Cerebral circulatory responses to arterial hypoxia in normal and chemodenervated dogs

Effects of dexmedetomidine on cerebral circulation and systemic hemodynamics after cardiopulmonary resuscitation in dogs

PURPOSE

Our purpose was to examine the effect of dexmedetomidine, when used with phenylephrine during cardiopulmonary resuscitation (CPR), on the cerebral and systemic circulations.

METHODS

In pentobarbital-anesthetized, mechanically ventilated dogs, we evaluated pial vessel diameters, cerebral oxygen extraction, and systemic hemodynamics before and after cardiac arrest (5 min) and resuscitation, in the presence or absence of dexmedetomidine (n = 7 each; dexmedetomidine or control group).

RESULTS

In both groups: (a) pial arterioles were dilated at 5 and 15 min after CPR, and had returned to baseline diameters at 30 min; (b) sagittal sinus pressure was significantly raised at 5 and 15 min after CPR; and (c) cerebral oxygen extraction was decreased at 5, 15, and 30 min after CPR, and had returned to baseline level at 60 min after CPR. We could find no differences between the two groups in the cerebral circulation after CPR. However, the number of defibrillation electric shocks required to restore spontaneous circulation (5.5 vs 3.6; P < 0.05), the dose of phenylephrine used for CPR (1193 microg vs 409 microg; P < 0.01), and the number of postresuscitation ventricular ectopic beats observed during the first 120 min after successful resuscitation (1606 vs 348; P < 0.05) were all significantly lower in the dexmedetomidine group.

CONCLUSION

Although intravenous dexmedetomidine, as used for CPR, does not have a beneficial effect on either cerebral vessels or cerebral oxygen extraction, it may reduce the number of defibrillation shocks needed and the number of postresuscitation ventricular ectopic beats, and help to bring about stable systemic circulation after CPR.

Cerebrovascular effects of carbon monoxide

This review examines the influence of endogenous and exogenous carbon monoxide (CO) on the cerebral circulation. Although CO generated from neuronal heme oxygenase can modulate neurotransmission, evidence supporting its role in cerebral vasodilation is limited. In newborn piglets, heme oxygenase is enriched in microvessels and contributes to hypoxic vasodilation. Low CO concentrations dilate piglet arterioles by opening calcium-activated potassium channels. With inhalation of CO and formation of carboxyhemoglobin, cerebral vasodilation can be greater than that occurring with hypoxic hypoxia at equivalent reductions of arterial oxygen content. This additional vasodilation is probably attributable to additional release of hypoxic vasodilators secondary to increased oxyhemoglobin affinity, although direct effects of CO on cerebral arterioles may also occur. When CO exposure is prolonged, cerebral endothelium undergoes oxidant stress as evident by nitrotyrosine formation. As CO levels increase, modest decreases in oxygen consumption are detectable, which may reflect CO or nitric oxide interactions with cytochrome oxidase in regions with very low oxygen availability. If subsequent CO concentration increases sufficiently to depress cardiac function and limit cerebral perfusion, cerebral oxygen consumption becomes further reduced, and oxidant stress becomes amplified by leukocyte sequestration and xanthine oxidase activity with consequent lipid peroxidation. Specific regions of the brain, such as central white matter, globus pallidus, and hippocampus, are selectively vulnerable to CO toxicity, but whether the mechanisms involved in selective injury differ from other forms of hypoxia-ischemia needs to be clarified.

Determinants of cerebral fractional oxygen extraction using near infrared spectroscopy in preterm neonates

Cerebral fractional oxygen extraction (FOE) represents the balance between cerebral oxygen delivery and consumption. This study aimed to determine cerebral FOE in preterm infants during hypotension, during moderate anemia, and with changes in the PaCO2. Three groups of neonates were studied: stable control neonates (n = 43), anemic neonates (n = 46), and hypotensive neonates (n = 19). Cerebral FOE was calculated from the arterial oxygen saturation measured by pulse oximetry, and cerebral venous oxygen saturation was measured using near infrared spectroscopy with partial jugular venous occlusion. Mean +/- SD cerebral FOE was similar in control (0.292+/-0.06), anemic (0.310+/-0.08; P = 0.26), and hypotensive (0.278+/-0.06; P = 0.41) neonates. After anemic neonates were transfused, mean +/- SD cerebral FOE decreased to 0.274+/-0.05 (P = 0.02). There was a weak negative correlation with the hemoglobin concentration (n = 89, r = -0.24, P = 0.04) but not with the hemoglobin F fraction (n = 56, r = 0.24, P = 0.09). In the hypotensive neonates, there was no relationship between cerebral FOE and blood pressure (n = 19, r = 0.34, P = 0.15). There was a significant negative correlation between cerebral FOE and PaCO2 within individuals (n = 14, r = -0.63, P = 0.01), but there was no relationship between individuals (n = 14, r = 0, P = 1). Cerebral FOE was not significantly altered in neonates with either mild anemia or hypotension. There were, however, changes in cerebral FOE when physiological changes occurred over a relatively short period: Cerebral FOE decreased after blood transfusion and increased with decreasing PaCO2. As no change in cerebral FOE was seen during hypotension, it was speculated that cerebral oxygen delivery may have been maintained by cerebral blood flow autoregulation.

