The ability to track the functional residual capacity (FRC) volume for a patient treated with ventilator therapy may be useful for guidance in improving or preserving gas exchange and for avoiding ventilator-associated injury to the lung. We tested the repeatability of the FRC measurement at the bedside for mechanically ventilated patients in the ICU.

All data needed for the FRC measurement were collected using a volumetric capnography system (NICO₂; Respironics, Wallingford, CT, USA), which had been modified to contain a fast, on-airway oxygen sensor. The nitrogen washout and washin method was used to calculate the FRC for 13 ICU patients. The protocol for a measurement set called for increasing the FIO₂ from the clinically determined baseline to 100% for 5 minutes, returning the FIO₂ to the baseline level for 5 minutes, and then repeating the FIO₂ change. After approximately 1 hour, the measurement set was repeated. The differences between the first and second measurements in each set were analyzed.

Bland–Altman analysis yielded a bias between repeated measurements of 85 ml (2.8%) and a standard deviation of the differences of ± 278 ml (9.0%). The r² of the repeated measurements was 0.92 (n = 28). See Figure 1.