Monitoring of functional residual capacity by an oxygen washin/washout; technical description and evaluation

It was the goal of this study to develop and test an automated method for measuring functional residual capacity (FRC) by an oxygen washin/washout in intensive care settings. Such a method is required to work with conventional ventilator breathing systems and to use only medical grade sensors.

The oxygen setting on a standard intensive care ventilator is changed by at least 10%. Ventilatory pressure and flow are measured by the built-in sensors of the intensive care ventilator. Oxygen concentration is measured by a diverting medical oxygen analyzer. In order to overcome the known problem that synchrony between flow and concentration measurement is corrupted by the change of gas viscosity and by the cyclic change of airway pressure, a physical/mathematical model of the pneumatic circuit of the analyzer was developed. With this model, the change of sample flow is calculated continuously. Thus, synchrony between flow and gas concentration measurement is restored. This allows the determination of volumetric gas fluxes as needed for the FRC measurement. The setup was tested in the laboratory with a lung simulator. Simulated lung compliance, breathing frequency and tidal volume were varied. Results. The mean difference between measured and simulated FRC (range 1.7 to 5 L) was less than 1% at tidal volumes greater than 400 mL. This difference ranged from -5% to 8%, depending on simulated lung compliance and ventilator setting. The variability of consecutive measurements was about 2.5%.

A method has been developed for reliable measurement of the FRC with an oxygen washin/washout technique. This method is sufficiently easy to use to suit for application in intensive care units. It does not require any action by the operator except a manual change of inspired oxygen concentration. Accuracy and sensitivity of the method have been proven sufficient to meet clinical and scientific requirements. Future clinical studies will reveal the applicability of the chosen procedure under clinical conditions.