Accuracy of End-tidal CO2 Measurement Through the Nose and Pharynx in Nonintubated Patients During Digital Subtraction Cerebral Angiography

Objective: To determine the accuracy of end-tidal CO2 (PETCO2) obtained in the nose through the Smart CapnoLine and in the pharynx through the modified Filterline H Set with supplemental oxygen at 5 L/min in nonintubated patients undergoing digital subtraction cerebral angiography (DSA). Type of Study: Prospective, observational. Patients: Twenty patients with disturbance of consciousness because of brain disease, who will receive DSA. Methods: PETCO2 was measured in the nose through the Smart CapnoLine and in the pharynx using the modified Filterline H Set that was inserted through the nasopharyngeal airway. Oxygen was administered through the Smart CapnoLine at a rate of 5 L/min. Five minutes after a constant and normally shaped capnography waveform, arterial blood was drawn from an indwelling femoral catheter for analyzing arterial CO2 partial pressure (PaCO2), and PETCO2 that was measured through the nose and the pharynx were simultaneously recorded. After the DSA procedure, PaCO2 was analyzed again. Data were analyzed with Pearson correlation and Bland-Altman analysis. Results: PETCO2 sampled from both the nose and the pharynx was significantly correlated with PaCO2, and the correlation coefficients had approximate values, 0.832 (P < 0.0001) for PaCO2 with PETCO2 through the nose and 0.836 (P < 0.0001) for PaCO2 with PETCO2 through the pharynx. The mean bias +/- SD for PETCO2 and PaCO2 was 4.53 +/- 2.76 mm Hg (nose) and 3.22 +/- 2.86 mm Hg (pharynx). The 95% level of agreement for PETCO2 and PaCO2 ranged from -0.90 to 9.95 mm Hg (nose) and from -2.39 to 8.82 mm Hg (pharynx). End-tidal CO2 measurements through the nose and the pharynx had comparable performance. The correlation of PETCO2 measured through the nose and the pharynx was 0.971 (P < 0.001). The difference between PETCO2 measured through the nose and the pharynx was 1.31 +/- 1.25 mm Hg, and t test results showed that arterial to end-tidal CO2 pressure difference (Pa-ETCO2) in sampling through the nose was significantly greater than Pa-ETCO2 sampling through the pharynx (P < 0.05). Conclusions: In a clinical setting, end-tidal CO2 measurements sampled from the nose and the pharynx were accurate and reliable in nonintubated patients with a nasopharynx airway in place during DSA.