Head position modifies cerebrovascular response to orthostatic stress

Proprioceptive manipulations in orthograde posture modulate postural control in low back pain patients: a pilot study

As we stand upright, perceptual afferences are crucial to successfully help generating postural motor commands. Non-Specific Low Back Pain patients frequently demonstrate a lack of proprioceptive acuity, often translating into postural control deficiencies. For the first time, to our knowledge, we studied the postural effects of proprioceptive manipulations in orthograde posture on Non-Specific Low Back Pain patients. Using static posturography recordings, we computed sway speed, speed variance, and the main direction of sway. We also addressed the patient’s subjective feedbacks after being manipulated. Five minutes after the proprioceptive manipulations, our results revealed decreased speed and speed variance outcomes, but the main direction of sway was not modulated. Furthermore, after the proprioceptive manipulations, the patients also self-reported improved clinical outcomes. These findings provide new knowledge opening new fields of research as well as potential treatment strategies in Low Back Pain patients.

Sodium nitroglycerin induces middle cerebral artery vasodilatation in young, healthy adults

NEW FINDINGS

What is the central question of this study? Nitric oxide causes dilatation in peripheral vessels; however, whether nitric oxide affects basal cerebral artery dilatation has not been explored. What is the main finding and its importance? This study demonstrated that vasodilatation occurs in the right middle cerebral artery in response to exogenous nitric oxide. However, blood velocity decreased and, therefore, overall cerebral blood flow remained unchanged. This study provides new insight into the role of nitric oxide in cerebral blood flow control.

ABSTRACT

Recent evidence indicates that basal cerebral conduit vessels dilate with hypercapnia, with a nitric oxide (NO) mechanism explaining one way in which parenchymal cerebral arterioles dilate. However, whether NO affects basal cerebral artery dilatation remains unknown. This study quantified the effect of an exogenous NO donor [sodium nitroglycerin (NTG); 0.4 mg sublingual spray] on the right middle cerebral artery (rMCA) cross-sectional area (CSA), blood velocity and overall blood flow. Measures of vessel CSA (7 T magnetic resonance imaging) and MCA blood velocity (transcranial Doppler ultrasound) were made at baseline (BL) and after exogenous NTG or placebo (PLO) administration in young, healthy individuals (n = 10, two males, age range 20-23 years). The CSA increased in the rMCA [BL, 5.2 ± 1.2 mm² ; PLO, 5.4 ± 1.5 mm² ; NTG, 6.6 ± 1.5 mm² , P < 0.05; mean ± SD]. Concurrently, rMCA blood velocity decreased from BL during NTG compared with PLO (BL, 67 ± 10 cm s^-1 ; PLO, 62 ± 10 cm s^-1 ; NTG, 59 ± 9.3 cm s^-1 , P < 0.05; mean ± SD]. However, total MCA blood flow did not change with NTG or PLO [BL, 221 ± 37.4 ml min^-1 ; PLO, 218 ± 35.0 ml min^-1 ; NTG, 213 ± 46.4 ml min^-1 ). Therefore, exogenous NO mediates a dilatory response in the rMCA, but not in its downstream vascular bed.

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.

Carotid artery blood flow decreases after rapid head rotation in piglets

Modification of cerebral perfusion pressure and cerebral blood flow (CBF) are crucial components of the therapies designed to reduce secondary damage after traumatic brain injury (TBI). Previously we documented a robust decrease in CBF after rapid sagittal head rotation in our well-validated animal model of diffuse TBI. Mechanisms responsible for this immediate (<10 min) and sustained (∼24 h) reduction in CBF have not been explored. Because the carotid arteries are a major source of CBF, we hypothesized that blood flow through the carotid arteries (Q) and vessel diameter (D) would decrease after rapid nonimpact head rotation without cervical spine injury. Four-week-old (toddler) female piglets underwent rapid (<20 msec) sagittal head rotation without impact, previously shown to produce diffuse TBI with reductions in CBF. Ultrasonographic images of the bilateral carotid arteries were recorded at baseline (pre-injury), as well as immediately after head rotation and 15, 30, 45, and 60 min after injury. Diameter (D) and waveform velocity (V) were used to calculate blood flow (Q) through the carotid arteries using the equation Q=(0.25)πD(2)V. D, V, and Q were normalized to the pre-injury baseline values to obtain a relative change after injury in right and left carotid arteries. Three-way analysis of variance and post-hoc Tukey-Kramer analyses were used to assess statistical significance of injury, time, and side. The relative change in carotid artery diameter and flow was significantly decreased in injured animals in comparison with uninjured sham controls (p<0.0001 and p=0.0093, respectively) and did not vary with side (p>0.39). The average carotid blood velocity did not differ between sham and injured animals (p=0.91). These data suggest that a reduction in global CBF after rapid sagittal head rotation may be partially mediated by a reduction in carotid artery flow, via vasoconstriction.

