R. Mirro

Relationship between mean airway pressure, cardiac output, and organ blood flow with normal and decreased respiratory compliance

We investigated the relation between blood flow and mean airway pressure in two groups of anesthetized newborn piglets. The first group had normal respiratory compliance; the second group had pulmonary surfactant depleted by repeated saline lavage, which decreased static respiratory compliance by 42%. In the normal group, cardiac output decreased linearly from 292 +/- 43 mL/min/kg at 5 cm H2O airway pressure to 134 +/- 37 ml/min/kg at 20 cm H2O airway pressure, a drop of 43% (r2 = 0.79). Blood flow to the heart, kidney, and intestines had a similar decline, but brain, hepatic artery, and adrenal flow were constant. Mean arterial blood pressure did not decrease significantly until the highest airway pressure was reached, whereas sagittal sinus pressure increased as mean airway pressure increased. In contrast, the surfactant-depleted group maintained cardiac output up to a mean airway pressure of 15 cm H2O. At 20 cm H2O, cardiac output fell to 40% of the original value. Blood flow to the heart and kidneys fell at a mean airway pressure of 20 cm H2O; intestinal blood flow decreased beginning at 10 cm H2O. As in the normal piglets, brain, hepatic arterial, and adrenal blood flow were not affected by increasing ventilation pressure. Our data show that positive pressure ventilation in the neonate has important cardiovascular effects that are blunted when respiratory compliance is decreased. More important, because cardiac output decreased prior to a significant decline in arterial blood pressure, these data suggest that in a clinical setting considerable cardiovascular alterations can occur before a decline in arterial blood pressure is detected.

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.

Ischemia alters cerebral vascular responses to hypercapnia and acetylcholine in piglets.

ABSTRACT: Effects of ischemia (20 min) on cerebral cortical prostanoid synthesis and microvascular responses to hypercapnia and topical acetylcholine were examined in anesthetized newborn pigs. Pial arteriolar dilation in response to hypercapnia (10% CO2 ventilation, 10 min) was absent 2 h after ischemia and reversed toward constriction by 24 h postischemia. In sham control piglets, hypercapnia increased cortical periarachnoid fluid prostanoid concentrations. After ischemia, hypercapnia did not affect prostanoid concentrations on the brain surface. Acetylcholine (10-3 M)-induced pial arteriolar constriction was reversed toward dilation 24 h after cerebral ischemia. Further, acetylcholine-induced prostanoid synthesis was markedly attenuated after ischemia. We conclude that cerebral ischemia- reperfusion alters cerebral prostanoid synthesis and microvascular control in newborn pigs. These abnormalities persist for at least 24 h.

Platelet activating factor: a potent constrictor of cerebral arterioles in newborn pigs.

This study characterized the nature of the response to platelet activating factor (PAF) in the cerebral microcirculation of the newborn pig. Pig arterioles were observed directly using a closed cranial window in chloralose-anesthetized piglets. Topical application of 10-100 ng/ml PAF produced dose-dependent decreases in pial arteriolar diameter; diameters were 193 ± 27 γ for control, 167 ± 25 γ at 10 ng/ml, and 129 ±21 γ at 100 ng/ml. Topical application of 30-300 ng/ml norepinephrine and 3-30 ng/ml U46619, a purported thromboxane A2 receptor agonist, also produced dose-dependent decreases in pial arteriolar diameter. After topical administration of U66985 (1 /mg/ml), a putative PAF antagonist, responses to PAF were attenuated significantly, but responses to norepinephrine and U46619 were unchanged. Moreover, intravenously administered U66985 (0.1 mg/kg) antagonized PAF responses as well. Responses to PAF were unchanged after cyclooxygenase and leukotriene receptor inhibition. Further, PAF did not increase cortical subarachnoid cerebrospinal fluid prostaglandin or leukotriene levels. These data indicate that PAF is a potent constrictor of cerebral arterioles in newborn pigs and that its mechanism of action is independent of formation of cyclooxygenase and lipoxygenase products of arachidonic acid metabolism. These data also suggest that U66985 may be a selective PAF antagonist that crosses the blood-brain barrier. Since PAF is an endogenous lipid released from a variety of tissues and may be an important mediator of inflammation and allergic reaction, PAF could be involved in the pathophysiology of the cerebral circulation in the perinatal period. (Circulation Research 1988;62:1-7)

Opioids in cerebrospinal fluid in hypotensive newborn pigs.

