The sympathetic nervous system and the regulation of cerebral blood flow in man

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.

Cerebrovascular regulation in patients with vasovagal syncope and autonomic failure due to familial amyloidotic polyneuropathy

INTRODUCTION

While there is strong evidence for autonomic involvement in cerebrovascular function acutely, long-term role of autonomic nervous system in cerebrovascular function has been controversial. We assessed autoregulation in 10 healthy individuals, nine patients with vasovagal syncope (VVS), and nine with Familial Amyloidotic Polyneuropathy (FAP), in response to head-up tilt test (HUTT).

METHODS

Arterial blood pressure heart rate, cardiac output, and bilateral cerebral blood flow velocity (CBFV) at the M1 segment of middle cerebral artery (transcranial Doppler ultrasound) were recorded during supine rest and 70° HUTT. Autoregulation was quantified using a validated nonlinear and nonparametric approach based on projection pursuit regression. Plasma adrenaline and noradrenaline were also measured at rest and during HUTT.

RESULTS

During supine rest and HUTT, plasma noradrenaline content was lower in FAP patients. During HUTT, VVS patients had a hyperadrenergic status; CBFV decreased in all groups, which was greater in FAP patients (p < 0.01). Healthy controls responded to HUTT with a reduction in CBFV responses to increases (p = 0.01) and decreases (p < 0.01) in arterial pressure without any change in the range or effectiveness of autoregulation. VVS patients responded to HUTT with a reduction in falling (p = 0.02), but not rising slope (p = 0.40). Autoregulatory range (p < 0.01) and effectiveness increased (p = 0.09), consistent with the rapid increase in levels of catecholamines. In FAP patients, the level of increase in range of autoregulation was significantly related to the magnitude of increase in plasma noradrenaline in response to HUTT (R² = 0.26, p = 0.05).

CONCLUSION

Autonomic dysfunction affects the cerebral autoregulatory response orthostatic to challenge.

Stellate ganglion block used to treat reversible cerebral vasoconstriction syndrome

BACKGROUND

We present a case report of a patient who developed severe reversible cerebral vasoconstriction syndrome, which was worsening despite typical interventional and supportive care. We administered a stellate ganglion block (SGB) and monitored the vasospasm with transcranial Doppler measurements.

CASE REPORT

A 25-year-old woman was admitted with recurrent headaches and neurological symptoms, which angiography showed to be caused by diffuse, multifocal, segmental narrowing of the cerebral arteries leading to severe ischemia in multiple regions. Typical treatment was initiated with arterial verapamil followed by supportive critical care, including nimodipine, intravenous fluids, permissive hypertension, and analgesia. Vasospasm was monitored daily via transcranial Doppler ultrasound (TCD). After symptoms and monitoring suggested worsening vasospasm, an SGB was administered under ultrasound guidance. Block success was confirmed via pupillometry, and repeat TCD showed improved flow through the cerebral vasculature. Improvement in vascular flow was accompanied by a gradual reduction in acute neurological symptoms, with the patient reporting no headaches the following morning.

CONCLUSIONS

For patients with reversible cerebral vasoconstriction syndrome who develop severe signs or symptoms despite typical treatment, sympathetic blockade may be a possible rescue therapy. This may extend to other causes of severe vasospasm as well, and further study is needed to determine if the SGB should be included in routine or rescue therapy.

A comparison of static and dynamic cerebral autoregulation during mild whole-body cold stress in individuals with and without cervical spinal cord injury: a pilot study

STUDY DESIGN

Experimental study.

OBJECTIVES

To characterize static and dynamic cerebral autoregulation (CA) of individuals with cervical spinal cord injury (SCI) compared to able-bodied controls in response to moderate increases in mean arterial pressure (MAP) caused by mild whole-body cold stress.

