Table 2. Calculations Illustrating Sevoflurane Vaporizer Dial Settings Required to Maintain 1.3% End-expired Sevoflurane during Minimal-flow and Low-flow Anesthesia with or without nitrous oxide
FGF = fresh gas flow; Vo2= oxygen consumption; Vn2o = nitrous oxide uptake; Sevo = sevoflurane; Vsevo= sevoflurane uptake by the patient; Vap%= vaporizer dial setting.
(a) Oxygen consumption is approximately 250 ml/min. 10
(b) Nitrous oxide requirements to attain an end-expired concentration of 60–65% are high the first few minutes because of circuit and lung wash-in but then gradually decrease to 250 ml/min after 10 min. These nitrous oxide requirements have been mathematically expressed as 1000 · t−1/2ml/min. 3
(c) Assumes that the concentration of sevoflurane in the scavenged gas is 1.3%. In reality, it may be slightly higher (between inspired and expired concentrations).
(d) Sevoflurane uptake (vapor) after 15 min remains approximately constant at 10.5 ml/min during the ensuing 45 min (Etsevo, 1.3%).
(e) Calculated as (ml sevoflurane lost through bellows block overflow valve + ml sevoflurane taken up by the patient)/total FGF.
(f) Calculated in each group from the dialed-in concentration at the common gas outlet and FGF.
The vaporization effect (sevoflurane vapor is added to carrier gas, thereby increasing total fresh gas flow) is ignored because it is not mathematically significant.