BACKGROUND:The arterial partial pressure of CO 2 (P aCO 2 ) can be grossly estimated by the end-tidal partial pressure of CO 2 (P ETCO 2 ). This principle is used in SmartCare (Dräger, Lü beck, Germany), which is an automated closed-loop system that uses P ETCO 2 to estimate alveolar ventilation during mechanical ventilation. OBJECTIVE: To assess whether the maximum P ETCO 2 value (instead of the averaged P ETCO 2 value) over 2-min or 5-min periods improves P aCO 2 estimation, and determine the consequences for the SmartCare system. METHODS: We continuously monitored breath-by-breath P ETCO 2 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 ETCO 2 values over 2-min and 5-min periods to the P aCO 2 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 ETCO 2 values to what the classifications would have been with the maximum P ETCO 2 values. RESULTS: Mean P aCO 2 was 44 ؎ 11 mm Hg. P aCO 2 was higher than averaged P ETCO 2 by 10 ؎ 6 mm Hg, and this difference was reduced to 6 ؎ 6 mm Hg with maximum P ETCO 2 . 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 ETCO 2 value instead of the averaged P ETCO 2 value. CONCLUSIONS: Use of maximum P ETCO 2 reduces the difference between P aCO 2 and P ETCO 2 and improves SmartCare's classification of patient ventilatory status.
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