Effects of mild hypothermia on the cortical release of excitatory amino acids and nitric oxide synthesis following hypoxia

Studies concerning neurotransmitter release following cerebral hypoxia are scarce, and the effects of mild hypothermia on hypoxia-induced neurotransmitter release are unknown. The purpose of this study was to investigate changes in excitatory amino acid (EAA) concentrations and nitric oxide (NO) synthesis following cerebral hypoxia in rats, and the effects of mild hypothermia on both. Cerebral hypoxia (PaO2, 30-40 mm Hg) was induced in each rat for 60 min. Cerebral blood flow (CBF) was measured by laser-Doppler flowmetry, and the extracellular concentrations of EAAs and NO end-products (nitrite and nitrate) were measured by in vivo microdialysis in normothermic (37 degrees C) and hypothermic (32 degrees C) rats. In both groups, CBF showed modest increases during hypoxia and returned to baseline during reoxygenation. The EAA levels of the normothermic rats increased markedly after hypoxia induction and returned to baseline levels during reoxygenation. Hypothermia abolished these increases completely. The NO end-product levels under normothermic conditions declined slightly during hypoxia, and then increased transiently during reoxygenation. Hypothermia appeared to attenuate the NO end-product level and to delay the peak. When the relationship between glutamate and the NO end-products was examined on an individual-animal basis, glutamate release did not parallel NO synthesis. The results indicate that hypothermic neuroprotection during cerebral hypoxia may be attributable to the amelioration of damage by reduction of presynaptic EAA release. Although it is unclear from the present results alone whether endothelial NO synthase, neuronal NO synthase or both caused the elevation of the NO end-products during reoxygenation, it is possible that the attenuation and delay of the peak of the NO end-product level plays a role in protection from NO-induced neuronal damage.

Brain blood flow and blood pressure during hypoxia in the epaulette shark Hemiscyllium ocellatum, a hypoxia-tolerant elasmobranch

The key to surviving hypoxia is to protect the brain from energy depletion. The epaulette shark (Hemiscyllium ocellatum) is an elasmobranch able to resist energy depletion and to survive hypoxia. Using epi-illumination microscopy in vivo to observe cerebral blood flow velocity on the brain surface, we show that cerebral blood flow in the epaulette shark is unaffected by 2 h of severe hypoxia (0.35 mg O2 l-1 in the respiratory water, 24 C). Thus, the epaulette shark differs from other hypoxia- and anoxia-tolerant species studied: there is no adenosine-mediated increase in cerebral blood flow such as that occurring in freshwater turtles and cyprinid fish. However, blood pressure showed a 50 % decrease in the epaulette shark during hypoxia, indicating that a compensatory cerebral vasodilatation occurs to maintain cerebral blood flow. We observed an increase in cerebral blood flow velocity when superfusing the normoxic brain with adenosine (making sharks the oldest vertebrate group in which this mechanism has been found). The adenosine-induced increase in cerebral blood flow velocity was reduced by the adenosine receptor antagonist aminophylline. Aminophylline had no effect upon the maintenance of cerebral blood flow during hypoxia, however, indicating that adenosine is not involved in maintaining cerebral blood flow in the epaulette shark during hypoxic hypotension.

Does NO regulate the cerebral blood flow response in hypoxia?

OBJECTIVES

To study whether nitric oxide (NO) affects the CBF response to hypoxic and carbon monoxide (CO) hypoxia.

MATERIAL AND METHODS

We incrementally reduced arterial oxygen content in rats, by decreasing the concentration of inspired oxygen (20 rats) or by repeated CO inhalation (20 rats), and measured local CBF using the hydrogen clearance method. Ten animals of each group received 80 mg/kg NO synthase (NOS) inhibitor N-monomethyl-L-arginine intravenously prior to hypoxia, while 10 rats served as controls.

RESULTS

Inhibition of NOS decreased mean CBF by 30% and increased cerebrovascular resistance by 70%. Under hypoxic hypoxia, mean oxygen reactivity of CBF (relative change of CBF to a change of arterial oxygen content) was 7.8%/vol% in control animals and 3.3%/vol% after NOS inhibition (P < 0.02). Under CO hypoxia, mean oxygen reactivity was 7.3%/vol% in control animals and 5.1%/vol% after NOS inhibition (P < 0.05). Inhibition of NOS diminished significantly the cerebral vasodilatory response during hypoxic hypoxia (P < 0.05) but only to a lesser extent during CO hypoxia.

CONCLUSION

These observations suggest that NO is involved in cerebral oxygen vasoreactivity, particularly in severe hypoxia.

Measurement of cerebral blood flow in dogs with near infrared spectroscopy in the reflectance mode is invalid

Near infrared spectroscopy (NIRS) is used to measure CBF (CBFNIRS) in humans, based on Fick's principle, using oxygen as an intravascular tracer. We compared CBFNIRS with CBF measured by microspheres (CBF mu) and the venous outflow technique (CBFv) in 15 dogs, altering CBF with ventilation-induced changes in PaCO2. Five hundred forty-nine CBFNIRS measurements were attempted using an integration time of 2.5 s on the saturation signal from the tongue. One hundred ninety-eight (36.1%) of the measurements fulfilled predefined criteria. The coefficient of variation (CV) for six measurements under stable conditions was 29.1%. The CBFNIRS measurements correlated best with microsphere-measured blood flows in the cortical gray matter (median 0.43, range 0.16-0.93); the contributions of the skull and dura were variable. The CBFv varied by a médian of 12% (range 0-67%) during the CBFNIRS measurements. The percentage of acceptable CBFNIRS measurements, the CV, and the correlation coefficients of the CBFNIRS were improved by using saturation signal directly from the artery and varying the integration time with an estimate of the minimum transit time. The current method of measuring CBFNIRS in the reflectance mode is in-accurate when compared with other accepted techniques.

Vascular reactivity in middle cerebral artery and basilar artery by transcranial Doppler in normals subjects during hypoxia

The anatomical and physiological differences between the carotid and vertebrobasilar circulations suggest the possibility of a different response to variations in systemic pO2. We evaluated cerebrovascular response (CR) in these two systems by monitoring variations in the blood flow velocities in the middle cerebral and basilar arteries during hypoxia. Eighteen healthy, non-smoking volunteers underwent transcranial Doppler study during a state of hypoxia obtained by means of the rebreathing method. Oxyhaemoglobin saturation (SaO2) was monitored using a pulsoxymeter in the 88-94% range. The cerebral blood flow velocity (BFV) was measured in the right middle cerebral artery (MCA) and the basilar artery (BA). Our findings indicate that the mean blood flow velocity (MFV) in the BA changes at a lower rate than that in the MCA during hypoxia.

