Absence of autoregulation of cerebral blood flow during halothane and enflurane anesthesia

Anesthesia in Experimental Stroke Research

Anesthetics have enabled major advances in development of experimental models of human stroke. Yet, their profound pharmacologic effects on neural function can confound the interpretation of experimental stroke research. Anesthetics have species-, drug-, and dose-specific effects on cerebral blood flow and metabolism, neurovascular coupling, autoregulation, ischemic depolarizations, excitotoxicity, inflammation, neural networks, and numerous molecular pathways relevant for stroke outcome. Both preconditioning and postconditioning properties have been described. Anesthetics also modulate systemic arterial blood pressure, lung ventilation, and thermoregulation, all of which may interact with the ischemic insult as well as the therapeutic interventions. These confounds present a dilemma. Here, we provide an overview of the anesthetic mechanisms of action and molecular and physiologic effects on factors relevant to stroke outcomes that can guide the choice and optimization of the anesthetic regimen in experimental stroke.

Effect of xenon on cerebral autoregulation in pigs

There are little data on the effect of anaesthetic concentrations of xenon on cerebral pressure autoregulation. In this study, we have investigated the effect of 79% xenon inhalation on cerebral pressure autoregulation and CO2 response in pigs. Ten pigs were randomly allocated to receive xenon 79% or halothane anaesthesia, respectively, in a crossover designed study. Halothane was used to validate the experimental set-up. Transcranial Doppler was performed to determine the mean flow velocities in the middle cerebral artery (vMCA) during defined cerebral perfusion pressures and during normo-, hyper- and hypoventilation. The results showed that the inhalation of 79% xenon preserved cerebral autoregulation during conditions of normo-, hyper- and hypoventilation and at different cerebral perfusion pressures in pigs. These results suggest that with the inhalation of xenon, in the highest concentration suitable for a safe clinical use, cerebral autoregulation is preserved.

Impaired autoregulation of cerebral blood flow in an experimental model of traumatic brain injury

In order to study the pathophysiology and the intracranial hemodynamics of traumatic brain injury, we have developed a modified closed-head injury model of impact-acceleration that expresses several features of severe head injury in humans, including acute and long-lasting intracranial hypertension, diffuse axonal injury, neuronal necrosis, bleeding, and edema. In view of the clinical relevance of impaired autoregulation of cerebral blood flow after traumatic brain injury, and aiming at further characterization of the model, we investigated the autoregulation efficiency 24 h after experimental closed-head injury. Cortical blood flow was continuously monitored with a laser-Doppler flowmeter, and the mean arterial blood pressure was progressively decreased by controlled hemorrhage. Relative laser-Doppler flow was plotted against the corresponding mean arterial blood pressure, and a two-line segmented model was applied to determine the break point and slopes of the autoregulation curves. The slope of the curve at the right hand of the break point was significantly increased in the closed head injury group (0.751 +/- 0.966%/mm Hg versus -0.104 +/- 0.425%/mm Hg,p = 0.028). The break point tended towards higher values in the closed head injury group (62.2 +/- 20.8 mm Hg versus 46.9 +/- 12.7 mm Hg; mean +/- SD, p = 0.198). It is concluded that cerebral autoregulation in this modified closed head injury model is impaired 24 h after traumatic brain injury. This finding, in addition to other characteristic features of severe head injury established earlier in this model, significantly contributes to its clinical relevance.

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.

Cerebral cortex regional blood flow and tissue oxygen tension during the trigeminal depressor response in rabbits

