Stable xenon versus radiolabeled microsphere cerebral blood flow measurements in baboons

The ischemic penumbra: From concept to reality

The discovery that brain tissue could potentially be salvaged from ischaemia due to stroke, has led to major advances in the development of therapies for ischemic stroke. In this review, we detail the advances in the understanding of this area termed the ischaemic penumbra, from its discovery to the evolution of imaging techniques, and finally some of the treatments developed. Evolving from animal studies from the 70s and 80s and translated to clinical practice, the field of ischemic reperfusion therapy has largely been guided by an array of imaging techniques developed to positively identify the ischemic penumbra, including positron emission tomography, computed tomography and magnetic resonance imaging. More recently, numerous penumbral identification imaging studies have allowed for a better understanding of the progression of the ischaemic core at the expense of the penumbra, and identification of patients than can benefit from reperfusion therapies in the acute phase. Importantly, 40 years of critical imaging research on the ischaemic penumbra have allowed for considerable extension of the treatment time window and better patient selection for reperfusion therapy. The translation of the penumbra concept into routine clinical practice has shown that "tissue is at least as important as time."

A Review of Ancillary Tests in Evaluating Brain Death

The neurological determination of death (NDD) is primarily considered to be clinical. However, situations may arise where confounding factors make this clinical assessment difficult or impossible. As a result, ancillary tests have been developed in order to aid in the confirmation of brain death. As assessment of neuronal electrical activity; electroencephalography (EEG) is no longer recommended in this determination, tools assessing cerebral perfusion, as reflected by the presence or absence of cerebral blood flow (CBF), are the mainstay of NDD. The preferred ancillary test currently is Hexamethylpropylene amine oxime-single photon emission computed tomography (HMPAO SPECT) radionuclide angiography. When this is not available, or is equivocal, 4-vessel cerebral angiography can be used to determine the presence or absence of intracranial blood flow. However, as cerebral angiography has its own limitations, other techniques are sought by physicians in the Intensive Care and Neuro-intensive Care settings to replace cerebral angiography. In this article, we briefly review the history of diagnosis of brain death, pathophysiologic issues in making this determination, and currently available CBF imaging techniques, discussing each in turn with respect to their utility in the diagnosis of brain death.

Physiologic Effects of Xenon in Xenon-CT Cerebral Blood Flow Studies on Comatose Patients

Despite more than 30 years of clinical use, questions remain about the safety of xenon gas in Xenon-CT cerebral blood flow (XeCTCBF) studies. In particular, xenon's effect on brain oxygen (PbtO2) in comatose patients is not well defined. Our objective was to assess the effect of a 4.5-min inhalation of 28 % stable xenon on several physiologic variables, including intracranial pressure (ICP), cerebral perfusion pressure (CPP), and PbtO2 in comatose patients (Glasgow Coma Scale [GCS] ≤ 8). Thirty-seven comatose patients who underwent 73 XeCTCBF studies were identified retrospectively from a prospective observational database. Changes in MAP, HR, SaO2, EtCO2, ICP, CPP, and PbtO2 measured at the start of xenon administration and every minute for 5 min thereafter were assessed. The maximum change in each variable also was determined for each scan to tabulate clinically relevant changes. Statistically, but not clinically significant changes in MAP, HR, and EtCO2 were seen. Xenon had no effect on ICP, and a small, but clinically insignificant decrease in CPP and PbtO2, was observed. There was a varied response to xenon in most measured variables. Clinically significant changes in each were infrequent, and readily reversed with the cessation of the gas. We conclude that xenon does not appear to have a clinically significant effect on ICP, CPP, and PbtO2 and so appears safe to evaluate cerebral blood flow in comatose patients.

CBF measurements using multidelay pseudocontinuous and velocity-selective arterial spin labeling in patients with long arterial transit delays: comparison with xenon CT CBF

PURPOSE

To test the theory that velocity-selective arterial spin labeling (VSASL) is insensitive to transit delay.

