Isovolemic hemodilution in experimental focal cerebral ischemia. Part 2: Effects on regional cerebral blood flow and size of infarction

The effect of the volemic and cardiac status on brain oxygenation in patients with subarachnoid hemorrhage: a bi-center cohort study

Background

Fluid management in patients after subarachnoid hemorrhage (SAH) aims at the optimization of cerebral blood flow and brain oxygenation. In this study, we investigated the effects of hemodynamic management on brain oxygenation by integrating advanced hemodynamic and invasive neuromonitoring.

Methods

This observational cohort bi-center study included data of consecutive poor-grade SAH patients who underwent pulse contour cardiac output (PiCCO) monitoring and invasive neuromonitoring. Fluid management was guided by the transpulmonary thermodilution system and aimed at euvolemia (cardiac index, CI ≥ 3.0 L/min/m²; global end-diastolic index, GEDI 680–800 mL/m²; stroke volume variation, SVV < 10%). Patients were managed using a brain tissue oxygenation (P_btO₂) targeted protocol to prevent brain tissue hypoxia (BTH, P_btO₂ < 20 mmHg). To assess the association between CI and P_btO₂ and the effect of fluid challenges on CI and P_btO₂, we used generalized estimating equations to account for repeated measurements.

Results

Among a total of 60 included patients (median age 56 [IQRs 47–65] years), BTH occurred in 23% of  the monitoring time during the first 10 days since admission. Overall, mean CI was within normal ranges (ranging from 3.1 ± 1.3 on day 0 to 4.1 ± 1.1 L/min/m² on day 4). Higher CI levels were associated with higher P_btO₂ levels (Wald = 14.2; p < 0.001). Neither daily fluid input nor fluid balance was associated with absolute P_btO₂ levels (p = 0.94 and p = 0.85, respectively) or the occurrence of BTH (p = 0.68 and p = 0.71, respectively). P_btO₂ levels were not significantly different in preload dependent patients compared to episodes of euvolemia. P_btO₂ increased as a response to fluid boluses only if BTH was present at baseline (from 13 ± 6 to 16 ± 11 mmHg, OR = 13.3 [95% CI 2.6–67.4], p = 0.002), but not when all boluses were considered (p = 0.154).

Conclusions

In this study a moderate association between increased cardiac output and brain oxygenation was observed. Fluid challenges may improve P_btO₂ only in the presence of baseline BTH. Individualized hemodynamic management requires advanced cardiac and brain monitoring in critically ill SAH patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13613-021-00960-z.

Clinical response to hypertensive hypervolemic therapy and outcome after subarachnoid hemorrhage

OBJECTIVE

Hypertensive hypervolemic therapy is widely used to treat symptomatic vasospasm after subarachnoid hemorrhage. Few data exist to support a relationship between early clinical response and mortality or functional outcome.

METHODS

In a prospective cohort of 580 subarachnoid hemorrhage patients, we studied 95 patients with acute symptomatic vasospasm who received stepwise volume expansion with crystalloid and/or 5% albumin solution followed by intravenous pressors to maintain systolic blood pressure between 180 and 220 mm Hg. We separately assessed the effects of volume expansion and induced hypertension on the neurological examination during the first 2 hours of each intervention. We used multivariate logistic regression analysis to calculate adjusted odds ratios assessing the relationship between clinical response to hypertensive hypervolemic therapy and 3-month outcome, as measured by the modified Rankin Scale.

RESULTS

Of 95 patients with symptomatic vasospasm, volume expansion was used in 94% (n = 89), of whom 43% had a clinical response; 85% of the patients (n = 81) received pressors, of whom 68% responded. Early clinical improvement attributable to either volume expansion or pressors was not related to the development of infarction on computed tomography, but response to either modality within 2 hours was independently protective against death (adjusted odds ratio, 0.03; P < 0.05) and death-or-severe-disability (modified Rankin Scale score, 4-6; adjusted odds ratio, 0.1; P < 0.05) after adjusting for age, Hunt-Hess grade, angioplasty, and aneurysm size.

CONCLUSION

Subarachnoid hemorrhage patients with symptomatic vasospasm who fail to demonstrate early clinical improvement in response to volume or pressor therapy are at high risk for death or disability. Urgent endovascular intervention in this high-risk patient cohort may be justified.

Resuscitation of traumatic brain injury and hemorrhagic shock with polynitroxylated albumin, hextend, hypertonic saline, and lactated Ringer's: Effects on acute hemodynamics, survival, and neuronal death in mice

Outcome after traumatic brain injury (TBI) is worsened by hemorrhagic shock (HS), but the optimal resuscitation approach is unclear. In particular, treatment of TBI patients with colloids remains controversial. We hypothesized that resuscitation with the colloids polynitroxylated albumin (PNA) or Hextend (HEX) is equal or superior to resuscitation with the crystalloids hypertonic (3%) saline (HTS) or lactated Ringer's solution (LR) after TBI plus HS in mice. C57/BL6 mice (n = 30) underwent controlled cortical impact (CCI) and 90 min of volume-controlled HS (2 mL/100 g). The mice were randomized to resuscitation with LR, HEX, HTS, or PNA, followed by 30 min of test fluid administration targeting a mean arterial pressure (MAP) of >50 mm Hg. Shed blood was re-infused to target a MAP >70 mm Hg. At 7 days post-insult, hippocampal neuron counts were assessed in hematoxylin and eosin-stained sections to quantify neuronal damage. Prehospital MAP was higher, and prehospital and total fluid requirements were lower in the PNA and HEX groups (p < 0.05 versus HTS or LR). Also, 7-day survival was highest in the PNA group, but was not significantly different than the other groups. Ipsilateral hippocampal CA1 and CA3 neuron loss did not differ between groups. We conclude that the colloids PNA and HEX exhibited more favorable effects on acute resuscitation parameters than HTS or LR, and did not increase hippocampal neuronal death in this model.

Acute normovolaemic haemodilution with a novel hydroxyethyl starch (130/0.4) reduces focal cerebral ischaemic injury in rats

BACKGROUND AND OBJECTIVES

This study investigated the neuroprotective effect of acute normovolaemic haemodilution with a novel hydroxyethyl starch (130/0.4, HES) on focal cerebral ischaemia-reperfusion injury and determined optimum degree for haemodilution in the rat.

METHODS

Male Sprague-Dawley rats were randomly divided into three groups (n = 10 each): hydroxyethyl starch, saline and controls. Animals in the hydroxyethyl starch and saline groups were haemodiluted until haematocrit decreased to 30% of baseline. In another experiment male Sprague-Dawley rats were randomly assigned to four groups (n = 10 each): HES30, HES25 and HES20 and controls. Rats in the HES30, HES25 and HES20 groups were haemodiluted with hydroxyethyl starch until the haematocrit decreased to 30%, 25% and 20%, respectively. At 15 min after haemodilution, all the rats were subjected to a right middle cerebral artery occlusion for 120 min. Neurologic deficit scores and infarct volumes were assessed 24 h after reperfusion.

