THE EFFECT OF METABOLIC ACIDOSIS AND ALKALOSIS ON THE BLOOD FLOW THROUGH THE CEREBRAL CORTEX

Acute Ischemic Stroke during Extracorporeal Membrane Oxygenation (ECMO): A Narrative Review of the Literature

Ischemic stroke (IS) is a severe complication and leading cause of mortality in patients under extracorporeal membrane oxygenation (ECMO). The aim of our narrative review is to summarize the existing evidence and provide a deep examination of the diagnosis and treatment of acute ischemic stroke patients undergoing ECMO support. The incidence rate of ISs is estimated to be between 1 and 8%, while the mortality rate ranges from 44 to 76%, depending on several factors, including ECMO type, duration of support and patient characteristics. Several mechanisms leading to ISs during ECMO have been identified, with thromboembolic events and cerebral hypoperfusion being the most common causes. However, considering that most of the ECMO patients are severely ill or under sedation, stroke symptoms are often underdiagnosed. Multimodal monitoring and daily clinical assessment could be useful preventive techniques. Early recognition of neurological deficits is of paramount importance for prompt therapeutic interventions. All ECMO patients with suspected strokes should immediately receive brain computed tomography (CT) and CT angiography (CTA) for the identification of large vessel occlusion (LVO) and assessment of collateral blood flow. CT perfusion (CTP) can further assist in the detection of viable tissue (penumbra), especially in cases of strokes of unknown onset. Catheter angiography is required to confirm LVO detected on CTA. Intravenous thrombolytic therapy is usually contraindicated in ECMO as most patients are on active anticoagulation treatment. Therefore, mechanical thrombectomy is the preferred treatment option in cases where there is evidence of LVO. The choice of the arterial vascular access used to perform mechanical thrombectomy should be discussed between interventional radiologists and an ECMO team. Anticoagulation management during the acute phase of IS should be individualized after the thromboembolic risk has been carefully balanced against hemorrhagic risk. A multidisciplinary approach is essential for the optimal management of ISs in patients treated with ECMO.

Neurological monitoring and management for adult extracorporeal membrane oxygenation patients: Extracorporeal Life Support Organization consensus guidelines

BACKGROUND

Critical care of patients on extracorporeal membrane oxygenation (ECMO) with acute brain injury (ABI) is notable for a lack of high-quality clinical evidence. Here, we offer guidelines for neurological care (neurological monitoring and management) of adults during and after ECMO support.

METHODS

These guidelines are based on clinical practice consensus recommendations and scientific statements. We convened an international multidisciplinary consensus panel including 30 clinician-scientists with expertise in ECMO from all chapters of the Extracorporeal Life Support Organization (ELSO). We used a modified Delphi process with three rounds of voting and asked panelists to assess the recommendation levels.

RESULTS

We identified five key clinical areas needing guidance: (1) neurological monitoring, (2) post-cannulation early physiological targets and ABI, (3) neurological therapy including medical and surgical intervention, (4) neurological prognostication, and (5) neurological follow-up and outcomes. The consensus produced 30 statements and recommendations regarding key clinical areas. We identified several knowledge gaps to shape future research efforts.

CONCLUSIONS

The impact of ABI on morbidity and mortality in ECMO patients is significant. Particularly, early detection and timely intervention are crucial for improving outcomes. These consensus recommendations and scientific statements serve to guide the neurological monitoring and prevention of ABI, and management strategy of ECMO-associated ABI.

Hypoxic-Ischemic Brain Injury in ECMO: Pathophysiology, Neuromonitoring, and Therapeutic Opportunities

Extracorporeal membrane oxygenation (ECMO), in conjunction with its life-saving benefits, carries a significant risk of acute brain injury (ABI). Hypoxic-ischemic brain injury (HIBI) is one of the most common types of ABI in ECMO patients. Various risk factors, such as history of hypertension, high day 1 lactate level, low pH, cannulation technique, large peri-cannulation PaCO2 drop (∆PaCO2), and early low pulse pressure, have been associated with the development of HIBI in ECMO patients. The pathogenic mechanisms of HIBI in ECMO are complex and multifactorial, attributing to the underlying pathology requiring initiation of ECMO and the risk of HIBI associated with ECMO itself. HIBI is likely to occur in the peri-cannulation or peri-decannulation time secondary to underlying refractory cardiopulmonary failure before or after ECMO. Current therapeutics target pathological mechanisms, cerebral hypoxia and ischemia, by employing targeted temperature management in the case of extracorporeal cardiopulmonary resuscitation (eCPR), and optimizing cerebral O2 saturations and cerebral perfusion. This review describes the pathophysiology, neuromonitoring, and therapeutic techniques to improve neurological outcomes in ECMO patients in order to prevent and minimize the morbidity of HIBI. Further studies aimed at standardizing the most relevant neuromonitoring techniques, optimizing cerebral perfusion, and minimizing the severity of HIBI once it occurs will improve long-term neurological outcomes in ECMO patients.

Lower Oxygen Tension and Intracranial Hemorrhage in Veno-venous Extracorporeal Membrane Oxygenation

INTRODUCTION AND METHODS

We examined the relationship between 24-h pre- and post-cannulation arterial oxygen tension (PaO2) and arterial carbon dioxide tension (PaCO2) and subsequent acute brain injury (ABI) in patients receiving veno-venous extracorporeal membrane oxygenation (VV-ECMO) with granular arterial blood gas (ABG) data and institutional standardized neuromonitoring.

RESULTS

Eighty-nine patients underwent VV-ECMO (median age = 50, 63% male). Twenty (22%) patients experienced ABI; intracranial hemorrhage (ICH) was the most common diagnosis (n = 14, 16%). Lower post-cannulation PaO2 levels were significantly associated with ICH (66 vs. 81 mmHg, p = 0.007) and a post-cannulation PaO2 level < 70 mmHg was more frequent in these patients (71% vs. 33%, p = 0.007). PaCO2 parameters were not associated with ABI. By multivariable logistic regression, hypoxemia post-cannulation increased the odds of ICH (OR = 5.06, 95% CI:1.41-18.17; p = 0.01).

CONCLUSION

In summary, lower oxygen tension in the 24-h post-cannulation was associated with ICH development. The precise roles of peri-cannulation ABG changes deserve further investigation, as they may influence the management of VV-ECMO patients.

Arterial oxygen and carbon dioxide tension and acute brain injury in extracorporeal cardiopulmonary resuscitation patients: Analysis of the extracorporeal life support organization registry

BACKGROUND

Acute brain injury (ABI) remains common after extracorporeal cardiopulmonary resuscitation (ECPR). Using a large international multicenter cohort, we investigated the impact of peri-cannulation arterial oxygen (PaO2) and carbon dioxide (PaCO2) on ABI occurrence.

METHODS

We retrospectively analyzed adult (≥18 years old) ECPR patients in the Extracorporeal Life Support Organization registry from 1/2009 through 12/2020. Composite ABI included ischemic stroke, intracranial hemorrhage (ICH), seizures, and brain death. The registry collects 2 blood gas data pre- (6 hours) and post- (24 hours) cannulation. Blood gas parameters were classified as: hypoxia (<60mm Hg), normoxia (60-119mm Hg), and mild (120-199mm Hg), moderate (200-299mm Hg), and severe hyperoxia (≥300mm Hg); hypocarbia (<35mm Hg), normocarbia (35-44mm Hg), mild (45-54mm Hg) and severe hypercarbia (≥55mm Hg). Missing values were handled using multiple imputation. Multivariable logistic regression analysis was used to assess the relationship of PaO2 and PaCO2 with ABI.

