Measurement of Functional Residual Capacity of the Lung Before and During Acute Lung Injury

Background: We measured Functional Residual Capacity (FRC) of the lungs 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 by oleic acid. We compared the FRC from partial CO2 rebreathing with those of a nitrogen washout reference method. Materials and Methods: Using an approved animal protocol, general anesthesia was induced and maintained with propofol in six swine (38.8-50.8 kg). In both the mechanical lung analog and the animals, a partial CO2 rebreathing monitor (NICO2, Respironics Inc., Wallingford, CT) was placed in the breathing circuit between the endotracheal tube and the Y-piece. The partial CO2 rebreathing signal obtained from this monitor was used to calculate FRC. FRC was also measured with a nitrogen washout measurement technique. In the animals, we collected data from healthy lungs and then subsequent to a lung injury that simulated the conditions of ARDS/ALI which was created by intravenously infusing 0.09 mL/kg of oleic acid over a 15-minute period. At each stage of the experiment, the positive end-expiratory pressure (PEEP) was set to 0, 5, 10, and 15 mmHg H2O. At each PEEP level, we compared the average of three FRC measurements from CO2 rebreathing to the average of three nitrogen washout reference measurements. Results: The correlation coefficient for the linear regression between CO2 rebreathing and nitrogen washout measurements in the animals was r= 0.89 (n = 50). The average error of the CO2 washout system was -87 mL with limits of agreement (LOA) ± 263 mL. In the mechanical lung, the average error in the FRC measured by the CO2 wash-in system was 37 mL with LOA ± 103 mL, which was equivalent to 1.7% of the true FRC. The correlation coefficient was r= 0.96. Conclusion: These results indicate FRC measurement by CO2 rebreathing can reliably detect a decrease in FRC during lung injury and can reflect the response of the FRC to treatment with PEEP.