Effect of deep hypothermia and circulatory arrest on cerebral blood flow and metabolism

Early Impairment of Cerebral Bioenergetics After Cardiopulmonary Bypass in Neonatal Swine

Objectives: We previously demonstrated cerebral mitochondrial dysfunction in neonatal swine immediately following a period of full-flow cardiopulmonary bypass (CPB). The extent to which this dysfunction persists in the postoperative period and its correlation with other markers of cerebral bioenergetic failure and injury is unknown. We utilized a neonatal swine model to investigate the early evolution of mitochondrial function and cerebral bioenergetic failure after CPB. Methods: Twenty piglets (mean weight 4.4 ± 0.5 kg) underwent 3 h of CPB at 34 °C via cervical cannulation and were followed for 8, 12, 18, or 24 h (n = 5 per group). Markers of brain tissue damage (glycerol) and bioenergetic dysfunction (lactate to pyruvate ratio) were continuously measured in cerebral microdialysate samples. Control animals (n = 3, mean weight 4.1 ± 1.2 kg) did not undergo cannulation or CPB. Brain tissue was extracted immediately after euthanasia to obtain ex-vivo cortical mitochondrial respiration and frequency of cortical microglial nodules (indicative of cerebral microinfarctions) via neuropathology. Results: Both the lactate to pyruvate ratio (P < .0001) and glycerol levels (P = .01) increased in cerebral microdialysate within 8 h after CPB. At 24 h post-CPB, cortical mitochondrial respiration was significantly decreased compared with controls (P = .046). The presence of microglial nodules increased throughout the study period (24 h) (P = .01, R2 = 0.9). Conclusion: CPB results in impaired cerebral bioenergetics that persist for at least 24 h. During this period of bioenergetic impairment, there may be increased susceptibility to secondary injury related to alterations in metabolic delivery or demand, such as hypoglycemia, seizures, and decreased cerebral blood flow.

The incidence, risk factors and outcomes of impaired cerebral autoregulation in aortic arch surgery: a single-center, retrospective cohort study

BACKGROUND

Impairment of cerebral autoregulation (CA) has been observed in patients undergoing cardiopulmonary bypass (CPB), but little is known about its risks and associations with outcomes. The cerebral oximetry index (COx), which is a moving linear correlation coefficient between regional cerebral oxygen saturation (rScO2) and mean blood pressure (MAP), may reflect CA function. When COx approaches 1, it implies that CA is damaged, whereas the CA is functional when the COx value approaches 0. The objective of this study was to analyze the incidence and risks of impaired CA, based on COx assessment, in patients undergoing total aortic arch replacement under systemic moderate hypothermia and circulatory arrest of the lower body (MHCA). We also evaluated the association between impaired CA and patient outcomes.

METHODS

One hundred and fifty-four adult patients who underwent total aortic arch replacement with stented elephant trunk implantation under MHCA at our hospital were retrospectively analyzed. Patients were defined as having new-onset impaired CA if pre-CPB COx < 0.3 and post-CPB COx > 0.3. Pre- and intraoperative factors were tested for independent association with impaired CA. Postoperative outcomes were compared between patients with normal and impaired CA.

RESULTS

In our 154 patients, 46(29.9%) developed new-onset impaired CA after CPB. Multivariable analysis revealed a prolonged low rScO2 (rScO2 < 55%) independently associated with onset of impaired CA, and receiver operating charactoristic curve showed a cutoff value at 40 min (sensitivity, 89.5%; specificity, 68.0%). Compared with normal CA patients, those with impaired CA showed a significantly higher rates of in-hospital mortality and postoperative complications.

CONCLUSIONS

Prolonged low rScO2 (rScO2 < 55%) during aortic arch surgery was closely related to onset of impaired CA. Impaired CA remained associated with the increased rates of postoperative complications and in-hospital mortality.

TRIAL REGISTRATION

ChiCTR1800014545 with registered date 20/01/2018.

Modification of pulmonary endarterectomy to prevent neurologic adverse events

PURPOSE

Neurologic adverse events (NAEs) are a major complication after pulmonary endarterectomy (PEA) performed under periods of deep hypothermic circulatory arrest (HCA) for chronic thromboembolic pulmonary hypertension. We modified the PEA strategy to prevent NAEs and evaluated the effectiveness of these modifications.

METHODS

We reviewed the surgical outcomes of 87 patients divided into the following three groups based on the surgical strategy used: group S (n = 49), periods of deep HCA with alpha-stat strategy; group M1 (n = 19), deep HCA with modifications of slower cooling and rewarming rates and the pH-stat strategy for cooling: and group M2 (n = 13), multiple short periods of moderate HCA.

RESULTS

PEA provided significant improvement of pulmonary hemodynamics in each group. Sixteen (29%) of the 49 group S patients suffered NAEs, associated with total circulatory arrest time (cutoff, 57 min) and Jamieson type I disease. The Group M1 and M2 patients did not suffer NAEs, although the group M1 patients had prolonged cardiopulmonary bypass (CPB) and more frequent respiratory failure.

CONCLUSIONS

NAEs were common after PEA performed under periods of deep HCA. The modified surgical strategy could decrease the risk of NAEs but increase the risk of respiratory failure. Multiple short periods of moderate HCA may be useful for patients at risk of NAEs.

Effects of Arterial Carbon Dioxide Tension on Cerebral and Somatic Regional Tissue Oxygenation and Blood Flow in Neonates After the Norwood Procedure With Deep Hypothermic Cardiopulmonary Bypass

Neonates undergoing the Norwood procedure for hypoplastic left heart syndrome are at higher risk of impaired systemic oxygen delivery with resultant brain, kidney, and intestinal ischemic injury, shock, and death. Complex developmental, anatomic, and treatment-related influences on cerebral and renal-somatic circulations make individualized treatment strategies physiologically attractive. Monitoring cerebral and renal circulations with near infrared spectroscopy can help drive rational therapeutic interventions. The primary aim of this study was to describe the differential effects of carbon dioxide tension on cerebral and renal circulations in neonates after the Norwood procedure. Using a prospectively-maintained database of postoperative physiologic and hemodynamic parameters, we analyzed the relationship between postoperative arterial carbon dioxide tension and tissue oxygen saturation and arteriovenous saturation difference in cerebral and renal regions, applying univariate and multivariate multilevel mixed regression techniques. Results were available from 7,644 h of data in 178 patients. Increases in arterial carbon dioxide tension were associated with increased cerebral and decreased renal oxygen saturation. Differential changes in arteriovenous saturation difference explained these effects. The cerebral circulation showed more carbon dioxide sensitivity in the early postoperative period, while sensitivity in the renal circulation increased over time. Multivariate models supported the univariate findings and defined complex time-dependent interactions presented graphically. The cerebral and renal circulations may compete for blood flow with critical limitations of cardiac output. The cerebral and renal-somatic beds have different circulatory control mechanisms that can be manipulated to change the distribution of cardiac output by altering the arterial carbon dioxide tension. Monitoring cerebral and renal circulations with near infrared spectroscopy can provide rational physiologic targets for individualized treatment.