SETTING

Japan METHODS: Five men with complete autonomic cervical SCI (sustained > 5 y) and six age-matched able-bodied men participated in hemodynamic, temperature, catecholamine and respiratory measurements for 60 min during three consecutive stages: baseline (10 min; 33 °C water through a thin-tubed whole-body suit), mild cold stress (20 min; 25 °C water), and post-cold recovery (30 min; 33 °C water). Static CA was determined as the ratio between mean changes in middle cerebral artery blood velocity and MAP, dynamic CA as transfer function coherence, gain, and phase between spontaneous changes in MAP to middle cerebral artery blood velocity.

RESULTS

MAP increased in both groups during cold and post-cold recovery (mean differences: 5-10 mm Hg; main effect of time: p = 0.001). Static CA was not different between the able-bodied vs. the cervical SCI group (mean (95% confidence interval (CI)) of between-group difference: -4 (-11 to 3) and -2 (-5 to 1) cm/s/mm Hg for cold (p = 0.22) and post-cold (p = 0.24), respectively). At baseline, transfer function phase was shorter in the cervical SCI group (mean (95% CI) of between-group difference: 0.6 (0.2 to 1.0) rad; p = 0.006), while between-group differences in changes in phase were not different in response to the cold stress (interaction term: p = 0.06).

CONCLUSIONS

This pilot study suggests that static CA is similar between individuals with cervical SCI and able-bodied controls in response to moderate increases in MAP, while dynamic CA may be impaired in cervical SCI because of disturbed sympathetic control.

Cerebral autoregulation is minimally influenced by the superior cervical ganglion in two- week-old lambs, and absent in preterm lambs immediately following delivery

Cerebral vessels in the premature newborn brain are well supplied with adrenergic nerves, stemming from the superior cervical ganglia (SCG), but their role in regulation of blood flow remains uncertain. To test this function twelve premature or two-week-old lambs were instrumented with laser Doppler flow probes in the parietal cortices to measure changes in blood flow during changes in systemic blood pressure and electrical stimulation of the SCG. In lambs delivered prematurely at ∼129 days gestation cerebral perfusion and driving pressure demonstrated a direct linear relationship throughout the physiologic range, indicating lack of autoregulation. In contrast, in lambs two-weeks of age, surgical removal of one SCG resulted in ipsilateral loss of autoregulation during pronounced hypertension. Electrical stimulation of one SCG elicited unilateral increases in cerebral resistance to blood flow in both pre-term and two-week-old lambs, indicating functioning neural pathways in the instrumented, anesthetized lambs. We conclude cerebral autoregulation is non-functional in preterm lambs following cesarean delivery. Adrenergic control of cerebral vascular resistance becomes effective in newborn lambs within two-weeks after birth but SCG-dependent autoregulation is essential only during pronounced hypertension, well above the normal range of blood pressure.

Initial orthostatic hypotension and cerebral blood flow regulation: effect of α1-adrenoreceptor activity

We examined the hypothesis that α1-adrenergic blockade would lead to an inability to correct initial orthostatic hypotension (IOH) and cerebral hypoperfusion, leading to symptoms of presyncope. Twelve normotensive humans (aged 25 ± 1 yr; means ± SE) attempted to complete a 3-min upright stand, 90 min after the administration of either α1-blockade (prazosin, 1 mg/20 kg body wt) or placebo. Continuous beat-to-beat measurements of middle cerebral artery velocity (MCAv; Doppler), blood pressure (finometer), heart rate, and end-tidal Pco2were obtained. Compared with placebo, the α1-blockade reduced resting mean arterial blood pressure (MAP) (−15%; P < 0.01); MCAv remained unaltered ( P ≥ 0.28). Upon standing, although the absolute level of MAP was lower following α1-blockade (39 ± 10 mmHg vs. 51 ± 14 mmHg), the relative difference in IOH was negligible in both trials (mean difference in MAP: 2 ± 2 mmHg; P = 0.50). Compared with the placebo trial, the declines in MCAv and PetCO2during IOH were greater in the α1-blockade trial by 12 ± 4 cm/s and 4.4 ± 1.3 mmHg, respectively ( P ≤ 0.01). Standing tolerance was markedly reduced in the α1-blockade trial (75 ± 17 s vs. 180 ± 0 s; P < 0.001). In summary, while IOH was little affected by α1-blockade, the associated decline in MCAv was greater in the blockade condition. Unlike in the placebo trial, the extent of IOH and cerebral hypoperfusion failed to recover toward baseline in the α1-blockade trial leading to presyncope. Although the development of IOH is not influenced by the α1-adrenergic receptor pathway, this pathway is critical in the recovery from IOH to prevent cerebral hypoperfusion and ultimately syncope.

