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.

Neonatal intensive care applications of near-infrared spectroscopy.

Near-infrared spectroscopy (NIRS) is a noninvasive technique for assessing cerebral hemodynamic variables and oxidative status in the neonatal intensive care setting. It can be performed for extended periods of time at the bedside without interfering with routine patient care. NIRS appears to have the ability to not only assess relative changes in oxygenated and deoxygenated hemoglobin, total hemoglobin, and cytochrome aa3, but it can also produce estimates of cerebral blood volume and cerebral blood flow. Research data document significant changes in these hemodynamic variables with patient activity and clinical interventions in both premature and term infants. NIRS may evolve into an important diagnostic and prognostic tool for neonatal neurologic outcome.

Effects of perfusion mode on regional and global organ blood flow in a neonatal piglet model.

BACKGROUND: Organ injury (brain, kidney, and heart) has been reported in up to 30% of pediatric open heart surgery patients after conventional hypothermic non-pulsatile cardiopulmonary bypass (CPB) support with or without deep hypothermic circulatory arrest (DHCA). The effects of pulsatile (with a Food and Drug Administration approved modified roller pump) versus non-pulsatile perfusion on regional and global cerebral, renal, and myocardial blood flow were investigated during and after CPB with 60 minutes of DHCA in a neonatal piglet model. METHODS: Piglets, mean weight 3 kg, were used in both pulsatile (n = 7) and non-pulsatile (n = 7) groups. After initiation of CPB, all animals were subjected to hypothermia for 25 minutes, reducing the rectal temperatures to 18 degrees C, 60 minutes of DHCA followed by 10 minutes of cold reperfusion and 40 minutes of rewarming with a pump flow of 150 mL/kg/min. During cooling and rewarming, alpha-stat acid-base management was used. Differently labeled radioactive microspheres were injected pre-CPB, on normothermic CPB, pre-DHCA, post-DHCA, and after CPB to measure the regional and global cerebral, renal, and myocardial blood flows. RESULTS: Global cerebral blood flow was significantly higher in the pulsatile group compared to the non-pulsatile group at normothermic CPB (100.4 +/- 6.3 mL/100 gm/min versus 70.2 +/- 8.1 mL/100 gm/min, p < 0.05) and pre-DHCA (77.2 +/- 5.2 mL/100 gm/min versus 56.1 +/- 6.7 mL/100 gm/min, p < 0.05). Blood flow in cerebellum, basal ganglia, brain stem, and right and left cerebral hemispheres had an identical pattern with the global cerebral blood flow. Renal blood flow appeared higher in the pulsatile group compared to the non-pulsatile group during CPB, but the results were statistically significant only at post-CPB (94.8 +/- 9 mL/100 gm/min versus 22.5 +/- 22 mL/100 gm/min, p < 0.05). Pulsatile flow better maintained the myocardial blood flow compared to the non-pulsatile flow after CPB (316.6 +/- 45.5 mL/100 gm/min versus 188.2 +/- 19.5 mL/100 gm/min, p < 0.05). CONCLUSIONS: Pulsatile perfusion provides superior vital organ blood flow compared to non-pulsatile perfusion in this model.

Correlation of near infrared spectroscopy cerebral blood flow estimations and microsphere quantitations in newborn piglets.

We compared cerebral blood flow (CBF) estimated using transmission mode near infrared spectroscopy (NIRS) and a modification of the Fick principle with CBF quantitations by radioactive microspheres (MSs) in newborn piglets. Thirteen piglets were studied during steady state, ischemia, and during two reflow periods. NIRS and MS flows were not significantly different during any measurement period. NIRS flows were compared to total brain blood flows and to regional brain blood flows quantitated with MSs and correlated best with temporal cortical flows. Linear regression analysis of the NIRS flows plotted against MS-quantitated temporal cortical flows showed r = 0.71. Thus, CBFs obtained with NIRS were not significantly different from, showed the same directional changes, and correlated acceptably with flows quantitated by MSs.

