Effect of baroreflex loading on the responsiveness of the vestibulosympathetic reflex in humans

Human cardiovascular adaptation to hypergravity

Despite decades of experience from high-G exposures in aircraft and centrifuges, information is scarce regarding primary cardiovascular adaptations to +Gz loads in relaxed humans. Thus, effects of G-training are typically evaluated after regimens that are confounded by concomitant use of anti-G straining maneuvers, anti-G suits and pressure breathing. Accordingly, the aim was to evaluate cardiovascular adaptations to repeated +Gz exposures in the relaxed state. Eleven men underwent 5 weeks of centrifuge G training, consisting of 15 × 40 min +Gz exposures at G levels close to their individual relaxed G-level tolerance. Before and after the training regimen, relaxed G-level tolerance was investigated during rapid (ROR) and gradual (GOR) onset-rate G exposures, and cardiovascular responses were investigated during orthostatic provocation and vascular pressure-distension tests. The G training resulted in: (i) a 13% increase in relaxed ROR G tolerance (P < 0.001), but no change in GOR G tolerance, (ii) increased pressure resistance in the arteries and arterioles of the legs (P < 0.001), but not the arms, (iii) a reduced initial drop in arterial pressure upon ROR high G, but no change in arterial pressure under basal resting conditions or during GOR G loading, or orthostatic provocation. The results suggest +Gz adaptation via enhanced pressure resistance in dependent arteries/arterioles. Presumably this reflects local adaptations to high transmural pressures, resulting from the +Gz-induced exaggeration of the intravascular hydrostatic pressure gradients.

Ageing of the Diaphragm Muscle

The diaphragm muscle is the most important contractile district used for breathing. Like other muscles in the human body, it is subject to ageing and sarcopenia. Sarcopenia can be classified as primary (or age-related) when there are no local or systemic pathologies that cause a functional and morphological detriment of skeletal musculature. Secondary sarcopenia occurs when there is a cause or more pathological causes (illness, malnutrition, immobility) related or unrelated to ageing. In the elderly population, transdiaphragmatic pressure (Pdi) decreases by 20-41%, with a decline in the overall strength of 30% (the strength of the expiratory muscles also decreases). The article discusses the adaptation of the diaphragm muscle to ageing and some other ailments and co-morbidities, such as back pain, emotional alterations, motor incoordination, and cognitive disorders, which are related to breathing.

Interaction between graviception and carotid baroreflex function in humans during parabolic flight-induced microgravity

The aim of the present study was to assess carotid baroreflex (CBR) function during acute changes in otolithic activity in humans. To address this question, we designed a set of experiments to identify the modulatory effects of microgravity on CBR function at a tilt angle of -2°, which was identified to minimize changes in central blood volume during parabolic flight. During parabolic flight at 0 and 1 g, CBR function curves were modeled from the heart rate (HR) and mean arterial pressure (MAP) responses to rapid pulse trains of neck pressure and neck suction ranging from +40 to -80 Torr; CBR control of HR (carotid-HR) and MAP (carotid-MAP) function curves, respectively. The maximal gain of both carotid-HR and carotid-MAP baroreflex function curves were augmented during microgravity compared with 1 g (carotid-HR, -0.53 to -0.80 beats·min^-1·mmHg^-1, P < 0.05; carotid-MAP, -0.24 to -0.30 mmHg/mmHg, P < 0.05). These findings suggest that parabolic flight-induced acute change of otolithic activity may modify CBR function and identifies that the vestibular system contributes to blood pressure regulation under fluctuations in gravitational forces. NEW & NOTEWORTHY The effect of acute changes in vestibular activity on arterial baroreflex function remains unclear. In the present study, we assessed carotid baroreflex function without changes in central blood volume during parabolic flight, which causes acute changes in otolithic activity. The sensitivity of both carotid heart rate and carotid mean arterial pressure baroreflex function was augmented in microgravity compared with 1 g, suggesting that the vestibular system contributes to blood pressure regulation in humans on Earth.

