Sequestration of Dexmedetomidine in Ex Vivo Cardiopulmonary Bypass Circuits.

Dexmedetomidine (DEX) is a sedative used in combination with other drugs in neonates and infants undergoing cardiac surgery using cardiopulmonary bypass (CPB). This study aimed to evaluate the disposition of DEX after administration to the ex vivo CPB circuits following different bolus doses and continuous infusion of DEX, including the effect of circuit coating, temperature, and modified ultrafiltration (MUF). Cardiopulmonary bypass circuits were setup ex vivo and primed with reconstituted blood. Dexmedetomidine was administered to the circuit (as a single bolus or single bolus along with continuous infusion). The circuit was allowed to equilibrate during the first 5 minutes, blood samples were collected at multiple time points (5-240 minutes). Blood samples were processed to collect plasma and analyzed for DEX with a validated assay. The majority of DEX sequestration in ex vivo CPB circuits occurred within the first 15 minutes. The percent of DEX remained in plasma pre-MUF (16-71%) and post-MUF (22-92%) varied depending on the dose and dosing scheme. Modified ultrafiltration significantly increased the plasma concentration of DEX in 19 of 23 circuits by an average of 12.1 ± 4.25% (p < 0.05). The percent sequestration of DEX was lower in CPB circuits at lower DEX doses compared to higher doses. A combination of DEX initial loading dose and continuous infusion resulted in steady concentrations of DEX over 4 hours. At therapeutically relevant concentrations of DEX (485-1,013 pg/ml), lower sequestration was observed in ex vivo CPB circuits compared to higher doses. The sequestration of DEX to circuits should be considered to achieve the optimal concentration of DEX during CPB surgery.

Critical Closing Pressure by Diffuse Correlation Spectroscopy in a Neonatal Piglet Model.

The critical closing pressure (CrCP) of the cerebral vasculature is the arterial blood pressure (ABP) at which cerebral blood flow (CBF) ceases. Because the ABP of preterm infants is low and close to the CrCP, there is often no CBF during diastole. Thus, estimation of CrCP may become clinically relevant in preterm neonates. Transcranial Doppler (TCD) ultrasound has been used to estimate CrCP in preterm infants. Diffuse correlation spectroscopy (DCS) is a continuous, noninvasive optical technique that measures microvascular CBF. Our objective was to compare and validate CrCP measured by DCS versus TCD ultrasound. Hemorrhagic shock was induced in 13 neonatal piglets, and CBF was measured continuously by both modalities. CrCP was calculated using a model of cerebrovascular impedance, and CrCP determined by the two modalities showed good correlation by linear regression, median r 2 = 0.8 (interquartile range (IQR) 0.71-0.87), and Bland-Altman analysis showed a median bias of -3.5 (IQR -4.6 to -0.28). This is the first comparison of CrCP determined by DCS versus TCD ultrasound in a neonatal piglet model of hemorrhagic shock. The difference in CrCP between the two modalities may be due to differences in vasomotor tone within the microvasculature of the cerebral arterioles versus the macrovasculature of a major cerebral artery.

Elevated arterial blood pressure after superior cavo-pulmonary anastomosis is associated with elevated pulmonary artery pressure and cerebrovascular dysautoregulation.

BACKGROUND: Elevated arterial blood pressure (ABP) is common after superior bidirectional cavopulmonary anastomosis (BCPA). The effects of elevated ABP after BCPA on cerebrovascular hemodynamics are unknown. We sought to determine the relationship between elevated ABP and cerebrovascular autoregulation after BCPA. METHODS: Prospective, observational study on infants with single-ventricle physiology after BCPA surgery. Continuous recordings of mean ABP, mean cavopulmonary artery pressure (PAP), near-infrared spectroscopy measures of cerebral oximetry (regional cerebral oxygen saturation (rSO2)), and relative cerebral blood volume index were obtained from admission to extubation. Autoregulation was measured as hemoglobin volume index (HVx). Physiologic variables, including the HVx, were tested for variance across ABP. RESULTS: Sixteen subjects were included in the study. Elevated ABP post-BCPA was associated with both, elevated PAP (P<0.0001) and positive HVx (dysautoregulation; P<0.0001). No association was observed between ABP and alterations in rSO2. Using piecewise regression, the relationship of PAP to ABP demonstrated a breakpoint at 68 mm Hg (interquartile range (IQR) 62-70 mm Hg). Curve fit of HVx as a function of ABP identified optimal ABP supporting robust autoregulation at a median ABP of 55 mm Hg (IQR 51-64 mm Hg). CONCLUSIONS: Elevated ABP post-BCPA is associated with cerebrovascular dysautoregulation, and elevated PAP. The effects, of prolonged dysautoregulation within this population, require further study.

