Comparative studies of regional CNS blood flow autoregulation and responses to CO2 in the cat. Effects of altering arterial blood pressure and PaCO2 on rCBF of cerebrum, cerebellum, and spinal cord

Apparent diffusion coefficient of different areas of brain in foetuses with intrauterine growth restriction

Purpose

This study aimed to compare the apparent diffusion coefficient (ADC) values of different brain areas between two groups of intrauterine growth restricted (IUGR) foetuses and control cases.

Material and methods

A total of 38 foetuses with IUGR and 18 normal control foetuses with similar gestational age were compared using a 3T magnetic resonance scanner. IUGR cases included 23 foetuses with clinical severity signs (group A) and 15 foetuses without clinical severity signs (group B). ADC values were measured in different brain regions and compared among groups. Foetuses with structural brain abnormalities were excluded from the study.

Results

All foetuses had normal foetal structural brain anatomy. Head circumference (HC) < 5% was more common in IUGR group A compared to IUGR group B (56.5% vs. 13.3%, p < 0.0001). In comparison to the normal group, the ADC values in IUGR foetuses were significantly lower in cerebellar hemispheres (CH) (1.239 vs. 1.280.5 × 10^-3 mm²/s, p = 0.045), thalami (1.205 vs. 1.285 × 10^-3 mm²/s, p = 0.031) and caudate nucleus (CN) (1.319 vs. 1.394 × 10^-3 mm²/s, p = 0.04). However, there were no significant differences in ADC values between IUGR subtypes. Among all brain regions, pons had the lowest ADC values.

Conclusions

ADC values of thalami, CN, and CH were significantly lower in IUGR than control foetuses, while there was no significant difference among IUGR groups. Further studies are needed to evaluate the prognostic value of ADC changes in IUGR foetuses.

Isoflurane anesthesia does not affect spinal cord neurovascular coupling: evidence from decerebrated rats

Neurological examination remains the primary clinical investigation in patients with spinal cord injury. However, neuroimaging methods such as functional magnetic resonance imaging (fMRI) are promising tools for following functional changes in the course of injury, disease and rehabilitation. However, the relationship between neuronal activity and blood flow in the spinal cord on which fMRI relies has been largely overlooked. The objective of this study was to examine neurovascular coupling in the spinal cord of decerebrated rats during electrical stimulation of the sciatic nerve with and without isoflurane anesthesia (1.2%). Local field potentials (LFP) and spinal cord blood flow (SCBF) were recorded simultaneously in the lumbosacral enlargement. Isoflurane did not significantly alter LFP (p = 0.53) and SCBF (p = 0.57) amplitude. Accordingly, neurovascular coupling remained comparable with or without isoflurane anesthesia (p = 0.39). These results support the use of isoflurane in rodents to investigate nociceptive functions of the spinal cord using fMRI.

Functional Neuroimaging of Nociceptive and Pain-Related Activity in the Spinal Cord and Brain: Insights From Neurovascular Coupling Studies

Spinal cord and brain processes underlie pain perception, which produces systemic cardiovascular changes. In turn, the autonomic nervous system regulates vascular function in the spinal cord and brain in order to adapt to these systemic changes, while neuronal activity induces local vascular changes. Thus, autonomic regulation and pain processes in the brain and spinal cord are tightly linked and interrelated. The objective of this topical review is to discuss work on neurovascular coupling during nociceptive processing in order to highlight supporting evidence and limitations for the use of cerebral and spinal fMRI to investigate pain mechanisms and spinal nociceptive processes. Work on functional neuroimaging of pain is presented and discussed in relation to available neurovascular coupling studies and related issues. Perspectives on future work are also discussed with an emphasis on differences between the brain and the spinal cord and on different approaches that may be useful to improve current methods, data analyses and interpretation. In summary, this review highlights the lack of data on neurovascular coupling during nociceptive stimulation and indicates that hemodynamic and BOLD responses measured with fMRI may be biased by nonspecific vascular changes. Future neuroimaging studies on nociceptive and pain-related processes would gain further understanding of neurovascular coupling in the brain and spinal cord and should take into account the effects of systemic vascular changes that may affect hemodynamic responses. Anat Rec, 301:1585-1595, 2018. © 2018 Wiley Periodicals, Inc.

Relationship between Hypotension and Cerebral Ischemia during Hemodialysis

The relationship between BP and downstream ischemia during hemodialysis has not been characterized. We studied the dynamic relationship between BP, real-time symptoms, and cerebral oxygenation during hemodialysis, using continuous BP and cerebral oxygenation measurements prospectively gathered from 635 real-world hemodialysis sessions in 58 prevalent patients. We examined the relationship between BP and cerebral ischemia (relative drop in cerebral saturation >15%) and explored the lower limit of cerebral autoregulation at patient and population levels. Furthermore, we estimated intradialytic exposure to cerebral ischemia and hypotension for each patient, and entered these values into multivariate models predicting change in cognitive function. In all, 23.5% of hemodialysis sessions featured cerebral ischemia; 31.9% of these events were symptomatic. Episodes of hypotension were common, with mean arterial pressure falling by a median of 22 mmHg (interquartile range, 14.3-31.9 mmHg) and dropping below 60 mmHg in 24% of sessions. Every 10 mmHg drop from baseline in mean arterial pressure associated with a 3% increase in ischemic events (P<0.001), and the incidence of ischemic events rose rapidly below an absolute mean arterial pressure of 60 mmHg. Overall, however, BP poorly predicted downstream ischemia. The lower limit of cerebral autoregulation varied substantially (mean 74.1 mmHg, SD 17.6 mmHg). Intradialytic cerebral ischemia, but not hypotension, correlated with decreased executive cognitive function at 12 months (P=0.03). This pilot study demonstrates that intradialytic cerebral ischemia occurs frequently, is not easily predicted from BP, and may be clinically significant.

