October 17, 2009
3:00 PM - 4:30 PM
Room Room 357
Nathaniel M. Birgenheier, M.D., Dwayne R. Westenskow, Ph.D., Joseph A. Orr, Ph.D.
Anesthesiology, University of Utah, Salt Lake City, Utah
Introduction: Modern anesthesia machines include plastic components that absorb anesthetic agents and therefore require prolonged flush times in preparation for MH-susceptible patients. These machines could be made instantly safe for use in MH-susceptible patients by installing activated charcoal filters on both limbs of the breathing circuit to reduce residual anesthetic concentrations to < 5 ppm.
Materials & Methods: Activated Charcoal Filters (ACFs) were prepared by placing granular activated carbon between mesh filter paper and enclosing it in a plastic filter housing. Drager Apollo and Datex- Ohmeda Aestiva anesthesia machines were primed with 1.5% isoflurane for 2 hours. Control: The machines were prepared for MH-susceptible patients by replacing the circuit and breathing bag and setting the oxygen flow to 10L/min. The ventilator was set at: VT: 600mL, RR: 10/min, I:E 1:2. A Miran SapphIRe XL trace gas analyzer measured the inspired isoflurane concentration every 45 sec. Time to < 5 ppm was measured. Experimental: The machines were prepared as above and ACFs were placed on the inspiratory and expiratory limbs. Isoflurane concentrations were recorded for 45 minutes.
Results: Without the activated charcoal filters it took 78 min before the inspired isoflurane concentrations fell below 5 ppm in the Apollo machine. It took 55 min with the Aestiva. With the activated charcoal filters the inspired isoflurane concentration was immediately less than 5 ppm and remained below 0.63 ppm for 45 min in the Apollo machine and below 0.39 ppm in the Aestiva.[figure1][figure2] Conclusions: The triggering level for MH is unknown in humans; previous studies use 5 ppm as a conservative estimate. Activated charcoal filters provide an effective way to prepare modern anesthesia machines for MH-susceptible patients. No flush was needed for the machines and inspired agent concentrations were < 5 ppm, independent of fresh gas flow.