Strangulation injuries in children. Part 2. Cerebrovascular hemodynamics

The cerebrovascular hemodynamics were recorded in two children with comparable hypoxic-ischemic injuries after strangulation. Monitoring was initiated within 13 hours of injury and continued for at least 38 hours. The profile included continuous measurements of cortical regional cerebral blood flow (rCBF) with a subdural thermal diffusion probe, intracranial pressure, mean arterial pressure, and expired CO2 tension. Data sets were obtained every 15 minutes or every 5 minutes during epochs of hyperventilation and inotropic support. Arterial CO2 and oxygen content and pH and, in the second patient, cardiac output (and cardiac index) were determined every 3 to 6 hours. Both children showed cortical hyperemia with a gradual rise of rCBF during the study; neither child showed elevated intracranial pressure. Mean CO2 reactivities were 1.8 and 2.1 mL/100 g/minute/mm Hg, with gradual elevations during the study. Mean cerebrovascular resistances were 0.7 and 0.9 mL/100 g/minute/mm Hg, respectively. Dissociative vasoparalysis with loss of autoregulation and preservation of CO2 reactivity was observed in both children. In the second child, during two periods of hyperventilation, an inverse steal occurred with rCBF indirectly related to expired CO2 tension; the rCBF was not related to changes in cardiac output or cardiac index. Neurologic outcome was not related to mean levels of rCBF, CPP, and CO2 reactivity, or clinical dissociative vasoparalysis. Lower initial and mean values of rCBF and an inverse steal after hyperventilation were associated with a poor outcome in the second patient.

ATP-sensitive K+ channels mediate dilatation of cerebral arterioles during hypoxia

We tested the hypothesis that dilatation of cerebral arterioles during hypoxia is mediated by activation of ATP-sensitive K+ channels. The diameter of pial arterioles was measured through a closed cranial window in anesthetized rabbits. Topical application of aprikalim (10(-6) mol/L), a direct activator of ATP-sensitive K+ channels, dilated pial arterioles by 18 +/- 3% (mean +/- SEM). Glibenclamide (10(-6) mol/L), an inhibitor of ATP-sensitive K+ channels, virtually abolished aprikalim-induced vasodilatation. When arterial PO2 was reduced from 129 +/- 3 to 25 +/- 1 mm Hg, the diameter of cerebral arterioles increased by 66 +/- 9% (P < .05). Glibenclamide inhibited dilatation of pial arterioles during hypoxia by 46 +/- 5% (P < .05). In contrast, vasodilatation in response to sodium nitroprusside was not altered by glibenclamide. Topical application of adenosine (10(-4) mol/L) increased arteriolar diameter by 21 +/- 4%. Glibenclamide did not affect adenosine-induced vasodilatation. These findings suggest that dilatation of cerebral arterioles in response to hypoxia is mediated, in part, by activation of ATP-sensitive K+ channels.

Lesions of the rostral ventrolateral medulla reduce the cerebrovascular response to hypoxia

Sympathoexcitatory neurons of the rostral ventrolateral medulla are tonically active and required for maintenance of resting levels of arterial pressure. They are also selectively excited by hypoxia and responsible for the associated sympathoexcitation. Since electrical or chemical stimulation of RVL will increase regional cerebral blood flow (rCBF) independently of changes in regional cerebral glucose utilization (rCGU) we investigated whether the RVL was also required to maintain resting levels of rCBF and also participated in the cerebrovascular vasodilation elicited by hypoxia. Rats were anesthetized (chloralose; 40 mg/kg, s.c.), paralyzed (tubocurarine) and ventilated (100% O2). rCBF was measured in 10 dissected brain regions using [14C]iodoantipyrine; rCGU was measured by 2-deoxy-D-[14C]glucose. In controls (n = 6) rCBF ranged from 56 +/- 5 in corpus callosum to 101 +/- 6 ml/min x 100 g in inferior colliculus. Hypoxic-hypoxia (PaO2 = 36 +/- 1 mmHg, n = 6) increased rCBF in all structures maximally, at 204% of control, in occipital cortex. Hypercapnia (PaCO2 = 63.5 +/- 0.9, n = 5) also increased rCBF (P < 0.01) maximally to 299% of control in superior colliculus. Spinal cord transection with maintenance of arterial pressure did not affect resting rCBF and increased the vasodilation to hypoxia (PaO2 = 39 +/- 1 mmHg, n = 5) from 2- to 3-fold in all structures (P < 0.01). Bilateral lesions within the RVL had no effect on resting rCBF or rCGU. However, they significantly reduced, in all areas by 50-69% (P < 0.01, n = 5), the cerebrovascular dilation elicited by hypoxia but not hypercapnia.(ABSTRACT TRUNCATED AT 250 WORDS)

Intravital microreflectometry of individual pial vessels and capillary region of rat

A microscopic reflectance spectrophotometer was constructed to obtain the spectra of single pial vessels and of a region containing only capillaries (capillary region). The difference in the oxygen saturation (SO2) of hemoglobin between the regional arteriole and venule [R(A - V)] and that between the regional arteriole or capillaries [R(A - C)] were calculated. The reduction of cytochrome aa3 was also estimated in the capillary region. This method was applied to the brain surface of spontaneously breathing rats subjected to hypoxic and anemic hypoxia. On decreasing the inhaled O2 from 100 to 15%, elevation of R(A - V) and R(A - C) with slight arteriolar dilatation (though statistically not significant) was observed. Below 10% O2 (especially at 4 and 3% O2), the R(A - V) and R(A - C) decreased in spite of significant arteriolar dilatation with progressive reduction of cytochrome aa3, indicating suppression of oxygen transport to mitochondria. In the case of hemodilution down to 37% hematocrit (Ht), elevation of R(A - V) and R(A - C) occurred with a slight tendency toward arteriolar dilatation. Below 32% Ht, the R(A - V) decreased but the R(A - C) remained steady, while reduction of cytochrome aa3 progressed. Altogether, the SO2 in the capillary region decreased and the reduction of cytochrome aa3 progressed with the decline of arteriolar O2 supply in both hypoxic and anemic hypoxia.