This study was designed to determine if opioids were detectable in cerebrospinal fluid (CSF) and if these concentrations were altered by hemorrhagic hypotension. This study was further designed to determine the effects of topically administered opioids on pial arteriolar diameter during normotension and hypotension. Closed cranial windows were used to determine pial arteriolar diameter. Periarachnoid cortical and cisterna magna CSF was collected from piglets during normotension and hypotension (systemic arterial pressure decreased from 63 +/- 1 to 33 +/- 1 mm Hg). Opioid profiles were assessed qualitatively by radioreceptor assay, and individual opioids were measured quantitatively by radioimmunoassay. Periarachnoid cortical and cisterna magna CSF methionine enkephalin-, leucine enkephalin-, dynorphin-, and beta-endorphin-like receptor active values all were increased by hypotension. When quantified by radioimmunoassay, periarachnoid cortical CSF values for methionine enkephalin-like immunoreactivity were 1,167 +/- 58 and 2,975 +/- 139 pg/ml for normotension and hypotension, respectively. Periarachnoid cortical CSF radioimmunoassay values for dynorphin-like immunoreactivity were 15 +/- 2 and 28 +/- 2 pg/ml for normotension and hypotension, respectively. When applied topically to the cortical surface, synthetic methionine enkephalin increased pial arteriolar diameter (134 +/- 4, 158 +/- 4, and 163 +/- 4 microns for control, 574 pg/ml [10(-10) M], and 5,740 pg/ml [10(-9) M], respectively). Similarly, topical synthetic leucine enkephalin and dynorphin elicited pial arteriolar dilation. However, beta-endorphin produced arteriolar constriction. Hypotension attenuated methionine and leucine enkephalin-induced dilation and reversed dynorphin-induced dilation to concentration-dependent constriction. beta-Endorphin-induced constriction was not changed by hypotension. Therefore, opioids could contribute to the control of the cerebral circulation during hypotension.

Vascular responses to vasopressin are tone-dependent in the cerebral circulation of the newborn pig.

The effects of lysine vasopressin (LVP) on pial arteriolar diameter and cortical periarachnoid fluid prostanoid concentrations were investigated in newborn pigs. Chloralose-anesthetized piglets were equipped with closed cranial windows over the parietal cortex for observation of pial arterioles and collection of cerebrospinal fluid (CSF) passing over the cerebral surface. Prostanoids in the CSF were determined by radioimmunoassay. LVP (10-1,000 μU/ml) elicited concentration-dependent increases in pial arteriolar diameter associated with increased levels of 6-keto-prostaglandin (PG)F1α, PGE2, thromboxane B2, and PGF2α. LVP-induced pial arteriolar dilation was unchanged after intravenous indomethacin (5 mg/kg). Conversely, LVP constricts pial arterioles previously dilated by physiological (hemorrhagic hypotension) and pharmacological (topically applied PGE2 or isoproterenol) intervention. This constriction is potentiated by indomethacin. Vascular and biochemical changes elicited by LVP were blocked by intravenous [l-(β-mercapto-ββ-cyclopentamethylene propionic acid), 2, (O-methyl)-Tyr-AVP] (5 μg/kg), a putative V1 receptor antagonist, whereas vascular effects of norepinephrine and U46619, a thromboxane A2 mimic, were unchanged. Therefore, the degree of vascular tone appears to influence responses of the newborn pig cerebral circulation to LVP.

Prostanoid synthesis and vascular responses to exogenous arachidonic acid following cerebral ischemia in piglets.

In newborn pigs, cerebral ischemia abolishes both increased cerebral prostanoid production and cerebral vasodilation in response to hypercapnia and hypotension. Attenuation of prostaglandin endoperoxide synthase activity could account for the failure to increase prostanoid synthesis and loss of responses to these stimuli. To test this possibility, arachidonic acid (3, 6, or 30 micrograms/ml) was placed under cranial windows in newborn pigs that had been exposed to 20 min of cerebral ischemia. The conversion to prostanoids and pial arteriolar responses to the arachidonic acid were measured. At all three concentrations, arachidonic acid caused similar increases in pial arteriolar diameter in sham control piglets and piglets 1 hr postischemia. Topical arachidonic acid caused dose-dependent increases of PGE2 in cortical periarachnoid cerebral spinal fluid. 6-keto-PGF1 alpha and TXB2 only increased at the highest concentration of arachidonic acid (30 micrograms/ml). Cerebral ischemia did not decrease the conversion of any concentration of arachidonic acid to PGE2, 6-keto-PGF1 alpha, or TXB2. We conclude that ischemia and subsequent reperfusion do not result in inhibition of prostaglandin endoperoxide synthase in the newborn pig brain. Therefore, the mechanism for the impaired prostanoid production in response to hypercapnia and hypotension following cerebral ischemia appears to involve reduction in release of free arachidonic acid.