Vestibulo-sympathetic responses

Evidence accumulated over 30 years, from experiments on animals and human subjects, has conclusively demonstrated that inputs from the vestibular otolith organs contribute to the control of blood pressure during movement and changes in posture. This review considers the effects of gravity on the body axis, and the consequences of postural changes on blood distribution in the body. It then separately considers findings collected in experiments on animals and human subjects demonstrating that the vestibular system regulates blood distribution in the body during movement. Vestibulosympathetic reflexes differ from responses triggered by unloading of cardiovascular receptors such as baroreceptors and cardiopulmonary receptors, as they can be elicited before a change in blood distribution occurs in the body. Dissimilarities in the expression of vestibulosympathetic reflexes in humans and animals are also described. In particular, there is evidence from experiments in animals, but not humans, that vestibulosympathetic reflexes are patterned, and differ between body regions. Results from neurophysiological and neuroanatomical studies in animals are discussed that identify the neurons that mediate vestibulosympathetic responses, which include cells in the caudal aspect of the vestibular nucleus complex, interneurons in the lateral medullary reticular formation, and bulbospinal neurons in the rostral ventrolateral medulla. Recent findings showing that cognition can modify the gain of vestibulosympathetic responses are also presented, and neural pathways that could mediate adaptive plasticity in the responses are proposed, including connections of the posterior cerebellar vermis with the vestibular nuclei and brainstem nuclei that regulate blood pressure.

Neck proprioceptors contribute to the modulation of muscle sympathetic nerve activity to the lower limbs of humans

Several different strategies have now been used to demonstrate that the vestibular system can modulate muscle sympathetic nerve activity (MSNA) in humans and thereby contribute to the regulation of blood pressure during changes in posture. However, it remains to be determined how the brain differentiates between head-only movements that do not require changes in vasomotor tone in the lower limbs from body movements that do require vasomotor changes. We tested the hypothesis that neck movements modulate MSNA in the lower limbs of humans. MSNA was recorded in 10 supine young adult subjects, at rest, during sinusoidal stretching of neck muscles (100 cycles, 35° peak to peak at 0.37 ± 0.02 Hz) and during a ramp-and-hold (17.5° for 54 ± 9 s) static neck muscle stretch, while their heads were held fixed in space. Cross-correlation analysis revealed cyclical modulation of MSNA during sinusoidal neck muscle stretch (modulation index 45.4 ± 5.3 %), which was significantly less than the cardiac modulation of MSNA at rest (78.7 ± 4.2 %). Interestingly, cardiac modulation decreased significantly during sinusoidal neck displacement (63.0 ± 9.3 %). By contrast, there was no significant difference in MSNA activity during static ramp-and-hold displacements of the neck to the right or left compared with that with the head and neck aligned. These data suggest that dynamic, but not static, neck movements can modulate MSNA, presumably via projections of muscle spindle afferents to the vestibular nuclei, and may thus contribute to the regulation of blood pressure during orthostatic challenges.

Is there diurnal variation of the vestibulosympathetic reflex: implications for orthostatic hypotension

Incidences of adverse cardiac events and orthostatic hypotension are associated with diurnal variations. The primary purpose of the present study was to determine if the vestibulosympathetic reflex (VSR) follows a diurnal variation in humans. We hypothesized that the VSR would be attenuated at night based on the relation between melatonin and the VSR. Arterial blood pressure, heart rate, calf blood flow, and muscle sympathetic nerve activity (MSNA) were measured in nine healthy subjects (28 ± 1 yr, 5 men and 4 women) at rest and during head-down rotation. Each subject was tested during the day at 11:34 ± 13 and again at night 22:10 ± 5. MSNA was significantly decreased at night compared with day (8 ± 1 vs. 11 ± 2 bursts/min, respectively, P < 0.02). Heart rate and arterial blood pressure at rest were significantly increased at night compared with day (heart rate: 70 ± 4 vs. 66 ± 4 beats/min and mean arterial blood pressure: 91 ± 2 vs. 87 ± 1 mmHg, respectively). MSNA and hemodynamic responses to head-down rotation were not significantly altered at night compared with day (changes of 3 ± 1 bursts/min and 25 ± 6% for MSNA and calf blood flow, respectively). The data indicate that MSNA at rest decreases during the late evening hours and exhibits a diurnal variation, whereas the VSR does not. In summary, diurnal variation of orthostatic hypotension in humans does not appear to be associated with changes in the VSR and MSNA at rest.

Acute cardiovascular responses in preterm infants at 34-39 weeks of gestational age

BACKGROUND

Premature infants demonstrate immature physiological control mechanisms; however their acute cardiovascular control has not yet been widely studied.

