Autoregulation of cerebral blood flow in the newborn beagle puppy

Regulation of the Cerebral Circulation During Development

The cerebral microcirculation undergoes dynamic changes in parallel with the development of neurons, glia, and their energy metabolism throughout gestation and postnatally. Cerebral blood flow (CBF), oxygen consumption, and glucose consumption are as low as 20% of adult levels in humans born prematurely but eventually exceed adult levels at ages 3 to 11 years, which coincide with the period of continued brain growth, synapse formation, synapse pruning, and myelination. Neurovascular coupling to sensory activation is present but attenuated at birth. By 2 postnatal months, the increase in CBF often is disproportionately smaller than the increase in oxygen consumption, in contrast to the relative hyperemia seen in adults. Vascular smooth muscle myogenic tone increases in parallel with developmental increases in arterial pressure. CBF autoregulatory response to increased arterial pressure is intact at birth but has a more limited range with arterial hypotension. Hypoxia-induced vasodilation in preterm fetal sheep with low oxygen consumption does not sustain cerebral oxygen transport, but the response becomes better developed for sustaining oxygen transport by term. Nitric oxide tonically inhibits vasomotor tone, and glutamate receptor activation can evoke its release in lambs and piglets. In piglets, astrocyte-derived carbon monoxide plays a central role in vasodilation evoked by glutamate, ADP, and seizures, and prostanoids play a large role in endothelial-dependent and hypercapnic vasodilation. Overall, homeostatic mechanisms of CBF regulation in response to arterial pressure, neuronal activity, carbon dioxide, and oxygenation are present at birth but continue to develop postnatally as neurovascular signaling pathways are dynamically altered and integrated. © 2021 American Physiological Society. Compr Physiol 11:1-62, 2021.

Perinatal stroke

Perinatal stroke is a heterogeneous syndrome resulting from brain injury of vascular origin that occurs between 20 weeks of gestation and 28 days of postnatal life. The incidence of perinatal stroke is estimated to be between 1:1600 and 1:3000 live births (approximately 2500 children per year in the United States), though its actual incidence is difficult to estimate because it is likely underdiagnosed. Perinatal arterial ischemic stroke (PAIS) accounts for approximately 70% of cases of perinatal stroke. Cerebral sinovenous thrombosis, while less common, also accounts for a large proportion of the morbidity and mortality seen with perinatal stroke. Hemorrhagic stroke leads to disruption of neurologic function due to intracerebral hemorrhage that is nontraumatic in origin. While most cases of PAIS fall into one of these three categories, other patterns of injury should also be considered perinatal stroke. In some cases, the etiology of PAIS is not known but is idiopathic. This chapter will review the classification, risk factors, pathogenesis, clinical presentation, management, and long-term sequelae of perinatal stroke.

Fetal Cerebrovascular Maturation: Effects of Hypoxia

The human cerebral vasculature originates in the fourth week of gestation and continues to expand and diversify well into the first few years of postnatal life. A key feature of this growth is smooth muscle differentiation, whereby smooth muscle cells within cerebral arteries transform from migratory to proliferative to synthetic and finally to contractile phenotypes. These phenotypic transformations can be reversed by pathophysiological perturbations such as hypoxia, which causes loss of contractile capacity in immature cerebral arteries. In turn, loss of contractility affects all whole-brain cerebrovascular responses, including those involved in flow-metabolism coupling, vasodilatory responses to acute hypoxia and hypercapnia, cerebral autoregulation, and reactivity to activation of perivascular nerves. Future strategies to minimize cerebral injury following hypoxia-ischemic insults in the immature brain might benefit by targeting treatments to preserve and promote contractile differentiation in the fetal cerebrovasculature. This could potentially be achieved through inhibition of receptor tyrosine kinase-mediated growth factors, such as vascular endothelial growth factor and platelet-derived growth factor, which are mobilized by hypoxic and ischemic injury and which facilitate contractile dedifferentiation. Interruption of the effects of other vascular mitogens, such as endothelin and angiotensin-II, and even some miRNA species, also could be beneficial. Future experimental work that addresses these possibilities offers promise to improve current clinical management of neonates who have suffered and survived hypoxic, ischemic, asphyxic, or inflammatory cerebrovascular insults.