Beneficial Effects of Receiving Johrei on General Health or Hypothermia Tendency

OBJECTIVES

Johrei is a type of biofield therapy that is said to bring physical and mental well-being to the recipient. This study sought to measure changes in body temperature and circulation resulting from Johrei treatment, for generally healthy subjects and for individuals with a tendency toward hypothermia.

PARTICIPANTS

A total of 199 qualified Johrei practitioners and 144 non-qualified operators provided Johrei and placebo treatments, respectively. Volunteer subjects -186 in general health and 39 with a hypothermia tendency - participated in this study to receive either or both of these treatments.

METHODS

Each subject was given a 10 min treatment daily by either a qualified practitioner or a non-qualified operator. The effects on subjects of receiving each treatment were compared by observing quantitative changes in blood flow and surface body temperature after a course of treatment.

RESULTS

A total of 107 healthy subjects were randomly assigned to the qualified-practitioner group or the non-qualified operator group. Treatment by qualified practitioners significantly enhanced blood flow and surface body temperature in the subjects' designated neck area compared to that in treatment by non-qualified operators. This finding was further corroborated by a comparative experiment in which each healthy subject was treated by both a qualified practitioner and a non-qualified operator. These results indicate that only the qualified-practitioner treatment increased the subject's-blood flow and surface body temperature. Similarly, in a comparative study of qualified-practitioner treatment against non-qualified-operator treatment, subjects tending toward hypothermia showed increased blood flow and elevated body temperature with only the authentic Johrei treatment.

Avoiding use of total circulatory arrest in the practice of congenital heart surgery

Deep hypothermic circulatory arrest (DHCA) technique has been an important armamentarium in the correction of congenital heart diseases. There have been many controversies and concerns associated with DHCA, particularly neurological damage. Selective ante grade cerebral perfusion (SACP) was introduced as an adjunct to DHCA with the objective of limiting the neurologic injury during aortic arch repairs. Over the past two decades, various aspects of cardiopulmonary bypass and DHCA have been studied and modified such as optimisation of flows, anti-inflammatory interventions, haematocrit, and temperature to improve neurologic outcomes. With the changes in practice of DHCA, outcomes have significantly improved but SACP intuitively appears attractive to offer better neuroprotection. The strategy of conduct of SACP is evolving and needs to be standardised for comparing outcomes. In this review we have discussed the various physiological and technical factors involved in conduct of SACP in paediatric cardiac surgery and outcomes with SACP.

Global REACH 2018: The influence of acute and chronic hypoxia on cerebral haemodynamics and related functional outcomes during cold and heat stress

KEY POINTS

Thermal and hypoxic stress commonly coexist in environmental, occupational and clinical settings, yet how the brain tolerates these multi-stressor environments is unknown Core cooling by 1.0°C reduced cerebral blood flow (CBF) by 20-30% and cerebral oxygen delivery (CDO₂ ) by 12-19% at sea level and high altitude, whereas core heating by 1.5°C did not reliably reduce CBF or CDO₂ Oxygen content in arterial blood was fully restored with acclimatisation to 4330 m, but concurrent cold stress reduced CBF and CDO₂ Gross indices of cognition were not impaired by any combination of thermal and hypoxic stress despite large reductions in CDO₂ Chronic hypoxia renders the brain susceptible to large reductions in oxygen delivery with concurrent cold stress, which might make monitoring core temperature more important in this context ABSTRACT: Real-world settings are composed of multiple environmental stressors, yet the majority of research in environmental physiology investigates these stressors in isolation. The brain is central in both behavioural and physiological responses to threatening stimuli and, given its tight metabolic and haemodynamic requirements, is particularly susceptible to environmental stress. We measured cerebral blood flow (CBF, duplex ultrasound), cerebral oxygen delivery (CDO₂ ), oesophageal temperature, and arterial blood gases during exposure to three commonly experienced environmental stressors - heat, cold and hypoxia - in isolation, and in combination. Twelve healthy male subjects (27 ± 11 years) underwent core cooling by 1.0°C and core heating by 1.5°C in randomised order at sea level; acute hypoxia ( P ET , O 2  = 50 mm Hg) was imposed at baseline and at each thermal extreme. Core cooling and heating protocols were repeated after 16 ± 4 days residing at 4330 m to investigate any interactions with high altitude acclimatisation. Cold stress decreased CBF by 20-30% and CDO₂ by 12-19% (both P < 0.01) irrespective of altitude, whereas heating did not reliably change either CBF or CDO₂ (both P > 0.08). The increases in CBF with acute hypoxia during thermal stress were appropriate to maintain CDO₂ at normothermic, normoxic values. Reaction time was faster and slower by 6-9% with heating and cooling, respectively (both P < 0.01), but central (brain) processes were not impaired by any combination of environmental stressors. These findings highlight the powerful influence of core cooling in reducing CDO₂ . Despite these large reductions in CDO₂ with cold stress, gross indices of cognition remained stable.

Static cerebrovascular pressure autoregulation remains intact during deep hypothermia

BACKGROUND

Clinical studies measuring cerebral blood flow in infants during deep hypothermia have demonstrated diminished cerebrovascular pressure autoregulation. The coexistence of hypotension in these cohorts confounds the conclusion that deep hypothermia impairs cerebrovascular pressure autoregulation.

AIM

We sought to compare the lower limit of autoregulation and the static rate of autoregulation between normothermic and hypothermic piglets.

METHODS

Twenty anesthetized neonatal piglets (5-7 days old; 10 normothermic and 10 hypothermic to 20°C) had continuous measurements of cortical red cell flux using laser Doppler flowmetry, while hemorrhagic hypotension was induced without cardiopulmonary bypass. Lower limit of autoregulation was determined for each subject using piecewise regression and SRoR was determined above and below each lower limit of autoregulation as (%change cerebrovascular resistance/%change cerebral perfusion pressure).

RESULTS

The estimated difference in lower limit of autoregulation was 1.4 mm Hg (lower in the hypothermic piglets; 95% C.I. -10 to 14 mm Hg; P=0.6). The median lower limit of autoregulation in the normothermic group was 39 mm Hg [IQR 38-51] vs 35 mm Hg [31-50] in the hypothermic group. Intact steady-state pressure autoregulation was defined as static rate of autoregulation >0.5 and was demonstrated in all normothermic subjects (static rate of autoregulation=0.72 [0.65-0.87]) and in 9/10 of the hypothermic subjects (static rate of autoregulation=0.65 [0.52-0.87]). This difference in static rate of autoregulation of 0.06 (95% C.I. -0.3 to 0.1) was not significant (P=0.4).

CONCLUSION

Intact steady-state cerebrovascular pressure autoregulation is demonstrated in a swine model of profound hypothermia. Lower limit of autoregulation and static rate of autoregulation were similar in hypothermic and normothermic subjects.