RESULTS

Haemodilution with hydroxyethyl starch improved neurologic outcome (P < 0.01) and reduced infarct volume (P < 0.01) compared to saline and controls. Haemodilution with saline augmented infarct volume (P < 0.05 vs. controls). Neurologic deficit scores in HES30 and HES25 groups were significantly lower compared to controls (P < 0.05) while infarct volumes in controls and HES20 were significantly larger compared to HES30 and HES25 (P < 0.01).

CONCLUSIONS

This study demonstrates that acute normovolaemic haemodilution with a novel hydroxyethyl starch (130/0.4) reduces infarct volume and improves neurological outcome after focal cerebral ischaemia in a rat stroke model. Neuroprotection by haemodilution was highest at haematocrit dilution between 25% and 30%, while ischaemic injury was exacerbates when haematocrit was diluted to 20%.

Effects of hypervolemia and hypertension on regional cerebral blood flow, intracranial pressure, and brain tissue oxygenation after subarachnoid hemorrhage

OBJECTIVE

Hypertensive, hypervolemic, hemodilution therapy (triple-H therapy) is a generally accepted treatment for cerebral vasospasm after subarachnoid hemorrhage. However, the particular role of the three components of triple-H therapy remains controversial. The aim of the study was to investigate the influence of the three arms of triple-H therapy on regional cerebral blood flow and brain tissue oxygenation.

DESIGN

Animal research and clinical intervention study.

SETTING

Surgical intensive care unit of a university hospital.

SUBJECTS AND PATIENTS

Experiments were carried out in five healthy pigs, followed by a clinical investigation of ten patients with subarachnoid hemorrhage.

INTERVENTIONS

First, we investigated the effect of the three components of triple-H therapy under physiologic conditions in an experimental pig model. In the next step we applied the same study protocol to patients following aneurysmal subarachnoid hemorrhage. Mean arterial pressure, intracranial pressure, cerebral perfusion pressure, cardiac output, regional cerebral blood flow, and brain tissue oxygenation were continuously recorded. Intrathoracic blood volume and central venous pressure were measured intermittently. Vasopressors and/or colloids and crystalloids were administered to stepwise establish the three components of triple-H therapy.

MEASUREMENTS AND MAIN RESULTS

In the animals, neither induced hypertension nor hypervolemia had an effect on intracranial pressure, brain tissue oxygenation, or regional cerebral blood flow. In the patient population, induction of hypertension (mean arterial pressure 143 +/- 10 mm Hg) resulted in a significant (p < .05) increase of regional cerebral blood flow and brain tissue oxygenation at all observation time points. In contrast, hypervolemia/hemodilution (intrathoracic blood volume index 1123 +/- 152 mL/m) induced only a slight increase of regional cerebral blood flow while brain tissue oxygenation did not improve. Finally, triple-H therapy failed to improve regional cerebral blood flow more than hypertension alone and was characterized by the drawback that the hypervolemia/hemodilution component reversed the effect of induced hypertension on brain tissue oxygenation.

CONCLUSIONS

Vasopressor-induced elevation of mean arterial pressure caused a significant increase of regional cerebral blood flow and brain tissue oxygenation in all patients with subarachnoid hemorrhage. Volume expansion resulted in a slight effect on regional cerebral blood flow only but reversed the effect on brain tissue oxygenation. In view of the questionable benefit of hypervolemia on regional cerebral blood flow and the negative consequences on brain tissue oxygenation together with the increased risk of complications, hypervolemic therapy as a part of triple-H therapy should be applied with utmost caution.

A review of red cell transfusion in the neurological intensive care unit

The treatment of anemia in critically ill patients has changed significantly in the past decade with a major shift toward restrictive blood-transfusion strategy. There is a paucity of studies regarding the approach toward anemia in the neurological critical care population. Anemia is a complex problem in this group of patients because of the extreme sensitivity of brain tissue to changes in the cerebral perfusion pressure and oxygen deficit. Most of the evidence regarding management of anemia and optimal hematocrit threshold is based on animal experiments and observational studies. Recent studies have shown a mixed response in the local oxygen saturations and patient outcomes after blood transfusion in neurological critically ill patients. Although there is little reason to suspect that restrictive transfusion protocols would be detrimental, further studies are needed to determine optimal transfusion threshold in this population.

High dose human serum albumin for the treatment of acute ischemic stroke: a safety study

INTRODUCTION

In animal models of focal cerebral ischemia, albumin infusions at doses ranging from 0.6 to 2.5 g/kg are neuroprotective. It is not known whether patients with stroke, often elderly and with underlying cardiovascular disease, can safely tolerate such degrees of volume expansion. Therefore, we retrospectively reviewed the safety of high-dose albumin treatment in patients with acute ischemic stroke.

MATERIALS AND METHODS

Within 24 hours of ischemic stroke onset, patients who received at least 0.7 g/kg albumin were identified by a review of medical records. Each albumin recipient was assigned two control patients, who received standard fluid management. Controls were matched by age, number of stroke risk factors, stroke severity, and stroke subtype. Medical records were reviewed for treatment-related adverse events, defined as cardiopulmonary complications and mortality.

RESULTS

Thirty cases (mean age 62.9+/-11.4 years) and 60 controls (mean age 62.5+/-11.8 years) were identified between July 1999 and November 2001. The two groups were evenly matched. The mean dose of albumin infusion was 171 g (2.4 g/kg). Cardiopulmonary complications or death developed in 37% of cases and 18% of controls (p=0.056). Mortality was 7% in both groups. Multivariate regression analysis showed that a history of congestive heart failure and higher total albumin dose were independently associated with the occurrence of adverse events.

CONCLUSION

Albumin treatment was associated with a nonsignificant trend toward increased cardiopulmonary adverse events. However, these adverse events did not result in excess mortality.

Relationship of initial hematocrit level to discharge destination and resource utilization after ischemic stroke: a pilot study

OBJECTIVE

To examine the association between initial hematocrit level at the time of ischemic stroke, discharge destination, and resource utilization.

DESIGN

Case series.

SETTING

University hospital.

PARTICIPANTS

A total of 1012 consecutive patients with ischemic stroke admitted to a university health system between August 3, 1995, and June 24, 1999.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Length of stay, hospital cost, and discharge disposition.

RESULTS

Of 1012 patients presenting with ischemic stroke, 58% were discharged home, 10% were discharged home with home care services, 15% were discharged to a rehabilitation hospital, 11% were discharged to a skilled or intermediate care facility, and 6% died. After adjusting for age, sex, race, and comorbidities, a significant association (P=.009) existed between discharge outcome and initial hematocrit level. The probability of achieving an equivalent or less favorable outcome increased at both high and low hematocrit levels, with a minimum probability at a hematocrit level of approximately 45%.

