Hypoxic vasoconstriction increases with postnatal age in lungs from newborn rabbits

Physiology of One-Lung Ventilation

The understanding of the physiology and management of one-lung ventilation (OLV) has advanced over the last two decades. OLV induces an obligatory shunt through the nonventilated lung that causes varying degrees of arterial hypoxemia. Shunt may also occur in the venti lated lung. The optimal mode of ventilation of the dependent lung has not been well defined. The optimal tidal volume, respiratory rate, inspired oxygen concen tration, and positive end-expiratory pressure (PEEP) during OLV are not known. Functional residual capacity (FRC) of the ventilated lung can be lower than during two-lung ventilation, causing atelectasis and arterial hypoxemia. Patients who desaturate might be expected to show improvement in oxygenation with dependent lung PEEP, because of increased FRC and reduced V/Q mismatch. Not all patients have low lung volumes, and not all patients who have low lung volumes will desatu rate. Therefore, prophylactic PEEP is not usually neces sary or appropriate. Because the predominant cause of hypoxemia during OLV is shunt in the nondependent lung, therapies to improve arterial oxygenation during OLV should be primarily directed toward the nondepen dent lung. Partial reinflation of the nondependent lung with O2will reduce the physiological shunt fraction of the lung. Continuous positive airways pressure (CPAP) is an effective prophylactic and therapeutic treatment for hypoxemia. All studies examining CPAP have found it to be effective, provided it is preceded by lung reinflation.

Antenatal hypoxia and pulmonary vascular function and remodeling

This review provides evidence that antenatal hypoxia, which represents a significant and worldwide problem, causes prenatal programming of the lung. A general overview of lung development is provided along with some background regarding transcriptional and signaling systems of the lung. The review illustrates that antenatal hypoxic stress can induce a continuum of responses depending on the species examined. Fetuses and newborns of certain species and specific human populations are well acclimated to antenatal hypoxia. However, antenatal hypoxia causes pulmonary vascular disease in fetuses and newborns of most mammalian species and humans. Disease can range from mild pulmonary hypertension, to severe vascular remodeling and dangerous elevations in pressure. The timing, length, and magnitude of the intrauterine hypoxic stress are important to disease development, however there is also a genetic-environmental relationship that is not yet completely understood. Determining the origins of pulmonary vascular remodeling and pulmonary hypertension and their associated effects is a challenging task, but is necessary in order to develop targeted therapies for pulmonary hypertension in the newborn due to antenatal hypoxia that can both treat the symptoms and curtail or reverse disease progression.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Sildenafil acutely reverses the hypoxic pulmonary vasoconstriction response of the newborn pig

Sildenafil is a pulmonary vasodilator shown to be effective in neonates, but conflicting data exist regarding its effect on arterial oxygenation. To address this issue, we tested the sildenafil effect on the piglet's hypoxic pulmonary vasoconstriction (HPV) response. A segmental lung atelectasis was created by obstructing the corresponding bronchus. Total pulmonary and specific flows to the atelectatic and contra-lateral lobes were measured by magnetic resonance (MR) before and 30-min post sildenafil (0.2 and 1 mg/kg i.v.) or saline administration. Flow was reduced (p < 0.01) in the atelectatic and increased in the contra-lateral lobe indicating an effective HPV response. Sildenafil at both doses significantly (p < 0.01) increased flow solely to the atelectatic lobe. At a dose of 1 mg/kg, sildenafil induced a decrease in Pao2 from 285 +/- 37 to 161 +/- 22 mm Hg (p < 0.01). We conclude that the HPV response in the newborn is capable of almost completely reducing blood flow to nonventilated lung units and is reversed following sildenafil i.v. administration in a dose-dependent manner. In the presence of lung parenchymal disease, the use of i.v. sildenafil as a pulmonary vasodilator may worsen arterial oxygenation by reversing the HPV response in nonventilated lung units.

Models of persistent pulmonary hypertension of the newborn (PPHN) and the role of cyclic guanosine monophosphate (GMP) in pulmonary vasorelaxation

At birth, a marked decrease in pulmonary vascular resistance allows the lung to establish gas exchange. Persistent pulmonary hypertension of the newborn (PPHN) occurs when this normal adaptation of gas exchange does not occur. We review animal models used to study the pathogenesis and treatment of PPHN. Both acute models, such as acute hypoxia and infusion of vasoconstrictors, and chronic models of PPHN created both before and immediately after birth are described. Inhaled nitric oxide is an important emerging therapy for PPHN. We review nitric oxide receptor mechanisms, including soluble guanylate cyclase, which produces cGMP when stimulated by nitric oxide, and phosphodiesterases, which control the intensity and duration of cGMP signal transduction. A better understanding of these mechanisms of regulation of vascular tone may lead to safer use of nitric oxide and improved clinical outcomes.

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.