Risk factors associated with intracranial hemorrhage in neonates with persistent pulmonary hypertension on ECMO

Background

Up to 40% of infants with persistent pulmonary hypertension (PPHN) remains refractory to conventional therapies, and extracorporeal membrane oxygenation (ECMO) is offered as an effective support for this group. However, ECMO is a highly invasive and risky procedure with devastating complications such as intracranial hemorrhage (ICH). In this study, we aimed to determine the risk factors for ICH in infants with PPHN.

Methods

A case-control study of patients admitted to the pediatric intensive care unit (PICU) with PPHN requiring ECMO support was conducted. The study was carried out at a 25-bed PICU in large urban tertiary care children’s hospital. A total number of 32 subjects were studied. Patients with and without ICH during ECMO were evaluated for activated clotting time (ACT), heparin dosing, platelet count, coagulation profile such as activated partial thromboplastin time (aPTT), prothrombin time (PT), international normalized ratio (INR), fibrinogen level, vital signs including heart rate and mean arterial pressure (MAP), transfusion history, gestational age, and severity of pre-ECMO illness as possible risk factors.

Results

Low fibrinogen level (115 ± 13 mg/dl) and low platelet counts (37.4 ± 18.3 Thousand/μl) were associated with higher incidence of ICH (p = 0.009 and p = 0.005, respectively). Elevated MAP (69 ± 4.34 mmHg) was also noticed in ICH patients (p = 0.006).

Conclusions

Results demonstrated that low fibrinogen level and low platelet count were associated with ICH in PPHN patients on ECMO. While on ECMO support, maintaining fibrinogen and platelet counts within normal ranges seems crucial to prevent ICH in PPHN patients. This is the first report identifying low fibrinogen level among the risk factors for ICH in infants with PPHN on ECMO support.

The role of systemic hemodynamic disturbances in prematurity-related brain injury

Premature infants who experience cerebrovascular injury frequently have acute and long-term neurologic complications. In this article, we explore the relationship between systemic hemodynamic insults and brain injury in this patient population and the mechanisms that might be at play.

Cerebrovascular injury in premature infants: current understanding and challenges for future prevention

Cerebrovascular insults are a leading cause of brain injury in premature infants, contributing to the high prevalence of motor, cognitive, and behavioral deficits. Understanding the complex pathways linking circulatory immaturity to brain injury in premature infants remains incomplete. These mechanisms are significantly different from those causing injury in the mature brain. The gaps in knowledge of normal and disturbed cerebral vasoregulation need to be addressed. This article reviews current understanding of cerebral perfusion, in the sick premature infant in particular, and discusses challenges that lie ahead.

Animal models of germinal matrix hemorrhage

Germinal matrix hemorrhage refers to bleeding that arises from the subependymal (or periventricular) germinal region of the immature brain. Clinical studies have shown that infants who experience germinal matrix hemorrhage can develop hydrocephalus or suffer from long-term neurologic dysfunction, including cerebral palsy, seizures, and learning disabilities. Understanding the causative factors and the pathogenesis of subsequent brain damage is important if germinal matrix hemorrhage is to be prevented or treated. Appropriate animal models are necessary to achieve this understanding. A number of animal species, including mice, rats, rabbits, sheep, pigs, dogs, cats, and primates, have been used to model germinal matrix hemorrhage. This literature review critically evaluates the animal models of germinal matrix hemorrhage. Each model has its own advantages and disadvantages; no single model is suitable for the study of all aspects of brain damage.

