Single breath CO2 analysis: description and validation of a method

J H Arnold, J E Thompson, L W Arnold
Critical Care Medicine 1996, 24 (1): 96-102

OBJECTIVES: To evaluate the performance of a newly developed single breath CO2 analysis station in measuring the airway deadspace in a lung model (study 1), and then to quantify the bias and precision of the physiologic deadspace measurement in a surfactant-depleted animal model (study 2).

DESIGN: A prospective bench validation of a new technique of airway deadspace measurement using a criterion standard (study 1); a prospective, animal cohort study comparing a new technique of physiologic deadspace measurement with a reference method (Bohr-Enghoff method) (study 2).

SETTING: A bench laboratory and animal laboratory in a university-affiliated medical center.

SUBJECTS: A lung model (study 1), and adult sheep with induced surfactant deficiency (saline lavage) (study 2).

METHODS: 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. Study 1: We evaluated the accuracy of the airway deadspace calculation using a plexiglass lung model. The capnometer and pneumotachometer were placed at the ventilator Y-piece with polyvinyl chloride tubing added to simulate increased airway deadspace. Segments of tubing were sequentially removed during each testing session to simulate decreasing deadspace. The calculated airway deadspace was derived from the single breath CO2 plot and compared with the actual tubing volume using least-squares linear regression and paired t-tests. Study 2: The accuracy of the physiologic deadspace measurement was examined in a saline-lavaged animal model by comparing the physiologic deadspace calculated from the single breath CO2 analysis station with values obtained using the Enghoff modification of the Bohr equation: deadspace/tidal volume ratio = (PaCO2-mixed expired PCO2)/PaCO2.

MEASUREMENTS AND MAIN RESULTS: Study 1: Thirty-six measurements of calculated airway deadspace were made and compared with actual circuit deadspace during four different testing conditions. Measured airway deadspace correlated significantly with actual circuit deadspace (r2 = .99). The proportional error of the method was -0.8% with a 95% confidence interval from -3.6% to 1.9%. Study 2: A total of 27 pairs of measurements in four different animals were available for analysis. The derived physiologic deadspace/tidal volume ratio significantly correlated with the value obtained using the Bohr-Enghoff method (r2 = .84). The bias and precision of our physiologic deadspace calculation were .02 and .02, respectively, and the mean percent difference for the physiologic deadspace calculated from the single breath CO2 analysis station was 2.4%.

CONCLUSIONS: Our initial experience with the single breath CO2 analysis station indicates that this device can reliably provide on-line evaluation of the single-breath CO2 waveform. In particular, estimation of the airway and physiologic deadspace under a variety of testing conditions was consistently within 5% of actual values. We feel that with further application and refinement of the technique, single breath CO2 analysis may provide a noninvasive, on-line monitor of changes in pulmonary blood flow.

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