William M. Armstead

Cerebral Blood Flow and Autoregulation After Pediatric Traumatic Brain Injury

Traumatic brain injury is a global health concern and is the leading cause of traumatic morbidity and mortality in children. Despite a lower overall mortality than in adult traumatic brain injury, the cost to society from the sequelae of pediatric traumatic brain injury is very high. Predictors of poor outcome after traumatic brain injury include altered systemic and cerebral physiology, including altered cerebral hemodynamics. Cerebral autoregulation is often impaired after traumatic brain injury and may adversely impact the outcome. Although altered cerebrovascular hemodynamics early after traumatic brain injury may contribute to disability in children, there is little information regarding changes in cerebral blood flow and cerebral autoregulation after pediatric traumatic brain injury. This review addresses normal pediatric cerebral physiology and cerebrovascular pathophysiology after pediatric traumatic brain injury.

Neutralizing the neurotoxic effects of exogenous and endogenous tPA

The clinical use of tissue-type plasminogen activator (tPA) in the treatment of stroke is profoundly constrained by its serious side effects. We report that the deleterious effects of tPA on cerebral edema and intracranial bleeding are separable from its fibrinolytic activity and can be neutralized. A hexapeptide (EEIIMD) corresponding to amino acids 350–355 of plasminogen activator inhibitor type 1 (PAI-1) abolished the tPA-induced increase in infarct size and intracranial bleeding in both mechanical and embolic models of stroke in rats, and reduced brain edema and neuronal loss after traumatic brain injury in pigs. These experiments suggest mechanisms to reduce the neurotoxic effects of tPA without compromising its fibrinolytic activity, through the use of selective antagonists and new tPA formulations.

Superoxide Generation Links Protein Kinase C Activation to Impaired ATP-Sensitive K+ Channel Function After Brain Injury

BACKGROUND AND PURPOSE--Endothelin-1, in concentrations similar to that present in cerebrospinal fluid after fluid percussion brain injury (FPI), increases superoxide anion (O2-) production. Endothelin-1 also contributes to altered cerebral hemodynamics after FPI through impairment of ATP-sensitive K+ (KATP) channel function through protein kinase C (PKC) activation. Generation of O2- additionally occurs after FPI. Nitric oxide and cGMP elicit pial artery dilation through KATP channel activation. The present study was designed to determine whether PKC activation generates O2-, which, in turn, could link such activation to impaired KATP channel function after FPI. METHODS--Injury of moderate severity (1.9 to 2.1 atm) was produced by the lateral FPI technique in anesthetized newborn pigs equipped with a closed cranial window. Superoxide dismutase-inhibitable nitroblue tetrazolium (NBT) reduction was determined as an index of O2- generation. RESULTS--Phorbol 12, 13-dibutyrate (10(-6) mol/L), a PKC activator, increased superoxide dismutase-inhibitable NBT reduction from 1+/-1 to 37+/-5 pmol/mm2. Staurosporine (10(-7) mol/L), a PKC antagonist, blocked the NBT reduction after phorbol 12,13-dibutyrate and blunted the NBT reduction observed after FPI (1+/-1 to 15+/-2 versus 1+/-1 to 5+/-1 pmol/mm2 after FPI in the absence versus presence of staurosporine). Exposure of the cerebral cortex to a xanthine oxidase O2--generating system increased NBT reduction in a manner similar to FPI and blunted pial artery dilation to the KATP channel agonists cromakalim and calcitonin gene-related peptide, the nitric oxide releasers sodium nitroprusside and S-nitroso-N-acetylpenicillamine, and the cGMP analogue 8-bromo-cGMP (10+/-1% and 21+/-1% versus 4+/-1% and 9+/-1% for 10(-8) and 10(-6) mol/L cromakalim before and after activated oxygen-generating system exposure). CONCLUSIONS--These data show that PKC activation increases O2- production and contributes to such production observed after FPI. These data also show that an activated system that generates an amount of O2- similar to that observed with FPI blunted pial artery dilation to KATP channel agonists and nitric oxide/cGMP. These data suggest, therefore, that O2- generation links PKC activation to impaired KATP channel function after FPI.