Morphometric assessment of collagen accumulation in germinal matrix vessels of premature human neonates

Germinal matrix haemorrhage in premature neonates is commonly attributed to vascular immaturity, possibly related to an abbreviated process of angiogenesis. Terminal steps in the progression of angiogenesis are the formation of a subendothelial basal lamina containing collagen IV and an extracellular matrix containing collagens I and III. Immature vessels would predictably be deficient in these collagen subtypes. We analysed germinal matrix (GM), cortical, and white matter (WM) vessels with antibodies specific for collagens I, III and IV to test the hypothesis that GM vessels are immature. Brains were collected during post-mortem from prematurely born human neonates ranging in age from 17 weeks to 36 weeks postconception. All GM vessels were immunoreactive for collagen subtypes I, III and IV. Using digital image analysis, collagen IV immunoperoxidase-labelling was measured in vessels in GM, cortex and WM. Intensity values in GM and WM were normalized relative to cortical intensity within the same subject. At week 17 of gestation, GM vessels exhibited a higher concentration of collagen IV than did WM or cortical vessels. Regression analysis demonstrated that collagen intensity in GM was greater than that in cortex and WM at all stages. We conclude that GM vessels in even the youngest, prematurely born, viable neonates do not exhibit evidence of structural immaturity. The high incidence of GM haemorrhage in premature neonates may be related to factors other than a deficiency in accumulated collagen.

Cerebral pressure autoregulation and vasoreactivity in the newborn rat

Perinatal brain injury has been associated with impaired cerebral blood flow (CBF) pressure autoregulation. The brain of 3- to 5-d-old rat pups is immature and similar to that of a preterm infant, and therefore we tested cerebral vasoreactivity in that animal. CBF pressure autoregulation was tested in 20 Wistar pups during normocapnia and hypercapnia, respectively. Hypotension was induced by hemorrhage and cerebral perfusion was monitored with laser Doppler flowmetry and near-infrared spectroscopy. Systolic blood pressure was measured noninvasively from the tail. During normocapnia, the autoregulatory plateau was narrow. Resting systolic blood pressure (SBP) was 39.2 mm Hg and CBF remained constant until SBP decreased below 36.0 mm Hg (SE 0.8). Below the lower limit, CBF declined by a mean of 2.7% per mm Hg [95% confidence interval (CI), 2.4-3.0%], and hemoglobin difference (HbD) and total hemoglobin (HbT) changed proportionally to CBF. After inhalation of carbon dioxide, CBF increased significantly by a mean of 17.7% (95% CI, 13.7-22.8%). The CBF-CO2 reactivity was estimated to 13.4% per kPa (95% CI, 2-24.8%), p=0.026. Over the range of SBP (6-54 mm Hg), a linear relationship between CBF and SBP was found during hypercapnia, indicating abolished pressure autoregulation. A linear correlation between CBF and HbD was found (r=0.80). CBF pressure autoregulation and reactivity to CO2 operate in the newborn rat. This model may be useful for future investigations concerning perinatal pathophysiology in the immature brain.

Blood pressure in the very low birth weight neonate

Few aspects of management of very low birth weight (VLBW; <1500 g) neonates have generated as much controversy as the assessment of blood pressure (BP) and need for treatment of perceived abnormalities of this physiologic variable. The approach to this problem may differ greatly among various institutions and even among clinicians within a given center. The purpose of this manuscript is to review available information regarding physiologic determinants and measurement of BP in VLBW neonates, normative data for BP, clinical factors that may affect BP in these at-risk neonates and studies in which presumed abnormalities of BP resulted in adverse clinical outcomes. Options for treatment of low BP in VLBW neonates also will be discussed.

Neoteric physiologic and immunologic methods for assessing early-onset neonatal sepsis

Septicemia is a growing problem among low birth weight infants. Early identification and treatment of sepsis in these infants would help to reduce the high mortality and morbidity seen with this disorder. Newer techniques may make earlier diagnosis a reality. In the following review article, early-onset sepsis in the premature infant is described, specifically focusing on the neonatal inflammatory response, neutropenia, and its somewhat inconsistent and delayed role as a marker for sepsis risk factors. Physiological signs, laboratory indicators, skin temperature, peripheral perfusion, and the interaction of macro-environmental factors are also discussed. Newer (neoteric) immunologic and cytokine markers of sepsis are reviewed. Finally, thermography, a noninvasive bioinstrument measuring vasoactive peripheral perfusion, which has potential for early recognition of neonatal septicemia, is described.

