Developmental changes in vascular responses to histamine in normoxic and hypoxic lamb lungs

Effects of nociceptive stimuli on the pulmonary circulation in the ovine fetus

The fetus is able to exhibit a stress response to painful events, and stress hormones have been shown to modulate pulmonary vascular tone. At birth, the increased level of stress hormones plays a significant role in the adaptation to postnatal life. We therefore hypothesized that pain may alter pulmonary circulation in the perinatal period. The hemodynamic response to subcutaneous injection of formalin, which is used in experimental studies as nociceptive stimulus, was evaluated in chronically prepared, fetal lambs. Fetal lambs were operated on at 128 days gestation. Catheters were placed into the ascending aorta, superior vena cava, and main pulmonary artery. An ultrasonic flow transducer was placed around the left pulmonary artery. Three subcutaneous catheters were placed in the lambs' limb. The hemodynamic responses to subcutaneous injection of formalin, to formalin after fetal analgesia by sufentanil, and to sufentanil alone were recorded. Cortisol and catecholamine concentrations were also measured. Pulmonary vascular resistances (PVR) increased by 42% ( P < 0.0001) after formalin injection. Cortisol increased by 54% ( P = 0.05). During sufentanil infusion, PVR did not change significantly after formalin. Cortisol increased by 56% ( P < 0.05). PVR did not change during sufentanil infusion. Norepinephrine levels did not change during any of the protocols. Our results indicate that nociceptive stimuli may increase the pulmonary vascular tone. This response is not mediated by an increase in circulating catecholamine levels. Analgesia prevents this effect. We speculate that this pulmonary vascular response to nociceptive stimulation may explain some hypoxemic events observed in newborn infants during painful intensive care procedures.

Bronchial epithelium-associated pulmonary arterial muscle relaxation in the rat is absent in the fetus and suppressed by postnatal hypoxia

We recently reported the existence of a bronchial epithelium-derived relaxing factor (BrEpRF) capable of reducing pulmonary arterial smooth muscle force generation in the newborn rat. We reasoned in this study that BrEpRF has physiological significance in the control of pulmonary vascular tone. We hypothesized that the release and/or activity of this factor can be stimulated and is suppressed prenatally or under hypoxic conditions postnatally. Therefore, we evaluated the pathways stimulated by the BrEpRF in fetal and newborn rat intrapulmonary arteries mounted with their adjacent bronchi in a wire myograph under both normoxic and hypoxic conditions. Under normoxic conditions, BrEpRF release/activation was observed in newborn vessels following methacholine stimulation of M(2) muscarinic receptors, which was mediated via a nitric oxide (NO)-dependent mechanism involving the phosphatidylinositol 3-kinase pathway. Hypoxia suppressed the BrEpRF-dependent modulation of basal and methacholine-induced pulmonary arterial muscle tone in newborn vessels without altering endothelium-dependent or -independent NO-mediated relaxation. In fetal pulmonary arteries studied under normoxic conditions, BrEpRF neither was active under basal conditions nor could it be stimulated with methacholine. We conclude that release/activation of the BrEpRF occurs by an oxygen-dependent mechanism in the newborn and is suppressed during late fetal life. These results suggest that the BrEpRF may be involved in postnatal adaptation of the pulmonary circulation and that its suppression may contribute to hypoxic pulmonary vasoconstriction.

Pathophysiology, Diagnosis, and Management of Pulmonary Hypertension in Infants and Children

Pulmonary hypertension (PH) may occur as a primary process or as a complication of several diseases. In the pediatric population, PH secondary to congenital heart disease, chronic hypoxemia, or acute respiratory failure is more common than primary PH. Regardless of etiology, PH may lead to significant morbidity or mortality as a consequence of right-to-left shunting across cardiovascular channels or right heart failure. In this review, PH is defined in terms of the determinants of pulmonary blood flow: pulmonary artery pressure, downstream pressure, and pulmonary vascular resistance. Research addressing both normal developmental changes in pulmonary vascular resistance and abnormal pulmonary vascular reactivity is then reviewed, followed by a discussion of the etiologies of PH in children. Some of the more common clinical presentations of PH are presented focussing on the differences seen between patients with and without intracardiac communications. Assessment of the severity of PH using both noninvasive (electrocardiogram, echocardiogram, magnetic resonance imaging) and invasive (cardiac catheterization, lung biopsy) techniques is then discussed. Treatment of PH is presented, focussing on restoration of adequate pulmonary blood flow through use of both conventional and newer vasodilator therapies. The review concludes by noting the limits to our understanding of the pathogenesis and therapy of PH.