Cerebral hemodynamic changes induced by sympathetic stimulation tests

Effects of Postural Resonance on Skin Sympathetic Nerve Activity and Blood Pressure: A Pilot Study Evaluating Vascular Tone Baroreflex Stimulation Through Biofeedback

Heart rate and vascular tension baroreflex exhibit resonance characteristics at approximately 0.1 and 0.03 Hz. In this study, we aimed to induce postural resonance (PR) through rhythmic postural adjustments. To assess the viability of this technique, we investigated the acute impacts of postural resonance on blood pressure, sympathetic nerve activity, and mood. Fifteen healthy study participants, consisting of 8 males and 7 females, were selected for this self-controlled study. Skin sympathetic nerve activity was continuously monitored during both the intervention and stress test on the experimental day. After PR intervention, the diastolic blood pressure and mean arterial pressure in the PR group exhibited significant reductions compared to the CON group (P = 0.032, CON = 71.67 ± 2.348, PR = 64.08 ± 2.35; P = 0.041, CON = 75.00 ± 2.17, PR = 81.67 ± 2.17). After PR intervention both left brachial ankle pulse wave velocity and right brachial ankle pulse wave velocity exhibited a significant reduction compared to pre-intervention levels (from 1115.86 ± 150.08 to 1048.43 ± 127.40 cm/s, p < 0.001; 1103.86 ± 144.35 to 1060.43 ± 121.35 cm/s, p = 0.018). PR intervention also led to a significant decrease in burst frequency and duration (P = 0.049; CON = 8.96 ± 1.17, PR = 5.51 ± 1.17) and a noteworthy decrease in burst amplitude and burst threshold during the cold-pressor test (P = 0.002; P = 0.002). Additionally, VAS scores exhibited a substantial increase following PR (P = 0.035, CON = 28.4 ± 4.49, PR = 42.17 ± 4.10). PR can induce resonance effects within the cardiovascular system, resulting in the effective reduction of blood pressure, skin sympathetic nerve activity and pulse wave velocity, and decreased burst amplitude and burst threshold of the sympathetic nerve during the cold-pressor test.

Cerebral sympatholysis: experiments on in vivo cerebrovascular regulation and ex vivo cerebral vasomotor control

Whether cerebral sympathetic-mediated vasomotor control can be modulated by local brain activity remains unknown. This study tested the hypothesis that the application or removal of a cognitive task during a cold pressor test (CPT) would attenuate and restore decreases in cerebrovascular conductance (CVC), respectively. Middle cerebral artery blood velocity (transcranial Doppler) and mean arterial pressure (finger photoplethysmography) were examined in healthy adults (n = 16; 8 females and 8 males) who completed a control CPT, followed by a CPT coupled with a cognitive task administered either 1) 30 s after the onset of the CPT and for the duration of the CPT or 2) at the onset of the CPT and terminated 30 s before the end of the CPT (condition order was counterbalanced). The major finding was that the CPT decreased the index of CVC, and such decreases were abolished when a cognitive task was completed concurrently and restored when the cognitive task was removed. As a secondary experiment, vasomotor interactions between sympathetic transduction pathways (α1-adrenergic and Y1-peptidergic) and compounds implicated in cerebral blood flow control [adenosine, and adenosine triphosphate (ATP)] were explored in isolated porcine cerebral arteries (wire myography). The data reveal α1-receptor agonism potentiated vasorelaxation modestly in response to adenosine, and preexposure to ATP attenuated contractile responses to α1-agonism. Overall, the data suggest a cognitive task attenuates decreases in CVC during sympathoexcitation, possibly related to an interaction between purinergic and α1-adrenergic signaling pathways.NEW & NOTEWORTHY The present study demonstrates that the cerebrovascular conductance index decreases during sympathoexcitation and this response can be positively and negatively modulated by the application or withdrawal of a nonexercise cognitive task. Furthermore, isolated vessel experiments reveal that cerebral α1-adrenergic agonism potentiates adenosine-mediated vasorelaxation and ATP attenuates α1-adrenergic-mediated vasocontraction.