CONCLUSIONS

An association exists between hematocrit level at the time of ischemic stroke and discharge outcome. Midrange hematocrit levels appear to be associated with discharge to home rather than to an inpatient rehabilitation unit or to a nursing facility. Further study is indicated to examine the relationship among hematocrit level, stroke severity, and outcome.

Fluids and the neurosurgical patient

Few human data exist concerning the impact of fluid administration on brain pathophysiology. Those factors that influence water movement into the brain are examined, in order to provide reasonable recommendations for peri-operative fluid management in the patients with brain pathology.

Regional cerebral blood flow in cats with cross-linked hemoglobin transfusion during focal cerebral ischemia

The beneficial effect of hemodilution on cerebral blood flow (CBF) during focal cerebral ischemia is mitigated by reduced arterial oxygen content (CaO2). In anesthetized cats subjected to permanent middle cerebral artery occlusion, the time course of regional CBF was evaluated after isovolemic exchange transfusion with either albumin or a tetrameric hemoglobin-based oxygen carrier. The transfusion started 30 min after arterial occlusion. We tested the hypothesis that bulk oxygen transport (CBF x CaO2) to ischemic tissue is increased by hemoglobin transfusion at a hematocrit of 18% compared with albumin-transfused cats at a hematocrit of 18% or control cats at a hematocrit of 30% and equivalent arterial pressure. In the nonischemic hemisphere, CBF increased selectively after albumin transfusion, and oxygen transport was similar among groups. In the ischemic cortex, albumin transfusion increased CBF, but oxygen transport was not increased above that of the control group. Hemoglobin transfusion increased both CBF and oxygen transport in the ischemic cortex above values in the control group, but the increase was delayed until 4 h of ischemia. Consequently, acute injury volume measured at 6 h of ischemia was not significantly attenuated. In contrast to the cortex, CBF in the ischemic caudate nucleus was not substantially increased by either albumin or hemoglobin transfusion. Therefore, in a large animal model of permanent focal ischemia in which transfusion starts 30 min after ischemia, tetrameric cross-linked hemoglobin transfusion can augment oxygen transport to the ischemic cortex, but the increase can be delayed and not necessarily provide protection. Moreover, an end-artery region such as the caudate nucleus is less likely to benefit from hemodilution.

Intensive care unit management of the stroke patient

Although the majority of patients with acute stroke do not require intensive care, it is important to recognize when admission to an intensive care unit (ICU) is warranted. Patients undergoing thrombolytic therapy, those with brainstem infarcts referable to the basilar artery, those with large space occupying hemispheric infarcts, and those with fluctuating neurological examinations should be admitted to the ICU for monitoring and treatment.

The effect of hemodilution on cerebral blood flow velocity in anesthetized patients

Transcranial Doppler is used to estimate changes in cerebral blood flow, but the effect of hemodilution on cerebral blood flow velocity (CBFV) in anesthetized patients has not been evaluated. The aim of this study was to measure the effect of isovolemic hemodilution on CBFV and lumbar cerebrospinal fluid pressure (P(CSF)) in anesthetized patients without change in other physiological variables that may affect CBFV. Patients undergoing hemodilution were compared with a control group undergoing no hemodilution. With hemodilution, hematocrit decreased from 38% +/- 3% to 30% +/- 2%, arterial oxygen content (Cao2) decreased from 17.5 +/- 1.3 to 13.9 +/- 0.9 mL/dL, and CBFV increased from 50 +/- 10 to 58 +/- 10 cm/s. An equivalent of cerebral arterial O2 transport calculated as Cao2 x CBFV did not significantly change. Over the same time interval, there were no changes in the control group. There was no statistically significant change in P(CSF), pulsatility index, Paco2, blood pressure, heart rate, or body temperature in either group. We conclude that CBFV reflects cerebral blood flow changes after hemodilution.

IMPLICATIONS

Hemodilution increases cerebral blood flow but may change the cerebral artery diameter, which could confound perioperative measurement of cerebral blood flow velocity. This study found transcranial Doppler ultrasonography to accurately assess the effects of hemodilution on the cerebral circulation, but the hematocrit should be taken into account to fully understand perioperative cerebral blood flow velocity changes.

Effect of delayed albumin hemodilution on infarction volume and brain edema after transient middle cerebral artery occlusion in rats

The authors examined the effect of delayed high-concentration albumin therapy on ischemic injury in a highly reproducible model of middle cerebral artery (MCA) occlusion in rats. Male Sprague-Dawley rats weighing 270 to 320 g were anesthetized with halothane and subjected to 120 minutes of temporary MCA occlusion induced by means of a poly-L-lysine-coated intraluminal nylon suture inserted retrograde via the external carotid artery into the internal carotid artery and MCA. The agent (20% human serum albumin [HSA]) or control solution (sodium chloride 0.9%) was administered intravenously at a dosage of 1% of body weight immediately after suture removal following a 2-hour period of MCA occlusion. The animals' neurological status was evaluated during MCA occlusion (at 60 minutes) and daily for 3 days thereafter. The brains were perfusion-fixed, and infarct volumes and brain edema were determined. The HSA significantly improved the neurological score compared with saline at 24 hours after MCA occlusion. The rats treated with HSA also had significantly reduced total infarct volume (by 34%) and brain edema (by 81%) compared with saline-treated rats. There was a strong correlation between hematocrit level and brain edema (p < 0.01), and between total infarct volume or brain edema and neurological score at 24, 48, and 72 hours postinjury (p < 0.0002). These results strongly support the beneficial effect of delayed albumin therapy in transient focal ischemia and indicate its possible usefulness in treating patients with acute ischemic stroke.

