SINGLE-BREATH CO2 ANALYSIS AS A PREDICTOR OF LUNG-VOLUME IN A HEALTHYANIMAL-MODEL DURING CONTROLLED VENTILATION

Objective: To examine the utility of single-breath CO2 analysis as a m easure of lung volume. Design: A prospective, animal cohort study comp aring 21 parameters derived from single-breath CO2 analysis with lung volume measurements determined by nitrogen washout in animals during c ontrolled ventilation. Setting: An animal laboratory in a university-a ffiliate medical center. Subjects: Seven healthy lambs. Interventions: The single breath CO2 analysis station consists of a mainstream capno meter, a variable orifice pneumotachometer, a signal processor and com puter software with capability for both on- and off-line data analysis . Twenty-one derived components of the CO2 expirogram were evaluated a s predictors of lung volume. Lung Volume was manipulated by 3 cm H2O i ncremental increases in positive end-expiratory pressure from 0 to 21 cm H2O, and ranged between 147 and 942 mt. Measurements and Main Resul ts: Fifty-five measurements of lung Volume were available for comparis on with derived variables from the CO2 expirogam, Stepwise linear regr ession identified four variables that were most predictive of lung vol ume: 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 de adspace. The multivariate equation was highly statistically significan t and explained 94% of the variance (adjusted r(2) = .94, p < .0001), The bias and precision of the calculated lung volume was .00 and 51, r espectively. The mean percent difference for the lung volume estimate derived from the single-breath CO2 analysis station was 0.79%, Conclus ions: Our data indicate that analysis of the CO2 expirogram can yield accurate information about lung volume. Specifically, four variables d erived from a plot of expired CO2 concentration vs, expired volume pre dict changes in lung volume in healthy lambs with an adjusted coeffici ent of determination of .94. Prospective application of this technolog y in the setting of lung injury and rapidly changing physiology is ess ential in determining the clinical usefulness of the technique.