Influence of cerebrovascular sympathetic, parasympathetic, and sensory nerves on autoregulation and spontaneous vasomotion

Percutaneous Trigeminal Nerve Stimulation Induces Cerebral Vasodilation in a Dose-Dependent Manner

BACKGROUND

The trigeminal nerve directly innervates key vascular structures both centrally and peripherally. Centrally, it is known to innervate the brainstem and cavernous sinus, whereas peripherally the trigemino-cerebrovascular network innervates the majority of the cerebral vasculature. Upon stimulation, it permits direct modulation of cerebral blood flow (CBF), making the trigeminal nerve a promising target for the management of cerebral vasospasm. However, trigeminally mediated cerebral vasodilation has not been applied to the treatment of vasospasm.

OBJECTIVE

To determine the effect of percutaneous electrical stimulation of the infraorbital branch of the trigeminal nerve (pTNS) on the cerebral vasculature.

METHODS

In order to determine the stimulus-response function of pTNS on cerebral vasodilation, CBF, arterial blood pressure, cerebrovascular resistance, intracranial pressure, cerebral perfusion pressure, cerebrospinal fluid calcitonin gene-related peptide (CGRP) concentrations, and the diameter of cerebral vessels were measured in healthy and subarachnoid hemorrhage (SAH) rats.

RESULTS

The present study demonstrates, for the first time, that pTNS increases brain CGRP concentrations in a dose-dependent manner, thereby producing controllable cerebral vasodilation. This vasodilatory response appears to be independent of the pressor response induced by pTNS, as it is maintained even after transection of the spinal cord at the C5-C6 level and shown to be confined to the infraorbital nerve by administration of lidocaine or destroying it. Furthermore, such pTNS-induced vasodilatory response of cerebral vessels is retained after SAH-induced vasospasm.

CONCLUSION

Our study demonstrates that pTNS is a promising vasodilator and increases CBF, cerebral perfusion, and CGRP concentration both in normal and vasoconstrictive conditions.

Axial variation of deoxyhemoglobin density as a source of the low-frequency time lag structure in blood oxygenation level-dependent signals

Perfusion-related information is reportedly embedded in the low-frequency component of a blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal. The blood-propagation pattern through the cerebral vascular tree is detected as an interregional lag variation of spontaneous low-frequency oscillations (sLFOs). Mapping of this lag, or phase, has been implicitly treated as a projection of the vascular tree structure onto real space. While accumulating evidence supports the biological significance of this signal component, the physiological basis of the “perfusion lag structure,” a requirement for an integrative resting-state fMRI-signal model, is lacking. In this study, we conducted analyses furthering the hypothesis that the sLFO is not only largely of systemic origin, but also essentially intrinsic to blood, and hence behaves as a virtual tracer. By summing the small fluctuations of instantaneous phase differences between adjacent vascular regions, a velocity response to respiratory challenges was detected. Regarding the relationship to neurovascular coupling, the removal of the whole lag structure, which can be considered as an optimized global-signal regression, resulted in a reduction of inter-individual variance while preserving the fMRI response. Examination of the T2* and S₀, or non-BOLD, components of the fMRI signal revealed that the lag structure is deoxyhemoglobin dependent, while paradoxically presenting a signal-magnitude reduction in the venous side of the cerebral vasculature. These findings provide insight into the origin of BOLD sLFOs, suggesting that they are highly intrinsic to the circulating blood.

Neural control of choroidal blood flow

The choroid is richly innervated by parasympathetic, sympathetic and trigeminal sensory nerve fibers that regulate choroidal blood flow in birds and mammals, and presumably other vertebrate classes as well. The parasympathetic innervation has been shown to vasodilate and increase choroidal blood flow, the sympathetic input has been shown to vasoconstrict and decrease choroidal blood flow, and the sensory input has been shown to both convey pain and thermal information centrally and act locally to vasodilate and increase choroidal blood flow. As the choroid lies behind the retina and cannot respond readily to retinal metabolic signals, its innervation is important for adjustments in flow required by either retinal activity, by fluctuations in the systemic blood pressure driving choroidal perfusion, and possibly by retinal temperature. The former two appear to be mediated by the sympathetic and parasympathetic nervous systems, via central circuits responsive to retinal activity and systemic blood pressure, but adjustments for ocular perfusion pressure also appear to be influenced by local autoregulatory myogenic mechanisms. Adaptive choroidal responses to temperature may be mediated by trigeminal sensory fibers. Impairments in the neural control of choroidal blood flow occur with aging, and various ocular or systemic diseases such as glaucoma, age-related macular degeneration (AMD), hypertension, and diabetes, and may contribute to retinal pathology and dysfunction in these conditions, or in the case of AMD be a precondition. The present manuscript reviews findings in birds and mammals that contribute to the above-summarized understanding of the roles of the autonomic and sensory innervation of the choroid in controlling choroidal blood flow, and in the importance of such regulation for maintaining retinal health.

Dynamic cerebral autoregulation is unrelated to decrease in external carotid artery blood flow during acute hypotension in healthy young men

NEW FINDINGS

What is the central question of this study? Dynamic cerebral autoregulation (CA) is impaired by sympathetic blockade, and the external carotid artery (ECA) vascular bed may prevent adequate internal carotid artery blood flow. We examined whether α1 -receptor blockade-induced attenuation of dynamic CA is related to reduced ECA vasoconstriction. What is the main finding and its importance? α1 -Receptor blockade attenuated dynamic CA, but in contrast to our hypothesis did not affect the ECA blood flow response to acute hypotension. These findings suggest that the recovery of cerebral blood flow during acute hypotension is unrelated to vasoconstriction within the ECA territory. External carotid artery (ECA) vasoconstriction may defend internal carotid artery (ICA) blood flow during acute hypotension. We hypothesized that the α1 -receptor blockade-induced delay in ICA recovery to the baseline level from acute hypoperfusion is related to attenuated ECA vasoconstriction. The ICA and ECA blood flow were determined by duplex ultrasound during thigh-cuff release-induced acute hypotension while the α1 -receptor blocker prazosin [1 mg (20 kg)(-1) ] was administered to nine seated young healthy men. Both ICA (mean ± SD; by 17 ± 8%, P = 0.005) and ECA (by 37 ± 15%, P < 0.001) blood flow decreased immediately after occluded thigh-cuff release, with a more rapid ICA blood flow recovery to the baseline level (9 ± 5 s) than for the ECA blood flow (17 ± 5 s; P = 0.019). The ICA blood flow recovery from hypoperfusion was delayed with prazosin (17 ± 4 s versus control 9 ± 5 s, P = 0.006), whereas ECA recovery remained unchanged (P = 0.313) despite a similar reduction in mean arterial pressure (-20 ± 4 mmHg versus control -23 ± 7 mmHg, P = 0.148). These findings suggest that α1 -receptor blockade-induced attenuation of the ICA blood flow response to acute hypotension is unrelated to the reduction in ECA blood flow. The sympathetic nervous system via the ECA vascular bed does not contribute to dynamic CA during acute hypotension.

