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Pulmonary hypertension leading to hypoxemia is a potentially life-threatening condition in pediatric intensive care. Hypoxemia may also result from conditions not primarily related to pulmonary hypertension. The pediatric patients with these symptoms are:· Children after surgery on cardiopulmonary bypass with pulmonary hypertension attributable to congenital heart defects with high pulmonary blood flow.· Newborns with persistent pulmonary hypertension as a symptom of fetal illness or malformation or after a relapse into fetal circulation without obvious reason.· Children with acute respiratory failure and disturbed vascular/alveolar relation with atelectases and ventilation/perfusion mismatch with major pulmonary shunt.New methods to provide better ventilation and oxygenation have been developed over the last few years. The aims of the present thesis were to assess:· The effect of inhaled nitric oxide (iNO), a selective pulmonary vasodilator, on pulmonary hypertension and oxygenation in dose-response studies.· The effect of high frequency oscillatory ventilation (HFOV) and partial liquid ventilation (PLV) on alveolar recruitment and response to iNO.· Respiratory inductive plethysmography during dynamic ventilatory changes on high frequency oscillatory ventilation and on conventional ventilation (CV) as a new continuous and non-invasive method to optimize alveolar recruitment without interrupting ventilation.· Possible residual cardiopulmonary and neurological symptoms in a follow-up study after treatment with inhaled nitric oxide.The methods used were dose-response studies with iNO, lung volume recruitment with surfactant, high frequency oscillatory ventilation and partial liquid ventilation, lung volume measurements with respiratory inductive plethysmography and a four-center follow up study after iNO.Results and conclusions:· iNO decreased pulmonary artery pressure after cardiopulmonary bypass. No dose response relationship was found in the dose range of 3-80 ppm nitric oxide. Only a low dose of inhaled nitric oxide was needed, 5 ppm or less, which also concomitantly improved oxygenation in postoperative pulmonary hypertension in children.· iNO in doses up to 20 ppm immediately improved oxygenation in 68% of children with acute respiratory failure. Only 29% of non-responders survived and no delayed response was found in patients with acute respiratory failure. Non-responders need careful attention in order to improve ventilation and/or hemodynamics. Otherwise they should be transferred without delay to extracorporeal membrane oxygenation when eligible for such treatment.· Follow up after iNO treatment showed residual pulmonary hypertension in cardiac but not in lung patients. Residual respiratory disease and neurodevelopmental delay were not increased as compared with previous studies or owing to improved survival of severely ill patients.· In severe respiratory distress syndrome in preterm lambs, rescue treatment with alveolar recruitment strategies, HFOV and PLV, resulted in improved oxygenation. Alveolar recruitment was improved, as indicated by further improvement in oxygenation in response to iNO, particularly when HFOV and PLV were combined. Mean airway pressures on HFOV and CV when combined with PLV were significantly lower, reducing the risk of alveolar barotrauma.· Respiratory inductive plethysmography could be used to assess changes in lung volume during HFOV and CV in term and preterm lambs. This method provided the means to monitor and optimize lung volume continuously, non-invasively and without interruption of ventilation during mechanical ventilation. Observed changes in lung volume predicted changes in oxygenation.