Impaired cerebral autoregulation and elevation in plasma glial fibrillary acidic protein level during cardiopulmonary bypass surgery for CHD.

BACKGROUND: Cerebrovascular reactivity monitoring has been used to identify the lower limit of pressure autoregulation in adult patients with brain injury. We hypothesise that impaired cerebrovascular reactivity and time spent below the lower limit of autoregulation during cardiopulmonary bypass will result in hypoperfusion injuries to the brain detectable by elevation in serum glial fibrillary acidic protein level. METHODS: We designed a multicentre observational pilot study combining concurrent cerebrovascular reactivity and biomarker monitoring during cardiopulmonary bypass. All children undergoing bypass for CHD were eligible. Autoregulation was monitored with the haemoglobin volume index, a moving correlation coefficient between the mean arterial blood pressure and the near-infrared spectroscopy-based trend of cerebral blood volume. Both haemoglobin volume index and glial fibrillary acidic protein data were analysed by phases of bypass. Each patient's autoregulation curve was analysed to identify the lower limit of autoregulation and optimal arterial blood pressure. RESULTS: A total of 57 children had autoregulation and biomarker data for all phases of bypass. The mean baseline haemoglobin volume index was 0.084. Haemoglobin volume index increased with lowering of pressure with 82% demonstrating a lower limit of autoregulation (41±9 mmHg), whereas 100% demonstrated optimal blood pressure (48±11 mmHg). There was a significant association between an individual's peak autoregulation and biomarker values (p=0.01). CONCLUSIONS: Individual, dynamic non-invasive cerebrovascular reactivity monitoring demonstrated transient periods of impairment related to possible silent brain injury. The association between an impaired autoregulation burden and elevation in the serum brain biomarker may identify brain perfusion risk that could result in injury.

The Diastolic Closing Margin Is Associated with Intraventricular Hemorrhage in Premature Infants.

Premature infants are at an increased risk of intraventricular hemorrhage (IVH). The roles of hypotension and hyperemia are still debated. Critical closing pressure (CrCP) is the arterial blood pressure (ABP) at which cerebral blood flow (CBF) ceases. When diastolic ABP is equal to CrCP, CBF occurs only during systole. The difference between diastolic ABP and CrCP is the diastolic closing margin (DCM). We hypothesized that a low DCM was associated with IVH. One hundred eighty-six premature infants, with a gestational age (GA) range of 23-33 weeks, were monitored with umbilical artery catheters and transcranial Doppler insonation of middle cerebral artery flow velocity for 1-h sessions over the first week of life. CrCP was calculated linearly and using an impedance model. A multivariate generalized linear regression model was used to determine associations with severe IVH (grades 3-4). An elevated DCM by either method was associated with IVH (p < 0.0001 for the linear method; p < 0.001 for the impedance model). Lower 5-min Apgar scores, elevated mean CBF velocity, and lower mean ABP were also associated with IVH (p < 0.0001). Elevated DCM, not low DCM, was associated with severe IVH in this cohort.

Elevated Diastolic Closing Margin Is Associated with Intraventricular Hemorrhage in Premature Infants.

OBJECTIVE: To determine whether the diastolic closing margin (DCM), defined as diastolic blood pressure minus critical closing pressure, is associated with the development of early severe intraventricular hemorrhage (IVH). STUDY DESIGN: A reanalysis of prospectively collected data was conducted. Premature infants (gestational age 23-31 weeks) receiving mechanical ventilation (n = 185) had ∼1-hour continuous recordings of umbilical arterial blood pressure, middle cerebral artery cerebral blood flow velocity, and PaCO2 during the first week of life. Models using multivariate generalized linear regression and purposeful selection were used to determine associations with severe IVH. RESULTS: Severe IVH (grades 3-4) was observed in 14.6% of the infants. Irrespective of the model used, Apgar score at 5 minutes and DCM were significantly associated with severe IVH. A clinically relevant 5-mm Hg increase in DCM was associated with a 1.83- to 1.89-fold increased odds of developing severe IVH. CONCLUSION: Elevated DCM was associated with severe IVH, consistent with previous animal data showing that IVH is associated with hyperperfusion. Measurement of DCM may be more useful than blood pressure in defining cerebral perfusion in premature infants.

