The Arterial to End-Tidal Carbon Dioxide Gradient Increases with Uncorrected but Not with Temperature-Corrected PaCO2 Determination During Mild to Moderate Hypothermia

Abstract

End-tidal carbon dioxide (PETCO2) monitoring is recommended as a basic standard of care and is helpful in adjusting mechanical ventilation. Gas solubility changes with temperature, which might affect the PaCO2 and thereby the gradient between Paco2 and PETCO2 (PA-ETCO2) under hypothermic conditions. We investigated whether the PA-ETCO2 changes during mild to moderate hypothermia (36[degree sign]C-32[degree sign]C) using PaCO2 measured at 37[degree sign]C (uncorrected PaCO2) and PaCO2 corrected to actual body temperature. We preoperatively investigated 19 patients. After anesthesia had been induced, controlled ventilation was established to maintain normocarbia using constant uncorrected PaCO (2) to adjust ventilation (alpha-stat acid-base regimen). Body core temperature was reduced without surgical intervention to 32[degree sign]C by surface cooling. Continuous PETCO2 was monitored with a mainstream PETCO2 module. The PA-ETCO (2) was calculated using the uncorrected and corrected PaCO2 values. During body temperature reduction from 36[degree sign]C to 32[degree sign]C, the gradient between PETCO2 and uncorrected PaCO2 increased 2.5-fold, from 4.1 +/- 3.7 to 10.4 +/- 3.8 mm Hg (P < 0.002). The PA-ETCO2 remained unchanged when the corrected Paco2 was used for the calculation. We conclude that when the alpha-stat acid-base regimen is used to adjust ventilation, the PA-ETCO2 calculated with the uncorrected PaCO2 increases and should be added to the differential diagnosis of widened PA-ETCO2. In contrast, when the corrected PaCO2 is used for the calculation of the PA-ETCO2, the PA-ETCO2 remains unaltered during hypothermia. Implications: We investigated the impact of induced hypothermia (36[degree sign]C-32[degree sign]C) on the gradient between PaCO2 and PETCO2 (PA-ETCO (2)). The PA-ETCO2 increased 2.5-fold when CO2 determinations were not temperature-corrected. Hypothermia should be added to the differential diagnosis of an increased PA-ETCO2 when the alpha-stat acid-base regimen is used. (Anesth Analg 1998;86:1131-6)