The role of lung perfusate PO2 in the control of the pulmonary vascular resistance of exteriorized fetal lambs

Inhibition of 20-HETE abolishes the myogenic response during NOS antagonism in the ovine fetal pulmonary circulation

Mechanisms that maintain high pulmonary vascular resistance (PVR) and oppose vasodilation in the fetal lung are poorly understood. In fetal lambs, increased pulmonary artery pressure evokes a potent vasoconstriction, suggesting that a myogenic response contributes to high PVR in the fetus. In adult systemic circulations, the arachidonic acid metabolite 20-hydroxyeicosatetraenoic acid (20-HETE) has been shown to modulate the myogenic response, but its role in the fetal lung is unknown. We hypothesized that acute increases in pulmonary artery pressure release 20-HETE, which causes vasoconstriction, or a myogenic response, in the fetal lung. To address this hypothesis, we studied the hemodynamic effects of N-methylsufonyl-12,12-dibromododec-11-enamide (DDMS), a specific inhibitor of 20-HETE production, on the pulmonary vasoconstriction caused by acute compression of the ductus arteriosus (DA) in chronically prepared fetal sheep. An inflatable vascular occluder around the DA was used to increase pulmonary artery pressure under three study conditions: control, after pretreatment with nitro-l-arginine (l-NA; to inhibit shear-stress vasodilation), and after combined treatment with both l-NA and a specific 20-HETE inhibitor, DDMS. We found that DA compression after l-NA treatment increased PVR by 44 ± 12%. Although intrapulmonary DDMS infusion did not affect basal PVR, DDMS completely abolished the vasoconstrictor response to DA compression in the presence of l-NA (44 ± 12% vs. 2 ± 4% change in PVR, l-NA vs. l-NA + DDMS, P < 0.05). We conclude that 20-HETE mediates the myogenic response in the fetal pulmonary circulation and speculate that pharmacological inhibition of 20-HETE might have a therapeutic role in neonatal conditions characterized by pulmonary hypertension.

Effects of nitric oxide on hyperinflation-induced pulmonary hypertension in the isolated-perfused lung

OBJECTIVE

To determine if nitric oxide decreases pulmonary vascular resistance in hyperinflation-induced pulmonary hypertension.

DESIGN

Isolated-perfused lamb lung model.

SETTING

Experimental animal laboratory in a university setting.

SUBJECTS

Ten isolated-perfused lamb lungs harvested from subjects with a mean age of 29 days.

INTERVENTIONS

After induction of anesthesia, endotracheal intubation, and mechanical ventilation, lungs were perfused via an extracorporeal circuit. Ventilatory pressures were set to provide tidal volumes of 10 mL/kg and ventilatory rates were adjusted to maintain a Paco2 of 40 +/- 5 torr (3.5 +/- 0.7 kPa). The perfusion system consisted of a blood reservoir, a membrane oxygenator, and a nonocclusive roller pump. Blood flow was increased progressively to 50 mL/kg/min, maintaining a pulmonary arterial pressure of < 25 mm Hg and a left atrial pressure between 2 and 5 mm Hg. End-expiratory lung volume was measured using a nitrogen washout method. Baseline data were collected after a 1-hr stabilization period. Lung volume was increased to achieve 25% (moderate hyperinflation) and 50% (severe hyperinflation) increments in pulmonary vascular resistance. Nitric oxide (80 parts per million) was administered to the preparation after each increment in lung volume.

MEASUREMENTS AND MAIN RESULTS

Mean pulmonary arterial pressure, mean left atrial pressure, pulmonary vascular resistance, and static lung compliance were measured at baseline and after moderate and severe hyperinflation, both before and after nitric oxide administration. Significant decreases in pulmonary vascular resistance were found when the preparation was ventilated with nitric oxide at baseline (43% decrease) and during hyperinflation induced pulmonary hypertension at both moderate (31% decrease) and severe (23% decrease) levels of hyperinflation.

CONCLUSIONS

Inhaled nitric oxide significantly reduces pulmonary vascular resistance, even when pulmonary hypertension is induced by airway hyperinflation and supraphysiologic lung volumes. These data suggest that the use of nitric oxide following lung transplantation may allow for effective management of pulmonary hypertension in patients who receive allografts from undersized donors. Further clinical experience will be crucial in precisely defining the range of donor-recipient size mismatch that can be adequately managed and the time course over which nitric oxide can be administered safely and effectively to these patients.

Hypoxic vasoconstriction in buffer-perfused rabbit lungs

Isolated rabbit lungs were buffer-perfused under constant flow-conditions with separate control of alveolar (PAO2) and mixed venous (PvO2) O2 tension. Alveolar hypoxia caused an increase in pulmonary artery pressure (PAP) with sigmoidal dose-dependency. Erythrocytes increased the strength of the hypoxic pulmonary vasoconstriction (HPV). The contractile and vasorelaxant responses to the onset and release of alveolar hypoxia, respectively, occurred within seconds. Kinetics of the PAP increase, but not the magnitude of response, were related to the velocity of PAO2 decline. In contrast, changes in PvO2, both in the absence and presence of erythrocytes, did neither provoke any pressor response nor amplify the response to concomitant alveolar hypoxia. Repeatedly performed HPV manoeuvres revealed excellent reproducibility, and long-term alveolar hypoxia (90 min) provoked a biphasic pressor response. We conclude that the isolated rabbit lung is a feasible model for the characterization of hypoxic vasoconstriction, with specific features hitherto not described for perfused lungs of other species.

The roles and vital importance of placental blood to the newborn infant

At 30 weeks' gestation, half of the approximately 110 ml/kg total blood volume (BV) of the feto-placental circulation is in the fetus, rising, by term, to about 90 ml/kg. In preterm infants at birth, subnormal blood volume is the rule, because of immediate cord clamping. Blood volume, typically 50-60 ml/kg during critical care, limits systemic oxygen (O2) transport and, because of shunting, causes hepato-splanchnic ischaemia and impaired lung function. Haemoconcentration results from plasma extravasation because of vascular endothelial damage. This elevates the haematocrit, masking the red cell lack. By allowing placental transfusion at birth, delaying cord clamping by 30-60 seconds, initial oligovolaemia is obviated, and post-natal lung adaptation greatly facilitated. The complications and costs of care can thereby be much reduced. Losses of haemopoietic stem cells are reduced, vital for haematologic and immunologic constitution and for response to haemopoietic growth factors. Further work is urgently needed to determine how to optimize this vital opportunity in preventive medicine in perinatology, with the objective of preventing complications, and reducing costs of all kinds, in management of the infant born preterm.

The HPV response is different with constant pressure vs constant flow perfusion

Hypoxic pulmonary vasoconstriction (HPV) may be manifest in one of two ways: either an increase in the pulmonary artery pressure, or flow diversion away from the portion of the pulmonary bed with reduced conductance. We tested the hypothesis that the magnitude of the HPV response differs under conditions of constant flow perfusion, where pulmonary artery pressure (Ppa) rises during hypoxia, vs conditions of constant pressure perfusion, where Ppa remains constant and flow (Q) is diverted away from the lungs during hypoxia. In isolated, perfused rabbit lungs, the HPV response to four levels of hypoxia (12, 6, 3 and 0% oxygen) was of greater magnitude and more sustained under conditions of constant pressure perfusion as compared to constant flow perfusion. The possible significance of these findings as they relate to interpretation of studies in both the perinatal and mature pulmonary circulation is discussed.