Acute hypercarbia increases the lower limit of cerebral blood flow autoregulation in a porcine model.

OBJECTIVES: In the present study, our objective was to determine if hypercarbia would alter cerebral blood flow (CBF) autoregulation and reduce the ability of cerebrovascular reactivity monitoring to identify the lower limit of cerebrovascular autoregulation (LLA). METHODS: Anaesthetised juvenile pigs were assigned between two groups: normocarbia (control group, n = 10) or hypercarbia [high carbon dioxide (CO2) group, n = 8]. Normocarbia subjects were maintained with an arterial CO2 of 40 Torr, while the hypercarbia subjects had an increase of inspired CO2 to achieve an arterial pCO2 of >80 Torr. Gradual hypotension was induced by continuous haemorrhage from a catheter in the femoral vein, and the LLA was determined by monitoring cortical laser Doppler flux (LDF). Vascular reactivity monitoring was performed using the pressure reactivity index (PRx) and haemoglobin volume index (HVx). RESULTS: There were no sustained differences in ICP between groups. Autoregulation was present in both groups, despite elevation in pCO2.The control group had an average LLA of 45 mmHg (95% CI: 43-47 mmHg) and the high CO2 group had a LLA of 75 mmHg (95% CI: 73-77 mmHg). The detected LLA for each subject correlated with the level of pCO2 (spearman R = 0.8243, P < 0.0001). Both the PRx and HVx accurately detected the LLA despite the presence of hypercarbia. DISCUSSION: Hypercarbia without acidosis increases the observed LLA independent of alterations in ICP. Elevations in CO2 can impair cerebrovascular autoregulation, but if there is a sufficient increase in blood pressure above the CO2 altered LLA, then autoregulation persists.

Optic nerve sheath diameter measurement techniques: examination using a novel ex-vivo porcine model.

BACKGROUND: Ultrasound (U/S) and MRI measurements of the optic nerve sheath diameter (ONSD) have been proposed as intracranial pressure measurement surrogates, but these methods have not been fully evaluated or standardized. The purpose of this study was to develop an ex-vivo model for evaluating ONSD measurement techniques by comparing U/S and MRI measurements to physical measurements. METHODS: The left eye of post mortem juvenile pigs (N = 3) was excised and the subdural space of the optic nerve cannulated. Caliper measurements and U/S imaging measurements of the ONSD were acquired at baseline and following 1 cc saline infusion into the sheath. The samples were then embedded in 0.5% agarose and imaged in a 7 Tesla (7T) MRI. The ONSD was subsequently measured with digital calipers at locations and directions matching the U/S and direct measurements. RESULTS: Both MRI and sonographic measurements were in agreement with direct measurements. U/S data, especially axial images, exhibited a positive bias and more variance (bias: 1.318, 95% limit of agreement: 8.609) compared to MRI (bias: 0.3156, 95% limit of agreement: 2.773). In addition, U/S images were much more dependent on probe placement, distance between probe and target, and imaging plane. CONCLUSIONS: This model appears to be a valid test-bed for continued scrutiny of ONSD measurement techniques. In this model, 7T MRI was accurate and potentially useful for in-vivo measurements where direct measurements are not available. Current limitations with ultrasound imaging for ONSD measurement associated with image acquisition technique and equipment necessitate further standardization to improve its clinical utility.

Intracranial pressure and optic nerve sheath diameter as cephalic venous pressure increases in swine.

BACKGROUND: Nontraumatic, nonhydrocephalic increases in intracranial pressure (ICP) are often difficult to diagnose and may underlie spaceflight-related visual changes. This study looked at the utility of a porcine animal model of increasing cephalic venous pressure to mimic acute changes in ICP and optic nerve sheath diameter (ONSD) from cephalic venous fluid shifts observed during spaceflight. METHODS: Anesthetized juvenile piglets were assigned to groups of either naïve (N = 10) or elevated superior vena cava pressure (SVCP; N = 20). To elevate SVCP, a 6F custom latex balloon catheter was inserted and inflated to achieve SVCP of 20 and 40 mmHg for 1 h at each pressure. In both groups, serial measurements of ICP, internal jugular pressure (IJP), and external jugular pressure (EJP) were made hourly for 3 h, and ONSD of the right eye was measured hourly by ultrasound (US). RESULTS: There was a significant linear correlation between IJP and ICP (slope: 0.9614 +/- 0.0038, r = 0.9683). With increasing SVCP, resulting ONSD was also well correlated with the ICP (slope: 0.0958 +/- 0.0061, r = 0.7841). The receiver operating characteristic curve for ONSD in diagnosing elevated ICP had an area under the curve of 0.9632 with a sensitivity and specificity of 92% and 91%, respectively, for a cutoff of 5.45 mm. CONCLUSIONS: Increases in SVCP result in ICP changes that are well correlated with alteration in ONSD. These changes are consistent with observed ONSD changes monitored during spaceflight.