Cerebral oxygen transport and metabolism during graded isovolemic hemodilution in experimental global ischemia

To verify the optimal hematocrit (Hct) level in the treatment of cerebral ischemia, cerebral oxygen transport (CTO2) and cerebral oxygen metabolism (CMRO2) in graded isovolemic hemodilution were evaluated during cerebral ischemia. Isovolemic hemodilution with low molecular weight dextran to stepwise lower Hct from 43% to 36%, 31%, and 26% was carried out in 13 splenectomized dogs, 6 h after global cerebral ischemia. Global ischemia of the animals was produced by multiple intra- and extracranial ligations of cerebral arteries. Cerebral blood flow (CBF) was measured with radioisotope labeled microspheres. CTO2, CMRO2, and oxygen extraction fraction (OEF) were calculated from CBF, arterial oxygen content (CaO2), and venous oxygen content (CvO2). In dogs with global cerebral ischemia, CBF increased with graded isovolemic hemodilution (r=-0.73, P<0.05). CTO2 reached its highest value at a Hct level of 31.3%. CTO2 at Hct of 36.1% and 31.3% was statistically different from the value measured at a Hct of 43.3%, and there was a decrease when Hct was lowered to 25.9%. CMRO2 was the highest when Hct was at 31.3% and differed significantly from the value measured at a Hct of 43.3%. There was a 10% increase of OEF when Hct was at 25.9%; however this change was not statistically significant compared with the OEF at Hct of 36.1% and 31.3%, respectively. These findings indicate that CTO2 and CMRO2 were the highest when Hct was reduced to 31% in hemodilution. Hct at 31% is the optimum for cerebral metabolism in ischemic status. Uncoupling of CTO2, CMRO2 with CaO2 was also observed in this study. This phenomenon suggests that hemodilution to augment cerebral circulation may be at least partially attributed to the beneficial effects of hemorheologic improvement in the microcirculation of the ischemic brain.

Hypertensive, hypervolemic, hemodilutional therapy for aneurysmal subarachnoid hemorrhage. Is it efficacious? Yes

Vasospasm is an important contributor to death and disability after aneurysmal SAH. CBF is decreased after SAH and correlates inversely with the severity of the clinical grade. It is necessary to avoid hypotension and hypovolemia, which can exacerbate an already reduced CBF, resulting in critically low perfusion. There have been no human, prospective, randomized trials of HHH therapy. This is attributable, perhaps, to the fact that such trials are difficult to blind. Nevertheless, there is strong evidence that HHH therapy can reverse the delayed onset of profound neurologic deficits by restoring blood flow to ischemic regions, and its prophylactic use can reduce the incidence and severity of DID.

Optimal timing of hemodilution for brain protection in a canine model of focal cerebral ischemia

BACKGROUND AND PURPOSE

Hemodilution is known to ameliorate the effects of focal ischemia when used shortly after cerebral arterial occlusion; however, it remains to be proved whether hemodilution will be effective when used at more clinically relevant times, ie, with some delay between the onset of ischemia and initiation of therapy.

METHODS

Thirty-two dogs were selected for inclusion in this study. Cerebral infarction was induced by permanent occlusion of the middle cerebral and the azygos anterior cerebral arteries. The animals were allocated to 1 of 4 groups of eight animals each: arterial occlusion without hemodilution (group 1); hemodilution immediately after occlusion (group 2); hemodilution 3 hours after occlusion (group 3); and hemodilution 6 hours after occlusion (group 4). Isovolemic hemodilution to a hematocrit of 30% was performed. The animals were killed 6 days after induction of ischemia, and the infarct size was determined.

RESULTS

Groups 2 and 3 showed significant reduction of infarct size (P < .0001) when compared with group 1. The neurological grade of group 3 on postoperative days 4, 5, and 6 was significantly better than those of groups 1 and 4 (P < .01). Group 4 showed a significant increase in the incidence of hemorrhagic infarction when compared with groups 1 and 2 (P < .01).

CONCLUSIONS

The current study indicates that hemodilution administered as much as 3 hours after ischemia is effective in reducing infarct size and improving neurological status. When administered 6 hours after ischemia, hemodilution is not helpful and may be harmful.

Isovolemic hemodilution normalizes the prolonged passage of red cells and plasma through cerebral microvessels in the partially ischemic forebrain of rats

The objective of this study was to determine whether hemodilution could normalize the mean transit times of red blood cells (Tr) and plasma (Tp) through cerebral microvessels in a partially ischemic brain. Wistar-Kyoto (WKY) rats, aged 30-40 weeks, were divided randomly into three groups. The first group was the nonocclusion, nonhemodilution (NN) normal control group. The second group was the occlusion, nonhemodilution (ON) group, in which animals were treated with bilateral carotid artery ligation. The third group was the occlusion-hemodilution (OH) group, in which animals were treated with bilateral common carotid artery ligation and, then, isovolemic hemodilution by replacing blood with the same volume of 3% modified fluid gelatin. Local cerebral blood flow (lCBF) and microvascular volumes of red blood cells (Vr) and plasma (Vp) in 14 brain structures were measured using 14C-iodoantipyrine, iron-55 labeled red blood cells, and 14C-inulin, respectively. The amount of oxygen delivered to local brain structures (OD), cerebral microvascular blood volume (Vb), mean transit time of blood (Tb), Tr, and Tp through cerebral microvessels were calculated from the data. Two hours after carotid artery ligation, lCBF decreased by approximately 38% in forebrain structures, 22% in rostral hindbrain areas, and 8% in the caudal hindbrain (29% for all 14 structures). The decreases in ODs were parallel with those of lCBFs, at 33, 17, and 2% in the three regions, respectively (24% for all structures). In contrast, Vb increased by 68, 37, and 16% in the three regions, respectively (48% for all structures). Tr and Tp were markedly prolonged (180% for Tr and 154% for Tp) in the forebrain regions, moderately (91% for Tr and 73% for Tp) in the rostral hindbrain, and mildly (60% for Tr and 13% for Tp) in the caudal hindbrain, with a mean increase of 136% for Tr and 111% for Tp in all structures. When data in the OH and NN groups were compared, lCBF values tended to be slightly higher and Vb values were significantly higher (p < 0.05) in the OH group. ODs in the eight forebrain structures were all significantly less (p < 0.05) in the OH group than the NN group. Tr and Tp values in the forebrain were similar between the OH and the NN groups. In conclusion, occlusion of the bilateral common carotid arteries in WKY rats causes partial forebrain ischemia, in which both Tr and Tp are prolonged. These prolongations of Tr and Tp can be normalized by isovolemic hemodilution. However, the ischemic forebrain remains hypoxic after hemodilution.

Effects of isovolemic hemodilution on hemodynamics, cerebral perfusion, and cerebral vascular reactivity

BACKGROUND AND PURPOSE

To verify the hemorheological effects of isovolemic hemodilution on hemodynamics and cerebral perfusion of normal humans, we tested the efficacy of isovolemic hemodilution in systemic hemodynamics and cerebral bood flow augmentation and its influences in vascular reserve.

METHODS

Isovolemic hemodilution was studied in a total of 13 normal healthy subjects. Regional cerebral blood flow was measured by the xenon-enhanced CT method. Cerebral vascular activity was measured by acetazolamide challenge. These measurements, in association with hemorheological and hemodynamic monitoring, were analyzed before and after isovolemic hemodilution with low-molecular-weight dextran.