Common carotid artery responses to the cold-pressor test are impaired in individuals with cervical spinal cord injury

Cervical spinal cord injury (SCI) leads to autonomic cardiovascular dysfunction that underlies the three- to fourfold elevated risk of cardiovascular disease in this population. Reduced common carotid artery (CCA) dilatory responsiveness during the cold-pressor test (CPT) is associated with greater cardiovascular disease risk and progression. The cardiovascular and CCA responses to the CPT may provide insight into cardiovascular autonomic dysfunction and cardiovascular disease risk in individuals with cervical SCI. Here, we used CPT to perturb the autonomic nervous system in 14 individuals with cervical SCI and 12 uninjured controls, while measuring cardiovascular responses and CCA diameter. The CCA diameter responses were 55% impaired in those with SCI compared with uninjured controls (P = 0.019). The CCA flow, velocity, and shear response to CPT were reduced in SCI by 100% (P < 0.001), 113% (P = 0.001), and 125% (P = 0.002), respectively. The association between mean arterial pressure and CCA dilation observed in uninjured individuals (r = 0.54, P = 0.004) was absent in the SCI group (r = 0.22, P = 0.217). Steady-state systolic blood pressure (P = 0.020), heart rate (P = 0.003), and cardiac contractility (P < 0.001) were reduced in those with cervical SCI, whereas total peripheral resistance was increased compared with uninjured controls (P = 0.042). Relative cerebral blood velocity responses to CPT were increased in the SCI group and reduced in controls (middle cerebral artery, P = 0.010; posterior cerebral artery, P = 0.026). The CCA and cardiovascular responsiveness to CPT are impaired in those with cervical SCI.NEW & NOTEWORTHY This is the first study demonstrating that CCA responses during CPT are suppressed in SCI. Specifically, CCA diameter, flow, velocity, and shear rate were reduced. The relationship between changes in MAP and CCA dilatation in response to CPT was absent in individuals with SCI, despite similar cardiovascular activation between SCI and uninjured controls. These findings support the notion of elevated cardiovascular disease risk in SCI and that the cardiovascular responses to environmental stimuli are impaired.

Phase dynamics of cerebral blood flow in subarachnoid haemorrhage in response to sodium nitrite infusion

Aneurysmal subarachnoid haemorrhage (SAH) is a devastating subset of stroke. One of the major determinates of morbidity is the development of delayed cerebral ischemia (DCI). Disruption of the nitric oxide (NO) pathway and consequently the control of cerebral blood flow (CBF), known as cerebral autoregulation, is believed to play a role in its pathophysiology. Through the pharmacological manipulation of in vivo NO levels using an exogenous NO donor we sought to explore this relationship. Phase synchronisation index (PSI), an expression of the interdependence between CBF and arterial blood pressure (ABP) and thus cerebral autoregulation, was calculated before and during sodium nitrite administration in 10 high-grade SAH patients acutely post-rupture. In patients that did not develop DCI, there was a significant increase in PSI around 0.1 Hz during the administration of sodium nitrite (33%; p-value 0.006). In patients that developed DCI, PSI did not change significantly. Synchronisation between ABP and CBF at 0.1 Hz has been proposed as a mechanism by which organ perfusion is maintained, during periods of physiological stress. These findings suggest that functional NO depletion plays a role in impaired cerebral autoregulation following SAH, but the development of DCI may have a distinct pathophysiological aetiology.

Cerebrovascular Dynamics During Continuous Motor Task

We investigated the cerebral autoregulation (CA) dynamics parameter phase and gain change when exposed to a longlasting motor task. 25 healthy subjects (mean age ± SE, 38±2.6 years, 13 females) underwent simultaneous recordings of spontaneous fluctuations in blood pressure (BP), cerebral blood flow velocity (CBFV), and end-tidal CO(2) (ETCO(2)) over 5 min of rest followed by 5 min of left elbow flexion at a frequency of 1 Hz. Tansfer function gain and phase between BP and CBFV were assessed in the frequency ranges of very low frequencies (VLF, 0.02-0.07 Hz), low frequencies (LF, 0.07-0.15), and high frequencies (HF, >0.15). CBFV increased on both sides rapidly to maintain an elevated steady state until movement stopped. Cerebrovascular resistance fell on the right side (rest 1.35±0.06, movement 1.28±0.06, p<0.01), LF gain decreased from baseline (right side 0.97±0.07 %/mm Hg, left 1.01±0.09) to movement epoch (right 0.73±0.08, left 0.76±0.06, p</=0.01). VLF phase decreased from baseline (right 1.03±0.05 radians, left 1.10±0.06) to the movement epoch (right 0.81±0.07, left 0.82±0.10, p?0.05). CA regulates continuous motor efforts by changes in resistance, gain and phase.