Magnitude of arterial carbon dioxide change at initiation of extracorporeal membrane oxygenation support is associated with survival.

Many patient factors have been associated with mortality from extracorporeal membrane oxygenation (ECMO) therapy. Pre-ECMO patient pH and arterial carbon dioxide (paCO2) have been associated with poor outcome and can be significantly altered by ECMO initiation. We hypothesized that the magnitude of change in paCO2 and pH with ECMO initiation could be associated with survival. We designed a retrospective observational study from a single tertiary care center and included all pediatric patients (age younger than 18 years) undergoing ECMO between 2002 and 2010. Electronic records were queried for demographics and clinical characteristics, including the arterial blood gas (ABG) pre- and post-ECMO initiation. Bivariate analysis compared ECMO course characteristics by outcome (survivor vs. nonsurvivor). Multivariable logistic regression was performed on factors associated with the outcome in the bivariate analysis at the significance level of p < .1. Odds ratios (ORs) and 95% confidence intervals (95% CIs) were reported. We identified 201 patients with a median age of 10 days (range, 1 day to 16 years). Indications for ECMO were: respiratory failure (51%), cardiac failure (23%), extracorporeal cardiopulmonary resuscitation (21%), and sepsis (5%). Mortality, defined by death before discharge, was 37% (74 of 201). ABG data pre- and post-ECMO initiations were available in 84% (169 of 201). Age, pH, paCO2, indication, and intracranial hemorrhage were significantly associated with mortality (p < .05). After adjusting for potential confounders (age, use of epinephrine, volume of fluid administered, year of ECMO, ECMO indication, and duration of ECMO) by multivariable logistic regression, the magnitude of paCO2 change (> or =25 mmHg) was associated with mortality (adjusted OR, 2.21; 95% CI, 1.06-4.63; p = .036). The decrease in paCO2 with ECMO initiation was associated with mortality. Although this change in paCO2 is multifactorial, it represents a modifiable element of clinical management involving pre-ECMO ventilation, ECMO circuit priming, CO2 administration/removal, and may represent a future therapeutic target that could improve survival in pediatric ECMO.

Continuous cerebrovascular reactivity monitoring and autoregulation monitoring identify similar lower limits of autoregulation in patients undergoing cardiopulmonary bypass.

OBJECTIVES: Cerebrovascular autoregulation can be monitored with a moving linear correlation of blood pressure to cerebral blood flow velocity (mean velocity index, Mx) during cardiopulmonary bypass (CPB). Vascular reactivity can be monitored with a moving linear correlation of blood pressure to cerebral blood volume trended with near-infrared spectroscopy (hemoglobin volume index, HVx). We hypothesized that the lower limits of autoregulation (LLA) and the optimal blood pressure (ABPopt) associated with the most active autoregulation could be determined by HVx in patients undergoing CPB. METHODS: Adult patients (n = 109) who underwent CPB for cardiac surgery had monitoring of both autoregulation (Mx) and vascular reactivity (HVx). Individual curves of Mx and HVx were constructed by placing each in 5 mmHg bins. The LLA and ABPopt for each subject were then identified by both methods and compared for agreement by correlation analysis and Bland-Altman. RESULTS: The average LLA defined by Mx compared to HVx were comparable (66±13 and 66±12 mmHg). Correlation between the LLA defined by Mx and HVx was significant (Pearson r = 0.2867; P = 0.0068). The average ABPopt with the most robust autoregulation by Mx was comparable to HVx (75±11 and 74±13 mmHg) with significant correlation (Pearson r = 0.5915; P < or =0.0001). DISCUSSION: Autoregulation and vascular reactivity monitoring are expected to be distinct, as flow and volume have different phasic relationships to pressure when cerebrovascular autoregulation is active. However, the two metrics have good agreement when identifying the LLA and optimal blood pressure in patients during CPB.