Traumatic brain injury: pathophysiology for neurocritical care

Severe cases of traumatic brain injury (TBI) require neurocritical care, the goal being to stabilize hemodynamics and systemic oxygenation to prevent secondary brain injury. It is reported that approximately 45 % of dysoxygenation episodes during critical care have both extracranial and intracranial causes, such as intracranial hypertension and brain edema. For this reason, neurocritical care is incomplete if it only focuses on prevention of increased intracranial pressure (ICP) or decreased cerebral perfusion pressure (CPP). Arterial hypotension is a major risk factor for secondary brain injury, but hypertension with a loss of autoregulation response or excess hyperventilation to reduce ICP can also result in a critical condition in the brain and is associated with a poor outcome after TBI. Moreover, brain injury itself stimulates systemic inflammation, leading to increased permeability of the blood–brain barrier, exacerbated by secondary brain injury and resulting in increased ICP. Indeed, systemic inflammatory response syndrome after TBI reflects the extent of tissue damage at onset and predicts further tissue disruption, producing a worsening clinical condition and ultimately a poor outcome.

Elevation of blood catecholamine levels after severe brain damage has been reported to contribute to the regulation of the cytokine network, but this phenomenon is a systemic protective response against systemic insults. Catecholamines are directly involved in the regulation of cytokines, and elevated levels appear to influence the immune system during stress. Medical complications are the leading cause of late morbidity and mortality in many types of brain damage. Neurocritical care after severe TBI has therefore been refined to focus not only on secondary brain injury but also on systemic organ damage after excitation of sympathetic nerves following a stress reaction.

Hyperthermia modulates regional differences in cerebral blood flow to changes in CO2

The purpose of this study was to assess blood flow responses to changes in carbon dioxide (CO2) in the internal carotid artery (ICA), external carotid artery (ECA), and vertebral artery (VA) during normothermic and hyperthermic conditions. Eleven healthy subjects aged 22 ± 2 (SD) yr were exposed to passive whole body heating followed by spontaneous hypocapnic and hypercapnic challenges in normothermic and hyperthermic conditions. Right ICA, ECA, and VA blood flows, as well as left middle cerebral artery (MCA) mean blood velocity (Vmean), were measured. Esophageal temperature was elevated by 1.53 ± 0.09°C before hypocapnic and hypercapnic challenges during heat stress. Whole body heating increased ECA blood flow and cardiac output by 130 ± 78 and 47 ± 26%, respectively (P < 0.001), while blood flow (or velocity) in the ICA, MCA, and VA was reduced by 17 ± 14, 24 ± 18, and 12 ± 7%, respectively (P < 0.001). Regardless of the thermal conditions, ICA and VA blood flows and MCA Vmean were decreased by hypocapnic challenges and increased by hypercapnic challenges. Similar responses in ECA blood flow were observed in hyperthermia but not in normothermia. Heat stress did not alter CO2 reactivity in the MCA and VA. However, CO2 reactivity in the ICA was decreased (3.04 ± 1.17 vs. 2.23 ± 1.03%/mmHg; P = 0.039) but that in the ECA was enhanced (0.45 ± 0.47 vs. 0.95 ± 0.61%/mmHg; P = 0.032). These results indicate that hyperthermia is capable of altering dynamic cerebral blood flow regulation.

Infratentorial strokes for posterior circulation folks: clinical correlations for current translational therapeutics

Approximately 20 percent of all strokes will occur in the Infratentorial brain. This is within the vascular territory of the posterior vascular circulation. Very few clinical specifics are known about the therapeutic needs of this patient sub-population. Most evidence-based practices are founded from research about the treatment of anterior circulatory stroke. As a consequence, little is known about how stroke in the Infratentorial brain region would require a different approach. We characterized the neurovascular features of Infratentorial stroke, pathophysiological responses, and experimental models for further translational study.

Posterior circulation stroke: animal models and mechanism of disease

Posterior circulation stroke refers to the vascular occlusion or bleeding, arising from the vertebrobasilar vasculature of the brain. Clinical studies show that individuals who experience posterior circulation stroke will develop significant brain injury, neurologic dysfunction, or death. Yet the therapeutic needs of this patient subpopulation remain largely unknown. Thus understanding the causative factors and the pathogenesis of brain damage is important, if posterior circulation stroke is to be prevented or treated. Appropriate animal models are necessary to achieve this understanding. This paper critically integrates the neurovascular and pathophysiological features gleaned from posterior circulation stroke animal models into clinical correlations.

