Single-breath CO2 analysis as a predictor of lung volume in a healthy animal model during controlled ventilation.

OBJECTIVE: To examine the utility of single-breath CO2 analysis as a measure of lung volume.

DESIGN: A prospective, animal cohort study comparing 21 parameters derived from single-breath CO2 analysis with lung volume measurements determined by nitrogen washout in animals during controlled ventilation.

SETTING: An animal laboratory in a university-affiliated medical center.

SUBJECTS: Seven healthy lambs.

INTERVENTIONS: The single-breath CO2 analysis station consists of a mainstream capnometer, a variable orifice pneumotachometer, a signal processor and computer software with capability for both on- and off-line data analysis. Twenty-one derived components of the CO2 expirogram were evaluated as predictors of lung volume. Lung volume was manipulated by 3 cm H2O incremental increases in positive end-expiratory pressure from 0 to 21 cm H2O, and ranged between 147 and 942 mL.

MEASUREMENTS AND MAIN RESULTS: Fifty-five measurements of lung volume were available for comparison with derived variables from the CO2 expirogam. Stepwise linear regression identified four variables that were most predictive of lung volume: a) dynamic lung compliance; b) the slope of phase 3; c) the slope of phase 2 divided by the mixed expired CO2 tension; and d) airway deadspace. The multivariate equation was highly statistically significant and explained 94% of the variance (adjusted r2 =.94, p < .0001). The bias and precision of the calculated lung volume was .00 and 51, respectively. The mean percent difference for the lung volume estimate derived from the single-breath CO2 analysis station was 0.79%.

CONCLUSIONS: Our data indicate that analysis of the CO2 expirogram can yield accurate information about lung volume. Specifically, four variables derived from a plot of expired CO2 concentration vs. expired volume predict changes in lung volume in healthy lambs with an adjusted coefficient of determination of .94. Prospective application of this technology in the setting of lung injury and rapidly changing physiology is essential in determining the clinical usefulness of the technique.