Detection of neurologic injury using vascular reactivity monitoring and glial fibrillary acidic protein.

New noninvasive methods for monitoring cerebrovascular pressure reactivity coupled with a blood-based assay for brain-specific injury in preterm infants could allow early diagnosis of brain injury and set the stage for improved timing and effectiveness of interventions. Using an adaptation of near-infrared spectroscopy, we report a case of a very low birth weight infant undergoing hemoglobin volume index monitoring as a measure of cerebrovascular pressure reactivity. During the monitoring period, this infant demonstrated significant disturbances in cerebrovascular pressure reactivity that coincided with elevation of serum glial fibrillary acidic protein and new findings of brain injury on head ultrasound. This case report demonstrates the potential of emerging noninvasive monitoring methods to assist in both detection and therapeutic management to improve neurologic outcomes of the very low birth weight neonate.

Transfusion of hemoglobin-based oxygen carriers in the carboxy state is beneficial during transient focal cerebral ischemia.

Exchange transfusion of large volumes of hemoglobin (Hb)-based oxygen carriers can protect the brain from middle cerebral artery occlusion (MCAO). Hb in the carboxy state (COHb) may provide protection at relatively low volumes by enhancing vasodilation. We determined whether transfusion of rats with 10 ml/kg PEGylated COHb [polyethylene glycol (PEG)-COHb] at 20 min of 2-h MCAO was more effective in reducing infarct volume compared with non-carbon monoxide (CO) PEG-Hb. After PEG-COHb transfusion, whole blood and plasma COHb was <3%, indicating rapid release of CO. PEG-COHb transfusion significantly reduced infarct volume (15 ± 5% of hemisphere; mean ± SE) compared with that in the control group (35 ± 6%), but non-CO PEG-Hb did not (24 ± 5%). Chemically dissimilar COHb polymers were also effective. Induction of MCAO initially produced 34 ± 2% dilation of pial arterioles in the border region that subsided to 10 ± 1% at 2 h. Transfusion of PEG-COHb at 20 min of MCAO maintained pial arterioles in a dilated state (40 ± 5%) at 2 h, whereas transfusion of non-CO PEG-Hb had an intermediate effect (22 ± 3%). When transfusion of PEG-COHb was delayed by 90 min, laser-Doppler flow in the border region increased from 57 ± 9 to 82 ± 13% of preischemic baseline. These data demonstrate that PEG-COHb is more effective than non-CO PEG-Hb at reducing infarct volume, sustaining cerebral vasodilation, and improving collateral perfusion in a model of transient focal cerebral ischemia when given at a relatively low dose (plasma Hb concentration < 1 g/dl). Use of acellular Hb as a CO donor that is rapidly converted to an oxygen carrier in vivo may permit potent protection at low transfusion volumes.

Renovascular reactivity measured by near-infrared spectroscopy.

Hypotension and shock are risk factors for death, renal insufficiency, and stroke in preterm neonates. Goal-directed neonatal hemodynamic management lacks end-organ monitoring strategies to assess the adequacy of perfusion. Our aim is to develop a clinically viable, continuous metric of renovascular reactivity to gauge renal perfusion during shock. We present the renovascular reactivity index (RVx), which quantifies passivity of renal blood volume to spontaneous changes in arterial blood pressure. We tested the ability of the RVx to detect reductions in renal blood flow. Hemorrhagic shock was induced in 10 piglets. The RVx was monitored as a correlation between slow waves of arterial blood pressure and relative total hemoglobin (rTHb) obtained with reflectance near-infrared spectroscopy (NIRS) over the kidney. The RVx was compared with laser-Doppler measurements of red blood cell flux, and renal laser-Doppler measurements were compared with cerebral laser-Doppler measurements. Renal blood flow decreased to 75%, 50%, and 25% of baseline at perfusion pressures of 60, 45, and 40 mmHg, respectively, whereas in the brain these decrements occurred at pressures of 30, 25, and 15 mmHg, respectively. The RVx compared favorably to the renal laser-Doppler data. Areas under the receiver operator characteristic curves using renal blood flow thresholds of 50% and 25% of baseline were 0.85 (95% CI, 0.83-0.87) and 0.90 (95% CI, 0.88-0.92). Renovascular autoregulation can be monitored and is impaired in advance of cerebrovascular autoregulation during hemorrhagic shock.