Differential blood flow responses to CO₂ in human internal and external carotid and vertebral arteries

Arterial CO2 serves as a mediator of cerebral blood flow(CBF), and its relative influence on the regulation of CBF is defined as cerebral CO2 reactivity. Our previous studies have demonstrated that there are differences in CBF responses to physiological stimuli (i.e. dynamic exercise and orthostatic stress) between arteries in humans. These findings suggest that dynamic CBF regulation and cerebral CO2 reactivity may be different in the anterior and posterior cerebral circulation. The aim of this study was to identify cerebral CO2 reactivity by measuring blood flow and examine potential differences in CO2 reactivity between the internal carotid artery (ICA), external carotid artery (ECA) and vertebral artery (VA). In 10 healthy young subjects, we evaluated the ICA, ECA, and VA blood flow responses by duplex ultrasonography (Vivid-e, GE Healthcare), and mean blood flow velocity in middle cerebral artery (MCA) and basilar artery (BA) by transcranial Doppler (Vivid-7, GE healthcare) during two levels of hypercapnia (3% and 6% CO2), normocapnia and hypocapnia to estimate CO2 reactivity. To characterize cerebrovascular reactivity to CO2,we used both exponential and linear regression analysis between CBF and estimated partial pressure of arterial CO2, calculated by end-tidal partial pressure of CO2. CO2 reactivity in VA was significantly lower than in ICA (coefficient of exponential regression 0.021 ± 0.008 vs. 0.030 ± 0.008; slope of linear regression 2.11 ± 0.84 vs. 3.18 ± 1.09% mmHg−1: VA vs. ICA, P <0.01). Lower CO2 reactivity in the posterior cerebral circulation was persistent in distal intracranial arteries (exponent 0.023 ± 0.006 vs. 0.037 ± 0.009; linear 2.29 ± 0.56 vs. 3.31 ± 0.87% mmHg−1: BA vs. MCA). In contrast, CO2 reactivity in ECA was markedly lower than in the intra-cerebral circulation (exponent 0.006 ± 0.007; linear 0.63 ± 0.64% mmHg−1, P <0.01). These findings indicate that vertebro-basilar circulation has lower CO2 reactivity than internal carotid circulation, and that CO2 reactivity of the external carotid circulation is markedly diminished compared to that of the cerebral circulation, which may explain different CBF responses to physiological stress.

Validation of diffuse correlation spectroscopy measurements of rodent cerebral blood flow with simultaneous arterial spin labeling MRI; towards MRI-optical continuous cerebral metabolic monitoring

Cerebral blood flow (CBF) during stepped hypercapnia was measured simultaneously in the rat brain using near-infrared diffuse correlation spectroscopy (DCS) and arterial spin labeling MRI (ASL). DCS and ASL CBF values agree very well, with high correlation (R=0.86, p< 10(-9)), even when physiological instability perturbed the vascular response. A partial volume effect was evident in the smaller magnitude of the optical CBF response compared to the MRI values (averaged over the cortical area), primarily due to the inclusion of white matter in the optically sampled volume. The 8.2 and 11.7 mm mid-separation channels of the multi-distance optical probe had the lowest partial volume impact, reflecting ~75 % of the MR signal change. Using a multiplicative correction factor, the ASL CBF could be predicted with no more than 10% relative error, affording an opportunity for real-time relative cerebral metabolism monitoring in conjunction with MR measurement of cerebral blood volume using super paramagnetic contrast agents.

Cerebellar autoregulation dynamics in humans

Knowledge on autoregulation of cerebellar blood flow in humans is scarce. This study investigated whether cerebellar autoregulation dynamics and CO(2) reactivity differ from those of the supratentorial circulation. In 56 healthy young adults, transcranial Doppler (TCD) monitoring of the posterior inferior cerebellar artery (PICA) and, simultaneously, of the contralateral middle cerebral artery (MCA) was performed. Autoregulation dynamics were assessed by the correlation coefficient method (indices Dx and Mx) from spontaneous blood pressure fluctuations and by transfer function analysis (phase and gain) from respiratory-induced 0.1 Hz blood pressure oscillations. CO(2) reactivity was measured via inhalation of air mixed with 7% CO(2). The autoregulatory indices Dx and Mx did not differ between the cerebellar (PICA) and cerebral (MCA) vasculature. Phase and gain, which describe faster aspects of autoregulation, showed slightly better values in the PICA compared with the MCA (higher phase, P=0.005; lower gain, P=0.007). Correlation between absolute autoregulation values in the PICA and the MCA was significant (P<0.001). The TCD CO(2) reactivity was significantly lower in the PICA (P<0.001), which could be influenced by an assumed PICA dilation under hypercapnia. In conclusion, dynamic autoregulation in the human cerebellum is well operating and has slightly faster regulatory properties than the anterior cerebral circulation.

Increases in oxygen consumption without cerebral blood volume change during visual stimulation under hypotension condition

