These differential effects on Qs/Qt and Vd/Vt suggest that both high and low Paw may limit improvement in gas exchange with iNO. During iNO treatment, a reduction in Vd/Vt occurred only at Paw of 8–15 cmH 2O ( p < 0.05), and the positive relationship between Vd/Vt and Paw was maintained. Therapy with iNO significantly reduced Qs/Qt ( p < 0.001) at all levels of mean Paw, reaching a maximum reduction at 16–23 cmH 2O ( p < 0.05), such that Qs/Qt during iNO treatment was similar at all levels of Paw. Baseline Qs/Qt was slightly increased at Paw 16–23 cmH 2O ( p < 0.05), while Vd/Vt increased progressively with higher Paw ( p < 0.002). Data from 62 patients during 153 trials were analyzed for effects of Paw and iNO on Qs/Qt and Vd/Vt. Qs/Qt and Vd/Vt were derived from a tripartite lung model with correction for shunt-induced dead space. Mechanical ventilator settings were at the discretion of the attending physician. iNO 0–50 ppm was administered during mechanical ventilation. Because the improvement in V/Q matching must occur by redistribution of pulmonary blood flow, and because high airway pressure (Paw) increases physiologic dead space (Vd/Vt), we hypothesized that high Paw may limit the improvement in V/Q matching during iNO treatment. Inhaled nitric oxide (iNO) can improve oxygenation and ventilation–perfusion (V/Q) matching by reduction of shunt (Qs/Qt) in patients with hypoxemic lung disease.
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