Facial nerve stimulation in normal pigs and healthy human volunteers: transitional development of a medical device for the emergency treatment of ischemic stroke

BACKGROUND

Magnetic stimulation of the facial nerve has been tested in preclinical studies as a new, non-invasive emergency treatment of ischemic stroke that acts by increasing cerebral blood flow (CBF). The objective of the studies reported herein was to identify minimal stimulation parameters that increase CBF in large animals and then test those stimulation parameters in healthy volunteers for safety, tolerability, and effectiveness at increasing CBF. This translational research is necessary preparation for clinical studies in ischemic stroke patients.

METHODS

Initial experiments in anesthetized Yorkshire pigs were undertaken in order to identify the lowest stimulus power and duration that increase CBF. A full 3 × 3 factorial design was used to evaluate magnetic stimulation of the facial nerve at various stimulation powers (1.3, 1.6, and 1.9 Tesla field strength at coil surface) and for various durations (2, 3.5, and 5 min). CBF was measured with contrast MRI perfusion imaging and the internal carotid arteries were assessed with MR angiography. Magnetic facial nerve stimulation with parameters identified in the pig study was then applied to 35 healthy volunteers. Safety was assessed with adverse event reports and by medical examination. Tolerability was defined as each volunteer's ability to withstand at least 2 min of stimulation. Volunteers could determine the maximum power of stimulation they received during a ramp-up period.

RESULTS

In pigs, unilateral facial nerve stimulation increased CBF by as much as 77% over pre-stimulation baseline when administered across a range of 1.3-1.9 Tesla power and for 2- to 5-min duration. No clear dose-response relationship could be observed across this range, but lower powers and durations than these were markedly less effective. The effect of a single stimulation lasted 90 min. A second stimulation delivered 100 min after the first stimulation sustained the increased CBF without evidence of tachyphylaxis. In human, bilateral facial nerve stimulation caused only non-serious adverse events that were limited to the 2-min stimulation period. Tolerability was greatly improved by gentle encouragement from the study staff, which enabled most volunteers to tolerate 1.6-1.8 Tesla of stimulation power. CBF measures taken approximately 10 min after stimulation demonstrated on average a 32 ± 6% increase in CBF, with ≥ 25% increases in CBF occurring in 10 of the 31 volunteers who had adequate CBF measurements.

CONCLUSIONS

The minimal effective stimulation parameters defined by increased CBF, as identified in the pig study, translated into safe, tolerable, and effective stimulation of healthy volunteers. These results support the future development and evaluation of non-invasive facial nerve stimulation for the emergency treatment of ischemic stroke. Trial Registration retrospectively registered with clinicaltrials.gov NRV_P1_01_15 on June 6, 2017.

Extra- and intracranial blood flow regulation during the cold pressor test: influence of age

We determined how the extra- and intracranial circulations respond to generalized sympathetic activation evoked by a cold pressor test (CPT) and whether this is affected by healthy aging. Ten young [23 ± 2 yr (means ± SD)] and nine older (66 ± 3 yr) individuals performed a 3-min CPT by immersing the left foot into 0.8 ± 0.3°C water. Common carotid artery (CCA) and internal carotid artery (ICA) diameter, velocity, and flow were simultaneously measured (duplex ultrasound) along with middle cerebral artery and posterior cerebral artery mean blood velocity (MCAv_mean and PCAv_mean) and cardiorespiratory variables. The increases in heart rate (~6 beats/min) and mean arterial blood pressure (~14 mmHg) were similar in young and older groups during the CPT (P < 0.01 vs. baseline). In the young group, the CPT elicited an ~5% increase in CCA diameter (P < 0.01 vs. baseline) and a tendency for an increase in CCA flow (~12%, P = 0.08); in contrast, both diameter and flow remained unchanged in the older group. Although ICA diameter was not changed during the CPT in either group, ICA flow increased (~8%, P = 0.02) during the first minute of the CPT in both groups. Whereas the CPT elicited an increase in MCAv_mean and PCAv_mean in the young group (by ~20 and ~10%, respectively, P < 0.01 vs. baseline), these intracranial velocities were unchanged in the older group. Collectively, during the CPT, these findings suggest a differential mechanism(s) of regulation between the ICA compared with the CCA in young individuals and a blunting of the CCA and intracranial responses in older individuals.NEW & NOTEWORTHY Sympathetic activation evoked by a cold pressor test elicits heterogeneous extra- and intracranial blood vessel responses in young individuals that may serve an important protective role. The extra- and intracranial responses to the cold pressor test are blunted in older individuals.