Cytochrome oxidase is decreased in piglet hippocampus following hypoxia-ischemia.

We assessed cytochrome oxidase (CytOx) staining in sham-operated control piglets and in piglets subjected to 30 minutes of cerebral hypoxia-ischemia (H-I) plus 4 hours of reperfusion (REP). The 1-day-old piglets were sedated, anesthetized, and ventilated. Cerebral blood flows (CBFs) were quantitated using microspheres. H-I was induced by a combination of phlebotomy and cervical tourniquet; the brain was reperfused for four hours after 30 minutes of H-I. CBF was reduced during ischemia in experimental animals from 42 + 13 to 12 + 5 ml/min/100g. CytOx staining of hippocampal sections from 3 control and 3 experimental animals was compared. The staining of the stratum pyramidale neurons of the same portion of the CA1 sector in a single high power field was assessed in a blinded fashion in 4 corresponding sections from each animal, and graded from 0 = no staining to 3 = heavy staining. The results were compared using one-way analysis of variance. Cells with grade 3 staining were significantly more numerous in controls compared to H-I/REP animals (p = 0.03). There were significantly more cells with no CytOx staining in the experimental animals (p = 0.01). These findings suggest that CytOx staining in newborn piglet CA1 is a reliable method of assessing cell dysfunction after H-I.

Cerebral hypoxia-ischemia increases microsomal iron in newborn piglets.

The primary cause of neurologic impairment in newborn infants is hypoxic-ischemic injury. Studies of the mechanisms involved in the damaging effects of hypoxia-ischemia and reperfusion in brain tissue indicate significant contributions from reactive oxygen species, with the loss of homeostatic control of intracellular iron an important determinant of oxidant-mediated damage. We investigated the effects of cerebral hypoxia-ischemia and reperfusion on the redistribution of nonheme iron in newborn piglets. Anesthetized newborn piglets were subjected to reductions in cerebral blood flow by phlebotomy and cervical cuff compression. Control animals were sham-operated. Subcellular fractions were isolated from brain tissue homogenates by differential centrifugation, and nonheme iron contents of these fractions were measured with ferene-S. Iron contents in the homogenates were not altered. However, iron contents of the microsomal fractions of animals subjected to 30 minutes of hypoxia-ischemia increased from 0.517 +/- 0.053 to 0.930 +/- 0.061 nmol/mg protein (p < 0.01); 120 minutes of reperfusion caused no further changes. This translocation of iron may be linked to oxidative alterations of brain proteins, which we investigated by detection of dinitrophenylhydrazine-derivitized protein carbonyls, which are characteristic of iron-catalyzed oxidation reactions.

Hexokinase binding in ischemic and reperfused piglet brain.

Hexokinase catalyzes the first step in cerebral glucose utilization and is a rate-limiting enzyme in glycolysis. Glucose utilization is tightly coupled to cerebral blood flow so that during ischemia the brain has a decreased supply of glucose, as well as oxygen. We studied hexokinase enzymatic activity in a newborn piglet model of ischemia-reperfusion to determine if any changes in the activity or mitochondrial binding of the enzyme occurred. We observed that mitochondrial binding of cortical HK increased from 55 to 71% with ischemia and returned toward control levels, but did not completely recover, after 2 h of reperfusion.

Dietary cholesterol supplementation improves growth and behavioral response of pigs selected for genetically high and low serum cholesterol.

