Electronic monitoring of patient status as a key element of anesthesia patient safety is well recognized in discussions on standards of anesthesia cam Perhaps less recognized, however, is the contribution of anesthesia machine design in promoting anesthesia safety and the importance of assessing the adequacy of old equipment which may lack the safety features inherent in current technology.

During the past 75 years, the anesthesia delivery system has undergone near-total technological change, with its primary advancements directed toward prevention of anesthesia mishaps. The anesthesia machine has evolved from a simple mechanical gas delivery device to a sophisticated system designed to still deliver gas but also minimize anesthesia risk and enhance human vigilance. Some of the more significant safety features which have been incorporated into the design of the anesthesia system include

1. Safety mechanisms which help to ensure the presence of at least a nominal 25 percent oxygen in oxygen/nitrous oxide gas mixtures, thus preventing delivery of a hypoxic mixture

2. A standing-bellows-type ventilator (i.e., the bellows rises, rather than falls, during each patient exhalation) which will not cycle if an accidental disconnection occurs within the patient circuit. Thus, monitoring the bellows can provide an immediate indication of a breathing system problem.

3. An integral vaporizer interlock designed to prevent the simultaneous operation of more than one vaporizer, thus reducing the potential for accidental administration of multiple anesthetic agents at the same time.

4. Ventilator low pressure alarms which help protect the patient from the danger of circuit disconnection or simple hypoventilation; and ventilator high pressure alarms and automatic pressure relief valves which help protect the patient against excessive airway pressure.

5. Latching and bayonet-type connections on gas hoses and pressure sensing tubes which help prevent accidental disconnection.

6. Size-dissimilar fittings and varied hose sizes which help ensure correct connection of gas, scavenging, and breathing system connections.

7. Isolation of vaporizers and special vaporizer manifolds that allow fresh gas to flow through only the vaporizer in use, minimizing the potential for contamination of effluent gas with another volatile agent.

8. An integrated bas-to-ventilator switch valve which removes the automatic pressure limiting (“pop-off”) valve from the patient circuit during ventilator operation, rather than relying on the user to remember to close the valve when switching from manual to mechanical ventilation.

Integral Monitors

Perhaps the most important safety improvement in current anesthesia machines is the integration of monitors into the anesthesia system, to provide added vigilance over both the machine and the patient. For example, circuit oxygen concentration and exhaled patient volume monitors may provide early warning of hypoxic mixtures, circuit leaks, or accidental disconnections. Integration of these and other monitors (e-g., capnography, pulse oximetry, noninvasive blood pressure, etc.) into the anesthesia system helps to ensure that monitors are on and functioning prior to activation of gas delivery and improves the management of the multiple machine and patient connections.

The integration of electronic monitors into advanced anesthesia systems and the adoption of the safety devices outlined above has occurred primarily during the past decade; many crucial safety devices have been developed within only the past five years. Thus, while current advanced anesthesia systems incorporate these safety features as standard components, many anesthesia machines ,which remain in use today lack the majority, if not all of them.

The problem is age. One market survey showed that approximately one-third of all anesthesia machines in U.S. hospitals are more than ten years old and, therefore were manufactured prior to the development of the newer safety technology. Barring extensive upgrading, which is often not feasible due to the initial design of older units, they may lack crucial safety features.

Many hospitals have acted in recent years to remove older anesthesia machines from service at a faster rate than occurred in the past in favor of newer equipment with enhanced safety and performance features. However, it is clear that there are still many units in use which do not have these guards against anesthesia mishaps. Replacement of this old anesthesia equipment would seem to be an essential part of any plan to improve anesthesia patient safety.

Discussions of anesthesia safety and standards of anesthesia care should include this important issue as all involved strive to improve patient cam

Anne Berssenbrugge Ph.D. is Product Manager and Kay Plantes, Ph.D. is North American Marketing Manager, Anesthesia Systems, Ohrneda, Madison, WI.