Sympathetic regulation of the human cerebrovascular response to carbon dioxide

Although the cerebrovasculature is known to be exquisitely sensitive to CO2, there is no consensus on whether the sympathetic nervous system plays a role in regulating cerebrovascular responses to changes in arterial CO2. To address this question, we investigated human cerebrovascular CO2 reactivity in healthy participants randomly assigned to the α1-adrenoreceptor blockade group (9 participants; oral prazosin, 0.05 mg/kg) or the placebo control (9 participants) group. We recorded mean arterial blood pressure (MAP), heart rate (HR), mean middle cerebral artery flow velocity (MCAV mean), and partial pressure of end-tidal CO2 (PetCO2) during 5% CO2 inhalation and voluntary hyperventilation. CO2 reactivity was quantified as the slope of the linear relationship between breath-to-breath PetCO2 and the average MCAvmean within successive breathes after accounting for MAP as a covariate. Prazosin did not alter resting HR, PetCO2, MAP, or MCAV mean. The reduction in hypocapnic CO2 reactivity following prazosin (−0.48 ± 0.093 cm·s−1·mmHg−1) was greater compared with placebo (−0.19 ± 0.087 cm·s−1·mmHg−1; P < 0.05 for interaction). In contrast, the change in hypercapnic CO2 reactivity following prazosin (−0.23 cm·s−1·mmHg−1) was similar to placebo (−0.31 cm·s−1·mmHg−1; P = 0.50 for interaction). These data indicate that the sympathetic nervous system contributes to CO2 reactivity via α1-adrenoreceptors; blocking this pathway with prazosin reduces CO2 reactivity to hypocapnia but not hypercapnia.

Methamphetamine causes sustained depression in cerebral blood flow

The use prevalence of the highly addictive psychostimulant methamphetamine (MA) has been steadily increasing over the past decade. MA abuse has been associated with both transient and permanent alterations in cerebral blood flow (CBF), hemorrhage, cerebrovascular accidents and death. To understand MA-induced changes in CBF, we exposed C56BL/6 mice to an acute bolus of MA (5mg/kg MA, delivered IP). This elicited a biphasic CBF response, characterized by an initial transient increase (~ 5 minutes) followed by a prolonged decrease (~ 30 minutes) of approximately 25% relative to baseline CBF--as measured by laser Doppler flowmetry over the somatosensory cortex. To assess if this was due to catecholamine derived vasoconstriction, phentolamine, an α-adrenergic antagonist was administered prior to MA treatment. This reduced the initial increase in CBF but failed to prevent the subsequent, sustained decrease in CBF. Consistent with prior reports, MA caused a transient increase in mean arterial blood pressure, body temperature and respiratory rate. Elevated respiratory rate resulted in hypocapnia. When respiratory rate was controlled by artificially ventilating mice, blood PaCO(2) levels after MA exposure remained unchanged from physiologic levels, and the MA-induced decrease in CBF was abolished. In vivo two-photon imaging of cerebral blood vessels revealed sustained MA-induced vasoconstriction of pial arterioles, consistent with laser Doppler flowmetry data. These findings show that even a single, acute exposure to MA can result in profound changes in CBF, with potentially deleterious consequences for brain function.

Sympathetic control of the cerebral vasculature in humans

BACKGROUND AND PURPOSE

The role of the sympathetic nervous system in cerebral autoregulation remains poorly characterized. We examined cerebral blood flow responses to augmented arterial pressure oscillations with and without sympathetic blockade and compared them with responses in the forearm circulation.

METHODS

An oscillatory lower body negative pressure of 40 mm Hg was used at 6 frequencies from 0.03 to 0.08 Hz in 11 healthy subjects with and without alpha-adrenergic blockade by phentolamine.

