Nasopharyngeal versus transcutaneous CO₂ monitoring during ERCP

Transcutaneous carbon dioxide monitoring (TCO₂) has been shown to have a strong correlation to arterial CO₂ levels. It has been suggested that TCO₂ may detect hypoventilation better than pulse oximetry and other standard monitoring techniques. However, TCO₂ is cumbersome and impractical for routine endoscopic procedures. Recently, a sidestream CO₂ monitor became available that allows for continuous CO₂ measurement via nasal prongs. Purpose: To assess the correlation between transcutaneous and nasopharyngeal CO₂ monitoring during ERCP. Methods: Patients underwent simultaneous TCO₂ and nasopharyngeal (NCO₂) monitoring during ERCP utilizing a sidestream CO₂ detector coupled with nasal cannula sampling device, providing a real time graphic assessment of respiratory rate and pattern. Patients received standard sedation and analgesia with meperidine and midazolam. Results: Ten patients were studied. There is a significant correlation between the simultaneous NCO₂ and TCO₂ measurement (r = 0.6, p < 0.001) Placement of the nasopharyngeal cannula was well-tolerated and did not require topical anesthesia. There were much weaker albeit significant correlations between the respiratory rate and both NCO₂ (r = -0.27, p < 0.01) and TCO₂ (r = -0.26, p < 0.01). Conclusions: 1) There is a strong correlation between NCO₂ and TCO₂. 2) NCO₂ is a well-tolerated, easy to use method for monitoring alveolar hypo ventilation 2) NCO₂ provides a real time graphic assessment of respiratory rate and pattern which supplants the need for visual assessment of respiratory excursion during ERCP.