The magnitude of the blood oxygenation level-dependent (BOLD) signal depends on cerebral blood flow (CBF), cerebral blood volume (CBV) and cerebral metabolic rate of oxygen (CMRO2). Thus, it is difficult to separate CMRO2 changes from CBF and CBV changes. To detect the BOLD signal changes induced only by CMRO2 responses without significant evoked CBF and CBV changes, BOLD and CBV functional magnetic resonance imaging (fMRI) responses to visual stimulation were measured under normal and hypotension conditions in isoflurane-anesthetized cats at 4.7 T. When the mean arterial blood pressure (MABP) decreased from 89+/-10 to 50+/-1 mm Hg (mean+/-standard deviation, n=5) by infusion of vasodilator sodium nitroprusside, baseline CBV in the visual cortex increased by 28.4%+/-8.3%. The neural activity-evoked CBV increase in the visual cortex was 10.8%+/-3.9% at normal MABP, but was negligible at hypotension. Positive BOLD changes of +1.8%+/-0.5% (gradient echo time=25 ms) at normal MABP condition became prolonged negative changes of -1.2%+/-0.3% at hypotension. The negative BOLD response at hypotension starts approximately 1 sec earlier than positive BOLD response, but similar to CBV change at normal MABP condition. Our finding shows that the negative BOLD signals in an absence of CBV changes are indicative of an increase in CMRO2. The vasodilator-induced hypotension model simplifies the physiological source of the BOLD fMRI signals, providing an insight into spatial and temporal CMRO2 changes.

Influence of moderate and profound hyperventilation on cerebral blood flow, oxygenation and metabolism

OBJECTIVE

The aim of the present study was to examine the impact of moderate and profound hyperventilation on regional cerebral blood flow (rCBF), oxygenation and metabolism.

MATERIALS AND METHODS

Twelve anesthetized pigs were subjected to moderate (mHV) and profound (pHV) hyperventilation (target arterial pO(2): 30 and 20 mmHg, respectively) for 30 min each, after baseline normoventilation (BL) for 1 h. Local cerebral extracellular fluid (ECF) concentrations of glucose, lactate, pyruvate and glutamate as well as brain tissue oxygenation (p(ti)O(2)) were monitored using microdialysis and a Licox oxygen sensor, respectively. In nine pigs, regional cerebral blood flow (rCBF) was also continuously measured via a thermal diffusion system.

RESULTS

Both moderate and profound hyperventilation resulted in a significant decrease in rCBF (BL: 37.9+/-4.3 ml/100 g/min; mHV: 29.4+/-3.6 ml/100 g/min; pHV: 23.6+/-4.7 ml/100 g/min; p<0.05) and p(ti)O(2) (BL: 22.7+/-4.1 mmHg; mHV: 18.9+/-4.9 mmHg; pHV: 13.0+/-2.2 mmHg; p<0.05). A p(ti)O(2) decrease below the critical threshold of 10 mmHg was induced in three animals by moderate hyperventilation and in five animals by profound hyperventilation. Furthermore, significant increases in lactate (BL: 1.06+/-0.18 mmol/l; mHV: 1.36+/-0.20 mmol/l; pHV: 1.67+/-0.17 mmol/l; p<0.005), pyruvate (BL: 46.4+/-7.8 micromol/l; mHV: 58.0+/-10.3 micromol/l; pHV: 66.1+/-12.7 micromol/l; p<0.05), and lactate/glucose ratio were observed during hyperventilation. (Data are presented as mean+/-S.E.M.)

CONCLUSIONS

Both moderate and profound hyperventilation may result in insufficient regional oxygen supply and anaerobic metabolism, even in the uninjured brain. Therefore, the use of hyperventilation cannot be considered as a safe procedure and should either be avoided or used with extreme caution.

Comparison of regional vasomotor responses to acetazolamide and CO2 in rabbit cerebrum and cerebellum, measured by a hydrogen clearance method

AIM

Many investigators have proved the usefulness of acetazolamide provocation and the carbon dioxide test for assessment of the local cerebrovascular reactivity by measurement of the regional cerebral blood flow in patients with occlusive cerebrovascular disease. Data originating from a comparison of these two different vasomotor stimuli as concerns the differences in sensitivity to them in various parts of the central nervous system are scarce. Our aim was to compare the cerebral blood flow responses to hypercapnic and acetazolamide stimuli in different brain regions.

METHODS

The cerebral blood flow was measured in the cerebrum (cortex and caudate nucleus) and cerebellum (cortex), as measured by a hydrogen clearance method in anaesthetized, artificially ventilated rabbits.

RESULTS

In normocapnia, the cerebral blood flow values in the cerebrum and the cerebellum differed significantly. The cerebral blood flow responses to both vasodilatory stimuli were to be significantly higher in the cerebrum than in the cerebellum, but the relative increases, i.e. the mean relative reactivities, were similar in the different regions measured.

CONCLUSION

The regional dissimilarity might explain to some extent the different sensitivities of the various brain areas to sudden blood pressure changes (infarction or haemorrhage). The results further suggest that heterogeneity in cerebrovascular reactivity should be considered in the assessment of vasoreactivity in patients with occlusive cerebrovascular disease. Since the comparison of the carbon dioxide and acetazolamide-induced cerebrovascular reactivities revealed a strong linear relationship, it was concluded that acetazolamide provocation is equivalent to the carbon dioxide test in the evaluation of cerebrovascular reactivity.