The trigeminal depressor response (TDR) is characterized with profound hypotension caused by sympathetic outflow decreases. The purpose of this study was: (1) to compare the hemodynamic changes during acute hypotension induced by the TDR with those induced by electrical stimulation of the vagal nerve (VS) as models of sympathetic inhibition and vagal activation and (2) to investigate the effects of nitrous oxide (N2O), a well-known sympathomimetic, on the hemodynamic changes during the TDR. Male Japan White rabbits were anesthetized with halothane in oxygen and mechanically ventilated. The electrode pair for the TDR was inserted into the submucosal tissue of the animal's tongue. The pair for the VS was applied to the isolated left vagal trunk. Both the TDR and the VS produced similar decreases in mean arterial pressure (MAP) and cerebral cortex regional blood flow (rCBF), although the decreases in heart rate (HR) and aortic blood flow were greater during the VS. Cerebral cortex tissue oxygen tension in both groups decreased slightly. 50% N2O, producing MAP elevation and HR decrease, ameliorated the hemodynamic changes including rCBF reduction during the TDR. A sudden diminution of sympathetic tone following noxious stimulation of the orofacial area, as in the case of the TDR, may be a possible trigger mechanism for vasodepressor reactions in dental patients. The sympathomimetic and possible antinociceptive effects of N2O may explain, at least in part, the preventive effects on vasodepressor reactions during dental procedures.

[Anesthetics: total intravenous anesthesia or inhalation anesthesia in neurosurgery]

In this review article the pro's and contra's of the use of either inhalational or intravenous anaesthetics for neurosurgical procedures are discussed. The objective is to stimulate thoughts concerning controversial subjects, rather than to resolve issues. It is much less complicated to approach the practice of neuroanaesthesia with a few straight forward "rules" based on laboratory measurements (such as intravenous drugs are good because they reduce CBF and ICP, whereas inhalational agents are bad because they increase CBF and ICP). It should also be noted that whereas statements about potential detrimental or beneficial effects of different anaesthetic agents are relatively common, there is a dearth of well-designed prospective studies of sufficient power to substantiate the outcome advantages or disadvantages. The choice of an anaesthetic should include more than just a consideration of the potential intracranial effects of a drug: it should also include experience with a drug and, more important a consideration of the patient as a whole.

Prevention of cerebral ischaemia under general anaesthesia

General anaesthesia influences the oxygen supply/demand ratio of the brain and may thus be able to improve neurological outcome after carotid artery surgery. Among the anaesthetic agents, barbiturates and isoflurane produce a marked reduction of the cerebral metabolic rate and activity. But the incidence of neurological deficits in the immediate postoperative period is low, while the possible causes of any neurological deficits are multiple. Consequently, to date, no particular anaesthetic agent or management has been shown to influence the neurological results.

Cerebral autoregulation during moderate hypothermia in rats

BACKGROUND AND PURPOSE

Little is known about the effects of hypothermia on cerebral autoregulation. The present study was designed to examine cerebral blood flow responses to controlled hemorrhagic hypotension in normothermic and hypothermic rats.

METHODS

Cortical blood flow was measured with a laser-Doppler flowmeter in halothane-anesthetized rats assigned to one of three groups: normothermic group 1 (n = 8) with a pericranial temperature of approximately 36.5 degrees C or hypothermic group 2 (n = 8) or group 3 (n = 8) with a pericranial temperature of approximately 30.5 degrees C. In group 2, a PaCO2 of approximately 40 mm Hg was maintained without correction for body temperature. To evaluate the role of PaCO2, in group 3 animals PaCO2 was kept at approximately 40 mm Hg as corrected for body temperature. In all animals, the mean arterial blood pressure was reduced by hemorrhage in increments of 10 mm Hg every 2 minutes.

RESULTS

In group 1 animals, a typical autoregulatory curve was observed with cerebral blood flow first falling at or below 75% of baseline at a mean arterial pressure of 57 +/- 15 mm Hg (mean +/- SD). Absolute normotensive cerebral blood flow in group 2 fell to < or = 75% of baseline at a mean arterial pressure of 73 +/- 21 mm Hg. In group 3, no evidence of autoregulation was seen. Cerebral blood flow reached values < or = 75% of baseline at a mean arterial pressure of 82 +/- 14 mm Hg, whereas calculated cerebrovascular resistance failed to show any compensatory vasodilation as the mean arterial pressure decreased.

CONCLUSIONS

Different PaCO2 management schemes used during hypothermia may have profound effects on cerebral blood flow and on autoregulation. If PaCO2 is maintained at 40 mm Hg after correction for temperature, autoregulation is abolished. If uncorrected PaCO2 is maintained at approximately 40 mm Hg, some degree of autoregulation is preserved, albeit with a right-shifted "knee."

