Comparative changes in (a) PaCO2 (Δ PaCO2) and (b) alveolar dead space (ΔVDA/VT) induced by increasing inspiratory intratracheal concentrations of inhaled NO (Insp IT NO) in the presence (n = 7, filled circle) or absence (n = 8, ○) of septic shock in 15 patients with ARDS. PaCO2 and VDA/VT were measured: (1) before NO administration (C1); (2) following seven randomized concentrations of NO between 0.15 and 150 ppm, and (3) after the cessation of NO (C2). Δ PaCO2 and Δ VDA/VT are expressed as percentage variation from the control value. In each condition, minute ventilation was kept constant by adjusting the tidal volume. In both groups, NO induced a decrease in PaCO2 and VD A/VT which was statistically significant but dose-dependent in patients who only had septic shock (P < 0.02).