The effect of carbon dioxide on cerebral arteries

The constancy of cerebral blood flow and volume relies heavily upon the cerebral arteries' intrinsic ability to respond to changes in the partial pressure of arterial CO2. The physiologic mechanisms underlying these responses have not been determined, although changes in extracellular and intracellular pH, mediation by prostanoids and neural activity have been suggested. CO2 reactivity can be influenced by oxygen status and blood pressure and can vary according to age and brain region. In certain pathological conditions or diseases, it can be severely altered. Modern techniques, which measure CBF in cases of cerebral hemodynamic insufficiency, head injury or tumor, rely on the inherent ability of the cerebral circulation to respond to changing levels of CO2.

Local cerebral blood flow responses in rats to hypercapnia and hypoxia in the rostral ventrolateral medulla and in the cortex

The effects of hypercapnia and hypoxia on two local cerebral blood flows in the parietal cortex (PC-BF) and rostral ventrolateral medulla (RVLM-BF) were examined using laser Doppler flowmetry in anesthetized rats. Hypercapnia for 45 s duration at the end-tidal CO2 between 5% and 10%, induced by increasing inspiratory CO2, increased both cerebral blood flows and systemic blood pressure in a degree-dependent manner. The response of RVLM-BF was significantly stronger than that of PC-BF. Both cerebral blood flow responses to hypercapnia were not influenced by cutting peripheral chemoreceptor afferent nerves. Hypoxia for 45 s duration at the end-tidal O2 between 12% and 6%, induced by decreasing inspiratory O2, produced an increase of similar magnitude in both RVLM and PC local blood flows in a degree-dependent manner and a decrease in systemic blood pressure. The responses of both PC-BF and RVLM-BF to hypoxia were significantly diminished after cutting peripheral chemoreceptor afferent nerves. It is concluded that: (1) the RVLM-BF is much more sensitive to hypercapnia than the PC-BF; and (2) activation of peripheral arterial chemoreceptors possibly contributes to hypoxia-induced increase in the RVLM-BF and PC-BF.

Effects of endothelium-derived nitric oxide on cerebral circulation during normoxia and hypoxia in the rat

The aim of this study was to determine the effects of endogenous nitric oxide (NO) on cerebral blood flow (CBF) and cerebrovascular resistance (CVR) under conditions of normoxia and hypoxia. Experiments were performed on anesthetized, mechanically ventilated Wistar rats. CBF was measured using the intracarotid 133Xe injection technique. NO formation was inhibited by NG-monomethyl-L-arginine (L-NMMA). Administration of L-NMMA (100 mg kg-1 i.v.) during normoxia resulted in an increase in mean arterial blood pressure from 113 +/- 4 to 145 +/- 4 mm Hg (p less than 0.001), a decrease in CBF of 21% (from 91 +/- 4 to 75 +/- 5 ml 100 g-1 min-1, p less than 0.001), and an increase in CVR of 53% (from 1.3 +/- 0.1 to 2.0 +/- 0.2 mm Hg ml-1 100 g min, p less than 0.001). These effects were reversed by i.v. administration of 300 mg kg-1 of L-arginine but not D-arginine. Moreover, the administration of L-NMMA abolished the enhancement of CBF and the diminution in CVR observed during intracarotid infusion of acetylcholine (ACh). The increase in CBF and decrease in CVR during hypoxia in the group of rats that received L-NMMA were similar to that in the control group, although CBF and CVR levels attained during hypoxia in both groups were different. The results show that NO is involved in the maintenance of basal CBF and CVR, and is responsible for the ACh-elicited increase in CBF and the decrease in CVR in rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute carbon monoxide exposure and cerebral blood flow in rabbits

Carbon monoxide (CO) 1% was administered to anaesthetised rabbits for 15 minutes. Despite a 28% +/- 5.8 (SEM) fall in mean arterial blood pressure during the CO exposure, cerebral blood flow increased by 236% +/- 36.5 in the left and 287% +/- 28.9 in the right cortex. Cerebrovascular resistance was reduced by 70.6% +/- 2.8 in the left and by 76.2% +/- 3 in the right cortex. These changes were accompanied by an increase in intracranial pressure, a drop in body temperature and ventilation requirement, and a metabolic acidosis. When the CO was withdrawn all these parameters returned to normal over three hours. Hence, these vascular effects are reversible and consistent with the natural history of CO intoxication in humans. Carboxyhaemoglobin levels correlated well with hemispheric cerebral blood flow (r = 0.90; r = 0.98) and cerebrovascular resistance (r = 0.87; r = 0.97).

