Equipment complications in anesthetic practice

Seminars in ANESTHESIA

Vol 15, No 2 J u n e 1996

Equipment Complications in Anesthetic Practice Cynthia Anderson and Steven J. Barker

W HAT ROLE does anesthesia equipment malfunction play in patient care? Perhaps

the most-quoted study on the role of anesthesia equipment failures is that by Cooper et al, pub- lished in 1984.1 One thousand eighty-nine de- scriptions of "critical incidents" during anesthesia were collected from 139 anesthesia personnel. Thirty percent of these were related to equipment failure, which included breathing circuit disconnections, gas flow-control errors, loss of gas supply, leaks, misconnections, and ventilator mal- functions. Of these incidents, 70 resulted in significant negative outcome for the patient. However, only 3 of these were attributable solely to equipment failure. The rest were believed to be attributable to human error in the use of equipment or monitoring. A more recent Canadian study looked at medical equipment manufacturers recalls and warnings from 1987 to 1992. 2 They found that anesthesia equipment consti- tuted 2.3% of the newly marketed devices during the review period; but accounted for a dispropor- tionate 8.6% of the 471 problem reports and recalls. In addition, anesthesia devices were re- sponsible for 37.5% of the alerts issued. Basically this tells us that the current equipment we use is probably quite good (the overall inci- dence of true equipment failure and problems is small), but that it is not infallible. More importantly, our lack of understanding and misuse of equipment may be the biggest culprit when patient outcome is adversely affected!

Indeed, partially as result of the American So- ciety for Testing Materials (ASTM) 1988 Guide-

lines f o r Min imum Per formance and Safety in Anes thes ia Gas Machines done in conjunction with the American Society of Anesthesiologists

(ASA) and the 1991 ASA Standards f o r Basic Intraoperative Monitoring, 3'4 we have seen an escalating sophistication in our delivery systems. Any of us who trained before 1980 can attest to the fact that our specialty has become increas- ingly technical. Advances in anesthesia-related equipment (machines, monitors, ancillary devices) have been nothing short of staggering over the last 15 years. Just consider the number of electrical outlets we now require or the frustra- tion of interpreting the alarm messages on the modem anesthesia machines!

A thorough discussion of all anesthesia equipment problems is certainly beyond the scope of one article. Therefore, we will concentrate on the piece of equipment we use the most: the anesthesia machine. In addition, our discussion will briefly review the use of those specific monitors that help ensure our machines are functioning properly; namely, the oxygen analyzer, the airway pressure monitor, capnograph, and pulse oximeter.

THE MODERN ANESTHESIA MACHINE

Only two types of anesthesia machines, the Drager Narkomed series and the Ohmeda Modu- lus series, meet all the ASTM Fl161-88 standards. We thus examine a "gener ic" modern machine. Emphasis is placed on how the ma-

From the Department of Anesthesia, University of Califor- nia, lrvine, CA.

Address reprint requests to Cynthia Anderson, MD, De- partment of Anesthesi a, UCI Medical Center, 101 City Dr S, Bldg 53, Rm 227, Orange, CA 92668.

Copyright �9 1996 by W.B. Saunders Company 0277-0326/96/1502 -0001 $5.00/0

Seminars in Anesthesia, Vo115, No 2 (June), 1996: pp 109-121 109

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chines have been designed to prevent mishaps, differences in the machines (when they occur), and how the components may fail. Where appropriate, we point out our department 's experience with machine problems over the past 5 years.

Electrical System

Modern anesthesia machines rely on an electrical power source. They also have a back-up battery. The battery is designed to provide tempo- rary power until an emergency generator is running. The battery does not provide for long periods of use. For example, the Drager Nar- komed 3 battery will provide all machine functions except ETC02 Multispec for only 5 minutes. To preserve battery power, the display screen and BP will then cease to function. Power will continue only to the 02 Med, Baromed, 02 sat., Spiromed, main switch, and ventilator for 10 minutes. After that, all power will be shut off and only gas flow and manual ventilation will be possible.

As long as the machine is plugged in, the battery will charge. However, we have had several experiences in which the plug had been partially pulled from its socket during end-of-day room cleaning when the machine was moved. Thus, the machine had been running on battery power overnight. In one case, this was not immediately recognized (the "1o bat tery" and AC power indi- cators were not noted before starting). Conse- quently, there was a complete electrical shutdown of the machine shortly after inducing anesthesia! A discharged battery takes about 16 hours to recharge.

The Gas Supply

The standard generic anesthesia machine provides oxygen and nitrous oxide (and perhaps other gases such as air or helium) through two routes: cylinders and pipeline.

Pipeline. Gas enters the anesthesia machine through inlet connections that use the Diameter Index Safety System (DISS). That is, the connector to the pipe source is specific to a given gas to ensure that the correct medical gas enters the correct part of the anesthesia machine. Gas enters through the pipeline at about 55 psi. A check valve downstream from the inlet prevents reverse flow from the machine. The pipeline pressure

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gauge on the current machines is located on the pipeline side so that it truly measures pipeline (not machine) pressure.

