Physiologic implications of helium as a carrier gas for inhaled nitric oxide in a neonatal model of Bethanecol-induced bronchoconstriction

Objective: To compare heliox to nitrogen-oxygen (nitrox) as a carrier gas for inducible nitric oxide (iNO) in the presence of pharmacologically inhaled bronchoconstriction. We hypothesized that respiratory resistance and gas exchange would improve when iNO is delivered with heliox. Design: Interventional laboratory study. Setting: An academic medical research facility in the northeastern United States. Subjects: Sedated, ventilated newborn piglets. Interventions: Newborn piglets (n = 16; 2.3 ± 0.1 kg) were placed on a flow-controlled ventilator and given intravenous Bethanecol (2 x 1 mg/kg followed by 1 mg/kg/hr) to induce bronchoconstriction. Piglets were randomized to heliox or nitrox (FIO₂ = 0.3) and given 80 ppm iNO. Measurements and Main Results: Hemodynamics, blood chemistry, and pulmonary mechanics were recorded at 30-min intervals for 2 hrs. Bethanecol dosing increased inspiratory respiratory resistance (cm H₂O/L/min; p < .01) and decreased respiratory compliance (mL/cm H₂O/kg; p < .01). Following carrier gas assignment, hemodynamics and respiratory compliance were similar between groups and respiratory resistance decreased (p < .01) in the heliox group. Over 2 hrs with iNO therapy, Paco₂ increased (p < .01) whereas blood pH decreased (p < .01) in the heliox group. Respiratory resistance trended downward, oxygenation index improved (p < .01), and blood methemoglobin levels trended higher for nitrox compared with heliox. Conclusions: The INOvent was effective for controlling heliox delivery of iNO. Despite marked reduction in respiratory resistance with heliox gas ventilation in a neonatal model of pharmacologic bronchoconstriction, nitrox might perform better as a delivery vehicle for iNO.