We hypothesized that, in pigs selected for low (L) or high (H) serum cholesterol for four generations, neonatal endogenous cholesterol synthesis would be sufficient to meet requirements for brain and body growth. In Experiment 1, eight 16-wk-old L pigs received a diet with or without 200 mg cholesterol/100 g diet for 35 d. Supplemented pigs grew approximately 25% faster and had a significantly greater concentration of free cholesterol in the cerebrum. In Experiment 2, 16 H and 16 L newborn pigs were fed a milk replacer with or without 200 mg cholesterol/100 g diet for 28 d. Pigs fed cholesterol had greater average daily gain (P < or = 0.09), significantly reduced liver 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity, and significantly increased cerebral cholesterol content than pigs not fed cholesterol. One of three indices of exploratory behavior was significantly greater in the L pigs that received cholesterol compared with L pigs that did not receive cholesterol. These data suggest that these neonatal pigs are unable to produce sufficient cholesterol to meet requirements for normal growth and brain development and are dependent on dietary cholesterol in milk.

Brain blood flow responses to indomethacin during hemorrhagic hypotension in newborn piglets.

Indomethacin has been shown to reduce cerebral blood flow and cerebral blood flow velocities in newborn infants and animals of various species. To answer the question of whether there may be a compromise of cerebral perfusion in hypotensive infants who have been treated with indomethacin, cerebral blood flow and cerebral vascular resistance were determined in 10 control and 16 indomethacin-treated 1-day-old piglets during (1) steady state conditions; (2) 10 min after the administration of saline or a 0.2-mg/kg dose of indomethacin; (3) 1 h after saline or indomethacin administration, and (4) 10 min after induction of moderate hemorrhagic hypotension. Mean arterial blood pressures increased immediately after the infusion of indomethacin in the experimental group. Cerebral blood flows did not change throughout the study despite hemorrhagic hypotension in controls; cerebral blood flows were significantly decreased 10 min after indomethacin infusion in the experimental animals. However, total and regional cerebral blood flows were not further decreased in the presence of moderate hypotension. Cerebral vascular resistance increased 10 min after indomethacin infusion but returned to steady state 1 h following the indomethacin dose. These results suggest that indomethacin lowers baseline cerebral blood flow, but does not impair cerebrovascular regulatory responses during acute, moderate hemorrhagic hypotension in the newborn piglet.

Barbiturates and hyperventilation during intracranial hypertension.

OBJECTIVE: The purpose of this study was to determine the effect of hyperventilation alone and hyperventilation plus barbiturate therapy on intracranial pressure, global and regional cerebral blood flow rates, cerebrovascular resistance, and cerebral perfusion pressure in adult dogs with and without intracranial hypertension induced by epidural balloon. DESIGN: Prospective, randomized, controlled study. SETTING: An animal laboratory of a university hospital. Four sequential global and regional cerebral blood flow determinations were made in each animal during monitoring of heart rate and systemic arterial pressure, during respiratory control and arterial blood gas monitoring, intracranial pressure monitoring, and with or without inflation of an epidural balloon catheter. SUBJECTS: Acute mongrel dogs obtained from the Baylor Center for Comparative Medicine. Five groups of animals were studied. In group 1, the response to hyperventilation was assessed in dogs without increased intracranial pressure. In group 2, the response to hyperventilation was assessed in animals with acute intracranial hypertension. In group 3, the response to hyperventilation plus barbiturate therapy was assessed in dogs without increased intracranial pressure. In group 4, the response to hyperventilation plus barbiturate therapy was assessed in dogs with acute increased intracranial pressure. In group 5, a group of dogs with increased intracranial pressure was treated with neither hyperventilation nor barbiturates. INTERVENTIONS: Hyperventilation, hyperventilation plus barbiturate therapy, or no interventions were studied in these experimental paradigms. MEASUREMENTS AND MAIN RESULTS: The main outcome measures were changes in intracranial pressure and/or changes in regional or total cerebral blood flow. A significant decrease in intracranial pressure and cerebral blood flow rate was produced by hyperventilation alone in groups with intracranial hypertension. Combined hyperventilation and barbiturate therapy resulted in a significant further decrease in cerebral blood flow rate in animals with normal and increased intracranial pressure, but no greater decrease in intracranial pressure was seen compared with treatment with hyperventilation alone. Cerebral perfusion pressures remained normal despite significant decreases in cerebral blood flow rates. CONCLUSIONS: These studies suggest that barbiturate administration in this model of intracranial hypertension was no more effective in reducing increased intracranial pressure than hyperventilation alone.