Cerebral haemodynamic and electrocortical CO2 reactivity in pigs anaesthetized with fentanyl, nitrous oxide and pancuronium

Cerebral haemodynamic, metabolic and electrocortical reactivity to alterations in arterial CO2 tension (PaCO2) was assessed in seven mechanically ventilated juvenile pigs to test an experimental model designed for cerebral pharmacodynamic and pharmacokinetic studies. The animals were anaesthetized with fentanyl, nitrous oxide and pancuronium and sequentially normo- and hyperventilated over a 100-min period. Five measurements were made at 25-min intervals. The cerebral blood flow (CBF) was measured with the intra-arterial 133Xe technique and the cerebral metabolic rate for oxygen (CMRO2) determined from CBF and the cerebral arteriovenous oxygen content difference. A linear correlation (r = 0.845) was found between CBF and PaCO2. The cerebrovascular reactivity to hypocapnia (delta CBF/delta PaCO2) was maintained throughout the experimental period and amounted to (95% confidence interval) 9.1 (7.1-11.1) ml x 100 g-1 x min-1 x kPa-1 within the PaCO2 range 3.3-6.3 kPa. The CMRO2 was not influenced by hyperventilation. The baseline electroencephalographic (EEG) pattern was stable at normocapnia (mean PaCO2 5.6 kPa), whereas spectral values for delta and total average voltage increased significantly (P < 0.05) at extensive hypocapnia (3.5 kPa). Maintenance of cerebral CO2 reactivity and spectral EEG voltage at a stable plasma level of fentanyl is complementary to the cerebral haemodynamic and metabolic stability previously found at sustained normocapnia in this model.

Monitoring and cerebral protection during carotid endarterectomy

Two recently published multicentre trials have confirmed the overall benefit of carotid endarterectomy in symptomatic patients with severe carotid artery disease. The key to improving further the long-term advantages of carotid endarterectomy, however, remains the continued reduction of the initial operative risk. While the principal responsibility for this continues to be borne by the surgeon, specifically in reducing technical error, the time is perhaps approaching when he or she might also be able to apply some of the recent advances in cerebrovascular research to reduce operative morbidity still further in the future. This article summarizes the aetiology and pathophysiology of operation-related neurological deficits and reviews current approaches towards intraoperative monitoring, cerebral protection and assessment of quality control.

Cerebral blood flow response to increases in arterial CO2 tension during alfentanil anesthesia in the rabbit

The stability of cerebral function and blood flow (CBF), and the CBF response to changes in arterial carbon dioxide tension (CBF reactivity) during alfentanil anesthesia were examined in rabbits. This model was first shown to provide stable anesthesia, cortical function, and CBF for 4 h. CBF increased significantly to 159% [of baseline] in the left hemisphere and to 167% in the right within 5 min of an exposure to 5% CO2 (p = 0.009 on the left and p = 0.003 on the right), but then decreased to 123% on the left and to 137% on the right (not significantly different from baseline, p = 0.11 on the left and p = 0.07 on the right) while PaCO2 was still rising. Steady state reactivity levels (0.8 ml 100 g-1/min-1/mm Hg-1 CO2 on the left and 0.65 ml 100 g-1/min-1/mm Hg-1 CO2 on the right) were consistent with previous work and were reached at 20 min. These results suggest that mechanisms other than perivascular hydrogen ion concentration mediate the CBF response to changes in arterial CO2 tension during alfentanil anesthesia.

Circulatory and metabolic changes in the brain during induced hypotension--comparison among trimetaphan, glycerin trinitrate and prostaglandin E1

Induced hypotension was carried out using trimetaphan (TMP), glycerin trinitrate (GTN) and prostaglandin E(1) (PGE(1)) in 45 patients received elective abdominal surgery under anesthesia with enflurane in N(2)O/O(2) in order to evaluate and compare the effects of these three agents on cerebral circulation and metabolism. Upon reduction of mean arterial blood pressure to 60-65 mmHg, cerebral blood flow decreased in the TMP and GTN groups but increased in the PGE(1) group. The changes were quite proportional to those in cardiac index in the three groups. Cerebral oxygen consumption decreased only in the TMP group. Changes in cerebrospinal fluid pressure were not in parallel with those in cerebral blood flow. The former decreased slightly in the TMP group but increased in the GTN and PGE(1) groups. These results offered a great caution for induction of artificial hypotension using these agents.