Unfortunately, the hospital 's central supply of gas is not infallible. There are cases in the literature of patients receiving hypoxic gas mixtures because of pipeline crossovers. 5 This has generally occurred during hospital construction or maintenance. Hospital oxygen supplies have also been contaminated by cleaning solutions, neces- sitating shutdown of the pipeline supplies. The central piped gas source can be lost in the operating room (OR) because of problems with central gases in other parts of the hospital. We have experienced such an event when a hyperbaric chamber developed a leak during maintenance work. Also, the gauge on the pipeline can stick and read 55 psi when no gas is flowing. Finally, there are several reports in the literature describ- ing a quick-connector that has been forced into the wrong DISS outlet or placed on the wrong gas hose. 6

Because of these reported problems, two cave- ats to safety become apparent. The first is that the only way to know your machine is delivering oxygen in adequate concentrations is to have a calibrated and functioning oxygen analyzer with an audible alarm on the inspiratory limb of your circuit. This monitor is absolutely essential and is included in the ASA Standards for Intraoperative Monitoring. The second is that you must have an adequate cylinder supply of oxygen. This means you must verify the tank pressures during your machine check after disconnecting the pipeline source and depressurizing the anesthesia machine by pressing the oxygen flush. 7

Cylinders. US anesthesia machines use color-coded E cylinders, which attach to the anesthesia machine with a hanger yoke that uses a Pin Index Safety System (PISS) If a double yoke is used, current machines have a check valve located downstream to minimize gas transfer from a cylinder with high pressure to one with low pressure and to prevent leaks when tanks are changed. I f one tank is left off of a double-tank yoke, a plug can be applied to prevent leaks from the machine. When the tank is open, the gas, which is under 2,200 psi for a full tank of oxygen, passes through a pressure regulator so that it enters the machine at about 45 lb/in 2.

EQUIPMENT COMPLICATIONS

The biggest problem associated with tanks is the possibility of delivering a hypoxic mixture of gas by one of several possible mechanisms. First, oxygen cylinders may be filled with some other gas. Second, despite universal use of the PISS in this country, there are reports in the literature of incorrect cylinders being placed on a yoke. This can especially occur if more than one washer is placed between the tank and the yoke. In addition, failing to remove the wrapper around the inlet or debris from the tank clogging the inlet can lead to inadequate delivery of oxygen. This can be prevented by thoroughly inspecting and partially opening a tank to "blow out" any debris before placing it on a machine.

Because the purpose of the cylinder oxygen is to supply a backup source of oxygen in the OR setting, it is important to realize two important facts. The first is that the reserve cylinder supply can be depleted if cylinders are left on and the internal machine pressure decreases below 45 lb/ in 2 (eg, flushing, ventilator use at high peak flow rates, problems with central piping). During these times, the machine will selectively use oxygen from an open tank. Hence the concept that it is safer for the patient if an oxygen tank is left open when the machine is in use is probably erroneous. The second fact is that if the gas from the oxygen pipeline is not thought to be oxygen, then opening the oxygen cylinder is not sufficient to prevent a hypoxic gas mixture. As long as the pipeline pressure is normal (which means it is entering the machine under higher pressure than the regulated cylinder oxygen), the machine will selectively draw gas from the oxygen pipeline instead of the cylinder. Hence, the pipeline source must be disconnected before the cylinder gas can be delivered to the patient.

Finally, check valves and pressure regulators can fail and result in leaks, which is why a yoke plug should always be placed when a cylinder is not attached to the anesthesia machine. In a deteriorated regulator, excessively high pressure can build up. A pressure relief valve in the regulator will then open and vent the gas to atmosphere. I f the valve' s diaphragm ruptures, oxygen under high pressure and velocity will flow to the atmosphere around the adjustment screw, causing a large leak that can result in hypoventilation of the patient. 5

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Oxygen Supply Pressure "Fail Safe" The ASTM-88 standards state: "The anesthe-

sia gas machine shall be designed so that whenever oxygen pressure is reduced from normal, and until flow ceases, the set oxygen concentration shall not decrease at the common outlet." This, in relation to N20 supply control, is the so- called fail-safe system designed to prevent delivery of a hypoxic mixture to the flowmeters. An audio and visual alarm should respond to decreased 02 pressure below a preset value.

This valve on the Drager Narkomed machines is known as the oxygen failure protection device (OFPD). As the oxygen supply falls below 45 to 50 psi, the OFPD proportionately reduces the supply pressure of nitrous oxide reaching the flowmeter. The supply of nitrous oxide becomes completely interrupted when the oxygen supply pressure falls to less than 12 + 4 psi. Narkomed machines have OFPDs that can interface the supply pressure of any gas supplied to the machine. One should realize that total flow through the fresh gas outlet will decrease as soon as oxygen pressure begins to fall below 50 psi. 8

The Ohmeda machines, however, have a pressure-sensor shut-off valve that is "all-or-nothing." That is, it remains open until oxygen falls to 20 psi. At that point it closes. In the current Ohmeda machines, there is a second-stage pressure regulator to the oxygen supply reaching the flowmeter. This actually opens as oxygen pressure decreases to maintain constant flow to the oxygen flowmeter. This will occur as long as oxygen pressure is greater than about 14 psi. Hence, in the Ohmeda machines, the total gas flow is constant at the fresh gas outlet as long as oxygen pressure exceeds 20 psi. 9

It is very important to realize that these valves do not require a flow of o x y g e n - - o n l y a pressurized s y s t e m - - t o remain open. Thus, some be- lieve that the term "fa i l -safe" is a misnomer. A hypoxic gas mixture can still be delivered if there is another gas besides oxygen in the oxygen pipeline, if there is failure of the regulator, or if the area around the regulator remains "pressurized" for any reason in the face of decreased flow (eg, debris in system).

Flowmeters Once our gases have reached the flowmeters,

it is important to be aware that these can pose a

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substantial hazard because the gases exiting the flowmeters are located downstream from all machine safety devices except the oxygen analyzer. Thus, several safety features have been developed over the years to prevent mishaps that were reported earlier in the literature.

The flowmeters are modular units; leaks can occur at the junction of the glass tube and manifold because of defective O-rings and gaskets. Therefore, one important safety feature is the po- sitioning of the gases in relation to each other. The oxygen will always be in the downstream position and closest to the common gas outlet. A leak in the flowmeter manifold would then be less likely to result in delivery of a hypoxic gas mixture through the common gas outlet.

