Orthostatic changes of cerebral blood flow velocity in patients with autonomic dysfunction

The role of the autonomic nervous system in cerebral blood flow regulation in stroke: A review

Stroke is a pathophysiological condition which results in alterations in cerebral blood flow (CBF). The mechanism by which the brain maintains adequate CBF in presence of fluctuating cerebral perfusion pressure (CPP) is known as cerebral autoregulation (CA). Disturbances in CA may be influenced by a number of physiological pathways including the autonomic nervous system (ANS). The cerebrovascular system is innervated by adrenergic and cholinergic nerve fibers. The role of the ANS in regulating CBF is widely disputed owing to several factors including the complexity of the ANS and cerebrovascular interactions, limitations to measurements, variation in methods to assess the ANS in relation to CBF as well as experimental approaches that can or cannot provide insight into the sympathetic control of CBF. CA is known to be impaired in stroke however the number of studies investigating the mechanisms by which this occurs are limited. This literature review will focus on highlighting the assessment of the ANS and CBF via indices derived from the analyses of heart rate variability (HRV), and baroreflex sensitivity (BRS), and providing a summary of both clinical and animal model studies investigating the role of the ANS in influencing CA in stroke. Understanding the mechanisms by which the ANS influences CBF in stroke patients may provide the foundation for novel therapeutic approaches to improve functional outcomes in stroke patients.

Pubertal Hormonal Changes and the Autonomic Nervous System: Potential Role in Pediatric Orthostatic Intolerance

Puberty is initiated by hormonal changes in the adolescent body that trigger physical and behavioral changes to reach adult maturation. As these changes occur, some adolescents experience concerning pubertal symptoms that are associated with dysfunction of the autonomic nervous system (ANS). Vasovagal syncope (VVS) and Postural Orthostatic Tachycardia Syndrome (POTS) are common disorders of the ANS associated with puberty that are related to orthostatic intolerance and share similar symptoms. Compared to young males, young females have decreased orthostatic tolerance and a higher incidence of VVS and POTS. As puberty is linked to changes in specific sex and non-sex hormones, and hormonal therapy sometimes improves orthostatic symptoms in female VVS patients, it is possible that pubertal hormones play a role in the increased susceptibility of young females to autonomic dysfunction. The purpose of this paper is to review the key hormonal changes associated with female puberty, their effects on the ANS, and their potential role in predisposing some adolescent females to cardiovascular autonomic dysfunctions such as VVS and POTS. Increases in pubertal hormones such as estrogen, thyroid hormones, growth hormone, insulin, and insulin-like growth factor-1 promote vasodilatation and decrease blood volume. This may be exacerbated by higher levels of progesterone, which suppresses catecholamine secretion and sympathetic outflow. Abnormal heart rate increases in POTS patients may be exacerbated by pubertal increases in leptin, insulin, and thyroid hormones acting to increase sympathetic nervous system activity and/or catecholamine levels. Given the coincidental timing of female pubertal hormone surges and adolescent onset of VVS and POTS in young women, coupled with the known roles of these hormones in modulating cardiovascular homeostasis, it is likely that female pubertal hormones play a role in predisposing females to VVS and POTS during puberty. Further research is necessary to confirm the effects of female pubertal hormones on autonomic function, and their role in pubertal autonomic disorders such as VVS and POTS, in order to inform the treatment and management of these debilitating disorders.

Pressure Autoregulation Measurement Techniques in Adult Traumatic Brain Injury, Part I: A Scoping Review of Intermittent/Semi-Intermittent Methods

The purpose of this study was to perform a systematic, scoping review of commonly described intermittent/semi-intermittent autoregulation measurement techniques in adult traumatic brain injury (TBI). Nine separate systematic reviews were conducted for each intermittent technique: computed tomographic perfusion (CTP)/Xenon-CT (Xe-CT), positron emission tomography (PET), magnetic resonance imaging (MRI), arteriovenous difference in oxygen (AVDO₂) technique, thigh cuff deflation technique (TCDT), transient hyperemic response test (THRT), orthostatic hypotension test (OHT), mean flow index (Mx), and transfer function autoregulation index (TF-ARI). MEDLINE^®, BIOSIS, EMBASE, Global Health, Scopus, Cochrane Library (inception to December 2016), and reference lists of relevant articles were searched. A two tier filter of references was conducted. The total number of articles utilizing each of the nine searched techniques for intermittent/semi-intermittent autoregulation techniques in adult TBI were: CTP/Xe-CT (10), PET (6), MRI (0), AVDO₂ (10), ARI-based TCDT (9), THRT (6), OHT (3), Mx (17), and TF-ARI (6). The premise behind all of the intermittent techniques is manipulation of systemic blood pressure/blood volume via either chemical (such as vasopressors) or mechanical (such as thigh cuffs or carotid compression) means. Exceptionally, Mx and TF-ARI are based on spontaneous fluctuations of cerebral perfusion pressure (CPP) or mean arterial pressure (MAP). The method for assessing the cerebral circulation during these manipulations varies, with both imaging-based techniques and TCD utilized. Despite the limited literature for intermittent/semi-intermittent techniques in adult TBI (minus Mx), it is important to acknowledge the availability of such tests. They have provided fundamental insight into human autoregulatory capacity, leading to the development of continuous and more commonly applied techniques in the intensive care unit (ICU). Numerous methods of intermittent/semi-intermittent pressure autoregulation assessment in adult TBI exist, including: CTP/Xe-CT, PET, AVDO₂ technique, TCDT-based ARI, THRT, OHT, Mx, and TF-ARI. MRI-based techniques in adult TBI are yet to be described, with the main focus of MRI techniques on metabolic-based cerebrovascular reactivity (CVR) and not pressure-based autoregulation.

Cerebral oxygenation monitoring for detecting critical cerebral hypoperfusion in patients with multiple system atrophy during the head-up tilt test

OBJECTIVE

Near infrared spectroscopy (NIRS) is a non-invasive optical technique to monitor cerebral tissue oxygen saturation (ScO(2)). The purpose of this study was to reveal the usefulness of ScO(2) monitoring in evaluating cerebral circulation in patients with autonomic failure.

METHODS

Nineteen patients with multiple system atrophy (MSA), who had autonomic failure, and 10 age-matched normal control subjects participated. In addition to blood pressure monitoring, ScO(2) was recorded by a near-infrared spectroscopy instrument during head-up tilt (HUT) test.

RESULTS

HUT tests induced postural symptoms in 9 MSA patients (presyncopal group), but not in 10 MSA patients (non-presyncopal group) or in any of the controls. ScO(2), which decreased slightly in the controls and MSA patients, did not differ significantly between the MSA and control groups. With regard to MSA subgroups, the ScO(2) reduction in the presyncopal group (-3.1+/-1.7%) was significantly larger than in the non-presyncopal group (-0.9+/-0.5%, P<0.005) and controls (-1.1+/-1.0%, P<0.05). The systolic blood pressure decreases during HUT in the non-presyncopal (-35.2+/-16.1 mmHg, P<0.01) and presyncopal (-54.3+/-27.5 mmHg, P<0.0005) groups were significantly greater than that in the control group (4.0+/-10.7 mmHg), but the difference between presyncopal and non-presyncopal groups was not significant.

CONCLUSION

In our study, ScO(2) reduction seemed to be associated with presyncopal symptoms. ScO(2) monitoring may be useful to detect cerebral hypoperfusion in MSA patients with autonomic failure.

