• Anne May, Chris Humston, Julie Rice, Christopher J Nemastil, Ann Salvator, and Joseph Tobias.
• Department of Pulmonary Medicine, Nationwide Children's Hospital, Columbus, OH, USA.
• J Anesth. 2020 Feb 1; 34 (1): 66-71.

IntroductionThe gold standard for measuring the partial pressure of carbon dioxide remains arterial blood gas (ABG) analysis. For patients with cystic fibrosis undergoing general anesthesia or polysomnography studies, continuous non-invasive carbon dioxide monitoring may be required. The current study compares end-tidal (ETCO2), transcutaneous (TCCO2), and capillary blood gas carbon dioxide (Cap-CO2) monitoring with the partial pressure of carbon dioxide (PaCO2) from an ABG in patients with cystic fibrosis.MethodsIntraoperatively, a single CO2 value was simultaneously obtained using ABG (PaCO2), capillary (Cap-CO2), TCCO2, and ETCO2 techniques. Tests for correlation (Pearson's coefficient) and agreement (Bland-Altman analysis) were performed. Data were further stratified into two subgroups based on body mass index (BMI) and percent predicted forced expiratory volume in 1 s (FEV1%). Additionally, the absolute difference in the TCCO2, ETCO2, and Cap-CO2 values versus PaCO2 was calculated. The mean ± SD differences were compared using a paired t test while the number of times the values were ≤ 3 mmHg and ≤ 5 mmHg from the PaCO2 were compared using a Fishers' exact test.ResultsThe study cohort included 47 patients (22 males, 47%) with a mean age of 13.4 ± 7.8 years, median (IQR) BMI of 18.7 kg/m2 (16.7, 21.4), and mean FEV1% of 87.3 ± 18.3%. Bias (SD) was 4.8 (5.7) mmHg with Cap-CO2 monitoring, 7.3 (9.7) mmHg with TCCO2 monitoring, and 9.7 (7.7) mmHg with ETCO2 monitoring. Although there was no difference between the degree of bias in the population as a whole, when divided based on FEV1% and BMI, there was greater bias with ETCO2 in patients with a lower FEV1% and a higher BMI. The Cap-CO2 vs. PaCO2 difference was 5.2 ± 5.3 mmHg (SD), with 16 (48%) ≤ 3 mmHg and 20 (61%) ≤ 5 mmHg from the ABG value. The TCCO2-PaCO2 difference was 9.1 ± 7.2 mmHg (SD), with 11 (27%) ≤ 3 mmHg and 15 (37%) ≤ 5 mmHg from the ABG value. The ETCO2-PaCO2 mean difference was 11.2 ± 7.9 mmHg (SD), with 5 (12%) ≤ 3 mmHg and 11 (26%) ≤ 5 mmHg from the ABG value.ConclusionsWhile Cap-CO2 most accurately reflects PaCO2 as measured on ABG, of the non-invasive continuous monitors, TCCO2 was a more accurate and reliable measure of PaCO2 than ETCO2, especially in patients with worsening pulmonary function (FEV1% ≤ 81%) and/or a higher BMI (≥ 18.7 kg/m2).

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