CEREBRAL VASOMOTOR PARALYSIS PRODUCED BY INTRACRANIAL HYPERTENSION

Intracranial pressure in patients with ruptured saccular aneurysm

✓ Intracranial pressure was recorded in 21 patients with subarachnoid hemorrhage due to rupture of a saccular aneurysm. Two different pressure patterns were found in nine patients who had verified recurrent hemorrhages while awaiting clinical improvement. One was associated with massive hematoma while the other occurred with edema but only minimal hematoma; the terms “hemorrhagic-compressive lesion” and “ischemic-edematous lesion” have been used for these two conditions. Four patients showed transient deterioration concomitant with marked pressure peaks in the continuous record. Although there was no evidence of fresh hemorrhage, three of these episodes were followed by a verified hemorrhage within 24 hours. Since no such “warning episode” was seen after the aneurysm had been clipped, the authors consider this pressure peak and concomitant clinical deterioration to be related to the mechanism of aneurysm rupture and possibly a forerunner of a life-threatening hemorrhage. These three pressure patterns showed the whole range from full spatial compensation to total decompensation. The determining factors are considered to be the volume of extravasated blood, the vasomoter reaction, and the intracranial spatial buffering capacity.

Effect of experimental brain injury on blood pressure, cerebral sinus pressure, cerebral venous oxygen tension, respiration, and acid-base balance

✓ Changes in the blood pressure, cerebral sinus pressure, cerebral venous oxygen tension, acid-base balance, respiratory frequency, and respiratory minute volume were studied in the rabbit after a lethal cold injury to the brain. About half of the animals responded to the injury with a quick rise in cerebral sinus pressure and in its relation to blood pressure (CSP/BP); in the other half, cerebral sinus pressure and the CSP/BP ratio rose more slowly. Changes in the CSP/BP ratio correlated well with criteria for changes in respiratory performance. The changes in cerebral venous oxygen tension were reasonably uniform: a dip during freezing, an overshoot to a mean of 1.6 times the original level (about 30 mm Hg) immediately after injury, a gradual return to the pretraumatic level, and then a drop to lower levels. The brain injury led to a respiratory alkalosis that became more pronounced the longer the animals lived. Considered with CSP/BP ratio, respiratory reaction to the brain injury may provide an early and accurate prognosis. The fact that at the time of death the cerebral perfusion pressure was still within a range believed safe for the brain shows that an actual brain injury, in addition to raised intracranial pressure, is important in such experiments and emphasizes the inappropriateness of comparing levels of intracranial pressure raised by a variety of methods.

Intracranial pressure gradients associated with experimental cerebral embolism

In a series of 20 cats unilateral cerebral oil embolism was followed by the development of clinical signs of increased intracranial pressure. Simultaneous bilateral epidural pressure measurements revealed marked pressure gradients between both cerebral hemispheres. Such local (tissue) pressure gradients possibly influence local cerebral blood flow (mainly in the diseased areas) by means of alterations in local tissue perfusion pressure.

Experimental effect of intracranial hypertension upon intraocular pressure

✓ Intracranial pressure was elevated acutely by inflation of an epidural balloon inside one side of the skull of monkeys. In most of the animals, intraocular pressure rose, beginning only after intracranial pressure had been elevated well above normal and continuing until the pressure in the expanding epidural balloon approached the level of the blood pressure. Thereafter intraocular pressure stabilized until it fell as vasomotor collapse ensued. The role of systemic arterial pressure elevations in the rising phase of intraocular pressure is thought to be less important than increases of ophthalmic venous pressure.

Effect of intracranial hypotension on cerebral blood flow

Intracranial hypotension increases cerebral blood flow. In dogs the average increase in cortical blood flow was 30 ml./100 g/min (47%) when the intracranial pressure was lowered acutely from 100 to 40 mm CSF. Permanent intracranial hypotension was established in seven demented patients using a ventriculoatrial shunt. The mean post shunt pressure was 50 mm CSF. In this group, the cerebral vascular resistance decreased 32%, the cortical blood flow increased 37%, and the relative weight of functional grey matter increased 44%. The systemic blood pressure was 8% lower. The increase in cerebral blood flow is the result of an increase in the pressure differential between the precapillary arterioles and the veins. In addition, the vessels dilate in response to the decreased external pressure. This increase in cerebral blood flow may be the mechanism for improvement in patients with normal pressure hydrocephalus who are shunted.

Vascular responses to acute intracranial hypertension

In 27 rhesus monkeys the cerebrospinal fluid pressure (CSFP) was raised by injections into the cisterna magna to about 40 to 50 mm Hg in steps of 5 mm Hg every five minutes. During the initial phase of the rise of the CSFP to about 15 mm Hg normal animals showed a significant fall in the systolic arterial blood pressure. With a further elevation of the CSFP the BP rose till the CSFP reached 30 to 40 mm Hg. If the CSFP were raised higher than that, a large number of the animals showed a significant fall in the BP. In animals which were shocked before the CSFP was raised there was no drop in the systolic BP during the initial phase. This study indicates that vascular decompensation occurs in the majority of animals when the CSFP goes higher than 30 to 40 mm Hg; there is a significant rise in the pulse rate, superior sagittal sinus pressure (SSP), and internal jugular vein pressure (JVP). The JVP was related to the SSP, indicating that the JVP most probably reflected the pressure changes in the intracranial venous sinuses. Four animals suddenly collapsed at the highest CSFP. In the remaining 23 animals, on a sudden lowering of the CSFP to zero from the highest level, 13 monkeys died in less than half an hour and four in about an hour, while six animals stood this elevation of the CSFP well, with a good recovery. This indicates that, once the vascular decompensation has set in, the prognosis is generally poor even after lowering the CSFP to normal. The drop of the CSFP to zero produced no significant change in the pulse rate but a significant fall in the BP. The SSP rose when its pre-lowering level was less than 7·5 mm Hg and fell when the level was at or above 7·5 mm Hg level. The JVP showed a significant correlation with the variations in the SSP. The fundus examination at the end of the experiment revealed no abnormality.

The effect of increased intracranial pressure on cerebrovascular hemodynamics

✓ Both brain edema (increased water content) and enlargement of the vascular compartment have been implicated as being responsible for intracranial hypertension following trauma. In this study pertinent cerebrovascular hemodynamic parameters have been investigated in states of increased intracranial pressure (ICP) and graded trauma to determine whether cerebral edema or vascular factors are of major importance. Utilizing the monkey-epidural balloon experimental model, continuous measurements of the mean arterial pressure (MABP) , jugular outflow pressure (MJVP), and sagittal sinus wedge pressure (SSWP) were obtained. Shulman's observations that the sagittal sinus wedge pressure accurately reflects the intracranial pressure have been confirmed. The total cerebral blood flow (CBF) and mean transit time (t̄) were determined and the total cerebral blood volume (CBV) computed. From these data the venous (Rv), arterial (Ra), and total resistances (Rt) were calculated. Analysis of these parameters during both the acute elevation of ICP and that following graded trauma has demonstrated: 1) a progressive decrease in the total cerebral blood flow and volume and a concomitant increase in the mean transit time; 2) a progressive increase in the total resistance with a shift from the arterial to the venous side; 3) a progressive decrease in the perfusion pressure (PP = MABP-SSWP); 4) impairment of CO2 reactivity pari passu with vasomotor activity and autoregulation of flow to pressure. The findings did not support the concept that increased intracranial pressure following trauma is the result of an increase in the size of the cerebrovascular compartment.

Effects of hyperbaric oxygen on intracranial pressure and cerebral blood flow in experimental cerebral oedema

Increased intracranial pressure was induced in anaesthetized dogs by application of liquid nitrogen to the dura mater. Intracranial pressure and cerebral blood flow were measured, together with arterial blood pressure and arterial and cerebral venous blood gases.Carbon dioxide was administered intermittently to test the responsiveness of the cerebral circulation, and hyperbaric oxygen was delivered at intervals in a walk-in hyperbaric chamber, pressurized to two atmospheres absolute.Hyperbaric oxygen caused a 30% reduction of intracranial pressure and a 19% reduction of cerebral blood flow in the absence of changes in arterial PCO(2) or blood pressure, but only as long as administration of carbon dioxide caused an increase in both intracranial pressure and cerebral blood flow. When carbon dioxide failed to influence intracranial pressure or cerebral blood flow then hyperbaric oxygen had no effect. This unresponsive state was reached at high levels of intracranial pressure.

Experimental intracranial hypertension due to vascular blockade

✓ High intracranial hypertension was induced in dogs by intracarotid injections of oil. Cerebrospinal fluid pressures continued to rise as Cushing pressor responses were evoked, but were not exceeded by the blood pressure. Transmission of blood pressure through a dilated vascular bed has been suggested as the mechanism. There was no correlation between levels of cerebral edema and the rise in intracranial pressure. This increase in pressure due to vascular blockade has been differentiated from that caused by subarachnoid blockade.