Partial pharmacologic blockade shows sympathetic connection between blood pressure and cerebral blood flow velocity fluctuations

Cerebral autoregulation (CA) dampens transfer of blood pressure (BP)-fluctuations onto cerebral blood flow velocity (CBFV). Thus, CBFV-oscillations precede BP-oscillations. The phase angle (PA) between sympathetically mediated low-frequency (LF: 0.03-0.15Hz) BP- and CBFV-oscillations is a measure of CA quality. To evaluate whether PA depends on sympathetic modulation, we assessed PA-changes upon sympathetic stimulation with and without pharmacologic sympathetic blockade. In 10 healthy, young men, we monitored mean BP and CBFV before and during 120-second cold pressor stimulation (CPS) of one foot (0°C ice-water). We calculated mean values, standard deviations and sympathetic LF-powers of all signals, and PAs between LF-BP- and LF-CBFV-oscillations. We repeated measurements after ingestion of the adrenoceptor-blocker carvedilol (25mg). We compared parameters before and during CPS, without and after carvedilol (analysis of variance, post-hoc t-tests, significance: p<0.05). Without carvedilol, CPS increased BP, CBFV, BP-LF- and CBFV-LF-powers, and shortened PA. Carvedilol decreased resting BP, CBFV, BP-LF- and CBFV-LF-powers, while PAs remained unchanged. During CPS, BPs, CBFVs, BP-LF- and CBFV-LF-powers were lower, while PAs were longer with than without carvedilol. With carvedilol, CPS no longer shortened resting PA. Sympathetic activation shortens PA. Partial adrenoceptor blockade abolishes this PA-shortening. Thus, PA-measurements provide a subtle marker of sympathetic influences on CA and might refine CA evaluation.

Global brain blood-oxygen level responses to autonomic challenges in obstructive sleep apnea

Obstructive sleep apnea (OSA) is accompanied by brain injury, perhaps resulting from apnea-related hypoxia or periods of impaired cerebral perfusion. Perfusion changes can be determined indirectly by evaluation of cerebral blood volume and oxygenation alterations, which can be measured rapidly and non-invasively with the global blood oxygen level dependent (BOLD) signal, a magnetic resonance imaging procedure. We assessed acute BOLD responses in OSA subjects to pressor challenges that elicit cerebral blood flow changes, using a two-group comparative design with healthy subjects as a reference. We separately assessed female and male patterns, since OSA characteristics and brain injury differ between sexes. We studied 94 subjects, 37 with newly-diagnosed, untreated OSA (6 female (age mean ± std: 52.1±8.1 yrs; apnea/hypopnea index [AHI]: 27.7±15.6 events/hr and 31 male 54.3±8.4 yrs; AHI: 37.4±19.6 events/hr), and 20 female (age 50.5±8.1 yrs) and 37 male (age 45.6±9.2 yrs) healthy control subjects. We measured brain BOLD responses every 2 s while subjects underwent cold pressor, hand grip, and Valsalva maneuver challenges. The global BOLD signal rapidly changed after the first 2 s of each challenge, and differed in magnitude between groups to two challenges (cold pressor, hand grip), but not to the Valsalva maneuver (repeated measures ANOVA, p<0.05). OSA females showed greater differences from males in response magnitude and pattern, relative to healthy counterparts. Cold pressor BOLD signal increases (mean ± adjusted standard error) at the 8 s peak were: OSA 0.14±0.08% vs. Control 0.31±0.06%, and hand grip at 6 s were: OSA 0.08±0.03% vs. Control at 0.30±0.02%. These findings, indicative of reduced cerebral blood flow changes to autonomic challenges in OSA, complement earlier reports of altered resting blood flow and reduced cerebral artery responsiveness. Females are more affected than males, an outcome which may contribute to the sex-specific brain injury in the syndrome.