Comment on “Changing cerebral blood flow velocity by transcranial Doppler during head up tilt in patients with diabetes mellitus” by Asil et al

Transcranial Doppler is often employed for the assessment of cerebral autoregulation. The study by Asil et al. [Asil T, Utku U, Balci K, Uzunca I. Changing cerebral blood flow velocity by transcranial Doppler during head up tilt in patients with diabetes mellitus. Clin Neurol Neurosurg 2007;109:1-6] raises some important issues regarding the better understanding of the influence of autonomic nervous system in cerebral autoregulation. We suggest including some additional parameters, especially carbon dioxide levels, transcranial Doppler waveform patterns and their characteristics to further elucidate cerebral hemodynamics and the mechanisms of cerebral autoregulation. Vasomotor reactivity testing under controlled circumstances may add to the quantification of cerebral autoregulation.

Cerebral blood flow and oxygenation during head-up tilt in patients with multiple system atrophy and healthy control subjects

To assess cerebral hemodynamics in patients with multiple system atrophy (MSA), cerebral blood flow and oxygenation were evaluated in 7 MSA patients and 9 healthy controls during a head-up tilt test (HUT) by means of transcranial Doppler ultrasonography and near-infrared spectrophotometry. In the MSA patients examined, the perfusion pressure reduction during HUT was marked, but severe reduction in blood flow velocity was prevented because of a decrease in cerebrovascular resistance. The MSA patients showed no severe reduction in cerebral oxygenation during HUT. These findings indicate that our MSA patients exhibited a compensatory cerebral vasodilatation response to orthostatic hypotension.

Compromised Autoregulatory Control of Ocular Hemodynamics in Glaucoma Patients after Postural Change

PURPOSE

The autoregulatory control of retrobulbar blood flow in response to postural challenge was investigated in normal-tension glaucoma (NTG) patients in comparison with primary open-angle glaucoma (POAG) patients and healthy volunteers.

DESIGN

Prospective cohort study.

PARTICIPANTS AND CONTROLS

Twenty POAG patients, 20 NTG patients, and 20 control subjects.

METHODS

Peak systolic velocity (PSV), end diastolic velocity (EDV), and resistivity index (RI) in the short posterior ciliary artery (SPCA), central retinal artery (CRA) and ophthalmic artery (OA) were recorded after a change from sitting upright to a supine body position using color Doppler imaging.

MAIN OUTCOME MEASURES

Peak systolic velocity, EDV, and RI.

RESULTS

Ten minutes after postural change to a supine position, blood flow velocities in the SPCA remained unchanged in controls, whereas a significant increase of PSV and EDV was found in both glaucoma groups. The RI in the SPCA was significantly lowered in the NTG group. Recordings for the OA and CRA showed a significant increase in EDV and significant decrease in RI in all 3 groups; a significant increase in PSV in the CRA was detected only in the NTG group.

CONCLUSIONS

The unaltered flow velocities in the SPCA of healthy controls may indicate tight autoregulatory control, whereas the flow velocities in the CRA and OA appeared to follow alterations in hydrostatic pressure. In contrast, NTG and POAG patients demonstrated an insufficient compensatory response to postural change, leading to accelerated flow in the SPCA. This compromised autoregulatory control could represent another contributing factor in the pathogenesis of glaucoma.

Continuous progression of orthostatic tachycardia as a further feature of the postural tachycardia syndrome

BACKGROUND

The clinical diagnosis of the postural tachycardia syndrome (POTS) includes the demonstration of an upright heart rate (HR) of at least 30 beats per minute (bpm) above supine HR. The dynamic behavior of HR during the course of standing has not yet been studied systematically in POTS.

METHODS

HR and arterial blood pressure (ABP) were continuously monitored in 17 POTS patients and in 24 age-matched controls at rest and during an 11-minute phase of 80 degrees tilt.

RESULTS

ABP values at different time intervals of the protocol did not differ between the subgroups with the exception of higher diastolic pressures in POTS after 5 and 10 minutes of tilt. POTS patients showed a higher resting HR (80.6 +/- 17.0 bpm vs 67.8 +/- 10.9 bpm in controls, P < 0.05) and there was a continuous HR acceleration in the course of the 11-minute tilt phase. In control subjects, the tilt-induced HR increase was nearly completed after 1 minute with only a minimal further rise between minute 1 and minute 10 (from 83.7 +/- 11.5 to 85.3 +/- 11.9 bpm vs from 106.1 +/- 15.6 to 120.1 +/- 13.8 bpm in POTS).

CONCLUSIONS

Continuously progressing orthostatic tachycardia can serve as an additional criterion in the diagnosis of POTS. It may be related to the recently observed increased orthostatic capillary filtration rate in POTS.

Effect of Generalized Sympathetic Activation by Cold Pressor Test on Cerebral Hemodynamics in Diabetics with Autonomic Dysfunction

BACKGROUND

We examined the effects of the cold pressor test on the cerebral circulation in diabetics with autonomic dysfunction without orthostatic hypotension using transcranial Doppler.

METHODS

Twenty diabetics with autonomic dysfunction and 19 age-matched healthy controls participated in the study. The mean arterial blood velocity was measured in the middle cerebral artery during the cold pressor test together with the mean arterial blood pressure.

RESULTS

The mean arterial blood velocity significantly (p < 0.01) increased during the 1st, 2nd, and 3rd min of the cold pressor test by 10.6, 14.1, and 13.4%, respectively, in the control subjects and by 5.8, 7.2, and 6.8%, respectively, in the diabetics. Simultaneously, the mean arterial blood pressure significantly (p < 0.01) increased by 12, 26, and 23%, respectively, in the controls and by 9.4, 12.4 and 12.9%, respectively, in the diabetics. The increases in the mean arterial velocity as well as in the mean arterial blood pressure were significantly higher in the controls than in the diabetics (p < 0.01). The change in the mean arterial blood pressure related significantly to the change in the mean arterial blood velocity both in the controls (p < 0.01, r = 0.76) and in the diabetics (p < 0.01; r = 0.59). The slope of the regression line was significantly steeper in the controls (b = 0.42, SE = 0.05) as compared with the diabetics with autonomic dysfunction (b = 0.27, SE = 0.05; p = 0.02). Moreover, also the relative increase in the cerebrovascular resistance index was higher in the controls than in the diabetics (p < 0.05).

CONCLUSION

These findings in the diabetics with autonomic neuropathy, but without orthostatic hypotension, suggest a failure in the cerebral autoregulation due to impaired cerebrovascular neurogenic control.

Orthostatic posture affects brain hemodynamics and metabolism in cerebrovascular disease patients with and without coronary artery disease: a positron emission tomography study

To investigate whether a physiological change in the orthostatic condition is associated with a deterioration of cerebrovascular and metabolic homeostasis in patients with neurocardiovascular compromises, we examined 10 patients with unilateral carotid artery occlusive disease (CVD), 6 CVD patients with coronary artery disease (CVDC), and 10 healthy subjects scanned twice under supine and sitting conditions by positron emission tomography (PET). Repeated measures analysis of variance showed significant reductions in regional cerebral blood flow (rCBF) and cerebral oxygen metabolism (rCMRO2) and tendency of increase in oxygen extraction fraction (OEF) in the affected-side parietal cortex during assuming of upright posture in the CVDC group, and there was a significant OEF increase to maintain rCMRO2 constant during sitting in the CVD counterpart. In this ischemic region, there were negative correlations between changes in OEF and rCBF in the CVD (P < 0.05) and CVDC groups (P < 0.01). Postural reductions in rCBF and CMRO2 in the parietal region were significantly greater in the CVDC group than those in the CVD group. While rCBF remained constant with mean arterial blood pressure (MABP) in healthy subjects, an rCBF reduction was found in the affected parietal cortex in proportion to the upright posture-induced MABP decrease in the CVDC group. These results indicate that patients suffering from both cerebral and coronary artery diseases may be at greater risk of deterioration of local perfusion pressure and metabolic regulation in the hemodynamically susceptible brain region during upright posture.