RESULTS

Of 3,125 patients with ECPR intervention (median age=58, 69% male), 488 (16%) experienced ABI (7% ischemic stroke; 3% ICH). In multivariable analysis, on-ECMO moderate (aOR=1.42, 95%CI: 1.02-1.97) and severe hyperoxia (aOR=1.59, 95%CI: 1.20-2.10) were associated with composite ABI. Additionally, severe hyperoxia was associated with ischemic stroke (aOR=1.63, 95%CI: 1.11-2.40), ICH (aOR=1.92, 95%CI: 1.08-3.40), and in-hospital mortality (aOR=1.58, 95%CI: 1.21-2.06). Mild hypercarbia pre-ECMO was protective of composite ABI (aOR=0.61, 95%CI: 0.44-0.84) and ischemic stroke (aOR=0.56, 95%CI: 0.35-0.89).

CONCLUSIONS

Early severe hyperoxia (≥300mm Hg) on ECMO was a significant risk factor for ABI and mortality. Careful consideration should be given in early oxygen delivery in ECPR patients who are at risk of reperfusion injury.

Beyond monoamines: II. Novel applications for PET imaging in psychiatric disorders

Early applications of positron emission tomography (PET) in psychiatry sought to identify derangements of cerebral blood flow and metabolism. The need for more specific neurochemical imaging probes was soon evident, and these probes initially targeted the sites of action of neuroleptic (dopamine D₂ receptors) and psychoactive (serotonin receptors) drugs. For nearly 30 years, the centrality of monoamine dysfunction in psychiatric disorders drove the development of an armamentarium of monoaminergic PET radiopharmaceuticals and imaging methodologies. However, continued investments in monoamine-enhancing drug development realized only modest gains in efficacy and tolerability. As patent protection for many widely prescribed and profitable psychiatric drugs lapsed, drug development pipelines shifted away from monoamines in search of novel targets with the promises of improved efficacy, or abandoned altogether. Over this period, PET radiopharmaceutical development activities closely parallelled drug development priorities, resulting in the development of new PET imaging agents for non-monoamine targets. In part two of this review, we survey clinical research studies using the novel targets and radiotracers described in part one across major psychiatric application areas such as substance use disorders, anxiety disorders, eating disorders, personality disorders, mood disorders, and schizophrenia. Important limitations of the studies described are discussed, as well as key methodologic issues, challenges to the field, and the status of clinical trials seeking to exploit these targets for novel therapeutics.

Arterial Carbon Dioxide and Acute Brain Injury in Venoarterial Extracorporeal Membrane Oxygenation

Acute brain injury (ABI) occurs frequently in patients receiving venoarterial extracorporeal membrane oxygenation (VA-ECMO). We examined the association between peri-cannulation arterial carbon dioxide tension (PaCO 2 ) and ABI with granular blood gas data. We retrospectively analyzed adult patients who underwent VA-ECMO at a tertiary care center with standardized neuromonitoring. Pre- and post-cannulation PaCO 2 were defined as the mean of all PaCO 2 values in the 12 hours before and after cannulation, respectively. Peri-cannulation PaCO 2 drop (∆PaCO 2 ) equaled pre- minus post-cannulation PaCO 2 . ABI included intracranial hemorrhage (ICH), ischemic stroke, hypoxic-ischemic brain injury, cerebral edema, seizure, and brain death. Univariable logistic regression analysis was performed for the presence of ABI. Out of 129 VA-ECMO patients (median age = 60, 63% male), 43 (33%) patients experienced ABI. Patients had a median of 11 (interquartile range: 8-14) peri-cannulation PaCO 2 values. Comparing patients with and without ABI, pre-cannulation (39 vs. 42 mm Hg; p = 0.38) and post-cannulation (37 vs. 36 mm Hg; p = 0.82) PaCO 2 were not different. However, higher pre-cannulation PaCO 2 (odds ratio [OR] = 2.10; 95% confidence interval [CI] = 1.10-4.00; p = 0.02) and larger ∆PaCO 2 (OR = 2.69; 95% CI = 1.18-6.13; p = 0.02) were associated with ICH. In conclusion, in a cohort with granular arterial blood gas (ABG) data and a standardized neuromonitoring protocol, higher pre-cannulation PaCO 2 and larger ∆PaCO 2 were associated with increased prevalence of ICH.

Acid-base balance and cerebrovascular regulation

The regulation and defence of intracellular pH is essential for homeostasis. Indeed, alterations in cerebrovascular acid-base balance directly affect cerebral blood flow (CBF) which has implications for human health and disease. For example, changes in CBF regulation during acid-base disturbances are evident in conditions such as chronic obstructive pulmonary disease and diabetic ketoacidosis. The classic experimental studies from the past 75+ years are utilized to describe the integrative relationships between CBF, carbon dioxide tension (PCO₂ ), bicarbonate (HCO₃ ^- ) and pH. These factors interact to influence (1) the time course of acid-base compensatory changes and the respective cerebrovascular responses (due to rapid exchange kinetics between arterial blood, extracellular fluid and intracellular brain tissue). We propose that alterations in arterial [HCO₃ ^- ] during acute respiratory acidosis/alkalosis contribute to cerebrovascular acid-base regulation; and (2) the regulation of CBF by direct changes in arterial vs. extravascular/interstitial PCO₂ and pH - the latter recognized as the proximal compartment which alters vascular smooth muscle cell regulation of CBF. Taken together, these results substantiate two key ideas: first, that the regulation of CBF is affected by the severity of metabolic/respiratory disturbances, including the extent of partial/full acid-base compensation; and second, that the regulation of CBF is independent of arterial pH and that diffusion of CO₂ across the blood-brain barrier is integral to altering perivascular extracellular pH. Overall, by realizing the integrative relationships between CBF, PCO₂ , HCO₃ ^- and pH, experimental studies may provide insights to improve CBF regulation in clinical practice with treatment of systemic acid-base disorders.

The evidence for the physiological effects of lactate on the cerebral microcirculation: a systematic review

Abstract

Lactate's role in the brain is understood as a contributor to brain energy metabolism, but it may also regulate the cerebral microcirculation. The purpose of this systematic review was to evaluate evidence of lactate as a physiological effector within the normal cerebral microcirculation in reports ranging from in vitro experiments to in vivo studies in animals and humans. Following pre‐registration of a review protocol, we systematically searched the PubMed, EMBASE, and Cochrane databases for literature covering themes of ‘lactate’, ‘the brain’, and ‘microcirculation’. Abstracts were screened, and data extracted independently by two individuals. We excluded studies evaluating lactate in disease models. Twenty‐eight papers were identified, 18 of which were in vivo animal experiments (65%), four on human studies (14%), and six on in vitro or ex vivo experiments (21%). Approximately half of the papers identified lactate as an augmenter of the hyperemic response to functional activation by a visual stimulus or as an instigator of hyperemia in a dose‐dependent manner, without external stimulation. The mechanisms are likely to be coupled to NAD⁺/NADH redox state influencing the production of nitric oxide. Unfortunately, only 38% of these studies demonstrated any control for bias, which makes reliable generalizations of the conclusions insecure. This systematic review identifies that lactate may act as a dose‐dependent regulator of cerebral microcirculation by augmenting the hyperemic response to functional activation below 5 mmol/kg, and by initiating a hyperemic response above 5 mmol/kg.

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Correlation of arterial blood gas markers and lactate levels with outcomes in pediatric traumatic brain injury

Background:

Various physical markers have been used to predict outcome of traumatic brain injury in children. However, the utility of metabolic alterations for prognostication has been poorly described. Thus, we aim to correlate arterial blood gas markers and lactate levels with outcomes in children with moderate to severe traumatic brain injury.

