Non-invasive carbon dioxide monitoring in patients with cystic fibrosis during general anesthesia: end-tidal versus transcutaneous techniques.

INTRODUCTION

The 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 (ETCO), transcutaneous (TCCO), and capillary blood gas carbon dioxide (Cap-CO) monitoring with the partial pressure of carbon dioxide (PaCO) from an ABG in patients with cystic fibrosis.

METHODS

Intraoperatively, a single CO value was simultaneously obtained using ABG (PaCO), capillary (Cap-CO), TCCO, and ETCO 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 (FEV%). Additionally, the absolute difference in the TCCO, ETCO, and Cap-CO values versus PaCO 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 PaCO were compared using a Fishers' exact test.

RESULTS

The 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/m (16.7, 21.4), and mean FEV% of 87.3 ± 18.3%. Bias (SD) was 4.8 (5.7) mmHg with Cap-CO monitoring, 7.3 (9.7) mmHg with TCCO monitoring, and 9.7 (7.7) mmHg with ETCO monitoring. Although there was no difference between the degree of bias in the population as a whole, when divided based on FEV% and BMI, there was greater bias with ETCO in patients with a lower FEV% and a higher BMI. The Cap-CO vs. PaCO difference was 5.2 ± 5.3 mmHg (SD), with 16 (48%) ≤ 3 mmHg and 20 (61%) ≤ 5 mmHg from the ABG value. The TCCO-PaCO difference was 9.1 ± 7.2 mmHg (SD), with 11 (27%) ≤ 3 mmHg and 15 (37%) ≤ 5 mmHg from the ABG value. The ETCO-PaCO mean difference was 11.2 ± 7.9 mmHg (SD), with 5 (12%) ≤ 3 mmHg and 11 (26%) ≤ 5 mmHg from the ABG value.

CONCLUSIONS

While Cap-CO most accurately reflects PaCO as measured on ABG, of the non-invasive continuous monitors, TCCO was a more accurate and reliable measure of PaCO than ETCO, especially in patients with worsening pulmonary function (FEV% ≤ 81%) and/or a higher BMI (≥ 18.7 kg/m).