Non-invasive Assessment of Cerebral Blood Flow and Oxygen Metabolism in Neonates during Hypothermic Cardiopulmonary Bypass: Feasibility and Clinical Implications

The neonatal brain is extremely vulnerable to injury during periods of hypoxia and/or ischemia. Risk of brain injury is increased during neonatal cardiac surgery, where pre-existing hemodynamic instability and metabolic abnormalities are combined with long periods of low cerebral blood flow and/or circulatory arrest. Our understanding of events associated with cerebral hypoxia-ischemia during cardiopulmonary bypass (CPB) remains limited, largely due to inadequate tools to quantify cerebral oxygen delivery and consumption non-invasively and in real-time. This pilot study aims to evaluate cerebral blood flow (CBF) and oxygen metabolism (CMRO₂) intraoperatively in neonates by combining two novel non-invasive optical techniques: frequency-domain near-infrared spectroscopy (FD-NIRS) and diffuse correlation spectroscopy (DCS). CBF and CMRO₂ were quantified before, during and after deep hypothermic cardiopulmonary bypass (CPB) in nine neonates. Our results show significantly decreased CBF and CMRO₂ during hypothermic CPB. More interestingly, a change of coupling between both variables is observed during deep hypothermic CPB in all subjects. Our results are consistent with previous studies using invasive techniques, supporting the concept of FD-NIRS/DCS as a promising technology to monitor cerebral physiology in neonates providing the potential for individual optimization of surgical management.

The Postoperative Care of Adult Patients Exposed to Deep Hypothermic Circulatory Arrest

Deep hypothermic circulatory arrest with cardiopulmonary bypass is indicated for complex surgical operations in adult patients involving the aortic arch, thoracoabdominal aorta, cerebral vasculature, and tumors extending into the vena cava and heart. Understanding the principles of ischemic-reperfusion injury and the effects of hypothermia in attenuating this process is fundamental to the delivery of effective postoperative care. Neurologic injury is the most troublesome adverse effect after the use of deep hypothermic circulatory arrest and cardiopulmonary bypass, presenting as either a transient neurologic deficit (5.9% to 28.1%) or an irreversible neurologic injury (1.8% to 13.6%). In patients with neurological injury, early postoperative mortality is markedly increased (18.2%), and for those patients that survive, long-term cognitive disability is still evident 6 months later. Early postoperative support of organ function, along with timely diagnosis and treatment of organ injury, is essential in minimizing perioperative morbidity, particularly neurologic morbidity. Meticulous management of fluids, maintaining stable cardiovascular hemodynamics with particular attention to systolic blood pressure, optimizing oxygen delivery, limiting ventilatorassociated lung injury, intensive insulin therapy for control of blood glucose levels, and avoidance of hyperthermia are essential in limiting organ injury and reducing perioperative morbidity and mortality.

Neurologic Injury Associated with Rewarming from Hypothermia: Is Mild Hypothermia on Bypass Better than Deep Hypothermic Circulatory Arrest?

Many known risk factors for adverse cardiovascular and neurological outcomes in children with congenital heart defects (CHD) are not modifiable; however, the temperature and blood flow during cardiopulmonary bypass (CPB), are two risk factors, which may be altered in an attempt to improve long-term neurological outcomes. Deep hypothermic circulatory arrest, traditionally used for aortic arch repair, has been associated with short-term and long-term neurologic sequelae. Therefore, there is a rising interest in using moderate hypothermia with selective antegrade cerebral blood flow on CPB during aortic arch repair. Rewarming from moderate-to-deep hypothermia has been shown to be associated with neuronal injury, neuroinflammation, and loss of cerebrovascular autoregulation. A significantly lesser degree of rewarming is required following mild (33-35°C) hypothermia as compared with moderate (28-32°C), deep (21-27°C), and profound (less than 20°C) hypothermia. Therefore, we believe that mild hypothermia is associated with a lower risk of rewarming-induced neurologic injury. We hypothesize that mild hypothermia with selective antegrade cerebral perfusion during CPB for neonatal aortic arch repair would be associated with improved neurologic outcome.

Near-infrared spectroscopy: exposing the dark (venous) side of the circulation

The safety of anesthesia has improved greatly in the past three decades. Standard perioperative monitoring, including pulse oximetry, has practically eliminated unrecognized arterial hypoxia as a cause for perioperative injury. However, most anesthesia-related cardiac arrests in children are now cardiovascular in origin, and standard monitoring is unable to detect many circulatory abnormalities. Near-infrared spectroscopy provides noninvasive continuous access to the venous side of regional circulations that can approximate organ-specific and global measures to facilitate the detection of circulatory abnormalities and drive goal-directed interventions to reduce end-organ ischemic injury.

Effects of phentolamine infusion during selective cerebral perfusion in neonatal piglets

BACKGROUND

An optimal selective cerebral perfusion protocol in pediatric cardiac surgery is unknown. Phentolamine is frequently used in pediatric cardiopulmonary bypass. We sought to determine the effects of continuous phentolamine infusion during selective cerebral perfusion.

METHODS

Twenty-seven neonatal piglets (3.38 ± 0.32 kg) were randomly assigned to 3 groups; sham (n = 7, anesthesia alone, no surgery or bypass), control (n = 10, saline infusion), or experimental (n = 10, phentolamine infusion 0.1 mg/kg per hour). Animals underwent 90 minutes of selective cerebral perfusion. Cerebral vascular resistance index (CVRI) and metabolic rate of oxygen (CMRO2) were determined every 15 minutes. Standardized sections of hippocampus, basal ganglia, and neo-cortex were obtained. Tissue samples were stained for caspase-3 and analyzed for positive apoptotic cell count. Data were analyzed with repeated measures and one-way analysis of variance.

RESULTS

The CVRI tended to increase over time in the control group and decrease over time in the experimental group, but difference was not statically significant (0.46 ± 0.24 vs 0.39 ± 0.10 mm Hg × min × kg(2/3)/mL, p = 0.15). Mean CMRO2 was higher in the control group compared with the experimental group (0.90 ± 0.27 vs 0.59 ± 0.12 mLO2/min × kg(2/3), p = 0.005) and decreased over time in both groups. The percentage of caspase-3 positive cells was significantly different among regions (hippocampus = 16.9 ± 8.8; basal ganglia = 14.6 ± 7.5; neocortex = 10.8 ± 6.3; p < 0.0001) but not significantly different among sham (11.8% ± 2.68%), control (14.4% ± 2.24%), and experimental (15.5% ± 2.24%) groups.

CONCLUSIONS

A continuous infusion of phentolamine during selective cerebral perfusion significantly decreases CMRO2 and tends to decrease CVRI when compared with control. At the dose studied and at the time of tissue sampling, phentolamine does not appear to decrease apoptosis during or early after selective cerebral perfusion.