Another safety factor is the control knob. The oxygen flowmeter control knob is shaped differ- ently than those of the other gases. In addition, there is a bar across the base of the knobs on the modern machines. Manufacturers also offer the option of an oxygen flow that cannot be completely discontinued, to ensure a minimum flow of 200 to 300 cc/min. (On the Drager Narkomed, this minimum oxygen flow feature functions only in the O2/N20 mode and not in the "all gases" mode!) All of this is designed to prevent a not uncommon human mistake: inadvertantly turning the oxygen flow down or off when one is not looking.

Proportioning Devices

A flowmeter proportioning device for nitrous oxide and oxygen is present on modem machines. That is, the oxygen and nitrous flows are physically interlinked. In the Drager Narkomed machines, the Oxygen Ratio Monitor Controller (ORMC) serves to limit the flow of nitrous oxide in the face of decreasing oxygen flow so that concentrations of at least 25% oxygen are maintained. The Ohmeda Modulus machines use a Link-25 Proportion Limiting Control System. A gear on the nitrous oxide flowmeter control valve is linked to a larger one on the oxygen knob. I f nitrous oxide is increased above 75%, the gear link is engaged and oxygen is increased to provide a minimum oxygen concentration of 25%.

There are several important points to note con- cerning these propertioning systems. The first is that the ORMC and Link-25 systems are only

between nitrous oxide and oxygen. I f an additional flowmeter gas such as helium is added, these systems will not prevent a hypoxic mixture secondary to that gas. In addition, the audio alarm on the ORMC will not work when the Narkomed is in the "all gases" mode, so that you may not know immediately that the system has been activated. Also, there have been case reports of breakage of the link chain and improper mount- ing, making the system dysfunctional. 1~ At best, these systems only guarantee 25% oxygen if functioning properly; and a leak downstream of the flowmeters can still result in a hypoxic mixture despite these proportioning devices. Again, the take-home message is: You must have a functioning oxygen analyzer.

Vaporizers

Modem anesthesia vaporizers are variable bypass. That is, once the gases have exited the flowmeters, a varying amount will be diverted through the vaporizer to pick up saturated vapor concentrations and then join the main flow of gas bypassing the vaporizer. The amount of gas diverted is controlled by the concentration dial. Hence, our modern vaporizers are concentration calibrated. Because temperature changes can influence the saturated vapor pressure, these vaporizers are also temperature compensated. I f temperature rises, a temperature-sensitive valve in the bypass opens wider so that less gas passes through the vapor chamber. When vaporizers are mounted in series, the ASTM standards require that an interlock system be used so that only one vaporizer can be opened at a time. This prevents cross-contamination of vaporizers. It should be noted that these systems can break and should therefore be checked periodically. All modem vaporizers open and close in the same direction. Before such standardization, incidences occurred in which an operator who intended to turn off a vaporizer could inadvertently turn it maximally on. 6 Finally, most modem vaporizers have locks that must be pushed to allow opening; this prevents them from being accidently turned on.

Desflurane's vapor pressure at 20~ is 669 mm Hg. It boils at 22.8~ To ensure controlled va- porization of this agent, the Ohmeda Tec 6 is electrically heated and pressurized. Its operating principles are reviewed elsewhere. 12


Generally, the biggest concern with vaporizer use is the possibility of delivering an inadvertent overdose of volatile agent. This is possible if an agent of higher vapor pressure is put into a vaporizer intended for an equipotent agent of lower vapor pressure. For example, erroneously filling an enflurane vaporizer with halothane and setting the dial at 2% will result in delivery of 3.3% halothane. 7 The ASTM-88 guidelines rec- ommend that the vaporizer-filling mechanism be fitted with a permanently attached standard, agent-specific, keyed filling device to prevent ac- cidental filling. Such devices do exist but have not been particularly popular. The obvious ex- ception is desflurane. Incorrectly filling another vaporizer with desflurane could result in excessively high output of this agent, especially if there is even a small increase in temperature. The desflurane bottle has an agent-specific filler cap that cannot be used on other vaporizers. Incorrect agent accidents can also be prevented by using "agent-specific" respired gas monitors, which can identify the agent in the breathing circuit.

Another way that excess vapor can be delivered is through spilling of the agent into the bypass section if the vaporizer is tipped during removal and attachment. The Ohmeda vaporizers have anti-spill mechanisms. The Drager Vapor 19.1 is not easily removed and should only be done by service personnel. 13

A third way that overdose could occur is from the "pumping effect". Theoretically, positive pressure in the common gas outflow, such as occurs during mechanical ventilation, could result in retrograde travel of some of the bypass gas back into the vaporizing chamber. It would then exit the inlet to join the rest of the bypass gas. This potential problem has been dealt with in two different ways. The Ohmeda machine has a back-check valve on the common gas outlet, which prevents the vaporizer from "see ing" this pressure. The Drager Vapor 19.1, however, has a specialized long spiral tube that serves as the inlet to the vaporizing chamber. Gas cannot travel retrograde through the entire length of the tube during the inspiratory ventilator cycle.

Finally, a vaporizer should never be used out of circuit. I f attached downstream of the common gas outlet, activating the oxygen flush can result in potentially dangerous overdose of that agent.

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Vaporizers can also be a notorious source of leaks. We have seen two instances in which old O-rings on a vaporizer caused a major leak in the anesthesia machine. Remember, if the vaporizer is not opened during a leak test, then there is no guarantee there is no leak at that site.