Effect of chemodenervation on the cerebral vascular and microvascular response to hypoxia

This study evaluated the effect of bilateral carotid chemodenervation on the cerebrovascular response to hypoxia in conscious rats. Cerebral blood flow was measured using 4-iodo[N-methyl-14C]antipyrine, and the total and perfused microvasculature was studied by injection of fluorescein isothiocyanate dextran and alkaline phosphatase staining. To maintain constant PCO2, hypoxia was achieved in chemoreceptor-intact rats by the use of 4% CO2-8% O2-88% N2 and in chemodenervated rats by the administration of 8% O2-92% N2. Blood gas and hemodynamic parameters were similar in the two groups of rats. Chemodenervation had no significant effect on either resting blood flow or the perfused microvasculature during normoxia. A significant increase in cerebral blood flow (from 71 +/- 3 to 138 +/- 9 ml/min/100 g in control and from 91 +/- 5 to 127 +/- 7 ml/min/100 g in chemodenervated rats) and in the percent of cerebral arterioles and capillaries perfused occurred in both hypoxic control and chemodenervated rats. In chemoreceptor-intact rats, the greatest increase in blood flow and in perfused microvasculature occurred in caudal structures (medulla and pons) in comparison with rostral structures (cortex, thalamus, and hypothalamus). In chemodenervated rats, a similar increase in blood flow and perfused microvasculature occurred in all brain regions, with no regional differences. Thus, chemodenervation did not affect the overall cerebral blood flow or the microvascular response to hypoxia; however, rostral-to-caudal regional differences in the hypoxic response were lost after chemodenervation.

Acute carbon monoxide poisoning: animal models: a review

Animals have been used for well over a century in an attempt to understand the toxicology, physiology, and pathology of acute carbon monoxide poisoning. Whether the toxic effects of this gas result from primary hypoxia, as in hypoxic hypoxia to which it is frequently compared, or from direct tissue effects since it enters cells and binds to certain vital components, remains a point of controversy. Acute severe poisoning in man and animals affects primarily the cardiovascular and nervous systems, and frequently produces neurologic dysfunction. Morphologically, tissue damage is usually confined to the white matter. The root cause is at best poorly understood and major investigative efforts have been made toward its elucidation. Many studies with rats, cats and primates indicate a major role for CO-induced hypotension, which serves to compromise blood flow and exacerbate acidosis. The likely cellular mechanisms in this process are only now becoming apparent. This review critically examines the recent as well as a few older CO-animal studies. In scope, they fall into several broad categories: general cardiopulmonary effects, metabolic and tissue effects, general resistance (i.e. tolerance), effects on the central nervous system including blood flow, neurochemistry, morphology and behavior, and finally, experimental therapeutic approaches.

Distribution of systemic blood flow during anoxia in the turtle, Chrysemys scripta

Hypoxia causes a reflex redistribution of regional blood flow in mammals that maintains delivery of oxygen to vital organs such as the brain during periods of decreased oxygen availability. The present study was performed to test if this response is developed in lower vertebrates. Regional organ blood flow and arterial blood gases were measured during normoxia (room air) and anoxia (nitrogen breathing) in anesthetized turtles, Chrysemys scripta. Organ blood flow was measured by the distribution of radioactive microspheres injected into the left atrium. The concentration of the microspheres in the organ is directly related to the blood flow rate. By knowing the reference blood flow rate, the reference microsphere concentration, and the total counts in the tissue, the tissue blood flow rate can be calculated. Anoxia caused a redistribution of blood flow away from the kidneys and splanchnic bed to the brain. Coronary blood flow and skeletal muscle blood flow remained constant. Brain blood flow increased approximately 260%. Blood flow to the kidneys and stomach was reduced approximately 50%. Blood flow to the pancreas, small intestine, and liver decreased almost to zero. The observation of anoxia-induced reflex redistribution of organ blood flow in a lower vertebrate suggests that this mechanism could be characteristic of vertebrates in general.

Extended latency of the cortical component of the somatosensory-evoked potential accompanying moderate increases in cerebral blood flow during systemic hypoxia in cats

In 24 adult cats, the somatosensory-evoked potential (SEP) and cerebral blood flow (CBF) were measured under paralyzed, anesthetized conditions during exposure to two different ventilatory regimens. Group I cats (ventilated from 20 to 2% oxygen) responded with a significant increase in white matter blood flow from 25.0 +/- 7.8 to 43.8 +/- 10.5 ml/100 g/min recorded at 7% O2. Gray matter blood flows in these animals increased but not to significant levels above the control blood flow measured at 20%. No significant changes in blood flow were observed in group II animals ventilated over the range of 25-3% oxygen as gray matter rose slightly (but not significantly) with hypoxia and white matter flows remained at levels of 25-30 ml/100 g/min. The latency of the cortical component of the SEP was related to the degree of hypoxia. For both groups, significant extensions in the latency to the occurrence of the cortical component of the SEP (normalized to the % of control SEP) occurred in each case (P less than 0.05). An inverse, linear relationship existed between the latency to the appearance of cortical component (ms) and the percentage oxygen concentration of the ventilatory mixture. No significant changes in thalamocortical conduction times were found, which indicates that hypoxia may have generalized effects on the synaptic pathways supporting the conduction of the SEP. The variation in blood flow and the latency of the cortical component observed between groups I and II may reflect the oxygen concentration used at the beginning of the experiment (25 vs 20%) and the gradations between them vs 3 and 2%.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebrocortical microcirculation in different stages of hypoxic hypoxia

This study investigated the relation between local cerebrocortical oxygen tension (PO2) and cerebrocortical microflow (CMF) during normoxia (FiO2 = 0.3) and hypoxic hypoxemia (FiO2 = 0.16 and 0.1). Measurements were performed on mechanically ventilated rats and rabbits anesthetized with 0.8% Ethrane and maintained within normocapnic limits. Polarographic techniques based on the use of multiwire surface electrodes were applied for measurements of local PO2 and CMF. In both species the mean tissue PO2 values were similar under normoxia (26.0 and 31.5 mm Hg for rats and rabbits, respectively). CMF histograms showed pronounced heterogeneity. The highest CMF values exceeded the lowest ones by a factor of 36 in the rat and by a factor of 26 in the rabbit. Mean CMF values were 6.67 +/- 0.72 and 4.09 +/- 0.14 relative units (for definition see text) in rats and rabbits, respectively. During hypoxemia, if the mean tissue PO2 was greater than 5 mm Hg, mean CMF did not change but a change in the pattern of microflow distribution was observed with increases in some CMF values (up to 670% of control) and decreases in others (down to 12% of control). When mean tissue PO2 values of less than 5 mm Hg were observed during hypoxemia, mean CMF increased in both species by approximately 50% on average. The increase in CMF could be seen in each individual CMF recording. We conclude that in the brain cortex local regulatory mechanisms are responsible for the change in the pattern of distribution of microcirculation during moderate tissue hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in human cerebral blood flow due to step changes in PAO2 and PACO2