Early postoperative changes in cerebral oxygen metabolism following neonatal cardiac surgery: effects of surgical duration

OBJECTIVE

The early postoperative period following neonatal cardiac surgery is a time of increased risk for brain injury, yet the mechanisms underlying this risk are unknown. To understand these risks more completely, we quantified changes in postoperative cerebral metabolic rate of oxygen (CMRO(2)), oxygen extraction fraction (OEF), and cerebral blood flow (CBF) compared with preoperative levels by using noninvasive optical modalities.

METHODS

Diffuse optical spectroscopy and diffuse correlation spectroscopy were used concurrently to derive cerebral blood flow and oxygen utilization postoperatively for 12 hours. Relative changes in CMRO(2), OEF, and CBF were quantified with reference to preoperative data. A mixed-effect model was used to investigate the influence of total support time and deep hypothermic circulatory arrest duration on relative changes in CMRO(2), OEF, and CBF.

RESULTS

Relative changes in CMRO(2), OEF, and CBF were assessed in 36 patients, 21 with single-ventricle defects and 15 with 2-ventricle defects. Among patients with single-ventricle lesions, deep hypothermic circulatory arrest duration did not affect relative changes in CMRO(2), CBF, or OEF (P > .05). Among 2-ventricle patients, total support time was not a significant predictor of relative changes in CMRO(2) or CBF (P > .05), although longer total support time was associated significantly with greater increases in relative change of postoperative OEF (P = .008).

CONCLUSIONS

Noninvasive diffuse optical techniques were used to quantify postoperative relative changes in CMRO(2), CBF, and OEF for the first time in this observational pilot study. Pilot data suggest that surgical duration does not account for observed variability in the relative change in CMRO(2), and that more comprehensive clinical studies using the new technology are feasible and warranted to elucidate these issues further.

Neurocognitive monitoring and care during pediatric cardiopulmonary bypass-current and future directions

Neurologic injury in patients with congenital heart disease remains an important source of morbidity and mortality. Advances in surgical repair and perioperative management have resulted in longer life expectancies for these patients. Current practice and research must focus on identifying treatable risk factors for neurocognitive dysfunction, advancing methods for perioperative neuromonitoring, and refining treatment and care of the congenital heart patient with potential neurologic injury. Techniques for neuromonitoring and future directions will be discussed.

Relationship of intraoperative cerebral oxygen saturation to neurodevelopmental outcome and brain magnetic resonance imaging at 1 year of age in infants undergoing biventricular repair

BACKGROUND

Near-infrared spectroscopy monitoring of cerebral oxygen saturation (rSo(2)) has become routine in many centers, but no studies have reported the relationship of intraoperative near-infrared spectroscopy to long-term neurodevelopmental outcomes after cardiac surgery.

METHODS AND RESULTS

Of 104 infants undergoing biventricular repair without aortic arch reconstruction, 89 (86%) returned for neurodevelopmental testing at 1 year of age. The primary near-infrared spectroscopy variable was the integrated rSo(2) (area under the curve) for rSo(2) <or=45%; secondary variables were the average and minimum rSo(2) by perfusion phase and at specific time points. Psychomotor and mental development indexes of the Bayley scales, head circumference, neurological examination, and abnormalities on brain magnetic resonance imaging did not differ between subjects according to a threshold level for rSo(2) of 45%. Lower Psychomotor Development Index scores were modestly associated with lower average (r=0.23, P=0.03) and minimum (r=0.22, P=0.04) rSo(2) during the 60-minute period after cardiopulmonary bypass but not with other perfusion phases. Hemosiderin foci on brain magnetic resonance imaging were associated with lower average rSo(2) from postinduction to 60 minutes post cardiopulmonary bypass (71+/-10% versus 78+/-6%, P=0.01) and with lower average rSO(2) during the rewarming phase (72+/-12% versus 83+/-9%, P=.003) and during the 60-minute period following cardiopulmonary bypass (65+/-11% versus 75+/-10%, P=0.009). In regression analyses that adjusted for age <or=30 days, Psychomotor Development Index score (P=0.02) and brain hemosiderin (P=0.04) remained significantly associated with rSo(2) during the 60-minute period following cardiopulmonary bypass.

CONCLUSIONS

Perioperative periods of diminished cerebral oxygen delivery, as indicated by rSo(2), are associated with 1-year Psychomotor Development Index and brain magnetic resonance imaging abnormalities among infants undergoing reparative heart surgery. Clinical Trial Registration- URL: http://clinicaltrials.gov. Unique identifier: NCT00006183.

Body and brain temperature coupling: the critical role of cerebral blood flow

Direct measurements of deep-brain and body-core temperature were performed on rats to determine the influence of cerebral blood flow (CBF) on brain temperature regulation under static and dynamic conditions. Static changes of CBF were achieved using different anesthetics (chloral hydrate, CH; alpha-chloralose, alphaCS; and isoflurane, IF) with alphaCS causing larger decreases in CBF than CH and IF; dynamic changes were achieved by inducing transient hypercapnia (5% CO(2) in 40% O(2) and 55% N(2)). Initial deep-brain/body-core temperature differentials were anesthetic-type dependent with the largest differential observed with rats under alphaCS anesthesia (ca. 2 degrees C). Hypercapnia induction raised rat brain temperature under all three anesthesia regimes, but by different anesthetic-dependent amounts correlated with the initial differentials--alphaCS anesthesia resulted in the largest brain temperature increase (0.32 +/- 0.08 degrees C), while CH and IF anesthesia lead to smaller increases (0.12 +/- 0.03 and 0.16 +/- 0.05 degrees C, respectively). The characteristic temperature transition time for the hypercapnia-induced temperature increase was 2-3 min under CH and IF anesthesia and approximately 4 min under alphaCS anesthesia. We conclude that both, the deep-brain/body-core temperature differential and the characteristic temperature transition time correlate with CBF: a lower CBF promotes higher deep-brain/body-core temperature differentials and, upon hypercapnia challenge, longer characteristic transition times to increased temperatures.

Treatment of the term newborn with brain injury: simplicity as the mother of invention

Neonatal brain injury remains a common cause of developmental disability, despite tremendously enhanced obstetrical and neonatal care. The timing of brain injury occurs throughout gestation, labor, and delivery, providing an evolving form of brain injury and a moving target for therapeutic intervention. Nonetheless, markedly improved methods are available to identify those infants injured at birth, via clinical presentation with neonatal encephalopathy and neuroimaging techniques. Postischemic hypothermia has been shown to be of tremendous clinical promise in several completed and ongoing trials. As part of this approach to the treatment of the newborn, other parameters of physiologic homeostasis can and should be attended to, with strong animal and clinical evidence that their correction will have dramatic influence on the outcome of the newborn infant. This review addresses aspects of newborn care to which we can direct our attention currently, and which should result in a safe and efficacious improvement in the prognosis of the newborn with neonatal encephalopathy.