Static autoregulation is intact early after severe unilateral brain injury in a neonatal Swine model.

BACKGROUND: Autoregulation is impaired by traumatic brain injury. Cerebral blood flow disturbances are spatially heterogeneous, but autoregulation is often reported as a global metric. OBJECTIVE: We tested lateralization of autoregulatory responses in the neonatal piglet brain during hypotension early after unilateral injury. METHODS: Neonatal piglets (5-7 days old) had controlled cortical impact (severe, n = 12; moderate, n = 13; sham, n = 13) and recovery for 6 hours. The lower limit of autoregulation (LLA) and static rate of autoregulation (SRoR) were determined for each subject and compared among groups and between the ipsilateral and contralateral hemispheres. RESULTS: The LLA was not increased by injury (sham, 34 mm Hg [29-39 mm Hg]; moderate injury, 37 mm Hg [33-41 mm Hg]; severe injury, 35 mm Hg [32-38 mm Hg]; P = .93, mean [95% confidence interval]). SRoR, when measured ipsilateral to injury and above the LLA, showed intact autoregulation and was not lower than SRoR in uninjured subjects (sham, 0.82 [0.53-1.1]; moderate injury, 1.0 [0.60-1.5]; severe, 0.91 [0.33-1.5]; P = .44). The average hemispheric LLA difference was 2.7 mm Hg, (95% limits of agreement, -7.5 to 7.0; bias, -0.25; Spearman r = 0.73; P < .0001). Ipsilateral and contralateral SRoR measurements also showed correlation in the injured groups (Spearman r = 0.85, P < .0001). CONCLUSION: LLA was not increased by controlled cortical impact, nor did SRoR measurements demonstrate ineffective autoregulation when cerebral perfusion pressure was greater than and within 10 mm Hg of the LLA. Cerebral perfusion pressure optimization, indicated by autoregulation measurements, was significantly similar in the 2 hemispheres despite severe unilateral injury.

Noninvasive autoregulation monitoring in a swine model of pediatric cardiac arrest.

BACKGROUND: Cerebrovascular autoregulation after resuscitation has not been well studied in an experimental model of pediatric cardiac arrest. Furthermore, developing noninvasive methods of monitoring autoregulation using near-infrared spectroscopy (NIRS) would be clinically useful in guiding neuroprotective hemodynamic management after pediatric cardiac arrest. We tested the hypotheses that the lower limit of autoregulation (LLA) would shift to a higher arterial blood pressure between 1 and 2 days of recovery after cardiac arrest and that the LLA would be detected by NIRS-derived indices of autoregulation in a swine model of pediatric cardiac arrest. We also tested the hypothesis that autoregulation with hypertension would be impaired after cardiac arrest. METHODS: Data on LLA were obtained from neonatal piglets that had undergone hypoxic-asphyxic cardiac arrest and recovery for 1 day (n = 8) or 2 days (n = 8), or that had undergone sham surgery with 2 days of recovery (n = 8). Autoregulation with hypertension was examined in a separate cohort of piglets that underwent hypoxic-asphyxic cardiac arrest (n = 5) or sham surgery (n = 5) with 2 days of recovery. After the recovery period, piglets were reanesthetized, and autoregulation was monitored by standard laser-Doppler flowmetry and autoregulation indices derived from NIRS (the cerebral oximetry [COx] and hemoglobin volume [HVx] indices). The LLA was determined by decreasing blood pressure through inflation of a balloon catheter in the inferior vena cava. Autoregulation during hypertension was evaluated by inflation of an aortic balloon catheter. RESULTS: The LLAs were similar between sham-operated piglets and piglets that recovered for 1 or 2 days after arrest. The NIRS-derived indices accurately detected the LLA determined by laser-Doppler flowmetry. The area under the curve of the receiver operator characteristic curve for cerebral oximetry index was 0.91 at 1 day and 0.92 at 2 days after arrest. The area under the curve for hemoglobin volume index was 0.92 and 0.89 at the respective time points. During induced hypertension, the static rate of autoregulation, defined as the percentage change in cerebrovascular resistance divided by the percentage change in cerebral perfusion pressure, was not different between postarrest and sham-operated piglets. At 2 days recovery from arrest, piglets exhibited neurobehavioral deficits and histologic neuronal injury. CONCLUSIONS: In a swine model of pediatric hypoxic-asphyxic cardiac arrest with confirmed brain damage, the LLA did not differ 1 and 2 days after resuscitation. The NIRS-derived indices accurately detected the LLA in comparison with laser-Doppler flow measurements at those time points. Autoregulation remained functional during hypertension.