Developmentally increased cerebrovascular NO in newborn pigs curtails cerebral blood flow autoregulation

We tested the hypothesis that a reduced ability of the newborn (1-2 d old) to autoregulate cerebral blood flow (CBF) during acute hypertension is contributed by an increased synthesis of nitric oxide (NO) from endothelial (e) and neuronal NO synthase (nNOS). As previously reported, CBF (measured by radiolabeled microsphere technique) in newborn pigs remained constant only between 50 and 90 mm Hg of mean arterial blood pressure. Treatment of newborn pigs with Nomega-monomethyl-L-arginine or specific nNOS inhibitors 7-nitroindazole monosodium, 3-bromo-7-nitroindazole, and 1-(2-trifluoromethylphenyl) imidazole extended the upper limit of CBF autoregulation as seen in saline-treated (control) juvenile (4-6-wk-old) animals. Cerebrovascular production of nitrite (stable NO oxidation product) in vivo was markedly increased during hypertension (mean arterial blood pressure > 90 mm Hg) in newborn but not in the juvenile pigs. Inhibition of NOS with Nomega-monomethyl-L-arginine, 7-nitroindazole monosodium, 3-bromo-7-nitroindazole, or 1-(2-trifluoromethylphenyl) imidazole prevented the hypertension-induced increase in nitrite levels. In addition, eNOS and nNOS protein expression and activity were 2- to 3-fold higher (p < 0.05) in the cerebral microvasculature of newborn than in the tissues of juvenile pigs. It is concluded that during acute hypertension, excess production of NO associated with increased activity of NOS curtails the upper limit of CBF autoregulation in the newborn subject; in addition, nNOS seems to serve a significant role in this important physiologic function.

Intrinsic tone of cerebral artery segments of human infants between 23 weeks of gestation and term

Segments of basilar and middle cerebral arteries of eight human preterm and early postnatal infants have been examined using the resistance artery myograph technique for wire-mounted segments and the pressure perfusion arteriograph. Myograph-mounted segments spontaneously developed tone of varying duration and time course. Perfused segments showed maintained tone levels of approximately 40% of maximum possible constriction when the intraluminal pressure was 60 mm Hg. This level is not different from that found in adult human pial arteries of similar lumen diameter. Indomethacin (10[-5] M) either initiated tone increase or potentiated existing tone in the isometrically mounted segments. After washout of vasoconstrictors norepinephrine (10[-6] M) and angiotensin II (10[-8] M), indomethacin caused a pronounced, long lasting increase in basal tone. Spontaneous tone was reversed by acetylcholine (10[-6] M), isoproterenol (10[-8] to 10[-5] M), histamine (10[-8] to 10[-5] M), and papaverine (10[-5] M). Low levels of tone were increased and higher levels decreased by intraluminal flow. The pressure/diameter curves of these vessels were of similar shape as those of the equivalent size in the adult. It is concluded that intrinsic tone is a prominent feature of these large cerebral arteries, and it is modified by an endogenous indomethacin-sensitive process.

Cerebral blood flow in the newborn infant

Studies of CBF have provided some insight into cerebrovascular physiology and pharmacology. However, the precise relation between CBF and cerebral damage remains elusive, and there is no definition of a threshold CBF below which ischaemic brain damage always occurs. Measurement of CBF thus does not currently provide a secure guide in the clinical management of sick infants. Further work, particularly using techniques like magnetic resonance imaging and NIRS, which provide data in addition to CBF measurements, may yet disclose strategies which manipulate CBF to reduce cerebral ischaemia. While cerebral injury remains a substantial problem in neonatal intensive care, such research is urgently needed.