Methods:

This is a retrospective cohort study that included all patients <16 years old who presented to the Emergency Department with moderate to severe traumatic brain injury (Glasgow Coma Scale ⩽13). Serial arterial blood gas results and lactate levels in the first five days of admission to a pediatric intensive care unit (PICU) were reviewed. Primary outcome was in-hospital mortality. Secondary outcomes were 28-day ventilator-free and PICU-free days. A stepwise logistic regression analysis in conjunction with receiver operating characteristic analysis were used to identify variables that were associated with in-hospital mortality. Secondary outcomes were analyzed using multiple linear regression.

Results:

Among the 43 patients analyzed, more than half of the patients (60%) had severe traumatic brain injury (Glasgow Coma Scale 8). Twenty-seven of the 43 (65%) patients underwent neurosurgical intervention and overall mortality was 9/43 (20.9%). The worst base excess and lactate levels of Day 2 of PICU stay were found to be most predictive for mortality with maximal area-under-curve (95% confidence interval) of 0.967 (0.906, 1.000). Worst lactate level on day 2 of PICU stay was also found to be associated with ventilator-free days and PICU-free days.

Conclusion:

In children with moderate to severe traumatic brain injury, base excess and lactate on Day 2 of PICU stay were predictors of mortality, duration of mechanical ventilation and length of PICU stay.

Where are we? The anatomy of the murine cortical meninges revisited for intravital imaging, immunology, and clearance of waste from the brain

Rapid progress is being made in understanding the roles of the cerebral meninges in the maintenance of normal brain function, in immune surveillance, and as a site of disease. Most basic research on the meninges and the neural brain is now done on mice, major attractions being the availability of reporter mice with fluorescent cells, and of a huge range of antibodies useful for immunocytochemistry and the characterization of isolated cells. In addition, two-photon microscopy through the unperforated calvaria allows intravital imaging of the undisturbed meninges with sub-micron resolution. The anatomy of the dorsal meninges of the mouse (and, indeed, of all mammals) differs considerably from that shown in many published diagrams: over cortical convexities, the outer layer, the dura, is usually thicker than the inner layer, the leptomeninx, and both layers are richly vascularized and innervated, and communicate with the lymphatic system. A membrane barrier separates them and, in disease, inflammation can be localized to one layer or the other, so experimentalists must be able to identify the compartment they are studying. Here, we present current knowledge of the functional anatomy of the meninges, particularly as it appears in intravital imaging, and review their role as a gateway between the brain, blood, and lymphatics, drawing on information that is scattered among works on different pathologies.

Does metabolic alkalosis influence cerebral oxygenation in infantile hypertrophic pyloric stenosis?

BACKGROUND

This pilot study focuses on regional tissue oxygenation (rSO₂) in patients with infantile hypertrophic pyloric stenosis in a perioperative setting. To investigate the influence of enhanced metabolic alkalosis (MA) on cerebral (c-rSO₂) and renal (r-rSO₂) tissue oxygenation, two-site near-infrared spectroscopy (NIRS) technology was applied.

MATERIALS AND METHODS

Perioperative c-rSO₂, r-rSO₂, capillary blood gases, and electrolytes from 12 infants were retrospectively compared before and after correction of MA at admission (T1), before surgery (T2), and after surgery (T3).

RESULTS

Correction of MA was associated with an alteration of cerebral oxygenation without affecting renal oxygenation. When compared to T1, 5-min mean (± standard deviation) c-rSO₂ increased after correction of MA at T2 (72.74 ± 4.60% versus 77.89 ± 5.84%; P = 0.058), reaching significance at T3 (80.79 ± 5.29%; P = 0.003). Furthermore, relative 30-min c-rSO₂ values at first 3 h of metabolic compensation were significantly lowered compared with postsurgical states at 16 and 24 h. Cerebral oxygenation was positively correlated with levels of sodium (r = 0.37; P = 0.03) and inversely correlated with levels of bicarbonate (r = -0.34; P = 0.05) and base excess (r = -0.36; P = 0.04). Analysis of preoperative and postoperative cerebral and renal hypoxic burden yielded no differences. However, a negative correlation (r = -0.40; P = 0.03) regarding hematocrite and mean r-rSO₂, indirectly indicative of an increased renal blood flow under hemodilution, was obtained.

CONCLUSIONS

NIRS seems suitable for the detection of a transiently impaired cerebral oxygenation under state of pronounced MA in infants with infantile hypertrophic pyloric stenosis. Correction of MA led to normalization of c-rSO₂. NIRS technology constitutes a promising tool for optimizing perioperative management, especially in the context of a possible diminished neurodevelopmental outcome after pyloromyotomy.

Two-week normobaric intermittent-hypoxic exposures stabilize cerebral perfusion during hypocapnia and hypercapnia

The effect of moderately extended, intermittent-hypoxia (IH) on cerebral perfusion during changes in CO2 was unknown. Thus, we assessed the changes in cerebral vascular conductance (CVC) and cerebral tissue oxygenation (ScO2) during experimental hypocapnia and hypercapnia following 14-day normobaric exposures to IH (10% O2). CVC was estimated from the ratio of mean middle cerebral arterial blood flow velocity (transcranial Doppler sonography) to mean arterial pressure (tonometry), and ScO2 in the prefrontal cortex was monitored by near-infrared spectroscopy. Changes in CVC and ScO2 during changes in partial pressure of end-tidal CO2 (PETCO2, mass spectrometry) induced by 30-s paced-hyperventilation (hypocapnia) and during 6-min CO2 rebreathing (hypercapnia) were compared before and after 14-day IH exposures in eight young nonsmokers. Repetitive IH exposures reduced the ratio of %ΔCVC/ΔPETCO2 during hypocapnia (1.00 ± 0.13 vs 1.94 ± 0.35 vs %/mmHg, P = 0.026) and the slope of ΔCVC/ΔPETCO2 during hypercapnia (1.79 ± 0.37 vs 2.97 ± 0.64 %/mmHg, P = 0.021), but had no significant effect on ΔScO2/ΔPETCO2. The ventilatory response to hypercapnia during CO2 rebreathing was significantly diminished following 14-day IH exposures (0.83 ± 0.07 vs 1.14 ± 0.09 L/min/mmHg, P = 0.009). We conclude that repetitive normobaric IH exposures significantly diminish variations of cerebral perfusion in response to hypercapnia and hypocapnia without compromising cerebral tissue oxygenation. This IH-induced blunting of cerebral vasoreactivity during CO2 variations helps buffer excessive oscillations of cerebral underperfusion and overperfusion while sustaining cerebral O2 homeostasis.