AIM

The aim of this study was to analyze heart rate (HR) and blood pressure (BP) control in preterm infants.

SUBJECTS

Twenty preterm infants with a mean gestational age of 31 ± 2.4 (26-34) weeks at birth were evaluated at a gestational age of 36 ± 1.5 (34-39) weeks. Results were compared to twenty, healthy, full-term, control infants studied at the age of 12 ± 3 weeks.

OUTCOME MEASURES

HR and BP responses to 45° head-up tilt and side motion tests during non-rapid eye movement sleep were analyzed. In addition, HR responses to spontaneous arousals from non-rapid eye movement sleep were evaluated.

RESULTS

Preterm infants showed significantly smaller initial HR and BP responses compared with controls in head-up tilt (HR p=0.0005, systolic BP p=0.02, diastolic BP p=0.01) and side motion tests (HR p=0.002, systolic BP p<0.0001, diastolic BP p<0.0001). Furthermore, in tilt tests, preterm infants presented with greater intersubject variability in BP responses than controls (systolic BP p=0.009, diastolic BP p=0005). Preterm HR responses to spontaneous arousals were similar to controls.

CONCLUSIONS

This study indicates immature vestibulo-mediated cardiovascular control in preterm infants compared with term infants. This is seen as attenuated BP responses to side motion test and more labile acute BP control to postural challenge.

Effects of short-term and prolonged bed rest on the vestibulosympathetic reflex

The mechanism(s) for post-bed rest (BR) orthostatic intolerance is equivocal. The vestibulosympathetic reflex contributes to postural blood pressure regulation. It was hypothesized that muscle sympathetic nerve responses to otolith stimulation would be attenuated by prolonged head-down BR. Arterial blood pressure, heart rate, muscle sympathetic nerve activity (MSNA), and peripheral vascular conductance were measured during head-down rotation (HDR; otolith organ stimulation) in the prone posture before and after short-duration (24 h; n = 22) and prolonged (36 ± 1 day; n = 8) BR. Head-up tilt at 80° was performed to assess orthostatic tolerance. After short-duration BR, MSNA responses to HDR were preserved (Δ5 ± 1 bursts/min, Δ53 ± 13% burst frequency, Δ65 ± 13% total activity; P < 0.001). After prolonged BR, MSNA responses to HDR were attenuated ∼50%. MSNA increased by Δ8 ± 2 vs. Δ3 ± 2 bursts/min and Δ83 ± 12 vs. Δ34 ± 22% total activity during HDR before and after prolonged BR, respectively. Moreover, these results were observed in three subjects tested again after 75 ± 1 days of BR. This reduction in MSNA responses to otolith organ stimulation at 5 wk occurred with reductions in head-up tilt duration. These results indicate that prolonged BR (∼5 wk) unlike short-term BR (24 h) attenuates the vestibulosympathetic reflex and possibly contributes to orthostatic intolerance following BR in humans. These results suggest a novel mechanism in the development of orthostatic intolerance in humans.

Influence of increased plasma osmolality on sympathetic outflow during apnea

Animal models have shown that peripheral chemoreceptors alter their firing patterns in response to changes in plasma osmolality, which, in turn, may modulate sympathetic outflow. The purpose of this study was to test the hypothesis that increases in plasma osmolality augment muscle sympathetic nerve activity (MSNA) responses to chemoreceptor activation. MSNA was recorded from the peroneal nerve (microneurography) during a 23-min intravenous hypertonic saline infusion (3% NaCl; HSI). Chemoreceptor activation was elicited by voluntary end-expiratory apnea. MSNA responses to end-expiratory apnea were calculated as the absolute increase from the preceding baseline period. Plasma osmolality significantly increased from pre- to post-HSI (284 ± 1 to 290 ± 1 mOsm/kg H(2)O; P < 0.01). There was a significant overall effect of osmolality on sympathetic activity (P < 0.01). Duration of the voluntary end-expiratory apnea was not different after HSI (pre = 40 ± 5 s; post = 41 ± 4 s). MSNA responses to end-expiratory apnea were not different after HSI, expressed as an absolute change in burst frequency (n = 11; pre = 8 ± 2; post = 11 ± 1 burst/min) and as a percent increase in total activity (pre = 51 ± 4% AU; post = 53 ± 4% AU). A second group of subjects (n = 8) participated in 23-min volume/time-control intravenous isotonic saline infusions (0.9% NaCl). Isotonic saline volume-control infusions yielded no change in plasma osmolality or MSNA at rest. Furthermore, MSNA responses to apnea following isotonic saline infusion were not different. In summary, elevated plasma osmolality increased MSNA at rest and during apnea, but contrary to the hypothesis, MSNA responsiveness to apnea was not augmented. Therefore, this study does not support a neural interaction between plasma osmolality and chemoreceptor stimulation.