Desflurane Increases Intracranial Pressure More and Sevoflurane Less Than Isoflurane in Pigs Subjected to Intracranial Hypertension

Desflurane and sevoflurane may have advantages over isoflurane in neuroanesthesia, but this is still under debate. A porcine model with experimental intracranial hypertension was used for paired comparison of desflurane, sevoflurane, and isoflurane with respect to the effects on cerebral blood flow (CBF), cerebrovascular resistance (CVR), and intracranial pressure (ICP). The agents, given in sequence to each of six pigs, were compared at 0.5 and 1.0 minimal alveolar concentrations (MAC) and three mean arterial blood pressure (MAP) levels (50, 70, and 90 mm Hg) at normocapnia and one MAP level (70 mm Hg) at hypocapnia. MAC for each agent had been previously determined in a standardized manner for comparison reliability. CBF was measured with Xe. MAP was lowered by inflation of a balloon catheter in the inferior caval vein and raised by inflation of a balloon catheter in the descending aorta. ICP was measured intraparenchymally. Two Fogarty catheters positioned extradurally were inflated to a baseline ICP of 20 to 22 mm Hg at 0.2 MAC of each agent. CBF and ICP with the three agents at normocapnia and MAP 70 and 90 mm Hg at both 0.5 and 1.0 MAC were as follows (P < 0.05): desflurane > isoflurane > sevoflurane. None of the agents abolished CO2 reactivity. High-dose desflurane resulted in a higher CBF at hypocapnia than corresponding doses of sevoflurane or isoflurane, but there were no significant differences between the agents in ICP at hypocapnia. The present study showed that desflurane increased ICP more and sevoflurane less than isoflurane during normoventilation, but the differences disappeared with hyperventilation.

Checkpoints and pitfalls in the experimental neuropathology of circulatory disturbance

In neural tissue injury many pathological processes are common to different neurological disorders, including cerebral ischemia. Because ischemia has a fundamentally simple impact on neural tissue, good laboratory modeling can help improve the general understanding of the neuropathological processes involved. Summarized here are some basic principles that should be followed to ensure that cerebral ischemia studies are reproducible and informative: (i) selection of an appropriate model of cerebral ischemia in an appropriate species (although rodents are widely used for genomic studies, the use of larger animals, with brain structures macroscopically similar to those of humans, is appropriate for many studies, e.g. of white matter lesions or the pathophysiology of cerebral edema); (ii) correct maintenance of physiological parameters, including body temperature, systemic blood pressure, and blood gas tensions, under appropriate general anesthesia; (iii) selection of an appropriate method of cerebral blood flow (CBF) monitoring (decisions include whether or not the experiment requires real-time monitoring, in vivo measurement, and CBF mapping); (iv) appropriate timing of drug application in therapeutic studies (many drugs that are effective when given immediately after a short period of ischemia are ineffective in clinical trials, probably because of longer periods of ischemia and delayed drug delivery in clinical settings); and (v) multiparametric evaluation of therapeutic effect (with the recent increase in diagnosis of cases of mild stroke, measurement of mortality and infarct size have proven to be insufficient for the evaluation of therapeutic effect). Use of mild ischemia models and batteries of neurological tests for individual neurological functions, such as motor, somatosensory, and visual function, are becoming important in experimental ischemia research. In histological evaluation, assessment of the extent of both selective neuronal loss and the infarct will become mandatory. Regional analysis of each brain structure and coordination of the results with the apparent neurological dysfunction is a promising approach.

Mild hypothermia disturbs regional cerebrovascular autoregulation in awake rats

The effects of mild hypothermia on regional CBF (rCBF) and autoregulation were investigated in 60 awake and spontaneously breathing Wistar rats. They were divided into normothermic (rectal and brain temperatures: 37.0 +/- 0.5 degrees C) and mildly hypothermic (33.0 +/- 0.5 degrees C) groups the temperature of the latter group was controlled by cooling a lead cast around each rat with ice-cold water. rCBF was measured by means of an autoradiographic technique with 14C-iodoantipyrine. In normothermia, rCBF in most of the supratentorial cortical regions was maintained down to a mean arterial blood pressure (MABP) of 50 mmHg, produced by exsanguination, while rCBF in most of the brain stem regions showed a tendency to increase despite this reduction of MABP (predysautoregulatory overshoot of CBF). In the mildly hypothermic group, pre-exsanguination rCBF values were lower than those in normothermia, and rCBF in all brain regions declined significantly in proportion to decreasing MABP, produced by exsanguination. It is, therefore, concluded that mild hypothermia disturbs cerebrovascular autoregulation in awake rats.

Measurement of local cerebral blood flow by magnetic resonance imaging: in vivo autoradiographic strategy using 17O-labeled water

Local cerebral blood flow (LCBF) was measured in cats by magnetic resonance imaging (MRI), using 17O-labeled water (H[2]17O) as a tracer. Cat brain images were obtained using a 256 x 240 matrix fast spin-echo sequence with a total acquisition time of 124 seconds per image. Intravenous injection of H(2)17O and sampling of arterial blood were simultaneously performed during the MRI scan. Injection of H(2)17O over 40 seconds caused a transient decrease in brain signal intensity, from which the changes in H(2)17O concentration of brain tissue were calculated. The concentration of H(2)17O in arterial blood was measured directly by 17O nuclear magnetic resonance (NMR) spectroscopy, and used as an input function for the calculation of LCBF. LCBF was successfully determined using an in vivo autoradiographic strategy in a total of three cats. In one, LCBF measurements were also performed under hypercapnic conditions, and LCBF maps of the cat brain during normocapnia and hypercapnia were constructed. These LCBF maps reflected well the changes in LCBF induced by hypercapnia, which indicated the validity of the used method.