The prevalence and pathological correlates of orthostatic hypotension and its subtypes when measured using beat-to-beat technology in a sample of older adults living in the community

BACKGROUND

beat-to-beat technology is increasingly used for investigating orthostatic intolerance (OI) but the prevalence of orthostatic hypotension (OH) diagnosed with this technology is unclear.

OBJECTIVES

(i) to use beat-to-beat technology to define the prevalence of OH, (ii) to investigate the pathological correlates of OH, (iii) to report the diversity of postural BP responses.

METHODS

cross-sectional study of adults ≥ 65 years. BP responses to a 3-min head-up tilt were analysed.

RESULTS

of 326 participants, 203(62.3%) were females. The median (IQR) age was 73 (70-78). One hundred and ninety-one (58.6%) met standard (20 mmHg systolic/10 mmHg diastolic) criteria for OH. The prevalence was higher in females (60.1% F versus 56.1% M); 47% were arteriolar subtype, 33% were venular, 9% were mixed and 11.0% could not be classified. Morphological analysis identified 102 subjects with 'small drop, overshoot', 131 with 'medium drop, slow recovery' and 31 with 'large drop, nonrecovery'. Those with OH had a lower BMI (P = 0.02), a higher resting heart rate (P = 0.005), were more likely to take a psychotropic (P = 0.02), have vertigo (P = 0.004) and report OI (P = 0.02). The 95th centile for the duration of systolic BP (SYSBP) decay >20 mmHg was 175 s and the slope of systolic BP decay was 4.75 mmHg/s. The 5th centile for percentage recovery of SYSBP was 81.4%.

CONCLUSION

(i) beat-to-beat methods identify a higher prevalence of OH than sphygmomanometry, (ii) the pathological correlates of OH diagnosed in this manner are similar to those described for sphygmomanometry, (iii) there is a diverse pattern of orthostatic BP decay that could be used in future research to predict adverse outcomes in OH.

Effect of head and body positioning on cerebral blood flow velocity in patients who underwent cranial surgery

AIM AND OBJECTIVES

The aim of this study was to investigate the effects of head and neck positions on the cerebral blood flow velocity by transcranial Doppler ultrasound in patients who underwent cranial surgery.

BACKGROUND

Inappropriate head elevation and body positioning in patients who undergo cranial surgery may affect cerebral blood flow and cerebral perfusion pressure. DESIGNED: Experimental clinical study.

METHOD

Our sample consisted of 38 patients who underwent cranial surgery between October 2009 and May 2010. The measurments of mean cerebral blood flow velocity were taken by the transcranial Doppler ultrasound through the temporal window. The mean cerebral blood flow velocity of the patients was measured in supine position with 0° and 30° head elevations, right and left lateral positions, right and left lateral positions with head flexion and extension. The measurements were taken before surgery and within 72 hours after surgery.

RESULTS

The mean cerebral blood flow velocity of the middle cerebral arteries was increased in head elevations from 0° to 30°, in right and lateral positions with 30° head elevations, but the velocity was decreased in head flexion and extension positions in preoperative and postoperative periods.

DISCUSSION

Head and body positioning, which is one of the nursing care activities, may affect intracranial pressure and cerebral perfusion pressure. Our results are similar with those of previous studies, which showed that head elevation did not affect the cerebral blood flow velocity.

RELEVANCE TO CLINICAL PRACTICE

By the results of this study, the head elevation of the patients, who underwent cranial surgery, should be 30° during the nursing care to provide optimum cerebral blood flow. Right and left lateral positioning is safe and recommended for these patients if there is no medical contraindication.