Anesthesia for craniotomy: total intravenous anesthesia with propofol and alfentanil compared to anesthesia with thiopental sodium, isoflurane, fentanyl, and nitrous oxide

STUDY OBJECTIVE

To compare a total intravenous (IV) anesthetic technique based on propofol and alfentanil with a commonly used anesthetic technique for craniotomy.

DESIGN

Open-label, randomized, clinical study.

SETTING

Neurosurgical clinic at a university hospital.

PATIENTS

Forty patients, aged 18 to 55 years, scheduled for brain tumor surgery.

INTERVENTIONS

In 20 patients, anesthesia was induced with fentanyl and thiopental sodium and maintained with fentanyl, dehydrobenzperidol, isoflurane, nitrous oxide (N2O), and a thiopental sodium infusion. Twenty patients were anesthetized with a propofol loading infusion followed by a maintenance infusion at a fixed rate. In addition, alfentanil was administered as a loading bolus, followed by a variable-rate infusion, with additional doses as necessary to maintain hemodynamic stability.

MEASUREMENTS AND MAIN RESULTS

A decrease in blood pressure (BP) after induction with thiopental sodium was followed by a significant increase in BP and heart rate (HR) during intubation. BP and HR did not change during the propofol loading infusion. However, the administration of alfentanil was followed by a similar decrease in BP with a return to baseline values during the intubation period. Return of normal orientation (7 +/- 5 minutes vs 27 +/- 23 minutes) and concentration (12 +/- 12 minutes vs 35 +/- 37 minutes) was shorter and more predictable for the propofol-alfentanil-treated patients than for the thiopental sodium patients. Maintenance propofol concentration (nine patients) was between 3 +/- 0.69 micrograms/ml and 3.36 +/- 1.17 micrograms/ml, while the concentration at awakening was 1.09 microgram/ml. Alfentanil concentration at extubation (nine patients) was 79 +/- 34 ng/ml.

CONCLUSION

A total IV anesthetic technique with propofol and alfentanil is a valuable alternative to a more commonly used technique based on thiopental sodium, N2O, fentanyl, and isoflurane.

Sodium nitroprusside decreases spinal cord perfusion pressure during descending thoracic aortic cross-clamping in the dog

Paraplegia is a devastating complication of surgery on the descending thoracic aorta. During surgical repair, the aorta is cross-clamped, and nitroprusside is often used to treat arterial hypertension that can occur above the cross-clamp. Twenty-one dogs were studied to determine the effects of nitroprusside on intraspinal pressures, mean aortic pressures below the cross-clamp, and spinal cord perfusion pressure. Perfusion pressure in spinal radicular arteries originating below the aortic cross-clamp was estimated as the distal aortic pressure minus intraspinal pressure. Nitroprusside was used to return the mean arterial pressure above the cross-clamp to values similar to the pre-cross-clamp levels in 7 dogs. Fourteen animals did not receive sodium nitroprusside. Aortic cross-clamping resulted in small but significant increases in intraspinal pressure (4.3 +/- 0.8 to 7.5 +/- 0.9 mm Hg in non-nitroprusside-treated dogs, and 3.4 +/- 1.0 to 5.6 +/- 1.5 mm Hg in the nitroprusside group before nitroprusside). Nitroprusside caused a further increase in intraspinal pressure (5.6 +/- 1.5 to 8.3 +/- 2.2 mm Hg) and a decrease in aortic pressure below the cross-clamp (26 +/- 5 to 18 +/- 4 mm Hg). The increase in intraspinal pressure and the decrease in aortic pressure below the cross-clamp after nitroprusside resulted in a decrease in spinal cord perfusion pressure from 19 +/- 5 mm Hg to 11 +/- 4 mm Hg. Because nitroprusside decreases spinal cord perfusion pressure and may increase the risk of spinal cord ischemia, the avoidance of large doses of nitroprusside to arbitrarily return mean arterial pressure above the cross-clamp to pre-cross-clamp levels is recommended.