Acute brain injury and therapeutic hypothermia in the PICU: A rehabilitation perspective

Acquired brain injury from traumatic brain injury, cardiac arrest (CA), stroke, and central nervous system infection is a leading cause of morbidity and mortality in the pediatric population and reason for admission to inpatient rehabilitation. Therapeutic hypothermia is the only intervention shown to have efficacy from bench to bedside in improving neurological outcome after birth asphyxia and adult arrhythmia-induced CA, thought to be due to its multiple mechanisms of action. Research to determine if therapeutic hypothermia should be applied to other causes of brain injury and how to best apply it is underway in children and adults. Changes in clinical practice in the hospitalized brain-injured child may have effects on rehabilitation referral practices, goals and strategies of therapies offered, and may increase the degree of complex medical problems seen in children referred to inpatient rehabilitation.

Neurologic monitoring on cardiopulmonary bypass: what are we obligated to do?

Improving survival from congenital cardiac repairs using cardiopulmonary bypass has appropriately shifted focus to neurologic outcomes. Hypoxic-ischemic mechanisms are the major cause of neurologic injury in neonatal cardiac surgery, and modifications of techniques of cardiopulmonary bypass can affect organ oxygen delivery and the propensity to injury both during and after surgery. Through successive refinements in the techniques of cardiopulmonary bypass, the risk factors for hypoxic-ischemic injury have been reduced, but not eliminated. The application of specific monitoring to enhance detection of hypoxic conditions associated with neurologic injury would both allow intervention on individual patients and drive refinements in strategies to further reduce risk. Specific neurologic monitoring techniques that can be used during cardiopulmonary bypass include near-infrared spectroscopy, transcranial Doppler ultrasonography, and electroencephalographic techniques. Of these, only near-infrared spectroscopy provides a continuous quantitative signal of the physiologic variable most related to injury and most amenable to intervention. This review will advocate wide adoption of near-infrared spectroscopy monitoring throughout the perioperative period, to enhance detection of hypoxic conditions and to drive patient-specific interventions.

Capacity of brain cooling via ventilating oxygen at low temperature over respiratory tract

Cerebral hypothermia is a rather useful way to improve outcome after brain injury. In this paper, the capacity of cooling oxygen ventilation (COV) during cerebral circulation arrest was theoretically evaluated. A transient two-dimensional mathematical model for the heat and water vapor transport through the respiratory tract of human body was established to predict the temperature response of the brain tissue. Calculation results indicate that COV will help in some extent to reduce the inner temperature of patient brain subject to cerebral circulation arrest. However, its capacity on lowering the deep brain temperature is very limited. Therefore more powerful cooling strategies should be investigated to realize an efficient cooling on the target cerebral tissue.

Aortic Arch Reconstruction: Safety of Moderate Hypothermia and Antegrade Cerebral Perfusion During Systemic Circulatory Arrest

BACKGROUND AND AIM

The ideal strategy for cerebral protection during aortic arch (AA) reconstructive surgery remains undefined. Antegrade cerebral perfusion (ACP) during systemic circulatory arrest (SCA) may provide superior results; however, optimal systemic temperature is undetermined. Our objective was to determine whether "deep" hypothermia is necessary during ACP with SCA, and whether the degree of hypothermia is associated with neurologic outcomes postoperatively.

METHODS

Retrospective series of 72 consecutive patients (aged 65.9 +/- 3.2 years) who underwent AA reconstructive surgery at Vancouver General Hospital using a cerebral protection strategy of ACP with SCA between December 1995 and December 2002. Patients were divided into two groups according to lowest systemic temperature: <22 degrees C (n = 52) and > or =22 degrees C (n = 20).

RESULTS

ACP was via right axillary or innominate artery, +/- left common carotid cannulation. Median SCA time with ACP was not different between groups. There were four hospital deaths (5.6%) (three from the <22 degrees C group). Eight patients (11.2%) had major neurologic injuries (seven from the <22 degrees C group): 4 (5.6%) permanent (1 fatal) and 4 (5.6%) temporary. There was a trend toward a significantly higher incidence of delirium in the <22 degrees C group than the > or =22 degrees C group (30.8 vs 10.0%, respectively, p = 0.07).

CONCLUSIONS

In our experience, SCA with ACP was a safe technique for AA reconstructive surgery. The observation of a larger number of major neurologic injuries, and a trend toward a higher incidence of delirium in the <22 degrees C group, suggests that systemic temperatures below 22 degrees C may not be necessary and may be associated with a higher incidence of neurologic injury when using ACP during SCA.

Systemic venous oxygen saturation after the Norwood procedure and childhood neurodevelopmental outcome

OBJECTIVE

Neonates with hypoplastic left heart syndrome have impaired systemic oxygen delivery and also have a high risk of hypoxic ischemic brain injury with resultant neurodevelopmental impairment. We hypothesized that decreased postoperative oxygen delivery, as measured on the basis of systemic venous oxyhemoglobin saturation, would be related to persistent neurodevelopmental abnormality assessed in childhood.

METHODS

Early perioperative hemodynamic data, prospectively acquired from neonates undergoing staged palliation of hypoplastic left heart syndrome by using deep hypothermic circulatory arrest with uniform perioperative management, were tested for relationship to later neurodevelopmental outcome assessed at age 4 years.

RESULTS

Complete hemodynamic and neurodevelopmental data were available in 13 patients aged 7 +/- 8 days at the time of the Norwood procedure and aged 4.5 +/- 0.7 years at follow-up assessment. The subjects scored significantly below the population mean for motor, visual-motor integration, and composite neurodevelopmental outcomes. The 5 (38%) patients with abnormal outcomes had significantly lower postoperative systemic venous oxygen saturation values than those with normal outcomes (46% +/- 8% vs 56% +/- 6%, P = .024). Standard hemodynamic parameters did not differentiate patient outcomes. The risk of abnormal outcome increased with increasing time at a systemic venous oxygen saturation of less than 40% (P < .001). A multivariate model of deep hypothermic circulatory arrest time, systemic venous oxygen saturation, blood pressure, and carbon dioxide tension accounted for 79% of the observed variance (P < .001).

CONCLUSIONS

Decreased systemic oxygen delivery in the neonatal postoperative period is associated with hypoxic-ischemic brain injury and childhood neurodevelopmental abnormality. Measures of systemic oxygen delivery should be used to guide perioperative strategies to reduce the risk of hypoxic-ischemic brain injury.

