Relationship between Venous Pressure and Cortical Blood Flow

Myogenic and metabolic feedback in cerebral autoregulation: Putative involvement of arachidonic acid-dependent pathways

The present paper presents a mechanistic model of cerebral autoregulation, in which the dual effects of the arachidonic acid metabolites 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) on vascular smooth muscle mediate the cerebrovascular adjustments to a change in cerebral perfusion pressure (CPP). 20-HETE signalling in vascular smooth muscle mediates myogenic feedback to changes in vessel wall stretch, which may be modulated by metabolic feedback through EETs released from astrocytes and endothelial cells in response to changes in brain tissue oxygen tension. The metabolic feedback pathway is much faster than 20-HETE-dependent myogenic feedback, and the former thus initiates the cerebral autoregulatory response, while myogenic feedback comprises a relatively slower mechanism that functions to set the basal cerebrovascular tone. Therefore, assessments of dynamic cerebral autoregulation, which may provide information on the response time of the cerebrovasculature, may specifically be used to yield information on metabolic feedback mechanisms, while data based on assessments of static cerebral autoregulation represent the integrated functionality of myogenic and metabolic feedback.

Cerebral Venous Infarction: The Pathophysiological Concept

Cerebral venous occlusion represents an often underdiagnosed cause for acute or slowly progressive neurological deterioration. The underlying pathophysiological basis is not well understood, but is different from those of arterial occlusion reflecting therefore different anatomical and physiological features of the cerebral venous system. Extensive collateral circulation within the cerebral venous system allows for a significant degree of compensation in the early stages of venous occlusion. Elevated cerebral venous pressure due to cerebral venous occlusion can result in a spectrum of phenomena including a dilated venous and capillary bed, development of interstitial edema, increased cerebrospinal fluid production, decreased cerebrospinal fluid absorption and rupture of venous structures (hematoma). All of these pathophysiological changes may explain the clinical observation that cerebral venous occlusion, if promptly diagnosed and adequately managed, contains reversible alterations and need not always lead to venous infarction. The present review outlines this different pathophysiological behavior of venous compared to arterial occlusion in the cerebral vasculature; special reference is given to the effect of these changes on the therapeutic impact.

The impact of raised intracranial pressure on cerebral venous hemodynamics: a prospective venous transcranial Doppler ultrasonography study

OBJECT

The effect of increased intracranial pressure (ICP) on cerebral venous blood flow has been the subject of very few clinical and experimental studies. The authors assessed the usefulness of venous transcranial Doppler (TCD) ultrasonography as a noninvasive monitoring tool for predicting raised ICP.

METHODS

Serial venous TCD studies of the basal vein of Rosenthal and the straight sinus (SS) were prospectively performed in 30 control volunteers and 25 patients with raised ICP. Correlations with ICP data were calculated using a multivariate regression model. Venous blood flow velocities (BFVs) in the basal vein of Rosenthal showed, within a certain range, a linear relationship between mean ICP and maximal venous BFV (r = 0.645; p<0.002). Moreover, a linear relationship was found for maximal venous BFVs in the SS and mean ICP (r = 0.928; p<0.0003).

CONCLUSIONS

Venous TCD studies may provide an additional noninvasive monitoring tool for raised ICP and give further insights into the cerebral venous hemodynamics present during raised ICP.

Correlation between venous stump pressure and brain damage after cortical vein occlusion: an experimental study

A canine model of cortical vein occlusion was used to evaluate whether data obtained from monitoring venous stump pressure could help predict cerebral infarction after venous obstruction. Following bilateral parasagittal craniotomy, the cortical vein in each hemisphere was temporarily occluded and the increase in pressure was directly measured. Permanent venous obstruction was subsequently produced, and parenchymal brain damage 24 hours later was classified as: Stage 0, no parenchymal damage; Stage I, mild edema; Stage II, moderate parenchymal edema and/or ischemic changes in neurons; and Stage III, moderate-to-severe hemorrhage. The histological stages correlated closely with the rise in venous pressure: mean pressure increases (+/- standard deviation) were 5.5 +/- 2.9 mm Hg in hemispheres graded as Stage 0 (12 hemispheres), 7.7 +/- 3.2 mm Hg in those graded as Stage I (five), 11.2 +/- 4.1 mm Hg in those classed as Stage II (five), and 16.4 +/- 5 in those categorized as Stage III (seven). There were significant differences between Stages 0 and II (p < 0.01) and between Stages 0 and III (p < 0.001). Disruption of the blood-brain barrier as indicated by extravasation of Evans blue dye correlated well with the pressure increment. These results may indicate the threshold for injury after cortical venous occlusion. Venous stump pressure measurements obtained during a test occlusion may be a useful adjunct in predicting brain damage and may be helpful for intraoperative vessel selection for venous resection.