MATERIALS AND METHODS

Cerebral blood flow (CBF) was measured in ten Moyamoya disease patients using xenon computed tomography (xeCT) and magnetic resonance imaging (MRI), which included multiple pseudo-continuous ASL (pcASL) with different postlabel delays, VSASL, and dynamic susceptibility contrast (DSC) imaging. Correlation coefficient, root-mean-square difference, mean CBF error between ASL, and gold-standard xeCT CBF measurements as well the dependence of this error on transit delay (TD) as estimated by DSC time-to-peak of the residue function (Tmax) were determined.

RESULTS

For pcASL with different postlabel delay time (PLD), CBF measurement with short PLD (1.5-2 sec) had the strongest correlations with xeCT; VSASL had a lower but still significant correlation with a mean coefficient of 0.55. We noted the theoretically predicted dependence of CBF error on Tmax and on PLD for pcASL; VSASL CBF measurements had the least dependence of the error on TD. We also noted effects suggesting that the location of the label decay (blood vs. tissue) impacted the measurement, which was worse for pcASL than for VSASL.

CONCLUSION

We conclude that VSASL is less sensitive to TD than conventional ASL techniques and holds promise for CBF measurements in cerebrovascular diseases with slow flow.

Variability of clinical CT perfusion measurements in patients with carotid stenosis

INTRODUCTION

CT perfusion imaging (pCT) may be used to detect and monitor hemodynamic abnormalities due to cerebrovascular disease. The magnitude of variability in clinical measurements has been insufficiently evaluated. The purpose of this study was to measure the long-term variability of clinical pCT measurements in patients with cerebrovascular disease.

METHODS

pCT parameters were calculated for the cerebral hemisphere contralateral to a carotid stenosis before and after stent treatment of stenosis in 33 consecutive patients. Mean transit time (MTT), cerebral blood flow (CBF), and cerebral blood volume (CBV) calculated from pCT data from both a small and large region of interest (ROI) using both manual and automated methods were compared before and after stent treatment. Differences between the first and second measurement were tested for statistical significance with at-test. Variability was calculated as the standard deviation of the differences divided by the mean of the pre- and post-stent treatment values. To adjust for proportional bias, the Bland-Altman analysis was applied.

RESULTS

The differences between the two measurements of MTT, CBF, and CBV averaged 2.5 to 7.7% when a manual method was used and was higher with automatic methods (p > 0.07). The variability of the values was 18% for MTT, 19% for CBV, and 25% for CBF with the large ROI and the manual method of calculation. The magnitude was larger when the small ROI and automatic methods were employed.

CONCLUSION

Longitudinal measurements of MTT, CBV, or CBF by pCT may vary by 20-25%. To detect changes in treatment-related changes in perfusion, pCT studies must be designed to achieve statistical significance based on this variability.

Cerebral perfusion imaging in vasospasm

Vasospasm following cerebral aneurysm rupture is one of the most devastating sequelae and the most common cause of delayed ischemic neurological deficit (DIND). Because vasospasm also is the most common cause of morbidity and mortality in patients who survive the initial bleeding episode, it is imperative not only to diagnose the condition but also to predict which patients are likely to become symptomatic. The exact pathophysiology of vasospasm is complex and incompletely elucidated. Early recognition of vasospasm is essential because the timely use of several therapeutic interventions can counteract this disease and prevent the occurrence of DIND. However, the prompt implementation of these therapies depends on the ability to predict impending vasospasm or to diagnose it at its early stages. A number of techniques have been developed during the past several decades to evaluate cerebral perfusion, including positron emission tomography, xenon-enhanced computed tomography, single-photon emission computed tomography, perfusion- and diffusion-weighted magnetic resonance imaging, and perfusion computed tomography. In this article, the authors provide a general overview of the currently available perfusion imaging techniques and their applications in treating vasospasm after a patient has suffered a subarachnoid hemorrhage. The use of cerebral perfusion imaging techniques for the early detection of vasospasm is becoming more common and may provide opportunities for early therapeutic intervention to counteract vasospasm in its earliest stages and prevent the occurrence of DINDs.