Murine orthostatic response during prolonged vertical studies: Effect on cerebral blood flow measured by arterial spin-labeled MRI

High-field MRI scanners are, in principle, well suited for mouse studies; however, many high-field magnets employ a vertical design that may influence the physiological state of the rodent. The purpose of this study was to investigate the orthostatic response of cerebral blood flow (CBF) in mice during a prolonged MR experiment in the vertical position. Arterial spin-labeled (ASL) MRI was performed at 4.7-Tesla with a 15-cm gradient insert that allowed horizontal and vertical CBF measurements to be obtained with the same scanner. For mice in the head-up (HU) vertical position, CBF decreased by approximately 40% compared to the horizontal position, although blood pressure did not differ. Furthermore, CBF values for vertically positioned mice treated with phenylephrine remained constant while blood pressure increased. These results support the conclusion that cerebral autoregulation was intact, albeit at a lower level. Since CBF recovers to near horizontal values by volume loading with saline, it appears that a decrease in central venous pressure (CVP) leading to an increase in sympathetic tone may be a contributing mechanism for lowered CBF. This suggests that using an HU vertical position for MRI in mice may have broader implications, especially for studies that rely on CBF (such as BOLD and fMRI).

Tilt-table test during transcranial Doppler monitoring in Parkinson's disease

Cardiovascular autonomic dysfunction can occur in Parkinson's disease (PD) and intracranial vascular modifications following orthostatism may be relevant to diagnostic and therapeutic decision-making. We performed transcranial Doppler monitoring of right middle cerebral artery (MCA) at rest and during passive 70 degrees tilt in 19 patients with idiopathic PD and in 19 age-matched normal controls. Brachial arterial blood pressure (systolic, diastolic and mean), cardiac frequency (CF), respiratory frequency and mean velocity (MV) of the MCA were recorded after 10 min of rest in supine position, and each minute during 9 min of tilting and 5 min of restored clinostatic position. The pulsatility and cerebrovascular resistances (CVR) indexes were calculated. At rest there was no significant difference in blood pressure, CF, respiratory frequency and MCA mean velocity between patients and controls. During tilt test, PD patients showed a trend to higher pulsatility index values (p=0.09) and significant lower diastolic blood pressure (p=0.001), while there was no significant difference in CVR index. In conclusion, PD patients showed mild hypotensive response to orthostatic stress, with intracranial compensatory vasodilation. Our findings suggest a preserved intracerebral autoregulation in PD without symptoms of orthostatic intolerance.

The middle cerebral artery flow velocities during head-up tilt testing in diabetic patients with autonomic nervous system dysfunction

BACKGROUND

The goal of this study was to examine the effects of diabetes mellitus on the trend of mean arterial velocity (v(m)) in both middle cerebral arteries during head-up tilt (HUT).

METHODS

The study was performed in 20 patients, 9 females and 11 males (mean age 51 +/- 12 years) with an average 17-year history of insulin-dependent diabetes mellitus type I or II and a dysfunction of the autonomic nervous system confirmed by cardiocirculatory tests [Valsalva maneuver, deep breathing test, handgrip test, orthostatic test and spectral analysis of heart rate (HR) variability], and 19 age-matched healthy volunteers, 9 females and 10 males (mean age 48 +/- 6.8 years). v(m) was measured by a transcranial Doppler monitoring system during a 5-min baseline period, followed by a 5-min HUT in the upright position (90 degrees ). Mean arterial blood pressure (MAP), HR and end-tidal CO(2) (Et-CO(2)) were monitored concomitantly.

RESULTS

In healthy volunteers, v(m) decreased stepwise during the first minute of HUT, reaching a minimum during the last 2 min of the test (v(m): basal 63.0 +/- 11.7 cm/s, 1st min 57.6 +/- 12.2 cm/s, 2nd min 55.9 +/- 12.6 cm/s, 3rd min 53.4 +/- 12.6 cm/s, 4th min 52.1 +/- 12.7 cm/s, 5th min 51.3 +/- 13.5 cm/s). In the supine position, v(m) recovered and reached the resting v(m) values. It declined gradually during HUT and less steeply in diabetic (v(m): basal 54.4 +/- 10.1 cm/s, 1st min 51.96 +/- 9.3 cm/s, 2nd min 50.7 +/- 11.6 cm/s, 3rd min 50.5 +/- 11.4 cm/s, 4th min 49.5 +/- 10.7 cm/s, 5th min 48.8 +/- 11.5 cm/s) than in healthy subjects. v(m) differed significantly (p = 0.00) between rest and HUT in both groups. The differences in MAP, HR and Et-CO(2) during rest and HUT between the groups were not statistically significant (p DeltaMAP = 0.36, p DeltaHR = 0.86, p DeltaEt-CO(2) = 0.97). The results of the analysis of variance of v(m) for repeated measurements between the two groups of subjects were highly significant (p = 0.00). The model of linear regression analysis was significant (p = 0.007). Diabetes was significant in the model (p = 0.00), while DeltaMAP, DeltaHR and DeltaEt-CO(2) were not.

CONCLUSIONS

These findings may indicate that vasomotor responses during HUT testing are decreased in diabetic patients.

Cerebral vasomotor reactivity testing in head injury: the link between pressure and flow

BACKGROUND

It has been suggested that a moving correlation index between mean arterial blood pressure and intracranial pressure, called PRx, can be used to monitor and quantify cerebral vasomotor reactivity in patients with head injury.

OBJECTIVES

To validate this index and study its relation with cerebral blood flow velocity and cerebral autoregulation; and to identify variables associated with impairment or preservation of cerebral vasomotor reactivity.

METHODS

The PRx was validated in a prospective study of 40 head injured patients. A PRx value of less than 0.3 indicates intact cerebral vasomotor reactivity, and a value of more than 0.3, impaired reactivity. Arterial blood pressure, intracranial pressure, mean cerebral perfusion pressure, and cerebral blood flow velocity, measured bilaterally with transcranial Doppler ultrasound, were recorded. Dynamic cerebrovascular autoregulation was measured using a moving correlation coefficient between arterial blood pressure and cerebral blood flow velocity, the Mx, for each cerebral hemisphere. All variables were compared in patients with intact and impaired cerebral vasomotor reactivity.

RESULTS

No correlation between arterial blood pressure or cerebral perfusion pressure and cerebral blood flow velocity was seen in 19 patients with intact cerebral vasomotor reactivity. In contrast, the correlation between these variables was significant in 21 patients with impaired cerebral vasomotor reactivity, whose cerebral autoregulation was reduced. There was no correlation with intracranial pressure, arterial blood pressure, cerebral perfusion pressure, or interhemispheric cerebral autoregulation differences, but the values for these indices were largely within normal limits.

