Expired gas backflow in oxygen delivery tubing

Using capnography, we investigated the backflow of expired gas into oxygen delivery tubing in 18 sedated patients receiving supplementary oxygen via a disposable facemask during spinal anaesthesia. At 5 cm from the mask, with an oxygen flow of 2 l.min(-1), carbon dioxide was detected in the oxygen delivery tubing of three patients (18%) on coughing. With the patient wearing the facemask but the oxygen tubing disconnected from the oxygen source, carbon dioxide was detectable 1 m from the facemask (i.e. at the fresh gas outlet) in 14 patients. In a second group of 20 similar patients, no carbon dioxide was detected sampling at the fresh gas outlet with flows of 2 and 4 l.min(-1). Assuming that detection of carbon dioxide indicates at least the possibility of contamination, oxygen tubing should be strictly single-use in this setting. Our findings suggest that filters are not required to protect gas supply outlets, provided that the patient's facemask is removed when the fresh gas outlet is disconnected.