Volume 4, No. 3 • Fall 1989

Older Anesthesia Machines Targeted for Component Replacement

Arthur J. L. Schneider, M.D.

Editor’s Note: Members of the APSF Education Committee recently discussed a number of issues relative to making older anesthesia machines safer without necessarily incurring the cost of entirely new equipment. following is a summary of the discussion “resenting the personal opinions of Arthur J.L. Schneider, M.D., Joseph S. Arcarese, Paul E. Berkebile, M.D., J. S. Gravenstein, M.D., Beverly Nichols, CRNA, Peter 1. Schrieber, Terry Spraker, Ph.D. and Charles Witcher, M.D. Comments and opposing opinions are welcome and will be considered for publication in a future issue.

by Arthur J. L. Schneider, M.D.

With the full realization that well understood and maintained older anesthesia equipment is not, by definition, unsafe and that a practitioner accustomed to the limitations of one piece of equipment is probably not a safer practitioner when he uses a now and unfamiliar piece, certain components and features of older anesthesia machines are worthy of consideration for replacement.

At the time of its marketing, older equipment was held to be safe, considering extant education and common practice. However, a hazardous situation may exist if a person accustomed to operating an anesthesia system with modem safety feature uses a piece of equipment of older vintage which does not incorporate the same safety features. Unquestionably, a great deal of time and effort has been expended in an effort to improve the safety and convenience features of modern machines.

Anesthesia machine features to be considered for replacement include: 22 mm Scavenger Connections, ANSI standards have been enacted that require 19 mm diameter hoses and for gas scavenging. Gas scavenging systems originally used connectors of non-standard diameter. The great danger is in using 22 mm hoses, standard for gas delivery, in scavenger applications. Here the unintentional connection of the patient’s airway to the negative pressure of the scavenger system can be very injurious. It is preferable to switch to 19 mm connectors.

Vaporizers with Nonstandard Direction of Turn

ANSI standards have advocated, for a decade, that the standard direction of turn to increase the output of calibrated vaporizers should be counterclockwise. Some vaporizers remain in service in which output is increased by a clockwise rotation of the knob. It is extremely disconcerting to discover that a vaporizer manipulation that was intended to reduce or discontinue the administration of volatile anesthetic has, in fact, greatly increased it. Here, again, the problem is more acute with unfamiliar machines. Vaporizers with nonstandard direction of turn should be considered for replacement.

Vaporizer Selection Switches

Older machines which contain flowmeter controlled vaporizers are often equipped with multifunction valves for vaporizer operation. These valves might be used to select a particular vaporizer or to open an oxygen flush valve. This leads to confusion in not always remembering to turn the vaporizer on again after an oxygen flush. In addition, many older machines contain two flowmeter controlled vaporizers that are not interlocked, leading to the possibility that both vaporizers can be turned on at the same time. Problems associated with vaporizer control (selector) valves are eliminated with anesthesia systems designed to accept two or more calibrated vaporizers which are interlocked so that only one vaporizer can be turned on at a time. Anesthesia systems using multifunction vaporizer control valves should be considered for replacement.

Carbon Dioxide Cylinders and Yokes

Modern anesthesia machines have been equipped with safety interlocks in their oxygen/nitrous oxide systems which act to prevent the delivery of gas mixtures containing less than 25 percent oxygen to the common gas outlet. The practice of adding carbon dioxide, or any third gas other than air, to these systems may cause the unintentional administration of hypoxic gas mixtures and is discouraged. Furthermore few, if any, clinical indications for the addition of carbon dioxide to the inspired gas mixture now seem to exist, particularly when intraoperative ventilation is monitored by capnography

Flowmeter Controlled Vaporizers

While there is no doubt that flowmeter controlled vaporizers have been used successfully to administer volatile agents for millions of anesthetics, newer calibrated vaporizers are simply better, safer, and easier to operate. The use of flowmeter controlled vaporizers requires consideration of vaporizer flow, agent partial pressure, diluting gas How, and temperature Calibrated vaporizers provide a single control for agent concentration. When flowmeter controlled vaporizers are used with vaporizer selection switches it is possible to set the vaporizer Dow before establishing the diluting How and to deliver high vapor concentrations to the patient. Similarly, a reduction in dilution flow correspondingly increases the concentration of anesthetic vapor delivered. Although a flowmeter controlled vaporizer can theoretically be used to deliver any volatile agent, this practice has been questioned because of the danger of mixing agents or of using a vaporizer filled with an unexpected agent. Agent specific calibrated vaporizers are a better choice

Vaporizers Downstream of Common Gas Outlet

This arrangement seems to have originated with machines embodying flowmeter controlled vaporizers physically built into the machine. These machines do not have convenient ways to accommodate calibrated vaporizers when they are added. This problem is sometimes resolved by installing the calibrated vaporizers downstream of the common gas outlet. Practice has shown that these installations induce situations where more than one vaporizer can be turned on at any time and that the output of these downstream vaporizers can be affected by use of the oxygen flush valve or by bumping or tipping the vaporizer. The arrangement invites undetected disconnection and is often mechanically unstable; vaporizers should not be placed downstream of the common gas outlet.

Carbon Dioxide Absorber Bypass Switch

Many carbon dioxide absorbers have been produced, and now exist in hospitals, which contain valves that permit the carbon dioxide absorbent to be bypassed. The design purpose of these valves was to permit the replacement of canisters without the loss of gas from the breathing circuit. The valves are also used for the purpose of budding up the carbon dioxide partial pressure within the circuit in order to stimulate spontaneous ventilation. In both instances, the reconnection of the carbon dioxide canister to the rebreathing circuit depends upon resetting the valve when bypass is no longer desired. The danger is in forgetting. These valves are no longer needed and, as they represent a hazard, should be considered for replacement.

Machines with Two Common Gas Outlets Some anesthesia machines, produced in the distant past, had two common gas outlets, the purpose of which was the maintenance of two breathing circuits, each separate, intact, and attached to the machine. Selection of the desired circuit was carried out by means of a se-lector switch that would direct the flow of gas to one outlet or the other. This procedure is no longer in common practice and anesthesia machine performance standards recommend against it. Machines of this type obviously should be considered for replacement in order to prevent mishaps encountered by persons not familiar with their use.

To be sure, old workhorse machines, with vigilance on the part of their operators, have been used in many successful anesthetic procedures. While these machines are still usable, practitioners likely will choose the prospect of safer care with the ease of operation and comprehensive monitoring of both machine and patient available in modern anesthesia systems.

Dr. Schneider is on the staff of the M. S. Hershey Medical Center in Hershey, PA.