TITLE: FILLING THE ANESTHETIC GAS WASHIN TO THE CIRCLE SYSTEM: FILLING THE RESERVOIR BAG AND HOSES

AUTHORS: Wendell C. Stevens, MD, Jorgen Dalsgaard, MD
AFFILIATION: Oregon Health Sciences University, Lake Oswego, OR.

INTRODUCTION: We have been analyzing the pattern of anesthetic gas washin to the circle C02 absorption system, washin one must do prior to inhalation induction, particularly a single breath induction where the entire first breath should contain the desired high gas concentration. If the y-piece opening at the patient end of the delivery hoses is occluded during washin, the reservoir bag and canister fill with anesthetic but none enters the delivery hoses. As a result, the first breath contains less than an optimal concentration because of dilution of anesthetic gas in the inspiratory limb. We measured the effect of inspiratory limb volume on dilution of anesthetic gas concentration which would occur in the first breath.
METHOD: With the y-piece occluded, we delivered 8% sevo- flurane (SEVO) to the circuit, filling and emptying the reservoir bag until the % SEVO in the reservoir bag was >7%. At this time no SEVO was detected at the y-piece. Then, with a 1500ml syringe, we measured how much gas had to be withdrawn at the y-piece (as a patient would have to do with single breath induction) before the % SEVO was nearly equal at the y-piece and reservoir bag. We compared volumes when the hoses were fully compressed and fully expanded and using 2 and 5 liter reservoir bags. Median values were computed from at least 5 trials with each approach.
RESULTS: The number of reservoir bag fillings emptyings to complete the washin varied from 3 to 7. The internal volume of the compressed delivery hose was 250ml, of the expanded hose 750ml. With the hoses compressed, 800 ml and with them expanded 1200ml gas had to be withdrawn for y-piece and reservoir % SEVO to be nearly equal. Withdrawal volumes were similar with 2 and 5 liter bags.
CONCLUSION: An unexpectedly large number of gas ex- changes may be required to completely fill the reservoir bag. A large volume of gas is required to wash agent-free gas out of the inspiratory limb. With the 5 liter but not the 2 liter reservoir, sufficient gas remains for a vital capacity inhalation after washout.