Summary

Background and objectives: Supplemental oxygen is commonly given via nasal cannulae in spontaneously breathing patients. Our modified nasal cannula with a clamp between the nasal prongs can provide O2 via one nostril and CO2 can be sampled through the other one. We have studied whether this cannula can provide oxygenation similar to a standard cannula without affecting end-tidal CO2 monitoring. Methods: Eighty-six patients were studied during spinal anaesthesia and sedation. In 15 patients, arterial blood was sampled while O2 was delivered at flow rates of 0, 2 and 4L min−1, with or without clamping between the prongs of our modified nasal cannula. In the remaining 71 patients, arterial O2 was measured while using our modified nasal cannula with the clamp applied. End-tidal CO2 was recorded on a capnograph and the correlation between end-tidal and arterial values with our modified nasal cannula was investigated. Results: No end-tidal CO2 waveforms were found with oxygen flow greater than 2L min−1 without clamping between the prongs. With clamping there was a significant correlation (r = 0.83) between arterial and end-tidal CO2. A Bland–Altman analysis revealed a bias of 0.49 kPa with precision of ±0.76 kPa. Arterial oxygenation was not affected by our modified nasal prongs with clamp as compared to the standard cannula. Conclusion: Our modified nasal cannula can provide continuous monitoring of end-tidal CO2 without affecting oxygen delivery in sedated, spontaneously breathing patients.