Phenylephrine alteration of cerebral blood flow during orthostasis: effect on n-back performance in chronic fatigue syndrome

Chronic fatigue syndrome (CFS) with orthostatic intolerance is characterized by neurocognitive deficits and impaired working memory, concentration, and information processing. In CFS, upright tilting [head-up tilt (HUT)] caused decreased cerebral blood flow velocity (CBFv) related to hyperventilation/hypocapnia and impaired cerebral autoregulation; increasing orthostatic stress resulted in decreased neurocognition. We loaded the baroreflex with phenylephrine to prevent hyperventilation and performed n-back neurocognition testing in 11 control subjects and 15 CFS patients. HUT caused a significant increase in heart rate (109.4 ± 3.9 vs. 77.2 ± 1.6 beats/min, P < 0.05) and respiratory rate (20.9 ± 1.7 vs. 14.2 ± 1.2 breaths/min, P < 0.05) and decrease in end-tidal CO2 (ETCO2; 42.8 ± 1.2 vs. 33.9 ± 1.1 Torr, P < 0.05) in CFS vs. control. HUT caused CBFv to decrease 8.7% in control subjects but fell 22.5% in CFS. In CFS, phenylephrine prevented the HUT-induced hyperventilation/hypocapnia and the significant drop in CBFv with HUT (-8.1% vs. -22.5% untreated). There was no difference in control subject n-back normalized response time (nRT) comparing supine to HUT (106.1 ± 6.9 vs. 97.6 ± 7.1 ms at n = 4), and no difference comparing control to CFS while supine (97.1 ± 7.1 vs 96.5 ± 3.9 ms at n = 4). However, HUT of CFS subjects caused a significant increase in nRT (148.0 ± 9.3 vs. 96.4 ± 6.0 ms at n = 4) compared with supine. Phenylephrine significantly reduced the HUT-induced increase in nRT in CFS to levels similar to supine (114.6 ± 7.1 vs. 114.6 ± 9.3 ms at n = 4). Compared with control subjects, CFS subjects are more sensitive both to orthostatic challenge and to baroreflex/chemoreflex-mediated interventions. Increasing blood pressure with phenylephrine can alter CBFv. In CFS subjects, mitigation of the HUT-induced CBFv decrease with phenylephrine has a beneficial effect on n-back outcome.

Hemodynamic and electrophysiological spontaneous low-frequency oscillations in the cortex: directional influences revealed by Granger causality

We used a combined electrophysiological/hemodynamic system to examine low-frequency oscillations (LFOs) in spontaneous neuronal activities (spike trains and local field potentials) and hemodynamic signals (cerebral blood flow) recorded from the anesthetized rat somatosensory and visual cortices. The laser Doppler flowmetry (LDF) probe was tilted slightly to approach the area in which a microelectrode array (MEA) was implanted for simultaneous recordings. Spike trains (STs) were converted into continuous-time rate functions (CRFs) using the ST instantaneous firing rates. LFOs were detected for all three of the components using the multi-taper method (MTM). The frequencies of these LFOs ranged from 0.052 to 0.167 Hz (mean±SD, 0.10±0.026 Hz) for cerebral blood flow (CBF), from 0.027 to 0.26 Hz (mean±SD, 0.12±0.041 Hz) for the CRFs of the STs and from 0.04 to 0.19 Hz (mean±SD, 0.11±0.035 Hz) for local field potentials (LFPs). We evaluated the Granger causal relationships of spontaneous LFOs among CBF, LFPs and CRFs using Granger causality (GC) analysis. Significant Granger causal relationships were observed from LFPs to CBF, from STs to CBF and from LFPs to STs at approximately 0.1 Hz. The present results indicate that spontaneous LFOs exist not only in hemodynamic components but also in neuronal activities of the rat cortex. To the best of our knowledge, the present study is the first to identify Granger causal influences among CBF, LFPs and STs and show that spontaneous LFOs carry important Granger causal influences from neural activities to hemodynamic signals.

The adaptation of the cerebral circulation to pregnancy: mechanisms and consequences

The adaptation of the cerebral circulation to pregnancy is unique from other vascular beds. Most notably, the growth and vasodilatory response to high levels of circulating growth factors and cytokines that promote substantial hemodynamic changes in other vascular beds is limited in the cerebral circulation. This is accomplished through several mechanisms, including downregulation of key receptors and transcription factors, and production of circulating factors that counteract the vasodilatory effects of vascular endothelial growth factor (VEGF) and placental growth factor. Pregnancy both prevents and reverses hypertensive inward remodeling of cerebral arteries, possibly through downregulation of the angiotensin type 1 receptor. The blood-brain barrier (BBB) importantly adapts to pregnancy by preventing the passage of seizure provoking serum into the brain and limiting the permeability effects of VEGF that is more highly expressed in cerebral vasculature during pregnancy. While the adaptation of the cerebral circulation to pregnancy provides for relatively normal cerebral blood flow and BBB properties in the face of substantial cardiovascular changes and high levels of circulating factors, under pathologic conditions, these adaptations appear to promote greater brain injury, including edema formation during acute hypertension, and greater sensitivity to bacterial endotoxin.

Autonomic blockade during sinusoidal baroreflex activation proves sympathetic modulation of cerebral blood flow velocity

BACKGROUND AND PURPOSE

Pharmacological blockade showed sympathetic origin of 0.03 to 0.15 Hz blood pressure (BP) oscillations and parasympathetic origin of 0.15 to 0.5 Hz RR-interval (RRI) oscillations, but has not been used to determine origin of cerebral blood flow velocity (CBFV) oscillations at these frequencies. This study evaluated by pharmacological blockade whether 0.1 Hz CBFV oscillations are related to sympathetic and 0.2 Hz CBFV oscillations to parasympathetic modulation.

METHODS

In 11 volunteers (24.6 ± 2.3 years), we monitored RRIs, BP, and proximal middle cerebral artery CBFV, at rest, during 180 s sympathetic BP activation by 0.1 Hz sinusoidal neck suction (NS), and during 180 s parasympathetic RRI activation by 0.2 Hz NS. We repeated recordings after 25 mg carvedilol, and after 0.04 mg/kg atropine. Autoregressive analysis quantified RRI-, BP-, and CBFV-spectral powers at 0.1 Hz and 0.2 Hz. We compared parameters at rest, during 0.1 Hz, or 0.2 Hz NS, with and without carvedilol or atropine (analysis of variance, post hoc testing; significance, P<0.05).

RESULTS

Carvedilol significantly increased RRIs and lowered BP, CBFV, and 0.1 Hz RRI-, BP-, and CBFV-powers at baseline (P=0.041 for CBFV-powers), and during 0.1 Hz NS-induced sympathetic activation (P<0.05). At baseline and during 0.2 Hz NS-induced parasympathetic activation, atropine lowered RRIs and 0.2 Hz RRI-powers, but did not change BP, CBFV, and 0.2 Hz BP- and CBFV-powers.

CONCLUSIONS

Attenuation of both 0.1 Hz CBFV and BP oscillations after carvedilol indicates a direct relation between 0.1 Hz CBFV oscillations and sympathetic modulation. Absent effects of atropine on BP, CBFV, and 0.2 Hz BP and CBFV oscillations suggest that there is no direct parasympathetic influence on 0.2 Hz BP and CBFV modulation.

Sympathetic activation increases basilar arterial blood flow in normotensive but not hypertensive rats

The close apposition between sympathetic and parasympathetic nerve terminals in the adventitia of cerebral arteries provides morphological evidence that sympathetic nerve activation causes parasympathetic nitrergic vasodilation via a sympathetic-parasympathetic interaction mechanism. The decreased parasympathetic nerve terminals in basilar arteries (BA) of spontaneously hypertensive rat (SHR) and renovascular hypertensive rats (RHR) compared with Wistar-Kyoto rats (WKY), therefore, would diminish this axo-axonal interaction-mediated neurogenic vasodilation in hypertension. Increased basilar arterial blood flow (BABF) via axo-axonal interaction during sympathetic activation was, therefore, examined in anesthetized rats by laser-Doppler flowmetry. Electrical stimulation (ES) of sympathetic nerves originating in superior cervical ganglion (SCG) and topical nicotine (10–30 μM) onto BA of WKY significantly increased BABF. Both increases were inhibited by tetrodotoxin, 7-nitroindazole (neuronal nitric oxide synthase inhibitor), and ICI-118,551 (β2-adrenoceptor antagonist), but not by atenolol (β1-adrenoceptor antagonist). Topical norepinephrine onto BA also increased BABF, which was abolished by atenolol combined with 7-nitroindazole or ICI-118,551. Similar results were found in prehypertensive SHR. However, in adult SHR and RHR, ES of sympathetic nerves or topical nicotine caused minimum or no increase of BABF. It is concluded that excitation of sympathetic nerves to BA in WKY causes parasympathetic nitrergic vasodilation with increased BABF. This finding indicates an endowed functional neurogenic mechanism for increasing the BABF or brain stem blood flow in coping with increased local sympathetic activities in acutely stressful situations such as the “fight-or-flight response.” This increased blood flow in defensive mechanism diminishes in genetic and nongenetic hypertensive rats due most likely to decreased parasympathetic nitrergic nerve terminals.

