Intratracheal Dead Space Wash Out in a Meconium Aspiration Model † 941

Abstract

Intratracheal dead space wash out (ITDSWO) has been developed as a method for reducing the effective dead space and has been studied in a number of low volume lung disease models. A study in preterm human neonates with hyaline membrane disease showed effective ventilation at lower airway pressures when ITDSWO was administered using a multiluminal endotracheal tube. Some models have however suggested that gas trapping may occur. We wished to determine whether ITDSWO with a multiluminal tube would be effective in a severe inhomogeneous lung disease model where elevated PaCO2 may be problematic and dead space washout would be potentially advantageous. Methods: Newborn piglets were instrumented under fentanyl anesthesia with arterial and pulmonary artery catheters. Assisted ventilation with a neonatal ventilator was commenced and after stability was assured, meconium was instilled as a 30% slurry in saline in a dose of 3 mL/kg followed after 20 minutes by a further 3 mL/kg. The animals were then monitored until PaO2 was less than 140 mmHg in an FiO2 of 1.0. At this time sequential 20 minute periods with and without extra flow delivered near the tip of the ETT via integral extra lumens built into the wall of the tube was commenced. No other changes in ventilation were permitted. Blood gas changes, airway pressures, and hemodynamics were monitored. In 4 animals flow was administered at 250 mL/min of gas with the same FiO2 as the main inspired gases, In 1animal a flow of 2 litres/min was used. Results: We confirmed that flow was being delivered to the tip of the endotracheal tube using end-tidal CO2 monitoring. There were no effects of the flow on cardiac output, pulmonary artery pressure or mean airway pressure. PaCO2 did not change when the extra flow was added. PaCO2 without flow = 42.3 (SD 9.3) mmHg, with flow = 42.8 (SD 10.7) mmHg. There were no effects on oxygenation; PaO2 with flow = 122 (SD 86) mmHg, without flow = 121(SD 86) mmHg. In the animal that received a flow of 2 litres/min PaCO2 increased during ITDSWO by 4 to 6 mmHg. Conclusion: In this lung disease model, which has airway obstruction and gas trapping as a prominent pathological feature, ITDSWO using the above methodology is ineffective possibly because of retardation of expiration and increased gas trapping. ITDSWO for meconium aspiration may require even lower flows or a different methodology such as the reverse thrust catheter.