Determination of functional residual capacity (FRC) by multibreath nitrogen washout in a lung model and in mechanically ventilated patients - Accuracy depends on continuous dynamic compensation for changes of gas sampling delay time

Objective: Validation of an open-circuit multibreath nitrogen washout technique (MBNW) for measurement of functional residual capacity (FRC), The accuracy of FRC measurement with and without continuous viscosity correction of mass spectrometer delay time (T-D) relative to gas flow signal and the influence of baseline FIO2 was investigated. Design: Laboratory study and measurements in mechanically ventilated patients. Setting: Experimental laboratory and anesthesiological intensive care unit of a university hospital, Patients: 16 postoperative patients with normal pulmonary function (NORM), 8 patients with acute lung injury (ALI) and 6 patients with chronic obstructive pulmonary disease (COPD) were included, Interventions: Change of FIO2 from baseline to 1.0. Measurements and main results: FRC was determined by MBNW using continuous viscosity correction of T-D (T-Ddyn), a constant T-D based on the viscosity of a calibration gas mixture (T-D0) and a constant T-D referring to the mean viscosity between onset and end of MBNW (T-Dmean). Using T-Ddyn, the mean deviation between 15 measurements of three different lung model FRCs (FRCmeasured) and absolute volumes (FRCmodel) was 0.2 %, For baseline FIO2 ranging from 0.21 to 0.8, the mean deviation between FRCmeasured and FRCmodel was -0.8 %, However, depending on baseline FIO2, the calculation of FRC using T-Dmean and T-D0 increased the mean deviation between FRCmeasured and FRCmodel to 2-4 % and 8-12 %, respectively. In patients (n = 30) the average repeatability coefficient was 6.0 %, FRC determinations with T-Dmean and T-D0 were 0.8-13.3 % and 4.2-23.9 % (median 2.7 % and 8.7 %) smaller than those calculated with T-Ddyn. Conclusion: A dynamic viscosity correction of TD improves the accuracy of FRC determinations by MBNW considerably, when gas concentrations are measured in a sidestream. If dynamic T-D correction cannot be performed, the use of constant T-Dmean might be suitable. However, in patient measurements this can cause an FRC underestimation of up to 13 %.