Age-related impairment in choroidal blood flow compensation for arterial blood pressure fluctuation in pigeons

PURPOSE

Choroidal vessels compensate for changes in systemic blood pressure (BP) so that choroidal blood flow (ChBF) remains stable over a BP range of approximately 40 mm Hg above and below basal. Because of the presumed importance of ChBF regulation for maintenance of retinal health, we investigated if ChBF compensation for BP fluctuation in pigeons fails with age.

METHODS

Transcleral laser Doppler flowmetry was used to measure ChBF during spontaneous BP fluctuation in anesthetized pigeons ranging in age from 0.5 to 17 years (pigeons can live approximately 20 years in captivity).

RESULTS

ChBF in <8-year-old pigeons remained near 100% of basal ChBF at BPs ranging 40 mm Hg above and below basal BP (95 mm Hg). Baroregulation failed below approximately 50 mm Hg BP. In ≥8-year-old pigeons, ChBF compensation was absent at >90 mm Hg BP, with ChBF linearly following BP. Over the 60 to 90 mm Hg range, ChBF in ≥8-year-old pigeons was maintained at 60-70% of young basal ChBF. Below approximately 55 mm Hg, baroregulation again followed BP linearly.

CONCLUSIONS

Age-related ChBF baroregulatory impairment occurs in pigeons, with ChBF linear with above-basal BP, and ChBF failing to adequately maintain ChBF during below-basal BP. Defective autonomic sympathetic and parasympathetic neurogenic control, or defective myogenic control, may cause these baroregulatory defects. In either case, overperfusion during high BP may cause oxidative injury to the outer retina, whereas underperfusion during low BP may result in deficient nutrient supply and waste removal, with both abnormalities contributing to age-related retinal pathology and vision loss.

Linear superposition of sensory-evoked and ongoing cortical hemodynamics

Modern non-invasive brain imaging techniques utilize changes in cerebral blood flow, volume and oxygenation that accompany brain activation. However, stimulus-evoked hemodynamic responses display considerable inter-trial variability even when identical stimuli are presented and the sources of this variability are poorly understood. One of the sources of this response variation could be ongoing spontaneous hemodynamic fluctuations. To investigate this issue, 2-dimensional optical imaging spectroscopy was used to measure cortical hemodynamics in response to sensory stimuli in anesthetized rodents. Pre-stimulus cortical hemodynamics displayed spontaneous periodic fluctuations and as such, data from individual stimulus presentation trials were assigned to one of four groups depending on the phase angle of pre-stimulus hemodynamic fluctuations and averaged. This analysis revealed that sensory evoked cortical hemodynamics displayed distinctive response characteristics and magnitudes depending on the phase angle of ongoing fluctuations at stimulus onset. To investigate the origin of this phenomenon, "null-trials" were collected without stimulus presentation. Subtraction of phase averaged "null trials" from their phase averaged stimulus-evoked counterparts resulted in four similar time series that resembled the mean stimulus-evoked response. These analyses suggest that linear superposition of evoked and ongoing cortical hemodynamic changes may be a property of the structure of inter-trial variability.

Postoperative blindness associated with posterior reversible encephalopathy syndrome: a case report

We report the case of a 46-year-old woman presenting with postoperative bilateral cerebral visual loss that was initially misinterpreted as an irreversible ischemic event. Magnetic resonance imaging of the brain showed high signal intensity on T2-weighted and fluid-attenuated inversion recovery images and normal signal intensity on diffusion-weighted images of the posterior lobe, which mostly disappeared with the improvement of clinical symptoms. Subsequent diagnosis revealed posterior reversible encephalopathy syndrome (PRES). Recognition of PRES as the correct diagnosis led to the appropriate management strategy and the recovery of normal vision. Differentiation from acute cerebral ischemia is important in order to prevent permanent vision loss due to delay in initiating prompt and vigorous treatment of exacerbating factors, such as intermittent hypertension. We believe that it is important for anesthesiologists and critical care physicians to accurately diagnose PRES in view of the key differences in the management of similarly presenting conditions.

Impaired cerebral autoregulation during acute alcohol withdrawal

Heavy alcohol consumption increases the risk for all major types of stroke and is associated with autonomic dysfunction during alcohol withdrawal syndrome (AWS). Cerebral autoregulation is the mechanism by which cerebral perfusion is maintained stable, representing an intrinsic protective system of the cerebral circulation. Here, we aimed to analyze the influence of acute AWS on cerebral hemodynamics in alcohol-dependent patients. We investigated 20 men in the unmedicated acute state of AWS and repeated the investigation 24h after initiation of clomethiazole treatment. Dynamic cerebral autoregulation (dCA) was assessed by the correlation coefficient index and transfer function analysis (phase and gain) from oscillations of arterial blood pressure and cerebral blood flow velocity (CBFV). The vasomotor reserve (VMR) was measured by the CO(2)-reactivity test. In addition, we assessed autonomic modulation by means of heart rate variability and baroreflex sensitivity. We observed impaired dynamic autoregulation as shown by a multivariate analysis of variance (p<0.038) including all parameters of dCA. Similar results were found for VMR at admission (p<0.05). Pair-wise comparison between baseline and treatment with clomethiazole revealed a significant improvement for the systolic correlation coefficient index (Sx; p<0.001). Furthermore, we found a strong association of autonomic dysfunction and impaired autoregulation indicated by a correlation between the LF/HF ratio and Sx (p<0.001). In conclusion, cerebral autoregulation and VMR are disturbed during acute AWS. Influences of autonomic dysbalance and mental state during withdrawal are suggested. The finding of an affected autoregulation during acute withdrawal might indicate an increased risk for cerebro-vascular disease.

Choroidal blood flow compensation in rats for arterial blood pressure decreases is neuronal nitric oxide-dependent but compensation for arterial blood pressure increases is not

Choroidal blood flow (ChBF) compensates for changes in arterial blood pressure (ABP) and thereby remains relatively stable within a +/-40 mmHg range of basal ABP in rabbits, humans and pigeons. In the present study, we investigated if ChBF can compensate for increases and decreases in ABP in rats. ChBF was continuously monitored using laser Doppler flowmetry in anesthetized rats, and ABP measured via the femoral artery. At multiple intervals over a 2-4 h period during which ABP varied freely, ChBF and ABP were sampled and the results compiled across rats. We found that ChBF remained near baseline over an ABP range from 40 mmHg above basal ABP (90-100 mmHg) to 40 mmHg below basal ABP, but largely followed ABP linearly below 60 mmHg. Choroidal vascular resistance increased linearly as BP increased above 100 mmHg, and decreased linearly as BP declined from basal to 60 mmHg, but resistance declined no further below 60 mmHg. Inhibition of nitric oxide (NO) formation by either a selective inhibitor of neuronal nitric oxide synthase (NOS) (N(omega)-propyl-L-arginine) or a nonselective inhibitor of both neuronal NOS and endothelial NOS (N(omega)-nitro-l-arginine methyl ester) did not affect compensation above 100 mmHg ABP, but did cause ChBF to linearly follow declines in BP below 90 mmHg. In NOS-inhibited rats, vascular resistance increased linearly with BP above 100 mmHg, but remained at baseline below 90 mmHg. These findings reveal that ChBF in rats, as in rabbits, humans and pigeons, compensates for rises and/or declines in arterial blood pressure so as to remain relatively stable within a physiological range of ABPs. The ChBF compensation for low ABP in rats is dependent on choroidal vasodilation caused by neuronal NO formation but not the compensation for elevated BP, implicating parasympathetic nervous system vasodilation in the ChBF compensation to low ABP.