Congenital Heart Surgery 2005: The brain: It’s the heart of the matter

Operative mortality after repair of even the most complex congenital heart lesions has become rare. As such, the gaze of the surgical team has been diverted beyond that of early survival to focus on decreasing early and late morbidity. Important and concerning information is accumulating delineating the vulnerability of the neonatal brain to injury as the result of congenital heart disease and/or the techniques employed to correct the lesions. For many years the prevention of neurologic injury associated with congenital heart surgery has concentrated on "unraveling" the mysteries of the deleterious effects of intentional brain ischemia (in the form of deep hypothermic circulatory arrest) and developing methods to interrupt the pathway of irreversible injury. In the late 1990s, alternative perfusion techniques were developed to minimize or theoretically avoid the use of deep hypothermic circulatory arrest where it was once thought to be mandatory. Simultaneously, the rather routine use of noninvasive, real-time, neurologic monitoring has provided surgical teams the opportunity to intervene and prevent brain injury , thus eliminating the historic reliance on postoperative surrogate markers to define the presence of brain injury. It is yet undetermined whether these strategies will translate into improved short- and long-term neurologic outcome. Common to all surgical disciplines is a trend that as mortality decreases for a particular disease process, focus is adjusted, and refinements in treatment protocols are designed to minimize morbidity of the disease and its treatment. This natural refining process of a discipline's maturation is increasingly present in the field of congenital heart surgery.

Regional low-flow perfusion improves neurologic outcome compared with deep hypothermic circulatory arrest in neonatal piglets

BACKGROUND

Regional low-flow perfusion is an alternative to deep hypothermic circulatory arrest, but whether regional low-flow perfusion improves neurologic outcome after deep hypothermic circulatory arrest in neonates remains unknown. We tested neurologic recovery after regional low-flow perfusion compared with deep hypothermic circulatory arrest in a neonatal piglet model.

METHODS

Sixteen neonatal piglets underwent cardiopulmonary bypass, were randomized to 90 minutes of deep hypothermic circulatory arrest or regional low-flow perfusion (10 mL.kg(-1).min(-1)) at 18 degrees C, and survived for 1 week. Standardized neurobehavioral scores were obtained on postoperative days 1, 3, and 7 (0 = no deficit to 90 = brain death). Histopathologic scores were determined on the basis of the percentage of injured and apoptotic neurons in the neocortex and hippocampus by hematoxylin and eosin and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick-end labeling (0 = no injury to 4 = diffuse injury). Differences between groups were tested by using the Wilcoxon rank sum test, and results are listed as medians within a range.

RESULTS

There were no significant differences between groups during cardiopulmonary bypass. Postoperative neurobehavioral scores were abnormal in 25% (2/8) of the regional low-flow perfusion animals versus 88% (7/8) of controls. Regional low-flow perfusion animals had significantly less neurologic injury compared with controls on postoperative day 1 (0.00 [range, 0-5] vs 12.5 [range, 0-52]; P <.008). There was a trend for less severe injury in the regional low-flow perfusion group (2.0 [range, 1-4] vs 0.0 [range, 0-50]; P =.08) on hematoxylin and eosin. The degree of apoptosis was significantly less in the regional low-flow perfusion group (0.0 [range, 0-1] vs 2.5 [range, 0-4]; P =.03).

CONCLUSIONS

Regional low-flow perfusion decreases neuronal injury and improves early postoperative neurologic function after deep hypothermic circulatory arrest in neonatal piglets.

Practice patterns in neonatal cardiopulmonary bypass

This article reviews practice patterns of numerous congenital heart surgeons, as collected from surveys (the Congenital Heart Surgeon's Society) and from audience response at the American Association of Thoracic Surgery (May 2003) and the Society of Thoracic Surgeons (January 2004). The information shows that there are numerous practice patterns, with unanimity only in the use of cardioplegia, although the delivery, type, and timing of doses varies. Hypothermic circulatory arrest continues to be used by the majority of congenital heart surgeons, although strategies for delivery have evolved in a way that reflects research contributions over the past decade.

Hypothermia and stroke: the pathophysiological background

Hypothermia to mitigate ischemic brain tissue damage has a history of about six decades. Both in clinical and experimental studies of hypothermia, two principal arbitrary patterns of core temperature lowering have been defined: mild (32-35 degrees C) and moderate hypothermia (30-33 degrees C). The neuroprotective effectiveness of postischemic hypothermia is typically viewed with skepticism because of conflicting experimental data. The questions to be resolved include the: (i) postischemic delay; (ii) depth; and (iii) duration of hypothermia. However, more recent experimental data have revealed that a protected reduction in brain temperature can provide sustained behavioral and histological neuroprotection, especially when thermoregulatory responses are suppressed by sedation or anesthesia. Conversely, brief or very mild hypothermia may only delay neuronal damage. Accordingly, protracted hypothermia of 32-34 degrees C may be beneficial following acute cerebral ischemia. But the pathophysiological mechanism of this protection remains yet unclear. Although reduction of metabolism could explain protection by deep hypothermia, it does not explain the robust protection connected with mild hypothermia. A thorough understanding of the experimental data of postischemic hypothermia would lead to a more selective and effective clinical therapy. For this reason, we here summarize recent experimental data on the application of hypothermia in cerebral ischemia, discuss problems to be solved in the experimental field, and try to draw parallels to therapeutic potentials and limitations.

Current strategies for optimizing the use of cardiopulmonary bypass in neonates and infants

The use of cardiopulmonary bypass is still necessary for the repair of many congenital cardiac defects. However, exposure to cardiopulmonary bypass can still lead to major morbidity and sometimes mortality, especially in neonates and infants, despite a perfect surgical repair. Various research-based strategies have been used to minimize some of the complications related to cardiopulmonary bypass, including the systemic inflammatory response, hemodilution, and transfusion requirement. This overview provides some of the strategies that we use in our practice in applying cardiopulmonary bypass in the repair of congenital cardiac defects in neonates and infants.

Numerical studies on the effect of lowering temperature on the oxygen transport during brain hypothermia resuscitation

There have been arguments about the advantage and shortcoming of hypothermia on the brain resuscitation during circulation arrest. People usually accepted that hypothermia may decrease the cerebral oxygen demands, which is beneficial for the patient to sustain longer time when subjected to a hypoxia. However, there are also quite a few disputes claiming that the blood viscosity would increase with the reduction of temperature, which may lead to an increase of cerebral vascular resistance and thus worsen the hypoxia state. To resolve this critical issue, a heat transfer model was established to characterize the thermal response of brain tissue during hypothermia resuscitation. Combined with this model, a compartmental model taking account of the temperature effect was further developed to analyze the transient oxygen partial pressure (PO(2)) distribution over the successive branches of the vascular network during circulation arrest. Using the morphological and physiological data of a sheep brain, effects of lowering temperature on the oxygen consumption dynamics were studied. Calculations indicated that the lower the temperature, the slower the decreasing rate for the PO(2). Although immediately lowering the brain temperature may induce an evident increase in blood viscosity and subsequently a decrease in blood flow rate, which is responsible for oxygen delivery, it seems to always result in a monotonic increase of PO(2). The results show a good qualitative accord with the experimental data. They also present better understanding on the transient oxygen transport in brain hypothermia during circulation arrest.

