INHALED NITRIC-OXIDE IMPROVES OXYGENATION IN PIGLETS WITH MECONIUM ASPIRATION

We hypothesized that nitric oxide (NO) inhalation in a model of meconium aspiration in newborn piglets would decrease pulmonary vascular resistance. Seven neonatal piglets were obtained at less than 48 hr of age and instrumented under fentanyl anesthesia. inhaled NO (40 parts per million) was administered during normoxia and again after hypoxia was induced by reducing FiO(2) to 0.13. During normoxia NO inhalation caused a fall in pulmonary artery pressure from a mean of 3.15 (SD 0.8) kPa to 2.84 (SD 0.7) kPa (P < 0.01). Hypoxia (mean arterial O-2 saturation 35%) increased PA pressures to a mean of 5.4 (SD 1.6) kPa and NO administration during hypoxia decreased PA pressures to 3.6 (SD 1.2) kPa (P < 0.001). In order to determine the effects of NO in a model of meconium aspiration, 6 to 7 mL/kg of 20% human meconium in normal saline was instilled into the trachea. This procedure induced hypoxemia (mean SaO(2) 43.4%, SD 19), respiratory acidosis, (mean PaCO2 12.1 kPa, SD 0.5; mean pH 7.04, SD 0.03), and pulmonary arterial hypertension (mean pulmonary artery pressure 6.0 kPa, SD 1.3) despite ventilation with 90% oxygen. Inhaled NO was then administered in concentrations of 5, 10, 20, 30, 40, 60, and 80 parts per million in random order according to a Latin square design. After meconium instillation, NO inhalation led to significant increases in mean arterial saturation of between 18 and 24 percentage points at all doses, and to statistically significant decreases in pulmonary arterial pressure (from 6.0 kPa, SD 1.3 to 5.1 kPa, SD 1.5, P < 0.01) and pulmonary vascular resistance index (from 0.033 kPa/mL/min/kg, SD 0.003 to 0.029 kPa/mL/min/kg, SD 0.003, P < 0.01). Systemic blood pressure, cardiac index and systemic vascular resistance index did not change. The ratio between pulmonary and systemic vascular resistances fell, therefore, from a mean of 0.54 (SD 0.18) to 0.46 (SD 0.14). The effects of NO inhalation after meconium aspiration were qualitatively different to the effects during aveolar hypoxia, in that there was a lesser fall in the PAP and PVRI and a significant increase in SaO(2). These results suggest that inhaled NO may be beneficial in meconium aspiration syndrome in human neonates, improving oxygenation by decreasing extrapulmonary shunting and by improving ventilation/perfusion ratios. (C) 1995 Wiiey-Liss, Inc.