Regional ischemia in cerebral venous hypertension due to embolic occlusion of the superior sagittal sinus in the rat

To determine the pathophysiological changes in brain tissue that characterize damage following cerebral venous hypertension, a model of cerebral venous hypertension in the rat was devised. This experimental model has the advantage of simultaneously measuring the regional changes in cerebral blood flow as well as the metabolism. The ischemic area demonstrated by the accumulation of NADH is confined to the cerebral cortex and becomes enlarged in proportion to the increase in venous pressure. This metabolic disturbance appears even in the very early period following cerebral venous hypertension. These pathophysiological features are different from those observed in the case of intracranial hypertension.

Effect of the Trendelenburg position on the distribution of arterial air emboli in dogs

We examined the effects of buoyancy on the distribution of arterial gas bubbles using in vitro and in vivo techniques in dogs. A simulated carotid artery preparation was used to determine the effects of bubble size and vessel angle on the velocity and direction of bubble movement in flowing blood. Because buoyancy tends to float bubbles away from dependent areas, bubble velocity would be expected to decrease as the vessel angle increased. We found that larger bubbles increased in velocity in the same direction as the blood flow at 0-, 10-, and 30-degree vessel angles and decreased when the vessel was positioned at 90 degrees. Smaller bubbles did not change velocity from 0 to 30 degrees and increased in velocity in the same direction as blood flow at 90 degrees. In 10 anesthetized dogs, we studied the effects of 0-, 10-, 15-, and 30-degree Trendelenburg's position on carotid artery distribution of gas bubbles injected into the left ventricle or ascending aorta. Regardless of the degree of the Trendelenburg position, the bubbles passed into the carotid artery simultaneously with passage into the abdominal aorta. We conclude that the forces of buoyancy do not overcome the force of arterial blood flow and that the Trendelenburg position does not prevent arterial bubbles from reaching the brain.

Brain and lungs at risk after cervical spinal cord transection: intracranial pressure, brain water, blood-brain barrier permeability, cerebral blood flow, and extravascular lung water changes

The early physiopathologic responses to transection of the cervical spinal cord (C-4) were studied in the experimental animal. After transection, increases were seen in the mean arterial pressure, pulmonary capillary wedge pressure, intracranial pressure, brain water, blood--brain barrier permeability, and extravascular lung water with a marked decrease occurring in cerebral blood flow. Pretreatment with an alpha-adrenergic blocker, phentolamine (Regitine Ciba-Geigy Corp.), followed by transection blocked the rise in mean arterial blood pressure and pulmonary capillary wedge pressure but did not affect the increases in intracranial pressure, brain water, blood--brain barrier permeability, and extravascular lung water and decreases in cerebral blood flow. Transection of the cervical spinal cord initiates a complex series of events involving intracranial compliance and pulmonary permeability, placing both brain and lungs at risk.

Evidence of myogenic vascular control in the rat cerebral cortex

The potential presence of myogenic regulation in the cerebral microvasculature of the rat was investigated using a method which alters intravascular pressure without appreciably changing cerebral perfusion pressure (arterial minus venous pressure). The entire rat was placed in a sealed box, with the cranial cavity open to the atmosphere and prepared for in vivo microscopy. By increasing the ambient pressure in the box, both systemic arterial and venous pressure could be changed by nearly equal amounts (+/- 20 mm Hg). Heart and respiratory rates were not influenced by changing ambient pressure by +/- 20 mm Hg. At elevated ambient pressures, cortical arterioles constricted in linear proportion to the ambient pressure, whereas subatmospheric ambient pressures caused vasodilation whose magnitude was about equal at ambient pressures of -6 to -18 mm Hg. The calculated vessel wall tension typically remained within about +/- 10-15% of control during changes of transmural pressure of +/- 20-40%. In all cases, arteriolar responses to changes in ambient and intravascular pressure reached a new steady state within 10-15 seconds and were sustained for up to 30 minutes. These data are interpreted to indicate the presence of a myogenic vascular response in the brain vasculature of the rat.