Age, aerobic fitness, and cerebral perfusion during exercise: role of carbon dioxide

Middle cerebral artery mean velocity (MCAvmean) is attenuated with increasing age both at rest and during exercise. The aim of this study was to determine the influence of the age-dependent reduction in arterial Pco2 (PaCO2) and physical fitness herein. We administered supplemental CO2 (CO2 trial) or no additional gas (control trial) to the inspired air in a blinded and randomized manner, and assessed middle cerebral artery mean flow velocity during graded exercise in 1) 21 young [Y; age 24 ± 3 yr (±SD)] volunteers of whom 11 were trained (YT) and 10 considered untrained (YUT), and 2) 17 old (O; 66 ± 4 yr) volunteers of whom 8 and 9 were considered trained (OT) and untrained (OUT), respectively. A resting hypercapnic reactivity test was also performed. MCAvmean and PaCO2 were lower in O [44.9 ± 3.1 cm/s and 30 ± 1 mmHg (±SE)] compared with Y (59.3 ± 2.3 cm/s and 34 ± 1 mmHg, P < 0.01) at rest, independent of aerobic fitness level. The age-related decreases in MCAvmean and PaCO2 persisted during exercise. Supplemental CO2 reduced the age-associated decline in MCAvmean by 50%, suggesting that PaCO2 is a major component in the decline. On the other hand, relative hypercapnic reactivity was neither influenced by age ( P = 0.46) nor aerobic fitness ( P = 0.36). Although supplemental CO2 attenuated exercise-induced reduction in cerebral oxygenation (near-infrared spectroscopy), this did not influence exercise performance. In conclusion, PaCO2 contributes to the age-associated decline in MCAvmean at rest and during exercise; however exercise capacity did not diminish this age effect.

Integrative regulation of human brain blood flow

Herein, we review mechanisms regulating cerebral blood flow (CBF), with specific focus on humans. We revisit important concepts from the older literature and describe the interaction of various mechanisms of cerebrovascular control. We amalgamate this broad scope of information into a brief review, rather than detailing any one mechanism or area of research. The relationship between regulatory mechanisms is emphasized, but the following three broad categories of control are explicated: (1) the effect of blood gases and neuronal metabolism on CBF; (2) buffering of CBF with changes in blood pressure, termed cerebral autoregulation; and (3) the role of the autonomic nervous system in CBF regulation. With respect to these control mechanisms, we provide evidence against several canonized paradigms of CBF control. Specifically, we corroborate the following four key theses: (1) that cerebral autoregulation does not maintain constant perfusion through a mean arterial pressure range of 60-150 mmHg; (2) that there is important stimulatory synergism and regulatory interdependence of arterial blood gases and blood pressure on CBF regulation; (3) that cerebral autoregulation and cerebrovascular sensitivity to changes in arterial blood gases are not modulated solely at the pial arterioles; and (4) that neurogenic control of the cerebral vasculature is an important player in autoregulatory function and, crucially, acts to buffer surges in perfusion pressure. Finally, we summarize the state of our knowledge with respect to these areas, outline important gaps in the literature and suggest avenues for future research.

Hemodynamic and metabolic correlates of perinatal white matter injury severity

Background and Purpose

Although the spectrum of perinatal white matter injury (WMI) in preterm infants is shifting from cystic encephalomalacia to milder forms of WMI, the factors that contribute to this changing spectrum are unclear. We hypothesized that the variability in WMI quantified by immunohistochemical markers of inflammation could be correlated with the severity of impaired blood oxygen, glucose and lactate.

Methods

We employed a preterm fetal sheep model of in utero moderate hypoxemia and global severe but not complete cerebral ischemia that reproduces the spectrum of human WMI.

Since there is small but measurable residual brain blood flow during occlusion, we sought to determine if the metabolic state of the residual arterial blood was associated with severity of WMI. Near the conclusion of hypoxia-ischemia, we recorded cephalic arterial blood pressure, blood oxygen, glucose and lactate levels. To define the spectrum of WMI, an ordinal WMI rating scale was compared against an unbiased quantitative image analysis protocol that provided continuous histo-pathological outcome measures for astrogliosis and microgliosis derived from the entire white matter.

Results

A spectrum of WMI was observed that ranged from diffuse non-necrotic lesions to more severe injury that comprised discrete foci of microscopic or macroscopic necrosis. Residual arterial pressure, oxygen content and blood glucose displayed a significant inverse association with WMI and lactate concentrations were directly related. Elevated glucose levels were the most significantly associated with less severe WMI.

Conclusions

Our results suggest that under conditions of hypoxemia and severe cephalic hypotension, WMI severity measured using unbiased immunohistochemical measurements correlated with several physiologic parameters, including glucose, which may be a useful marker of fetal response to hypoxia or provide protection against energy failure and more severe WMI.

Inner retinal oxygen extraction fraction in rat

PURPOSE

Oxygen extraction fraction (OEF), defined by the ratio of oxygen consumption to delivery, may be a useful parameter for assessing the retinal tissue status under impaired circulation. We report a method for measurement of inner retinal OEF in rats under normoxia and hypoxia based on vascular oxygen tension (PO(2)) imaging.

METHODS

Retinal vascular PO(2) measurements were obtained in 10 rats, using our previously developed optical section phosphorescence lifetime imaging system. Inner retinal OEF was derived from retinal vascular PO(2) measurements based on Fick's principle. Measurements of inner retinal OEF obtained under normoxia were compared between nasal and temporal retinal sectors and repeatability was determined. Inner retinal OEF measurements obtained under normoxia and hypoxia were compared.

RESULTS

Retinal vascular PO(2) and inner retinal OEF measurements were repeatable (ICC ≥ 0.83). Inner retinal OEF measurements at nasal and temporal retinal sectors were correlated (R = 0.71; P = 0.02; n = 10). Under hypoxia, both retinal arterial and venous PO(2) decreased significantly as compared with normoxia (P < 0.001; n = 10). Inner retinal OEF was 0.46 ± 0.13 under normoxia and increased significantly to 0.67 ± 0.16 under hypoxia (mean ± SD; P < 0.001; n = 10).

CONCLUSIONS

Inner retinal OEF is a promising quantitative biomarker for the adequacy of oxygen supply for metabolism under physiologically and pathologically altered conditions.

pCO(2) and pH regulation of cerebral blood flow

CO₂ serves as one of the fundamental regulators of cerebral blood flow (CBF). It is widely considered that this regulation occurs through pCO₂-driven changes in pH of the cerebral spinal fluid (CSF), with elevated and lowered pH causing direct relaxation and contraction of the smooth muscle, respectively. However, some findings also suggest that pCO₂ acts independently of and/or in conjunction with altered pH. This action may be due to a direct effect of CSF pCO₂ on the smooth muscle as well as on the endothelium, nerves, and astrocytes. Findings may also point to an action of arterial pCO₂ on the endothelium to regulate smooth muscle contractility. Thus, the effects of pH and pCO₂ may be influenced by the absence/presence of different cell types in the various experimental preparations. Results may also be influenced by experimental parameters including myogenic tone as well as solutions containing significantly altered HCO₃⁻ concentrations, i.e., solutions routinely employed to differentiate the effects of pH from pCO₂. In sum, it appears that pCO₂, independently and in conjunction with pH, may regulate CBF.

Adenosine and the regulation of cerebral blood flow

OBJECTIVES

This review summarizes the 30 year effort of my collaborator and mentor Dr J. W. Phillis to establish the role of adenosine in the regulation of cerebral blood flow.

METHODS

While most of the experiments described utilized the rat cerebral cortex as a model, several different and complementary methodologies were employed. Superfusate samples were collected from the cortical surface and analysed for purines using HPLC. Laser-Doppler flowmetry was utilized to measure blood flow in the pial vasculature, while pial diameters were monitored by videomicroscopy. An additional series of experiments looked at coronary blood flow in a Langendorff preparation.

RESULTS

Adenosine is released from the cortex in response to decreased nutrient supply (hypoxia/ ischemia) and during conditions that mimic alterations in the extracellular environment associated with increased metabolism. The application of pharmacological agents that alter adenosine metabolism resulted in the appropriate alterations in ECF adenosine levels and also in blood flow. Selective blockade of the adenosine A(2A) receptor reduced the pial vasodilation evoked by hypercapnoea. Results from the isolated rat heart, utilizing similar agents, support a role for adenosine in the regulation of coronary blood flow during respiratory and metabolic acidosis.

