A simple strategy for faster induction and more cost-effective use of anesthetic vapor

Introduction. Inducing general anesthesia often involves mask ventilation u sing high fresh gas flow (FGF) to administer anesthetic vapor prior to endo tracheal intubation. A common practice is to turn the vaporizer off when th e mask is removed from the patient's face to avoid room contamination (VAPO ff). An alternative approach is to leave the vaporizer on and turn the FGF to minimum to reduce the amount of vapor laden gas that can enter the room (FGFOff). The objective of this study is to compare the relative induction times and vapor costs associated with each induction strategy. Methods. Eac h induction method was simulated using Gasman (R) (MedMan Simulations, Ches tnut Hill, MA) for Windows assuming a 70 kg patient. To simulate a period o f mask ventilation with anesthetic vapor prior to intubation, the FGF was s et to 6 l/min and the isoflurane vapor concentration to 1.2% (1 MAC) for th ree minutes with an alveolar ventilation of 5 l/min and cardiac output of 5 l/min. For the rst simulation of the intubation period (FGFOff), FGF was t urned to 150 ml/min, minute ventilation was set to zero and the vaporizer s etting unchanged for one minute. Initial settings were then restored and th e rate of change of anesthetic vapor concentration in the circuit (V-ckt) a nd alveolus (V-alv) followed for 10 minutes along with the cost of delivere d vapor (V-$). For the second simulation (VAPOff), after the initial three minutes of vapor delivery, the vaporizer was set to zero, minute ventilatio n was set to zero and the FGF left unchanged for one minute. The initial se ttings were then restored and V-ckt, V-alv and V-$ followed for ten minutes . The cost calculation was based upon a 100 ml bottle of Isoflurane at $72/ bottle. Actual gas flow was measured at the y-piece of a circle system for fresh gas flows from 0.15 to 6 l/min. Results. At the end of the simulated intubation period (minute 4), V-ckt was unchanged with the FGFOff method wh ereas it had fallen by more than half with the VAPOff method. Using VAPOff, it took until the 6-minute mark for V-ckt to return to the same concentrat ion that existed prior to intubation at minute three. Throughout the 10 min ute simulated induction, V-alv using FGFOff exceeded V-alv using VAPOff alt hough the difference became small at the end of the period. V-$ was essenti ally identical at all time points. No flow into the room was measured at th e minimum fresh gas flow whereas higher fresh gas flows resulted in a signi ficant portion of the fresh gas flowing into the room. Conclusions. The str ategy of turning the FGF to minimum and leaving the vaporizer on during int ubation does not contaminate the room and speeds induction by fostering a g reater alveolar concentration than the VAPOff method. Cost savings are deri ved using FGFOff since a higher alveolar concentration is achieved at the s ame vapor cost. Additional cost savings are demonstrated since a low flow t echnique is possible immediately after intubation when using FGFOff. The pr actice of turning off the vaporizer during endotracheal intubation while FG F remains high should be abandoned.