Attenuation of neonatal ischemic brain damage using a 20-HETE synthesis inhibitor.

20-Hydroxyeicosatetraenoic acid (20-HETE) is a cytochrome P450 metabolite of arachidonic acid that that contributes to infarct size following focal cerebral ischemia. However, little is known about the role of 20-HETE in global cerebral ischemia or neonatal hypoxia-ischemia (H-I). The present study examined the effects of blockade of the synthesis of 20-HETE with N-hydroxy-N'-(4-n-butyl-2-methylphenyl) formamidine (HET0016) in neonatal piglets after H-I to determine if it protects highly vulnerable striatal neurons. Administration of HET0016 after H-I improved early neurological recovery and protected neurons in putamen after 4 days of recovery. HET0016 had no significant effect on cerebral blood flow. cytochrome P450 4A immunoreactivity was detected in putamen neurons, and direct infusion of 20-HETE in the putamen increased phosphorylation of Na(+), K(+) -ATPase and NMDA receptor NR1 subunit selectively at protein kinase C-sensitive sites but not at protein kinase A-sensitive sites. HET0016 selectively inhibited the H-I induced phosphorylation at these same sites at 3 h of recovery and improved Na(+), K(+) -ATPase activity. At 3 h, HET0016 also suppressed H-I induced extracellular signal-regulated kinase 1/2 activation and protein markers of nitrosative and oxidative stress. Thus, 20-HETE can exert direct effects on key proteins involved in neuronal excitotoxicity in vivo and contributes to neurodegeneration after global cerebral ischemia in immature brain.

Cerebral blood flow and cerebrovascular autoregulation in a swine model of pediatric cardiac arrest and hypothermia.

OBJECTIVE: Knowledge remains limited regarding cerebral blood flow autoregulation after cardiac arrest and during postresuscitation hypothermia. We determined the relationship of cerebral blood flow to cerebral perfusion pressure in a swine model of pediatric hypoxic-asphyxic cardiac arrest during normothermia and hypothermia and tested novel measures of autoregulation derived from near-infrared spectroscopy. DESIGN: Prospective, balanced animal study. SETTING: Basic physiology laboratory at an academic institution. SUBJECTS: Eighty-four neonatal swine. INTERVENTIONS: Piglets underwent hypoxic-asphyxic cardiac arrest or sham surgery and recovered for 2 hrs with normothermia followed by 4 hrs of either moderate hypothermia or normothermia. In half of the groups, blood pressure was slowly decreased through inflation of a balloon catheter in the inferior vena cava to identify the lower limit of cerebral autoregulation at 6 hrs postresuscitation. In the remaining groups, blood pressure was gradually increased by inflation of a balloon catheter in the aorta to determine the autoregulatory response to hypertension. Measures of autoregulation obtained from standard laser-Doppler flowmetry and indices derived from near-infrared spectroscopy were compared. MEASUREMENTS AND MAIN RESULTS: Laser-Doppler flux was lower in postarrest animals compared to sham-operated controls during the 2-hr normothermic period after resuscitation. During the subsequent 4-hr recovery, hypothermia decreased laser-Doppler flux in both the sham surgery and postarrest groups. Autoregulation was intact during hypertension in all groups. With arterial hypotension, postarrest, hypothermic piglets had a significant decrease in the perfusion pressure lower limit of autoregulation compared to postarrest, normothermic piglets. The near-infrared spectroscopy-derived measures of autoregulation accurately detected loss of autoregulation during hypotension. CONCLUSIONS: In a pediatric model of cardiac arrest and resuscitation, delayed induction of hypothermia decreased cerebral perfusion and decreased the lower limit of autoregulation. Metrics derived from noninvasive near-infrared spectroscopy accurately identified the lower limit of autoregulation during normothermia and hypothermia in piglets resuscitated from arrest.

Cytosolic phospholipase A2 alpha amplifies early cyclooxygenase-2 expression, oxidative stress and MAP kinase phosphorylation after cerebral ischemia in mice.