RESULT

Our results showed significant change in hemodynamic parameters after isovolemic hemodilution, including tachycardia, a 24% increase of cardiac index, and decrease of peripheral vascular resistance. Both left and right heart work index increased as a consequence of increased cardiac index. Regional cerebral blood flow increased 35.0 +/- 2.5% at 3 hours after hemodilution and 20.2 +/- 3.9% at 1 week after hemodilution. Cerebral vascular reactivity decreased from 32.1 +/- 4.1% to 25.3 +/- 4.0% after hemodilution, implicating a certain degree of vasodilation in the process of hemodilution. The whole procedure of hemodilution was completed in 52 +/- 6 minutes, and the subjects did not report discomfort during the procedure.

CONCLUSIONS

Isovolemic hemodilution in subjects with normal cerebral perfusion can augment cerebral blood flow efficiently in a rapid fashion, and this effect can last for at least a week. The mechanism of flow augmentation may be partially attributed to vasodilation, which could be manifested as tachycardia, increased cardiac output, and decreased cerebral vascular reactivity.

Marked hemodilution increases neurologic injury after focal cerebral ischemia in rabbits

Moderate hemodilution (hematocrit approximately 30%) reduces neurologic injury after focal cerebral ischemia. In contrast, both clinical and experimental studies suggest that marked hemodilution (hematocrit < 30%) may exacerbate neurologic injury. We compared the effect of marked versus minimal hemodilution on cerebral infarct volume after focal cerebral ischemia in rabbits. Anesthetized New Zealand White rabbits underwent hemodilution by exchange of arterial blood with 6% high molecular weight hydroxyethyl starch. In the marked hemodilution group (n = 15) the target hemoglobin concentration was 6 g/100 mL. In the minimal hemodilution group (n = 15) the target hemoglobin concentration was 11 g/100 mL. After hemodilution, middle cerebral artery occlusion was achieved by embolizing an autologous blood clot via the internal carotid artery. Four hours after embolization, the animals were killed and their brains removed. Brains were sectioned, stained with 2,3,5-triphenyltetrazolium chloride, and infarct volumes determined via quantitative image analysis. Systemic physiologic variables were similar between groups, except for arterial hemoglobin concentration. The percentage of hemispheric infarct was significantly larger in the marked hemodilution group as compared to the minimal hemodilution group, 70% +/- 19% vs 51% +/- 23%, respectively (mean +/- SD); P = 0.02. Similarly, the percentage of infarct was greater in the hemodilution group as compared to the minimal hemodilution group in both cortex (73% +/- 18% vs 54% +/- 23%, respectively; P = 0.02) and subcortex (62% +/- 25% vs 44% +/- 23%, respectively; P = 0.04). These findings indicate that marked hemodilution exacerbates neurologic injury resulting from permanent focal ischemia. Although some degree of hemodilution may improve neurologic outcome, the advantage is lost at an extreme level of therapy.

Cerebral metabolic consequences of hypotensive challenges in hemodiluted pigs with and without cardiopulmonary bypass

We tested the hypothesis that progressive aortic hypotension with bicarotid occlusion produces greater reductions in cerebral blood flow (CBF) and more flow-metabolism mismatching with hemodilution during cardiopulmonary bypass (CPB) than with hemodilution alone. In Yorkshire pigs randomized to hemodilution with CPB (n = 10) or hemodilution without CPB (control; n = 9), the effects of bicarotid ligation and graded hypotension on CBF (microspheres), the electroencephalogram (EEG), and cortical energy metabolites were examined. After bicarotid ligation, systemic flow was reduced for 15-min intervals of 80, 60, and 40 mm Hg aortic pressure, followed by a cortical brain biopsy. At baseline, CBF was lower in CPB (58 +/- 3 mL.100g-1.min-1) than control (90 +/- 3 mL.100 g-1.min-1., P < 0.05) animals, as was cerebral oxygen metabolism (3.1 +/- 0.1 vs 4.2 +/- 0.2 mL.min-1.100g-1; P < 0.05). Although CBF remained 40% lower at each level of hypotension in CPB than control animals (P < 0.05), EEG scores showed no intergroup differences, indicating similar flow-metabolism matching. Brain metabolites were similar between CPB and control groups (adenosine triphosphate, 9.6 +/- 2.4 vs 12.4 +/- 1.9 mumol/g; adenosine diphosphate, 6.0 +/- 0.7 vs 6.3 +/- 0.4 mumol/g; adenosine monophosphate, 4.8 +/- 0.9 vs 3.8 +/- 0.8 mumol/g; creatine phosphate, 8.3 +/- 1.8 vs 7.9 +/- 1.0 mumol/g; and lactate, 178.4 +/- 20.2 vs 150.8 +/- 13.9 mumol/g). Thus, despite significantly lower CBF during hypotension with bicarotid occlusion in hemodiluted animals during normothermic CPB, cortical electrical activity and the balance between flow and metabolism did not differ from those in control animals without CPB.

Intra-aortic balloon counterpulsation augments cerebral blood flow in a canine model of subarachnoid hemorrhage-induced cerebral vasospasm

We tested the effect of intra-aortic balloon counterpulsation (IABC) on cerebral blood flow (CBF) in a canine model of cerebral vasospasm. Cerebral vasospasm was induced in ten adult mongrel dogs using a "two-hemorrhage" model. CBF was then measured using radiolabeled microspheres, before and after activation of an intra-aortic balloon pump. Physiologic parameters including pCO2 and cardiac filling pressures were maintained constant during the experiment. Cardiac output was monitored in each animal. CBF increased with IABC in all ten animals. The mean CBF was 78.5 milliliters per 100 grams per minute (ml/100g/min) before versus 93.3ml/100g/min after IABC (P = 0.0001). Increases in CBF were associated in most, but not all, cases with increases in cardiac output. This study supports the ability of IABC to raise CBF in the setting of cerebral vasospasm. IABC may represent an important clinical option in cases of refractory vasospasm following aneurysmal subarachnoid hemorrhage.

Arterial microsphere concentrations in cats are not affected by changes in hematocrit

BACKGROUND AND PURPOSE

Acute anemia may lead to erroneously low arterial reference sample concentrations of radioactive microspheres, depending on the sampling rate and the size of the artery from which the reference samples are withdrawn. Because this error would lead to falsely high cerebral blood flow values in studies involving hemodilution caused by hemorrhage and fluid resuscitation, we studied the effects of hematocrit, withdrawal rate, and vessel location and size on arterial microsphere concentrations in anesthetized adult cats.

METHODS

Cats were anesthetized with ketamine, isoflurane, and nitrous oxide; both brachial arteries were cannulated with polyethylene tubing, as was the abdominal aorta through the femoral artery. Sequential left atrial microsphere injections were made using several doses of each of five isotopes. The rate of reference sample withdrawal from the three sampling catheters was randomized to 1.03 mL.min-1 or 2.06 mL.min-1. We analyzed the ratio of the number of microspheres in paired reference samples using the factors hematocrit, rate of withdrawal, and site. A ratio less than 1 indicates an underestimation of arterial microsphere concentration, which would lead to erroneously high cerebral blood flow values. The procedure was repeated after isovolemic hemodilution with 10% hetastarch to hemoglobin levels approximating 85%, 70%, 55%, and 40% of baseline.