Cerebral Superoxide Anion Generation during Seizures in Newborn Pigs

Cerebral superoxide anion generation during bicuculline-induced seizures was measured in newborn pigs. Using two closed cranial windows inserted over the parietal cortices, superoxide dismutase (SOD)-inhibitable nitroblue tetrazolium (NBT) reduction was determined during 20 min of seizure activity induced by bicuculline, 5 mg/kg i. v. A modest increase in SOD-inhibitable NBT reduction was observed in piglets subjected to bicuculline-induced seizure activity (2.4 ± 0.6 pmol/mm2 in 20 min) when compared to control piglets (0.4 ± 0.3 pmol/mm2 in 20 min). Pretreatment with indomethacin (5 mg/kg i.v.) reduced SOD-inhibitable NBT reduction during seizures to the control level (0.5 ± 0.4 pmol/mm2 in 20 min). We conclude that small quantities of superoxide anion radical are produced by newborn pig brain during bicuculline-induced seizures and that cyclooxygenase metabolism of arachidonic acid appears to be a major source.

Young age as a risk factor for impaired cerebral autoregulation after moderate to severe pediatric traumatic brain injury.

Background:Little is known about age and cerebral autoregulation in children with traumatic brain injury (TBI). The authors compared cerebral autoregulation between young (aged <4 yr) and older (aged ≥4 yr) children with TBI. Methods:After University of Washingtons institutional review board approval, a retrospective analysis of prospectively collected data (May 2002 and June 2007) was performed. Eligibility criteria included age 16 yr or younger, moderate to severe (admission Glasgow Coma Scale score <13) TBI, TBI on computed tomography scan, and tracheal intubation. Cerebral autoregulation testing was performed within 72 h after TBI, and autoregulation was quantified using the autoregulatory index. An autoregulatory index less than 0.4 represents impaired cerebral autoregulation. The 12-month Glasgow outcome score was measured. Data are presented as mean ± SD or range. Results:Thirty-seven children (8.9 ± 5.1 yr; 0.8–16 yr) were enrolled. Children younger than 4 yr had a higher incidence of impaired cerebral autoregulation (8 of 10 vs. 7 of 27; P = 0.006) and worse 12-month outcome (Glasgow outcome score 3.0 ± 1.0 vs. 4.0 ± 1.0; P = 0.02) than older children. Age less than 4 yr (adjusted odds ratio, 12.2; 95% confidence interval, 1.5–98.5) and low Glasgow Coma Scale score (adjusted odds ratio for higher Glasgow Coma Scale, 0.53; 95% confidence interval, 0.30–0.96) were independently associated with impaired cerebral autoregulation. Conclusions:Age less than 4 yr was a risk factor for impaired cerebral autoregulation, independent of TBI severity. Age-related factors may play a role in the mechanisms maintaining or worsening cerebral autoregulation in children after TBI.

Role of endothelin-1 in age-dependent cerebrovascular hypotensive responses after brain injury

