Nocturnal pulse-oximetry (NPO) is a technique widely used in the diagnostic process and in the follow-up of patients with sleep-disordered breathing (SDB) due to its availability and low cost; however, it does not allow differentiation between purely hypoxaemic disorders and those with predominantly ventilatory failure. Incorporating nocturnal transcutaneous capnography (CpTC) could optimise the information obtained for therapeutic decision-making.Our aim was to measure nocturnal pCO2 using CpTC in patients with a T90>30% and make treatment changes based on the results. A cross-sectional study in which patients with SDB with NPO at diagnosis or control, with a T90%>30% and a daytime arterial blood gas with a pCO2<49 mmHg were included. A nocturnal study was performed with a capnograph with a recording of at least 6h. T50 (time with pCO2 >49mmHg) and mean PCO2 were analysed. A pathological CpTc was considered as T50 >10% and/or mean PCO2 >49mmHg. 47 cases were included with a mean age of 64 years (SD ± 11 years). The rest of the variables are attached in table 1.CpTc detected nocturnal hypercapnia in 36% of patients  and therapeutic changes were made in 34% of them. NIV was initiated in 12 cases, parametric changes in 2 and O2 was added in 2 cases. Adding transcutaneous capnography to the monitoring of patients with nocturnal hypoxaemia has detected 34% of cases with unknown hypercapnia. This has provided information for therapeutic changes in 1/3 of patients.