Phenobarbital and cerebral blood flow during hypotension in newborn pigs.

Phenobarbital sodium (PhS) has been used in anticonvulsant concentrations in premature newborns in attempts to prevent peri- and intraventricular hemorrhages (PIVH). Its effectiveness in preventing PIVH in clinical situations is still uncertain; however, PhS has reduced PIVH after hypertension in newborn beagles, and it has lowered cerebral blood flow (CBF) during hypertension in newborn beagles and piglets. We hypothesized that PhS might reduce CBF during systemic hypotension. Twelve control and 12 PhS-treated piglets (1 to 2 d old) were used for microsphere determinations of CBF during 1) steady state; 2) 30 min after PhS (treatment group) or saline infusion (controls); and 3 and 4) during two levels of graded hypotension. Mean arterial blood pressure (MABP) was 61 +/- 13 (SD) mm Hg (controls) and 57 +/- 13 (SD) mm Hg (PhS) during steady state. Thirty min after the PhS or saline infusion, MABP and CBF remained unchanged in both groups. CBF during hypotension at MABP of 41 +/- 5 (SD) mm Hg was significantly higher in controls than was CBF at MABP of 39 +/- 6 (SD) mm Hg in the PhS-treated group (p = 0.044); CBF in the two groups during the second hypotensive phase was not significantly different. However, LOWESS regression suggested that the CBF from the controls dropped as the arterial pressure decreased to less than 37 mm Hg, whereas PhS treatment lowered CBF during hemorrhagic hypotension compared with controls at blood pressures greater than 37 mm Hg but did not lower CBF further at lower systemic blood pressures.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain vasoactive effects of phenobarbital during hypertension and hypoxia in newborn pigs.

Phenobarbital in anticonvulsant concentrations has been shown to lower cerebral blood flow (CBF) during hypertension and to reduce the incidence of intraventricular hemorrhage in newborn beagles after hypertensive insult. We proposed that hypoxic dilatation of brain blood vessels might alter the effect of phenobarbital (PBS) on blood flow during hypertension. Thus, in 14 control and nine PBS-treated 1- to 2-d-old newborn piglets, the radio-active microsphere technique was used to determine CBF during 1) steady state (SS), 2) hypertension (HT), and 3) HT plus hypoxia of 5 min duration. In seven controls and in four PBS-treated piglets, CBF was also determined during recovery from hypoxia. PBS was infused after SS in a 20-mg/kg dose, and serum levels were obtained 30 min later. Blood pressures were not significantly different between groups when compared during SS, HT, hypoxia, and recovery. Similarly, pH, PO2, and PCO2 were not significantly different between groups when compared during normoxia and hypoxia, and hematocrits were maintained by transfusions after reference sample withdrawals. CBF in control animals increased significantly during HT and remained significantly higher than SS values throughout the 5 min of hypoxia and into the recovery period. In PBS-treated piglets, however, there was no significant increase in CBF during HT. Blood flows also stayed at SS levels during HT plus 5 min hypoxia and recovery from hypoxia. Thus, in newborn piglets, PBS lowered CBF during HT. Furthermore, this blood flow lowering effect persisted during hypoxia, preventing compensatory increases in CBF that might have otherwise occurred.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebrovascular regulation and neonatal brain injury.

Neuropathology occurring as a result of hemodynamic injury occurs in up to 25% of preterm newborns of less than 1,500 gm birth weight and in a much smaller, but nonetheless meaningful, proportion of more mature infants. Abnormalities in cerebrovascular regulation have been proposed as major contributing factors to both ischemic and hemorrhagic injuries in the newborn brain. In this review we explore several factors that play a role in cerebrovascular regulation in the immature brain and relate them to what is known about vascular regulation in the mature brain and to the types of pathology that occur in the newborn brain. One goal in this "decade of the brain" should be to increase our basic and clinical knowledge about the cerebrovasculature of the newborn in order to enhance our ability to predict and prevent perinatal brain injury.

