Measurement of lung volume in mechanically ventilated monkeys with an ultrasonic flow meter and the nitrogen washout method.

OBJECTIVE

Measurement of functional residual capacity (FRC) during mechanical ventilation is important to standardise respiratory system compliance and adjust the ventilator settings to optimise lung recruitment. In the present study we compared three methods to measure FRC.

DESIGN

The bias flow nitrogen washout technique (FRC(N2MC)), the multiple breath nitrogen washout (FRC(MBNW)) and the multiple breath sulphur-hexafluoride washout using the molar mass signal of an ultrasonic flow meter (FRC(MBSF6)) were compared in six adult monkeys after endotracheal intubation and during spontaneous breathing and mechanical ventilation at three different positive end-expiratory pressure (PEEP) levels of 0, 5 and 10 cmH2O.

SETTING

Animal research laboratory.

RESULTS

We found good agreement between all three methods and they all accurately measured changes in FRC when PEEP was increased. The coefficients of variance of the three measurement techniques were in the same range (1.3-9.2%).

CONCLUSION

The measurement of the tracer gas concentration with the molar mass signal of the ultrasonic flow meter provides a good and simple alternative to respiratory mass spectrometer for FRC measurements in ventilated subjects.