RESULTS

No significant effects of hematocrit on ratios of microsphere concentrations existed at any withdrawal rate or site. Ratios of microsphere concentrations in reference samples withdrawn slowly (1.03 mL.min-1) from the aorta and ratios of microsphere concentrations withdrawn either rapidly (2.06 mL.min-1) or slowly from the brachial arteries were significantly (P < .001) less than 1.

CONCLUSIONS

Hemodilution did not affect microsphere concentrations in arterial reference samples at any withdrawal site or rate and therefore does not affect the accuracy of microsphere blood flow determinations. However, slow withdrawal from a large vessel may underestimate actual microsphere concentrations.

Effect of acute and chronic arecoline treatment on cerebral metabolism and blood flow in the conscious rat

Treatment with the muscarinic agonist arecoline improves memory retention in patients with Alzheimer's disease (AD). In animal models, arecoline selectively increases local cerebral glucose utilization (LCGU). We examined (1) whether these focal increases in metabolism were coupled to local cerebral blood flow (LCBF) and (2) whether the effect of arecoline on LCGU and LCBF was dependent upon duration of drug administration. In groups of young Fischer-344 rats, LCGU and LCBF were determined in 59 brain regions by the [14C]2-deoxyglucose and the [14C]iodoantipyrine autoradiographic methods following either the acute administration of arecoline (2 mg/kg and 15 mg/kg) or the chronic three week administration of arecoline (50 mg/kg/day). In general, LCBF correlated closely with LCGU following arecoline 2 mg/kg administration, but heterogeneous regions were present. Following treatment with arecoline 15 mg/kg, the two parameters became uncoupled with LCBF increasing disproportionately in relation to LCGU. Coupling between LCBF and LCGU was preserved during chronic arecoline treatment (50 mg/kg/day) but some regions, such as the hippocampus, were uncoupled with LCGU increasing to a greater extent than LCBF. Thus, we demonstrate that acute and chronic administration of arecoline can differentially modulate LCBF and LCGU. Since clinical administration of arecoline can improve cognitive function in patients with AD, understanding the ability of arecoline to selectively alter LCBF and LCGU in regions such as the hippocampus may offer insight into the pathophysiology of AD and provide direction for the development of definitive therapy for neurodegenerative disorders.

Optimum degree of hemodilution for brain protection in a canine model of focal cerebral ischemia

The ability of hemodilution to lower blood viscosity and increase cerebral blood flow has been proven experimentally; however, the optimum hematocrit for maximum oxygen delivery to ischemic brain tissue is not known, and a study was designed to determine this. Fifty dogs were selected for inclusion in the study using criteria based on changes in somatosensory evoked potentials at the time of arterial occlusion, which were found in a previous study to predict the development of a moderate infarction of relatively constant size. Infarctions were induced by permanent occlusion of the left middle cerebral artery and the azygous anterior cerebral artery. The animals selected for inclusion were divided into five groups of 10 dogs each: 1) a control group; 2) a group with 25% hematocrit; 3) a group with 30% hematocrit; 4) a group with 35% hematocrit; and 5) a group with 40% hematocrit. Isovolemic hemodilution was accomplished 1 hour after occlusion of vessels using dextran infusion and blood withdrawal. The animals were sacrificed after 6 days and infarction volume was determined from fluorescein-stained sections. Statistical analysis was performed using Student's t-test and one-way analysis of variance. Mean infarction volume for each group, expressed as a percentage of total hemispheric volume +/- 1 standard error of the mean, was 28.3% +/- 2.8% for the control group, 33.6% +/- 3.4% for the 25% hematocrit group, 17.1% +/- 2.2% for the 30% hematocrit group, 29.2% +/- 4.3% for the 35% hematocrit group, and 29.9% +/- 2.1% for the 40% hematocrit group. The 30% hematocrit group showed the smallest average infarction size and this size differed significantly (p = 0.02) from the average infarction size in the control animals. These results show that, in this model of focal ischemia, a hematocrit of approximately 30% is optimum for protecting the brain.

Diaspirin crosslinked hemoglobin (DCLHb): effect of hemodilution during focal cerebral ischemia in rats

The efficacy of hemodilution therapy, to ameliorate cerebral ischemia, is limited by an accompanying decrease in oxygen content. We assessed the effect of hemodilution, with diaspirin cross-linked hemoglobin (DCLHb), on cerebral blood flow (CBF) and infarct after middle cerebral artery occlusion (MCAo). Rats (n = 36) were alloted to a control group in which hematocrit (Hct) was not manipulated, or reduced with DCLHb to 30% (30/DCLHb), 16% (16/DCLHb), or 9% (9/DCLHb). After MCAo, the brain area with a CBF of 0-10 ml.100g-1.min-1 was determined. This area was decreased in the 30/DCLHb and 16/DCLHb groups vs the Control group; and was less in the 9/DCLHb group vs the other groups. Different rats (n = 49) were hemodiluted with DCLHb or Albumin (Alb): Control, 30/Alb, 30/DCLHb, 16/DCLHb, or 9/DCLHb. After 3-hr of MCAo and 2-hr of reperfusion, infarct area was determined. Brain infarct was less in the 30/DCLHb and 16/DCLHb groups vs the Control and 30/Alb groups; and was less in the 9/DCLHb group vs the other groups. The results of this study support the hypothesis that hemodilution with DCLHb decreases cerebral ischemia in a dose-dependent manner, and in terms of brain ischemia is a more proficient hemodiluting fluid than albumin.

The perfluorocarbon fluoromethyloadamantane offers cerebral protection in a model of isovolemic hemodilution in rabbits

BACKGROUND AND PURPOSE

Perfluorocarbons (PFCs) are considered promising cerebral protection agents because they could combine the beneficial effects of decreased blood viscosity with enhanced oxygen-carrying capacity and oxygen tissue delivery, but trials of PFCs as hemodilutants have been very limited. We evaluated fluoromethyloadamantane (FMA), a new perfluorocarbon compound, as an isovolemic hemodilutant and compared it with low-molecular-weight dextran 40 (D40) and a control group.

METHODS

Through a transorbital craniectomy, the internal carotid, anterior, and middle cerebral arteries were coagulated to create a cerebral infarction in anesthetized, mechanically ventilated rabbits. No other experimental procedure was performed in control animals. In the two other groups, hemodilution was commenced 30 minutes after the arterial occlusion with either D40 or FMA. Hemodynamic parameters and brain and systemic temperature were monitored throughout the experiments. All animals were killed 6 hours after the arterial occlusion.