CONCLUSIONS

The PRx is valid for monitoring and quantifying cerebral vasomotor reactivity in patients with head injury. This intracranial pressure based index reflects changes in cerebral blood flow and cerebral autoregulatory capacity, suggesting a close link between blood flow and intracranial pressure in head injured patients. This explains why increases in arterial blood pressure and cerebral perfusion pressure may be useful for reducing intracranial pressure in selected head injured patients (those with intact cerebral vasomotor reactivity).

Impairment of cerebral autoregulation in diabetic patients with cardiovascular autonomic neuropathy and orthostatic hypotension

AIMS

Impaired cerebrovascular reactivity and autoregulation has been previously reported in patients with diabetes mellitus. However, the contribution of cardiovascular diabetic autonomic neuropathy and orthostatic hypotension to the pathogenesis of such disturbances is not known. The purpose of this study was to evaluate cerebral blood flow velocity in response to standing in patients with diabetes and cardiovascular autonomic neuropathy with or without orthostatic hypotension.

METHODS

We studied 27 patients with diabetes--eight had cardiovascular autonomic neuropathy and orthostatic hypotension (age 46.4 +/- 13.5 years, diabetes duration 25.0 +/- 11.0 years), seven had autonomic neuropathy without hypotension (age 47.3 +/- 12.7 years, diabetes duration 26.4 +/- 12.1 years), and 12 had no evidence of autonomic neuropathy (age 44.1 +/- 13.8 years, diabetes duration 17.1 +/- 10.2 years)-and 12 control subjects (age 42.6 +/- 9.7 years). Flow velocity was recorded in the right middle cerebral artery using transcranial Doppler sonography in the supine position and after active standing.

RESULTS

Cerebral flow velocity in the supine position was not different between the groups studied. Active standing resulted in a significant drop of mean and diastolic flow velocities in autonomic neuropathy patients with orthostatic hypotension, while there were no such changes in the other groups. The relative changes in mean flow velocity 1 min after standing up were -22.7 +/- 16.25% in patients with neuropathy and orthostatic hypotension, +0.02 +/- 9.8% in those with neuropathy without hypotension, -2.8 +/- 14.05% in patients without neuropathy, and -9.2 +/- 15.1% in controls.

CONCLUSIONS

Patients with diabetes and cardiovascular autonomic neuropathy with orthostatic hypotension show instability in cerebral blood flow upon active standing, which suggests impaired cerebral autoregulation.

Continuous cerebral autoregulation monitoring by cross-correlation analysis

In order to validate cross-correlation analysis between spontaneous slow oscillations of arterial blood pressure (aBP) and intracranial pressure (ICP) or flow velocity as a means to assess the status of cerebral autoregulation continuously, we compared its results with different autoregulation bedside tests. The second aim was to check the method's stability over longer time periods. aBP, ICP, and flow velocity in the middle cerebral artery (FV(MCA)) was measured continuously in 13 critically ill comatose patients. Cross-correlation analysis was performed online and offline between aBP and ICP (CC [aBP --> ICP]) and aBP/FV(MCA) (CC [aBP --> FV(MCA)]). Three different autoregulation bedside tests (cuff deflation, transient hyperemic response, orthostatic hypotension) were performed immediately before a 29-min cross-correlation test period. In addition, continuous cross-correlation autoregulation monitoring was performed over multiple hours (in order to analyze for stability and to assess the influence of other factors). Cluster analysis revealed two main clusters. Cluster 1 (indicative for disturbed autoregulation) showed a centroid at t = -0.21 +/- 3.32 sec, r = 0.43 +/- 0.18 for CC [aBP --> ICP], and t = 0 +/- 3.14 sec, r = 0.44 +/- 0.18 for CC [aBP --> FV(MCA)]. Cluster 2 (indicative for normal autoregulation) revealed a centroid at t = 4.94 +/- 3.74 sec, r =- 0.4 +/- 0.16 for CC [aBP --> ICP], and t = 3.38 +/- 4.44 sec, r = -0.38 +/- 0.18 for CC [aBP --> FV(MCA)]. Comparison between the cross-correlation test results and the bedside tests showed a sensitivity of 44-73% for CC [aBP --> FV(MCA)], whereas CC [aBP --> ICP] was more specific (60-80%). Long-term monitoring revealed stable cross-correlation tests in about 45% of the measurement time. It is concluded that cross-correlation between aBP, ICP, and FV(MCA) is a valid means to monitor the autoregulation status continuously, although further improvement of sensitivity and specificity is needed to make it reliable for clinical decision making.

Continuous cerebral autoregulation monitoring by cross-correlation analysis: evaluation in healthy volunteers

OBJECTIVE

In a former study, we applied cross-correlation (CC) analysis to recordings of arterial blood pressure (BP), intracranial pressure (ICP), and intracranial blood flow velocity (FV). A lack of significant time delay and a positive correlation coefficient of slow oscillations between these parameters was interpreted as indicative of impaired cerebral autoregulation, whereas a significant time delay and a negative correlation was regarded as preserved autoregulation. To test this hypothesis, cross-correlation was applied on recordings of BP and FV (CC [BP --> FV]) in healthy volunteers with a presumably preserved cerebral autoregulation.

DESIGN

Study of a diagnostic test.

SUBJECTS

A total of 17 healthy volunteers.

MEASUREMENTS AND MAIN RESULTS

BP was recorded by using a tonometric device, and bilateral FV in the middle cerebral arteries (MCA) was measured by transcranial Doppler sonography. Signals were sampled at a resting horizontal position for 29 mins. Cluster analysis showed a mean +/- sd time delay for CC [BP --> FV(MCA right)] of 6.45 +/- 2.1 secs, and for CC [BP --> FV(MCA left) ] of 6.09 +/- 1.8 secs. The mean correlation coefficient was -.33 +/-.17 for the left and -.36 +/-.09 for the right side. In about 30%, differing results with a correlation coefficient between -.2 and.2 and a time delay near zero were found. Cross-correlation between left and right FV showed a mean time delay of 0.09 +/- 0.18 secs, with a mean correlation coefficient of.82 +/-.16. CONCLUSION Spontaneous slow oscillations of BP and FV were detected, and cross-correlation analysis showed a negative correlation and a positive time delay in about 70% of the examinations. These findings corroborate the hypothesis that CC [BP --> FV] might be able to assess the status of cerebral autoregulation continuously. The observed time delay between BP and FV oscillations is in good agreement with former studies on the dynamic properties of cerebral autoregulation.

Normal cerebral hemodynamic response to orthostasis in Parkinson's disease

In patients with Idiopathic Parkinson's Disease (IPD) without a history of syncope the cardiovascular and cerebrovascular response to orthostatic stress was studied to search for subclinical impairment of autoregulatory mechanisms. Fifteen patients with IPD and 15 healthy age-matched controls were studied at rest and during head-up tilt (HUT). Heart rate, mean arterial blood pressure (MAP) and mean blood flow velocity (MBFV) in the middle cerebral artery were measured simultaneously. Changes of MAP and MBFV and the relationship between both were assessed. During HUT, heart rate increased less in patients than in healthy subjects (16.3% versus 24.2%, p=0.03). In the first minute of orthostasis MAP decreased more in patients than in healthy subjects (-4.0% versus -0.6%, p=0.04). MAP reached the pre-tilt values within 2 min in healthy subjects and 5 min in patients. Cerebral blood flow velocities fell to a similar degree and with similar time characteristics in patients and controls (-15.4% versus -16.7%, p=0.3). In both groups, patients and controls, changes of MAP did not correlate with changes of MBFV. It can be concluded that in IPD patients without symptoms of orthostatic dysregulation the autonomic circulatory control is impaired while the cerebral hemodynamic regulation during orthostasis is unaffected.

