Microprocessor-controlled alveolar gas sampling system

The composition of alveolar gas is an important parameter in many pulmonary functions tests. Most alveolar gas sampling devices are difficult to interface to noncooperative patients and do not allow averaging over several breaths. A microprocess-controlled gas sampler is described that is further development of a gas sampler built by Farr (1974) which solves both these problems. The gas sampler employs a stepper motor drive syringe pump to pull a gas sample through a flexible catheter. The catheter has a think film thermistor mounted on the tip to provide a pneumographic signal for triggering. The catheter tip assembly is easily placed in a patient's nose of other open airways and does not occlude the passage of air. Samples are collected over several breath cycles and then emptied into a reservoir. This automatically averages breath-to-breath values and reduced the effect of irregular breathing. An algorithm is described that allows sampling to take place over a variable portion of expiration and also compensates for different breathing rates. A description is given of Clark's (1978) synchronous tracer insertion (STI) method for monitoring the alveolar ventilation, carbon dioxide production, oxygen consumption, pulmonary blood flow, and ventilation-perfusion distributions. The STI method employs a device which inserts a bolus of several tracer gases during inspiration in such a way that the gases are carried into the alveolar compartments. The gases are washed in the washed out of the lungs on a cyclical manner and the washin/washout curves for each gas are analyzed to obtain a measurement of the monitored lung parameters. A gas collection system has been developed to assist in the analysis of the washin/washout curves of the STI method. The gas collection system is interfaced to the gas insertion device and to the gas sampler. This allows alveolar gas samples collected over many washin/washout cycles at predetermined points to be stored n separate reservoirs. The gas samples in the reservoirs are then analyzed and the lung parameters are determined by mathematical analysis of the measured gas fractions. A mass spectrometer has been used to compare gas fractions in gas samples collected over the middle half of expiration with directly measured midexpiration gas fractions. The gas samples were collected over several breath cycles and mechanically averaged n the collection syringe. A simultaneous record of the expired gas frication was used to measure midexpiration values of the sample expirations, and an average was calculated. The results of this test show an average percent error between the two measurements of .06 percent with the largest error being about 1 percent.