Vestibular effects on cerebral blood flow

Background

Humans demonstrate a number of unique adaptations that allow for the maintenance of blood pressure and brain blood flow when upright. While several physiological systems, including cerebral autoregulation, are involved in this adaptation the unique role the vestibular system plays in helping to maintain brain blood flow is just beginning to be elucidated. In this study, we tested the hypothesis that stimulation of the vestibular system, specifically the otoliths organs, would result in changes in cerebral blood flow.

Results

To test our hypothesis, we stimulated the vestibular organs of 25 healthy subjects by pitch tilt (stimulates both canals and otoliths) and by translation on a centrifuge (stimulates otoliths and not the canals) at five frequencies: 0.5, 0.25, 0.125 and 0.0625 Hz for 80 sec and 0.03125 Hz for 160 sec. Changes in cerebral flow velocity (by transcranial Doppler) and blood pressure (by Finapres) were similar during both stimuli and dependent on frequency of stimulation (P < 0.01). However, changes in cerebral blood flow were in opposition to changes in blood pressure and not fully dependent on changes in end tidal CO₂.

Conclusion

The experimental results support our hypothesis and provide evidence that activation of the vestibular apparatus, specifically the otolith organs, directly affects cerebral blood flow regulation, independent of blood pressure and end tidal CO₂ changes.

Vestibular control of arterial blood pressure during head-down postural change in anesthetized rabbits

This study was undertaken to elucidate neural control of the arterial blood pressure (ABP) in head-down postural change which causes both stimulation to the vestibular system and head-ward fluid shift. Experiments were carried out with urethane-anesthetized rabbits. The animal was mounted on a tilting table, tilted to 45 degrees head-down in 5 s, and kept at the position for 5 min. The head-down rotation (HDR) induced a transient decrease in ABP (10 +/- 3 mmHg; mean +/- SE), and then the pressure gradually recovered toward the pre-HDR level during the 5 min at the head-down position. Pretreatment with hexamethonium bromide, a ganglionic transmission blocker, suppressed the HDR-induced drop of ABP, suggesting that the ABP drop was induced by an inhibition of autonomic neural outflows. Renal sympathetic nerve activity (RSNA) decreased considerably after 1.6 +/- 0.2 s from the onset of HDR, which was followed by the ABP drop. Aortic depressor nerve activity (ADNA), an afferent for baroreceptor reflex, increased significantly during the rotation, but the peak of ADNA increase was 3.2 +/- 0.5 s after the initiation of the HDR. Therefore, the suppression of RSNA seems to be induced mainly by a quicker mechanism than baroreceptor reflex. In order to test the possibility, we examined changes in ABP and RSNA during HDR using vestibular-lesioned rabbits. In these rabbits, RSNA and ABP did not change significantly during HDR. These results suggest that vestibular organs play a role in the transient drop in ABP induced by HDR through the suppression of sympathetic nerve outflows.

Optokinetic nystagmus as related to neonatal position

The objective of this study was to assess the role of the newborn vestibular system on the infant's preferred position. Neonatal electronystagmography was recorded from 80 full-term healthy neonates in the prone and supine positions. Records were analyzed by the clinical ranking of dysmetria and dysrhythmia and computerized fractal analysis. A significantly (P < .002) decreased organization of the electronystagmography signal was observed in the prone compared with the supine position. These results concur with the previously documented, more optimal physiologic functioning in the supine compared with prone position in infancy. It is possible that the vestibular system, among other factors, plays a role in the more protective supine position in infancy.

