Pial arterial response to systemic hypoxia in anaesthetised cats

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.

Cerebrovascular injury in premature infants: current understanding and challenges for future prevention

Cerebrovascular insults are a leading cause of brain injury in premature infants, contributing to the high prevalence of motor, cognitive, and behavioral deficits. Understanding the complex pathways linking circulatory immaturity to brain injury in premature infants remains incomplete. These mechanisms are significantly different from those causing injury in the mature brain. The gaps in knowledge of normal and disturbed cerebral vasoregulation need to be addressed. This article reviews current understanding of cerebral perfusion, in the sick premature infant in particular, and discusses challenges that lie ahead.

Cerebral blood flow response in adenosine 2a receptor knockout mice during transient hypoxic hypoxia

We evaluated cerebral blood flow by laser Doppler during 30 secs of hypoxia (0.10 FiO(2)) in anesthetized, ventilated adenosine 2a receptor knockout (A2aR KO) and wild-type (WT) mice to test the hypothesis that cerebral hypoxic hyperemia in KO mice would be attenuated. We also studied the effects of selective and nonselective A2aR antagonists. During 30 secs of hypoxia, P(a)O(2) decreased significantly (P<0.05) and to a similar degree in both types of mice, whereas P(a)CO(2) remained relatively stable. However, mean arterial blood pressure (MABP) decreased to a greater extent (P<0.05) during hypoxia in KO mice (58.6+/-1.5 mm Hg) than in WT animals (76.1+/-3.2 mm Hg). Consequently, in a separate group of mice, we stabilized and matched MABP during hypoxia. Hypoxic hyperemia was attenuated by 38% (P<0.05) in KO animals whose MABP was uncontrolled, and by 81% (P<0.05) in KO animals whose MABP changes were matched to the MABP in the hypoxic WT mice. In animals treated with adenosine antagonists, hypoxic hyperemia was decreased by 44% to 48% (P<0.05) in WT mice, but was without effect in KO mice. We conclude that adenosine via A2aR is responsible for a significant proportion of the hyperemia during hypoxia.

GM1 ganglioside induces vasodilation and increases catalase content in the brain

Monosialoganglioside (GM1) is a glycosphingolipid present in most cell membranes that displays antioxidant and neuroprotective properties. GM1 increases catalase activity in cerebral cortices in vivo, but the mechanisms underlying this effect of GM1 are not known. In the current study we investigated the effect of GM1 (50 mg/kg, ip) on the content of hemoglobin and catalase activity of hippocampus, cortex, and striatum of rats. GM1 administration increased catalase activity and hemoglobin content in brain samples after 30 min, but had no effect on blood catalase activity. GM1-induced increase in catalase activity was abolished by brain perfusion with heparinized saline. Brain catalase activity in the absence of blood, estimated by regression analysis of data from perfused and nonperfused animals, was not altered by the systemic injection of GM1. Moreover, the addition of GM1 (30 or 100 microM) did not increase catalase activity in slices of cerebral cortex in situ, further suggesting that blood circulation is required for this effect. The GM1-induced vasodilation was confirmed in vivo, because the systemic injection of GM1 (50 mg/kg, ip) increased (1.2-1.6 times) the width of pial vessels.

Dynamic observation of oxygenation-induced contraction of and transient fiber-network formation-disassembly in cultured human brain microvascular endothelial cells

Oxygenation-induced contraction of nonconfluent cultured human brain microvascular endothelial cells (HBECs, n = 30) was examined by video-enhanced contrast-differential interferential contrast microscopy. After administering a continuous gentle blow of pure oxygen gas to the surface of the medium just above the flattened HBEC, the plasma membrane exhibited tensioning and wrinkling, resulting in a strong contraction of the cell body by 14 +/- 7% (P < 0.001). When the cell stopped contracting, transient formation of a fiber network starting from certain spots (possibly adhesion plaques, though these were not visible in the majority of cases) and expanding to the whole cell was observed. The occurrence of fiber network formation was statistically significant (26 of 30 separate cells, P < 0.05). After cessation of oxygen delivery, the observed network of fibers broke up rapidly (in a period of 3.3 +/- 1.2 seconds) into small particles of <0.5 microm in diameter, which subsequently fused into the cellular structure. The HBEC completely recovered the control appearance. The sequential process was completed within 30 seconds and was reproduced in individual cells each time that oxygen gas was supplied. The authors conclude that the HBEC strongly contracts in response to a transient oxygenation stimulus, followed by rapid formation/disassembly of a network structure.