Effect of adenosine-induced hypotension on the cerebral autoregulation in the anesthetized pig

The influence on cerebral blood flow (CBF) and autoregulation of systemic adenosine infusion, at doses that produced a 29 +/- 4% (0.28 +/- 0.06 mg/kg/min) or a 55 +/- 2% (0.49 +/- 0.07 mg/kg/min) reduction of mean arterial blood pressure (MABP), was evaluated in 12 normoventilated fentanyl/N2) anesthetized pigs. CBF was determined as sagittal sinus outflow and recorded continuously by an electromagnetic technique. Autoregulation was evaluated by two formal tests: infusion of angiotensin for elevation of MABP, and reduction of myocardial filling pressure by caval block for graded MABP decrease before, during and after adenosine infusion. CBF as well as cerebral metabolic rate of oxygen were unaffected during both levels of hypotension and were not significantly altered after the hypotension. Signs of impaired autoregulation were found during the angiotensin test as well as during the caval block at light hypotension (92 +/- 3 mmHg, 12.3 +/- 0.4 kPa), while autoregulation was completely abolished at moderate hypotension (59 +/- 2 mmHg, 7.9 +/- 0.3 kPa). After termination of adenosine-induced hypotension, autoregulation was restored in all animals within 60 min. It is concluded that systemically administered adenosine preserves CBF, even at low MABP levels, by a direct cerebral vasodilatory effect. However, the cerebral autoregulatory mechanisms are impaired or abolished in a dose-dependent and reversible manner.

Cerebral blood flow, cerebral metabolic rate of oxygen and relative CO2-reactivity during craniotomy for supratentorial cerebral tumours in halothane anaesthesia. A dose-response study

Fourteen patients were studied during craniotomy for small supratentorial cerebral tumours. Cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) were measured twice by a modification of the Kety-Schmidt technique using 133Xe intravenously. Anaesthesia was induced with thiopental 4-6 mg kg-1, fentanyl and pancuronium, and maintained with an inspiratory halothane concentration of 0.45% in nitrous oxide 67% at a moderate hypocapnic level. In one group of patients (n = 7) the inspiratory halothane concentration was maintained at 0.45% throughout anaesthesia. About 1 h after induction of anaesthesia CBF and CMRO2 averaged 35 +/- 2 ml 100 g-1 min-1 and 2.7 +/- 0.3 ml O2 100 g-1 min-1 (mean +/- s.c. mean), respectively. During repeat studies 1 h later CBF and CMRO2 did not change. In another group of patients (n = 7) an increase in halothane concentration from 0.45% to 0.90% was associated with a significant decrease in CMRO2 from 2.3 +/- 0.1 to 2.0 +/- 0.1 ml O2 100 g-1 min-1. The CO2-reactivity measured after the second flow measurement was preserved. It is concluded that halothane in this study induces a dose-dependent decrease in cerebral metabolism, an increase in CBF while CO2-reactivity is maintained.

Induced hypotension with labetalol for orthognathic surgery

Induced hypotension is an accepted technique for reducing blood loss in various surgical procedures. This study evaluates the effectiveness of labetalol in producing controlled reduction in mean arterial pressure during orthognathic surgery. The potential advantages of this technique are ease of administration, decreased pulmonary shunting, and absence of tachycardia or rebound hypertension compared to other commonly used agents.

Effect of preischemia cyclooxygenase inhibition by zomepirac sodium on reflow, cerebral autoregulation, and EEG recovery in the cat after global ischemia

