Carbon monoxide generation in carbon dioxide absorbents.

Several cases of unexpected high carboxyhemoglobin (COHb) levels in patients undergoing general anesthesia were observed. To avoid carbon monoxide (CO) intoxication, the use of high fresh gas flows and frequent changes of the absorbent were recommended. However, due to economic and ecologic considerations, low-flow anesthetic techniques have advantages. Thus, the subject urgently needed to be reexamined. In 1001 patients undergoing enflurane or isoflurane anesthesia, blood samples were taken 30 min after fresh gas flow reduction to 0.5 L/min and analyzed for COHb. The absorbent canisters, containing soda lime, were used for several days. The statistical mean and SD of COHb was 1.17% +/- 0.97% in the range of 0%-7.6%. There was no statistical difference between the COHb values when broken down by the duration of use of the absorbent canisters. In no case were dangerously high COHb levels observed. As recently revealed, only dry absorbents produce CO if exposed to volatile anesthetics containing a CHF2-moiety. Thus, all measures must be avoided that dry out the absorbent. Low-flow anesthesia preserves the moisture content of the absorbent and, thus, seems to be a factor protecting from CO generation.