Phenobarbital and cerebral blood flow during hypertension in the newborn beagle.

Phenobarbital sodium has been used in anticonvulsant concentrations (15 to 40 micrograms/mL serum) in premature newborns in attempts to prevent periventricular and intraventricular hemorrhages. Although its clinical usefulness in this regard is controversial, phenobarbital treatment has been shown to reduce periventricular and intraventricular hemorrhages after hypertensive insult in newborn beagles. In this study cerebral blood flow values in steady state and during phenylephrine-induced hypertension with and without phenobarbital pretreatment were measured in newborn beagles. At anticonvulsant dosage, phenobarbital sodium decreased mean arterial blood pressure transiently during steady state and significantly reduced total cerebral blood flow during phenylephrine-induced hypertension without reducing mean arterial blood pressure. This phenobarbital sodium effect on cerebral blood flow was not as great in the presence of acidosis, and the initial hypotensive effect of phenobarbital sodium was sustained for a longer period of time during acidosis. Phenobarbital sodium may reduce the incidence of hemorrhages in the newborn brain by providing protection against isolated hemodynamic stresses characterized by acute increases in cerebral blood flow, with or without increased mean arterial blood pressure.

Prediction of brain blood flow using pulsed Doppler ultrasonography in newborn lambs.

The purpose of this study was to evaluate the ability of pulsed Doppler ultrasonography to predict brain blood flow by comparing measurements made in basal cerebral arteries using a pulsed Doppler system with measurements of brain blood flow using radioactive microspheres in lambs. We measured the instantaneous maximum velocity during peak systole and end diastole and the mean velocity over the pulse cycle in basal cerebral arteries, calculated Pourcelot's index of resistance and Gosling's pulsatility index, and used regression analysis to compare the pulsed Doppler measurements with brain blood flow measured with radioactive microspheres. Pulsed Doppler measurements of the peak systolic, end diastolic, and mean flow velocity in basal cerebral arteries were directly related to brain blood flow. In contrast, no linear relationship was detected between the resistance or pulsatility indices and brain blood flow measured by microspheres (p greater than 0.14). Prediction of brain blood flow for individual subjects lacks the quantitative precision necessary for use as a clinical tool. However, the direct relationship between brain blood flow and the peak systolic, end diastolic, and mean flow velocities in basal cerebral arteries supports the use of these measurements in clinical research for the qualitative assessment of change in brain blood flow.

Lipid-containing cells in brains of normal and hypoxic infant monkeys: a quantitative and ultrastructural study.

The significance of lipid-containing cells found at autopsy in the white matter of infant brains is controversial, particularly with respect to their postulated role as markers of the "sudden infant death syndrome." To determine whether such cells are indicative of prior nonlethal hypoxic insult, we quantitated them in the brains of control infant monkeys and in two groups of infant monkeys that were subjected to 30 minutes of hypoxic insult. One group consisting of monkeys that died less than 48 hours after the hypoxia, and the other of those that survived 7 to 13 days following the insult. The quantification of lipid-containing cells was undertaken in frozen brain sections stained with Oil red O; sections of brains from 4 perfusion-fixed animals were evaluated by electron microscopy. Lipid-containing cells were found in the corpus callosum, in the septum, and in periventricular white matter in both posthypoxic and control animals. There was a relationship between numbers of lipid-containing cells and the age of the animal; animals with large numbers were less than 28 days old. Decreases in numbers of lipid-containing cells correlated with advancing myelination as well as with age. Electron microscopic evaluation revealed lipid droplets in the cytoplasm of cells with irregularly shaped nuclei, densely clumped chromatin, occasional microtubules, and narrow cytoplasmic processes. We suggest that lipid-containing cells in the white matter of the brains of infants are related to age and to maturational factors and, in the absence of other pathologic signs, are not related to prior hypoxic injury.