RESULTS

Sympathetic blockade resulted in unchanged mean pressure and cerebral flow. The transfer function relationship to arterial pressure at frequencies >0.05 Hz was significantly increased in both the cerebral and brachial circulations, but the coherence of the relation remained weak at the lowest frequencies in the cerebral circulation.

CONCLUSIONS

Our data demonstrate a strong, frequency-dependent role for sympathetic regulation of blood flow in both cerebral and brachial circulations. However, marked differences in the response to blockade suggest the control of the cerebral circulation at longer time scales is characterized by important nonlinearities and relies on regulatory mechanisms other than the sympathetic system.

Autoregulatory cerebral vasodilation occurs during orthostatic hypotension in patients with primary autonomic failure

It is unclear whether patients with autonomic failure autoregulate cerebral blood flow during hypotension. The objective in this study was to examine cerebral autoregulatory capacity in patients with autonomic failure by studying changes in middle cerebral artery blood flow velocity using transcranial Doppler ultrasonography before, during, and after tilt-induced hypotension. Nine patients with primary autonomic failure were evaluated. Mean arterial pressure and middle cerebral artery blood flow velocity were simultaneously recorded while the patients were in the supine position, during 60 degrees head-up tilt, and after they were returned to the horizontal position. The results were as follows: during tilt-induced hypotension, mean arterial pressure decreased significantly more than middle cerebral artery mean blood flow velocity (58% versus 36%, p <0.0002). After return to the horizontal position, mean arterial pressure returned to baseline, and middle cerebral artery blood flow velocity transiently increased above pretilt value (p <0.02). It is concluded that cerebral autoregulatory vasodilation occurs in patients with autonomic failure. This was demonstrated by a more pronounced decline in mean arterial pressure than in middle cerebral artery blood flow velocity during hypotension and by a transient increase in middle cerebral artery blood flow velocity (ie, hyperemic response) after blood pressure was restored.

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

Simultaneous registrations of intracranial blood flow velocity parameters achieved by transcranial Doppler sonography and basic cardiovascular parameters were carried out during orthostatic changes in normal controls, diabetic patients and patients with pandysautonomia. Normal subjects had a rapid increase in heart rate at a constant blood pressure and a slight decrease in cerebral blood flow velocities associated with a mild increase of the pulsatility index (PI) after being tilted from a horizontal to a vertical position. Diabetics showed a fixed heart rate reflecting the disturbed autonomic innervation but only minor changes of cerebral blood flow velocity, which is similar to normal cerebrovascular autonomic regulation. Patients with pandysautonomia had a fixed heart rate associated with a decrease of systemic blood pressure but a failure of compensatory cerebral autoregulation to maintain normal flow velocity values after standing up. The results suggest that in diabetics cerebrovascular autonomic regulation is intact in contrast to cardiac autonomic function, while in patients with pandysautonomia both functions are disturbed. Criteria for the interpretation of autonomic regulatory mechanisms involved in cerebrovascular flow measurements are discussed.

Effect of peripheral and central alpha-adrenoceptor blockade on cerebral microvascular and blood flow responses to hypoxia

The purpose of this study was to evaluate the effects of vascular and central alpha-adrenoceptor blockade on cerebral blood flow (CBF) and utilization of brain arteriolar and capillary reserve in conscious rats during normoxia and hypoxia (8% O2 in N2). Animals were divided into three groups and administered either saline, N-methyl chlorpromazine (does not cross the blood-brain barrier), or phenoxybenzamine (crosses the blood-brain barrier) in equipotent doses. Neither agent affected regional CBF and the utilization of brain microvascular reserve during normoxia. CBF increased from 70.9 +/- 2.9 (SEM) ml/min/100 g in the control normoxic group to 123.8 +/- 4.2 ml/min/100 g in control hypoxic animals. In control, hypoxic flow to pons and medulla of the brain was higher than to cortex, hypothalamus or thalamus. The percent of arterioles/mm2 perfused increased from 49.6 +/- 2.0% during control normoxia to 65.6 +/- 3.0% during control hypoxia. The percentage of capillaries/mm2 perfused changed similarly. Hypoxic CBF was increased similarly after administration of N-methyl chlorpromazine or phenoxybenzamine. Administration of N-methyl chlorpromazine or phenoxybenzamine eliminated regional differences in hypoxic CBF and the utilization of arterioles, and did not affect capillary response. There was no difference between the effect of N-methyl chlorpromazine and phenoxybenzamine on cerebral microvascular and blood flow responses to hypoxia. It was concluded that peripheral alpha-adrenoceptors affect the distribution of regional microvascular and blood flow responses to hypoxia, and central alpha-adrenoceptors probably do not participate in this effect.

