Study objective: Mechanical ventilation may damage the lung. Low tidal volume (Vt) is protective, but Vt is scaled to body weight (BW) and may be high in functionally small ARDS lungs. We hypothesized that exhaled breath condensate (EBC) nitrite (NO2−) concentration may increase with lung distension.
Design: Prospective, noncontrolled study.
Setting: University hospital and medical ICU.
Patients: Thirty-five ICU patients requiring mechanical ventilation (severe pneumonia, n = 31; exacerbated COPD, n = 4). Patients were scored according to American and European Consensus Conference on ARDS criteria (AECC) [no lung injury, n = 7; acute lung injury, n = 13; ARDS, n = 15], as well as the Murray lung injury severity score (LISS) [score 0, n = 3; score 0.1 to 2.5, n = 19; score > 2.5, n = 13].
Interventions: EBC was collected and analyzed for NO2−, interleukin (IL)-6, and IL-8. Serum was analyzed for IL-6, IL-8, and procalcitonin.
Results and measurements: EBC NO2− correlated well with Vt (milliliters per kilogram of BW; r = 0.79, p < 0.0001) and expiratory minute volume (r = 0.60, p < 0.0001) but not with other ventilatory parameters or parameters of pulmonary (EBC IL-6, EBC IL-8) or systemic (serum IL-6, IL-8, and procalcitonin) inflammation. The ratio of EBC NO2− and the size of the Vt correlated directly with lung injury (AECC, r = 0.66, p < 0.0001; LISS, r = 0.84, p < 0.0001).
Conclusion: EBC NO2− increased linearly with Vt. The ratio of EBC NO2− to Vt is assumed to reflect NO2− release at a given Vt. An increase in this ratio indicates an inappropriate increase of NO2− production most likely due to mechanical stress of the remaining open lung units in injured lungs. We conclude that the EBC NO2−/vt ratio may help to identify situations of critical mechanical stress.