Cerebral extracellular lactate concentration and blood flow during chemical stimulation of the nucleus tractus solitarii in anesthetized rats

The extracellular lactate concentration and blood flow in the cerebral cortex of urethane-anesthetized, paralyzed and artificially ventilated rats were monitored continuously and simultaneously using an enzyme electrode and a laser Doppler flowmeter (LDF), respectively, during chemical stimulation of the nucleus tractus solitarii (NTS) by microinjection of L-glutamate (1.7 nmol 50 nl). Chemical stimulation of the NTS significantly decreased the arterial blood pressure (ABP) from 85 +/- 17 to 68 +/- 14 mmHg, heart rate from 418 +/- 13 to 402 +/- 19 beats x min(-1) and cerebral blood flow (CBF) by 17.9 +/- 6.2% (P < 0.001). However, chemical stimulation of the NTS significantly increased the lactate concentration by 58.9 +/- 17.3 microM (P < 0.001). Barostat maneuver, which held systemic ABP constant during chemical stimulation of the NTS attenuated the responses in CBF and lactate concentration by 30 and 27%, respectively. The onset of the increase in lactate concentration was delayed about 19 s after that of the CBF decrease. Circulatory lactate produced no significant change in the cerebral extracellular lactate concentration. These results indicate that chemical stimulation of the NTS induces an increase in extracellular lactate concentration in the cerebral cortex through a decrease in CBF via cerebral vasoconstriction.

Hemodilution impairs hypocapnia-induced vasoconstrictor responses in the brain and spinal cord in dogs

Despite the increasing use of plasma expanders in the perioperative period, there have been few studies of cerebrovascular responsiveness during hemodilution. The present study was performed to evaluate the influence of isovolemic hemodilution on vasoconstrictor responses in the brain and spinal cord during hypocapnia. Sixteen mechanically ventilated, halothane-anesthetized dogs were randomly divided into two equal groups: Group 1, control group (hematocrit [Hct], 42% +/- 2%); Group 2, isovolemic hemodilution with 5% dextran 40 (Hct, 19% +/- 2%). Hypocapnia (22 +/- 1 mm Hg) was induced in both groups by removal of dead space tubing without altering mechanical ventilation. Regional blood flow in the brain and spinal cord was measured with 15-microns radioactive microspheres and used to calculate regional vascular resistance (RVR). In Group 1, hypocapnia caused increases in RVR (ranging from 44% +/- 10% in the cerebral cortex to 93% +/- 17% in the thoracic spinal cord). In Group 2, hemodilution itself decreased RVR relatively uniformly throughout the brain and spinal cord. After hemodilution, hypocapnia had no significant effect on RVR in the cerebral cortex, cerebellum, pons, and medulla, and caused less pronounced increases in RVR within the spinal cord. We conclude that hemodilution either attenuated or completely abolished vasoconstrictor responses within the brain and spinal cord during hypocapnia. Furthermore, the present findings suggest that induced hypocapnia may be less effective as a clinical maneuver to reduce increased intracranial pressure during hemodilution.

[Is there still a place for routine deep hypocapnia in intracranial surgery?]

Deliberate hypocapnia during the anaesthetic management of the patient undergoing craniotomy has become an accepted standard of care. However there has been a resurgence of interest, in how hypocapnia should be applied in intra- and extra-operative settings. There are three possible therapeutic effects of hypocapnia, namely, (a) reduction of brain bulk through a reduction in cerebral blood volume, with a decrease cerebral blood flow; (b) developing an "inverse steal" by redistribution of blood from normal to ischaemic regions and (c) acting to offset cerebral acidosis by increasing pH in the extracellular space. In anaesthetic intraoperative practice, hypocapnia is used as a specific treatment of, or prophylaxis against, intracranial hypertension during induction of anaesthesia and the period before dural exposure. More commonly, hypocapnia is used for intraoperative brain relaxation (intracranial pressure = 0). Severe hypocapnia (< 20 mmHg) may result in cerebral production of lactate; however no studies have shown that a Paco2 in the range of 23-28 mmHg has deleterious effects. Recent studies in head-injured patients suggest that routine long-term hyperventilation, without an objective index of cerebral flow/metabolism coupling, may place the brain at risk for adverse outcome. The few data available for intraoperative management suggest that Paco2 figures of 30-35 mmHg result in acceptable operating conditions. Unless otherwise specifically indicated by surgical conditions or cerebral flow/metabolism coupling (e.g. jugular O2 saturation), routine application of profound (Paco2 < 28-30 mmHg) hyperventilation should probably be avoided and its use needs reevaluation.

Changes in cranial and rectal temperature, blood pressure and arterial blood gas during and after unilateral and bilateral forebrain ischemia in Mongolian gerbils

Transient forebrain ischemia was produced by occluding the common carotid arteries, either unilaterally or bilaterally, in Mongolian gerbils under halothane anesthesia. After 20-min ischemia, the cranial temperature measured in the temporal muscle decreased compared to the preischemic level, the decrease being larger after the bilateral than after the unilateral occlusion. After recirculation, cranial temperature recovered promptly to the preischemic level in the unilateral group, while it was elevated to above the preischemic level in the bilateral group. The rectal temperature also decreased with a similar time course. During 30-min ischemia, the blood pressure of both groups increased to above the preischemic level, the increase being larger in the bilateral group than in the unilateral group. After recirculation, blood pressure of the unilateral group recovered promptly to the preischemic level, while that of the bilateral group decreased to below the preischemic level. When forebrain ischemia was produced immediately after cessation of halothane inhalation, blood pH, PaO2 or PaCO2 did not change significantly from the control level. However, these values showed larger variation in the bilateral group than in the unilateral group. Unilateral occlusion in preselected gerbils provided a good model of transient brain ischemia, giving rise to uniform experimental results.