CT perfusion imaging in the syndrome of the sinking skin flap before and after cranioplasty

The syndrome of the sinking skin flap (SSSF) has been described as one of the causes of neurological deficits after decompressive craniectomy We report a case of a 57-year-old woman with SSSF. Two years earlier, this patient, with no neurological deficits, underwent removal of the bone flap during treatment of an epidural abscess due to wound infection after a clipping operation for a ruptured aneurysm. The patient, who subsequently developed a sinking skin flap, gradually presented with gait disturbance and poor activity around 1 year before she came to our facility. On admission, neurological examination showed left hemiparesis and mild confusion. Cranioplasty with titanium mesh plate was performed. The cerebral blood flow (CBF) value in CT perfusion imaging in the symptomatic hemisphere increased from 23 to 31 cm3/100 g/min, and the value in the contralateral side increased from 37 to 41 cm3/100 g/min after cranioplasty. CT perfusion imaging after cranioplasty revealed the improvement of cerebral blood flow not only on the symptomatic side but also on the contralateral side. The patient recovered well and was discharged without hemiparesis and confusion 2 weeks after cranioplasty. As far as we know, this is the first reported case of SSSF examined with CT perfusion imaging before and after cranioplasty.

CT perfusion imaging for childhood moyamoya disease before and after surgical revascularization

Moyamoya disease is a progressive occlusive disease of the circle of Willis with prominent collateral arterial formation. We report on a 12-year-old girl with moyamoya disease presenting with transient ischemic attacks (TIAs). Surgical indirect revascularization was performed. The patient did not suffer further TIAs at 12 month follow-up. Pre and postoperative cerebral perfusion were studied in quantitative single photon emission computerized tomography (SPECT) and CT perfusion imaging. CT perfusion imaging demonstrated postoperatively increased cerebral blood flow as well as SPECT before and after revascularization. Furthermore, the area of decreased vascular reserve in SPECT with acetazolamide corresponded to areas of increased cerebral blood volume in CT perfusion imaging. CT perfusion imaging was equivalent to SPECT in accuracy, and superior in spatial resolution. CT perfusion imaging is likely to become more widely available as an easy-to-perform technique for assessing cerebral perfusion in a patients with moyamoya disease.

Cerebral Perfusion Imaging

There are multiple imaging techniques available to assess cerebral perfusion, including positron emission tomography (PET), xenon computed tomography (XeCT), single photon emission computed tomography (SPECT), perfusion-weighted MRI (PWI), and perfusion computed tomography (PCT). Current interest has focused mainly on their use in the setting of acute brain ischemia. Perfusion imaging may be able to distinguish infarcted from salvageable ischemic tissue as a guide to treatment. Perfusion techniques may also be helpful in cases of chronic ischemia, post-subarachnoid hemorrhage vasospasm, trauma, and contemplated therapeutic carotid artery occlusion.

Comparison of cerebral blood flow between perfusion computed tomography and xenon-enhanced computed tomography for normal subjects: territorial analysis

OBJECTIVE

The purpose of this study was to clarify the difference between cerebral blood flow (CBF) by perfusion computed tomography (CT) and that by xenon-enhanced CT (Xe-CT) through simultaneous measurement.

METHODS

Xenon-enhanced CT and perfusion CT were continually performed on 7 normal subjects. Ratios of CBF by perfusion CT (P-CBF) to CBF by Xe-CT (Xe-CBF) were measured for 5 arterial territories; 3 were territories of 3 major arteries (the anterior [ACA], middle [MCA], and posterior [PCA] cerebral arteries), and the other 2 were areas of the thalamus and putamen.

RESULTS

The ratios were 1.30 +/- 0.10, 1.26 +/- 0.15, 1.61 +/- 0.15, 0.801 +/- 0.087, and 0.798 +/- 0.080 for the ACA, MCA, PCA, thalamus, and putamen, respectively. Although a good correlation was observed between P-CBF and Xe-CBF for each territory, the ratios were significantly different (P < 0.0001) between 3 territory groups (group 1: ACA and MCA, group 2: PCA, and group 3: thalamus and putamen).

CONCLUSIONS

The difference in the ratio of P-CBF to Xe-CBF between the 3 territory groups was considered to result principally from the features of P-CBF. To evaluate P-CBF properly, its territorial characteristics should be taken into account.