Autonomic neural control of cerebral hemodynamics

Despite the rich innervation of the cerebral vasculature by both sympathetic and parasympathetic nerves, the role of autonomic control in cerebral circulation and, particularly, cerebral hemodynamics is not entirely clear. Previous animal studies have reported inconsistent results regarding the effects of electrical stimulation or denervation on cerebral blood flow (CBF), cerebral pressure-flow relationship, and cerebral vessel response to metabolic stimuli. Moreover, with the advance of transcranial Doppler ultrasound (TCD), which yields accurate measurements of CBF velocity (CBFV) with high time resolution, it has been found that in humans CBFV in the middle cerebral artery decreased substantially during lower body negative pressure (LBNP) and head-up tilt in the absence of systemic hypotension, which suggests the presence of cerebral vasoconstriction associated with augmented sympathetic nerve activity during orthostatic stress. These observations were based on assessing static measures of cerebral circulation, i.e., mean values of artevial blood pressure (ABP) and CBF with a low time resolution.

Effects of heat stress on dynamic cerebral autoregulation during large fluctuations in arterial blood pressure

Impaired cerebral autoregulation during marked reductions in arterial blood pressure may contribute to heat stress-induced orthostatic intolerance. This study tested the hypothesis that passive heat stress attenuates dynamic cerebral autoregulation during pronounced swings in arterial blood pressure. Mean arterial blood pressure (MAP) and middle cerebral artery blood velocity were continuously recorded for approximately 6 min during normothermia and heat stress (core body temperature = 36.9 +/- 0.1 degrees C and 38.0 +/- 0.1 degrees C, respectively, P < 0.001) in nine healthy individuals. Swings in MAP were induced by 70-mmHg oscillatory lower body negative pressure (OLBNP) during normothermia and at a sufficient lower body negative pressure to cause similar swings in MAP during heat stress. OLBNP was applied at a very low frequency ( approximately 0.03 Hz, i.e., 15 s on-15 s off) and a low frequency ( approximately 0.1 Hz, i.e., 5 s on-5 s off). For each thermal condition, transfer gain, phase, and coherence function were calculated at both frequencies of OLBNP. During very low-frequency OLBNP, transfer function gain was reduced by heat stress (0.55 +/- 0.20 and 0.31 +/- 0.07 cm x s(-1) x mmHg(-1) during normothermia and heat stress, respectively, P = 0.02), which is reflective of improved cerebrovascular autoregulation. During low-frequency OLBNP, transfer function gain was similar between thermal conditions (1.19 +/- 0.53 and 1.01 +/- 0.20 cm x s(-1) x mmHg(-1) during normothermia and heat stress, respectively, P = 0.32). Estimates of phase and coherence were similar between thermal conditions at both frequencies of OLBNP. Contrary to our hypothesis, dynamic cerebral autoregulation during large swings in arterial blood pressure during very low-frequency (i.e., 0.03 Hz) OLBNP is improved during heat stress, but it is unchanged during low-frequency (i.e., 0.1 Hz) OLBNP.

The influence of pregnancy and gender on perivascular innervation of rat posterior cerebral arteries

The authors investigated the influence of pregnancy and gender on the density of trigeminal and sympathetic perivascular nerves in posterior cerebral arteries (PCA) and the reactivity to norepinephrine and calcitonin gene-related peptide (CGRP). PCAs were isolated from nonpregnant, late-pregnant, postpartum, and male rats, mounted and pressurized on an arteriograph chamber to obtain concentration-response curves to norepinephrine and CGRP. Arteries were immunostained for CGRP-, tyrosine hydroxylase-, and protein gene product 9.5 (PGP 9.5)-containing perivascular nerves, and nerve density was determined morphologically. Pregnancy had a trophic effect on trigeminal perivascular innervation (P < .01 vs male); however, this was not accompanied by a change in reactivity to CGRP. Sympathetic and PGP 9.5 nerve densities were not altered by pregnancy or gender, and there were no differences in reactivity to norepinephrine. Together, these results suggest that the increase in trigeminal innervation during pregnancy is more related to nociception than in controlling resting cerebral blood flow.

Autonomic neural control of the cerebral vasculature: acute hypotension

BACKGROUND AND PURPOSE

The effect of antihypertensive drugs on autonomic neural control of the cerebral circulation remains unclear. This study was designed to compare middle cerebral artery mean blood velocity responses to acute hypotension with and without alpha(1)-adrenoreceptor blockade (Prazosin) in young, healthy humans.

METHODS

Acute hypotension was induced nonpharmacologically in 6 healthy subjects (mean+/-SE; 28+/-2 years) by releasing bilateral thigh cuffs after 9 minutes of suprasystolic resting ischemia before and after an oral dose of Prazosin (1 mg/20 kg body weight).

RESULTS

Prazosin had no effect on thigh cuff release-induced reductions in mean arterial pressure and middle cerebral artery mean blood velocity. However, Prazosin attenuated the amount of peripheral vasoconstriction through the arterial baroreflex as evidenced by a slower return of mean arterial pressure to baseline (P=0.03). Immediately after cuff release, cerebral vascular conductance index increased through cerebral autoregulation and returned to resting values as a result of an increased perfusion pressure mediated through arterial baroreflex mechanisms. The rate of regulation, an index of cerebral autoregulation, was attenuated with Prazosin (control versus Prazosin; rate of regulation=0.204+/-0.020 versus 0.006+/-0.053/s, P=0.037). In addition, as mean arterial pressure was returning to resting values, the rate of change in cerebral vascular conductance index was decreased with Prazosin (0.005+/-0.006/s) compared with control (0.025+/-0.005/s; P=0.010).

CONCLUSIONS

These data suggest that during recovery from acute hypotension, decreases in cerebral vascular conductance index were mediated by increases in arterial blood pressure and sympathetically mediated cerebral vasoconstriction.

Impairment of cerebral hemodynamic response to the cold pressor test in patients with Parkinson's disease

BACKGROUND AND PURPOSE

Disturbance of the autonomic nervous system (ANS) is frequently encountered in Parkinson's disease (PD). In this study, we examined changes in systemic and cerebral hemodynamics during the cold pressor test (CPT) to determine whether cerebrovascular reactivity, controlled by the sympathetic nervous system, is intact or impaired in patients with PD.

METHODS

Forty-nine patients with PD and 49 sex- and age-matched non-PD subjects were evaluated. Measurements were performed in the resting state and over a period of 1min of CPT. The cerebral blood flow velocity (CBFV) and pulsatility index (PI) of the middle cerebral artery (MCA) were recorded by transcranial color-coded Doppler ultrasonography (TCCS). Mean arterial blood pressure (MAP), heart rate (HR), and end-tidal CO(2) (Et-CO(2)) were investigated simultaneously. The resistance of the cerebrovascular bed (CVR) was calculated as the ratio of mean arterial blood pressure to mean cerebral blood flow velocity (Vm). Changes of Vm, PI and CVR in response to the cold pressor test were evaluated.

