Baroreflex control of muscle sympathetic nerve activity in postural orthostatic tachycardia syndrome

Abstract

-Postural orthostatic tachycardia syndrome (POTS) is characterized by excessive tachycardia during orthostasis. To test the hypothesis that patients with POTS have decreased sympathetic neural responses to baroreflex stimuli, we measured heart rate (HR) and muscle sympathetic nerve activity (MSNA) responses to three baroreflex stimuli including vasoactive drug boluses (modified Oxford technique), Valsalva maneuver, and head-up tilt (HUT) in POTS patients and healthy control subjects. The MSNA response to the Valsalva maneuver was significantly greater in the POTS group (controls, 26 Ϯ 7 vs. POTS, 48 Ϯ 6% of baseline MSNA/mmHg; P ϭ 0.03). POTS patients also had an exaggerated MSNA response to 30°HUT (controls, 123 Ϯ 24 vs. POTS, 208 Ϯ 30% of baseline MSNA; P ϭ 0.03) and tended to have an exaggerated response to 45°HUT (controls, 137 Ϯ 27 vs. POTS, 248 Ϯ 58% of baseline MSNA; P ϭ 0.10). Sympathetic baroreflex sensitivity calculated during administration of the vasoactive drug boluses also tended to be greater in the POTS patients; however, this did not reach statistical significance (P ϭ 0.15). Baseline MSNA values during supine rest were not different between the groups (controls, 23 Ϯ 4 vs. POTS, 16 Ϯ 5 bursts/100 heartbeats; P ϭ 0.30); however, resting HR was significantly higher in the POTS group (controls, 58 Ϯ 3 vs. POTS, 82 Ϯ 4 beats/min; P ϭ 0.0001). Our results suggest that POTS patients have exaggerated MSNA responses to baroreflex challenges compared with healthy control subjects, although resting supine MSNA values did not differ between the groups. intolerance; blood pressure; heart rate; Valsalva maneuver; head-up tilt POSTURAL ORTHOSTATIC TACHYCARDIA syndrome (POTS) is a chronic, potentially debilitating condition that often occurs in otherwise healthy individuals who are most often young women. Patients with this condition (also known as idiopathic orthostatic intolerance) exhibit excessive tachycardia [increase in heart rate (HR) of Ͼ30 beats/min] during head-up tilt (HUT) in the absence of significant orthostatic hypotension. They also develop dizziness, fatigue, headache, and/or anxiety with standing or HUT (24,34). Although POTS has become better recognized clinically in recent years, the mechanisms underlying the pathophysiology of this condition remain unclear.In humans, normal baroreflex physiology involves increased HR, increased vascular sympathetic nerve activity, and peripheral vasoconstriction in response to decreased venous return during upright posture (31). These normal responses result in maintenance of arterial pressure with the upright posture and an absence of symptoms such as those reported by POTS patients. It is not clear whether the excessive tachycardia with orthostasis in POTS patients is the cause or the consequence of one or more abnormal steps in the baroreflex pathway. For example, evidence is conflicting regarding baroreflex control of HR in patients with orthostatic intolerance. Some researchers have reported decreased sensitivity of baroreflex control of HR in these ...

Neuronal and hormonal perturbations in postural tachycardia syndrome

“…Using muscle sympathetic nerve activity (MSNA) as measured by microneurography, sympathetic nervous activity (SNA) in POTS patients has been shown to differ from that of healthy subjects at rest (Furlan et al, 1998), during orthostasis (Muenter et al, 2005), or induced hypotension (Bonyhay and Freeman, 2004). Although data available is conflicting regarding resting SNA activity in POTS patients, POTS patients have an exaggerated SNA response compared to healthy subjects during orthostatic and hypotensive challenge (Bonyhay and Freeman, 2004; Muenter et al, 2005).…”

“…Although data available is conflicting regarding resting SNA activity in POTS patients, POTS patients have an exaggerated SNA response compared to healthy subjects during orthostatic and hypotensive challenge (Bonyhay and Freeman, 2004; Muenter et al, 2005). …”

Reduced central blood volume and cardiac output and increased vascular resistance during static handgrip exercise in postural tachycardia syndrome

“…The data do not directly convey information about baroreflex regulation of peripheral resistance or of cardiac contractility (31). However, prior work has suggested at least blunting of both sympathetic and cardiovagal baroreflex sensitivities in subsets of POTS (9,36,56).…”

“…Abnormalities in baroreflex control also typify forms of POTS, resulting in baseline sympathoexcitation (9,36,56). To obtain an index of sympathetic and parasympathetic activity, we used indexes of HRV and BPV to investigate the effects of handgrip on the cardiovagal baroreflex regulation of HR.…”

Postural orthostatic tachycardia syndrome (POTS)

“…Proposed mechanisms underlying this include α-1 receptor hyposensitivity and denervation insensitivity of the capacitance vessels, β-receptor hypersensitivity with vasodilation and reflex tachycardia, increased capillary permeability with fluid loss from the intravascular to the interstitial space 9 10. Other hypotheses suggest impaired baroreceptor functioning at different levels including its ability to maintain blood pressure when there is reduced venous return to the heart 5 11. Some researchers have postulated that the excessive tachycardia is the trade off for attempts by the neurovascular machinery to maintain a normal blood pressure in the face of excessive pooling and inadequate physiological compensatory mechanisms 12…”