Zomepirac sodium (ZS) (5 mg/kg i.v.) was used to evaluate the effects of preischemia cyclooxygenase inhibition on CBF (as assessed by 133Xe clearance), CBF-PaCO2 responsiveness, and electrophysiologic (EEG) parameters before and after a 15-min period of complete global ischemia produced by four-vessel occlusion and mild hypotension. During the 15-min period of ischemia, CBF was essentially zero. Following reflow all groups displayed an initial hyperemia as compared with control (92 +/- 11 vs. 141-146 ml/100 g/min). Saline-treated animals during reflow displayed a delayed hypoperfusion (26 +/- 3 ml/100 g/min), which showed no improvement during the 2-h reflow period prior to death. In contrast, ZS-treated animals during reflow displayed significantly higher flows during the hypoperfusion phase (72 +/- 9 ml/100 g/min). The CBF-PaCO2 response displayed an approximately sevenfold reduction in slope at 2 h after reflow in saline-treated animals. This decrease in PaCO2 reactivity was not observed in the ZS-pretreated animals. With regard to EEG, all animals showed a total flattening during the 15 min of ischemia. In saline-treated animals only one of seven showed any sign of even marginal recovery. In ZS-treated animals EEG activity showed prominent recovery in seven of seven. Brainstem auditory evoked potentials were monitored and showed prominent recovery of amplitude and latency in ZS but not saline-treated animals during reflow.

Anesthesia for pediatric neurological and neuromuscular diseases

Children with neurological and neuromuscular diseases often present anesthetic problems in the perioperative period. The anesthetic technique can play a significant role in altering the state of the brain during neurosurgical procedures through effects on the cerebral circulation and metabolism. Pre-existing neuromuscular disease may also have specific anesthetic implications such as cardiorespiratory involvement (eg, myotonia dystrophica), the potential for drug interactions (eg, myasthenia gravis) or abnormal responses to commonly used drugs (eg, malignant hyperthermia). In this review, the perioperative anesthetic considerations in a number of common neurological and neuromuscular conditions in the pediatric patient are discussed.

Cellular actions of halothane on cat cerebral arterial muscle

The effects of halothane on intracellular membrane potential (Em) and force development in cat MCA were studied. Halothane (0.07-0.14 mM/1) relaxed isolated MCA which had developed myogenic tone. Measurement of Em showed that halothane depolarized this preparation in a dose-dependent fashion in the face of vessel relaxation, demonstrating uncoupling of electrical and mechanical activity. Halothane markedly inhibited the contractile effects of histamine and serotonin suggesting that, apart from its direct action on cerebral arterial tone, it also blunts the action of vasoactive agents. When this preparation is partially depolarized from -62 to -50 mV with excess K+, halothane, while having only a small (1.2 mV) additional depolarizing effect, consistently elicits contraction rather than relaxation. Thus, the action of this particular volatile anesthetic on cerebral arteries can depend upon the resting level of Em. These studies indicate that halothane relaxes myogenic tone in cat MCA by an intracellular mechanism, but that the direction of its effect (i.e., relaxation vs. contraction) may depend upon the prior level of Em and muscle cell activation.

Microvascular response in cancellous bone to halothane-induced hypotension in pigs

The microvascular response in cancellous bone to halothane-induced hypotension was studied simultaneously using Laser Doppler Flowmetry (LDF) and clearance of locally injected 133Xe in the subapical mandibular bone of 16 young pigs. During halothane inhalation, a significant reduction in mean blood pressure to values ranging from 22 to 60 mm Hg was observed. The blood flow, as measured with LDF, increased in more than 60% of the animals with hypotension levels above 30 mm Hg. At hypotension levels below 30 mm Hg, blood flow decreased in all cases. The individual regional vascular resistance, calculated as the quotient between blood pressure and recorded blood flow value, was significantly reduced (p less than 0.001) during hypotension. Rhythmical flow variations, observed in some animals at normotension, disappeared immediately following the start of halothane inhalation, but returned when blood pressure was stabilized at a lower level. For recordings made with 133Xe-clearance, a significant reduction (p less than 0.001) in the logarithmic decay of the washout curves was observed during hypotension. This logarithmic decay was correlated to the reduced blood pressure levels (r = 0.73). No average change in regional vascular resistance could, however, be demonstrated. The lack of correlation between results obtained by the 2 methods may be due to the fact that LDF is sensitive to the total blood flow in a limited tissue volume, while the 133Xe isotope seems to be washed out mainly by the capillary blood flow. It can be further concluded that halothane-induced hypotension levels, applicable to clinical routines, may produce an increase rather than a decrease in total blood flow in cancellous bone tissue.