RESULTS

Baseline values for control and PD subjects showed no statistical difference. CPT induced a significant increase in MAP, HR, and Vm in both groups. Pulsatility index (PI) and CVR were decreased in both groups during CPT. Percent increases of Vm (P<0.001) and MAP (P=0.011) were significantly higher while the percent decreases of PI (P=0.002) and CVR (P=0.007) were significantly decreased more in the non-PD group.

CONCLUSIONS

This study indirectly shows that ANS-mediated cerebrovascular reactivity is impaired in patients with PD. Further investigations are needed to confirm the hypothesis that using the cold pressor test to evaluate cerebrovascular reactivity might be beneficial in early diagnosis of impairment of ANS-mediated cerebrovascular autoregulation in patients with PD.

Cerebral Autoregulation and Syncope

Whatever the pathogenesis of syncope is, the ultimate common cause leading to loss of consciousness is insufficient cerebral perfusion with a critical reduction of blood flow to the reticular activating system. Brain circulation has an autoregulation system that keeps cerebral blood flow constant over a wide range of systemic blood pressures. Normally, if blood pressure decreases, autoregulation reacts with a reduction in cerebral vascular resistance, in an attempt to prevent cerebral hypoperfusion. However, in some cases, particularly in neurally mediated syncope, it can also be harmful, being actively implicated in a paradox reflex that induces an increase in cerebrovascular resistance and contributes to the critical reduction of cerebral blood flow. This review outlines the anatomic structures involved in cerebral autoregulation, its mechanisms, in normal and pathologic conditions, and the noninvasive neuroimaging techniques used in the study of cerebral circulation and autoregulation. An emphasis is placed on the description of autoregulation pathophysiology in orthostatic and neurally mediated syncope.

Effects of sympathetically induced vasomotion on tissue-capillary fluid exchange

The spontaneous and rhythmic constriction of peripheral arterioles, which is not associated with the cardiac or respiratory cycles, is called vasomotion. Vasomotion is observed in various tissues of various species, but the physiological role of vasomotion has not been clarified because of the difficulty in controlling the appearance of vasomotion in in vivo preparations. We developed a method of controlling vasomotion in in vivo experiments. The electrical stimulation of the cervical sympathetic nerve could reproducibly evoke vasomotion in rabbit ear skin. The frequencies of the evoked vasomotion were 0.04–0.07 Hz, which corresponded to spontaneously occurring vasomotion that has been reported before. Vasomotion was always evoked between 25 and 35°C. At lower than 17°C or higher than 37°C, vasomotion was not evoked. With the use of this method of evoking vasomotion in vivo, the role of vasomotion in tissue perfusion was examined. A tracer (Cr-EDTA) was injected into the ear tissue, and tracer fading was then measured by using a camera. The rates of fading (clearance) of the tracer with vasomotion were significantly greater (1.7 to 8.1 times) than those without vasomotion. These results provided evidence that vasomotion enhanced tissue perfusion.

Large-amplitude, spatially correlated fluctuations in BOLD fMRI signals during extended rest and early sleep stages

A number of recent studies of human brain activity using blood-oxygen-level-dependent (BOLD) fMRI and EEG have reported the presence of spatiotemporal patterns of correlated activity in the absence of external stimuli. Although these patterns have been hypothesized to contain important information about brain architecture, little is known about their origin or about their relationship to active cognitive processes such as conscious awareness and monitoring of the environment. In this study, we have investigated the amplitude and spatiotemporal characteristics of resting-state activity patterns and their dependence on the subjects' alertness. For this purpose, BOLD fMRI was performed at 3.0 T on 12 normal subjects using a visual stimulation protocol, followed by a 27 min rest period, during which subjects were allowed to fall asleep. In subjects who were asleep at the end of the scan, we found (a) a higher amplitude of BOLD signal fluctuation during rest compared with subjects who were awake at the end of the scan; (b) spatially independent patterns of correlated activity that involve all of gray matter, including deep brain nuclei; (c) many patterns that were consistent across subjects; (d) that average percentage levels of fluctuation in visual cortex (VC) and whole brain were higher in subjects who were asleep (up to 1.71% and 1.16%, respectively) than in those who were awake (up to 1.15% and 0.96%) at the end of the scan and were comparable with those levels evoked by intense visual stimulation (up to 1.85% and 0.76% for two subject groups); (e) no confirmation of correlation, positive or negative, between thalamus and VC found in earlier studies. These findings suggest that resting-state activity continues during sleep and does not require active cognitive processes or conscious awareness.

Autonomic dysfunction and impaired cerebral autoregulation in cirrhosis

Cerebral blood flow autoregulation is lost in patients with severe liver cirrhosis. The cause of this is unknown. We determined whether autonomic dysfunction was related to impaired cerebral autoregulation in patients with cirrhosis. Fourteen patients with liver cirrhosis and 11 healthy volunteers were recruited. Autonomic function was assessed in response to deep breathing, head-up tilt and during 24-h Holter monitoring. Cerebral autoregulation was assessed by determining the change in mean cerebral blood flow velocity (MCAVm, transcranial Doppler) during an increase in blood pressure induced by norepinephrine infusion (NE). The severity of liver disease was assessed using the Child-Pugh scale (class A, mild; class B, moderate; class C, severe liver dysfunction).NE increased blood pressure similarly in the controls (27 (24-32) mmHg) and patients with the most severe liver cirrhosis (Child-Pugh C, 31 (26-44) mmHg, p=0.405 Mann-Whitney). However, the increase in MCAVm was greater in cirrhosis patients compared to the controls (Child-Pugh C, 26 (24-39) %; controls, 3 (-1.3 to 3) %; respectively, p=0.016, Mann-Whitney). HRV during deep breathing was reduced in the cirrhosis patients (Child-Pugh C, 6.0+/-2.0 bpm) compared to the controls (21.7+/-2.2 bpm, p=0.001, Tukey' test). Systolic blood pressure fell during head-up tilt only in patients with severe cirrhosis. Our results imply that cerebral autoregulation was impaired in the most severe cases of liver cirrhosis, and that those with impaired cerebral autoregulation also had severe parasympathetic and sympathetic autonomic dysfunction. Furthermore, the degree of liver dysfunction was associated with increasing severity of autonomic dysfunction. Although this association is not necessarily causal, we postulate that the loss of sympathetic innervation to the cerebral resistance vessels may contribute to the impairment of cerebral autoregulation in patients with end-stage liver disease.

Cortical blindness: Clinical and radiologic findings in reversible posterior leukoencephalopathy syndrome

PURPOSE

To alert ophthalmologists to the recognition of cortical visual loss as the presenting feature in patients with reversible posterior leukoencephalopathy syndrome (RPLES). Unique radiologic findings are paramount to the diagnosis.

DESIGN

Interventional case report.

METHODS

A patient was seen with perioperative bilateral cerebral visual loss that was misinterpreted initially as an irreversible ischemic event. Further detailed analysis of the radiologic findings and clinical history led to the correct diagnosis.

MAIN OUTCOME MEASURES

Visual acuity and magnetic resonance imaging (MRI) of the brain.

RESULTS

Recognition of the correct diagnosis of RPLES led to the institution of antihypertensive therapy and recovery of normal vision.

CONCLUSIONS

The diagnosis of RPLES should be considered in all patients with acute cerebral visual loss, especially in the setting of recent surgery, blood transfusion, chemotherapy, immunosuppressant use, hypertension, eclampsia, or seizures. Prompt diagnosis requires close collaboration with a radiologist and an emergent MRI study, which ideally should include diffusion-weighted imaging with calculation of an apparent diffusion coefficient map. Differentiation from acute cerebral ischemia is important in order to avoid permanent visual loss by prompt and vigorous treatment of exacerbating factors such as intermittent hypertension. Prompt diagnosis will also help to avoid potentially dangerous invasive procedures such as thrombolytic therapy.