Volume therapy in orthostatic transient ischemic attacks

We report the case of an 83-year-old man with recurrent orthostatic transient ischemic attacks despite anticoagulation and crystalloid therapy. An initial cerebral angiogram revealed a nearly occluded right carotid artery with a string sign. Following aggressive volume expansion with albumin, the patient became asymptomatic. A second angiogram demonstrated the resolution of the carotid string sign and unmasked a high-grade, very tight, surgically approachable stenosis. The role of a high intravascular volume status is discussed.

Transcranial Doppler sonographic studies of cerebral autoregulation in Shy-Drager syndrome

A study is reported of mean arterial blood pressure and heart rate in four patients suffering from Shy-Drager syndrome. Blood flow velocity in the middle cerebral artery (MCA) was recorded by transcranial Doppler sonography. Concomitant changes in cerebral blood flow and the effect of cerebral autoregulation were thus examined. During tilt (60 degrees, head up) mean arterial blood pressure decreased by 40 mm Hg or 35%, while MCA blood flow velocity dropped by 14 cm/s or 28% (mean values). The lower percentage reduction in flow velocity may indicate a preserved cerebral autoregulation in central autonomic insufficiency.

Effect of reducing elevated blood pressure on cerebral circulation

Circulation to the brain is protected not only by autoregulation, which maintains cerebral blood flow (CBF) in spite of a decrease in mean arterial pressure, but also by baroreflexes that tend to prevent or minimize hypotension. Hypertensive patients have impaired cerebral autoregulation and depressed baroreflex sensitivity, suggesting that the brain might be vulnerable to a reduction in blood pressure. Nevertheless, in clinical experience, studies of cerebral blood flow during drug-induced reduction of elevated blood pressure have shown that control of hypertension can usually be achieved without jeopardizing cerebral blood flow or inducing signs and symptoms of cerebral ischemia, even in patients with cerebrovascular disease. Indeed, control of hypertension reduces both the risk of stroke and the recurrence rate for second strokes.

Treatment of hypertension with labetalol in neurosurgical practice. Influence of labetalol on cerebral perfusion pressure in dogs without and with intracranial mass lesions

In neurosurgical patients autoregulation of cerebral perfusion is often lost. Therefore, a sudden increase in blood pressure may lead to an increase in cerebral blood flow and cerebral oedema may follow. The influence of labetalol, a new alpha- and beta-adrenoceptor blocking agent, on intracranial pressure and cerebral perfusion pressure was investigated in dogs without and with mass lesions. During hypotension with labetalol the intracranial pressure remained unchanged and the cerebral perfusion pressure decreased to the same extent as mean arterial pressure (30%). Labetalol seems to be suitable to treat hypertension perioperatively in neurosurgical patients but it is not a suitable drug for induced hypotension.

