Arterial blood gas analysis (ABG) is the gold standard test for carbon dioxide measurement. End-tidal PCO2 (PetCO2) and transcutaneous PCO2 (PtcCO2) are non-invasive alternative methods. Objective: to examine the use of PetCO2 and PtcCO2 as PaCO2surrogates in awake children. Methods: Prospective observational study. Consecutive awake children in stable condition referred to the Sleep Unit of Hospital de Pediatría Prof. Dr. J. P. Garrahan with suspected or confirmed SRRD requiring ABG were included. PetCO2 and PtcCO2 were recorded simultaneously during arterial puncture. PetCO2 and PtCO2 values were compared with PaCO2. Correlation coefficient and Bland-Altman analysis were applied. Sample size was calculated considering a mean difference ≤ 3 mmHg as clinically acceptable. Results: 68 sample sets were obtained from 67 patients. Median age was 9.11 years (0.23-18.76). During 94.1% of the procedures patients breathed spontaneously, 30% needed multiple punctures and 92% resulted in pain. Median (IQR) PaCO2 (mmHg) was 36.3 (31.45; 40.90), PetCO2 33.0 (29;39) and PtcCO2 38.8 (32.95;43.32). Correlation and agreement for PaCO2/PetCO2 and PaCO2/PtcCO2was: r= 0.6 and0.9, and media of bias=2.83(-9.97;15.64) and-1.88 (-9.01;5.24), respectively. Hypercapnia(PaCO2>45.0 mmHg) was present in 8/68 (11.8%) samples. Sensitivity, specificity, positive predictive value and negative predictive value to detect hypercapnia with PetCO2 was 38 %,98%,75% and 92%, respectively, and with PtcCO2, 100%, 90%, 57% and 100%, respectively. Conclusion: PtcCO2 showed better agreement with PaCO2 than PetCO2,.but because of the wide dispersion of values, neither method can replace the gold standard. Transcutaneous CO2might be a good screening tool to detect hypercapnia in awake children.