The effect of moderate hypoxia on cerebral blood flow (CBF) in man has not been well described, and little is known about the interaction of changes in arterial PO2 and PCO2 as regards CBF. Using a non-invasive doppler ultrasound method we have measured the instantaneous mean blood velocity (which is proportional to CBF as long as the cross-section of the vessel is constant) in the carotid artery in four healthy unanaesthetized subjects. We found in all subjects that a reduction in alveolar PO2 from about 13 to about 8.7 kPa with maintained constant alveolar PCO2 (PA, CO2) caused CBF to increase gradually over 10 min (half-time about 4 min) to about 125% of control. The CBF decreased quickly (half-time about 45 s) towards control when alveolar PO2 was reset to 13 kPa. As measured 5 min after a step-change in PA, O2, the change in CBF was independent of PA, CO2 within the range 3.3-6.7 kPa. An increase in PA, O2 to about 33 kPa reduced CBF only if PA, CO2 was in the hypercapnic range. Unexpectedly we found that the CBF response showed 'adaptation' during both maintained increase and decrease in PA, CO2. The CBF started to return towards control level within 10 min after induction of hypo- or hypercapnia. We conclude that also moderate hypoxia causes increased CBF in unanaesthetized man within a wide range of PA, CO2.

Relationship of somatosensory evoked potentials and cerebral oxygen consumption during hypoxic hypoxia in dogs

The effects of hypoxic hypoxia on cerebral hemodynamics and somatosensory evoked potential (SEP) were studied in 10 pentobarbital anestheteized dogs. Cerebral blood flow (CBF) was measured using the venous outflow technique and cerebral oxygen consumption (CMRO2) was calculated from the arterio-cerebro-venous oxygen difference times CBF. SEP was evaluated by percutaneous stimulation of an upper extremity nerve and was recorded over the contralateral somatosensory cortex. The latencies of the initial negative wave (N1), second positive wave (P2) and the amplitude of the primary complex (P1N1) were measured. Animals were breathed sequentially with oxygen concentrations of 21, 10, 6, 5, and 4.5% for five minutes each. Animals were returned to room air breathing when the amplitude of the SEP decreased to less than 20% of control and were observed for 30 minutes following reoxygenation. Severe hypoxia (4.5% O2) increased CBF to 200% of control, decreased CMRO2 to 45% of control, decreased amplitude and increased latency of SEP. Following reoxygenation, as CMRO2 increased toward control, latency of SEP decreased and amplitude increased and CBF returned to baseline within 30 min. During hypoxia and reoxygenation, the latencies of N1 and P2 and the amplitude of P1N1 were correlated with CMRO2 in individual animals. We conclude that changes in SEP amplitude and latency reflect changes in CMRO2 despite high CBF during rapidly progressive hypoxic hypoxia and following reoxygenation.

Response of vertebral and carotid blood flow to isocapnic changes in end-tidal oxygen tension

The response of the vertebral and carotid blood flow to isocapnic hypoxia was measured in 9 cats anaesthetized with chloralose-urethane using perivascular electromagnetic flow probes. The carotid flow was already significantly increased when going from hyperoxia (PETO2 55 kPa) to normoxia. For the vertebral blood flow a significant increase compared to hyperoxia was observed at a moderate level of hypoxia (PETO2 9 kPa). The time course of the response of the blood flow to isocapnic step-like changes in PETO2 was fitted with a first order model. The mean time constant (+/- SD) for steps into hypoxia for the carotid flow was 35 +/- 38 sec(8 cats) and for the vertebral flow, 44 +/- 37 sec (5 cats). The mean time constant (+/- SD) for steps out of hypoxia was significantly smaller and found to be 23 +/- 22 sec (8 cats) and 19 +/- 18 sec (4 cats), respectively. We argue that a major part of the changes in vertebral and carotid blood flow to steps into hypoxia goes to brain tissue.

Transient analysis of the canine cerebrovascular response to carbon dioxide

Cerebral venous outflow and carbon dioxide transients were studied during five different transitional states: (1) on and off 10% carbon dioxide breathing, (2) on and off hyperventilation, (3) on 7% carbon dioxide breathing, (4) on 10% carbon dioxide breathing initiated from 7% carbon dioxide breathing, and (5) on 10% carbon dioxide breathing initiated during intracarotid papaverine infusion, in pentobarbital anesthetized, paralyzed, mechanically ventilated dogs. Plots of the temporal relationships between these variables indicated that cerebral blood flow is closely related with cerebral venous carbon dioxide tension but not arterial carbon dioxide tension. The rate at which flow changed upon transition from one steady state to another was phase dependent, in that longer times were required to establish stable conditions in the on phase than in the off phase. The magnitude of the maximum rates of change in cerebral blood flow achieved during transition was influenced both by the size of the forcing function and the level of flow present at the time the response was initiated. Directional changes had no effect upon the maximum rate of the flow change as long as equivalent-sized forcing functions were employed and the initial blood flow levels were similar between responses. However, faster flow transients could be produced by increasing either of the latter two factors. These findings are consistent with the hypothesis that it is either tissue carbon dioxide tension or cerebral venous carbon dioxide tension that is the important variable regulated by cerebral blood flow. The rate-limiting factor in the response appears to be carbon dioxide delivery rate and not the rate of carbon dioxide diffusion.