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

Activation of the vestibular otolith organs with head-down rotation (HDR) increases muscle sympathetic nerve activity (MSNA) in humans. Previously, we demonstrated this vestibulosympathetic reflex (VSR) elicits increases in MSNA during baroreflex unloading (i.e., lower body negative pressure) in humans. Whether such an effect persists during baroreflex loading is unknown. We tested the hypothesis that the ability of the VSR to increase MSNA is preserved during baroreflex unloading and inhibited during baroreflex loading. Ten subjects (26 +/- 1 yr) performed three trials of HDR to activate the VSR. These trials were performed after a period of sustained saline (control), nitroprusside (baroreflex unloading: 0.8-1.0 microg.kg(-1).min(-1)), and phenylephrine (baroreflex loading: 0.6-0.8 microg.kg(-1).min(-1)) infusion. Nitroprusside infusion decreased (Delta7 +/- 1 mmHg, where Delta is change; P < 0.001) and phenylephrine infusion increased mean arterial pressure (Delta8 +/- 1 mmHg; P < 0.001) at rest. HDR performed during the control [Delta3 +/- 2 bursts/min, Delta314 +/- 154 arbitrary units (au) total activity, Delta41 +/- 18% total activity; P < 0.05] and nitroprusside trials [Delta5 +/- 2 bursts/min, Delta713 +/- 241 au total activity, Delta49 +/- 20% total activity; P < 0.05] increased MSNA similarly despite significantly elevated levels at rest (13 +/- 2 to 26 +/- 3 bursts/min) in the latter. In contrast, HDR performed during the phenylephrine trial failed to increase MSNA (Delta0 +/- 1 bursts/min, Delta-15 +/- 33 au total activity, Delta-8 +/- 21% total activity). These results confirm previous findings that the ability of the VSR to increase MSNA is preserved during baroreflex unloading. In contrast, the ability of the VSR to increase MSNA is abolished during baroreflex loading. These results provide further support for the concept that the VSR may act primarily to defend against hypotension in humans.

Effects of postural changes and removal of vestibular inputs on blood flow to the head of conscious felines

Prior studies have shown that removal of vestibular inputs produces lability in blood pressure during orthostatic challenges (Holmes MJ, Cotter LA, Arendt HE, Cass SP, and Yates BJ. Brain Res 938: 62-72, 2002; Jian BJ, Cotter LA, Emanuel BA, Cass SP, and Yates BJ. J Appl Physiol 86: 1552-1560, 1999). Furthermore, these studies led to the prediction that the blood pressure instability results in susceptibility for orthostatic intolerance. The present experiments tested this hypothesis by recording common carotid blood flow (CCBF) in conscious cats during head-up tilts of 20, 40, and 60 degrees amplitudes, before and after the surgical elimination of labyrinthine inputs through a bilateral vestibular neurectomy. Before vestibular lesions in most animals, CCBF remained stable during head-up rotations. Unexpectedly, in five of six animals, the vestibular neurectomy resulted in a significant increase in baseline CCBF, particularly when the laboratory was illuminated; on average, basal blood flow measured when the animals were in the prone position was 41 +/- 17 (SE) % higher after the first week after the lesions. As a result, even when posturally related lability in CCBF occurred after removal of vestibular inputs, blood supply to the head was not lower than when labyrinthine inputs were present. These data suggest that vestibular influences on cardiovascular regulation are more complex than previously appreciated, because labyrinthine signals appear to participate in setting basal rates of blood flow to the head in addition to triggering dynamic changes in the circulation to compensate for orthostatic challenges.

Cerebral blood flow during orthostasis: role of arterial CO2

Reductions in end-tidal Pco(2) (Pet(CO(2))) during upright posture have been suggested to be the result of hyperventilation and the cause of decreases in cerebral blood flow (CBF). The goal of this study was to determine whether decreases in Pet(CO(2)) reflected decreases in arterial Pco(2) (Pa(CO(2))) and their relation to increases in alveolar ventilation (Va) and decreases in CBF. Fifteen healthy subjects (10 women and 5 men) were subjected to a 10-min head-up tilt (HUT) protocol. Pa(CO(2)), Va, and cerebral flow velocity (CFV) in the middle and anterior cerebral arteries were examined. In 12 subjects who completed the protocol, reductions in Pet(CO(2)) and Pa(CO(2)) (-1.7 +/- 0.5 and -1.1 +/- 0.4 mmHg, P < 0.05) during minute 1 of HUT were associated with a significant increase in Va (+0.7 +/- 0.3 l/min, P < 0.05). However, further decreases in Pa(CO(2)) (-0.5 +/- 0.5 mmHg, P < 0.05), from minute 1 to the last minute of HUT, occurred even though Va did not change significantly (-0.2 +/- 0.3 l/min, P = not significant). Similarly, CFV in the middle and anterior cerebral arteries decreased (-7 +/- 2 and -8 +/- 2%, P < 0.05) from minute 1 to the last minute of HUT, despite minimal changes in Pa(CO(2)). These data suggest that decreases in Pet(CO(2)) and Pa(CO(2)) during upright posture are not solely due to increased Va but could be due to ventilation-perfusion mismatch or a redistribution of CO(2) stores. Furthermore, the reduction in Pa(CO(2)) did not fully explain the decrease in CFV throughout HUT. These data suggest that factors in addition to a reduction in Pa(CO(2)) play a role in the CBF response to orthostatic stress.

