The effect of perfluorocarbon vapour on the measurement of respiratorytidal volume during partial liquid ventilation

Abstract

During partial liquid ventilation perfluorocarbon vapour is present in theexhaled gases. The volumes of these gases are measured by pneumotachometers.Error in measuring tidal volumes will give erroneous measurement of lungcompliance during partial liquid ventilation. We aim to compare measured tidalvolumes with and without perfluorocarbon vapour using tidal volumes suitablefor use in neonates.Tidal volumes were produced with a 100 ml calibrationsyringe from 20 to 100 ml and with a calibrated Harvard rodent ventilator from2.5 to 20 ml. Control tidal volumes were drawn from a humidifier chambercontaining water vapour and the PFC tidal volumes were drawn from a humidifierchamber containing water and perfluorocarbon (FC-77) vapour. Tidal volumeswere measured by a fixed orifice, target, differential pressure flowmeter(VenTrak) or a hot-wire anenometer (Bear Cub) placed between the calibrationsyringe or ventilator and the humidifier chamber.All tidal volumes measuredwith perfluorocarbon vapour were increased compared with control (ANOVA p < 0.001 and post t-test p < 0.0001). Measured tidal volume increased from 7to 16% with the fixed orifice type flow-meter, and from 35 to 41% with thehot-wire type. In conclusion, perfluorocarbon vapour flowing throughpneumotachometers gives falsely high tidal volume measurements. Calculation oflung compliance must take into account the effect of perfluorocarbon vapour onthe measurement of tidal volume.