Common Gas Outlet

Before leaving the anesthesia machine, there are a few miscellaneous items to mention:

Leak Test. Part of the FDA 1993 Checkout Procedure for Anesthesia Apparatus includes checking the anesthesia machine for low-pressure leaks. This is perhaps the poorest understood (and in our institution the most missed) part of the anesthesia checklist. 12 This may be because machines differ in that some have a back-check valve in the common gas outlet to prevent the retrograde flow of gases when there is positive pressure in the circuit. A positive-pressure "oxygen flush test" will not pick up leaks in anesthesia machines that contain a common outlet back- check valve. The FDA has recommended a "universal" pressure leak test that is based on the Ohmeda's negative pressure leak guidelines. The master switch, flow control valves, and vaporizers are turned off. A suction bulb is attached to the common fresh gas outlet and squeezed repeatedly until it collapses. This creates a vaccuum inside the machine (and opens up the check valve if there is one). I f there is no leak, then the bulb will stay collapsed for at least 10 sec- onds. Again, the vaporizers should be turned ON to be included in the check for leaks. This test will work on machines with or without back- check valves.

Oxygen Flush. The oxygen flush allows a high flow of oxygen to exit the common gas outlet through a separate conduit within the machine. The gas flow can vary from 35 to 75 L/ min. Although one route has been suggested for providing a mode of transtracheal jet ventilation, not all machines can generate the driving pressure needed. 14 The Narkomed 2 and Ohmeda Modulus II have a one-way check valve between the oxygen flush and the vaporizers. Thus the entire flow of oxygen out the common gas outlet is at 50 psig. Conversely, the Narkomed 2A and the Ohmeda Modulus II Plus have no such check valves. Activating the oxygen flush will drive

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some of the oxygen back toward the vaporizers. Each of these machines has a pressure relief valve to protect the vaporizers so that gas exits under only 18 and 7 psi, respectively.

Activating the flush valve can also lead to barotrauma if the patient's airway is exposed to the majority of the pressure. This mainly occurs when the APL ("pop-off") valve is closed. One should realize that the anesthesia ventilator also has a "pop-off" valve of its own. This is closed on inspiration and during the first part of expiration. Using the flush to fill the ventilator bellows when this valve is closed can result in high peak airway pressures.

THE BREATHING CIRCUIT

All That Tubing

The circle system that we use consists of a fresh gas inflow source, inspiratory and expiratory unidirectional valves, inspiratory and expiratory corrugated tubes, a Y connector, an overflow or pop-off valve (APL), a reservoir bag, and a carbon dioxide cannister. Often, a heated humidifier also is in the circuit. In other words, there are a minimum of 10 connections! Breathing circuit disconnections and leaks are among the most common of 6 anesthesia mishaps. Seventy percent of disconnects occur at the junction of the endo- tracheal tube to the Y piece. The next most common sites for disconnect are anywhere two non- like materials are joined. That is, metal-to-metal and plastic-to-plastic are better then metal-to- plastic connections. 15 Fortunately, the past problem of disconnects at the fresh gas flow outlet of the machine are now rare because of the re- taining bars on the newer machines.

Obstruction to flow can also be a problem in the circle system. This can occur because of kinking of the tubing (eg, common gas outlet tubing in the drawer of the anesthesia machine, scavenge tubing under a wheel, etc.), sticking of the APL or unidirectional valves, or when misconnections occur. The most common example that we have seen is obstruction to flow because the humidifier tubing is not attached in proper se- quence to the inspiratory limb of the circuit. Other cases of obstruction cited in the literature involve the placement of positive end-expiratory pressure (PEEP) valves on the inspiratory limb of the circuit or the placement in horizontal, in-

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stead of vertical, position. Adjustable PEEP valves placed on the machine by the manufacturer prevent this. Obstruction has also been caused by a filter added to the circuit. The bottom line is: the more attachments on the system, the more you must beware.

Inspiratory and Expiratory Valves

The circle system consists of two one-way valves to ensure that flow of inhaled and exhaled gas follows a unidirectional course. Should either of these valves be faulty, the patient will re- breathe expired carbon dioxide. We have seen two instances of hypercarbia, diagnosed by end- tidal carbon dioxide monitoring, related to valve malfunction. One was on a machine that had a broken inspiratory valve (half of the valve was missing and not noted on several preoperative machine checks). The other involved a Nar- komed machine with a nonfunctioning respiratory volume monitor. Because the monitor sits between the carbon dioxide cannister and the hose terminal containing the expiratory valve, the expiratory valve was removed with the unit. Unfortunately, the new volume sensor was replaced without the expiratory valve! The volume monitor was screwed into the top of the hose terminal, which is where the expiratory valve usually sits. This was not noted preoperatively, but picked up intraoperatively, when the capnogram showed increasing inspired carbon dioxide.

These valves also can stick, most often in the open position. In our experience, it is more common with the expiratory valve, probably because of the moisture from exhaled gases that collects on it. If these valves stick in a closed position, flow will be obstructed. Observing the presence and function of the valves should be part of the routine machine check.

The Carbon Dioxide Absorber

Despite the presence of a CO2 cannister in the circuit, problems can occur with carbon dioxide rebreathing. Some older machines contain a carbon dioxide bypass valve on the absorber. If this is open, then gas will not flow through the absorbent. More recently it has been shown that the fluorescent lights in the operating room can deac- tivate the indicator in the asorbent around the outside of the cannister, so that it does not appear


exhausted when it actually i s . 16 This probably accounts for those times (usually the first case of the day) when the absorbent appears mostly white but changes color almost completely within the first hour of the anesthetic. Finally, there are case reports of CO2 rebreathing because no indicator has been added to the absorbent, when gases have apparent ly "channe led through" exhausted absorbent in the cannister, or when the the cannister has been improperly designed. Obviously, the best way to know that rebreathing is not occurring is by observation of the capnogram.