Electrical impedance plethysmography: its use in studying the cerebral circulation of the rabbit

Electrical impedance plethysmography (EIP) is a noninvasive method that may be useful for both the continuous and serial measurement of changes in pulsatile cerebral blood volume and perhaps cerebral blood flow (CBF). It has not been well validated by comparison with other methods. To attempt to validate the EIP technique, the relationship between the peak amplitude of the transcranial, cardiac-synchronous impedance waveform (dZp) and cerebral blood flow measured by the radiolabelled microsphere technique (CBFrlm) and laser Doppler spectroscopy (CBFlds) was studied in rabbits. CBF was altered by inducing hypertension using metaraminol, hypotension by controlled haemorrhage or hypocarbia by hyperventilation. Twenty-three comparisons between dZp and CBFlds and 19 comparisons with CBFrlm were made in eight rabbits. The percentage change between each measurement using the three techniques in each animal was calculated. Using pooled data from all the animals, the linear regression equations were dZp = 0.5 CBFrlm + 33 (r = 0.38, p = 0.22, SE = 79) and dZp = 0.84 CBFlds + 19.6 (r = 0.46, p = 0.09, SE = 72). It is concluded that, in the anaesthetised rabbit, when large changes in CBF are induced by the manoeuvres described above, changes in dZp correlate very weakly with changes in either cortical or global CBF, and are influenced by other factors such as pulsatile intracranial blood volume.

Changes in regional cerebral blood flow after hyperventilation in the pig with an induced focal cerebral contusion

Changes in regional cerebral blood flow in anaesthetized pigs with an induced focal cerebral contusion were studied before and after two grades of hyperventilation. A reduction in arterial tension of CO2 with 0.70 mmHg and a further reduction of 0.55 mmHg did not change the CO2 reactivity. Reactivity in both injured and macroscopically normal regions was the same, revealing an average of 39.3% flow change per kPa change in CO2 tension. Regions with low flow after the contusion had an equally big reduction apparently leading to hypoxia because global metabolic rate was unchanged.

The effects of acute proximal basilar artery occlusion on the primate cerebral circulation

The effects of acute proximal basilar artery occlusion on blood flow, autoregulation and CO2 reactivity in four separate regions of the brain (cerebral cortex, thalamus, brainstem and caudal pons) were studied and compared in 30 anaesthetised baboons. Significant flow changes were seen in all areas of the basilar territory, even in instances where the posterior communicating artery was observed to be relatively large. Flow changes were also seen in regions of the brain remote from the basilar territory. Areas furthest from the collateral blood supply showed the largest changes in blood flow, as has previously been shown in the case of proximal middle cerebral artery occlusion. From this, one can predict that in surgery, the more rostral the occlusion of the artery, the safer the procedure should be. At normal blood pressure, while the collateral circulation to the brainstem and thalamus was adequate to maintain normal electrical function after basilar occlusion, the flow was totally inadequate to maintain autoregulation or CO2 reactivity in the basilar territory.

A comparison of measurements of cerebral blood flow in the rabbit using laser Doppler spectroscopy and radionuclide labelled microspheres

Laser Doppler spectroscopy has been evaluated for the measurement of cerebral blood flow (CBF) by correlation with simultaneous measurements by radionuclide labelled microspheres. The experimental procedures were carried out on five anaesthetised rabbits. The cortical tissue was exposed by means of a small burr hole and illuminated by a helium neon laser (632.8 nm). Reflected light was detected using a silicon photodiode, and CBF was calculated continuously from the power of the frequency weighted Doppler spectrum in the reflected light. Three successive measurements of CBF were made using the microsphere technique. Following an initial baseline measurement, CBF was increased by an infusion of metaraminol and then reduced by controlled haemorrhage. Laser Doppler spectroscopy provided continuous monitoring of blood flow fluctuations and during the haemorrhage it was possible to demonstrate CBF autoregulation until the mean blood pressure fell below 6.7 kPa (50 mmHg). A regression analysis was performed between the simultaneous CBF measurements from the two techniques using a least squares best fit straight line analysis (r = 0.92, P less than 0.001). It was concluded that the flow computed from laser Doppler spectroscopy varied linearly with CBF and offers the unique advantage of continuous and instantaneous measurements even during nonsteady state flow.

