Use of Maximum End-Tidal CO2 Values to Improve End-Tidal CO2 Monitoring Accuracy

BACKGROUND: The arterial partial pressure of CO2 (P-aCO2) can be grossly estimated by the end-tidal partial pressure of CO2 (P-ETCO2). This principle is used in SmartCare (Drager, Lubeck, Germany), which is an automated closed-loop system that uses P-ETCO2 to estimate alveolar ventilation during mechanical ventilation. OBJECTIVE: To assess whether the maximum P-ETCO2, value (instead of the averaged P-ETCO2 value) over 2-min or 5-min periods improves P-aCO2 estimation, and determine the consequences for the SmartCare system. METHODS: We continuously monitored breath-by-breath P-ETCO2 during ventilation with SmartCare in 36 patients mechanically ventilated for various disorders, including 14 patients with COPD. Data were collected simultaneously from SmartCare recordings, every 2-min or 5 min, and through a dedicated software that recorded ventilation data every 10 s. We compared the maximum and averaged P-ETCO2 values over 2-min and 5-min periods to the P-aCO2 measured from 80 arterial blood samples clinically indicated in 26 patients. We also compared SmartCare's classifications of patient ventilatory status based on averaged P-ETCO2 values to what the classifications would have been with the maximum P-ETCO2 values. RESULTS: Mean P-aCO2 was 44 +/- 11 mm Hg. P-aCO2 was higher than averaged P-ETCO2 by 10 +/- 6 mm Hg, and this difference was reduced to 6 +/- 6 mm Hg with maximum P-ETCO2. The results were similar whether patients had COPD or not. Very few aberrant values (< 0.01%) needed to be discarded. Among the 3,137 classifications made by the SmartCare system, 1.6% were changed by using the maximum P-ETCO2 value instead of the averaged P-ETCO2 value. CONCLUSIONS: Use of maximum P-ETCO2 reduces the difference between P-aCO2 and P-ETCO2 and improves SmartCare's classification of patient ventilatory status.