Evaluation of a transcutaneous carbon dioxide monitor in severe obesity.

Objective: To determine the reliability of estimating arterial CO2 pressure (PaCO2) using a recently introduced transcutaneous CO2 pressure (PtcCO2) monitor in severe obese patients. Design: Observational and interventional study. Setting: District hospital with respiratory ward and bariatric surgery unit. Patients and methods: PtcCO2 was measured in 35 obese patients with varied pathology, including chronic obstructive pulmonary disease, obstructive sleep apnea syndrome and hypoventilation syndrome. Ten minutes after the probe had been attached to an earlobe, PtcCO2 was recorded immediately before arterial blood sampling. The PtcCO2 and PaCO2 values obtained with two methods were compared by Bland–Altman analysis. In a subgroup of 18 obese patients with chronic obstructive pulmonary disease and/or obstructive sleep apnea syndrome with moderate to severe hypercapnia both PtcCO2 and PaCO2 were re-evaluated during continuous positive airways pressure (CPAP) or bi-level positive airway pressure (Bi-PAP) treatment. Results: The mean difference between PaCO2 and PtcCO2 was –1.4 mmHg, and the standard deviation of the difference was 1.3 mmHg. Bland–Altman analysis showed generally good agreement between the two methods with a 95% limit of agreement of –4 to 1.1. The agreement between methods did not significantly change before and during cPAP or Bi-PAP treatment in hypercapnic patients. Conclusions: The accuracy of estimation of PaCO2 by transcutaneous monitoring was generally good in comparison with standard arterial blood gases examination. The device appears to be promising for use in obese patients to evaluate abnormalities in their alveolar ventilation.