DISCUSSION

Adenosine is released when there is a mismatch between supply and demand. If the effects of adenosine are blocked with receptor antagonists, the vasodilation is also reduced. However, the effects of adenosine on the hyperemia evoked by hypercapnoea are complicated by the arousal evoked by adenosine receptor antagonists and the effects of upstream regulation.

Effects of adenosine receptor antagonists on pial arteriolar dilation during carbon dioxide inhalation

The role of adenosine in the cerebrovascular response to carbon dioxide inhalation was evaluated in two sets of experiments. The pial circulation was recorded by a Laser-Doppler flow probe placed over a closed cranial window in methoxyflurane anesthetized rats. Topical application of the nonselective adenosine receptor antagonist caffeine (1 mM), the selective A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX,1 microM), or the selective A2A receptor antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a]triazin-5-yl amino]ethyl) phenol (ZM 241385, 1 microM) all failed to affect mean arterial blood pressure, basal cerebral blood flow, or the carbon dioxide-evoked hyperemia. Systemically administered caffeine (20 mg/kg) also had no significant effects. However, following the systemic administration of the nonselective nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 20 mg/kg), the topical application of both caffeine and ZM 241385 (but not DPCPX) significantly reduced the carbon dioxide-evoked hyperemia. L-NAME (20 mg/kg) administered intravenously, evoked a significant increase in mean arterial blood pressure, a slow progressive decline in cerebral blood flow and, during brief (60-90 s) periods of 7.5% carbon dioxide inhalation, a significant decrease in arterial blood pressure. L-NAME failed to reduce the carbon dioxide-evoked increase in cerebral blood flow as measured by the area under the curve (AUC), although it did reduce the peak flow response. Topically applied L-NAME (1 mM) failed to alter mean arterial blood pressure, basal cerebral blood flow, or the carbon dioxide-evoked increases in cerebral blood flow. In a second series of experiments, we evaluated the ability of 10% carbon dioxide inhalation for 8 min to elicit a release of adenosine from the cerebral cortex. Adenosine levels in the cortical superfusates rose significantly during periods of carbon dioxide inhalation. The data suggest that following the removal of the confounding effects of nitric oxide, which are unlikely to be mediated locally, a significant contribution by adenosine A2A receptor activation to the carbon dioxide-evoked cortical hyperemia was evident.

Protection of focal cerebral ischemia by alkalinization of systemic pH

OBJECT

It has been demonstrated in many studies that intracellular brain acidosis during cerebral ischemia is a significant factor in perpetuating the cycle of cellular dysfunction leading to neuronal injury. The purpose of this study was to determine whether preischemic administration of alkalotic agents could reduce neuronal injury after focal cerebral ischemia.

METHODS

Fifteen fasted rabbits under 1.0% halothane anesthesia were randomized into three groups: Group 1 rabbits were ischemic controls (n = 5) that underwent 4 hours of focal cerebral ischemia. Groups 2 and 3 rabbits underwent a paradigm similar to that of Group 1, except that they were pretreated with either sodium bicarbonate or Carbicarb at similar buffering capacities. Intracellular brain pH (pH(i)), regional cortical blood flow (rCBF), and intrinsic reduced nicotinamide adenine dinucleotide (NADH) fluorescence were measured with in vivo fluorescence imaging. At the end of each experiment, infarct volume expressed as a percentage of hemispheric volume was measured by triphenyltetrazolium chloride staining.

RESULTS

Preischemic alkalinization did not alter brain pH(i), rCBF, or NADH fluorescence. After 4 hours of ischemia, brain pH(i), rCBF, NADH fluorescence, and infarct volume measured 6.40 +/- 0.09 (mean +/- standard error), 11 +/- 2 ml/100 g/min, 165 +/- 8% of baseline control, and 37 +/- 3% in ischemic controls, respectively. In Group 2 animals treated with sodium bicarbonate, brain pH(i), rCBF, NADH fluorescence, and infarct volume improved significantly (P < 0.05, analysis of variance) to 6.74 +/- 0.08, 24 +/- 6 ml/100 g/min, 137 +/- 6% of baseline control, and 22 +/- 4%, respectively. Group 3 Carbicarb animals demonstrated improvements in brain pH(i), rCBF, and NADH fluorescence, with a significant reduction in infarct volume.

CONCLUSION

These findings suggest that pretreatment with alkalinizing agents may be a useful intervention to provide intraoperative cerebral protection from ischemic injury.

Cerebral blood flow and vascular physiology

The cerebral circulation is tightly regulated to meet the brain's metabolic demands. Although the mechanism is not fully understood, the major physiologic influences on cerebral blood flow have been well documented. In this chapter the basic vascular anatomy, and physiologic control of the cerebral circulation are reviewed. Clinical implications are emphasized.

Endothelial nitric oxide synthase-dependent cerebral blood flow augmentation by L-arginine after chronic statin treatment

Nitric oxide, a product of nitric oxide synthase activity, relaxes vascular smooth muscle and elevates brain blood flow. We evaluated the importance of eNOS to cerebral blood flow augmentation after L-arginine infusion and increases in flow after eNOS upregulation in SV-129 mice. Blood flow was measured by laser-Doppler flowmetry before and after L-arginine infusion (450 mg/kg during a 15-minute period) or measured by 14C-iodoamphetamine indicator fractionation or 14C-iodoantipyrine tissue equilibration techniques. rCBF increased by 26% (laser Doppler flowmetry) after L-arginine infusion but did not change in mutant mice deficient in eNOS expression. After eNOS upregulation by chronic simvastatin treatment (2 mg/kg subcutaneously, daily for 14 days), L-arginine amplified and sustained the hyperemia (38%) and increased absolute brain blood flow from 86 +/- 7 to 119 +/- 10 mL/100 g per minute. Furthermore, pretreatment with simvastatin enhanced blood flow within ischemic brain tissue after middle cerebral artery occlusion. Together, these findings suggest that eNOS activity is critical for blood flow augmentation during acute L-arginine infusion, and chronic eNOS upregulation combined with L-arginine administration provides a novel strategy to elevate cerebral blood flow in the normal and ischemic brain.

Xenon-enhanced CT: clinical applications

A constant challenge for health care providers caring for the neurologically injured patient is to restore function and/or prevent permanent disability. Advances in technology have led to the development of diagnostic studies that assist in determining potential and actual neuronal injury. Xenon-enhanced computed tomography (CT) provides reproducible quantitative information coupled with anatomic CT imaging. Not only does it provide accurate estimation of cerebral blood flow, but it also reflects regional alterations in flow. This technique is useful for identifying those patients who have, or are at risk for, ischemic compromise. Xenon-enhanced computed tomography is useful when assessing the patient with an acute neurologic change who is being considered for thrombolytic therapy and for patients with carotid artery stenosis to evaluate cerebrovascular reserve. This article will focus on the clinical applications of xenon-enhanced CT.