Effect of differential spinal cord transection on human cerebral blood flow

Regional cerebral blood flow (rCBF) was measured by 133Xe inhalation in 17 patients with chronic spinal cord transection. This was done to investigate any effects such spinal cord deafferentation might have on resting rCBF and to test whether resulting chronic preganglionic sympathectomy influenced cerebral vasomotor CO2 responsiveness and autoregulation. Thirteen patients had complete cervical cord transection (CCT) at levels C4--C6 (age 37 +/- 15 years, time interval, 2 months--20 years). Four patients had complete thoracic cord transection at levels T3--4, T8 and T12 (TCT; age 49 +/- 22 years; time interval 2--5 months). CO2 responsiveness was tested by induced hypercapnia in 11 patients with CCT and 2 patients with TCT. Autoregulation was tested in 10 patients with CCT and 4 patients with TCT by decreasing cerebral perfusion pressure during postural tilting. Mean resting hemispheric Fg values (MHFg) were significantly reduced only in patients with CCT (MHFg = 69 +/- 12 ml/100 g brain/min), while brain stem-cerebellar Fg values (BSC Fg) were reduced significantly both in patients with CCT (BSC Fg = 85 +/- 10) and with TCT (BSC Fg = 88 +/- 12) compared to values measured in healthy normals (N = 21, MHFg = 81 +/- 10, BSC Fg = 98 +/- 10). Hemispheric CO2 responsiveness showed a trend toward reduction in patients with CCT but this was not statistically significant. Hemispheric autoregulation was significantly impaired in CCT compared to healthy normals but improved with time and rehabilitation.

Acute cerebral revascularization after regional cerebral ischemia in the dog. Part 2: Clinicopathological correlation

The efficacy of cerebral revascularization by anastomosis of the superficial temporal artery (STA) to the middle cerebral artery (MCA), performed 4 and 24 hours after a regional MCA infarction had been produced by combined occlusion of the MCA and internal carotid artery, was tested in 12 dogs. To control possible intercurrent variables, seven other dogs remained untreated and five had a sham operation. Clinical and pathological changes were recorded and analyzed. An incidence of 85% infarction was obtained in the untreated control group. The severity of the clinical deficits and pathological changes for the anastomosed groups were greater than those seen in the untreated control group. The extent of the infarction was significantly greater (p less than 0.05) in the anastomosed groups than in the sham-operated and control groups. Hemorrhagic infarcts occurred in most of the dogs in the anastomosed groups, but were not present in either control group (p less than 0.05). Two dogs in the 4-hour and one in the 24-hour group improved more than any control dog, but the difference was not statistically significant (p greater than 0.05). In two dogs with occluded anastomosis the clinical deficits and the pathological changes were less than those seen in animals with patent anastomosis. The severity of the pathological and clinical changes correlated well with the reestablishment of flow in the MCA territory. It is proposed that cerebral revascularization at 4 and 24 hours following a regional MCA infarct in the dog is followed by an exacerbation of the microcirculatory obstruction, cerebral edema, and infarction. From improvement noted in three animals the authors suggest that under special conditions the revascularization could benefit some cases following acute cerebral infarction.

Quantitative multiregional blood flow measurements during cervical sympathetic stimulation

The ethanol tissue sampling method for rCBF measurement was used to obtain information on the effects of cervical sympathetic stimulation in 8 cerebral structures in the non-anaesthetized rabbit. Sympathetic stimulation induced flow decreases of 12-29% according to structure, confirming the capability of this nerve to significantly reduce rCBF. Furthermore, a regional differentiation of cerebral structures into an 'anterior' group (mean decrease 22%) and a 'posterior' group (mean decrease 12%) with different reactivity to stimulation was established, thus confirming previous work in this laboratory with a local thermoclearance technique, and histochemical studies on sympathetic innervation to cerebral arteries and arterioles. These results provide strong evidence of the functional nature of the sympathetic nervous system in CBF regulation. The complementary nature of the ethanol technique (quantitative, multiregional measurement) and the local thermoclearance technique (continuous, semiquantitative measurements in 2-3 regions), and the absence of anaesthesia and significant trauma, means that combination of these two techniques offers considerable advantages in research on dynamic phenomena of the kind studied here.