Another recent suspected problem with the CO2 absorbent granules involves their interaction with inhalational anesthetics. There have been two recent case reports of unexplained appear- ance of carbon monoxide during general anesthesia. 17 Studies at the University of California, San Francisco using both soda lime and baralyme have shown that chemical degradation by these agents could result in the production of carbon monoxide. The amount is greatest when desflurane reacts with a cannister of dry baralyme; however, measurable levels are produced with enflurane and isoflurane with both types of granules. The clinical risk seems highest for the first patient who receives an anesthetic on a Mon- day morning with a machine that has not been used over the weekend (ie, the absorbent has had time to dry). Using soda lime; avoiding high gas flows, which dry the granules; and avoiding Des- flurane in the scenario described should minimize the risk of carbon monoxide toxicity.

Finally, the CO2 cannister is a frequent source of leaks. This is probably because it is the one part of the anesthesia machine that has to be opened repeatedly to change absorbent. Thus, one needs to ensure that the cannisters are prob- erly seated in the frame, the rubber sealant rings are intact, and the absorber frame is properly locked.

Humidifier

Before leaving the circle system, it would be wise to review some of the problems with the heated humidifiers in common use. Heated humidifiers are devices through which the inspired gases pass to make them saturated with water. The dry gases either pass over the surface of

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water or bubble through. Heat is provided electrically to the unit.

As gas saturated with water vapor passes out of the unit, it will cool and condense in the tubing. To maintain temperature and humidity at the airway, the unit may function in one of two ways: a heated wire in the tubing from the unit to the patient keeps the gases heated; or the unit may "super heat" the gases, which then cool to an appropriate temperature as they traverse the inspiratory tubing to reach the patient 's airway. It is essential in the latter case to monitor airway temperature to avoid airway burns.

The most frequent problems with humidifiers that we have seen are failure to ensure that sterile water has been added to the unit, leaks that result from vents left open in the unit or disconnects with the temperature probes, etc., and improper set-up of hoses leading from the inspiratory limb to the humidifier and from humidifier to the patient. All of these problems can be avoided intraoperatively if the humidifier is added to the circle system before performing a machine check. Fi- nally, these units should not be on until gas is flowing through them. We have seen one melt- down of the disposable cannister that sits on the heating unit caused by overheating in the presence of no gas flow. In addition, the inspiratory corrugated tubing leading from the humidifier can become soft and obstruct from overheating if the heater is on and no gas is flowing through the unit. 5

The Anesthesia Ventilator

The anesthesia ventilator is one of the components of the anesthesia delivery system that is least understood by the user. The contemporary anesthesia ventilators (Drager AV-E, Ohmeda 7000 and 7800 series) can be thought of as "bags in bottles." The reservoir bag of the circle system is replaced by a bellows. This bellows sits in a bellows housing (the bottle). The APL, or pop- off, valve is replaced by a ventilator pressure relief valve (PRV). Inspiration occurs when a driving gas enters the bellows housing. The increased pressure causes the bellows (bag containing gas from patients last exhaled tidal volume) to collapse and the PRV to remain closed. This forces the gas in the bellows, along with fresh gas flow from the machine, into the pa-

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tient's lungs. Expiration occurs when the driving gas exits the bellows housing. The bellows will passively fill once the airway pressure becomes greater than the pressure in the housing. Excess gas on expiration (remember that the common gas outlet is continuing to supply fresh gas during expiration) will then exit through the PRV into the scavenge unit.

These ventilators can be thought of as "double circuits." One circuit is the driving gas circuit, which is controlled electronically and pneumati- cally by the machine according to the settings for inspiratory flow rate, inspiratory:expiratory (I:E) ratio, and respiratory rate. The other is the patient breathing circuit with a " b a g " that will fill to a preset tidal volume. The interface between the two circuits is the bellows.

Although one should refer to the individual manuals for complete information on the functioning of specific ventilators, some common points are worthy of mention:

The first is that all of our modern anesthesia ventilators are designed as standing bellows. That is, they rise on exhalation, as compared with older bellows models that descend, or hang, on exhalation. The older hanging bellows would fill by gravity on exhalation. In the event of a patient circuit disconnect, the bellows could still fill through the leak, delaying recognition of the problem.

The driving gas is different for Drager and Ohmeda ventilators. The gas entering the housing unit is 100% oxygen in the Ohmeda models. The Drager ventilator uses a Venturi nozzle to entrain room air through a muffler as the main bulk of the compressing gas. I f a leak occurs in the bellows, driving gas can be entrained. The potential exists for providing a lower fractional inspired oxygen (Fi t2) to the patient in this situation if the Drager is used. Conversely, the Drager ventilator requires less oxygen, making it more efficient if cylinder oxygen is being used as the driving gas source. Obviously, the muffler on the Drager ventilator must remain clean; if it is blocked, then inspiration will not occur.

The PRVs differ in their location on the two types of ventilators. In the Ohmeda, the PRV is mounted inside the bellows, whereas the Drager PRV is external and has a pilot line. The pilot line is subject to kinking; however, the Drager

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PRV does offer the advantage of being observ- able.

One common "p rob lem" with the anesthesia ventilator is that it does not deliver the tidal volume (TV) for which it is set. If the problem is not caused by the patient (eg, bronchospasm), then it is most often caused by the fact that changes in the respiratory rate, fresh gas flow, or I:E ratio have profound effects on TV. Remem- ber that two sources contribute to a patient 's TV on inspiration: the bellows gas and the fresh gas from the machine. All of the above factors will alter the TV because of their influence on the amount of fresh gas entering the lungs during inspiration. The addition of PEEP can also cause a loss of TV (see below).

