Volumetric and reflective device dead space of anaesthetic reflectors under different conditions

Inhalation sedation is increasingly performed in intensive care units. For this purpose, two anaesthetic reflectors, AnaConDa™ and Mirus™ are commercially available. However, their internal volume (100 ml) and possible carbon dioxide reflection raised concerns. Therefore, we compared carbon dioxide elimination of both with a heat moisture exchanger (HME, 35 ml) in a test lung model. A constant flow of carbon dioxide was insufflated into the test lung, ventilated with 500 ml, 10 breaths per minute. HME, Mirus and AnaConDa were connected successively. Inspired (insp-CO₂) and end-tidal carbon dioxide concentrations (et-CO₂) were measured under four conditions: ambient temperature pressure (ATP), body temperature pressure saturated (BTPS), BTPS with 0.4 Vol% (ISO-0.4), and 1.2 Vol% isoflurane (ISO-1.2). Tidal volume increase to maintain normocapnia was also determined. Insp-CO₂ was higher with AnaConDa compared to Mirus and higher under ATP compared to BTPS. Isoflurane further decreased insp-CO₂ and abolished the difference between AnaConDa and Mirus. Et-CO₂ showed similar effects. In addition to volumetric dead space, reflective dead space was determined as 198 ± 6/58 ± 6/35 ± 0/25 ± 0 ml under ATP/BTPS/ISO-0.4/ISO-1.2 conditions for AnaConDa, and 92 ± 6/25 ± 0/25 ± 0/25 ± 0 ml under the same conditions for Mirus, respectively. Under BTPS conditions and with the use of moderate inhaled agent concentrations, reflective dead space is small and similar between the two devices.