Retroglenoid venoconstriction and its influence on canine intracranial venous pressures

Using standard in vitro techniques, we found that the canine retroglenoid vein, a vessel that drains a significant fraction of canine cerebral venous effluent, demonstrated the following: an average wall thickness of approximately 240 microns; a norepinephrine (NE) content of approximately 3 micrograms/g tissue; a NE uptake capacity (uptake 1) of approximately 8 nmol/g tissue; an ED50 for NE of 1.9 X 10(-8) M; and a phentolamine-sensitive constriction during electric transmural stimulation that had a median effective frequency of approximately 3 Hz and a maximum response that was approximately 84% of the maximum response to exogenous NE. In a separate series of in vivo experiments conducted in six alpha-chloralose-anesthetized dogs, we found that electrical stimulation of the left superior cervical ganglion produced a phentolamine-sensitive, frequency-dependent increase in cerebral venous pressure (CVP) of up to 19 mm Hg when all cerebral venous effluent was diverted through the left retroglenoid vein. Taken together, our findings suggest that the canine retroglenoid vein undergoes a marked vasoconstriction during physiological frequencies of electric sympathetic nerve stimulation in vivo. Although our data further suggest that the retroglenoid is not a dominant influence on CVP in the intact dog, they do encourage a cautious interpretation of cerebral venous outflow data obtained with techniques in which cerebral effluent is drained primarily by extracranial veins.

Cerebral infarction due to carotid occlusion and carbon monoxide exposure III. Influence of neck vein occlusion

Unilateral cerebral infarcts were produced in the rat by ligation of one common carotid artery and a subsequent exposure to carbon monoxide. In animals which had undergone an additional ligation of the external jugular veins leading to a moderate increase of the cephalic venous pressure the outcome of the procedure was ameliorated significantly. Venous pressure elevation was thought to reduce the venous vascular resistance effectively by preventing the leptomeningeal veins from collapsing. Collapse of the leptomeningeal veins probably occurred during the severe carbon monoxide-induced hypotension causing a steep increase of cerebral vascular resistance.

Experimental cerebral hemodynamics. Vasomotor tone, critical closing pressure, and vascular bed resistance

✓ Application of Burton's concept of the critical closing pressure to experimental data on brain-blood flow in the monkey suggests that perfusion pressure, not vascular bed resistance, is the primary variable affecting cerebral blood flow. Perfusion pressure for the cerebral circulation is the mean arterial pressure minus the critical closing pressure (MAP — CCP). Vasomotor tone and intracranial pressure are the major determinants of the critical closing pressure. Changes in either of these variables, therefore, affect perfusion pressure and flow. Data on brain-blood flow at fixed vasomotor tone obtained over wide pressure ranges show little change in vascular bed resistance despite significant changes in flow. The diameter of resistance vessels probably does not change significantly throughout the normal physiological range of cerebral blood flow. The limits of the critical closing pressure in the anesthetized monkey are from 10 to 95 mm Hg. Using these limits, and beginning with the average values for MAP and CCP in 11 awake monkeys breathing room air, the authors present theoretical flow curves in response to changes in intracranial pressure and mean arterial pressure that closely approximate the data reported in man.

Autoregulation of cerebral blood flow. Studies during drug-induced hypertension in normal subjects and in patients with cerebral vascular diseases

Autoregulation of cerebral blood flow (constant perfusion despite variations in blood pressure) was studied in 15 control subjects and in 24 patients with cerebral vascular lesions. Thirteen patients were studied within the first 2 days of an episode of cerebral ischemia ("acute" group); 11 were studied from 15 days to several months after an ischemic episode ("chronic" group). Measurements of regional cerebral blood flow (rCBF) were made by recording extracranially in four to six locations the clearance of 85 Kr dissolved in saline and injected into the internal carotid artery. Blood flow values were calculated with a digital computer, using a compartmental analysis of the clearance curves. After measuring rCBF in the resting state, the blood pressure was increased by 30 to 40 mm Hg by the intravenous infusion of angiotensin amide (Hypertensin, Ciba). While the blood pressure was kept constant at the new level, a second series of measurements was made. Autoregulation was present in all normal subjects: no significant differences were found in average or compartmental flow rates between values obtained at normal and at hypertensive levels of blood pressure.

Loss of autoregulation was demonstrated in 10 of 13 patients of the "acute" group and in two of 11 patients of the "chronic" group. These data support previous observations that autoregulation is impaired during the first few days after an ischemic episode. The three patients of the "acute" group with a preserved autoregulatory response to hypertension had only transient ischemic attacks. In each of the other subjects of this group loss of autoregulation was found in one to three different regions of the brain. Our results suggest that regional loss of autoregulation is found frequently near the periphery of the ischemic brain. Impairment of autoregulation is one of the types of derangement of cerebral circulation that occurs after an ischemic episode.