RESULTS

Hemodynamic and metabolic parameters and blood oxygen content were not affected by the infusion of either FMA or D40. Brain and systemic temperature remained constant. The ratio of infarct volume to the hemispheric volume was 19.6 +/- 3.7% in the FMA group (n = 17), 19.9 +/- 4.6% in the D40 group (n = 16), and 40.3 +/- 5.7% in the control group (n = 17). The difference in infarct volume of both FMA and D40 animals compared with controls was statistically significant (P < .01) when tested with Student's t test. There was no significant difference between FMA and D40 groups.

CONCLUSIONS

These results suggest that FMA has cerebral protective properties and should be purified, optimized, and further tested experimentally to develop a stable, efficient, and safe oxygen carrier, potentially suitable for clinical trials.

Beneficial effect of prolonged administration of albumin on ischemic cerebral edema and infarction after occlusion of middle cerebral artery in rats

This study compared the therapeutical effect of the prolonged administration of albumin, 2 g/kg body weight per day, with that of saline or dextran, 0.8 g/kg body weight per day, on cerebral ischemia, using an occlusion of the middle cerebral artery in the rat. Brain water, sodium, and potassium contents were measured 72 hours after middle cerebral artery occlusion. The volume of infarction was represented as volume index, which is a total of the infarction area measured in the five brain slices at 168 hours after middle cerebral artery occlusion. The postischemic administration of albumin at the dose tested elucidated an antiedema effect and reduced the infarction size after regional ischemia in rats. These results strongly support the idea that hemodilution therapy with colloids such as albumin has wide usage as a treatment of patients with ischemic cerebral stroke.

Current concepts in cerebral protection

In the past, physicians viewed ischemic injury as an irreversible event. Modern science has shown that this view is incorrect and that ischemic neuronal damage is an ongoing, active process that might be amenable to various therapies. Figure 2 illustrates some of the possible sites where these therapies might be active. Pending evidence of their effectiveness, cerebral protection can best be achieved by maintaining adequate CPP and CBF during periods when patients are at risk for cerebral ischemia, restoring perfusion after ischemia occurs, and optimizing the metabolic milieu of the ischemic penumbra.

The hemodynamic effects of prolonged albumin administration in beagle dogs exposed to experimental subarachnoid hemorrhage

The hemodynamic effects of hyperdynamic hemodilution induced by prolonged (for 1 week) intravenous albumin or low-molecular-weight dextran administration were studied in the beagle two-hemorrhage model. Drug infusion was started immediately after the induction of a subarachnoid hemorrhage and continued for 7 days thereafter, according to the following treatment schedule: Group 1 (physiological saline); Group 2 (10% low-molecular-weight dextran, 0.8 g/kg per day); Groups 3, 4, and 5 (25% albumin, 1, 2, and 3 g/kg per day, respectively). On Day 0 (before subarachnoid hemorrhage) and Day 7, the cardiopulmonary parameters and the blood flow of the vertebral artery (VAF) were measured by the use of the Swan-Ganz catheter and an electromagnetic flowmeter, respectively. Cerebral angiography was carried out on Days 0 and 7. Only the albumin-treated groups showed a significant increase in plasma osmotic pressure, cardiac output, and VAF, as well as a decrease in the hematocrit on Day 7. The increase in cardiac output and VAF was maximal in Group 4. In Group 5, signs of pulmonary edema in the chest roentgenogram were noted in two of the four animals. The angiographic spasm of the basilar artery on Day 7 was not significantly ameliorated by either albumin or dextran. In the albumin-treated groups, the VAF was shown to increase in parallel with the increase of the cardiac output, within the range between 50 and 150%. Thus, this study indicates that the hemodynamic changes induced by prolonged albumin administration improve the cerebral circulation hindered by chronic vasospasm, whereas the increase in cardiac output is considered to be the most reliable indicator regarding the determination of the optimal dosage of albumin.

Hydroxyethyl starch 200/0.5 reduces infarct volume after embolic stroke in rats

BACKGROUND AND PURPOSE

We evaluated isovolumic hemodilution with hydroxyethyl starch 200/0.5 in a rat model of focal cerebral ischemia. This compound avoids the unfavorable viscosity and erythrocyte aggregation abnormalities of low molecular weight dextran during administration over a period of several days.

METHODS

Sprague-Dawley rats, anesthetized with 0.5-1% halothane and 70% N2O, were subjected to silicon cylinder (treated and control groups) or sham (sham group) embolization of the cerebral circulation. Thirty minutes after embolization, the treated group (n = 5) was infused with 11 ml/kg of 10% hydroxyethyl starch 200/0.5, and the control (n = 5) and sham (n = 4) groups were infused with saline for 1 hour. In the treated group, 7.1 ml/kg of blood was withdrawn. After 24 hours, the animals were reanesthetized, and cerebral blood flow was determined with [14C]iodoantipyrine. Alternative brain slices were either incubated with 2,3,5-triphenyltetrazolium chloride for infarct volume determination or frozen for ischemic volume and cerebral blood flow determination using autoradiography.

RESULTS

The hematocrit in the treated group was reduced from (mean +/- SEM) 46 +/- 1% to 35 +/- 2% at 1.5 hours (p < 0.01). Cortical blood flow was within the normal range of 115-185 ml/min/100 g, except for the ischemic cortex in the embolized groups, treated and control. The ischemic and infarct volume of the treated group was reduced by 74% (p < 0.05) and 89% (p < 0.05), respectively, from the control group. The treated and sham ischemic and infarct volumes were not statistically different.

CONCLUSIONS

These data suggest that hydroxyethyl starch 200/0.5 could be an effective treatment for ischemic stroke when administered early, because it reduces infarct and ischemic volumes from control values to levels indistinguishable from those of the sham group.

Focal cerebral ischemia in rats: effect of hemodilution with alpha-alpha cross-linked hemoglobin on CBF

Hemodilution has had limited success as a treatment of cerebral ischemia. When using a nonoxygen binding fluid, the therapeutic efficacy of hemodilution-induced increases in CBF are offset by concomitant decreases in oxygen content. The effect of hemodilution, with diaspirin alpha-alpha cross-linked hemoglobin (DCLHb), on CBF during middle cerebral artery occlusion was assessed. Rats were hemodiluted to one of the following hematocrits (Hct): (a) 44/Hct, (b) 37/Hct, (c) 30/Hct, (d) 23/Hct, (e) 16/Hct, or (f) 9/Hct. After 10 min of ischemia, CBF was determined with 14C-iodoantipyrine. Coronal brain sections were evaluated for areas with a CBF of 0-10 and 11-20 ml 100 g-1 min-1. In addition, oxygen delivery was calculated. In the center of the ischemic zone, both areas of low CBF were less in the 30/Hct, 23/Hct, and 16/Hct groups compared with the 44/Hct and 37/Hct groups; and both areas were less in the 9/Hct group compared with the other five groups (p < 0.05). For the hemisphere contralateral to occlusion, there was a direct correlation between hematocrit and oxygen delivery. However, for the hemisphere ipsilateral to occlusion, oxygen delivery increased as hematocrit decreased (44/Hct, 8.6 +/- 0.3 vs. 9/Hct, 13.6 +/- 0.4 [mean +/- SD, ml 100 g-1 min-1]). The results of this study support a hypothesis that hemodilution with DCLHb decreases the extent of focal cerebral ischemia.