Dynamic cardiorespiratory interaction during head-up tilt-mediated presyncope

In 28 healthy adults, we compared the dynamic interaction between respiration and cerebral autoregulation in 2 groups of subjects: those who did and did not develop presyncopal symptoms during 70 degrees passive head-up tilt (HUT), i.e., nonpresyncopal (23 subjects) and presyncopal (5 subjects). Airflow, CO2, cerebral blood flow velocity (CBF), ECG, and blood pressure (BP) were recorded. To determine whether influences of mean BP (MBP) and systolic SP (SBP) on CBF were altered in presyncopal subjects, coherencies and transfer functions between these variables and mean and peak CBF (CBFm and CBFp) were estimated. To determine the influence of end-tidal CO2 (ETco2) on CBF, the relative CO2 reactivity (%change in CBFm per mmHg change in ETco2) was calculated. We found that in presyncopal subjects before symptoms during HUT, coherence between SBP and CBFp was higher (P=0.02) and gains of transfer functions between BP (MBP and SBP) and CBFm were larger (MBP, P=0.01; SBP, P=0.01) in the respiratory frequency region. In the last 3 min before presyncope, presyncopals had a reduced relative CO2 reactivity (P=0.005), likely a consequence of the larger decrease in ETco2. We hypothesize that the CO2-mediated increase in resistance attenuates autoregulation such that the relationship between systemic and cerebral hemodynamics is enhanced. Our results suggest that an altered cardiorespiratory interaction involving cerebral hemodynamics may contribute in the cascade of events during tilt that culminate in unexplained syncope.

Human cutaneous vascular responses to whole-body tilting, Gzcentrifugation, and LBNP

We hypothesized that gravitational stimuli elicit cardiovascular responses in the following order with gravitational stress equalized at the level of the feet, from lowest to highest response: short-(SAC) and long-arm centrifugation (LAC), tilt, and lower body negative pressure (LBNP). Up to 15 healthy subjects underwent graded application of the four stimuli. Laser-Doppler flowmetry measured regional skin blood flow. At 0.6 Gz(60 mmHg LBNP), tilt and LBNP similarly reduced leg skin blood flow to ∼36% of supine baseline levels. Flow increased back toward baseline levels at 80–100 mmHg LBNP yet remained stable during 0.8–1.0 Gztilt. Centrifugation usually produced less leg vasoconstriction than tilt or LBNP. Surprisingly, SAC and LAC did not differ significantly. Thigh responses were less definitive than leg responses. No gravitational vasoconstriction occurred in the neck. All conditions except SAC increased heart rate, according to our hypothesized order. LBNP may be a more effective and practical means of simulating cardiovascular effects of gravity than centrifugation.

Human cerebrovascular and autonomic rhythms during vestibular activation

Otolith activation increases muscle sympathetic nerve activity (MSNA), and MSNA activation may alter associations among autonomic oscillators, including those modulating cerebral hemodynamics. The purpose of this study was to determine the influence of vestibulosympathetic activation on cerebral and autonomic rhythms. We recorded the ECG, finger arterial pressure, end-tidal CO2, respiration, cerebral blood flow velocity, and MSNA in eight subjects. Subjects breathed at 0.25 Hz for 5 min in the prone and head-down positions. We analyzed data in time and frequency domains and performed cross-spectral analyses to determine coherence and transfer function magnitude. Head-down rotation increased MSNA from 7 ± 1.3 to 12 ± 1.5 bursts/min ( P = 0.001) but did not affect R-R intervals, arterial pressures, mean cerebral blood flow velocities ( Vmean), or their power spectra. Vestibular activation with head-down rotation had no effect on mean arterial pressure and Vmean transfer function magnitude. The two new findings from this study are 1) head-down rotation independently activates the sympathetic nervous system with no effect on parasympathetic activity or Vmean; and 2) frequency-dependent associations between arterial pressures and Vmean are independent of vestibular activation. These findings support the concept that vestibular-autonomic interactions independently and redundantly serve to maintain steady-state hemodynamics.