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.

Subthalamic stimulation improves the cerebral hemodynamic response to the cold pressure test in patients with Parkinson's disease

PURPOSE

Disturbances of the autonomic nervous system are common in patients with Parkinson's disease (PD) but the effect of deep brain stimulation of the subthalamic nucleus on cerebrovascular reactivity is not entirely known.

METHODS

Seven patients in an advanced stage of the disease and seven healthy age-matched controls participated in the study, which took place after one night of drug withdrawal. Cerebral blood flow velocity was continuously monitored on both sides with transcranial Doppler ultrasound, and cerebrovascular reactivity (CR) was evaluated with the cold pressure test. The measurements were repeated and compared during the stimulation-on and -off phases.

RESULTS

The PD patients had significantly higher CR values in the stimulation-on than -off conditions (15.1% ± 6.9 versus 9.4% ± 6.2; p = 0.03). CR values were higher in controls than in patients in the stimulation-off condition (20.4% ± 12.5 versus 9.4% ± 6.2; p = 0.007) without a significant difference with the stimulation-on phase.

CONCLUSIONS

CR, evaluated by the response to the cold pressure test, is impaired in patients with advanced PD and improved by subthalamic nucleus.

Cerebral blood flow, sympathetic nerve activity and stroke risk in obstructive sleep apnoea. Is there a direct link?

Obstructive sleep apnoea (OSA) is significantly associated with the risk of stroke, and this association is independent of other risk factors, including hypertension, atrial fibrillation and diabetes mellitus. Therefore, additional pathogenic mechanisms may exist, which contribute to the increased risk of stroke. OSA is characterized by prolonged sympathetic overactivity; however the role of the sympathetic nervous system in regulating cerebral circulation remains a matter of controversy. Converging data indicate that brain perfusion is significantly distorted in OSA, with reported decreases in cerebral blood flow as well as intermittent surges in blood pressure and cerebral blood flow velocity. Based on recent research, there is accumulating evidence that sympathetic nerve activity is an important element in brain protection against excessive increases in perfusion pressure during blood pressure surges and flow during rapid eye movement sleep. The aim of this article was to review: (i) the current physiological knowledge related to the role of the sympathetic system in the regulation of cerebral blood flow, (ii) how the influence of the sympathetic system on cerebral vessels is affected by apnoea (increased PaCO(2)) and (iii) the potential significance of the pathological sympathetic system/PaCO(2) interplay in OSA. Sympathetic system seems to be at least partially involved in pathogenesis of distorted haemodynamics and stroke in OSA patients. However, there are still several open questions that need to be addressed before the effective therapeutic strategies can be implemented.

The effect of the cold pressor test on a visually evoked cerebral blood flow velocity response

We investigated the hypothesis that during tonic pain stimulus, neurovascular coupling (NVC) decreases, measuring visually evoked cerebral blood flow velocity response (VEFR) during cold pressor test (CPT) in healthy human subjects as a test. VEFR was calculated as a relative increase in blood flow velocity in the posterior cerebral artery from average values during the last 5 s of the stimulus-OFF period to average values during the last 10 s of the stimulus-ON period. Three consecutive experimental phases were compared: basal, CPT and recovery. During CPT, end-diastolic and mean VEFR increased from 20.2 to 23.6% (p < 0.05) and from 17.5 to 20.0% (p < 0.05), respectively. In recovery phase, end-diastolic and mean VEFR decreased to 17.7% and 15.5%, respectively. Both values were statistically significantly different from CPT phase (p < 0.05). Compared with the basal phase, only end-diastolic VEFR was statistically significantly different in the recovery phase (p < 0.05). Our results are consistent with the assumption that there is a change in the activity of NVC during CPT because of the modulatory influence of subcortical structures activated during tonic pain. Contrary to our expectations, the combined effect of such influences increases rather than decreases NVC.