Hypercapnia-induced increases in cerebral blood flow: roles of adenosine, nitric oxide and cortical arousal

The roles of nitric oxide, adenosine and cortical arousal in the response to 7.5% CO2 inhalation were investigated by measuring cerebral blood flow bilaterally in the rat somatosensory cortices with laser-Doppler flow probes. Administration of N(omega)-nitro-L-arginine methyl ester (L-NAME; 20 mg/kg, i.v.) significantly attenuated the response to hypercapnia (mean decrease of 47%). This effect was partially reversed by a subsequent administration of L-arginine. Caffeine (10 mg/kg, i.v.) also significantly reduced hypercapnic responses (mean decrease of 44%). Caffeine administration was also associated with a tendency for animals to exhibit electrocorticographic signs of arousal; often associated with a reduction in the attenuation of the flow response to CO2 inhalation. 8-(3-Chlorostyryl) caffeine (CSC, 1.0 mg/kg), a selective antagonist at adenosine A2a striatal receptors failed to attenuate CO2-evoked responses, whereas CGS 15943, a less selective A2a receptor antagonist, significantly reduced CO2 responses. These data from the rat suggest (1) that both nitric oxide and adenosine may contribute to pial arteriolar vasodilatation during hypercapnia, and (2) that CO2 inhalation acts as a potent stimulus for cortical arousal, with enhanced neuronal activity contributing to the vascular response. The effects of administration of adenosine antagonists, such as the methylxanthines antagonists caffeine and theophylline, on CBF responses to hypercapnia can potentially be negated by the ability of these agents to facilitate CO2-induced cortical arousal.

L-arginine infusion increases basal but not activated cerebral blood flow in humans

Nitric oxide is a potent vasodilator. Infusion of its precursor, L-arginine, results in increased cerebral blood flow (CBF) in experimental animals. We examined the effects of L-arginine infusion on CBF in humans using positron emission tomography and the quantitative H2(15)O method. Six subjects received 500 ml of 0.9% NaCl solution, and six subjects received an infusion of L-arginine (16.7 mg/kg/min; 500 mg/kg). Before and after the i.v. infusion, paired CBF measurements were performed at baseline and with vibrotactile stimulation of the right hand. In scans performed without vibrotactile stimulation, mean whole-brain CBF increased from 34.9 +/- 3.7 ml 100 g-1 min-1 to 38.2 +/- 4.4 ml 100 g-1 min-1. (9.5%; p < 0.005) after L-arginine infusion. The temporal pattern of CBF changes differed from that of plasma growth hormone and insulin levels and of arterial pH. In contrast, in the saline group, mean whole-brain CBF did not change significantly (35.8 +/- 5.9 ml 100 g-1 min-1 to 35.9 +/- 6.4 ml 100 g-1 min-1; 0.3%). Vibrotactile stimulation produced significant focal increases in CBF, which were unaffected by L-arginine infusion. L-arginine infusion was associated with an increase in plasma L-citrulline, a byproduct of nitric oxide synthesis.

Simultaneous measurements of lactate and blood flow during hypoxia and recovery from hypoxia in a localized region in the brain of the anesthetized rabbit

We observed simultaneous changes in lactate level and regional blood flow (rBF) in the brain of the anesthetized rabbit by using localized proton magnetic resonance spectroscopy (1H MRS) and laser Doppler flowmetry. The volume of interest of 0.5 ml for 1H MRS contained mostly thalamic nuclei. During hypoxia peak area for lactate increased up to 57% of that from N-Acetylaspartate. While the rBF increased during hypoxia up to 260% of the control, oxygen delivery (rBF x arterial oxygen content) decreased. In the normoxic recovery period following hypoxia, the rBF recovered slowly and a consequent overshoot of oxygen delivery was observed. The multiple and stepwise linear regression analyses revealed that the averaged decrease in oxygen delivery during hypoxia was the most significant independent variable for the increase in lactate during hypoxia (correlation coefficient; r2 = 0.68) and also that the increase in lactate during hypoxia was the most significant independent variable for the time for half-recovery of rBF (r2 = 0.75). These results suggest that the increase in lactate during hypoxia is due to the deficiency of oxygen delivery and that the increase in lactate during hypoxia prolongs the period of enhancement of rBF during recovery from hypoxia.

Effects of defibrination on hemorheology, cerebral blood flow velocity, and CO2 reactivity during hypocapnia in normal subjects

BACKGROUND AND PURPOSE

Plasma fibrinogen is reported to be an independent risk factor for stroke and cardiovascular diseases. The effects of defibrination on hemorheology, middle cerebral artery (MCA) blood flow velocity, and CO2 reactivity during hypocapnia were evaluated in normal subjects.

METHODS

Twenty-five healthy subjects (mean age, 31.8 +/- 5.7 years) were included in the study. Measurements were done at rest and repeated 24 hours after administration of 10 batroxobin units. Plasma fibrinogen, plasma viscosity, and whole blood viscosity were measured as hemorheological factors. MCA blood flow velocity was measured with a transcranial Doppler flowmeter. Blood flow velocity was corrected to 40 mm Hg of end-tidal CO2 partial pressure (PETCO2), and expressed as CV40. CO2 reactivity was measured as percent change in mean blood flow velocity per millimeter of mercury PETCO2.

RESULTS

Plasma fibrinogen (from 7.04 to 2.29 mumol/L; P < .001), whole blood viscosity, and plasma viscosity decreased after administration of batroxobin. Mean MCA blood flow velocity at rest, CV40. and CO2 reactivity during hypocapnia increased significantly (from 67.4 to 73.6 cm/s, from 71.7 to 77.7 cm/s, and from 2.9%/mm Hg to 3.2%/mm Hg, respectively; P < .01) after defibrination. Mean arterial blood pressure and PETCO2 at rest were constant before and 24 hours after administration of batroxobin. There was a significant positive correlation between CV40 and CO2 reactivity (r = .623, P < .0001).

CONCLUSIONS

The increase in MCA blood flow velocity was associated with improved CO2 reactivity and reduced blood viscosity after defibrination. The data may suggest that defibrination increases cerebral blood flow by reducing blood viscosity.

Effects of propranolol pretreatment on cerebral blood flow, oxygen uptake and catecholamines during metabolic acidosis following E. coli endotoxin in dogs

After an intravenous injection of E. coli endotoxin in dogs a decrease in cerebral blood flow (CBF) and an increase in cerebral metabolic rate of oxygen (CMRo2) have been shown to occur. In metabolic acidosis following endotoxin CMRo2 increased with decreasing pH. A possible explanation for the increased CMRo2 after endotoxin and metabolic acidosis seems to be a damage of the blood-brain barrier (BBB) by endotoxin. This gives possibilities for a leakage of hydrogen ions and circulating monoamines from the blood to the brain, thus affecting the cerebral blood flow and metabolism. The effects of an E. coli endotoxin injection on CBF and CMRo2 during metabolic acidosis and beta-adrenoceptor blockade were studied in eight anaesthetized dogs. All the dogs were pretreated with propranolol (PPL), per os 12.5 mg.kg-1 twice a day for one week. Metabolic acidosis (pH 7.01-7.30) was achieved by an intravenous infusion of hydrochloric acid. Endotoxin E. coli lipopolysaccharide O 111:B 4 was given as an intravenous injection of 1 mg.kg-1 bodyweight over a 5 min period. Another five animals, published earlier, with the same experimental protocol but without PPL, constituted a control group. After endotoxin no increase in CMRo2 or CBF was observed with increasing acidosis in the PPL-group. In the control group, after endotoxin, both CBF and CMRo2 increased with decreasing pH. This resulted in a significant difference in both CBF and CMRo2 between the groups in the pH range 7.01-7.15. The present results indicate that the increase in CMRo2 and CBF with metabolic acidodis in endotoxinaemia is mediated via beta-adrenoceptors.