The effect of hypocapnia and change of blood pressure on cerebral blood flow in men with cervical spinal cord transection

Cerebral blood flow (CBF) was measured by a 133Xe inhalation technique in 8 patients with chronic high spinal cord injuries. Six patients had a physiologically complete cervical cord transection and 1 subject had an incomplete C4-5 lesion but with evidence of interruption of sympathetic pathways. CBF and arterial blood pressure (BP) were measured in the supine position and then in the sitting, or feet up, position to produce a change of BP. In 4 patients CBF was measured during a suction manoeuvre applied to the lower half of the body to produce a fall of BP. There was no significant change of CBF in the patients during hypo- and hypertension. The response of CBF to hyperventilation for 5 min was measured in the supine position and did not differ significantly from that of 13 normal "control" subjects and 1 patient with a lesion at T2-3. It is concluded that the responses of the cerebral circulation to change of blood pressure and to hypocapnia are normal in patients with high spinal cord transection. The mechanisms involved in these responses are therefore independent of control via cervical sympathetic pathways.

Pharmacological control of local oxygen regulation mechanisms in brain tissue

The effect of several agents active on autonomic nervous system functions was tested on brain oxygen autoregulation parameters. It was found that atropine, propranolol and isoproterenol had no influence in abolishing the measured parameters. Phenoxybenzamine, tolazoline and dibenamine all suppress autoregulation. In an additional experimental series, a phenoxybenzamine infusion was given during O2 breathing. The infusion induced, in most cases, an additional rise in TpO2 (tissue pressure of oxygen, which refers to the partial pressure [in mm Hg] of this gas at the measuring tip of the electrode). It is concluded that an alpha-adrenergic mechanism is part of the autoregulation process. Also, the increase in brain TpO2 induced by 59% O2-5% Co2 breathing seems to be blocked or reversed by alpha-adrenolytic drugs, thus supporting the thinking that the effect of CO2 on cerebral blood flow is at least in part mediated through an alpha-adrenergic response.

Effects of decreasing arterial blood pressure on cerebral blood flow in the baboon. Influence of the sympathetic nervous system

The influence of the sympathetic nervous system on the cerebral circulatory response to graded reductions in mean arterial blood pressure was studied in anesthetized baboons. Cerebral blood flow was measured by the 133Xe clearance method, and arterial blood pressure was decreased by controlled hemorrhage. In normal baboons, the constancy of cerebral blood flow was maintained until mean arterial blood pressure was approximately 65% of the base-line value; thereafter, cerebral blood flow decreased when arterial blood pressure was reduced. Superior cervical sympathectomy of 2-3 weeks duration did not affect the normal response. In contrast, both acute surgical sympathectomy (cervical trunk division) and alpha-receptor blockade (1.5 mg/kg of phenoxybenzamine) enhanced the maintenance of cerebral blood flow in the face of hemorrhagic hypotension in that cerebral blood flow did not decrease until mean arterial blood pressure was approximately 35% of the base-line value. The results indicate that the sympathetic nervous system is not involved in the maintenance of cerebral blood flow in the face of a fall in arterial blood pressure. Indeed, the implication is that the sympathicoadrenal discharge accompanying hemorrhagic hypotension is detrimental to, rather than responsible for, cerebral autoregulation.

Chronically impaired autoregulation of cerebral blood flow in long-term diabetics

Using the arteriovenous oxygen difference method autoregulation of cerebral blood flow (CBF) was tested in 16 long-term diabetics and eight control patients. Blood pressure was raised by angiotensin infusion and lowered by trimethaphan camsylate infusion, in some cases combined with head-up tilting of the patient. Regression analysis was carried out on the results in order to quantify autoregulatory capacity. In the control patients CBF did not vary with moderate blood pressure variations, indicating normal autoregulation. In four of the 16 diabetic patients CBF showed significant pressure dependency, indicating impaired autoregulation. The cause of impaired autoregulation in some long-term diabetics is believed to be diffuse or multifocal dysfunction of cerebral arterioles due to diabetic vascular disease. Other conditions with impaired autoregulation are discussed and compared with that seen in long-term diabetes.

