Low-frequency oscillation of sympathetic nerve activity decreases during development of tilt-induced syncope preceding sympathetic withdrawal and bradycardia

Abstract

. Low-frequency oscillation of sympathetic nerve activity decreases during development of tiltinduced syncope preceding sympathetic withdrawal and bradycardia. Am J Physiol Heart Circ Physiol 289: H1758 -H1769, 2005. First published June 2, 2005; doi:10.1152/ajpheart.01027.2004.-Sympathetic activation during orthostatic stress is accompanied by a marked increase in low-frequency (LF, ϳ0.1-Hz) oscillation of sympathetic nerve activity (SNA) when arterial pressure (AP) is well maintained. However, LF oscillation of SNA during development of orthostatic neurally mediated syncope remains unknown. Ten healthy subjects who developed head-up tilt (HUT)-induced syncope and 10 agematched nonsyncopal controls were studied. Nonstationary timedependent changes in calf muscle SNA (MSNA, microneurography), R-R interval, and AP (finger photoplethysmography) variability during a 15-min 60°HUT test were assessed using complex demodulation. In both groups, HUT during the first 5 min increased heart rate, magnitude of MSNA, LF and respiratory high-frequency (HF) amplitudes of MSNA variability, and LF and HF amplitudes of AP variability but decreased HF amplitude of R-R interval variability (index of cardiac vagal nerve activity). In the nonsyncopal group, these changes were sustained throughout HUT. In the syncopal group, systolic AP decreased from 100 to 60 s before onset of syncope; LF amplitude of MSNA variability decreased, whereas magnitude of MSNA and LF amplitude of AP variability remained elevated. From 60 s before onset of syncope, MSNA and heart rate decreased, index of cardiac vagal nerve activity increased, and AP further decreased to the level at syncope. LF oscillation of MSNA variability decreased during development of orthostatic neurally mediated syncope, preceding sympathetic withdrawal, bradycardia, and severe hypotension, to the level at syncope. autonomic nervous system; baroreflex; blood pressure; heart rate variability; hemodynamics HUMANS HAVE BEEN SUBJECTED to ceaseless orthostatic stresses since they first evolved and assume an orthostatic posture for most of their lives. During standing, gravitational fluid shift toward the lower part of body (i.e., abdominal vascular bed and lower limbs) would cause severe orthostatic hypotension if it were not countered by compensatory mechanisms (23). Orthostatic sympathetic activation has a crucial role in preventing orthostatic hypotension and maintaining arterial blood pressure (AP) (23). Recent studies have reported that orthostatic sympathetic activation is accompanied by an increase in lowfrequency (LF, ϳ0.1-Hz) oscillation of sympathetic nerve activity (SNA) (1, 5). Tilt maneuvers of 75°and 80°greatly increase the LF oscillatory patterns of muscle SNA (MSNA), which mirrored similar changes in LF oscillation of AP (1, 5). However, LF oscillation of SNA has been investigated only in the steady-state orthostatic condition, when AP remains well maintained. It remains unclear whether LF oscillation of SNA changes during development of orthostatic neurally mediated s...

Indirect measures of human vagal withdrawal during head-up tilt with and without a respiratory acidosis

“…During HUT, the mean heart rate was 86 beats min -1 with HF and LF components of 28.6 NU and 63.4 NU respectively J Physiol Sci (2009) 59: 31-36 33 ventilation, as shown by others [23][24][25][26][27]. Also, in the current study, a decrease in HF cardiac variability was shown with HUT, again similar to the findings of others [14,[18][19][20]33]. However, in the present study, when using the HF component as an index of cardiac vagal tone, the expected vagal withdrawal during HUT was reduced when the HF component was already elevated by an increase in ventilation.…”

“…In humans, it is well established that moving from supine to an upright posture induces an increase in heart rate and a small decrease in end-tidal partial pressure of CO 2 (PET CO 2 ) [13][14][15][16][17]. Both postural change and hypercapnia are known to affect HRV, whereby the HF component has consistently been shown to decrease with head-up tilt (HUT) in normoxic healthy humans [14,[18][19][20]. However, directional changes in the LF component of HRV with tilting appear equivocal, with no change [14,18,21] or an increase [19,20,22] being reported for healthy subjects.…”

“…Both postural change and hypercapnia are known to affect HRV, whereby the HF component has consistently been shown to decrease with head-up tilt (HUT) in normoxic healthy humans [14,[18][19][20]. However, directional changes in the LF component of HRV with tilting appear equivocal, with no change [14,18,21] or an increase [19,20,22] being reported for healthy subjects. Hypercapnia has consistently been shown to increase HF cardiac variability [7,[23][24][25], suggestive of an increase in respiratory sinus arrhythmia [24,[26][27][28] but shown to have variable effects on LF variability [29].…”

Fatigue influence on dynamics of cardiovas­cular functional parameters during head-­up tilt table test in women non-­athletes cohort

“…Kamiya et al (2005) suggested that one or more control systems governing the muscle nerve sympathetic activity (MSNA) are modulated prior to the inhibition of MSNA during the development of orthostatic syncope. Therefore, modulation of ABR function under orthostatic stress, possibly, one of the mechanisms maintaining ABP and limiting orthostatic hypotension, and impairment of ABR control over sympathetic vasomotor activity leads to a severe hypotension associated with orthostatic syncope (Ichinose and Nishiyasu, 2012).…”

Zastosowanie testu pionizacyjnego i podciśnienia wokół dolnej połowy ciała w badaniach odruchowej regulacji krążenia u ludzi

“…Due to its simplicity, ease of application, and full control over the level of excitation, HUT has been widely used in the diagnostics of syncope [2]. Clinical manifestations of syncope include nausea, diaphoresis, yawning, monochromatic vision (grey out), vertigo, systolic blood pressure (SBP) < 80 mm Hg [3], and a decrease in SBP by 15 mm Hg/min and/or in bradycardia by 15 beats/min [4,5]. According to the latest guidelines of the European Society of Cardiology [5], the orthostatic test should include a 5-min rest in the supine position before examination and tilting for at least 2 minutes.…”