This study was designed to compare the effect of fluid percussion brain injury (FPI) on the hypotensive cerebrovascular response in newborn and juvenile pigs as a function of time postinsult and to determine the role of endothelin-1 (ET-1) in any age-dependent differences in hypotensive cerebrovascular regulation after injury. Ten minutes of hypotension (10-15 ml blood/kg) decreased mean arterial blood pressure uniformly in both groups (∼45%). In the newborn, hypotensive pial artery dilation (PAD) was blunted within 1 h, remained diminished for at least 72 h, but was resolved within 168 h postinjury (66 ± 4, 69 ± 4, 71 ± 4, and 64 ± 4% inhibition at 1, 4, 8, and 72 h post-FPI). During normotension, regional cerebral blood flow (rCBF) was decreased by FPI, and hypotension further reduced the already decremented rCBF for at least 72 h. Cerebrospinal fluid (CSF) ET-1 was increased from 26 ± 4 to 206 ± 25 pg/ml within 72 h post-FPI, whereas an ET-1 antagonist partially restored impaired hypotensive PAD and altered hypotensive rCBF. In contrast, hypotensive PAD and altered CBF were only inhibited for 4 h post-FPI in the juvenile (56 ± 3 and 34 ± 4% inhibition at 1 and 4 h post-FPI). CSF ET-1 was only increased from 27 ± 4 to 67 ± 9 pg/ml at 4 h, whereas the concentration returned to preinjury value by 8 h post-FPI. ET-1 antagonism similarly partially restored impaired hypotensive PAD and altered hypotensive rCBF. These data show that FPI disturbs cerebral autoregulation during hypotension both to a greater magnitude and for a longer duration in the newborn than in the juvenile. These data suggest that the greater FPI-induced ET-1 release in the newborn could contribute to age-dependent differences in impaired hypotensive cerebral autoregulation after FPI.This study was designed to compare the effect of fluid percussion brain injury (FPI) on the hypotensive cerebrovascular response in newborn and juvenile pigs as a function of time postinsult and to determine the role of endothelin-1 (ET-1) in any age-dependent differences in hypotensive cerebrovascular regulation after injury. Ten minutes of hypotension (10-15 ml blood/kg) decreased mean arterial blood pressure uniformly in both groups ( approximately 45%). In the newborn, hypotensive pial artery dilation (PAD) was blunted within 1 h, remained diminished for at least 72 h, but was resolved within 168 h postinjury (66 +/- 4, 69 +/- 4, 71 +/- 4, and 64 +/- 4% inhibition at 1, 4, 8, and 72 h post-FPI). During normotension, regional cerebral blood flow (rCBF) was decreased by FPI, and hypotension further reduced the already decremented rCBF for at least 72 h. Cerebrospinal fluid (CSF) ET-1 was increased from 26 +/- 4 to 206 +/- 25 pg/ml within 72 h post-FPI, whereas an ET-1 antagonist partially restored impaired hypotensive PAD and altered hypotensive rCBF. In contrast, hypotensive PAD and altered CBF were only inhibited for 4 h post-FPI in the juvenile (56 +/- 3 and 34 +/- 4% inhibition at 1 and 4 h post-FPI). CSF ET-1 was only increased from 27 +/- 4 to 67 +/- 9 pg/ml at 4 h, whereas the concentration returned to preinjury value by 8 h post-FPI. ET-1 antagonism similarly partially restored impaired hypotensive PAD and altered hypotensive rCBF. These data show that FPI disturbs cerebral autoregulation during hypotension both to a greater magnitude and for a longer duration in the newborn than in the juvenile. These data suggest that the greater FPI-induced ET-1 release in the newborn could contribute to age-dependent differences in impaired hypotensive cerebral autoregulation after FPI.

Cerebral hemodynamic predictors of poor 6-month Glasgow Outcome Score in severe pediatric traumatic brain injury.

Little is known regarding the cerebral autoregulation in pediatric traumatic brain injury (TBI). We examined the relationship between cerebral hemodynamic predictors, including cerebral autoregulation, and long-term outcome after severe pediatric TBI. After Institutional Review Board (IRB) approval, a retrospective analysis of prospectively collected data (May 2002 to October 2007) for children age < or =16 years with severe TBI (admission Glasgow Coma Scale [GCS] score <9) was performed. Cerebral autoregulation was assessed within 72 h after TBI. Cerebral hemodynamic predictors (intracranial pressure [ICP], systolic blood pressure [SBP], and cerebral perfusion pressure [CPP]) through the first 72 h after TBI were abstracted. Univariate and multivariate analyses examined the relationship between impaired cerebral autoregulation (autoregulatory index <0.4), intracranial hypertension (ICP >20 mm Hg), and hypotension (SBP <5th percentile and CPP <40 mm Hg). Six-month Glasgow Outcome Scale (GOS) score of <4 defined poor outcome. Ten (28%) of the 36 children examined (9.1 +/- 5.3 [0.8-16] years; 74% male) had poor outcome. Univariate factors associated with poor outcome were impaired cerebral autoregulation (p = 0.005), SBP <5(th) percentile for age and gender (p = 0.02), and low middle cerebral artery flow velocity (<2 SD for age and gender; p = 0.04). Independent risk factors for poor 6-month GOS were impaired cerebral autoregulation (adjusted odds ratio [aOR] 12.0; 95% confidence interval [CI] 1.4-99.4) and hypotension (SBP <5th percentile; aOR 8.8; 95% CI 1.1-70.5), respectively. Previous studies of TBI describing poor outcome with hemodynamics did not consider the status of cerebral autoregulation. In this study, both impaired cerebral autoregulation and SBP <5th percentile were independent risk factors for poor 6-month GOS.