Prostaglandin E1-associated pathology of pulmonary microvasculature in newborn pups: similarity to findings in prostaglandin E1-treated human newborns.

Prostaglandin E1 (PGE1) administration is a useful therapeutic measure for short-term maintenance of ductal patency in patients with obstructions to pulmonary or systemic blood flow. Such treatment is not without complications, however, and a report of three infants from our institution with abnormalities of the pulmonary microvasculature after varying periods of PGE1 therapy was recently published (Heffelfinger et al., Pediatr Pathol 1987;7:165-73). The vascular abnormalities appeared to be temporally related to the PGE1 administration. To test this hypothesis, we investigated the effects of PGE1 in newborn beagles by infusing PGE1 for periods of up to 21 days in four experimental pups. Two control pups were infused with saline for the same period of time. Five of the animals developed respiratory infection during the course of the infusions. One PGE1-treated pup was not infected. Both the PGE1- and saline-treated pups had bronchopneumonias of similar severity; however, pulmonary arteritis occurred only in the PGE1-treated pups. The severity of the arteritis varied with the amount of pulmonary parenchymal inflammation and not with the duration of PGE1 administration. Inflammatory and vascular lesions were found in organs other than the lung only in two pups receiving longer courses of PGE1 treatment. We conclude that systemic PGE1 infusion at therapeutic levels plays a role in the development of arterial lesions in small muscular arteries and that this is potentiated by the presence of infection.

Understanding and preventing perinatal, intracerebral, peri- and intraventricular hemorrhage.

Antenatal anticipation of problem pregnancies and improvements in resuscitation and care of newborns have led to increasing survival of babies born prematurely. Nevertheless, the potential for neurologic handicaps is significant in this population of children, and the prevention of intracerebral, peri- and intraventricular hemorrhages and associated brain lesions remains a high priority. In this review, we consider the clinical problem of periventricular, intraventricular hemorrhage; means of diagnosis; the EEG and periventricular, intraventricular hemorrhage; sequelae; hypotheses of pathogenesis; experimental approaches to understanding periventricular, intraventricular hemorrhage; agents being tested for use in preventing hemorrhage; and future areas for research toward the prevention of hemorrhage and other neonatal brain lesions.

Reduction in incidence of periventricular, intraventricular hemorrhages in hypertensive newborn beagles pretreated with phenobarbital.

Periventricular and intraventricular intracerebral hemorrhages occur frequently in premature newborns and are markers for, or contribute significantly to, neurologic morbidity in survivors. Hemorrhages are hypothesized to result from rapid fluctuations in cerebral perfusion pressure and/or cerebral blood flow. Phenobarbital sodium has been given to premature infants in anticonvulsant dosages in attempts to prevent hemorrhages, but its efficacy in clinical studies has been disputed. In this study, in the 24 to 72-hour-old newborn beagle, an animal model for periventricular and intraventricular intracerebral hemorrhage, phenobarbital sodium was administered to obtain anticonvulsant serum levels prior to phenylephrine-induced hypertensive insult. None of the animals was hypoxic or hypercarbic. Resulting hypertensive systemic mean arterial BPs were 99 +/- 8 mm Hg in the 15 control pups and 93 +/- 15 mm Hg in the 15 phenobarbital sodium-treated pups (not statistically significantly different). The duration of the hypertension was the same in both groups. Phenobarbital caused a significant decrease in mean arterial BP in the treated group just after its administration (P less than .05). Six of the 15 control pups (40%) and one of the 15 treated pups (7%) demonstrated macroscopic and microscopic periventricular and intraventricular hemorrhage (P less than .05, chi 2, Yates correction). Thus, in nonasphyxiated newborn beagles, phenobarbital sodium significantly reduced the incidence of hemorrhage after hypertensive insult.