Evidence-based neuroimaging in acute ischemic stroke

The imaging work-up of patients with acute neurologic deficits should begin with noncontrast CT to exclude intracerebral hemorrhage. Based on positive results from the NINDS t-PA trial, the overriding objectives of imaging in the selection of patients for t-PA treatment are the detection of hemorrhage and rapid evaluation (speed of imaging). Despite its limited sensitivity for the identification of an ischemic stroke lesion, CT has multiple advantages over MR imaging in the initial diagnostic work-up. Advanced MR techniques promise to provide anatomic, physiologic, and vascular information in a single examination, and the ability to increase treatment specificity and improve outcome. Clinical outcome data are lacking; therefore, the routine use of screening MR imaging before t-PA therapy is not supported. Rigorous validation and correlation to clinical outcomes will be required.

Contribution of edema and cerebral blood volume to traumatic brain swelling in head-injured patients

OBJECT

The pathogenesis of traumatic brain swelling remains unclear. The generally held view is that brain swelling is caused primarily by vascular engorgement and that edema plays a relatively minor role in the swelling process. The goal of this study was to examine the roles of cerebral blood volume (CBV) and edema in traumatic brain swelling.

METHODS

Both brain-tissue water and CBV were measured in 76 head-injured patients, and the relative contribution of edema and blood to total brain swelling was determined. Comparable measures of brain-tissue water were obtained in 30 healthy volunteers and CBV in seven volunteers. Brain edema was measured using magnetic resonance imaging, implementing a new technique for accurate measurement of total tissue water. Measurements of CBV in a subgroup of 31 head-injured patients were based on consecutive measures of cerebral blood flow (CBF) obtained using stable xenon and calculation of mean transit time by dynamic computerized tomography scanning after a rapid bolus injection of iodinated contrast material. The mean (+/- standard deviation) percentage of swelling due to water was 9.37+/-8.7%, whereas that due to blood was -0.8+/-1.32%.

CONCLUSIONS

The results of this study showed that brain edema is the major fluid component contributing to traumatic brain swelling. Moreover, CBV is reduced in proportion to CBF reduction following severe brain injury.

Quantitative assessment of cerebral hemodynamics using CT: stability, accuracy, and precision studies in dogs

PURPOSE

The limited clinical availability of currently used methods to measure regional cerebral blood volume (CBV) and cerebral blood flow (CBF) represents an important restriction. We undertook this study to evaluate a new dynamic CT method to measure CBV and CBF in normal and ischemic tissue.

METHOD

A total of 21 dynamic CT studies were performed in seven male beagles. The contrast enhancement curves of the carotid arteries and of various brain regions were deconvolved to obtain CBV and CBF. The stability of the deconvolution method employed was assessed by comparing three data sets obtained by analyses of one, two, and four regions of interest (ROIs), all covering the entire brain area. The accuracy of CT-derived CBF was analyzed for normal (n = 5 studies) and ischemic (n = 7 studies) brain tissue using fluorescent microspheres. Repetitive CT studies were performed to evaluate the precision of the CT measurements.

RESULTS

The stability of the deconvolution method was high with variabilities of 2.3% (CBV), 5.9% (CBF), and 8.9% (mean transit time), respectively. The correlation between the CT and the microsphere measurements was good for both normal and ischemia studies (r > 0.78, slope > 0.9). The variability of the CT CBF (30.6%) was higher than that of the CT CBV (12.3%) measurements.

CONCLUSION

Our novel dynamic CT method is stable with respect to the sizes of ROIs used, allowing for accurate measurements of CBV and CBF in both normal and ischemic tissue. Further studies are necessary to evaluate the variability of this method under controlled physiologic conditions.

Correlation of regional cerebral blood flow measured by stable xenon CT and perfusion MRI

PURPOSE

The purpose of this work was to investigate the validity of perfusion MRI in comparison with stable xenon CT for evaluating regional cerebral blood flow (rCBF).