Effect of cerebral blood flow and cerebrovascular autoregulation on the distribution, type and extent of cerebral injury

Global cerebral blood flow (GCBF) is low in the human neonate compared to the adult. It is even lower in mechanically ventilated, preterm infants: 10-12 ml/100 g/minute, a level associated with brain infarction in adults. The reactivity, however, of global CBF to changes in cerebral metabolism, PaCO2, and arterial blood pressure is normal, except following severe birth asphyxia, or in mechanically ventilated preterm infants, who subsequently develop major germinal layer hemorrhage. The low level of cerebral blood flow (CBF) matches a low cerebral metabolism of glucose and a relatively small number of cortical synapses in the perinatal period. It has not been possible to define a threshold for GCBF below which electrical dysfunction or brain damage occurs (such as white matter and thalamic-basal ganglia necrosis). Three explanations for the lack of clear relation between GCBF and electrical brain activity of the preterm infant must be examined more closely: 1) low levels of CBF are adequate; 2) GCBF does not adequately reflect critically low perfusion of the white matter, and 3) acute white matter ischemia does not result in electrical silence. Two clinical patterns of brain damage following asphyxia may be explained by changes in the blood flow distribution induced by asphyxia: brainstem sparing and parasagittal cerebral injury. Hours to days after severe asphyxia, a state of marked global hyperperfusion may prevail. It is associated with poor neurological outcome and may be an entry point for trials of interventions aiming sat blocking the translation of asphyctic injury to cellular death and tissue damage.

Regional cerebral blood flow following hypothermic circulatory arrest in newborn dogs

A model of hypothermic circulatory arrest has been developed in newborn dogs which simulates the procedure used for the operative repair of congenital cardiac defects in human infants. Hypothermic circulatory arrest for 1.0 h causes no brain damage, whereas cardiac arrest for 1.75 h results in damage of the cerebral cortex, basal ganglia and to a lesser extent the claustrum and amygdaloid nucleus. In the present study, we determined regional cerebral blood flow (rCBF) during 24 h of recovery from hypothermic circulatory arrest. Newborn nitrous oxide anesthetized and artificially ventilated dogs were cooled to 20 degrees C and subjected to cardiac arrest by the i.v. injection of KCl for either 1.0 or 1.75 h. Thereafter, animals were resuscitated, rewarmed to 37 degrees C, and rCBF measured with [14C]iodoantipyrine at either 2 or 18 h of recovery. Control animals were rendered hypothermic to 20 degrees C without cardiac arrest for 1.0 or 1.75 h prior to rewarming. No alterations in CBF at either 2 or 18 h of recovery were present in any of 16 analyzed structures in animals previously subjected to hypothermic circulatory arrest compared to controls rendered hypothermic alone. A direct linear correlation existed between mean arterial blood pressure and blood flow within frontal, parietal and occipital cortex, occipital white matter, hypothalamus and cerebellar vermis in puppies arrested for 1.75 h and recovered for 2 h, suggesting a loss of CBF autoregulation at this interval. No such association between blood pressure and CBF was apparent at 18 h of recovery.(ABSTRACT TRUNCATED AT 250 WORDS)

Neonatal periventricular-intraventricular hemorrhage after maternal beta-sympathomimetic tocolysis. The March of Dimes Multicenter Study Group

OBJECTIVE

Our objective was to determine if the rate of periventricular-intraventricular hemorrhage is increased in the offspring of women who received a beta-sympathomimetic agent as part of the management of preterm labor.

STUDY DESIGN

This retrospective study consists of 2827 women who were delivered of a singleton, live infant free of congenital neurologic anomalies between 25 and 36 completed weeks of gestation during a multicenter preterm birth prevention trial. The data were analyzed, adjusting for type of tocolytic agent, race, infant sex, gestational age, birth weight, health care center, route of delivery, indication for delivery, intrapartum fetal distress, respiratory distress syndrome, and neonatal sepsis.