Brain Injury Impairs ATP-Sensitive K+ Channel Function in Piglet Cerebral Arteries

BACKGROUND AND PURPOSE Traumatic injury is the leading cause of death for infants and children, and mortality is increased with head injury. Previous studies have shown that pial arteries constricted and that responses to several nitric oxide (NO)-dependent dilator stimuli were blunted after fluid percussion injury (FPI) in newborn pigs. Membrane potential of vascular muscle is a major determinant of vascular tone, and activity of K+ channels is a major regulator of membrane potential. Recent data show that the NO releasers sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP) and 8-bromo-cGMP elicit dilation via ATP-sensitive K+ channel (KATP) activation. The present study was designed to investigate the effect of FPI on KATP channel function. METHODS Chloralose-anesthetized newborn pigs equipped with a closed cranial window were connected to a percussion device that consisted of a saline-filled cylindrical reservoir and a metal pendulum. Brain injury of moderate severity (1.9 to 2.1 atm) was produced by allowing the pendulum to strike a piston on the cylinder. Pial artery diameter was measured with a video microscaler. Data were analyzed by repeated measures ANOVA. An alpha level of P < .05 was considered significant. RESULTS FPI blunted dilation to cromakalim (10(-8), 10(-6) mol/L), a KATP agonist (10 +/- 1% and 27 +/- 2% versus 3 +/- 1% and 7 +/- 2% before and after FPI, respectively, n = 8). Similarly, FPI blunted dilation to calcitonin gene-related peptide, an endogenous KATP activator. FPI also blunted dilator responses to SNP, S-nitroso-N-acetylpenicillamine, and 8-bromo-cGMP (10(-6) to 10(-8) mol/L) (10 +/- 1% and 20 +/- 1% versus 2 +/- 1% and 8 +/- 2% for SNP before and after FPI; 9 +/- 1% and 16 +/- 1% versus 2 +/- 1% and 4 +/- 1% for 8-bromo-cGMP before and after FPI, respectively, n = 8). In contrast, responses to papaverine and brain natriuretic peptide were unchanged after FPI. CONCLUSIONS These data show that KATP channel function is impaired after FPI. Furthermore, these data suggest that impaired function of mechanisms distal to NO synthase contribute to altered cerebral hemodynamics after FPI.

Endothelin Production Links Superoxide Generation to Altered Opioid-Induced Pial Artery Vasodilation After Brain Injury in Pigs