METHOD

The rCBF was measured by xenon CT and perfusion MRI within a 24 h interval in 10 patients (mean +/- SD age 63 +/- 10 years). For perfusion MRI, absolute values of rCBF were calculated based on the indicator dilution theory after injection of 0.1 mmol/kg of Gd-DTPA. Eight to 10 regions of interest (37 mm2) were located in the white and gray matter on the rCBF images for each of the 10 patients.

RESULTS

The mean +/- SD values of rCBF in gray matter were 48.5 +/- 14.1 ml/100 g/min measured by xenon CT and 52.2 +/- 16.4 ml/100 g/min measured by perfusion MRI. In the white matter, the rCBF was 22.6 +/- 9.1 ml/100 g/min by xenon CT and 27.4 +/- 6.8 ml/100 g/min by perfusion MRI. There was a good correlation of rCBF values between perfusion MRI and xenon CT (Pearson correlation coefficient 0.83; p < 0.0001).

CONCLUSION

Comparable to xenon CT, perfusion MRI provides relatively high resolution, quantitative local rCBF information coupled to MR anatomy.

[Monitoring and maintenance of blood volume in intracranial surgery]

Intraoperative monitoring usually does not assess intravascular blood volume of the patient. The variations of the systolic blood pressure and the right auricular pressure allow to appreciate the intravascular blood volume changes and their consequences on the cerebral perfusion pressure. However, when the autoregulation of the cerebral blood flow is strongly depressed, the evaluation of the cerebral perfusion pressure alone is inadequate. In this case, the optimal monitoring should be metabolic (jugular bulb venous oxygen saturation) to finally assess the cerebral flow-metabolic coupling.

Evaluation of time-dependent thresholds of cerebral blood flow and transit time during the acute stage of cerebral embolism: a retrospective study

BACKGROUND

Neurons within the ischemic penumbra are thought to be in a potentially reversible state of ischemic challenge. One therapeutic approach that is being actively explored is the recovery of function of cells within the ischemic penumbra through endovascular recanalization of cerebral arteries occluded with embolus. The purpose of this study was to determine the time-dependent hemodynamic threshold for the prevention of irreversible ischemia in patients with acutely symptomatic internal and middle cerebral artery (MCA) embolism.

METHODS

Thirty-six patients admitted within 6 hours of the onset of symptoms of acute cerebral ischemia, due to embolic occlusion of the major trunk of one of the arteries of the anterior cerebral circulation, were studied. On admission, both cerebral blood flow (CBF) and mean transit time (MTT) measurements were obtained following plain computed tomography (CT). All patients were treated by intraarterial administration of urokinase. MTT in the territory of the affected MCA divided by that in the territory of the unaffected MCA was defined as %MTT.

RESULTS

A significant negative correlation was found between MTT and CBF. In patients with at least 19 mL/100 g/minute CBF and a maximum of 1.6 %MTT, no cortical infarction occurred whether or not recanalization was obtained. Cortical infarction did not appear in patients with 9 mL/100 g/minute residual CBF and infinite %MTT in whom recanalization was achieved within 2 hours of onset, in patients with 13 mL/100 g/minute residual CBF and 3.7 %MTT in whom recanalization was achieved within 2.5 hours of onset, and in patients with 14 mL/100 g/minute residual CBF and 2.8 %MTT in whom recanalization could be achieved within 3.5 hours of onset.

CONCLUSIONS

CBF and MTT thresholds for conversion of reversible to irreversible ischemia can be rapidly determined by CT-based technologies. This type of information should be clinically relevant to guiding the management of patients with cerebral embolism.

Correlation between cerebral blood flow values obtained by Xenon/CT and Kety-Schmidt (N2O) methods