Mechanism of cerebral blood flow augmentation by hemodilution in rabbits

BACKGROUND AND PURPOSE

Hemodilution is known to increase cerebral blood flow, but it is not known whether the increase in flow is a direct result of a decrease in viscosity or whether it may be due to compensatory vasodilatation in response to the decrease in oxygen carrying capacity that results from hemodilution. This study is designed to investigate this question.

METHODS

Changes in regional cerebral blood flow were studied in normal and ischemic brains of 15 and 18 rabbits, respectively. In one group of rabbits graded hemodilution was used to reduce arterial oxygen content progressively in stages; in the second group the arterial oxygen content was reduced in similar stages by progressively larger reductions in the concentration of inspired oxygen (hypoxic hypoxia). In the ischemic animals focal ischemia was produced by embolic occlusion of the right middle cerebral artery.

RESULTS

In the normal rabbits, hypoxic hypoxia and hemodilution resulted in similar progressive increases in cerebral blood flow as arterial oxygen content fell. In the ischemic animals, there was a significant fall in cerebral blood flow in the ischemic region in all groups after arterial occlusion. Hemodilution resulted in a progressive increase in cerebral blood flow in both ischemic and nonischemic regions. With hypoxic hypoxia, however, cerebral blood flow in the ischemic region showed no increase or a slight decrease.

CONCLUSIONS

Even though hypoxic hypoxia results in a marked increase in cerebral blood flow in normal brain, it does not significantly change cerebral blood flow in ischemic brain. In contrast, hemodilution resulting in a comparable degree of hypoxemia is capable of significantly increasing cerebral blood flow in ischemic brain. Therefore, the mechanism of blood flow augmentation by hemodilution in ischemic brain is probably related to a direct hemorheologic effect rather than to the resulting hypoxemia.

Time course of the impairment of cerebral autoregulation during chronic cerebral vasospasm after subarachnoid hemorrhage in primates

The time course of the impairment of cerebral autoregulation during chronic cerebral vasospasm after subarachnoid hemorrhage was studied in 18 monkeys. Changes in cerebral blood flow (CBF) at the regional level and central conduction times during either graded hypo- or hypertension were evaluated in these animals at three stages (3, 7, and 14 days) following the introduction of an autologous blood clot around the right middle cerebral artery (MCA). Angiograms revealed a reduction in vessel caliber (compared to the baseline level in the involved MCA) of 30% at 3 days, 50% at 7 days, and 10% at 14 days. At all stages, CBF remained constant at mean arterial blood pressures (MABP) of 60 to 160 mm Hg in the noninvolved hemisphere. In contrast, at the 3- and 7-day stages, there was an impairment of autoregulation in the involved hemisphere at MABP of 40 to 180 mm Hg. The right hemispheric CBF was significantly (p less than 0.05) lower than that in the left throughout the period of investigation at MABP below 120 mm Hg, but rose to exceed the left CBF at MABP above 180 mm Hg at the 7-day stage and 160 mm Hg at the 14-day stage. The right-sided central conduction time showed significant (p less than 0.05) prolongation at MABP below 60 mm Hg at the 3-day stage and 40 mm Hg at the 7-day stage. It is suggested that these results may help to develop guidelines for hemodynamic therapy for vasospasm in its various stages.

Reduction in focal cerebral ischemia by agents acting at imidazole receptors

Treatment with the alpha 2-adrenergic antagonist idazoxan (IDA) can provide protection from global cerebral ischemia. However, IDA also recognizes another class of receptors, termed imidazole (IM) receptors, which differ from alpha 2-adrenergic receptors and are responsible for the hypotensive actions of some centrally acting agents such as the oxazole rilmenidine (RIL). We therefore sought to determine whether RIL, an agent highly selective for IM receptors, offered protection from focal cerebral ischemia elicited in rat by ligation of the middle cerebral artery (MCA). We compared the effects of RIL with the effects of IDA and the selective non-IM alpha 2-antagonist SKF 86466 (SKF). In addition, we examined whether the neuroprotective effects of RIL and IDA could be attributed to changes in local CBF (LCBF). The MCA was occluded and animals either received immediate administration of drug while arterial pressure was maintained for 1 h or had local CBF increased to 200% of control for 1 h by hypercapnia or hypertension. RIL elicited a significant dose-dependent preservation of tissue to 33% of control at optimal dose (0.75 mg/kg). IDA (3 mg/kg) significantly reduced the size of ischemic infarction by 22%. In contrast, SKF (15 mg/kg) as well as doubling of LCBF did not preserve ischemic tissue. We conclude that both RIL and IDA can reduce focal ischemic infarction but that the mechanism does not appear secondary to antagonism of alpha 2-adrenergic receptors or elevation of LCBF. Occupation of IM receptors, either in the ischemic zone or at remote brain sites, may be responsible for neuroprotection of RIL and IDA.

Cardiac performance indices during hypervolemic therapy for cerebral vasospasm

The effect of hypervolemic preload enhancement on cardiac performance was systematically analyzed in nine patients following aneurysmal subarachnoid hemorrhage. The patients ranged in age from 34 to 63 years, and none had a history of cardiac disease. Each patient underwent placement of a flow-directed balloon-tipped catheter and the following measurements were taken during hypervolemic therapy: pulmonary artery wedge pressure (PAWP), central venous pressure (CVP), cardiac index (CI), stroke volume index (SVI), and left ventricular stroke work index (LVSWI). After baseline measurements were recorded, hetastarch or plasmanate was infused intravenously at 300 cc/hr. Thermal output determination and pressures were measured every 15 minutes. The PAWP did not correlate in a statistically significant fashion with the CVP in the ranges recorded; however, a statistically significant correlation did exist between PAWP increases and increases in CI, SVI, and LVSWI (p less than 0.01). There was no statistical correlation between PAWP increases above 14 mm Hg and improvement in cardiac performance as evidenced by CI, SVI, and LVSWI measurements. It is concluded that CVP is an unreliable index of cardiac performance during hypervolemic therapy and that, in previously healthy individuals, a PAWP of 14 mm Hg is associated with maximum cardiac performance.