Hazard [Health Devices Sep-Oct 1987;16(9-10):343-4]

Problem

While investigating a heart-lung bypass accident involving gas embolism, ECRI discovered that some perfusionists use what may be unsafe techniques to scavenge waste gas from membrane oxygenators. In this accident, it appeared that the technique used to scavenge waste gas may have been responsible for generating back pressure in the gas jacket surrounding a microporous hollow-fiber membrane oxygenator. This back pressure can force gas through the fibers and into the blood flowing through them. The magnitude of the back pressure dictates the volume of gas introduced into the blood and may determine the extent of the sequelae. In this case, the patient died as a result of the embolism.

Although membrane oxygenators have been in existence since heart-lung bypass was introduced, the development of efficient gas-exchange membranes and the realization that direct contact with gas may traumatize blood have led to increased use of membrane oxygenators. Unlike bubble oxygenators, which rely on direct contact of blood and bubbles for gas exchange and are designed to separate undissolved gas from blood before the blood exits the oxygenator, membrane oxygenators are not designed to separate blood and bubbles of undissolved gas. Consequently, large volumes of undissolved gas entering with blood or being generated by back pressure in the membrane oxygenator will flow out of the oxygenator with the blood.

At the reporting hospital, a ¼-inch ID section of tubing had been connected to the gas outlet port of the hollow-fiber membrane oxygenator to scavenge waste anesthetic gas from the oxygenator. This section of tubing was in turn connected to a vacuum line. Several ½-inch diameter holes were cut in the tubing at evenly spaced intervals to prevent the oxygenator from being exposed to excessive negative pressure from the vacuum, which could possibly result in poor blood oxygenation. We believe that at some point during the bypass procedure, the gas scavenging tubing became kinked at the location of the hole nearest to the oxygenator gas vent port, causing back pressure to develop in the gas jacket of the oxygenator. Ironically, the holes cut in the tubing to prevent one problem may have caused another. As a result of the holes cut in the tubing wall, the tubing in the vicinity of the holes was less resistant to kinking and occlusion than sections of tubing where the wall was intact.

Although this accident may have been the result of a unique set of circumstances, it raises the concern that other hospitals may not be scavenging anesthetic gases safely from membrane oxygenators. In tests conducted in our laboratory on the same type of oxygenator that was used by the reporting hospital, we found that oxygen at a pressure of 26 mm Hg in the gas jacket could produce bubbles in the blood-carrying channels of the hollow-fiber oxygenator. Oxygen supplied from tanks or piped into the OR is typically delivered at a regulated pressure of 50 psi (2,600 mm Hg), and hospitals routinely connect gas lines from these sources to membrane oxygenators through gas blenders or flowmeters that do not introduce significant pressure drops in the gas. Clearly, the potential for gas embolism exists if the gas vent port of these oxygenators becomes either partially or totally occluded. Gas scavenging systems for these oxygenators must not cause the application of positive or negative pressures in the gas jackets as this may adversely affect the patient.

The American National Standards Institute (ANSI) standard (ANSI Z79.11) addressing scavenging systems for anesthetic gases applies to oxygenators as well as to anesthesia machines. This standard states that scavengers must not generate positive pressures exceeding 10 cm of water (7.4 mm Hg) or negative pressures exceeding 0.5 cm of water (0.37 mm Hg). While some manufacturers have informed ECRI that the design of their membrane oxygenators will prevent the development of back pressure in the gas jacket, we believe that precautions should still be taken to ensure that the gas scavenging system cannot adversely affect any membrane oxygenators.

1. Be sure to consult manufacturer's instructions when setting up a scavenging system for membrane oxygenators. Contact the manufacturer to ensure that the recommended scavenging technique meets the ANSI performance standard.
2. Do not exceed the gas flow rates recommended by the manufacturer for a particular oxygenator as excessive flow rates may cause excessive back pressure in the gas jacket of oxygenators.
3. If the manufacturer does not provide instructions on how to scavenge waste anesthetic gas from membrane oxygenators, use an anesthesia machine scavenging system that provides active scavenging (non-HVAC type) and meets the ANSI performance standard. (For more information on anesthetic gas scavengers, see Health Devices 12:267-80, September 1983.)
4. The section of tubing between the oxygenator and the scavenger must also prevent positive pressure from developing in the oxygenator. Be sure that this tubing is as short as possible to prevent kinking. When installing scavenging systems near oxygenators, verify that the scavenger's orientation will not affect its performance. If the manufacturer recommends the installation of a rigid plastic T-tubing connector or an open Luer-lock fitting in the tubing between the oxygenator and the gas scavenging system to provide positive pressure relief, be sure to install it as close to the oxygenator as possible.
5. Route all tubing, including blood lines, for a heart-lung bypass system in such a way that it will not be easily kinked, walked on, or pinched as equipment is moved near the pump console. Never cover or drape tubing so that it cannot be monitored by the perfusionist during bypass.

UMDNS Terms

• Anesthesia Unit Gas Scavengers [10-142]
• Oxygenators, Extracorporeal, Membrane [17-643]

Device factors: Design/labeling error; Device interaction; Improper maintenance

User errors: Failure to read label; Incorrect clinical use

External factor: Medical gas and vacuum supplies

Embolism (gaseous)