Dynamics of cerebral blood flow regulation explained using a lumped parameter model

The dynamic cerebral blood flow response to sudden hypotension during posture change is poorly understood. To better understand the cardiovascular response to hypotension, we used a windkessel model with two resistors and a capacitor to reproduce beat-to-beat changes in middle cerebral artery blood flow velocity (transcranial Doppler measurements) in response to arterial pressure changes measured in the finger (Finapres). The resistors represent lumped systemic and peripheral resistances in the cerebral vasculature, whereas the capacitor represents a lumped systemic compliance. Ten healthy young subjects were studied during posture change from sitting to standing. Dynamic variations of the peripheral and systemic resistances were extracted from the data on a beat-to-beat basis. The model shows an initial increase, followed approximately 10 s later by a decline in cerebrovascular resistance. The model also suggests that the initial increase in cerebrovascular resistance can explain the widening of the cerebral blood flow pulse observed in young subjects. This biphasic change in cerebrovascular resistance is consistent with an initial vasoconstriction, followed by cerebral autoregulatory vasodilation.

Postural effects on brain hemodynamics in unilateral cerebral artery occlusive disease: a positron emission tomography study

To investigate effect of assuming of upright posture on brain hemodynamics in patients with unilateral internal carotid or middle cerebral artery occlusion (MCAO), local tissue oxygen extraction fraction (OEF), and postural changes in regional cerebral blood flow (rCBF) during supine and sitting conditions were examined using positron emission tomography (PET) with (15)O-gas steady-state method and H(2)(15)O autoradiographic method. A total of 22 minor stroke patients at relatively early stages participated. The regions of interest method was used for analyzing levels of perfusion and oxygen metabolic parameters, and postural rCBF change within MCAO group was investigated using statistical parametric mapping. Region of interest analyses showed significant rCBF reduction in the cortical and subcortical regions distal to the artery occlusion in CAO patients during sitting. Regression analyses showed that magnitudes of rCBF reduction in those areas were correlated positively with OEF values and inversely with metabolic rates of oxygen (P < 0.05). Statistical parametric mapping for MCAO patients demonstrated further rCBF reduction by sitting in the occlusion-side MCA territory. The current study suggested that assumption of upright posture could exert an adverse effect on local perfusion in hemodynamically compromised patients with major cerebral vessel occlusion, possibly caused by impairment of local autoregulation.

Cerebrovascular reactivity and hypercapnic respiratory drive in diabetic autonomic neuropathy

Because abnormalities in cerebrovascular reactivity (CVR) in subjects with long-term diabetes could partly be ascribed to autonomic neuropathy and related to central chemosensitivity, CVR and the respiratory drive output during progressive hypercapnia were studied in 15 diabetic patients without (DAN-) and 30 with autonomic neuropathy (DAN+), of whom 15 had postural hypotension (PH) (DAN+PH+) and 15 did not (DAN+PH-), and in 15 control (C) subjects. During CO(2) rebreathing, changes in occlusion pressure and minute ventilation were assessed, and seven subjects in each group had simultaneous measurements of the middle cerebral artery mean blood velocity (MCAV) by transcranial Doppler. The respiratory output to CO(2) was greater in DAN+PH+ than in DAN+PH- and DAN- (P < 0.01), whereas a reduced chemosensitivity was found in DAN+PH- (P < 0.05 vs. C). MCAV increased linearly with the end-tidal PCO(2) (PET(CO(2))) in DAN+PH- but less than in C and DAN- (P < 0.01). In contrast, DAN+PH+ showed an exponential increment in MCAV with PET(CO(2)) mainly >55 Torr. Thus CVR was lower in DAN+ than in C at PET(CO(2)) <55 Torr (P < 0.01), whereas it was greater in DAN+PH+ than in DAN+PH- (P < 0.01) and DAN- (P < 0.05) at PET(CO(2)) >55 Torr. CVR and occlusion pressure during hypercapnia were correlated only in DAN+ (r = 0.91, P < 0.001). We conclude that, in diabetic patients with autonomic neuropathy, CVR to CO(2) is reduced or increased according to the severity of dysautonomy and intensity of stimulus and appears to modulate the hypercapnic respiratory drive.

Cerebral autoregulation testing after aneurysmal subarachnoid hemorrhage: the phase relationship between arterial blood pressure and cerebral blood flow velocity

OBJECTIVE

Impairment of cerebral autoregulation (CA) appears to be an important cause for secondary ischemia after subarachnoid hemorrhage (SAH). It has been shown that graded CA impairment is predictive of outcome. Little is known about whether such impairment is present, what causes CA impairment, whether it precedes vasospasm, and whether it is predictive of outcome in patients with severe aneurysmal SAH.

DESIGN

Prospective, controlled study.

SETTING

Neurosurgical intensive care unit.

PATIENTS

Twelve patients after aneurysmal subarachnoid hemorrhage, 40 controls.

INTERVENTIONS

Recording of cerebral blood flow velocities and continuous measurement of arterial blood pressure at a controlled ventilatory frequency of six per minute to standardize the influence of intrathoracic pressure changes on blood pressure.

MEASUREMENTS AND MAIN RESULTS

We calculated the phase shift angles (deltaphidegrees) between slow (0.1 Hz) arterial blood pressure and cerebral blood flow velocity waves measured by transcranial Doppler ultrasound in the middle cerebral artery during a) posthemorrhage days (PHD) 1-6 (early or prevasospasm phase), and b) during PHD 7-13 (late or vasospasm phase) using a 6/min ventilation protocol, and in 40 controls spontaneously ventilating at the same rate. deltaphi <30 degrees indicated lost CA. Mean flow velocities >100 cm/sec were considered vasospasm. We combined early and late measurements to assess the CA relationship with low cerebral perfusion pressure (CPP) and/or vasospasm. We assessed the Glasgow Outcome Scale (GOS) score at discharge (1 = worst, 5 = best). The admission Hunt and Hess score was 3.6 +/- 0.7. GOS scores were n = 3 (GOS 1), n = 2 (GOS 2), n = 5 (GOS 3), n = 1 (GOS 4), and n = 1 (GOS 5). In the early phase, deltaphi was 40.4 +/- 19.8 degrees (left), and 40.4 +/- 19.2 degrees (right). CPP was 69.4 +/- 10.9, intracranial pressure (ICP) was 6.7 +/- 2.8 mm Hg. In the late phase, deltaphi worsened in six patients and none improved: 32.1 +/- 21 degrees (left), and 26.9 +/- 17.2 degrees (right); CPP was 68.1 +/- 12.1, ICP was 7.5 +/- 3.7 mm Hg. CA was significantly impaired in both phases when compared with normal subjects (deltaphi: 65.7 +/- 24.5 degrees; p < .01 for early, p < .001 for late phase). In the early phase, seven of eight patients in whom autoregulation was intact had a GOS >2 at discharge and disturbed CA on at least one side was predictive of either vegetative condition at discharge or death (p < .01). In the late phase, deltaphi was no longer predictive of outcome. Spasm was present in 8 of 17 vessels (47%) in which CA was lost; no spasm was found in 25 of 28 vessels (89%) in which CA was intact (p < .01). A low CPP was present in 6 of 17 vessels (35%) in which CA was lost; a normal CPP was found in 21 of 27 vessels (78%) in which CA was intact (p > .05, NS). However, 14 of 17 vessels (82%) with lost CA showed spasm and/or low CPP while only 8 of 27 cases (30%) with intact CA had either spasm or low CPP (p < .001).