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.

Cold pressor test in tetraplegia and paraplegia suggests an independent role of the thoracic spinal cord in the hemodynamic responses to cold

BACKGROUND

Cold application to the hand (CAH) is associated in healthy people with increase in heart rate (HR) and blood pressure (BP).

OBJECTIVE

To study hemodynamic responses to CAH in humans following spinal cord injuries of various levels, and examine the effect of spinal cord integrity on the cold pressor response.

DESIGN

An experimental controlled study.

SETTING

The spinal research laboratory, Loewenstein Hospital, Raanana, Israel.

SUBJECTS

Thirteen healthy subjects, 10 patients with traumatic T(4-6) paraplegia and 11 patients with traumatic C(4-7) tetraplegia.

MAIN OUTCOME MEASURES

HR, BP, HR and BP spectral components (low frequency, LF; high frequency, HF; LF/HF), cerebral blood flow velocity (CBFV) and cerebrovascular resistance index (CVRi).

METHODS

The outcome measures of the three subject groups monitored for HR, BP and CBFV were compared from 5 min before to 5 min after 40-150 s of CAH. The recorded signals were digitized online and analyzed offline in both the time and frequency domains.

RESULTS

During CAH, HR and CVRi increased significantly in all subject groups (P<0.001), and BP in control subjects and in the tetraplegia group (P<0.01). BP increase was not statistically significant in paraplegia, and CBFV, HR LF, HR HF and BP LF did not change significantly during CAH in any group.

CONCLUSIONS

The CAH effect in tetraplegia and the suppressed BP increase in paraplegia, supported by the other findings, suggest a contribution of an independent thoracic spinal mechanism to the cold pressor response.

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.

Differential responses to CO2 and sympathetic stimulation in the cerebral and femoral circulations in humans

The relative importance of CO2 and sympathetic stimulation in the regulation of cerebral and peripheral vasculatures has not been previously studied in humans. We investigated the effect of sympathetic activation, produced by isometric handgrip (HG) exercise, on cerebral and femoral vasculatures during periods of isocapnia and hypercapnia. In 14 healthy males (28.1 +/- 3.7 (mean +/- S.D.) years), we measured flow velocity (VP; transcranial Doppler ultrasound) in the middle cerebral artery during euoxic isocapnia (ISO, +1 mmHg above rest) and two levels of euoxic hypercapnia (HC5, end-tidal P(CO(2)), P(ET,CO2), = +5 mmHg above ISO; HC10, P(ET,CO2) = +10 above ISO). Each P(ET,CO2) level was maintained for 10 min using the dynamic end-tidal forcing technique, during which increases in sympathetic activity were elicited by a 2-min HG at 30% of maximal voluntary contraction. Femoral blood flow (FBF; Doppler ultrasound), muscle sympathetic nerve activity (MSNA; microneurography) and mean arterial pressure (MAP; Portapres) were also measured. Hypercapnia increased VP and FBF by 5.0 and 0.6% mmHg-1, respectively, and MSNA by 20-220%. Isometric HG increased MSNA by 50% and MAP by 20%, with no differences between ISO, HC5 and HC10. During the ISO HG there was an increase in cerebral vascular resistance (CVR; 20 +/- 11%), while VP remained unchanged. During HC5 and HC10 HG, VP increased (13% and 14%, respectively), but CVR was unchanged. In contrast, HG-induced sympathetic stimulation increased femoral vascular resistance (FVR) during ISO, HC5 and HC10 (17-41%), while there was a general decrease in FBF below ISO. The HG-induced increases in MSNA were associated with increases in FVR in all conditions (r = 0.76-0.87), whereas increases in MSNA were associated with increases in CVR only during ISO (r = 0.91). In summary, in the absence of hypercapnia, HG exercise caused cerebral vasoconstriction, myogenically and/or neurally, which was reflected by increases in CVR and a maintained VP. In contrast, HG increased FVR during conditions of ISO, HC5 and HC10. Therefore, the cerebral circulation is more responsive to alterations in PCO2, and less responsive to sympathetic stimulation than the femoral circulation.

