Ventilatory factors affecting inhaled nitric oxide concentrations during continuous-flow administration.

Abstract

PURPOSE: Inhaled nitric oxide has been reported to be useful in acute respiratory distress syndrome and other lung diseases characterized by pulmonary hypertension and hypoxemia. The best site to inject the NO is still controversial, as is whether it can be affected by changes in ventilatory settings. The objective of this study was to examine the impact of various ventilator settings (tidal volume [Vt], inspiratory-to-expiratory [I:E] ratio, added dead space) in the final NO concentrations reached when NO is administered as a continuous flow near the endotracheal tube. METHODS: A test lung model was ventilated using a ServoVentilator 300 Siemens with square wave flow. A continuous flow of NO was administered between the Y piece and the endotracheal tube. The flow was adjusted with an NO precision flowmeter (range, 0.2 to 1.5 L/min) attached to a cylinder containing 800 ppm of NO. Actual NO concentrations in the lung model were measured by chemiluminescence (NOX 2000; Seres). The effects of changing Vt (from 300 to 1300 mL) and I:E ratio (from 1:4 to 2:1) and the addition of dead space (100 mL) were examined. RESULTS: At each step of the study, the measured NO concentration was higher than predicted. Biases from predicted NO concentration due to tidal volume were 17% at 1,300 mL Vt, 21% at 1,100 mL Vt, 25% at 900 mL Vt, 28% at 700 mL Vt, 35% at 500 mL Vt, and 48% at 300 mL Vt. Changes in I:E ratio induced increases in No concentration of 4% at 2:1, 5% at 1:1, 22% at 1:2, and 31% at 1:4. The addition of dead space enhanced these differences: 62% at 2:1, 104% at 1:1, 230% at 1:2, and 322% at 1:4. CONCLUSION: According to our lung model, continuous-flow administration of NO near the endotracheal tube is influenced strongly by changes in ventilatory settings.