An MRI Study of Neurological Injury Before and After Congenital Heart Surgery

Background Neuorological deficits are observed in patients with congenital heart disease (CHD) before and after neonatal surgery, the etiology being multifactorial. To understand the impact of preoperative events and to characterize the evaluation of neurological injury, we performed serial magnetic resonance imaging (MRI) studies of the brain in a cohort of neonates undergoing open-heart surgery.

Methods and Results Twenty-four term neonates with CHD were studied prospectively with brain MRI: before surgery, within 2 weeks of surgery, and several months after surgery. Preoperative MRI examinations showed periventricular leukomalacia (PVL) in 4 patients (16%) and infarct in 2 subjects (8%). MR spectroscopy was performed in 19 subjects preoperatively and revealed elevated brain lactate in 53%. An early postoperative MRI (n=21) identified new PVL in 48%, new infarct in 19%, and new parenchymal hemorrhage in 33%. New lesions or worsening of preoperative lesions occurred in 67% of subjects. No patient- or procedure-related factors for the development of early postoperative lesions were identified. A late postoperative MRI (n=17) demonstrated resolution of early lesions in 8 and mild cerebral atrophy in 2.

Conclusions Mild ischemic lesions, primarily in the form of PVL, occur in a number of neonates with CHD before surgery and >50% of patients postoperatively. Resolution of these lesions is common 4 to 6 months after surgery. Longer-term follow-up is needed to determine the significance of perioperative ischemic lesions on functional outcome.

Global and regional cerebral blood flow in neonatal piglets undergoing pulsatile cardiopulmonary bypass with continuous perfusion at 25 degrees C and circulatory arrest at 18 degrees C

To investigate the influence of hypothermic cardiopulmonary bypass (HCPB) at 25 degrees C and circulatory arrest at 18 degrees C on the global and regional cerebral blood flow (CBF) during pulsatile perfusion, we performed the following studies in a neonatal piglet model. Using a pediatric physiologic pulsatile pump, we subjected six piglets to deep hypothermic circulatory arrest (DHCA) and six other piglets to HCPB. The DHCA group underwent hypothermia for 25 min, DHCA for 60min, cold reperfusion for 10 min, and rewarming for 40 min. The HCPB group underwent 15 min of cooling, followed by 60 min of HCPB, 10min of cold reperfusion, and 30 min of rewarming. The following variables remained constant in both groups: pump flow (150 ml/kg/min), pump rate (150 bpm), and stroke volume (1 ml/kg). During the 60-min aortic crossclamp period, the temperature was kept at 18 degrees C for DHCA and at 25 degrees C for HCPB. The global and regional CBF (ml/100g/min) was assessed with radiolabeled microspheres. The CBF was 48% lower during deep hypothermia at 18degrees C (before DHCA) than during hypothermia at 25 degrees C (55.2 +/- 14.3ml/100g/min vs 106.4 +/- 19.7 ml/100 g/min; p < 0.05). After rewarming, the global CBF was 45% lower in the DHCA group than in the HCPB group 48.3 +/- 18.1 ml/100g/min vs (87 +/- 35.9ml/100g/min; p < 0.05). Fifteen minutes after the termination of CPB, the global CBF was only 25% lower in the DHCA group than in the HCPB group (42.2 +/- 20.7 ml/100 g/min vs 56.4 +/- 25.8ml/100g/min; p = NS). In the right and left hemispheres, cerebellum, basal ganglia, and brain stem, blood flow resembled the global CBF. In conclusion, both HCPB and DHCA significantly decrease the regional and global CBF during CPB. Unlike HCPB, DHCA has a continued negative impact on the CBF after rewarming. However, 15 min after the end of CPB, there are no significant intergroup differences in the CBF.

Dynamic changes in cerebral oxygenation related to deep hypothermia and circulatory arrest evaluated by near-infrared spectroscopy

BACKGROUND

Total circulatory arrest in deep hypothermia, which is used in corrective surgery of complex cardiovascular malformations, has been said to cause brain injury. Near-infrared spectroscopy (NIRS) is a new non-invasive method that potentially monitors changes in cerebral oxygenation and tissue oxygen utilisation. The aim of this experimental study in rabbits was to evaluate the change in intravascular and intracellular oxygenation patterns during cooling, deep hypothermic circulatory arrest and rewarming using a commercially available NIRS-cerebral oximeter.

METHODS

Ten New Zealand White male rabbits (weight, 3.1+/-0.25 kg BW) were included in this study. All animals underwent cardiopulmonary bypass (CPB), cooling to a rectal temperature below 15 degrees C, 60 min of deep hypothermic circulatory arrest (DHCA) followed by reperfusion and rewarming. Cerebral oxyhaemoglobin (HbO2), deoxyhaemoglobin (HHb) and cytochrome oxidase aa3 (CytOxaa3) concentrations were continuously measured during the entire procedure using the Cerebral RedOx Monitor 2020 (Criticon cerebral redox monitor 2020, Johnson & Johnson Medical). Total haemoglobin concentration (tHb) and regional cerebral oxygen saturation (rSO2) values were calculated by integrated algorithm.

RESULTS

In all animals an initial increase of oxygenated haemoglobin (HbO2, rSO2) and a fall in deoxygenated haemoglobin (HHb) were found during cooling on bypass. A slight decrease in CytOxaa3 signal was observed in response to initial cooling. Variation in intravascular haemoglobin oxygenation parameters (HbO2, HHb) were related to haemodynamic changes associated with fluid loading, initiation and termination of CPB, bypass flow rate and cooling and rewarming. When the pump flow was stopped all NIRS parameters, except the HHb value, decreased precipitously during the DHCA-period (P<0.01). After reperfusion and rewarming, all haemoglobin oxygen saturation readings returned nearly to pre-CPB levels (P=0.09), but the CytOxaa3 was still significantly lower than the pre-CPB levels (P<0.05).

CONCLUSION

The change in the NIRS-derived haemoglobin oxygenation parameters may reflect physiological changes in systemic and cerebral haemodynamics. CytOxaa3 values may represent related effects on cellular oxygenation. Thus, continuous, real-time NIRS-monitoring may identify critical periods with inadequate brain tissue oxygenation, particularly during DHCA. The neurological implications of the observed changes in NIRS oxygenation parameters, however, require further quantitative morphological evaluation of the brain in animals surviving a longer reperfusion and observation period.

