OBJECTIVE: To compare the PaCO(2) with the ETCO(2) obtained with the Smart Capnoline™ in the postoperative setting of cardiac surgery during ventilation and after extubation TYPE OF STUDY: Prospective, observational. PATIENTS: Twenty patients after cardiac surgery. METHODS: In the intensive care unit, arterial blood gases were measured concomitantly with ETCO(2), and difference between PaCO(2) and ETCO(2) were calculated. Three CO(2) sensors were utilized: Filterline H set for intubated patients, Smart Capnoline HO(2) (nasal version) and Smart Capnoline O(2) (bucconasal version) after extubation. Data were compared with Wilconson test and the intraclass correlation coefficient was calculated. RESULTS: The difference PaCO(2) - ETCO(2) was significantly larger in extubated patients compared to intubated patients, which is also confirmed for the bucconasal sensor (intubated patients: 6.6 ± 4.3 mmHg, nasal sensor: 9.3 ± 3.5 mmHg, bucconasal sensor: 15,4 ± 12.9 mmHg). CONCLUSION: In the postoperative setting of cardiac surgery, ETCO(2) measurements allow a reliable estimation of PaCO(2) in intubated patients in contrast to measurements in extubated patients. The bucconasal CO(2) sensor does not show more reliable measurements compared to nasal sensors in the postoperative setting of cardiac surgery.