The means of the cerebral blood flow (CBF) values obtained by the stable xenon enhanced CT (Xe/CT) method using two different CT scanners were compared with global CBF value obtained by the Kety-Schmidt (N2O) method as a reference. Xe/CT CBF values were obtained using a GE CT9200 (31 patients, 2 flow maps, 120 kV, absorption constant of 0.040) as well as a GE ProSeed Accell (38 patients, 4 flow maps, 80 kV, absorption constant of 0.028). The protocol of inhalation in the Xe/XT method consisted of 4 min wash-in and 4-min wash-out of 35% stable xenon. In the Kety-Schmidt method, 15% N2O gas was inhaled for 10 min. The N2O content of blood samples was measured using a van Slyke-Neill blood gas analyzer. We corrected all obtained CBF values for a PaCO2 of 34 mmHG (CBF34). The global CBF34 values obtained by the Kety-Schmidt method were linearly correlated with the CBF34 values obtained using the CT9200 and with those obtained using the ProSeed Accell, and the regression line equations were, respectively, Y = 0.64X + 13.7 (X: CT9200, Y: Kety-Schmidt, r = 0.666, p < 0.01) and Y = 0.99X + 11.2 (X: ProSeed Accell, Y: Kety Schmidt, r = 0.756, p < 0.01). Since the CBF values obtained by the Xe/CT method using different CT scanners are not always the same as the global CBF values obtained by the Kety-Schmidt method, CBF values obtained by the Xe/CT method should be corrected referring to the regression line obtained by applying both methods for each patient.

Why do all drugs work in animals but none in stroke patients? 1. Drugs promoting cerebral blood flow

Medical treatments which presumably alter cerebral blood flow (CBF) have been quite unimpressive in their effect on stroke outcome. In considering experimental and clinical data from the use of haemodilution and of the antiplatelet agent prostacyclin in focal cerebral ischaemia, and the current work with fibrinolytic agents in acute stroke, several lessons are apparent. Often agents hypothesized to affect CBF receive an underserved reputation based on sparse experimental evidence. Significant even unsuspected differences between species limit application to the clinical setting. Limitations of CBF measurements in experimental models and in humans raise questions about apparent responses to those agents. The failure to confirm a relationship between CBF enhancement and reduction in infarct development experimentally has plagued these approaches. The need for early application of agents which may modulate CBF during cerebral ischaemia is critical. Attention to these general issues and careful application of appropriate models are necessary so that a potentially useful therapeutic intervention is not overlooked.

Effect of stable xenon inhalation on intracranial pressure during measurement of cerebral blood flow in head injury

Xenon-enhanced computerized tomography (CT) is well suited for measurements of cerebral blood flow (CBF) in head-injured patients. Previous studies indicated divergent results on whether inhalation of xenon may cause a clinically relevant increase in intracranial pressure (ICP). The authors employed Xe-enhanced CT/CBF measurements to study the effect of 20 minutes of inhalation of 33% xenon in oxygen on ICP, cerebral perfusion pressure (CPP), and arteriovenous oxygen difference (AVDO2) in 13 patients 3 days (mean 1 to 5 days) after severe head injury (Glasgow Coma Scale score < or = 7). The patients were moderately hyperventilated (mean PaCO2 4.3 kPa or 32.3 mm Hg). Six patients were studied before and during additional hyperventilation. All 13 patients reacted with an increase in ICP and 11 with a decrease in CPP. The mean ICP increment was 6.9 +/- 7.7 (range 2 to 17 mm Hg). The mean CPP decrement was -9.7 +/- -14.6 (range 17 to 47 mm Hg). The time course of the ICP changes indicated that ICP increased rapidly during the first 5 to 6 minutes, then declined to a plateau (peak-plateau type in four of 13 patients), remained at a plateau (plateau type in six of 13), or continued to increase in three of 13, indicating individual variance in xenon reactivity. Additional hyperventilation had no effect on the xenon-induced increments in ICP but these occurred at lower ICP and higher CPP baseline levels. The AVDO2 values, an index of flow in relation to metabolism, indicated a complex effect of xenon on CBF as well as on metabolism. This study indicates that xenon inhalation for Xe-CT CBF measurements in head-injured patients according to our protocol causes clinically significant increments in ICP and decrements in CPP. It is suggested that the effect of xenon is analogous to anesthesia induction. Individual variations were observed indicating possible individual tolerance, possible influence of type and extent of the cerebral injury, disturbances in cerebrovascular reactivity, and possible influence of medication. These effects of xenon suggest that hyperventilation should be ensured in patients with evidence of reduced compliance or high ICP. On the other hand, inhalation of stable xenon is not believed to pose a risk because no signs of cerebral oligemia or ischemia were indicated in the AVDO2 values.