The effects of arterial blood gas values on lesion volumes in a graded rat spinal cord contusion model

The detrimental effects of extreme blood gas values are well documented. However, the range of normal values has not been rigorously defined. There is an ongoing debate concerning the need for ventilation and tight control of blood gas values in spinal cord injury models. Consequently, we performed a retrospective study of 84 rats using a graded rat spinal cord contusion model. Spinal cord ionic lesion volumes were calculated from Na and K shifts at 24 h after injury. Blood gas measurements were obtained 5 min before contusion. For pH values of 7.31-7.46, systemic acidosis was associated with a small but significant decrease in ionic lesion volumes in the 12.5 and 25 g.cm contusion groups (p < 0.05 and p < 0.03, respectively). pH had no effect on ionic lesion volumes in the 50 g.cm contusion group (p > 0.5). PaCO2 values from 23 to 53 mm Hg showed an effect only at 25 g.cm (p < 0.05). PaO2 values of 46-138 mm Hg and calculated HCO3 values of 13-28 mEq/L had no effect on ionic lesion volumes. Two conclusions may be derived from these data. First, mild systemic acidosis is associated with a small reduction in ionic lesion volumes after mild and moderate injury but not after severe injury. This suggests that secondary mechanisms play a greater role in mild injuries. Second, variations in arterial blood gases within clinically normal ranges do not strongly influence 24-h ionic lesion volumes in a graded spinal cord injury model. The effects of blood gas values on ionic lesion volumes are not statistically significant unless the data are adjusted for injury severity. Although blood gas values must be carefully monitored, ventilation may not be needed routinely in rat spinal cord injury models. We recommend maintaining pH values between 7.35 and 7.40, PaCO2 between 35 and 41 mm Hg, and PaO2 greater than 71 mm Hg.

Effects of hemodialysis on cerebral circulation evaluated by transcranial Doppler ultrasonography

BACKGROUND AND PURPOSE

The effects of hemodialysis on the cerebral circulation of humans and the correlation between changes in blood flow velocity in the basal cerebral arteries and those of several physiological variables influenced by hemodialysis have been inadequately studied.

METHODS

Blood flow velocities were obtained from the middle cerebral artery and the basilar artery by transcranial Doppler ultrasonography in 27 patients receiving chronic maintenance hemodialysis immediately before and after the procedure. Changes in body weight, hematocrit, blood pressure, and arterial blood gases were recorded simultaneously.

RESULTS

There was a significant reduction in mean flow velocity in the middle cerebral artery (P < .01) and the basilar artery (P < .01) after hemodialysis. We observed a significant negative correlation between the relative change in mean flow velocity and the loss of weight after hemodialysis, the amount of fluid removed, and the increase in hematocrit in the middle cerebral artery and the basilar artery.

CONCLUSIONS

Hemodialysis and the associated physiological changes can significantly affect the cerebral circulation. Blood flow velocities in the middle cerebral artery and the basilar artery decrease significantly with hemodialysis. The loss of body weight, the amount of fluid removed, and the change in hematocrit significantly correlate with the change in mean flow velocity. The transcranial Doppler method can effectively monitor rapid changes in the cerebral circulation during potentially harmful procedures.

Effects of metabolic pH-alterations on cerebral blood flow and oxygen uptake following E. coli endotoxin in dogs

The aim of the present study was to investigate if metabolic pH-alterations have an influence on cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) after an injection of E. coli endotoxin. Following endotoxin in dogs with normal pH a decreased CBF and an increased CMRO2 have earlier been found. Thirteen anaesthetized dogs were subjected to metabolic pH-variations in blood by infusion of hydrochloric acid or sodium bicarbonate. Ten dogs received E. coli endotoxin in a dose of 1 mg.kg-1 bodyweight. CBF, CMRO2 and noradrenaline and adrenaline concentrations in blood and cerebrospinal fluid were measured repeatedly during normoxia and normocarbia. Measurements before endotoxin served as controls, together with three additional animals, where endotoxin was never given. In control measurements pH showed no influence on the variables studied. After endotoxin CBF, CMRO2 and noradrenaline in cerebrospinal fluid increased with decreasing arterial blood pH. The influence exerted by metabolic pH alterations in blood after endotoxin may be explained by hydrogen ions and monoamines passing over a blood-brain barrier (BBB), damaged by endotoxin, into the brain tissue causing vasodilation and neuronal activation.

[Acid-base equilibrium and the brain]

In physiological conditions, the regulation of acid-base balance in brain maintains a noteworthy stability of cerebral pH. During systemic metabolic acid-base imbalances cerebral pH is well controlled as the blood/brain barrier is slowly and poorly permeable to electrolytes (HCO3- and H+). Cerebral pH is regulated by a modulation of the respiratory drive, triggered by the early alterations of interstitial fluid pH, close to medullary chemoreceptors. As blood/brain barrier is highly permeable to Co2, CSF pH is corrected in a few hours, even in case of severe metabolic acidosis and alkalosis. Conversely, during ventilatory acidosis and alkalosis the cerebral pH varies in the same direction and in the same range than blood pH. Therefore, the brain is better protected against metabolic than ventilatory acid-base imbalances. Ventilatory acidosis and alkalosis are able to impair cerebral blood flow and brain activity through interstitial pH alterations. During respiratory acidosis, [HCO3-] increases in extracellular fluids to control cerebral pH by two main ways: a carbonic anhydrase activation at the blood/brain and blood/CSF barriers level and an increase in chloride shift in glial cells (HCO3- exchanged for Cl-). During respiratory alkalosis, [HCO3-] decreases in extracellular fluids by the opposite changes in HCO3- transport and by an increase in lactic acid synthesis by cerebral cells. The treatment of metabolic acidosis with bicarbonates may induce a cerebral acidosis and worsen a cerebral oedema during ketoacidosis. Moderate hypocapnia carried out to treat intracranial hypertension is mainly effective when cerebral blood flow is high and vascular CO2 reactivity maintained. Hypocapnia may restore an altered cerebral blood flow autoregulation. Instrumental hypocapnia requires a control of cerebral perfusion pressure and cerebral arteriovenous difference for oxygen, to select patients for whom this kind of treatment may be of benefit, to choose the optimal level of hypocapnia and to avoid any deleterious effect. If hypocapnia is maintained over several days, an adaptation of CSF pH may limit the therapeutic effect on the cerebral blood flow and the intracranial pressure.

Plasma epinephrine modulates the cerebrovasodilation evoked by electrical stimulation of dorsal medulla

We examined whether plasma epinephrine contributes to the increase in regional cerebral blood flow (rCBF) evoked by electrical stimulation of the dorsal medullary reticular formation (DMRF). Rats were anesthetized (alpha-chloralose, 30 mg/kg, s.c.), paralyzed and artificially ventilated. The DMRF was electrically stimulated through microelectrodes stereotaxically implanted. During stimulation, blood gases and arterial pressure were monitored and maintained within normal range. rCBF was determined in 11 dissected brain regions using the [14C]iodoantipyrine technique. Plasma epinephrine and norepinephrine were measured radioenzymatically in rats with intact adrenals or adrenalectomy, and with or without infusion of epinephrine. DMRF stimulation induced widespread increases in rCBF associated with a 50-fold increase in plasma epinephrine and a 20-fold increase in norepinephrine without changes in the electroencephalogram. In contrast, stimulation of the adjacent medial longitudinal fasciculus had no effect upon rCBF or plasma catecholamines. Acute bilateral adrenalectomy produced regionally selective reductions in the stimulation-coupled increases in rCBF throughout brain (P less than 0.05). Infusion of epinephrine in adrenalectomized rats to levels comparable to those observed in intact animals during DMRF stimulation did not by itself modify rCBF. However, when infused in conjunction with stimulation of the DMRF, but not medial longitudinal fasciculus, epinephrine fully restored the stimulus-related increases in rCBF in all brain regions to levels comparable to those observed in intact rats. We conclude that stimulation of the DMRF elevates rCBF through two mechanisms; by a neurally-mediated increase in local metabolism and thereby flow (adrenal independent secondary vasodilation) and by releasing epinephrine from adrenal medulla which secondarily acts to increase rCBF by an action on brain.