Melatonin attenuates the vestibulosympathetic but not vestibulocollic reflexes in humans: selective impairment of the utricles

Melatonin has been reported to decrease nerve activity of medial vestibular nuclei in the rat and is associated with attenuated muscle sympathetic nerve activity (MSNA) responses to baroreceptor unloading in humans. The purpose of this study was to determine if melatonin alters the vestibulosympathetic reflex (VSR) and vestibulocollic reflex (VCR) in humans. In study 1, MSNA, arterial blood pressure, and heart rate were measured in 12 healthy subjects (28 ± 1 yr; 6 men, 6 women) during head-down rotation (HDR) before and 45 min after ingestion of either melatonin (3 mg) or placebo (sucrose). Subjects returned at least 2 days later at the same time of day to repeat the trial after ingesting the opposite treatment (melatonin or placebo). Melatonin significantly attenuated MSNA responses during HDR compared with placebo (burst frequency Δ 4 ± 1 vs. Δ 7 ± 1 bursts/min, and total MSNA Δ 51 ± 20 and Δ 96 ± 15%, respectively; P < 0.02). In study 2, vestibular evoked myogenic potentials (VEMP) were measured in 10 healthy subjects (26 ± 1 yr; 4 men and 6 women) before and after ingestion of 3 mg melatonin. Melatonin did not alter the timing of the p13 and n23 peaks (pre-melatonin 13.2 ± 0.4 and 21.3 ± 0.6 ms vs. post-melatonin 13.5 ± 0.4 and 21.4 ± 0.7 ms, respectively) or the p13-n23 interpeak amplitudes [pre-melatonin 22.5 ± 4.6 arbitrary units (au) and post-melatonin 22.7 ± 4.6 au]. In summary, melatonin attenuates the VSR and supports the concept that melatonin negatively affects orthostatic tolerance. However, melatonin does not alter the VCR in humans suggesting melatonin's effect on the VSR appears to be mediated by the utricles.

Heart rate and blood pressure control in infants exposed to maternal cigarette smoking

AIM

Exposure to maternal cigarette smoking is a major risk factor for sudden infant death syndrome (SIDS). Foetal and postnatal smoke-exposure may alter cardiovascular control in infants. We studied heart rate (HR) and blood pressure (BP) responses in smoke-exposed infants.

METHODS

Eleven infants exposed to maternal cigarette smoking were studied at the age of 12 +/- 2.1 (range 10-16) weeks. Twenty healthy, age-matched infants from non-smoking families served as controls. During confirmed slow-wave sleep (NREM3), 3-5 sec side motion and 45 sec 45 degrees head-up tilt tests were performed.

RESULTS

Control infants showed consistent biphasic HR and BP responses to side motion, with an initial 2-5% increase followed by a 2% decrease (p < 0.0001). In smoke-exposed infants, the initial HR (p = 0.009) and BP responses (p < 0.0001) were markedly reduced, and the subsequent decrease in BP was more prominent (systolic blood pressure, SBP, p = 0.005; diastolic blood pressure, DBP, p = 0.03). No differences were observed between the groups in tilt test results, HR variability or HR responses to spontaneous arousals.

CONCLUSION

Maternal cigarette smoking may alter vestibulo-mediated cardiovascular control in early infancy. This may contribute to increased SIDS risk.