BACKGROUND: The enzyme cytosolic phospholipase A2 alpha (cPLA2alpha) has been implicated in the progression of cerebral injury following ischemia and reperfusion. Previous studies in rodents suggest that cPLA2alpha enhances delayed injury extension and disruption of the blood brain barrier many hours after reperfusion. In this study we investigated the role of cPLA2alpha in early ischemic cerebral injury. METHODS: Middle cerebral artery occlusion (MCAO) was performed on cPLA2alpha+/+ and cPLA2alpha-/- mice for 2 hours followed by 0, 2, or 6 hours of reperfusion. The levels of cPLA2alpha, cyclooxygenase-2, neuronal morphology and reactive oxygen species in the ischemic and contralateral hemispheres were evaluated by light and fluorescent microscopy. PGE2 content was compared between genotypes and hemispheres after MCAO and MCAO and 6 hours reperfusion. Regional cerebral blood flow was measured during MCAO and phosphorylation of relevant MAPKs in brain protein homogenates was measured by Western analysis after 6 hours of reperfusion. RESULTS: Neuronal cPLA2alpha protein increased by 2-fold immediately after MCAO and returned to pre-MCAO levels after 2 hours reperfusion. Neuronal cyclooxygenase-2 induction and PGE2 concentration were greater in cPLA2alpha+/+ compared to cPLA2alpha-/- ischemic cortex. Neuronal swelling in ischemic regions was significantly greater in the cPLA2alpha+/+ than in cPLA2alpha-/- brains (+/+:2.2+/-0.3 fold vs. -/-:1.7+/-0.4 fold increase; P<0.01). The increase in reactive oxygen species following 2 hours of ischemia was also significantly greater in the cPLA2alpha+/+ ischemic core than in cPLA2alpha-/- (+/+:7.12+/-1.2 fold vs. -/-:3.1+/-1.4 fold; P<0.01). After 6 hours of reperfusion ischemic cortex of cPLA2alpha+/+, but not cPLA2alpha-/-, had disruption of neuron morphology and decreased PGE2 content. Phosphorylation of the MAPKs-p38, ERK 1/2, and MEK 1/2-was significantly greater in cPLA2a+/+ than in cPLA2alpha-/- ischemic cortex 6 hours after reperfusion. CONCLUSIONS: These results indicate that cPLA2alpha modulates the earliest molecular and injury responses after cerebral ischemia and have implications for the potential clinical use of cPLA2alpha inhibitors.

Noninvasive autoregulation monitoring with and without intracranial pressure in the naive piglet brain.

BACKGROUND: Cerebrovascular autoregulation monitoring is often desirable for critically ill patients in whom intracranial pressure (ICP) is not measured directly. Without ICP, arterial blood pressure (ABP) is a substitute for cerebral perfusion pressure (CPP) to gauge the constraint of cerebral blood flow across pressure changes. We compared the use of ABP versus CPP to measure autoregulation in a piglet model of arterial hypotension. METHODS: Our database of neonatal piglet (5-7 days old) experiments was queried for animals with naïve ICP that were made lethally hypotensive to determine the lower limit of autoregulation (LLA). Twenty-five piglets were identified, each with continuous recordings of ICP, regional cerebral oximetry (rSo2), and cortical red cell flux (laser Doppler). Autoregulation was assessed with the cerebral oximetry index (COx) in 2 ways: linear correlation between ABP and rSo2 (COx(ABP)) and between CPP and rSo2 (COx(CPP)). The lower limits of autoregulation were determined from plots of red cell flux versus ABP. Averaged values of COx(ABP) and COx(CPP) from 5 mm Hg ABP bins were used to show receiver operating characteristics for the 2 methods. RESULTS: COx(ABP) and COx(CPP) yielded identical receiver operating characteristic curve areas of 0.91 (95% confidence interval [CI], 0.88-0.95) for determining the LLA. However, the thresholds for the 2 methods differed: a threshold COx(ABP) of 0.5 was 89% sensitive (95% CI, 81%-94%) and 81% specific (95% CI, 73%-88%) for detecting ABP below the LLA. A threshold COx(CPP) of 0.42 gave the same 89% sensitivity (95% CI, 81%-94%) with 77% specificity (95% CI, 69%-84%). CONCLUSIONS: The use of ABP instead of CPP for autoregulation monitoring in the naïve brain with COx results in a higher threshold value to discriminate ABP above from ABP below the LLA. However, accuracy was similar with the 2 methods. These findings support and refine the use of near-infrared spectroscopy to monitor autoregulation in patients without ICP monitors.