Measurement of Functional Residual Capacity of the Lung by Partial CO 2 Rebreathing Method During Acute Lung Injury in Animals

Abstract

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 CO₂ wash-in method that allows automatic and continual FRC measurement in mechanically ventilated patients.

METHODS:

We measured FRC with a CO₂ 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 CO₂ 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 CO₂ rebreathing monitor was placed in the breathing circuit between the endotracheal tube and the Y-piece. The partial CO₂ 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 H₂O. At each PEEP level we compared the average of 3 CO₂ 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 CO₂ rebreathing and nitrogen wash-out measurements in the animals was r² = 0.89 (n = 50). The average error of the CO₂ 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 CO₂ 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 r² = 0.96.

CONCLUSION:

These results indicate that FRC measurement via CO₂ rebreathing can reliably detect an FRC decrease during lung injury and can reflect the response of the FRC to treatment with PEEP.