On-line techniques for perfluorochemical vapor sampling and measurement

The authors developed a compact gas sampling and perfluorochemical (PFC) measuring system for use in total and partial liquid ventilation systems, based on a precision two-thermistor thermal detector (TD). They describe the sensitivity and linearity of their on-line method for PFC analysis of expired gases and show how it may be used in partial liquid ventilation studies for determining PFC saturation and loss. Gas is sampled for a short time from a breathing circuit through a heated tube at a selectable point in the breathing cycle. Inspiration is sensed by a pressure transducer. The sample of gas is pulled into the heated (48F°C) thermistor chamber by suction and held there while the cooling effect of the vapor changes the thermistor temperature. Dry air in another chamber affects a second thermistor, and the difference of these responses is amplified. The raw signal is corrected for the effects of varying O₂ levels by a fuel cell. This signal is sampled and held and displayed on a front panel display. Calibration is performed in percentage saturation at 37°C using the PFC in use at that temperature, or another standard such as O₂. In-vitro testing showed a linear response in the thermal detector device (R₂ = 0.99) over the range of vapor pressures tested (0-14 mmHg) and was reproducible to within 3%. When electronically corrected for changes in O₂ concentration, there was less than a 2% change in PFC saturation. The TD responses to CO₂ (R₂ = 0.99) and water vapor (R₂ = 1.0) were linear and approximately equal and opposite over the normal operating ranges of expired gases. In-vivo results in rabbits showed a significant (R₂ = 0.73; p < 0.01) correlation between the (auto-sampler and manual collection modes for determination of PFC in expired gas.