RESULTS

The overall incidence of periventricular-intraventricular hemorrhage in this population was 5.6%. In a univariate analysis in which no adjustment was made for potentially confounding variables, beta-sympathomimetic tocolysis was found to be associated with nearly a fourfold increase in the incidence of periventricular-intraventricular hemorrhage when compared with the use of either magnesium sulfate or no tocolytic agent. The results of a multivariate regression analysis revealed that beta-sympathomimetic agents were associated with a statistically significant increase in the overall incidence of periventricular-intraventricular hemorrhage (odds ratio 2.47, 95% confidence interval 1.34 to 4.56, p = 0.004) and a similar, but not significant, increase in the incidence of grades 3 and 4 periventricular-intraventricular hemorrhage (odds ratio 2.50, 95% confidence interval 0.96 to 6.48, p = 0.06).

CONCLUSION

beta-Sympathomimetic tocolytic therapy may be associated with a more than twofold increase in the incidence of neonatal periventricular-intraventricular hemorrhage.

CO2 reactivity and autoregulation in fetal brain

Intraventricular hemorrhage (IVH) in preterm infants is well known to be associated with the high morbidity and mortality of this group. Previous studies have suggested altered cerebral blood flow (CBF) as an important pathologic factor. We measured the CBF in near-term rabbit fetuses using the hydrogen clearance technique. The local CBF of the rabbit fetuses was significantly low compared with that of the maternal rabbits. The response of CBF to changes in PaCO2 was observed in rabbit fetuses. The CO2 reactivity index of the fetal rabbit was lower than that of the maternal rabbit. This low CO2 reactivity might reflect the immaturity of the fetal brain and its low CBF. We were unable to monitor the fetal blood pressure, but the fetal CBF remained stable when the maternal blood pressure was altered. It is well known that IVH in preterm infants originates from the subependymal germinal matrix and that this has many fragile vessels. Our observation suggests that even a small increase of CBF during hypercapnia might have a large effect towards producing hemorrhage.

Control of cerebral circulation in the high-risk neonate

A knowledge of neonatal cerebrovascular physiology is essential to the understanding of diseases that frequently affect the subsequent development of the newborn brain. Recent observations indicate that the cerebral vessels of the healthy newborn infant, even the very preterm, respond to physiological stimuli in the same manner as in the mature organism. Thus, cerebral blood flow changes with changes in arterial carbon dioxide tension (PaCO2), oxygen concentration (CaO2), or glucose concentration, whereas cerebral blood flow remains constant at minor fluctuations in arterial blood pressure. In pathological states, pressure autoregulation may become impaired, and in severe cases the vessels do not react to chemical or metabolic stimuli. These infants are at high risk for developing cerebral lesion, and they may be candidates for new "brain-protecting regimens."

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.

Autoregulation of cochlear blood flow. A comparison of cerebral blood flow with muscular blood flow

The cochlear blood flow of healthy adult guinea pigs was measured with a laser Doppler flowmeter and flow dynamics were analyzed on the basis of autoregulation. Angiotensin II infusion was used to raise blood pressure, while phlebotomy was done to lower blood pressure. The characteristics of autoregulation of cerebral blood flow and muscular blood flow were also investigated. Cochlear blood flow was considered to have some autoregulation but was less than brain blood flow, which showed significant regulation. Muscular blood flow seemed to have no similar regulatory mechanism.

Postnatal changes in local cerebral blood flow measured by the quantitative autoradiographic [14C]iodoantipyrine technique in freely moving rats

The postnatal changes in local cerebral blood flow in freely moving rats were measured by means of the quantitative autoradiographic [14C]iodoantipyrine method. The animals were studied at 10, 14, 17, 21 and 35 days and at the adult stage. At 10 days after birth, rates of blood flow were very low and quite homogeneous in most cerebral structures except in a few posterior areas. From these relatively uniform levels, values of local cerebral blood flow rose notably to reach a peak at 17 days in all brain regions studied. Rates of blood flow decreased between 17 and 21 days after birth and then increased from weaning time to reach the known characteristic distribution of the adult rat. The postnatal evolution of local cerebral blood in the rat is in good agreement with previous studies in other species such as dog and humans that also show higher rates of cerebral blood flow and glucose utilization at immature stages. However, in the rat, local cerebral blood flow and local cerebral glucose utilization are not coupled over the whole postnatal period studied, since blood flow rates reach peak values at 17 days whereas glucose utilization remains still quite low at that stage. The high rate of cerebral blood flow in the 17-day-old rat may reflect the energetic and biosynthetic needs of the actively developing brain that are completed by the summation of glucose and ketone body utilization.