Regional circulatory responses to 96 hours of hypoxia in conscious sheep

Exposure of adult ewes to normobaric hypoxia (PaO2 40 mm Hg) for 96 h led to increases of VE (+ 54%), while VO2 decreased by 48%. PaCO2 declined progressively to stabilize at 24 (+/- 1.5 SE) mm Hg by 24-48 h. Cardiac output (thermodilution) was elevated temporarily for 24 h (23-34%) but then returned to normoxic levels, while heart rate (28-42%) and pulmonary artery pressure (38-56%) were increased for the duration of hypoxia. Cerebral blood flow (radiolabelled microspheres) increased transiently for 48 h from 65.9 (+/- 4.4) to 100.4 (+/- 9.9) ml X min-1 X 100 g-1 with no change in its regional distribution. Coronary flow was elevated for the duration of hypoxia from 181 (+/- 15) to between 280 (+/- 33) and 350 (+/- 37) ml X min-1 X 100 g-1 with a more pronounced increment in right heart flow, and a decline in the endocardial/epicardial flow ratio. These regional flow increases resulted from a sustained decrease in pancreatic flow from 234 (+/- 11) to 125 (+/- 13) ml X min-1 X 100 g-1 for 96 h, with persisting decreases in splenic flow from 249 (+/- 30) to 100 (+/- 18), and in renal cortical flow from 787 (+/- 70) to 540 (+/- 31) ml X min-1 X 100 g-1, occurring at 48 and 72 h, respectively. Therefore, there is a redistribution of cardiac output during 96 hours of hypoxia with increased flows to heart and brain, and decreased flows to abdominal viscera.

Comparison of cerebrovascular response to hypoxic and carbon monoxide hypoxia in newborn and adult sheep

Cerebral blood flow (CBF) responses to two types of isocapnic hypoxia, hypoxic hypoxia (HH) and carbon monoxide hypoxia (COH), were examined in seven unanesthetized adult sheep by the radiolabeled microsphere technique. Comparisons were made with newborn lambs (5-12 days old) previously studied under similar conditions. The arterial O2 content (CaO2) was reduced in a graded manner to 50-60% of the control value. During HH, CBF increased to maintain cerebral O2 delivery (CaO2 X CBF) in both adults and newborns; however, cerebral O2 uptake (CMRO2) did not change. Although CMRO2 was higher in newborns, the responses of CBF/CMRO2 to HH did not differ significantly in newborns and adults. In newborns, regional CBF showed that brainstem areas were particularly responsive to HH. In both age groups, CBF increased to a greater extent with COH than with HH for similar reductions in CaO2. This resulted in an increase in cerebral O2 delivery with COH. The degree to which COH differed from HH correlated with the magnitude of the leftward shift of the oxyhemoglobin dissociation curve that accompanies COH. In adults, CMRO2 fell by 16% with COH but was maintained in newborns. We conclude that maintenance of cerebral O2 delivery during acute, isocapnic HH is a property of CBF regulation common to both newborn and adult sheep. During COH, the position of the oxyhemoglobin dissociation curve is an additional factor that sets the level of O2 delivery. The fetal conditions of low CaO2 and a left-shifted oxyhemoglobin dissociation curve may have provided the newborn with a microcirculation better suited for maintaining CMRO2 during COH.

The role of adenosine in CBF increases during hypoxia in young vs aged rats

The role of adenosine in the regional cerebral blood flow (rCBF) response to hypoxia was evaluated in young (6 month) and aged (26-28 month) F344 rats using theophylline, an adenosine antagonist. Regional CBF was measured with radioactive microspheres under control anesthetized conditions (70% N2O, 30% O2) and at two levels of hypoxia (CaO2 = 8.7-9.0 ml . 100ml-1 and 3.2-3.7 ml . 100ml-1). Without theophylline infusion, CBF increases were similar between young and aged rats during moderate hypoxia but were increased more in young during severe hypoxia. Intracerebrovascular theophylline infusion significantly attenuated the increase in CBF during both moderate and severe hypoxia and decreased the difference between young and aged rats. Theophylline infusion produced no significant effect on the increase in CBF produced by hypercapnia, indicating the specificity of the treatment for hypoxic induced CBF changes and adenosine release. Intracerebrovascular infusion of adenosine had no effect on CBF, presumably due to the presence of the blood brain barrier. The results suggest that adenosine plays a major role in CBF increases during both moderate and severe hypoxia and in the difference in response to hypoxia between young and aged rats.

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)

Cardiac output and regional oxygen transport in the acutely hypoxic conscious sheep

We have studied the effects of severe acute hypoxemia (PaO2 = 25 torr) on cardiac output (Q), heart rate (HR), left ventricular contractility ((dP/dt)max/P), intravascular pressures and blood flow to the heart, brain, abdominal viscera, skin and respiratory and non-respiratory muscles in twelve conscious ewes that breathed a mixture of 8% O2 and 92% N2 for 20 min. Q, HR, (dP/dt)max/P) and systemic and pulmonary arterial pressures increased. Total peripheral resistance decreased while pulmonary vascular resistance remained unchanged. Coronary, cerebral, respiratory and nonrespiratory muscle and adrenal flows increased, in association with a decrease in regional vascular resistances, while the flows to the kidney and other abdominal viscera remained unchanged. The concentration of total plasma catecholamines doubled, indicating that the sympathetic nervous system plays a major role in the hemodynamic response to this level of hypoxia. Increased oxygen delivery to the heart (31%) and respiratory muscles (44%) were brought about by increases in both the magnitude and the redistribution of Q, the latter being the more important of the two mechanisms. In contrast, both mechanisms contributed equally to the amount of oxygen delivered to the brain and nonrespiratory muscles. We concluded that in acute hypoxemia, both the increase in Q and its regional redistribution contribute to the delivery of oxygen to the various tissues.