A method for recording effects of anti-epileptic drugs on interictal discharge in the cat's cerebral cortex. Factors determining the distribution of external carotid artery infusions

The method utilizes infusion via the external carotid (ECA), the internal maxillary arteries and their anastomoses to the cerebral circulation. It takes into account the ipsilateral distribution of the carotid blood supply. A regular interictal epileptiform spiking from foci on both hemispheres was provided by local application to the cortical surface of small pieces of filter paper soaked in sodium benzylpenicillin, 100,000 IE ml-1. The infused drug affects the ipsilateral foci, and the contralateral one functions as a simultaneous untreated control. The stability of the interictal frequency and the effect of non-oxygen carrying solvents are described. The effect of changes in blood pressure, temperature and PCO2 are considered as well as the coupling between activity in ipsi- and contralateral foci. Experiments with infused radioactive microspheres were performed to determine the strictness of the ipsilateral distribution and the conditions under which it was upheld. With mean arterial blood pressures between 70 mm Hg and 170 mm Hg and infusion speeds between 1.0 ml min-1 and 6.3 ml min-1 the distribution to the contralateral cerebral hemisphere was 0.3% (SD 0.2, SEM 0.1). Infusions of [125I]albumin were used to determine the blood flow in ECA. The flow varied between 20 ml min-1 and 68 ml min-1. The higher values were seen when the extracerebral shunting was high. Conditions influencing the dilution of the infusion and its distribution within the brain were investigated. Important factors were carotid and cerebral blood flow, arterial blood pressure, speed and duration of the infusion, recirculation and cerebral temperature. Arterial PCO2, pH and PO2 should be carefully controlled. Computer-supported treatment of interictal spike frequency and amplitude, as well as of circulatory and respiratory parameters, was utilized. The method was tested in experiments with infusions of 5 alpha-pregnanolone. It was shown that infusions, shorter than the estimated circulation time, reduced the interictal spike frequency and amplitude recorded from the ipsilateral foci without effects on the contralateral ones.

Regional cerebral perfusion during hypertension depends on the hypertensive agent

Cerebral blood flow (CBF) was measured in 40 regions of the rat central nervous system by the [14C]iodoantipyrine autoradiographic technique during a moderate elevation in mean arterial blood pressure (to ca. 150 mmHg), induced by i.v. infusion of either dopamine (DA) or noradrenaline (NA). Hypertension induced by DA resulted in significant increases (median = 44%) in local CBF in 38 of the 40 brain regions investigated. In contrast, during NA infusion, CBF was elevated only slightly (median = 15%) in a few (8 of 40) brain regions (P less than 0.05). The cerebrovascular response to hypertension appears to be dependent upon the catecholamine which is employed to elicit the elevation in arterial blood pressure.

Sensory evoked responses in head injury

Head trauma is a significant source of morbidity in the United States each year. Approximately 700 patients were admitted to our surgical intensive care unit with some degree of head trauma in a 24-month period. Glasgow Coma Score (GCS) was 8 or less in 90% of this group, and 3 or 4 in 43%. Sensory evoked responses were recorded in over 500 patients. This study is reported to demonstrate that optimum care of the injured brain depends on titration of care aimed at maintaining normal neuronal function. In our series, 25% of the patients with GCS of 3 or 4 returned home or to a rehabilitation unit, a significant decrease in morbidity over other reported series. Chemical paralysis and barbiturate coma were a factor in the decision to monitor in 50-60% of the series. In these patients, the auditory brainstem evoked response (ABR), a monitor of brainstem neuroelectrical function, and the somatosensory evoked response, a monitor of brainstem and cortical function, were used to follow the effectiveness of medical and surgical management in these patients, since neurologic examination was of limited value. Case reports are presented to demonstrate that even at high barbiturate levels, access to the integrity of the central nervous system is still possible. Relations among GCS, computerized tomography (CT), intracranial pressure (ICP), ABR, pupillary response, and outcome were studied for a subgroup of 114 patients. All of these clinical parameters, except CT findings, were significantly correlated with outcome using Chi-square analysis. When the data were further analyzed with linear regression analysis, however, the only parameters that significantly correlated with outcome were pupil reactivity and ABR. The principal conclusion of this report is that the main application of serial monitoring of the sensory central pathway in the head-injured patient is not in the prediction of outcome but in the titration of care of the patient for the preservation of neuronal function.

Comparative studies of regional CNS blood flow and evoked potentials in the cat. Effects of hypotensive ischemia on somatosensory evoked potentials in cerebral cortex and spinal cord

Functional resistance to graded hypotensive ischemia of various segments of the somatosensory pathway was determined in anesthetized cats by repeated concurrent recordings of regional blood flow measured by hydrogen clearance, and evoked potentials (EPs), of dorsal horn of lumbar spinal cord and cerebral cortex. During normal resting CNS blood flow (CBF), there were significant successive reductions of EP amplitudes, recorded from presynaptic spinal components (634, 424-949 microV; re-linearized mean and 95% confidence limits of log-transformed data) compared to postsynaptic spinal (359, 247-522 microV) and presynaptic cortical (50, 32-79 microV) and to postsynaptic cortical components (33, 22-50 microV). During ischemia amplitudes of EPs in spinal cord and cerebral cortex showed significantly different behaviors. The presynaptic spinal component was virtually independent of regional blood flow down to 12 percent of resting values, the postsynaptic cortical component exhibited strongest positive correlations (r = 0.45) with flow. In both regions postsynaptic amplitude was more sensitive to flow changes than respective presynaptic amplitudes. Despite similar regression coefficients for intermediate segments of somatosensory pathway, only postsynaptic spinal components were significantly correlated (r = 0.40) with regional flow. Presynaptic cortical amplitudes were variable and no significant flow dependence was demonstrated. Results suggested that in comparable degrees of regional ischemia of CNS functional integrity is determined by numbers of synaptic transmissions involved locally. Comparatively simple structures, e.g. the spinal cord, are less susceptible to ischemia and complex neuronal networks, e.g. the cerebral cortex, are more susceptible.