Vasoconstriction in telencephalic microvessels: a response to one model for intraventricular hemorrhage in beagle pups

Telencephalic microvessels were morphologically and morphometrically evaluated in beagle pups at 24, 48, and 72 hours of age following an experimental insult consisting of exposure to acute hypovolemic hypotension followed by rapid corrective blood reinfusion. Animals were anesthetized throughout the procedure and maintained for 75 minutes prior to sacrifice. Microvessels from the germinal matrix, where intraventricular hemorrhage (IVH) most commonly occurs, were compared to those of the adjacent cerebral cortex and choroid plexus. Perfusion, fixation and processing of the tissue followed our previously published protocol (Leuschen et al.: Anatomical Record 208:435-443, 1984). Both germinal matrix and cortical microvessels had collapsed lumens and normal endothelial cell morphology; mitochondria contained distinct cristae, junctional complexes were present and a basement lamina completely surrounded most vessels. Morphometric data on matrical and cortical microvessels supported vasoconstriction. Microvessels from choroid plexus were not vasoconstricted; tight junctions were infrequently seen and microvesicular elements were present. An increased cerebral blood flow to the germinal matrix and the adjacent cerebral cortex has been reported during and immediately following hypovolemic hypotensive insult. By 75 minutes following insult, our ultrastructural studies support a rebound phenomena including vasoconstriction of microvessels. The resulting vasoconstriction and associated cerebral ischemia may play an important role in subsequent tissue damage to the matrical area and ultimately to the etiology of IVH.

Cochlear blood flow autoregulation in Wistar-Kyoto rats

Wistar-Kyoto rats (WKY) were intra-arterially infused with angiotensin II (AII) or phenylephrine for 10 min. Both vasoactive compounds produced an initial increase in cochlear blood flow (CoBF) as measured by laser Doppler flowmetry, followed by a slow steady return to baseline, despite sustained elevations in systemic blood pressure. These results suggest autoregulation of CoBF in the WKY rat. In a second experiment. All was infused directly into the anterior inferior cerebellar artery (AICA) which feeds the cochlear artery. Significant reductions in CoBF were noted without changes in systemic blood pressure. Pretreatment with the specific angiotensin-receptor antagonist, sarthran (Sar1, Thr8-AII), diminished subsequent AII-induced reductions in CoBF. These results indicate that AII binding to vascular receptors may induce vasoconstriction in the supplying vessels of the cochlea, and thus, the interaction of blood-borne AII and vascular angiotensin receptors may participate in the autoregulation of CoBF.

Angiotensin II-induced changes in cochlear blood flow and blood pressure in normotensive and spontaneously hypertensive rats

Previous investigations in our laboratory have measured significant increases in the circulating levels of the potent vasoconstrictive hormone, angiotensin II (AII; 26 and 64 pg/100 microliters plasma, normal and noise exposed, respectively), during and following noise exposure in the alert rat (Wright et al., 1981). In the present study, these levels were approximated through intra-arterial infusion in the anesthetized spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto (WKY) rat. Laser Doppler flowmeter measurements of cochlear blood flow (CBF) indicated that despite equivalent AII-induced elevations in systemic blood pressure, CBF in the SHR did not increase to the levels measured in the WKY. Pretreatment with the specific angiotensin receptor antagonist sarile, (Sar1,Ile8-AII), reduced AII-induced elevations in systemic blood pressure in members of both strains, but did not change the overall pattern of CBF. These results indicate that SHRs may have a compromised cochlear circulation that is refractory to increases in systemic blood pressure.