Cerebral autoregulation is preserved in postural tachycardia syndrome

To test whether cerebral autoregulation is impaired in patients with postural tachycardia syndrome (POTS), we evaluated 17 healthy control subjects and 27 patients with POTS. Blood pressure, heart rate, and cerebral blood velocity (transcranial Doppler) were recorded at rest and during 80 degree head-up tilt (HUT). Static cerebral autoregulation, as assessed from the change in cerebrovascular resistance during HUT, was the same in POTS and in controls. The properties of dynamic cerebral autoregulation were inferred from transfer gain, coherence, and phase of the relationship between blood pressure and cerebral blood velocity estimated from filtered data segments (0.02-0.8 Hz). Dynamic cerebral autoregulation of patients with POTS did not differ from that of controls. The patients' dynamic cerebral autoregulation did not change over the course of HUT, despite increased tachycardia suggestive of worsening orthostatic stress. Inflation of military anti-shock trouser pants substantially reduced the tachycardia of patients with POTS without affecting cerebral autoregulation. Symptoms of orthostatic intolerance were reduced in one-half of the patients following military anti-shock trouser pants inflation. We conclude that cerebral perfusion and autoregulation in many patients with POTS do not differ from that of normal control subjects.

Evaluation of the Early Hemodynamic Changes in Carotid Arteries During Ventricular and Dual Chamber Pacing

In spite of a wide choice of pacemakers, there are some problems in making more rational clinical decisions for individual patients since mode selection and programming is usually performed on the basis of a clinical hunch. The aim of this study was to measure the differences in carotid flow in patients with a pacemaker programmed in the dual chamber and in the single chamber pacing modes. Sixty patients with implanted bipolar DDD pacemakers were enrolled in this study. Blood peak systolic velocity (PSV) and end-diastolic velocity (EDV), cross-sectional area, resistive index (RI), and pulsatility index (PI) were measured in the common (CCA), internal (ICA), and external (ECA) carotid arteries before pacemaker implantation and after dual chamber and ventricular pacing at 60 beats/min. PSVs in the left CCA (79.3 +/- 24.9 cm/s) and right CCA (84.1 +/- 18.7) were shown to significantly decrease after VVI pacing (60.1 +/- 16.6 and 62.1 +/- 20.0, respectively). There was also a similar significant decrease in PSV in the left and right ICAs and ECAs. Besides PSV, RI, and PI in the left and right CCAs, ICAs, and ECAs significantly decreased after VVI pacing. There was no similar decrease after DDD pacing. Cross-sectional area and flow volume in the CCA, ICA, and ECA were similar after DDD and VVI pacing and before pacemaker implantation suggesting that cardiac output was similar when the measurements were recorded. Carotid artery PSVs, pulsatility, and RIs were found to be significantly decreased during VVI pacing compared to baseline and DDD pacing. The greater incidence of adverse cerebral outcomes in patients with VVI rather than DDD pacing may be partly due to decreased carotid PSVs.

Ventilatory, cerebrovascular, and cardiovascular interactions in acute hypoxia: regulation by carbon dioxide

This study examined the effect of high, normal, and uncontrolled end-tidal Pco2 (PetCO2) on the ventilatory, peak cerebral blood flow velocity ( V̄p), and mean arterial blood pressure (MAP) responses to acute hypoxia. Nine healthy subjects undertook, in random order, three hypoxic protocols (end-tidal Po2 was held at eight steps between 300 and 45 Torr) in conditions of hypercapnia, isocapnia, or poikilocapnia (PetCO2 +7.5 Torr, +1.0 Torr, or uncontrolled, respectively). Transcranial Doppler ultrasound was used to measure V̄p in the middle cerebral artery. The slopes of the linear regressions of ventilation, V̄p, and MAP with arterial O2 saturation were significantly greater in hypercapnia than in both isocapnia and poikilocapnia ( P < 0.05). Strong, significant correlations were observed between ventilation, V̄p, and MAP with each PetCO2 condition. These data suggest that 1) a high acute hypoxic ventilatory response (AHVR) decreases the acute hypoxic cerebral blood flow responses during poikilocapnia hypoxia, due to hypocapnic-induced cerebral vasoconstriction; and 2) in hypercapnic hypoxia, a high AHVR is associated with a high acute hypoxic cerebral blood flow response, demonstrating a linkage of individual sensitivities of ventilation and cerebral blood flow to the interaction of PetCO2 and hypoxia. In summary, the between-individual variability in AHVR is shown to be firmly linked to the variability in V̄p and MAP responses to hypoxia. Individuals with a high AHVR are found also to have high V̄p and MAP responses to hypoxia.

