Measurement of functional residual capacity of the lung by partial CO2 rebreathing method during acute lung injury in animals.

BACKGROUND Several techniques for measuring the functional residual capacity (FRC) of the lungs in mechanically ventilated patients have been proposed, each of which is based on either nitrogen wash-out or dilution of tracer gases. These methods are expensive, difficult, time-consuming, impractical, or require an intolerably large change in the fraction of inspired oxygen. We propose a CO(2) wash-in method that allows automatic and continual FRC measurement in mechanically ventilated patients. METHODS We measured FRC with a CO(2) partial rebreathing technique, first in a mechanical lung analog, and then in mechanically ventilated animals, before, during, and subsequent to an acute lung injury induced with oleic acid. We compared FRC measurements from partial CO(2) rebreathing to measurements from a nitrogen wash-out reference method. Using an approved animal protocol, general anesthesia was induced and maintained with propofol in 6 swine (38.8-50.8 kg). A partial CO(2) rebreathing monitor was placed in the breathing circuit between the endotracheal tube and the Y-piece. The partial CO(2) rebreathing signal obtained from the monitor was used to calculate FRC. FRC was also measured with a nitrogen wash-out measurement technique. In the animal studies we collected data from healthy lungs, and then subsequent to a lung injury that simulated the conditions of acute lung injury/acute respiratory distress syndrome. The injury was created by intravenously infusing 0.09 mL/kg of oleic acid over a 15-min period. At each stage of the experiment, the positive end-expiratory pressure (PEEP) was set to 0, 5, 10, or 15 cm H(2)O. At each PEEP level we compared the average of 3 CO(2) rebreathing FRC measurements to the average of 3 nitrogen wash-out reference measurements. We also tested the FRC measurement system with a mechanical test lung in which the true FRC could be directly measured. RESULTS The squared correlation for the linear regression between CO(2) rebreathing and nitrogen wash-out measurements in the animals was r(2) = 0.89 (n = 50). The average error of the CO(2) wash-out system was -87 mL and the limits of agreement were+/- 263 mL. In the mechanical test lung, the average error of the FRC measured via the CO(2) wash-in system was 37 mL, and the limits of agreement were +/- 103 mL, which was equivalent to 1.7% of the true FRC. The squared correlation was r(2) = 0.96. CONCLUSION These results indicate that FRC measurement via CO(2) rebreathing can reliably detect an FRC decrease during lung injury and can reflect the response of the FRC to treatment with PEEP.