A novel central pathway links arterial baroreceptors and pontine parasympathetic neurons in cerebrovascular control

1. We tested the hypothesis that arterial baroreceptor reflexes modulate cerebrovascular tone through a pathway that connects the cardiovascular nucleus tractus solitarii with parasympathetic preganglionic neurons in the pons. 2. Anesthetized rats were used in all studies. Laser flowmetry was used to measure cerebral blood flow. We assessed cerebrovascular responses to increases in arterial blood pressure in animals with lesions of baroreceptor nerves, the nucleus tractus solitarii itself, the pontine preganglionic parasympathetic neurons, or the parasympathetic ganglionic nerves to the cerebral vessels. Similar assessments were made in animals after blockade of synthesis of nitric oxide, which is released by the parasympathetic nerves from the pterygopalatine ganglia. Finally the effects on cerebral blood flow of glutamate stimulation of pontine preganglionic parasympathetic neurons were evaluated. 3. We found that lesions at any one of the sites in the putative pathway or interruption of nitric oxide synthesis led to prolongation of autoregulation as mean arterial pressure was increased to levels as high as 200 mmHg. Conversely, stimulation of pontine parasympathetic preganglionic neurons led to cerebral vasodilatation. The second series of studies utilized classic anatomical tracing methods to determine at the light and electron microscopic level whether neurons in the cardiovascular nucleus tractus solitarii, the site of termination of baroreceptor afferents, projected to the pontine preganglionic neurons. Fibers were traced with anterograde tracer from the nucleus tractus solitarii to the pons and with retrograde tracer from the pons to the nucleus tractus solitarii. Using double labeling techniques we further studied synapses made between labeled projections from the nucleus tractus solitarii and preganglionic neurons that were themselves labeled with retrograde tracer placed into the pterygopalatine ganglion. 4. These anatomical studies showed that the nucleus tractus solitarii directly projects to pontine preganglionic neurons and makes asymmetric, seemingly excitatory, synapses with those neurons. These studies provide strong evidence that arterial baroreceptors may modulate cerebral blood flow through direct connections with pontine parasympathetic neurons. Further study is needed to clarify the role this pathway plays in integrative physiology.

M-wave analysis and passive tilt in patients with different degrees of carotid artery disease

BACKGROUND AND PURPOSE

Carotid artery disease (CAD) is able to critically impair cerebral autoregulation which increases the risk for stroke. As therapeutic strategy largely depends on the degree of CAD, we investigated whether this gradation is also related to significant changes in autoregulatory capacity. We applied cross-spectral analysis (CSA) of spontaneous Mayer-wave (M-wave) oscillations and passive tilting (PT) to test cerebral autoregulation.

METHODS

Cerebral autoregulation was tested in 102 patients with carotid stenosis (> or =70%) or occlusion and 14 controls by comparison of continuous transcranial Doppler sonography of the middle cerebral artery and beat-to-beat arterial blood pressure (ABP) during PT to 80 degrees head-up position as well as by CSA of M-waves (3-9 cpm).

RESULTS

The orthostatic decrease of cerebral blood flow velocity (CBFV) was not correlated with the degree of CAD and showed a lower sensitivity and specificity than phase angle shifts between M-waves in ABP and CBFV (sensitivity: 75-80%, specificity: 86%). Phase angles were gradually lowered in carotid stenoses > 70%, but apparently, they were only moderately correlated with the degree of CAD (r = -0.35, P < 0.01). An additional influencing factor seemed to be the sufficiency of collateralization.

CONCLUSIONS

The results show that CSA of M-waves is more appropriate for testing autoregulation than PT. CSA suggests that the capacity to autoregulate depends to a certain extent on the degree of CAD but is also influenced by the sufficiency of collateral pathways and pre-existing strokes.

Choroidal blood flow in pigeons compensates for decreases in arterial blood pressure

While it had once been thought that choroidal blood flow (ChBF) does not compensate for changes in perfusion pressure, recent studies have shown that ChBF in rabbits and humans does compensate for changes in arterial blood pressure (ABP) and thereby remains relatively stable within a physiological range of ABPs. In this study, we sought to determine if ChBF in birds can compensate for decreases in ABP, either spontaneously occuring or caused by blood withdrawal. ChBF was continuously monitored using laser Doppler flowmetry in anesthetized pigeons, and at the same time ABP was measured via the brachial artery. In studies of spontaneous fluctuation in ABP, ChBF and ABP were analyzed at regular intervals over a 2-3 hr period, while for blood withdrawal studies, blood was transiently withdrawn via the brachial artery. In both paradigms, ChBF remained near baseline over an ABP range from basal (about 90 mmHg) to about 55 mmHg, followed ABP nearly linearly below 50 mmHg, and showed no compensation below 40 mmHg. The blood withdrawal studies further showed that the compensation was more rapid with small acute declines in ABP than with larger declines. These findings reveal that ChBF in pigeons, as in rabbits and humans, compensates for declines in ABP so as to remain relatively stable within a physiological range of ABPs. Given the phylogenetic distance between humans and rabbits on one hand and birds on the other, these results suggest that choroidal compensation for ABP declines may be a common ocular mechanism among warm-blooded vertebrates.

Autonomic neural control of dynamic cerebral autoregulation in humans

BACKGROUND

The purpose of the present study was to determine the role of autonomic neural control of dynamic cerebral autoregulation in humans.

METHODS AND RESULTS

We measured arterial pressure and cerebral blood flow (CBF) velocity in 12 healthy subjects (aged 29+/-6 years) before and after ganglion blockade with trimethaphan. CBF velocity was measured in the middle cerebral artery using transcranial Doppler. The magnitude of spontaneous changes in mean blood pressure and CBF velocity were quantified by spectral analysis. The transfer function gain, phase, and coherence between these variables were estimated to quantify dynamic cerebral autoregulation. After ganglion blockade, systolic and pulse pressure decreased significantly by 13% and 26%, respectively. CBF velocity decreased by 6% (P<0.05). In the very low frequency range (0.02 to 0.07 Hz), mean blood pressure variability decreased significantly (by 82%), while CBF velocity variability persisted. Thus, transfer function gain increased by 81%. In addition, the phase lead of CBF velocity to arterial pressure diminished. These changes in transfer function gain and phase persisted despite restoration of arterial pressure by infusion of phenylephrine and normalization of mean blood pressure variability by oscillatory lower body negative pressure.

CONCLUSIONS

These data suggest that dynamic cerebral autoregulation is altered by ganglion blockade. We speculate that autonomic neural control of the cerebral circulation is tonically active and likely plays a significant role in the regulation of beat-to-beat CBF in humans.

Regulation of cerebral blood flow in patients with autonomic dysfunction and severe postural hypotension

BACKGROUND

Whether cerebral blood flow (CBF) autoregulation is maintained in autonomic dysfunction has been debated for a long time, and the rather sparse data available are equivocal. The relationship between CBF and mean arterial blood pressure (MABP) was therefore tested in eight patients with symptoms and signs of severe cardiovascular autonomic dysfunction.

PATIENTS AND METHODS

Eight patients were included, three of whom had Parkinson's disease, three diabetes, one pure autonomic failure and the last one had multiple system atrophy. By the use of two techniques, the arteriovenous oxygen [(a-v)O2] method and xenon-inhalation with single photon emission tomography, 15 measurements (range 10-20) and three to four CBF measurements, respectively, were obtained in each patient. Following CBF measurements during baseline, MABP was raised gradually using intravenous noradrenaline infusion, and then lowered by application of lower body negative pressure. From the (a-v)O2 samples the CBF response to changes in MABP was evaluated using a computer program fitting one or two regression lines through the plot.