Cardiovascular and respiratory adjustments at altitude sustain cerebral oxygen delivery -- Severinghaus revisited

Analysis of a paper by Severinghaus et al. (see text) has already shown that sea level oxygen delivery (D(a)O(2)) is sustained 8 h after ascent to 3810 m, despite low arterial oxygen content (C(a)O(2)), largely as a result of increased cerebral blood flow (CBF). The present study extends the analysis to show that D(a)O(2) is also sustained after 3 and 5 days at altitude, despite a progressively falling CBF. It is shown that this later compensation is a result of the improvement in C(a)O(2), which accompanies acclimatisation. Since less than 3% rise in haemoglobin occurred, the rise in C(a)O(2) was predominantly respiratory. It has been shown elsewhere that as acclimatisation occurs, the fall in arterial PCO(2) (P(a)CO(2)) results in increased arterial PO(2) (P(a)O(2)) until they are related according to P(a)CO(2)=0.25 P(a)O(2)+/-15 mmHg. The results from Severinghaus et al. at 3 and 5 days fall close to this line. We also report arterialised capillary blood gases from 18 normal subjects, acclimatised at 5300 m. The values fall in a group centred on the same line. In summary, soon after arrival at altitude (8 h), cerebral oxygen delivery is largely sustained by an increase in CBF. The present study shows that, although CBF declines during the 3-5 day period, D(a)O(2) is sustained as a result of the improvement in C(a)O(2), which is mainly due to respiratory acclimatisation.

Indomethacin in the management of elevated intracranial pressure: a review

Elevated intracranial pressure occurs frequently in patients with severe head injury. A number of studies in recent years suggest that indomethacin may be useful in the management of elevated intracranial pressure. Indomethacin acts primarily by reducing cerebral blood flow and decreasing cerebral edema following head injury. This review summarizes the basic and clinical studies of the effects of indomethacin on cerebral blood flow, brain edema, and intracranial pressure. The pharmacology of indomethacin, and issues for future investigation in the use of indomethacin in severe head injury, are discussed.

An investigation of the actions of diltiazem on rat aorta exposed to acute hypoxia followed by re-oxygenation

1 The effects of diltiazem and removal of extracellular Ca2+ were examined on contractions, of the rat isolated aorta, to noradrenaline (NA) and high K+, during exposure to oxygenated conditions and hypoxia followed by re-oxygenation. 2 Exposure to hypoxia caused a similar reduction of contractile responses to NA and KCl, while re-oxygenation restored contractile activity. 3 Ca2+-free conditions abolished responses to KCl but a transient response to NA remained which was resistant to hypoxia. 4 Diltiazem produced similar reductions of responses to NA during both oxygenated conditions and hypoxia, whereas during re-oxygenation the effects of diltiazem upon responses to NA were enhanced. 5 Diltiazem produced a more pronounced reduction of responses to KCl than of responses to NA. However, the reduction of responses to KCl by diltiazem was not modified by the changes in PO2 examined in the present study. 6 The present study indicates that contractions of the rat aorta mediated by intracellular Ca2+ are resistant to the hypoxic conditions studied in the present investigation, whereas those responses mediated by an influx of Ca2+ are reduced. The increase in the contractile response to NA following re-oxygenation may result from an increased influx of extracellular Ca2+ since such responses show an enhanced sensitivity to diltiazem.

Hemodynamic study of internal carotid artery stenosis and occlusion: value of combined isotopic measurements of regional cerebral blood flow and blood volume

The assessment of the intracranial hemodynamic consequences of obstructive lesions of the carotid artery by measuring resting rCBF is inadequate because cerebral blood flow may remain constant in spite of significant drops in the intraluminal pressure due to autoregulation. Moreover, flow may be permanently decreased following cerebral infarction, even if the arterial anatomical conditions have resumed their normal state because of the decreased metabolic demand of an infarcted area. Measurement of the regional cerebral blood volume (rCBV) helps with the hemodynamic assessment of these conditions, since there is a linear and inverse relationship between intraarterial pressure and intracranial blood volume. In 24 patients exhibiting various carotid and ischemic brain lesions we studied both rCBF and rCBV. The latter is a comparative measure between hemispheres obtained by single photon emission tomography after autotransfusion of 99m Technetium labeled erythrocytes. There was no correlation between rCBF and clinical status, CT scan or arterial lesions. There was no correlation between rCBV and clinical status or CT scan. There was, however, an interesting correlation between rCBV and the severity of the arterial lesion. The rCBV was symmetrical in all patients with normal or moderately stenotic carotid arteries before and after operation. In some patients with severe unilateral stenosis or occlusion, there was a significant relative increase of rCBV in the hemisphere downstream from the lesion, which disappeared after surgery (endarterectomy or extra-intracranial bypass). In some patients with severe and bilateral carotid lesions, we noted an asymmetry in rCBV that disappeared after a unilateral operation. Other patients with similar lesions develop asymmetry only after an operation that resulted in a relative increase in rCBV in the hemisphere supplied by the non-operated artery. In conclusion analysis of these results suggests that in this series of patients, rCBV modifications were the consequence of cerebral autoregulation distal to the arterial lesions and provided satisfactory assessment of hemodynamic improvement after surgical repair.