It is even more frustrating when the bellows does not seem to work at all. This may be because the machine has not been switched to automatic mode or because of disconnection of the ventilator hose to the circuit. Also, if the adjustable pressure limit is stuck in the open position, gas will exit from the bellows to the scavenge on inspiration. Diagnosis of this is made by observing the scavenge bag for inflation on inspiration. 6 We have had several episodes in which the bellows became stuck and would not rise. Two of these events occurred after repeated use of an adult bellows for infants on the previous day. Thus, the bellows had been receiving moist exhaled gas under minimal tidal volumes. We hy- pothesize that the bellows gas dried overnight and the rubber corrugations stuck together. In two situations, the bellows had been recently changed, and on replacement in the housing, had gotten stuck at the bottom of the casing. In another situation, the housing was not tightly secured, so ~that driving gas leaked out of the chamber.

There are many reports of ventilator malfunc- tions causing high circuit pressures. Remember that if the ventilator is not cycling from inspiration to expiration, the PRV will remain closed, and no gas will be released to the scavenge. As mentioned before, any flushing into the circuit on inspiration, when the PRV is closed, will result in dangerously high airway pressures. Both Ohmeda and Drager can incorporate circuit inspiratory pressure-limiting devices that will trigger exhalation (Ohmeda) or removal of driving


gas (Drager). 5 These devices require a functioning PRV.

PEEP Valves

Both Drager and Ohmeda now supply PEEP valves built into the delivery system. At the end of exhalation, the part of the circuit between the inspiratory valve and the PEEP valve will be at positive end-expiratory pressure. On inspiration, the ventilator bellows must first compress gas in the patient circuit to the level of PEEP that has been set before flow will begin. Because the tidal volume on the bellows is fixed, this will result in lost tidal volume to the patient if the PEEP is added after the ventilator settings have been made. The loss of volume attributable to com- pression of gas in the circuit is greater with the Ohmeda ventilators because their bellows have a large compressible volume of gas remaining after inspiration (1,600 cc TV). The Drager AV- E, however, empties almost completely on inspiration.

If the PEEP valve is located near the ventilator bellows, the amount of TV lost to gas compres- sion will be minimized. However, the valve will only function in the ventilator mode. A PEEP valve near the expiratory unidirectional valve will work in manual mode also.

A recurring problem we have seen with the older Drager Narkomed PEEP valves is that it is difficult to estimate the PEEP setting by looking at the dial control for PEEP. We have had several instances of patients being placed on the anesthesia ventilators with PEEP valves inadvertently turned to the maximum setting. We strongly rec- ommend making sure the PEEP dial is rotated counterclockwise to the closed position during the machine check. Newer Narkomed machines have a switch that must be opened before PEEP can be set.

SCAVENGING SYSTEM

The modem scavenging system is a closed system; that is, it has an interface that communicates to the "a tmosphere" through valves that ensure the patient' s circuit does not see extremes in pressure. When excess gas enters the scavenge tubing through the APL or PRV, part of the gas passes into the central vacuum; the remainder passes into a reservoir bag. The amount passing to each

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depends on the Vacuum Control Valve, which opens to central vacuum. There is one positive- pressure valve that allows gas to escape to room air when the reservoir bag is distended so that the breathing circuit will not be exposed to excessively high pressures. There are two negative- pressure valves to entrain room air so that the breathing circuit will not be exposed to excessive negative pressure (eg, when the control valve is maximally opened). The corrugated tubing con- necting the AP outlet and the ventilator outlet is 19 mm in diameter, instead of the 22-mm diameter tubing used in the breathing circuit. This is done intentionally to help prevent misconnections.

Obviously, if these valves are not functioning properly, then excessive positive pressure (if gas is not exposed to adequate wall vacuum) or excessive negative pressure (if the scavenged gas is exposed to excessive wall vaccuum) can occur. I f the 19-ram diameter tubing at the APL or ventilator PRV becomes occluded, then excessive circuit pressures will also occur.

The scavenge can indicate an APL or PRV malfunction. If these valves are stuck in the open position, then the reservoir bag will fill on inspiration (Normally it fills only on expiration). It is noteworthy that negative pressure in the circuit from excessive vacuum can actually hold the ventilator's PRV closed, resulting in high circuit pressures during mechanical ventilation.

MONITORS AND ALARMS

The Oxygen Analyzer and Alarm

It is obvious from the above discussion that the anesthesia machine must be equipped with a functioning oxygen analyzer in the inspiratory limb of the breathing circuit. This analyzer must be calibrated and equipped with a high-priority alarm when the Fie2 falls below the preset threshold. The fuel cells on these analyzers are specific for oxygen and are not " foo led" by other agents.

Monitoring Circuit Pressure and Volume

The ideal place to monitor airway pressure is at the airway; but on current machines, the airway pressure sensor is mounted on the absorber. Thus, there are some situations in which this


monitor will fail to measure true airway pressures. One example occurs during positive-pressure ventilation. The ventilator delivers its tidal volume in close proximity to the pressure-sensing monitor. It is therefore possible to have an unrecognized partial disconnect if the pressure- sensing alarm is not set at a sensitive enough level. One of the most frequent errors we see is that anesthesia personnel ignore the "threshold lo" warning on their machine. This warning occurs when the patient's peak inspiratory airway pressure (PIP) is more than 6 to 8 cm above the threshold PIP set for the low airway pressure alarm. In such a situation, a partial disconnect will not be picked up by the airway pressure alarm until the PIP has fallen below the threshold level. If the PIP is 30 and the low airway threshold is set at 12, that could be a while! Adjusting the threshold appropriately throughout the case - -because PIPs change- -wi l l ensure that the alarm is adequately sensitive.