RESULTS AND CONCLUSION

Preserved autoregulation was observed in three patients, while the remaining five patients showed a linear relationship between CBF and MABP. Comparison of the results of the tomographic CBF measurements to the (a-v)O2 data demonstrated that it is not possible to assess whether CBF is autoregulated or not with only three to four pairs of data.

Signal transduction of physiological concentrations of vasopressin in A7r5 vascular smooth muscle cells. A role for PYK2 and tyrosine phosphorylation of K+ channels in the stimulation of Ca2+ spiking

The signal transduction pathway linking physiological concentrations of [Arg(8)]vasopressin (AVP) to an increase in frequency of Ca(2+) spiking was examined in confluent cultures of A7r5 vascular smooth muscle cells. Immunoprecipitation/Western blot studies revealed a robust increase in tyrosine phosphorylation of the non-receptor tyrosine kinase, PYK2, in A7r5 cells treated with 4beta-phorbol 12-myristate 13-acetate or ionomycin. 100 pm AVP also induced PYK2 tyrosine phosphorylation, and this effect was inhibited by protein kinase C inhibitors Ro-31-8220 (1-10 microm) or chelerythrine chloride (1-20 microm). In fura-2-loaded A7r5 cells, the stimulation of Ca(2+) spiking by 100 pm AVP or 1 nm 4beta-phorbol 12-myristate 13-acetate was completely blocked by PP2 (10 microm, a Src family kinase inhibitor). Salicylate (20 mm, recently identified as a PYK2 inhibitor) and the tyrosine kinase inhibitor, tyrphostin A47 (50 microm), but not its inactive analog, tyrphostin A63, also blocked AVP-stimulated Ca(2+) spiking. PYK2 phosphorylation was inhibited by both PP2 and salicylate, whereas tyrphostin A47 failed to inhibit PYK2 tyrosine phosphorylation. ERK1/2 kinases did not appear to be involved because 1) 100 pm AVP did not appreciably increase ERK1/2 phosphorylation and U-0126 (2.5 microm) did not inhibit AVP-stimulated Ca(2+) spiking; and 2) epidermal growth factor (10 nm) robustly stimulated ERK1/2 phosphorylation but did not induce Ca(2+) spiking. Delayed rectifier K(+) channels may mediate the PYK2 activity because Kv1.2 channel protein co-immunoprecipitated with PYK2 and tyrosine phosphorylation of Kv1.2 was stimulated by AVP and inhibited by Ro-31-8220, PP2, and salicylate but not tyrphostin A47. Our findings are consistent with a role for PYK2 and phosphorylation of K(+) channels in the stimulation of Ca(2+) spiking by physiological concentrations of AVP.

Cerebral hemodynamics in carotid sinus syndrome and atrioventricular block

Carotid sinus syndrome (CSS) is a cause of syncope due to exaggerated baroreceptor-mediated cardioinhibitory/vasodepressive reflexes. We sought to determine if cerebral hemodynamics and regulation were specifically altered in these patients by comparison with pure asystole without vasodepression in patients with atrioventricular block (AVB). Mean blood flow velocity (transcranial Doppler sonography) and mean arterial blood pressure (Finapres) were recorded during cardioinhibition induced by carotid massage in patients with CSS (n = 14, 75 +/- SD 8 years) and asystole induced by temporary pacemaker inhibition in patients with complete AVB (n = 10, 69 +/- 11 years). Cerebrovascular resistance was estimated by the arterial pressure/cerebral flow velocity ratio, and dynamic cerebral autoregulatory responses were determined by the rate of regulation and autoregulatory index. Asystole and cardioinhibition each induced a decrease in arterial pressure (CSS 55 +/- 9% vs AVB 40 +/- 14%, p <0.05) and cerebral flow velocity (CSS 66 +/- 19% vs AVB 69 +/- 14%, p = NS), with an initial transient increase in cerebrovascular resistance (CSS 102 +/- 136% vs AVB 128 +/- 92%, p = NS) followed by a decrease (CSS 38 +/- 12%, AVB 29 +/- 13%, p = NS). The rate of regulation and autoregulatory index were higher with AVB (0.43 +/- 0.20 and 8.5 +/- 1.1 second(-1)) than CSS (0.20 +/- 0.12 and 4.8 +/- 1.3 second(-1), respectively, p <0.01 and p <0.001 vs AVB). During asystole and vasodepression, cerebral hypoperfusion in CSS is normally compensated for by cerebral autoregulation. The lower rate of regulation in CSS compared with AVB likely results from persistent peripheral vasodepression triggered by carotid massage.

Parasympathetic nerves influence cerebral blood flow during hypertension in rat

This study tested the hypothesis that cerebral vasodilatation during marked acute hypertension is mediated in part through the influence of parasympathetic nerves from the pterygopalatine ganglia. Blood pressure was increased slowly in anesthetized rats after bilateral transection of the parasympathetic nerves. Cerebral blood flow was measured by laser flowmetry. Acutely hypertensive denervated animals developed significantly less cerebral vasodilatation than did control animals with intact nerves. Thus, parasympathetic vasodilator nerves contribute to vasodilatation seen with acute hypertension.

Vasomotion in critically perfused muscle protects adjacent tissues from capillary perfusion failure

We analyzed the incidence and interaction of arteriolar vasomotion and capillary flow motion during critical perfusion conditions in neighboring peripheral tissues using intravital fluorescence microscopy. The gracilis and semitendinosus muscles and adjacent periosteum, subcutis, and skin of the left hindlimb of Sprague-Dawley rats were isolated at the femoral vessels. Critical perfusion conditions, achieved by stepwise reduction of femoral artery blood flow, induced capillary flow motion in muscle, but not in the periosteum, subcutis, and skin. Strikingly, blood flow within individual capillaries was decreased (P < 0.05) in muscle but was not affected in the periosteum, subcutis, and skin. However, despite the flow motion-induced reduction of muscle capillary blood flow during the critical perfusion conditions, functional capillary density remained preserved in all tissues analyzed, including the skeletal muscle. Abrogation of vasomotion in the muscle arterioles by the calcium channel blocker felodipine resulted in a redistribution of blood flow within individual capillaries from cutaneous, subcutaneous, and periosteal tissues toward skeletal muscle. As a consequence, shutdown of perfusion of individual capillaries was observed that resulted in a significant reduction (P < 0.05) of capillary density not only in the neighboring tissues but also in the muscle itself. We conclude that during critical perfusion conditions, vasomotion and flow motion in skeletal muscle preserve nutritive perfusion (functional capillary density) not only in the muscle itself but also in the neighboring tissues, which are not capable of developing this protective regulatory mechanism by themselves.

Impaired autoregulation of cerebral blood flow in an experimental model of traumatic brain injury

In order to study the pathophysiology and the intracranial hemodynamics of traumatic brain injury, we have developed a modified closed-head injury model of impact-acceleration that expresses several features of severe head injury in humans, including acute and long-lasting intracranial hypertension, diffuse axonal injury, neuronal necrosis, bleeding, and edema. In view of the clinical relevance of impaired autoregulation of cerebral blood flow after traumatic brain injury, and aiming at further characterization of the model, we investigated the autoregulation efficiency 24 h after experimental closed-head injury. Cortical blood flow was continuously monitored with a laser-Doppler flowmeter, and the mean arterial blood pressure was progressively decreased by controlled hemorrhage. Relative laser-Doppler flow was plotted against the corresponding mean arterial blood pressure, and a two-line segmented model was applied to determine the break point and slopes of the autoregulation curves. The slope of the curve at the right hand of the break point was significantly increased in the closed head injury group (0.751 +/- 0.966%/mm Hg versus -0.104 +/- 0.425%/mm Hg,p = 0.028). The break point tended towards higher values in the closed head injury group (62.2 +/- 20.8 mm Hg versus 46.9 +/- 12.7 mm Hg; mean +/- SD, p = 0.198). It is concluded that cerebral autoregulation in this modified closed head injury model is impaired 24 h after traumatic brain injury. This finding, in addition to other characteristic features of severe head injury established earlier in this model, significantly contributes to its clinical relevance.

