Automated evaluation of functional residual capacity by oxygen washout

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]).

For the evaluation of FRC[O2J, 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].

FRC[O2J 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 1). Mean FRC[O2] during mechanical ventilation decreased to 66.6 percent of the preoperative mean FRC[bp].

The automated oxygen washout technique is a simple method to measure FRC in the ICU patient.