CONCLUSIONS

CA can be assessed in a graded fashion in SAH patients. CA impairment precedes vasospasm; ongoing vasospasm worsens CA. CA assessment early after subarachnoid hemorrhage, within PHD 1-6, is predictive of outcome whereas late assessment is not. CA impairment is associated with cerebral vasospasm and low CPP.

Transcranial doppler sonography-ergometer test for the non-invasive assessment of cerebrovascular autoregulation in humans

Cerebrovascular hemodynamics during physical stress have been sparsely investigated, mostly through risky invasive techniques. The aim of this study was to determine the effect of ergometer stress on cerebrovascular hemodynamics in humans using the non-invasive and thus clinically-applicable method of transcranial Doppler sonography (TCD) combined with simultaneous non-invasive measurements of cardiovascular parameters. In eighteen healthy subjects (six women, twelve men; 29.3+/-4.6 years old) left midcerebral artery blood flow velocities (CBFVs) were continuously monitored using TCD during 3 min at rest, 3 min during ergometry and 3 min recovery. Simultaneously, systolic, diastolic, mean CBFVs, pulsatility index (PI), heart rate, beat-to-beat blood pressure (BP) and transcutaneous p(CO(2)) were measured. The subjects were supine with elevated trunk. Ergometry was performed by pedalling a Mühe-ergometer. In eight volunteers, the procedure was repeated within the next day to test the repeatability of the results. Heart rate increased significantly during ergometry (from 65.2+/-11 to 105. 3+/-12.3/min; P<0.05). The systolic BP increased significantly slightly later during ergometry (from 118.9+/-8.6 to 141.6+17.9 mmHg; P<0.05). Transcutaneous p(CO(2)) was initially within physiological ranges, but increased significantly after a delay during the 3rd min of cycling (from 39.7+/-3.7 to 41.1+/-4.7 mmHg; P<0.05). MFV started to rise significantly after 1 min of the exercise period (from 59.6+10.9 to 68.3+13.9 cm/s; P<0.05). PI increased immediately and significantly at the start of exercise (PI at rest 0.93+0.11; PI ergometry 1.1+0.13; P<0.05). The results were found to be reproducible in the eight volunteers. The cerebrovascular changes during ergometer exercise may reflect the combined activation of the cerebrovascular autoregulative mechanisms (neurogenic, myogenic and metabolic). The TCD-ergometer test presented here is non-invasive and would seem to present a low risk for patients who are judged fit enough for mild exercise. The test may contribute to the detection of cerebrovascular abnormalities in various diseases.

Simultaneous assessment of brain tissue oxygenation and cerebral perfusion during orthostatic stress

Simultaneous registration of cerebral tissue oxygenation parameters obtained by near infrared spectroscopy (NIRS), intracranial blood flow velocity (CBFV) measured by transcranial Doppler sonography (TCD) and basic cardiovascular parameters was carried out during a passive 80 degrees head-up tilt table test in 15 patients with a history of orthostatic syncope and 20 control subjects. In normals, the cardiovascular parameters showed a specific course after changing to a vertical position: the heart rate increased, the mean arterial blood pressure remained unchanged, and the CBFV decreased. The NIRS measurements showed an increase in deoxyhemoglobin (HHb) and a decline in oxyhemoglobin (O(2)Hb) and the regional oxygen saturation (RSAT). Patients had a significantly more prominent decline in arterial blood pressure (p < 0.001), CBFV (p < 0.001) and RSAT (p = 0.04). Five patients experienced symptoms of (pre)syncope during the experiment, which were associated with a further sudden and marked (>10%) drop of O(2)Hb. The results indicate that the combination of TCD and NIRS increases the understanding of hemodynamic and metabolic changes during orthostatic stress, which may lead to individually suited therapeutic procedures.

Spectral analysis of arterial blood pressure and cerebral blood flow velocity during supine rest and orthostasis

This study evaluates the effect of orthostasis on the low frequency (LF, 0.04 to 0.15 Hz) fluctuations in the blood flow velocity of the middle cerebral artery (MCAFV) in relation to its arterial blood pressure (ABP) equivalent to further define and quantify this relationship in cerebrovascular regulation. Spectral analysis was performed on 22 healthy subjects during supine rest and head-up tilt. The power in the LF range can be used to quantify the LF fluctuations, and four types of LF power data could be obtained for each individual: LF power of supine MCAFV, LF power of supine ABP, LF power of tilt MCAFV, and LF power of tilt ABP. By comparing LF power of MCAFV with LF power of ABP, two power ratios could be generated to describe the flow-pressure relationship during supine rest and head-up tilt, respectively, supine power ratio (LF power of supine MCAFV/ LF power of supine ABP) and tilt power ratio (LF power of tilt MCAFV/ LF power of tilt ABP). In addition, an index for dynamic autoregulation in response to orthostasis can be calculated from these two power ratios (tilt power ratio/supine power ratio). The authors found that this index was dependent on the extent of orthostatic MCAFV changes, and the dependency could be mathematically expressed (r = 0.61, P = .0001), suggesting its involvement in cerebrovascular regulation. Moreover, these data further support the previous observation that the LF fluctuations of MCAFV might result from modulation of its ABP equivalent, and the modulation effect could be quantified as the power ratio (LF power of MCAFV/ LF power of ABP). These observations could be an important step toward further insight into cerebrovascular regulation, which warrants more research in the future.

Autonomic control of the cerebral circulation during normal and impaired peripheral circulatory control

OBJECTIVE

To determine whether oscillations in the cerebrovascular circulation undergo autonomic modulation in the same way as cardiovascular oscillations.

DESIGN

Cardiovascular and cerebrovascular oscillations were monitored at rest and during sympathetic stimulation (head up tilt). The association with and transmission of the oscillations in the sympathetic (low frequency, LF) and respiratory (high frequency, HF) bands was assessed.

SUBJECTS

13 healthy volunteers, 10 subjects with vasovagal syncope, and 12 patients with complicated non-insulin dependent diabetes mellitus.

MAIN OUTCOME MEASURES

Power spectrum analysis of cerebral blood flow velocity, arterial blood pressure, and heart rate. Coherence analysis was used to study the association between each pair of oscillations. Phase analysis showed the delay of the oscillations in the cardiovascular signals with respect to the cerebrovascular signals.

RESULTS

The power in the sympathetic (LF) components in all the oscillations increased during head up tilt (p < 0.01) in the controls and in the subjects with vasovagal syncope, but not in patients with diabetes. Significant coherence (> 0.5) in the LF band was present between cerebrovascular and cardiovascular oscillations in most of the controls and in subjects with vasovagal syncope, but not in the diabetic patients (< 50% of the patients). In the LF band, cerebrovascular oscillations preceded the cardiovascular oscillations (p < 0.05) at rest in all groups: the phase shifts were reduced (p < 0.05) during head up tilt for all cardiovascular signals in healthy and syncopal subjects, but only for heart rate in diabetic patients.

CONCLUSIONS

The cerebrovascular resistance vessels are subject to autonomic modulation; low frequency oscillations in cerebral blood flow velocity precede the resulting fluctuations in other cardiovascular signals. Autonomic neuropathy and microvascular stiffness in diabetic patients reduces this modulation.