Local cerebral blood flow measured by xenon-enhanced CT during cryogenic brain edema and intracranial hypertension in monkeys

We developed a closed-skull model of freeze injury-induced brain edema, a model classically thought to produce vasogenic edema, and observed the natural course of changes in edema and blood flow using xenon-enhanced computed tomography (CT) in five rhesus monkeys before and for up to 6 h post insult. Intracranial pressure (ICP) gradually rose throughout the duration of the experiment. CT scans and CBF images permitted direct observation of the evolution of the lesion and revealed early ischemia in the periphery of the injury zone that progressed over time in association with edema. Frequency histogram analysis of local CBF (ICBF) demonstrated subtle but potentially important changes in distribution of ICBF between and within hemispheres at various times post insult. Changes in ICBF distribution were phasic and dissociated from increases in ICP in the latter stages of injury. The Xe/CT CBF method can be used to evaluate the effects of injury and therapy on CBF in this and other models of acute brain injury.

Multifocal cerebral blood flow by Xe-CT and global cerebral metabolism after prolonged cardiac arrest in dogs. Reperfusion with open-chest CPR or cardiopulmonary bypass

Using the stable xenon-enhanced computed tomography (Xe-CT) method in dogs, we studied local, regional and global cerebral blood flow (LCBF, rCBF and gCBF) in two sham experiments and nine cardiac arrest experiments. Within the same experiments without arrest, gCBF and rCBF values were reproducible and stable. LCBF values varied over time. In group I (n = 4), ventricular fibrillation cardiac arrest (no blood flow) of 10 min was reversed by open-chest cardiopulmonary resuscitation (CPR). In group II (n = 5), ventricular fibrillation cardiac arrest of 12.5 min was reversed by brief closed-chest cardiopulmonary bypass. This was followed by controlled ventilation, normotension, normoxia, normocarbia and normothermia to 4 h (n = 7) or 20 h (n = 2) postarrest. The postarrest CBF patterns were similar in both groups. Open-chest CPR during ventricular fibrillation generated near-baseline gCBF and lower LCBF ranges. During postarrest spontaneous circulation, transient diffuse hyperemia was without low-flow regions, longer in brain stem and basal ganglia than in neocortex. During delayed hypoperfusion at 1-4 h postarrest (n = 9), mean gCBF was 44-60% baseline, rCBF in primarily gray matter regions was 15-49 ml/100 cm3 per min and LCBF voxels with trickle-flow and low-flow values, in percent of CT cut area, were increased over baseline. Global CMRO2 (n = 3 of group II) recovered to near baseline values between 1 and 4 h postarrest, while gCBF and O2 delivery were about 50% baseline (mismatching of O2 uptake and O2 delivery).

A porcine model for sequential assessments of cerebral haemodynamics and metabolism

We present a physiologically stable porcine model designed for sequential assessments of pharmacological effects on mean hemispheric cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRO2) at sustained normocapnia. The dynamic influence of continuously administered fentanyl (0.040 mg.kg-1.h-1 i.v.), nitrous oxide (70%) and pancuronium (0.30 mg.kg-1.h-1 i.v.) on these variables was studied in eight normoventilated pigs. CBF was reliably assessable at 10-min intervals by clearance of intra-arterially injected 133Xe, monitored by an extracranial scintillation detector. CMRO2 was calculated from CBF and the simultaneously measured cerebral arteriovenous difference in blood oxygen content. The intracerebral distribution of a contrast medium injected into the external and internal carotid arteries was studied by angiography, and the cerebral venous outflow was investigated by measurements of the distribution of an intra-arterially administered non-diffusible tracer, [99mTc]pertechnetate, to the internal and external jugular veins. After a 3-h equilibration period, CBF and CMRO2 were determined on six occasions over a study period lasting 1 h 40 min. The mean ranges of these variables were 56-60 and 1.9-2.0 ml.100 g-1.min-1, respectively. We conclude that the model enables repeated assessments of CBF and CMRO2 under stable physiological background conditions and thus valid cerebral pharmacodynamic investigations of drugs given for anaesthesia.