BACKGROUND AND PURPOSE Traumatic brain injury conveys significant morbidity and mortality to infants and children. In the newborn pig, opioids contribute to pial artery vasconstriction after fluid percussion injury (FPI). FPI attenuates vasodilation and cGMP production by methionine enkephalin (Met) and leucine enkephalin (Leu) and reverse dynorphin (Dyn) from a dilator to a constrictor. Superoxide anion (O2-) production contributes to altered cerebral hemodynamics after FPI, and O2- scavengers partially restore decreased dilator responses after FPI. Endothelin-1 (ET-1), a purported mediator of cerebral vasospasm, has been suggested to alter nitric oxide function and cGMP concentration. The present study was designed to determine the contribution of ET-1 to altered opioid-induced dilation after FPI and the role of O2- in such altered responses. METHODS Injury of moderate severity (1.9 to 2.3 atm) was produced by the lateral FPI technique in anesthetized newborn pigs equipped with a closed cranial window. Superoxide dismutase (SOD)-inhibitable nitroblue tetrazolium (NBT) reduction was determined as an index of O2- generation. RESULTS FPI increased cerebrospinal fluid ET-1 from 20 +/- 2 to 93 +/- 6 pg/mL (approximately 10(-10) mol/L). Topical ET-1 (10(-10) mol/L) increased SOD-inhibitable NBT reduction from 1 +/- 1 to 16 +/- 3 pmol/mm2, similar to previously reported NBT reduction after FPI (14 +/- 2 pmol/mm2). BQ123 (10(-6) mol/L), an ET-1 antagonist, blunted the NBT reduction observed after FPI (4 +/- 1 pmol/mm2). Met produced pial vasodilation that was attenuated by FPI and partially restored by BQ123 pretreatment (7 +/- 1%, 11 +/- 1%, and 17 +/- 1% versus 3 +/- 1%, 6 +/- 1%, and 9 +/- 2% versus 5 +/- 1%, 9 +/- 1%, and 14 +/- 2% for 10(-10), 10(-8), and 10(-6) mol/L Met during control conditions, after FPI, and after FPI pretreated with BQ123, respectively). Met-induced dilation was associated with increased cerebrospinal fluid cGMP, and these biochemical changes were likewise blunted by FPI and partially restored by BQ123 (357 +/- 12, 455 +/- 15, 500 +/- 19, and 632 +/- 11 versus 264 +/- 4, 267 +/- 4, 295 +/- 12, and 305 +/- 15 versus 309 +/- 19, 432 +/- 11, 529 +/- 10, and 593 +/- 4 pg/mL for resting conditions, 10(-10), 10(-8), and 10(-6) mol/L Met during control conditions, after FPI, and after FPI pretreated with BQ123, respectively). Similar partial restoration of vascular and biochemical parameters was observed for Leu and Dyn. CONCLUSIONS These data show that ET-1, in concentrations similar to that present in cerebrospinal fluid after FPI, increases O2- production. These data also indicate the opioid-induced vasodilation and cGMP production are partially restored after FPI by ET-1 receptor blockade. These data suggest that ET-1 contributes to altered cerebral hemodynamics after FPI, at least in part, through elevated O2- production.

Nociceptin/orphanin FQ dilates pial arteries by KATP and Kca channel activation

Abstract Nociceptin/orphanin FQ (NOC/oFQ) is a recently discovered endogenous ligand for the opioid like receptor, ORL-1. In the piglet, cGMP activates the ATP sensitive (KATP) while cAMP activates both the KATP and the calcium sensitive (Kca) K+ channel to elicit vasodilation. The present study was designed to characterize the role of cGMP, cAMP, KATP, and Kca channel activation in NOC/oFQ-induced pial artery dilation in newborn pigs equipped with a closed cranial window. NOC/oFQ (10−8, 10−6 M) induced pial arteriole dilation was decreased by the protein kinase A inhibitor Rp 8-Br cAMPs (16±1 and 30±1 vs. 5±1 and 10±1%). NOC/oFQ dilation was associated with elevated CSF cAMP (1037±58 vs. 1919±209 fmol/ml for control and 10−6 M NOC/oFQ). Glibenclamide and iberiotoxin, KATP and Kca channel antagonists, attenuated NOC/oFQ induced dilation (15±1 and 28±1 vs. 10±1 and 19±1% before and after iberiotoxin). In contrast, the nitric oxide synthase inhibitor, l -NNA, and the protein kinase G inhibitor, Rp 8-Br cGMPs had no effect on NOC/oFQ dilation while such dilation was not associated with a change in CSF cGMP. The putative ORL-1 receptor antagonist [F/G] NOC/oFQ (1–13)–NH2 blocked NOC/oFQ dilation while responses were unchanged after naloxone (17±1 and 30±2 vs. 3±1 and 5±1%, before and after [F/G] NOC/oFQ (1–13)–NH2). Dilation to other opioids (e.g., methionine enkephalin) was unchanged by [F/G] NOC/oFQ (1–13)–NH2. These data show that NOC/oFQ elicits pial artery dilation, at least in part, via cAMP, KATP, and Kca channel dependent mechanisms. These data suggest that such a mechanism involves the sequential release of cAMP and subsequent KATP and Kca channel activation.