Noradrenergic constriction of cerebral arteries as detected by transcranial Doppler (TCD) in the rabbit

Interpretation of transcranial Doppler (TCD) recordings requires assumptions about flow or diameter of the insonated vessel. This study aimed at assessing if activation of the sympathetic system could affect blood velocity (bv) in basal cerebral arteries. In anaesthetized rabbits, stimulation of cervical sympathetic nerve (cervSN) was used selectively to activate the sympathetic pathway to the head while monitoring bv in all major cerebral arteries. cervSN stimulation at 10 Hz produced: 1. in internal carotid artery (ICA) and ICA-supplied arteries (ICA-s), a consistent bv increase ranging between 20 and 70%, 2. in the basilar artery, a transient decrease by 15-30%. These effects were mimicked, in both territories, by injection of phenylephrine into the ICA. Because cerebral blood flow is known to be reduced by cervSN stimulation, the increase in bv in ICA and ICA-s must be ascribed to constriction of the insonated vessels. These effects should be considered when monitoring bv during sympathetic activation tests or exercise.

The effect of age on cerebrovascular reactivity to cold pressor test and head-up tilt

OBJECTIVE

To compare the cold pressor test (CPT) and head-up tilt (HUT) responses of the older and younger healthy individuals by transcranial Doppler.

SUBJECTS AND METHODS

Forty healthy volunteers were divided into two age groups (18-39 years, 40-69 years). Mean blood velocity (v(m)) in both middle cerebral arteries was monitored during CPT and HUT. Mean arterial blood pressure, heart rate and end-tidal CO(2) (Et-CO(2)) were measured simultaneously.

RESULTS

The v(m) increased by 7.1% during CPT and decreased by 10.1% during HUT. The v(m) responses were significantly lower in the older group (P < 0.01). Linear regression analysis showed a significant effect of age on dv(m) during both CPT (P < 0.01) as well as HUT (P < 0.01).

CONCLUSION

The age affected the v(m) responses to CPT and HUT in the group of older subjects.

Evaluation of the cerebral hemodynamic response to rhythmic handgrip

The response of the cerebral circulation to exercise has been studied with transcranial Doppler ultrasound (TCD) because this modality provides continuous measurements of blood velocity and is well suited for the exercise environment. The use of TCD as an index of cerebral blood flow, however, requires the assumption that the diameter of the insonated vessel is constant. Here, we examine this assumption for rhythmic handgrip using a spectral index designed to measure trends in vessel flow. Nineteen normal subjects were studied during 5 min of volitional maximum rhythmic right handgrip at 1 Hz. TCD velocities from both middle arteries (left and right), blood pressure, and end-tidal PCO(2) were recorded every 10 s. A spectral weighted sum was also calculated as a flow index (FI). Averages were computed from the last 2 min of handgrip. Relative changes in velocity, FI, and pressure were calculated. The validity of FI was tested by comparing the change in diameter derived from equations relating flow and diameter. Mean blood pressure increased 23.8 +/- 17.8% (SD), and velocity increased 13.3 +/- 9.8% (left) and 9.6 +/- 8.3% (right). Although the mean change in FI was small [2.0 +/- 18. 2% (left) and 4.7 +/- 29.7% (right)], the variation was high: some subjects showed a significant increase in FI and others a significant decrease. Diameter estimates from two equations relating flow and luminal area were not significantly different. Decreases in FI were associated with estimated diameter decreases of 10%. Our data suggest that the cerebral blood flow (CBF) response to rhythmic handgrip is heterogeneous and that middle cerebral artery flow can decrease in some subjects, in agreement with prior studies using the Kety-Schmidt technique. We speculate that the velocity increase is due to sympathetically mediated vasoconstriction rather than a ubiquitous flow increase. Our data suggest that the use of ordinary TCD velocities to interpret the CBF response during exercise may be invalid.