Beagle puppy model of intraventricular hemorrhage. Effect of indomethacin on local cerebral glucose utilization

The newborn beagle puppy has been demonstrated to provide a good model for neonatal intraventricular hemorrhage (IVH). A study was designed to determine if indomethacin can prevent IVH, and if indomethacin would produce changes in local cerebral glucose utilization (LCGU). By computerized random design, newborn beagle puppies were pretreated with either indomethacin (a known inhibitor of prostaglandin synthetase) or saline, and then assigned either to receive a hemorrhagic hypotension/volume reexpansion insult or to receive no insult. Pretreatment with indomethacin produced a marked drop in the incidence of IVH as well as significant alterations in the blood pressure responses to the hemorrhagic hypotension/volume reexpansion insult. Carbon-14 autoradiography was used to determine LCGU: no alterations were demonstrated in cerebral metabolism in uninjured pups pretreated with indomethacin compared to saline-pretreated animals. In addition, although the hemorrhagic hypotension/volume reexpansion insult produced marked alterations in LCGU in both groups of traumatized pups, indomethacin prevented the changes in LCGU in the germinal matrix and white matter that were found in the saline-pretreated animals.

Cerebrovascular response to acute decreases in arterial PO2

The purpose of these studies was to examine the time course of the cerebrovascular response to acute hypoxia in unanesthetized ponies. An electromagnetic flow transducer chronically placed on the internal carotid artery of the pony allowed continuous recording of internal carotid artery blood flow (ICBF) which has been shown to be representative of cerebral blood flow (CBF). The ponies were subjected to three levels of acute isocapnic hypoxia (PaO2 = 62, 44, and 39 mm Hg for hypoxia level I, II, and III, respectively), and the temporal and steady-state cerebrovascular response was examined. ICBF increased significantly at all three hypoxia levels (8, 25, and 40% at hypoxia I, II, and III, respectively). This increase was rapid in the two most severe levels of hypoxia, beginning within 45 s, and was complete within 90 s. The increase lagged behind the reduction in PaO2 by 24-28 s. During the very mild level of hypoxia (I), no such rapid increase in flow was observed; rather, the increase occurred only after 5 min of hypoxia. Microsphere (15 microns diameter) measurements from six ponies during the most severe level of hypoxia (III) demonstrated that CBF increased 38%. Noncerebral tissues known to be vascularly connected to the circle of Willis, and thus capable of receiving blood flow via the internal carotid artery, either did not change or increased so slightly during hypoxia that their effect on ICBF was minimal.(ABSTRACT TRUNCATED AT 250 WORDS)

Beagle puppy model of intraventricular hemorrhage. Effect of indomethacin on cerebral blood flow

The newborn beagle puppy has been demonstrated to provide a good model for neonatal intraventricular hemorrhage (IVH). A study was designed to determine if indomethacin can prevent IVH and if indomethacin would produce changes in cerebral blood flow (CBF). Newborn beagle puppies were randomized by computer into two groups: one was pretreated with indomethacin, a known inhibitor of prostaglandin synthetase, and the other was saline. The dogs in both groups were then assigned either to undergo hemorrhagic hypotension/volume reexpansion insult or to receive no insult. Twenty percent of all pups receiving indomethacin and undergoing the insult experienced IVH, compared to 71% of the pups undergoing insult that had been pretreated with saline. Significant alterations in the blood pressure responses to the hemorrhagic hypotension/volume reexpansion insult were noted in the former group compared to the saline-pretreated pups subjected to insult. Finally, employing carbon-14 autoradiography for the determination of CBF, it was demonstrated that indomethacin decreases resting CBF of the newborn beagle pups and, in indomethacin-pretreated animals subjected to insult, prevents the increases in CBF seen in the saline-pretreated traumatized pups.

Cerebrovascular effects of prostaglandin inhibitors in the gerbil

Autoregulation of cerebral blood flow (CBF) to mean arterial blood pressure (MABP) of 40-50 mm Hg has been demonstrated in the spontaneously breathing gerbil anaesthetised with barbiturate (60 mg/kg). CO2 reactivity has also been assessed at 2.8% change CBF/mm Hg change in arterial PCO2. In six animals pretreated with indomethacin (3 mg/kg), autoregulation was preserved although the resting CBF was significantly reduced, but CO2 reactivity was completely abolished. 1-n-Butyl imidazole, a specific thromboxane synthetase inhibitor, was used in six other animals (3 mg/kg), and this abolished CO2 reactivity while preserving autoregulation; the effect of this agent has not been described previously. Both drugs inhibit different pathways of prostaglandin metabolism and may interfere with normal CO2 reactivity in several ways. Two explanations are that prostaglandins constitute the final common pathway in effecting cerebrovascular response to CO2 or, alternatively, that the free radicals and ionic fluxes generated during prostaglandin metabolism are a coincidental source of the hydrogen ion changes required.