Otolithic activation on visceral circulation in humans: effect of aging

Engagement of the otolith organs elicits differential activation of sympathetic nerve activity and vascular responses to muscle and skin in humans. Additionally, aging attenuates the otolith organ-mediated increases in muscle sympathetic nerve activity in older adults. In this study, we hypothesized that 1) the vestibulosympathetic reflex (VSR) would elicit visceral vascular vasoconstriction and 2) visceral vascular response to the VSR would be attenuated in older subjects compared with young. To test these hypotheses, heart rate, mean arterial blood pressure, and renal, celiac trunk, and superior mesenteric arterial blood velocity (Doppler ultrasound) were measured in 22 young (25+/-1 yr) and 18 older (65+/-2 yr) healthy subjects during head-down rotation (HDR), which selectively activates the otolith organs. Mean arterial pressure and heart rate did not change from baseline during HDR in young or older subjects. Renal blood velocity (Delta -2+/-1 cm/s) and vascular conductance (Delta -0.03+/-0.01 cm.s(-1).mmHg(-1)) significantly decreased from baseline during HDR (P<0.05) in young subjects. In contrast, renal blood velocity and conductance did not change in older subjects (Delta -0.2+/-1 cm/s and Delta0.02+/-0.08 mmHg.cm(-1).s(-1), respectively) during HDR. Superior mesenteric and celiac blood velocity and vascular conductance did not change in response to HDR in either the young or older subjects. These data suggest that renal vasoconstriction occurs during otolith organ activation in young but not older humans. Together with our previous studies, we conclude that the VSR elicits a diverse patterning of sympathetic outflow that results in heterogeneous vascular responses in humans and that these responses are significantly attenuated in older humans.

Greater sensitivity of the vestibulosympathetic reflex in the upright posture in humans

Otolith organs have been shown to activate the sympathetic nervous system in the prone position by head-down rotation (HDR) in humans. To date, otolithic stimulation by HDR has not been comprehensively studied in the upright posture. The purpose of the present study was to determine whether otolithic stimulation increases muscle sympathetic nerve activity (MSNA) in the upright posture. It was hypothesized that stimulation of the otolith organs would increase MSNA in the upright posture, despite increased baseline sympathetic activation due to unloading of the baroreceptors. MSNA, arterial blood pressure, heart rate, and degree of head rotation were measured during HDR in 18 volunteers (23 +/- 1 yr) in different postures. Study 1 (n = 11) examined HDR in the prone and sitting positions and study 2 (n = 7) examined HDR in the prone and 60 degrees head-up tilt positions. Baseline MSNA was 8 +/- 4, 15 +/- 4, and 33 +/- 2 bursts/min for prone, sitting, and head-up tilt, respectively. HDR significantly increased MSNA in the prone (Delta4 +/- 1 and Delta105 +/- 37% for burst frequency and total activity, respectively), sitting (Delta5 +/- 1 and Delta43 +/- 12%), and head-up tilt (Delta7 +/- 1 and Delta110 +/- 41%; P < 0.05). Sensitivity of the vestibulosympathetic reflex (%DeltaMSNA/DeltaHDR; degree of head rotation) was significantly greater in the sitting and head-up tilt than prone position (prone = 74 +/- 22; sitting = 109 +/- 30; head-up tilt = 276 +/- 103; P < 0.05). These data indicate that stimulation of the otolith organs can mediate increases in MSNA in the upright posture and suggest a greater sensitivity of the vestibulosympathetic reflex in the upright posture in humans.

Sympathetic responses to vestibular activation in humans

Activation of sympathetic neural traffic via the vestibular system is referred to as the vestibulosympathetic reflex. Investigations of the vestibulosympathetic reflex in humans have been limited to the past decade, and the importance of this reflex in arterial blood pressure regulation is still being determined. This review provides a summary of sympathetic neural responses to various techniques used to engage the vestibulosympathetic reflex. Studies suggest that activation of the semicircular canals using caloric stimulation and yaw rotation do not modulate muscle sympathetic nerve activity (MSNA) or skin sympathetic nerve activity (SSNA). In contrast, activation of the otolith organs appear to alter MSNA, but not SSNA. Specifically, head-down rotation and off-vertical axis rotation increase MSNA, while sinusoidal linear accelerations decrease MSNA. Galvanic stimulation, which results in a nonspecific activation of the vestibule, appears to increase MSNA if the mode of delivery is pulse trained. In conclusion, evidence strongly supports the existence of a vestibulosympathetic reflex in humans. Furthermore, attenuation of the vestibulosympathetic reflex is coupled with a drop in arterial blood pressure in the elderly, suggesting this reflex may be important in human blood pressure regulation.