Brain superoxide anion generation during asphyxia and reventilation in newborn pigs.

ABSTRACT: Superoxide anion generation during severe asphyxia and reventilation was assessed in newborn pigs. Using closed cranial windows over the parietal cortices, superoxide dismutase (SOD)-inhibitable nitroblue tetrazolium (NBT) reduction was determined during asphyxia/ reventilation. Asphyxia was induced by turning off the respirator and occluding the endotracheal tube. In each animal, 2.4 mM NBT dissolved in artificial cerebrospinal fluid was placed under one window and under the opposite window, NBT (2.4 mM) plus SOD (60 U/mL) dissolved in artificial cerebrospinal fluid was used. A significant increase in SOD-inhibitable NBT reduction was observed in asphyxiated piglets (14.67 ± 4.5 pmol/mm2 · 20 min) when compared with control piglets (2.82 ± 1.1 pmol/mm2 · 20 min). In another group in which the animals were treated with indomethacin (5 mg/kg i.v.) before asphyxia/reventilation, there was minimal SOD-inhibitable NBT reduction. Our results indicate that superoxide anion is generated on the cerebral cortex during asphyxia/reventilation via the prostaglandin endoperoxide synthase pathway.

Effects of indomethacin on cerebral vasodilator responses to arachidonic acid and hypercapnia in newborn pigs.

ABSTRACT: Responses of pial arterioles to topically applied arachidonic acid, conversion of exogenous arachidonic acid to prostanoids, and pial arteriolar dilation to hypercapnia were examined before and at progressive times after treatment with indomethacin (5 mg/kg i.v.) in chlor-alose-anesthetized newborn pigs with closed cranial windows. Before treatment with indomethacin, arachidonic acid and hypercapnia dilated pial arterioles and increased cortical periarachnoid cerebrospinal fluid concentrations of 6-keto-prostaglandin (PG) F1α and PGE2. One h after indomethacin treatment, the dilations and prostanoid synthesis were blocked. By 2 h after indomethacin treatment, hypercapnia produced significant dilation of pial arterioles, and dilation to both stimuli had returned to preindometha-cin levels by 3 h. Inhibition of conversion of exogenous arachidonic acid to prostanoids as monitored by increases in 6-keto-PGF1α and PGE2 in cerebrospinal fluid under the window also was reversed by 3 h after treatment with indomethacin. Repeated indomethacin treatment again blocked dilations and conversion of arachidonic acid to prostanoids on the brain surface. The possibility of short duration of vascular effectiveness of indomethacin when it is administered systemically needs to be considered, both when it is used as a probe for understanding contributions of PGH synthase products to control of cerebral circulation and when it is used therapeutically in attempts to alter the newborn cerebral circulation.

Opioids and the Prostanoid System in the Control of Cerebral Blood Flow in Hypotensive Piglets

The interaction between opioid and prostanoid mechanisms in the control of cerebral hemodynamics was investigated in the conscious hypotensive piglet. Radiomicrospheres were used to determine regional cerebral blood flow (rCBF) in piglets pretreated with the opioid receptor antagonist, naloxone, or its vehicle, saline, during normotension, hypotension, and after the administration of indomethacin, a cyclooxygenase inhibitor, during hypotension. Hemorrhage (30 ml/kg) decreased systemic arterial pressure from 68 ± 12 to 40 ± 10 mm Hg but did not decrease blood flow to any brain region. Indomethacin treatment (5 mg/kg) of hypotensive piglets decreased blood flow to all brain regions within 20 min; this decrease in CBF resulted from increases in cerebral vascular resistance of 65 and 281% at 20 and 40 min after treatment, respectively. In hypotensive piglets, cerebral oxygen consumption was reduced from 2.62 ± 0.71 to 0.53 ± 0.27 ml 100g−1 min−1 and to 0.11 ± 0.04 ml 100 g−1 min−1 at 20 and 40 min following indomethacin, respectively. Treatment with naloxone (1 mg/kg) had no effect on rCBF, calculated cerebral vascular resistance, or cerebral oxygen consumption of normotensive or hypotensive piglets. However, decreases in CBF and oxygen consumption and increases in cerebral vascular resistance upon treatment of hypotensive piglets with indomethacin were attenuated in animals pretreated with naloxone. These data indicate that the removal of prostanoid modulation of an opioid-mediated constrictor influence on the cerebral circulation is a potential mechanism for the increase in cerebral vascular resistance that follows indomethacin treatment of hypotensive piglets.