The pressure monitor and alarm on the Ohmeda and Drager machines are self-sealing. If the pressure sensor becomes disconnected from the absorber, then the low-pressure alarm will sound, but the circuit pressure gauge will display normal pressures. We have experienced one episode in which the alarm tubing became disconnected where it enters the anesthesia machine. This did result in a significant circuit leak.

There are a number of errors, some already mentioned, that can lead to high airway pressures in the circuit. Because the pressure sensors are on the CO2 absorber cannister, they reflect absorber pressure and not the true airway pressure. For example, if a PEEP valve sits between the expiratory limb and expiratory valve, this PEEP will not register on the sensor. Early subtle increases in airway pressure may not be immediately ob- served. Thus the anesthesiologist must be vigi- lant and ensure that the high-pressure alarm threshold, like the low-pressure alarm, is set appropriate to the actual clinical situation. One should not rely on the machine 's default settings for all patients.

Mechanical spirometers are provided on the modern Ohmeda and Drager machines. In Ohmeda, it is upstream of the exhalational valve; in the Drager it is downstream (absorber side). It will measure exhaled TV and also reverse f low.

Again, the low-volume alarm limit should be set so that it is sensitive enough to register an early problem.

Capnography and Pulse Oximetry

Capnography. As we have seen, anesthesia machines and their monitors have limitations. That is one reason why pulse oximetry and capnography are so important. They measure the end result of what we are trying to accompl ish- - adequate oxygenation and ventilation of the patient. In fact, the initial ASA Closed Malpractice Claims Study concluded that the pulse oximeter has become the single most efficacious additional monitor in preventing anesthetic morbidity and mortality. 18 In 40% of the reviewed cases deemed preventable, the authors conclude that the addition of pulse oximetry alone would have prevented the injury. If a pulse oximeter had been combined with a capnograph, more than 90% of these mishaps may have been avoided. These two monitors, along with our eyes and ears, are our best evidence that oxygenation and ventilation are adequate. Thus we should be familiar with some of the limitations of these monitors.

Capnographs can measure carbon dioxide in the airway through either in-line CO2 sensors or side-stream sampling devices. Side-stream capnographs aspirate respiratory gas from an airway sampling site and transport the gas sample through a tube to a remote CO2 analyzer. Main- stream capnograhs position the CO2 analyzer in the airway, and respiratory gas is analyzed as it passes through a special adapter. No gas is removed from the airway. 19

Besides being another site for a possible circuit disconnect, each system has its own set of problems. Side-stream capnographs will give falsely low CO2 readings if the sampling catheter has a leak, because room air will be entrained. More commonly, the sampling line gets plugged from moisture in the expired gas. There is a case report of a side-stream mass spectrometer creating negative pressure in the breathing circuit of a patient on cardiopulmonary bypass because it was sampling gas at 250 cc/min while the flowmeter was set at only 50 cc/min. 5 Thus, if you are consider- ing a low gas flow technique, you must know the flow rate of your side-stream sampler. Con- versely, the mainstream capnometer adds consid-


erably more bulk to the breathing system, so that disconnects are more likely. In addition, it cannot be used unless the patient is intubated; and it cannot measure any gases other than CO2. How- ever, the chief advantage of the mainstream capnograph is its superior frequency response, which provides better detail on the capnogram.

Evaluation of the capnogram by the anesthesiologist should not focus on the digital readout of end-tidal CO2, but on interpretation of the major features, including inspiratory baseline, expiratory upstroke, expiratory plateau, and inspiratory downstroke. This will enable the anesthesiologist to diagnose untoward events. For example, an elevated baseline in a calibrated monitor may mean CO2 rebreathing from a faulty expiratory valve or CO2 absorber. An incompetent inspiratory valve produces a slanted upstroke. Partial obstruction in the airway will produce a slanted upstroke of the plateau. Finally, capnography readily detects disconnection in the breathing circuit, as well as any condition that produces changes in alveolar deadspace, such as pulmo- nary embolism, shock, or cardiac arrest. Of the 25 most frequent critical incidents described in Cooper's study, more than 40% could have been quickly detected from the capnograph trace. 19

Good advises the clinician using capnography to avoid three major pitfalls2~ First, do not be totally instrument dependent, but rely on clinical observation and judgment. Second, interpret the entire capnogram; do not just follow a number. Whenever the end-tidal CO2 is inconsistent with the clinical situation, obtain a blood gas measurement. Many clinical conditions can cause variations in alveolar deadspace, which will in turn change the gap between arterial and end-tidal CO2.

Oximetry. Oximetry determines the concentrations of various hemoglobin species in blood by measuring the absorbance of light at various wavelengths. The concentration of one hemoglobin species can be determined from each light wavelength whose absorption is measured. A laboratory co-oximeter, which uses four or more wavelengths, can measure the concentrations of reduced hemoglobin, O2Hb, MetHb, and COHb.

The pulse oximeter is a two-wavelength oximeter that functions in vivo. It eliminates the effects of the solid tissues positioned between the light source and detector by determining the

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fluctuating or " A C " component of the absorbance signal. At each of its two wavelengths (660 nm and 940 nm), the pulse oximeter divides the AC signal by the corresponding mean, or " D C " component to obtain a "pulse-added absorbance." It then calculates the ratio of the two pulse-added absorbances, and this ratio (R) is related to arterial saturation by a built-in calibra- tion algorithm based on volunteer data. The resulting pulse oximeter saturation is called SpO2.

Pulse oximeters do not normally emit heat and are considered very safe as long as one does not cause ischemia by taping it on too tightly. There is a case report of serious burns to the earlobe and finger of an infant caused by inadvertent con- nection of a pulse oximeter to an incompatible sensor of another manufacturer. 2~

Most complications arising from pulse oximetry are related to errors in interpretation of the data. One must realize that an SpO2 value of 98% to 100% could correspond to an arterial PO2 of 80 or 400 mm Hg because of the sigmoidal shape of the oxyhemoglobin dissociation curve and the _+2% uncertainty of the oximeter. The PaO2 of 80 would definitely be abnormal in a patient breathing 100% oxygen; yet the clinician could be lulled into a false sense of security by the pulse oximeter reading.