Alpha-receptor stimulation by endogenous and exogenous norepinephrine and blockade by phentolamine in pial arteries of cats

The question regarding the existence of an alpha-adrenergic component of pial arterial tone was investigated using a microapplication technique combined with the measurement of vascular diameter. Concentration-response curves for the alpha-receptor blocker, phentolamine, revealed no vascular reaction for a concentration range from 2.5 x 10(-11) to 2.5 x 10(-7) M. At higher concentrations (up to 1.3 x 10(-3) M) concentration-dependent dilations were observed. Constrictions of pial arteries induced by perivascular injection of 2.5 x 10(-6) M norepinephrine could be reduced by 38% and 73% when phentolamine was applied simultaneously in concentrations of 2.5 x 10(-7) and 2.5 x 10(-6) M, respectively, whereas constrictions due to 2.5 x 10(-4) M norepinephrine were not reduced by 2.5 x 10(-6) M phentolamine, indicating a competitive antagonism between norepinephrine and phentolamine for pial arteries. Stimulation of the cervical sympathetic chain (90 seconds, 10 v, 1.4 msec, 20 Hz) induced constrictions of pial arteries (mean 12%) which could be reduced by two-thirds during the simultaneous application of 2.5 x 10(-7) M phentolamine. Since the constriction induced by norepinephrine applied exogenously or released endogenously could be reduced by a concentration of phentolamine which had no vascular effect per se, we conclude that the resting tone of the pial arteries is not influenced by an alpha-adrenergic component under our experimental conditions. The dilations induced by high concentrations of phentolamine are believed to be nonspecific.

A paradoxical cerebral hemodynamic effect of hydralazine

Hydralazine is shown to have a very complex cerebral hemodynamic effect. It raises the intracranial pressure which, together with its effect upon systemic blood pressure, reduces the cerebral perfusion pressure. In spite of this and a concomitantly induced hyperventilation by hydralazine, CBF increases with some delay. The conclusion is that hydralazine is a cerebral vasodilator acting immediately upon cerebral capacitance vessels but later upon the resistance vessels as well.

Effect of sympathetic nerve stimulation on cerebral and cephalic blood flow in dogs

The effect of sympathetic stimulation (stellate ganglion) on dog cerebral and cephalic blood flows was studied via a cervical or a thoracic approach to the stellate ganglion under sodium pentobarbital or chloralose anesthesia. Two different stimulation voltages (3v and 5v) of monophasic pulses were applied for 1 minute. Venous outflow was measured at the confluence of the sagittal, straight and lateral sinuses with the lateral sinuses occluded and with them patent. When the lateral sinuses were occluded, stellate ganglion stimulation resulted in a marked decrease in common carotid blood flow to 38 plus or minus 2.5% (SE) of control and dilation of the ipsilateral pupil, but cerebral blood flow did not change. Similar effects were observed with each of the anatomic approaches, anesthetics, and voltages used and in dogs with low cerebral vascular tone induced by hypercapnia. When the lateral sinuses were kept patent, sympathetic nerve stimulation decreased the venous outflow to 89 plus or minus 2.9% of control and clamping both of the external jugular veins increased venous outflow to 120 plus or minus 2.7% of control. When the lateral sinuses were kept patent and the extracranial venous pressure was increased by clamping both of the external jugular veins, the decrease in venous outflow in response to sympathetic stimulation was even larger: venous outflow was only 65 plus or minus 4.9% of control. We conclude that stimulation of the stellate ganglion has no effect on the cerebral vasculature. Sympathetic stimulation significantly decreases venous blood flow measured at the confluence of the sinuses only when communications between the intracranial and extracranial venous vasculatures are present.

[Neurological complications of heart surgery. Review of their pathogenesis and bases for their treatment]

São relatadas as complicações neurológicas assinaladas em 320 pacientes submetidos a cirurgias cardíacas, com uma incidência de 7,8%. As etiologias mais comuns encontradas foram a embolia aérea (4,0%) e isquemia cerebral após hipotensão sistêmica (2,4%). Os autores analisam a patogenia das complicações cerebrais e tentam correlacioná-las com o fluxo sangüíneo cerebral, com o metabolismo cerebral e com a dinâmica dos pequenos vasos cerebrais. Uma revisão a respeito dos métodos para tratamento da isquemia cerebral é apresentada.