Coupling of cerebral blood flow and oxygen metabolism in infant pigs during selective brain hypothermia

Studies documenting the cerebral hemodynamic consequences of selective brain hypothermia (SBH) have yielded conflicting data. Therefore, the authors have studied the effect of SBH on the relation of cerebral blood flow (CBF) and CMRO2 in the forebrain of pigs. Selective brain hypothermia was induced in seven juvenile pigs by bicarotid perfusion of the head with extracorporally cooled blood. Cooling and stepwise rewarming of the brain to a Tbrain of 38 degrees C, 25 degrees C, 30 degrees C, and 38 degrees C at normothermic Ttrunk (38 degrees C) decreased CBF from 71 + 12 mL 100 g(-1) min(-1) at normothermia to 26+/-3 mL 100 g(-1) min(-1) and 40+/-12 mL 100 g(-1) min(-1) at a Tbrain of 25 degrees C and 30 degrees C, respectively. The decrease of CMRO2 during cooling of the brain to a Tbrain of 25 degrees C resulted in a mean Q10 of 2.8. The ratio between CBF and CMRO2 was increased at a Tbrain of 25 degrees C indicating a change in coupling of flow and metabolism. Despite this change, regional perfusion remained coupled to regional temperatures during deep cerebral hypothermia. The data demonstrate that SBH decreases CBF and oxygen metabolism to a degree comparable with the cerebrovascular and metabolic effects of systemic hypothermia. The authors conclude that, irrespective of a change in coupling of blood flow and metabolism during deep cerebral hypothermia, cerebral metabolism is a main determinant of CBF during SBH.

Induced hypothermia in experimental pneumococcal meningitis

Pneumococcal meningitis resulting from Streptococcus pneumoniae has a death rate of 28% in adults. In severe head injury and stroke, inflammatory changes and intracranial hypertension are improved by induced hypothermia, which also is neuroprotective. We hypothesized that moderate hypothermia ameliorates inflammatory changes in experimental pneumococcal meningitis. Wistar rats were cooled systemically, and meningitis was induced by pneumococcal cell wall components. The increase of regional cerebral blood flow in the meningitis animals was blocked by hypothermia at 6 hours. The reduction of intracranial pressure correlated with temperature. The influx of leukocytes into the cerebrospinal fluid and levels of tumor necrosis factor alpha in the cerebrospinal fluid were decreased. Cooling the animals 2 hours after meningitis induction to 30.5 degrees C was also protective. We conclude that hypothermia is a new adjuvant approach to reduce meningitis-induced changes, in particular intracranial pressure, in the early phase of the disease.

Hypothermic circulatory arrest causes multisystem vascular endothelial dysfunction and apoptosis

BACKGROUND

Multiple organ failure after deep hypothermic circulatory arrest (DHCA) may occur secondary to endothelial dysfunction and apoptosis. We sought to determine if DHCA causes endothelial dysfunction and apoptosis in brain, kidney, lungs, and other tissues.

METHODS

Anesthetized pigs on cardiopulmonary bypass were: (1) cooled to 18 degrees C, and had their circulation arrested (60 minutes) and reperfused at 37 degrees C for 90 minutes (DHCA, n = 8); or (2) time-matched normothermic controls on bypass (CPB, n = 6). Endothelial function in cerebral, pulmonary, and renal vessels was assessed by vasorelaxation responses to endothelial-specific bradykinin (BK) or acetylcholine (ACh), and smooth muscle-specific nitroprusside.

RESULTS

In vivo transcranial vasorelaxation responses to ACh were similar between the two groups. In small-caliber cerebral arteries, endothelial relaxation (BK) was impaired in CPB vs DHCA (maximal 55% +/- 2% [p < 0.05] vs 100% +/- 6%). Pulmonary artery ACh responses were comparable between CPB (110% +/- 10%) and DHCA (83% +/- 6%), but responses in pulmonary vein were impaired in DHCA (109% +/- 3%, p < 0.05) relative to CPB (137% +/- 6%). In renal arteries, endothelial (ACh) responses were impaired in DHCA (71% +/- 13%) relative to CPB (129% +/- 14%). Apoptosis (DNA laddering) occurred primarily in duodenal tissue, with a greater frequency in DHCA (56%, p < 0.05) compared with normothermic CPB (17%) and nonbypass controls (0%).

CONCLUSIONS

DHCA is associated with endothelial dysfunction in cerebral microvessels but not in the in vivo transcranial vasculature; in addition, endothelial dysfunction was noted in large-caliber renal arteries and pulmonary veins. DHCA is also associated with duodenal apoptosis. Vascular endothelial dysfunction and apoptosis may be involved in the pathophysiology of multisystem organ failure after DHCA.

Postoperative brain complications following retrograde cerebral perfusion

This study was undertaken to investigate the neurological risk factors associated with the retrograde cerebral perfusion (RCP) technique, by examining the relationship between intraoperative parameters and post-operative brain complications. A total of 12 patients who underwent surgery for thoracic aortic aneurysms using the RCP technique were included in this study. Profound hypothermia was induced through cardiopulmonary bypass which was established with a femoral arterial cannula and bicaval return. During RCP, a venous drainage cannula from the superior vena cava (SVC) was switched over to the arterial return circuit, and oxygenated blood was retrogradely infused through the SVC. The perfusion flow rate was maintained at 273 +/- 113 ml/min and the SVC pressure was maintained at 15 +/- 6 mmHg. The RCP time was 68 +/- 27 min with a range of 27-130 min, and the lowest rectal temperature was 16 +/- 1 degrees C. The total elapsed time until emergence from anesthesia after the operation was 12 +/- 6 h. The operation time correlated with the awakening time (r = 0.729, P = 0.0088). Longer RCP times of up to 101 and 130 min tended to result in post-operative brain damage. The lowest rectal temperature also correlated with the awakening time (r = 0.697, P = 0.0149), and an inverse correlation between the SVC pressure and the awakening time was observed (r = -0. 727, P = 0.0091). These findings demonstrate the importance of reducing both the RCP and operation times to decrease the incidence of brain damage. If carried out under optimal conditions, including perfusion pressure and brain temperature, RCP could be marginally prolonged safely without causing major neurological complications.

Cerebral metabolic suppression during hypothermic circulatory arrest in humans

BACKGROUND

Hypothermic circulatory arrest (HCA) is used in surgery for aortic and congenital cardiac diseases. Although studies of the safety of HCA in animals have been carried out, the degree to which metabolism is suppressed in patients during hypothermia has been difficult to determine because of problems with serial measurements of cerebral blood flow in the clinical setting.

METHODS

To quantify the degree of metabolic suppression achieved by hypothermia, we studied 37 adults undergoing operations employing HCA. Cerebral blood flow was estimated using an ultrasonic flow probe on the left common carotid artery, and cerebral arteriovenous oxygen content differences were calculated from jugular venous bulb and arterial oxygen saturations. Cerebral metabolic rates while cooling were then ascertained. The temperature coefficient, Q10, which is the ratio of metabolic rates at temperatures 10 degrees C apart, was determined.

RESULTS

The human cerebral Q10 was found to be 2.3. The cerebral metabolic rate is still 17% of baseline at 15 degrees C. If one assumes that cerebral blood flow can safely be interrupted for 5 min at 37 degrees C, and that cerebral metabolic suppression accounts for the protective effects of hypothermia, the predicted safe duration of HCA at 15 degrees C is only 29 min.

CONCLUSIONS

The safe intervals calculated from measured cerebral oxygen consumption suggest that shorter intervals and lower temperatures than those currently used may be necessary to assure adequate cerebral protection during hypothermic circulatory arrest.