Pulse oximeters will give erroneous estimates of saturation under certain conditions, and the clinician must be aware of when these are likely to occur. 22 The pulse oximeter analyzes only two light wavelengths; hence, it can measure saturation in the presence of only two light absorbers: reduced and oxyhemoglobin. Dyshemoglobins create additional light absorbances in the blood, and the pulse oximeter cannot be relied on in their presence. Carboxyhemoglobin is " seen" by the pulse oximeter as if it were mostly oxyhemoglobin. Thus, a patient with a lethal carboxyhem- oglobinemia may have an SpO2 value of more than 90%. Methemoglobin absorbs well at both pulse oximeter wavelenths, and tends to drive SpO2 values toward 85%. That is, the oximeter will underestimate at high saturations and overes- timate at low ones. Intravenous dyes can have profound effects: methylene blue can produce SpO2 values as low as 4% in normoxemic volun- teers. Some nail polishes can also cause errors. Ambient light and reduced arterial pulsations will


lower the signal-to-noise ratio and degrade accuracy. Patient motion can cause very unpredictable errors, and poorly placed sensors can create either systematic overestimation or underestimation. 23

Another important point about pulse oximeters is that they may continue to display the last " readab le" 02 saturation during loss-of-signal periods (electrical interference, interruptions of pulsatile flow, cardiac arrest). We have seen several instances in which the 02 saturation was mistakenly thought to be " n o r m a l " during a patient crisis when, in fact, the oximeter was dis- playing an old value.

Finally, one should realize that the plethysmo- graph ( " p u l s e " ) one sees on the oximeter screen represents signal output after electronic amplifi- cation and does not correspond in any way to the patient 's actual pulse. It cannot be used to determine hemodynamic status.

Warning Alarms

In discussing complications of anesthesia equipment, we should consider the increasing number o f alarms to which we are subjected in our daily practice. With the progressive sophistication of our equipment has come the need to dis t inguish a m o n g a mul t i tude o f warning "no ises . " The danger that derives f rom this is the tendancy to " tune ou t" alarm signals. In fact, when anesthetists at a professional conference were asked to assess 10 c o m m o n operating room sounds, they were able to identify the monitor alarm sounds only 33% of the time. 24 In one reported case, a critical anesthesia incident re- suited in multiple alarms f rom an integrated monitoring system. This "a la rm over load" actually led to a delay in diagnosis o f the basic problem. 25 Until we have a truly coordinated alarm system, we must attempt not to tune out alarms. More importantly, we should not forget our basic clinical skills of looking at the patient, listening to the breath sounds, and feeling the pulses.

CONCLUSION: HOW DO WE STAY SAFE?

The weight of the literature, and certainly our experience, supports the fact that our own igno- rance and lack of vigilance can often be the cause of equipment complications. Buffington et a126 reported that 190 people attending an anesthesia meeting were, on the average, able to identify

only 2 of 10 faults created on a standard anesthesia machine. The responsibility is ours to know that our equipment is functioning before we begin an anesthetic and to have enough knowledge to safely troubleshoot problems. The improved features of anesthesia machines and advances in monitoring of the past 20 years are useless unless we do so. To accomplish this we should:

�9 Ensure that the equipment in use meets current ASTM-88 standards and that A S A basic monitoring guidelines are followed. Equip- ment should be maintained on a regular ba- sis by licensed technicians.

�9 Follow a thorough preinduction machine check religiously. This should be performed after all accessories (humidifier, etc,) have been added to the circuit. In our institution, all residents are given a test on the checkout procedure. The reported equipment c o m p l i - cations involving human error consistently come f rom the poorest performers on this test.

�9 Know your equipment. Use the manual.

Finally, no piece of equipment is perfect. Always have a backup mode of ventilation. This is the first step on the FDA Anesthesia Machine Check- List.

REFERENCES

l. Cooper JB, Newbower RS, Kitz R J: An analysis of major error and equipment failures in anesthesia manage- ment: Consideration for prevention and detection, Anesthesi- ology 60:34-42, 1984

2. Gilran I: Anesthesia equipment safety in Canada: The role of government regulation. Can J Anaesth 40:987-992, 1993

3. American Society for Testing and Materials: Minimum Performance and Safety Requirements for Components and Systems of Anesthesia Gas Machines Fl161-88. Philadel- phia, ASTM, March 1989

4. Standards for Basic Intraoperative Monitoring (last amended October 1990; effective Jan 1, 1991). Park Ridge, IL, American Society of Anesthesiologists; 1986

5. Eisencraft JB, Sommer RM: Equipment failure: Anes- thesia delivery systems, in Anesthesia and Perioperative Complications. St. Louis, MO, 1992, Mosby, pp 77-127

6. Dorsch JA, Dorsch SE: Hazards of the anesthesia machines and breathing systems, in Understanding Anesthesia Equipment: Construction, Care and Complications. (ed 3). Baltimore, MD, Williams and Wilkins, 1994, pp 325-360

7. Eisenkraft JB, Sommer RM: Hazards of the anesthesia


delivery system, in Anesthesia Equipment: Principles and Applications. St. Louis, MO, 1993, pp 321-349

8. Narkomed 3 Anesthesia System, Operator's Instruction Manual, Telford, PA, North American Drager Inc, 1986

9. Modulus II Plus R Anesthesia Machine, preoperative checklists, operation and maintenance manual, