Effects of arterial hypoxaemia, hypercapnia, and changes in cerebral perfusion pressure on mean cerebrospinal fluid and sagittal sinus pressure

Cerebral spinal fluid dynamics: effect of hypoxia and implications for high-altitude illness

The pathophysiology of acute mountain sickness and high-altitude cerebral edema, the cerebral forms of high-altitude illness, remain uncertain and controversial. Persistently elevated or pathological fluctuations in intracranial pressure are thought to cause symptoms similar to those reported by individuals suffering cerebral forms of high-altitude illness. This review first focuses on the basic physiology of the craniospinal system, including a detailed discussion of the long-term and dynamic regulation of intracranial pressure. Thereafter, we critically examine the available literature, based primarily on invasive pressure monitoring, that suggests intracranial pressure is acutely elevated at altitude due to brain swelling and/or elevated sagittal sinus pressure, but normalizes over time. We hypothesize that fluctuations in intracranial pressure occur around a slightly elevated or normal mean intracranial pressure, in conjunction with oscillations in arterial Po2 and arterial blood pressure. Then these modest fluctuations in intracranial pressure, in concert with direct vascular stretch due to dilatation and/or increased blood pressure transmission, activate the trigeminal vascular system and cause symptoms of acute mountain sickness. Elevated brain water (vasogenic edema) may be due to breakdown of the blood-brain barrier. However, new information suggests cerebral spinal fluid flux into the brain may be an important factor. Regardless of the source (or mechanisms responsible) for the excess brain water, brain swelling occurs, and a “tight fit” brain would be a major risk factor to produce symptoms; activities that produce large changes in brain volume and cause fluctuations in blood pressure are likely contributing factors.

Effects of duration of isoflurane anesthesia and mode of ventilation on intracranial and cerebral perfusion pressures in horses

OBJECTIVE

To test the hypothesis that isoflurane-anesthetized horses during controlled ventilation and spontaneous ventilation exhibit temporal changes in cerebral hemodynamics, as measured by intracranial pressure and cerebral perfusion pressure, that reflect temporal changes in systemic arterial pressure.

ANIMALS

6 healthy adult horses.

PROCEDURE

Horses were anesthetized in left lateral recumbency with 1.57% isoflurane in O2 for 5 hours in 2 experiments by use of either controlled ventilation (with normocapnia) or spontaneous ventilation (with hypercapnia) in a randomized crossover design. Intracranial pressure was measured with a subarachnoid strain-gauge transducer. Carotid artery pressure, central venous pressure, airway pressures, blood gases, and minute ventilation also were measured.

RESULTS

Intracranial pressure during controlled ventilation significantly increased during constant dose isoflurane anesthesia and thus contributed to decreasing cerebral perfusion pressure. Intracranial pressure was initially higher during spontaneous ventilation than during controlled ventilation, but this difference disappeared over time; no significant differences in cerebral perfusion pressures were observed between horses that had spontaneous or controlled ventilation.

CONCLUSIONS AND CLINICAL RELEVANCE

Cerebral hemodynamics and their association with ventilation mode are altered over time in isoflurane-anesthetized horses and could contribute to decreased cerebral perfusion during prolonged anesthesia.

Analysis of CO2 vasomotor reactivity and vessel diameter changes by simultaneous venous and arterial Doppler recordings

BACKGROUND AND PURPOSE

The use of flow velocity changes in the middle cerebral artery (MCA) measured by Doppler techniques as an index of corresponding cerebral blood flow (CBF) changes is based on the assumption that the insonated arterial diameter remains stable. The postulate of unchanging vessel calibers during CBF changes, however, is still under debate. We performed simultaneous measurements of arterial and venous blood flow velocities by transcranial Doppler ultrasound during various stages of hypercapnia to analyze diameter changes in the insonated vessels by comparing differences in the vasomotor reactivity.

METHODS

Simultaneous Doppler recordings of 1 MCA and of a contralateral venous vessel thought to represent the sphenoparietal sinus (SPS) were carried out with a pair of 2-MHz range-gated transducers in 16 young healthy subjects during variations of end-tidal PaCO2.

RESULTS

During hypercapnia the mean blood flow velocity of the MCA rose from 62. 5+/-10.2 to a maximum of 99+/-12.2 cm/s (vasomotor reactivity of 60. 1+/-17.3%). The corresponding values in the SPS were significantly higher (P<0.001), revealing a rise from 17.8+/-5.7 to 34.9+/-14.3 cm/s (vasomotor reactivity of 91.4+/-25.9%). Exponential and linear regression analyses revealed an identical high correlation (r2=0.97 and 0.98 for the MCA and SPS, respectively). Slopes were 0.034+/-0. 01 on the arterial and 0.048+/-0.01 on the venous side. The CO2 reactivity (percentage per mm Hg, EtCO2) was found to be 4.5+/-1%/mm Hg in the MCA and 6.8+/-1.5%/mm Hg in the SPS. This difference indicates a vasodilation of the MCA in comparison to the venous vessel.

CONCLUSIONS

We have demonstrated a different reaction pattern between intracranial venous and arterial vessels related to end-tidal CO2. Relating the flow velocities to the square of the vessel diameter and assuming a global rise of CBF and not extensible sinus walls, our results indicate that the MCA undergoes a vasodilation of 9.5+/-7% in maximal hypercapnia.

Marked episodic elevation of cerebrospinal fluid pressure during nocturnal sleep in patients with sleep apnea hypersomnia syndrome

The CSF pressure was measured continuously at the lumbar level during nocturnal sleep in 3 patients with sleep apnea hypersomnia syndrome. Nocturnal sleep was very unstable with frequent episodes of obstructive sleep apnea. When the patients were awake and relaxed in the supine position, their CSF pressure was stable and within the normal range. Episodic marked elevations of CSF pressure occurred frequently during sleep, and each elevation was preceded and accompanied by an episode of sleep apnea or hypopnea. Significant correlations were found between the duration of apneic episodes and increase of CSF pressure, and between decrease of SaO2 or TcPO2 and increase of CSF pressure. The duration of sleep apnea was longer, increase of CSF pressure was greater, and decreases of SaO2 and TcPO2 were more marked during REM sleep than during NREM sleep. It is suggested that the frequent marked episodic elevations of CSF pressure are caused by an increase in the intracranial vascular volume occurring mainly in response to transient hypercapnia and hypoxia, which are induced by pulmonary hypoventilation during the episodes of sleep apnea.

Effect of atropine on cerebrovascular responsiveness to carbon dioxide

Effects of cholinergic inhibition by atropine on cerebral circulation were studied in 15 baboons anesthetized with sodium pentobarbital. Intravertebral infusion of atropine, 0.1 mg/kg, did not cause any changes in cerebral blood flow (CBF), superior sagittal sinus wedge pressure (SSWP), epidural pressure (EDP), cerebral perfusion pressure, or cerebral vascular resistance under normal conditions. Cerebrovascular responsiveness to carbon dioxide (CO2) inhalation was not influenced by atropine. The presence of cholinergic nerve fibers has been proved in the cerebral blood vessels and the existence of cholinergic mechanism suggested in the brain stem, but it is not likely that the cholinergic nerves have tonic control of cerebral blood vessels in the resting state or affect cerebrovascular responsiveness to CO2. The changes in EDP and those in SSWP showed a very good correlation to each other. There was also a good correlation between the changes in CBF and those in EDP or SSWP.

Cerebral blood flow and oxidative brain metabolism during and after moderate and profound arterial hypoxaemia

In anaesthetized artificially ventilated dogs, the effect of graded arterial hypoxaemia on cerebral blood flow (CBF) and on the oxidative carbohydrate metabolism of the brain was tested. It is shown that the hypoxic vasodilatory influence on cerebral vessels is present even at moderate systemic hypoxaemia, provide that PaCO2 is kept within normal limits. At PaO2 of about 50 Torr, CBF increased from 56.6 to 89.7 ml/100g/min. With increasing cerebral hyperamia (CBF increased to 110.9 ml/100g/min, at PaO2 of 30 Torr), CMRO2 (4.2 ml/100g/min) was not significantly raised above its normal level (4.7 ml/100g/min) even with profound arterial hypoxaemia. This shows that CMRO2 levels are poor indices of hypoxic hypoxia. A disproportionately high increase in cerebral glucose uptake (CMR glucose levels rose from 4.4 to 10.4 mg/100g/min) and enhanced cerebral glycolysis (CMR lactate changed from 0.2 to 1.6 mg/100g/min) at moderately reduced PaO2 (50 Torr) indicated early metabolic changes which became more marked with further falls in arterial oxygen tension. However, 60 minutes after restoration of a normal PaO2 level, CBF and brain metabolism were found to have completely recovered. It is concluded that a short period of profound systemic hypoxaemia does not produce long lasting metabolic and circulatory disorders of the brain provided the cerebral perfusion pressure does not vary, and is kept at normal levels.