Glaucoma patients demonstrate faulty autoregulation of ocular blood flow during posture change

BACKGROUND/AIMS

Autoregulation of blood flow during posture change is important to ensure consistent organ circulation. The purpose of this study was to compare the change in retrobulbar ocular blood flow in glaucoma patients with normal subjects during supine and upright posture.

METHODS

20 open angle glaucoma patients and 20 normal subjects, similar in age and sex distribution, were evaluated. Blood pressure, intraocular pressure, and retrobulbar blood velocity were tested after 30 minutes of sitting and again after 30 minutes of lying. Retrobulbar haemodynamic measures of peak systolic velocity (PSV), end diastolic velocity (EDV), and resistance index (RI) were obtained in the ophthalmic and central retinal arteries using colour Doppler imaging (CDI).

RESULTS

When changing from the upright to supine posture, normal subjects demonstrated a significant increase in OA EDV (p = 0.016) and significant decrease in OA RI (p = 0.0006) and CRA RI (p = 0.016). Glaucoma patients demonstrated similar changes in OA measures of EDV (p = 0.02) and RI (p = 0.04), but no change in CRA measures.

CONCLUSION

Glaucoma patients exhibit faulty autoregulation of central retinal artery blood flow during posture change.

Multimodality monitoring during passive tilt and Valsalva maneuver under hypercapnia

In occlusive cerebrovascular disease cerebral blood flow (CBF) autoregulation can be impaired and constant CBF during fluctuations in blood pressure (BP) cannot be guaranteed. Therefore, an assessment of cerebral autoregulation should consider not only responsiveness to CO2 or Diamox. Passive tilting (PT) and Valsalva maneuver (VM) are established tests for cardiovascular autoregulatory function by provoking BP changes. To develop a comprehensive test for vasomotor reactivity with a potential increase of sensitivity and specificity, the authors combined these maneuvers. Blood pressure, corrected to represent arterial pressure at the level of the circle of Willis, middle cerebral artery Doppler frequencies (DF), heart rate (HR) and endtidal partial pressure of CO2 (PtCO2) were measured continuously and noninvasively in 81 healthy subjects (19-74 years). Passive tilt and Valsalva maneuver were performed under normocapnia (mean, 39 + 4 mmHg CO2) and under hypercapnia (mean, 51 + 5 mm Hg CO2). Resting BP, HR, and DF increased significantly under hypercapnia. Under normocapnia and hypercapnia, PT induced only minor, nonsignificant changes in mean BP at the level of the circle of Willis compared to baseline (normocapnia: + 2 + 15 mm Hg; hypercapnia: -3 +/- 13 mm Hg). This corresponded with a nonsignificant decrease of the mean of DF (normocapnia: -4 +/- 11%; hypercapnia -6 +/- 12%). Orthostasis reduced pulsatility of BP by a predominantly diastolic increase of BP without significant changes in pulsatility of DF. Valsalva maneuver, with its characteristic rapid changes of BP due to elevated intrathoracic pressure, showed no significant BP differences in changes to baseline between normocapnic and hypercapnic conditions. Under both conditions the decrease in BP in phase II was accompanied by significantly increased pulsatility index ratio (PIDF/PIBP). Valsalva maneuver and PT as established tests in autonomic control of circulation provoked not only changes in time-mean of BP but also in pulsatility of BP. The significant increase in pulsatility ratio and decrease of the DF/BP ratio during normocapnia and hypercapnia indicated preserved CBF autoregulation within a wide range of CO2 partial pressures. Hypercapnia did not significantly influence the autoregulatory indices during VM and PT. Physiologically submaximally dilated cerebral arterioles can guarantee unchanged dynamics of cerebral autoregulation. Combined BP and MCA-DF assessment under hypercapnia enables investigating the effect of rapid changes of blood pressure on CO2-induced predilated cerebral arterioles. Assuming no interference of hypercapnia-induced vasodilation, VM, with its rapid, distinct changes in BP, seems especially to be adequate provocation for CBF autoregulation. This combined vasomotor reactivity might provide a more sensitive diagnostic tool to detect impaired cerebral autoregulation very early.

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.

Cerebrovascular and cardiovascular measurements during neurally mediated syncope induced by head-up tilt

BACKGROUND AND PURPOSE

This study examines changes in systemic hemodynamics and in cerebral blood velocity that occur during neurally mediated syncope (NMS) to determine whether cerebral autoregulation is intact or impaired in patients with recurrent NMS.

METHODS

Beat-to-beat recordings of heart rate, blood pressure (volume clamp photoplethysmography), stroke volume (impedance cardiography), and right middle cerebral artery blood velocity (transcranial Doppler sonography) were performed at rest and during 80 degrees head-up tilt. Twelve patients with NMS and 10 healthy control subjects were studied.

RESULTS

Baseline values and the initial response to head-up tilt of control subjects and patients with NMS were similar. The mean latency to onset of syncope was 11.8 +/- 11.1 minutes. At syncope, heart rate, systolic and diastolic blood pressure, and diastolic cerebral blood velocity decreased significantly, whereas systolic cerebral blood velocity did not change. Calculated cerebrovascular resistance was significantly reduced from 1.85 +/- 0.60 to 1.32 +/- 0.27 mm Hg/cm per second, whereas the pulsatility index increased from 0.92 +/- 0.16 to 1.52 +/- 0.21. We never observed a change in cerebral blood velocity before the rapid decline in blood pressure, nor did we observe any significant change in respiratory pattern.

CONCLUSIONS

The decrease in cerebrovascular resistance during NMS indicates that the integrity of cerebrovascular autoregulation is maintained even when syncope is imminent. The selective loss of diastolic flow during syncope and the increase in pulsatility index are likely caused by collapse of downstream vessels as diastolic blood pressure decreases below the critical closing pressure of cerebral vessels.

Cerebral autoregulation in patients with cirrhosis and ascites. A transcranial Doppler study

BACKGROUND/AIMS

Patients with cirrhosis and ascites usually show alterations of systemic hemodynamics and are thus prone to develop arterial hypotension, which might result in cerebral hypoperfusion if cerebral autoregulation is impaired.

METHODS

We evaluated cerebral autoregulation in 15 patients with cirrhosis and ascites and 15 healthy subjects by monitoring mean blood flow velocity in the middle cerebral artery and arterial pressure during supine rest and passive tilting.

RESULTS

Tilt provoked a drop of arterial pressure in both groups. Control subjects had a prompt recovery of mean flow velocity and a progressive recovery of arterial pressure, so that, after 120 s, both parameters had returned to baseline: at 20 s the recovery of flow velocity was faster (p<0.01) than that of blood pressure. By contrast, patients with cirrhosis had a delayed and incomplete recovery of both parameters (p<0.01 vs healthy subjects). In eight patients, the recovery of mean flow velocity paralleled that of arterial pressure, indicating an impaired cerebral autoregulation. These patients had a worse liver function, a higher cardiac index and lower peripheral resistance.

CONCLUSIONS

Cerebral autoregulation is often impaired in patients with cirrhosis and ascites. These patients can develop cerebral hypoperfusion if arterial pressure falls abruptly.

Effect of PCO2 changes induced by head-upright tilt on transcranial Doppler recordings

BACKGROUND AND PURPOSE

Transcranial Doppler (TCD) monitoring of mean blood flow velocity (mV) during head-upright tilt can allow testing of cerebral autoregulation. Nonetheless, head-upright tilt can induce changes in the ventilation-perfusion relationship and/or respiratory activity that might influence TCD data.