Differences between cerebral and cerebellar autoregulation during progressive hypotension in rats

Autoregulation in the brain is essential to the maintenance of perfusion and hence to the normal functioning of the organism in the face of various hemodynamic challenges. The existence of prodromal symptoms preceding fainting suggests that cerebellar autoregulation could be altered earlier than cerebral autoregulation during the development of hypotension. The purpose of this study was to compare cerebral and cerebellar autoregulatory responses to hypotension induced by two rates of hemorrhage 1.5 and 2.0 ml/min. Cortical blood flows were measured simultaneously using laser Doppler flowmetry in rats. With increasing rate of hemorrhage, the kinetics of autoregulation were maintained in the cerebrum, whereas it caused a progressive loss in the efficacy of autoregulation in the cerebellum.

Orthostasis as a test for cerebral autoregulation in normal persons and patients with carotid artery disease

Orthostasis reduces mean flow velocity (FVmean) in cerebral arteries. This might be used as an alternative provocation test for cerebral hemodynamics in patients with carotid artery disease (CAD). In 21 unilateral CAD patients and 21 controls, FVmean in both middle cerebral arteries (MCA) was measured by transcranial Doppler, together with blood pressure (BP) and heart rate (HR) during a tilt table test. Cerebrovascular reserve (CVR) was measured by an acetazolamide test. In all cases, FVmean dropped to a lower level (controls: 81.9 +/- 9.4% of baseline; patients: 84.3 +/- 7.9% symptomatic side, 85.6 +/- 9.0% contralateral). Impaired CVR patients showed a smaller (p < 0.01) decrease (90.6 +/- 3.3%) compared to contralateral (84.9 +/- 6.0%), to normal CVR patients (81.1 +/- 7.8%) and to controls. Heart rate increased in both groups (controls: +16.6 +/- 9.9%, patients +10.3 +/- 9.9%; p < 0.01); BP showed no change. Orthostasis induces a decrease of MCA FVmean as already previously described. This decrease is significantly smaller in patients with impaired CVR. Since BP does not change, some authors explain the lower MCA Fvmean during orthostasis as caused by sympathetic induced vasoconstriction of cerebral resistance vessels. The authors speculate that in impaired CVR-patients autoregulative protection against ischemia might limit vasoconstriction. In combination with standard tests for measurement of CVR, this test might be useful for evaluation of cerebral autoregulation.

Cerebrovascular regulation in the postural orthostatic tachycardia syndrome (POTS)

Patients with the postural orthostatic tachycardia syndrome (POTS) have symptoms of orthostatic intolerance despite having a normal orthostatic blood pressure (BP), which suggests some impairment of cerebrovascular regulation. Cerebrovascular autoregulation refers to the maintenance of normal cerebral blood flow in spite of changing BP. Mechanisms of autoregulation include myogenic, metabolic and neurogenic vasoregulation. Beat-to-beat recording of blood-flow velocity (BFV) is possible using transcranial Doppler imaging. It is possible to evaluate autoregulation by regressing deltaBFV to deltaBP during head-up tilt. A number of dynamic methods, relating deltaBFV to deltaBP during sudden induced changes in BP by occluding then releasing peripheral arterial flow or by the Valsalva maneuver. The deltaBFV to deltaBP provides an index of autoregulation. In orthostatic hypotension, the autoregulated range is typically expanded. In contrast, paradoxical vasoconstriction occurs in POTS because of an increased depth of respiration, resulting in hypocapnic cerebrovascular constriction, and impaired autoregulation.

Vasomotion and blood flow regulation in hamster skeletal muscle microcirculation: A theoretical and experimental study

A mathematical model of a microvasculature was used to study the effects of myogenic and flow-dependent stimuli on the characteristics of vasomotion and microvascular perfusion regulation. The model includes three branching orders of arterioles derived from in vivo observations and incorporates a mechanism for terminal arteriolar closure during vasomotion. Simulations were performed to evaluate the effect of vasodilation and vasoconstriction on vasomotion pattern, and the changes in arteriolar effective diameter and flow in response to arterial blood pressure variations triggering the regulatory mechanisms. Vasomotion patterns were studied in the hamster cutaneous muscle, visualized by fluorescent microscopy, in control conditions and after injection of acetylcholine (Ach) or NG-monomethyl-L-arginine (L-NMMA). We have found that vasomotion may be caused by different combinations of feedback mechanisms, including a strong rate-dependent myogenic response or a strong flow-dependent mechanism with no rate-dependent response. Decreasing the rate-dependent component of the myogenic mechanism and increasing the time constant of the flow-dependent mechanism causes vessel stabilization and disappearance of vasomotion. In hamster microcirculation, Ach decreased vasomotion frequency and increased vasomotion amplitude and arteriolar effective diameter, whereas L-NMMA caused a slight increase in vasomotion frequency and decrease in effective diameter. Model simulations, under dilatory and constrictory stimuli, confirmed these results. Moreover, the model predicted that mean blood flow is maintained closer to normal despite arterial pressure changes (+/-15% flow changes versus +/-50% pressure variations) when the vessels were in nonoscillatory than when they are in oscillatory state. In conclusion, a large variety of vasomotion patterns affect arteriolar resistance and microvessel perfusion in skeletal muscle. Furthermore, in the presence of vasomotion the network exhibits a poorer aptitude for regulating blood flow during arterial pressure changes (i.e., worse autoregulation) than the nonoscillatory network.

Ca2+-dependence and nifedipine-sensitivity of vascular tone and contractility in the isolated superfused spiral modiolar artery in vitro

The regulation of the vascular diameter of the spiral modiolar artery may play a major role in the regulation of cochlear blood flow and tissue oxygenation since the spiral modiolar artery provides the main blood supply to the cochlea. The goal of the present study was to determine whether vascular tone and contractility of the spiral modiolar artery depend on the presence of extracellular Ca2+ and involves nifedipine-sensitive Ca2+ channels. The spiral modiolar artery was isolated and superfused in vitro and the diameter was measured continuously by video microscopy. Isolated segments of the spiral modiolar artery had an outer diameter of 61 +/- 3 microm (n = 59) and displayed vasomotion characterized by 5-15 clearly distinguishable constrictions per min. Removal of Ca2+ from the superfusion medium caused a reversible relaxation and cessation of vasomotion and was used to determine the magnitude of basal vascular tone. The basal vascular tone consisted of a sustained reduction of the vascular diameter to 95.1 +/- 0.3% (n = 51) of the maximal diameter in Ca2+-free medium. Nifedipine reduced the basal vascular tone with an IC50 of (1.1 +/- 0.3) x 10(-9)) M although 22% of the basal vascular tone was insensitive to nifedipine. Elevation of the K+ concentration from 3.6 to 150 mM caused a transient vasoconstriction which was dependent on the presence of extracellular Ca2+. Nifedipine fully inhibited K+-induced vasoconstriction with an IC50 of (2.0 +/- 0.7) x 10(-9) M. Norepinephrine (10(-4) M) caused a transient vasoconstriction and an increase of vasomotion at branch points of the spiral modiolar artery. Norepinephrine-induced vasoconstriction was fully inhibited in the absence of Ca2+ and partially inhibited by 10(-7) M nifedipine. These observations suggest that the spiral modiolar artery contains voltage-dependent nifedipine-sensitive Ca2+ channels which are involved in the maintenance of basal vascular tone as well as in the mediation of K+- and norepinephrine-induced contractility. Further, the data suggest that cytosolic Ca2+ stores, if present in the spiral modiolar artery, are of limited capacity compared to other vessels.