.05 vs baseline), although a rise in heart rate was not apparent (A, *p<0.05 vs Control). scBRS was reduced from baseline during hypoglycemia (^†p<0.01) and was lower in carotid body resected patients when compared with controls (*p<0.05). Data are presented as group averages of the slope of the relationship between heart rate and systolic blood pressure [C: R-R interval (ms) and systolic blood pressure (mmHg); D: Heart rate (beat/min) and systolic blood pressure (mmHg).]

Figure 4. Changes in blood pressure distribution with hypoglycemia

An increase in blood pressure distribution (%CV of systolic blood pressures) with hypoglycemia was not observed (A: *Main effect of group, p=0.05; Main effect of time, p=0.46; Interaction of group and time, p=0.52). Blood pressure distribution is also shown as a frequency distribution curve (B: Control subjects; C: Resected subjects).