Vasopressin secretion induced by hypoxia in sheep: developmental changes and relationship to beta-endorphin release

To investigate the developmental changes in the secretion of vasopressin and the potential role of beta-endorphin as a stimulus to the release of vasopressin, the concentrations of these peptides were measured in fetal, newborn, and adult sheep after episodes of induced hypoxia. The studies confirm that hypoxia is a potent stimulus to the release of both vasopressin and beta-endorphin in the fetal animal. In both the newborn lamb and the ewe, more profound hypoxia is necessary for a similar release. In the fetus, the release of both vasopressin and beta-endorphin after hypoxia increased with gestational maturation. A comparison of control concentrations of both peptides, the discordance of release in the newborn lamb, and the absence of a change in concentrations of vasopressin with infusion of beta-endorphin implies that these hormones are released in parallel but independently during hypoxic stress.

Pial arterial response to systemic hypoxia in anaesthetised cats

Pial arterial responses to reduction in arterial oxygen tension were studied in anaesthetised cats. In 12 cats under chloralose, dilatation occurred in vessels of all sizes between 20 and 200 micrometers, at variable levels of PaO2, and to a very variable extent. At PaO2 25-35 mm Hg, dilatation ranged from negligible to 175% above initial diameter. The variations in response were largely dependent on associated blood pressure (BP) changes. Increase in BP counteracted dilatation; dilatation was greater during hypoxia when the BP change was prevented. At the induction of hypoxia, the first response of the vessels was a constriction, which occurred about 5 s after the chemoreflex increase in BP. Dilatation was delayed a further 30-90 s, and this delay was similar when BP was prevented from rising. Vessels of all sizes responded in the appropriate manner when only BP was transiently changed within the autoregulatory range. In 3 cats in which similar procedures were compared under pentobarbitone anaesthesia, there were smaller and less consistent changes in responses to BP changes alone and to hypoxia and its associated BP changes. The findings were compatible with a local effect of lowered PO2. From the time course of the changes there was no indication of a chemoreflex component in the responses of these vessels at the induction of hypoxia.

Vasoactive intestinal polypeptide and the canine cerebral circulation

A potential role for cerebrovascular nerves containing vasoactive intestinal polypeptide (VIP) was examined in 24 anesthetized, ventilated dogs. Cerebral blood flow (CBF) was measured by either the cerebral venous outflow or microsphere method. Plasma VIP concentration was measured by radioimmunoassay. Hypercapnia (5% and 10% CO2) and hypoxia (7% O2) produced significant increases in cerebral venous outflow, but had no affect on arterial or cerebral venous VIP concentrations. Measurements of VIP in cerebrospinal fluid (CSF) made during 5% and 8% CO2 breathing also were not different from control values. VIP produced large dose-dependent increases in common carotid artery and temporalis muscle blood flow when injected or infused intraarterially; however, VIP had no effect on total or regional cerebral blood flow (rCBF) within the brain when administered in a similar manner. Unilateral perfusion of the cerebral ventricles with VIP produced significant increases (range: 11-80%) in rCBF. These data are consistent with the possibility that local release of VIP from perivascular nerve endings could affect CBF. The unresponsiveness of canine cerebral vessels to blood-borne VIP may be due to the blood-brain barrier, since VIP dilates cerebral vessels when the barrier is bypassed by intraventricular infusion. These studies do not support the hypothesis that CBF changes induced during hypercapnia or hypoxia are mediated by VIP.

Canine cerebral metabolism and blood flow during hypoxemia and normoxic recovery from hypoxemia

There are conflicting reports regarding the effects of hypoxemia on the cerebral metabolic rate for oxygen (CMRO2). Accordingly, we examined the changes in CMRO2 during normoxia, progressive hypoxia (PaO2 of 37, 27, and 23 mm Hg), and normoxic recovery from hypoxia. Measurements were made in dogs anesthetized with nitrous oxide (60-70%) and halothane (less than 0.1%) in oxygen. Arterial-cerebral venous blood oxygen content differences and cerebral blood flow (CBF) were measured simultaneously, the latter by a technique (collection of sagittal sinus outflow) previously validated for conditions of near-maximal CBF. The duration of each of the three hypoxic exposures was approximately 10 min. CMRO2 was significantly decreased (14%) only when the arterial blood oxygen tension was reduced to 23 mm Hg. CBF increased progressively to a maximum of 153% of control. Posthypoxemic brain biopsy values for cerebral metabolites obtained 40 min after normoxemia had been restored were normal. These results, in conjunction with an unchanged CMRO2 at 40 min normoxic recovery, suggest that no gross irreversible brain cell damage occurred. We conclude that with progressive hypoxemia, CMRO2 remains stable until oxygen demand exceeds oxygen delivery, resulting thereafter in a progressive reduction in CMRO2.

Enhancement of cerebrovascular effect of CO2 by hypoxia

Internal carotid blood flow, taken as an index of cerebral blood flow (CBF); arterial pressure; and respiratory O2 and CO2 concentrations, were measured in halothane (1%)-anesthetized, paralyzed and mechanically ventilated rabbits. CBF was determined at end-tidal CO2 of 4% (normocapnia) and 8%, as inspired O2 [O2]I) was varied stepwise over the range of 6.5 to 92%. Normocapnic CBF was constant over the range of 15 to 92% [O2]I, but it increased significantly to 240% and 380% of control when [O2]I was at 10% and 6.5%, respectively. Cerebrovascular response to CO2 was constant over the range of [O2]I tested, except for a significant elevation to 180% of control at [O2]I of 10%.