Measurement of the Respiratory Functional Residual Capacity on an Artificial Lung System

Salt Lake Community College

Decreases in functional residual capacity (FRC), the residual respiratory volume following an expiration, are associated with the application of anesthesia and supine body positioning, both common in the ICU. We are developing two non-invasive methods of measuring the FRC on patients under mechanical ventilation. FRC increases are typically accomplished by increasing the Positive End Expiratory Pressure (PEEP) until the arterial O2 content is saturated; however, increased PEEP may also cause a decrease in cardiac output due to the increased thoracic cavity pressure, resulting in a net decreased O2 delivery. Measurement of the FRC will be useful in optimizing the application of PEEP to maximize O2 delivery. FRC determination as a function of PEEP was made on a test lung using a partial CO2 rebreathing method and an N2 washout method in order to compare their accuracy. The CO2 rebreathing method uses Fick’s principle along with a perturbation of gas concentrations initiated by partial rebreathing. The N2 washout method also utilizes Fick’s principle, but creates the perturbation through an increase in inspired O2 concentration. Preliminary FRC measurements were made using the NICO2 system which includes a pneumotachograph for volumetric measurements and a CAPNOSTATTM sensor for CO2 concentration measurement. Although both methods correlated to measured FRC volumes in the test lung, the N2 washout method resulted in greater precision and less variability, most likely due to the greater magnitude of perturbation that is made and the use of data from multiple breaths. Both methods will require further bench testing to verify their accuracy within typical ranges of mechanical ventilation variables, followed by en vivo studies in order to characterize any inherent physiologic implications and to determine repeatability.