Automated evaluation of functional residual capacity by oxygen washout

Objective. Measurement of functional residual capacity (FRC) is of considerable interest for monitoring ventilated patients in ICUs. However, the lack of instruments that can be used in the clinical setting has so far impeded the routine application of this measurement. It was the aim of our study to evaluate the accuracy and the reproducibility of a simplified oxygen washout technique (FRC[O₂]). Materials and methods. For the evaluation of FRC[O₂], gas flow, CO₂ and O₂ concentrations were determined by the flow probe of an ICU ventilator, a mainstream capnometer and O₂ analyser. In 30 volunteers FRC[O₂] as measured during spontaneous breathing was compared to: 1. Helium dilution technique (FRC[He], n = 21), 2. Body plethysmography (FRC[bp], n = 9). In n = 7 male patients FRC[O₂] was repeatedly evaluated during mechanical ventilation and compared to the preoperative FRC[bp]. Results. FRC[O₂] corresponded well with FRC[He] (range: 1.9 to 6.0 l, bias of FRC[O₂]: 0.53 l (95% CI 0.24 l to 0.82 l)) and FRC[bp] (range: 2.1 to 4.3 l, bias of FRC[O₂] 0.03 l (95% CI -0.30 l to 0.37 l)). The mean of the repeated FRC[O₂]-measurements (basic range: 1.3 to 3.6 l) during mechanical ventilation with unchanged ventilator settings stayed unchanged. The within subject-between error ranged from 0.1 to 0.4 l (mean = 0.23 l). Mean FRC[O₂] during mechanical ventilation decreased to 66.6 percent of the preoperative mean FRC[bp]. Conclusions. The automated oxygen washout technique is a simple method to measure FRC in the ICU patient.