Technique

A Reliable Method for Determination of Initial Sweep Gas Flow Rates Using the CML Oxygenator

lzaak Walton Killam and Victoria General Hospitals, Halifax, Nova Scotia, Canada

^* Direct communications to: Gerard Myers, 12 Londra Court, Dartmouth, Nova Scotia, Canada, B2W 5A5

Abstract

(J. Extra-Corpor. Technol. 18[4] p.221–224 Winter 1986) The Cobe Membrane Lung Oxygenator is a Z folded flat plate membrane made of polypropylene material, with an effective surface area of 2.5m². Since its introduction to the market in 1982, it has provided perfusionists with all the benefits of the membrane technology, while utilizing an ease of operation that is equal to the bubble oxygenator. Cobe’s initial recommendation for use was to maintain a constant sweep gas flow rate of 10 liters/minute and control the pCO₂ by adding extraneous carbon dioxide to this ventilating gas. However, the use of Alpha Stat Control (nontemperature correction) for acid base balance requires the elimination of all extraneous carbon dioxide during bypass, and thereby has required perfusionists to rely on their own experience with this membrane, or follow Cobe’s second recommendation of establishing an initial sweep gas flow rate of 1:1 (gas:blood).

Presented here is a method of determining the initial sweep gas flow rate while using Alpha Stat Control. This method appears useful during the initiation of bypass and the use of moderate hypothermic temperature. A well-known parameter in respiratory medicine, the V/Q ratio, is used in the determination and must be established prior to bypass.

The results of this paper indicate that use of the V/Q ratio in establishing a desired PaCO₂ for the first 1 0 minutes of bypass is more reliable and consistent than the use of the 1:1 gas to blood flow ratio. However, the method has proved useful only for the initial blood gas sampling, after which sweep gas flows must be reduced to a V/Q of at least 1.0 for the remainder of the case. Use of the V/Q ratio appears to allow the perfusionist to predict the initial PaCO₂ within an accuracy of ± 4 mmHg.