METHODS

Forty-eight healthy volunteers underwent monitoring of mV and end-tidal CO2 in the horizontal position and during head-upright tilt.

RESULTS

Both mV and end-tidal CO2 significantly decreased in orthostasis (P < .01). Linear regression analysis showed a significant linkage between end-tidal CO2 and mV changes (r = .83, P < .01).

CONCLUSIONS

Changes in ventilation-perfusion ratio and in the respiratory pattern induced by head-upright tilt can significantly influence TCD data by determining a PCO2 decrease.

Cerebrovascular disorders in patients with diabetes mellitus

Diabetes mellitus is a risk factor for ischemic, but not hemorrhagic stroke. The frequency of transient ischemic attacks is not increased in patients with diabetes compared to the general population. Diabetes mellitus is associated with higher mortality, worse functional outcome, more severe disability after stroke and a higher frequency of recurrent stroke. Diabetes is not associated with an increased size of cerebral infarction. Controversy exists regarding whether hyperglycemia adversely affects stroke outcome or primarily reflects stroke severity. Cerebral blood flow disturbances, impaired cerebrovascular reactivity, and damage to large and small extra- and intracranial cerebral vessels have been found in humans and animals with diabetes. Combinations of some or all of these factors may underlie the high incidence and worse outcome of stroke in patients with diabetes. Knowledge of these pathophysiologic factors will assist in the design of future intervention strategies.

Which drug for which orthostatic hypotension?

This review focuses on the actual limits of the clinical pharmacology of drugs used for the treatment of orthostatic hypotension. The evidences for heterogeneity of the pathophysiological mechanisms of primary orthostatic hypotension and autonomic failure are discussed. The available data on the efficacy of some drugs used in orthostatic hypotension are also discussed.

Cerebral blood flow and resistances during hypotensive haemorrhage in the rabbit: transcranial Doppler and laser-Doppler flowmetry

Transcranial Doppler (TCD) determined cerebral blood flow velocity and laser Doppler flowmetry (LDF) measured cortical perfusion were simultaneously assessed during hypotensive haemorrhage in 15 anaesthetized rabbits. Systolic (Fsys), diastolic (Fdia) and mean (Fmean) blood flow velocities were recorded into the intracranial internal carotid (ICA) and basilar artery (BA). Resistance (RI = Fsys-Fdia/Fsys) and pulsatility (PI = Fsys-Fdia/Fmean) indices were calculated. Step decreases of 10 mmHg of mean arterial pressure (MAP) from 80 to less than 30 mmHg provoked a fall of LDF signal below 50 mmHg. Blood velocities decreased into BA below 40 mmHg, and below 50 mmHg into ICA indicating regional differences in cerebral autoregulation. Cortical resistances (resLDF = MAP/LDF) fell below 60 mmHg whereas RI and PI increased when MAP decreased into BA below 40 mmHg and ICA below 50 mmHg. A weak correlation was found between Fmean and LDF (BA: r = 0.55, P < 0.01 and ICA = 0.46, P < 0.01). Both RI and PI were poorly correlated to resLDF into BA (RI-resLDF: r = -0.39, P < 0.01; PI-resLDF: r = -0.39, P < 0.01) and ICA (RI-resLDF: r = -0.18, ns; PI-resLDF: r = -0.22, ns). Pulse pressure (systolic-diastolic pressure) correlated with RI (ICA: r = -0.62, P < 0.001; BA: r = -0.61, P < 0.001) and PI (ICA: r = -0.61, P < 0.001; BA: r = -0.62, P < 0.001). In conclusion, during haemorrhagic shock, TCD correlates with LDF and indicates regional differences in autoregulatory settings. However, Doppler indices do not reflect the changes in cerebral resistances because they are influenced by the changes in pulsatile pressure.

Cerebrovascular regulation and vasoneuronal coupling

Maintenance of cerebral perfusion pressure is a prerequisite for the prevention of cerebral ischemia. Physiological fluctuations in systemic perfusion pressure are compensated by cerebrovascular autoregulation. Cerebral hypoperfusion could result from (1) systemic hemodynamic failure (eg, distal to severe arterial stenosis), overcharging the vasoregulatory capacity; (2) dysfunction and exhaustion of cerebrovascular autoregulation; or (3) both. Ultrasound offers an excellent temporal resolution, is noninvasive, and is easily applicable for follow-up investigations. Despite its poor spatial resolution, transcranial Doppler sonography has been used for determination of cerebral perfusion reserve studies measuring cerebral blood flow velocity (CBFV) during hypercapnia or application of vasoactive agents (eg, acetazolamide). This approach evaluates vasomotor regulation in patients with hemodynamic compromise distal to severe stenosis or occlusion of the brain supplying arteries. Monitoring CBFV during tilt table examinations directly measures cerebral autoregulation. In patients with systemic orthostatic hypotension, maintainance or failure of cerebrovascular compensation and, even more importantly, cerebrovascular dysautoregulation, despite normal systemic blood pressure regulation, may be demonstrated. Vasoneuronal coupling is reflected by CBFV variations during appropriate neuronal stimulation. Neuronal dysfunction is associated with CBFV abnormalities as exemplified by preconditions of focal cerebral dysfunction in the posterior cerebral artery (PCA) in migraineurs with aura, where massive alteration of vasoneuronal coupling and ischemia is threatening during spreading depression. A highly significant asymmetric gain of vasoneuronal coupling in the interictal state may act as a trigger mechanism in these patients. Testing for vasoneuronal coupling within the middle cerebral artery (MCA) territory is more difficult due to the poor spatial resolution with various neuronal stimuli (eg, motorsensory or cognitive paradigms), only eliciting local neuronal areas underrepresented in the MCA CBFV global changes. However, motor stimulation evoked CBFV may be used to indicate dysintegration of vasoneuronal coupling in the course of acute cerebral ischemia with sensorimotor hemiparesis and, moreover, seems to be of prognostic value regarding the motor deficit.

Cerebral autoregulation in orthostatic hypotension. A transcranial Doppler study

BACKGROUND AND PURPOSE

Transcranial Doppler measurements of blood flow velocity permit an assessment of variations in intracranial hemodynamics in response to acute arterial pressure variations. The purpose of this study was to scan healthy volunteers and patients with autonomic failure for differences in cerebral hemodynamic patterns under an acute hypotensive stimulus.

METHODS

We used transcranial Doppler monitoring of blood flow velocity in the middle cerebral artery and noninvasive monitoring of arterial blood pressure and heart rate before, during, and after acute arterial hypotension induced by reactive hyperemia of the lower limbs.

RESULTS

After maximum hypotension, the mean blood flow velocity was higher in the healthy volunteers than in the patients. In the healthy subjects mean velocity rose significantly (P < .01) higher than arterial blood pressure after 30 seconds and 60 seconds; in the patients mean velocity and arterial pressure moved in parallel fashion. The diastolic blood flow velocity increased more in the control group than in the patients during the early stages of the test; furthermore, only in the healthy volunteers did it increase significantly more than arterial pressure after 30 seconds and 60 seconds. Regarding the pulsatility index, the differences between the two groups were similar to the diastolic velocity results.

CONCLUSIONS

(1) Monitoring of mean blood flow velocity showed the ability to maintain an adequate cerebral blood flow in healthy subjects; this mechanism was not efficient in the patients with autonomic failure. (2) Diastolic velocity and pulsatility index values clearly showed that only in healthy subjects were cerebral hemodynamics relatively independent of pressure values.