Aim. To evaluate the effect of iNO on gas exchange, cardiovascular profile and lung mechanics in a model of pulmonary hypertension(PH) induced by tracheal instillation of meconium (eMAS) in newborn lambs.

Subjects: Fifteen newborn lambs <6 days, on mechanical ventilation (IMV).

Intervention: Lambs were randomly assigned to 3 groups (n=6);Control: on IMV without further intervention; eMAS: PH induced by tracheal instillation of meconium (5mL/kg of a 25% meconium solution); iNO: PH exposed to iNO (20ppm). Meconiun was tagged with colored microspheres to study its lung distribution.

Measurements: Catheters were placed in the trachea, femoral and pulmonary arteries and right ventricle. Heart rate, systemic and pulmonary arterial pressures, arterial pH and blood gases, cardiac output and pulmonary mechanics were measured.

Results: eMAS lambs had severe hypoxemia (PaO2<80mmHg), PH (PAP>30 mmHg) and low dynamic lung compliance (<0.5 mL/cmH2O/Kg). The exposure to iNO in lambs with PH, produced a transient response. PaO2 increased (147±23 mmHg), and PAP decreased (24±1.6 mmHg), without reaching control values. There were not changes in cardiovascular profile or lung mechanics.

Conclusion. The transient and incomplete response to iNO in eMAS might be related to the fact that hypoxemia is not only due to pulmonary vasoconstriction but also to parenchymal lung disease. We hypothesize that this unexpected, poor and transient response could be avoided if strategies that increase lung recruitment, are applied prior to iNO exposure.Supported in part by Grants FIS 96/1348 and 96/1332.