Effect of thyrotropin-releasing hormone on cerebral blood flow in conscious rat

The effect of thyrotropin-releasing hormone (TRH) was studied on local CBF (LCBF) in normal conscious rats. LCBF was measured by the autoradiographic [14C]iodoantipyrine method 5 min after TRH (5 mg/kg, i.v.) administration. TRH significantly increased LCBF in 22 of 33 brain regions. This increase of LCBF exceeded 100% of the control values in the cerebral cortices, whereas there was no significant increase in white matter or in some gray matter structures. The increase of CBF following TRH administration was abolished by pretreatment with indomethacin (5 mg/kg, i.v.). The mechanisms underlying the increase of CBF following TRH administration are discussed in relation to prostaglandin metabolism.

Effects of lactic acid infusions and pH on cerebral blood flow and metabolism

To determine the effects of lactic acidemia versus lactate on CBF, we infused lactic acid, either buffered with NaOH (L + NaOH) or with added NaCl (L + NaCl), to attain similar osmolalities in 18 piglets. CBF (microsphere technique), pH, blood gases, plasma osmolality, and cerebral arteriovenous differences of O2 content and lactic acid concentrations were measured prior to, at 30 min of a lactic acid infusion, and 15 and 90 min after completion of the infusion. Control arterial pH was comparable between groups (7.50 +/- 0.02 vs. 7.49 +/- 0.02, X +/- SE); during and following L + NaCl and L + NaOH, values were (p less than 0.05) 7.09 +/- 0.03, 7.35 +/- 0.02, and 7.46 +/- 0.02 vs. 7.58 +/- 0.03, 7.61 +/- 0.01, and 7.57 +/- 0.03, respectively. PaCO2 remained unchanged and osmolality rose by 15% in both groups during infusions and persisted throughout the study period. For L + NaCl piglets, CBF (ml/min.100 g) rose from 136 +/- 15 to 198 +/- 26 (p less than 0.05) at 30 min of infusion and remained elevated at 201 +/- 25 and 207 +/- 28 at 15 and 90 min following the infusion, respectively. Similarly, for L + NaOH piglets, CBF rose from 130 +/- 25 to 196 +/- 31 (p less than 0.05) with the infusion and was 174 +/- 17 and 166 +/- 21 at 15 and 90 min afterward, respectively. Although lactic acid infusion increases CBF, the associated metabolic acidemia is not responsible for changes in CBF.

An involvement of adenosine in cerebral blood flow regulation during hypercapnia

The possibility that endogenously released adenosine, a potent vasodilator, is involved in the increase in cerebral blood flow (CBF) response to hypercapnia has been investigated in an anesthetized, paralyzed rat model. The left retroglenoid vein was cannulated and cerebral venous blood flow measured with a drop counter. Animals were ventilated with a 40% oxygen, 60% nitrogen gas mixture. At 20 min intervals, at a constant rate of flow, the inspired gas mixture was altered to 10% carbon dioxide, 40% oxygen, 50% nitrogen for periods of between 30-90 sec. This brief hypercapnic challenge induced a rapid increase in CBF in the absence of any change in MABP. An involvement of adenosine in this response was demonstrated using an adenosine antagonist, caffeine, an uptake inhibitor, dipyridamole and an adenosine deaminase inhibitor, deoxycoformycin. Caffeine (10 and 20 mg/kg i.p.) 15 min prior to hypercapnic challenges significantly decreased the peak increases in CBF. Dipyridamole (0.1 mg/kg) and deoxycoformycin (0.1 microgram/kg) enhanced the peak increases in flow. These results are consistent with an important role for adenosine in coupling PCO2 to cerebral blood flow.

Preferential blood flow to brainstem during generalized seizures in the newborn marmoset monkey

The effect of generalized seizures on local cerebral blood flow was studied autoradiographically in 21 immature marmoset monkeys, using either [123I]- or [131I]isopropyliodoamphetamine. Generalized convulsions were induced in ketamine-anesthetized and awake monkeys with bicuculline and continued for 4-59 min. During convulsions in marmosets less than 3 weeks of age, there was a striking rearrangement of blood flow in favor of the brainstem pontomedullary region. The ratios of blood flow in pons-medulla to blood flow in cerebral cortex, putamen, ventroposterior thalamic nuclei, lateral geniculate nuclei, cerebellum and hemispheric white matter increased 1 1/2 to 2 times compared to controls. In seizure animals 4-8 weeks of age, the redistribution of blood flow to brainstem did not occur. Although metabolic acidosis developed after 30 min of bicuculline-induced seizures, mean arterial blood pressure, temperature, arterial pO2 and pCO2 did not differ significantly from controls, indicating that hypoxemia, hypercapnia and hypotension cannot explain the altered cerebral blood flow pattern. The redistribution phenomenon could be explained by more pronounced vasodilatation in brainstem than many other brain regions during generalized seizures in newborn monkeys. Lack of significant vasodilatation in forebrain structures such as cerebral cortex could contribute to neuronal damage by limiting substrate supply at a time of increased metabolic activity.

Brain luxury perfusion during cardiopulmonary bypass in humans. A study of the cerebral blood flow response to changes in CO2, O2, and blood pressure

CBF and related parameters were studied in 68 patients before, during, and following cardiopulmonary bypass. CBF was measured using the intraarterial 133Xe injection method. The extracorporeal circuit was nonpulsatile with a bubble oxygenator administering 3-5% CO2 in the main group of hypercapnic patients (n = 59) and no CO2 in a second group of hypocapnic patients. In the hypercapnic patients, marked changes in CBF occurred during bypass. Evidence was found of a brain luxury perfusion that could not be related to the effect of CO2 per se. Mean CBF was 29 ml/100 g/min just before bypass, 49 ml/100 g/min at steady-state hypothermia (27 degrees C), reached a maximum of 73 ml/100 g/min during the rewarming phase (32 degrees C), fell to 56 ml/100 g/min at steady-state normothermic bypass (37 degrees C), and was 48 ml/100 g/min shortly after bypass was stopped. Addition of CO2 evoked systemic vasodilation with low blood pressure and a rebound hyperemia. The hypocapnic group responded more physiologically to the induced changes in hematocrit (Htc) and temperature, CBF being 25, 23, 25, 34, and 35 ml/100 g/min, respectively, during the five corresponding periods. Carbon dioxide was an important regulator of CBF during all phases of cardiac surgery, the responsiveness of CBF being approximately 4% for each 1-mm Hg change of PaCO2. The level of MABP was important for the CO2 response. At low blood pressure states, the CBF responsiveness to changes in PaCO2 was almost abolished. An optimal level of PaCO2 during hypothermic bypass of approximately 25 mm Hg (at actual temperature) is recommended. A normal autoregulatory response of CBF to changes in blood pressure was found during and following bypass. The lower limit of autoregulation was at pressure levels of approximately 50-60 mm Hg. CBF autoregulation was almost abolished at PaCO2 levels of greater than 50 mm Hg. The degree of hemodilution neither affected the CO2 response nor impaired CBF autoregulation, although, as would be expected, it influenced CBF: In 33 women CBF was 55 ml/100 g/min at an Htc of 24%, as compared with 42 ml/100 g/min in 35 men (Htc = 28%). High PaO2 was a vasoconstrictor, the autoregulatory plateau being narrowed. The lower limit of autoregulation was shifted to a higher pressure when PaO2 was low.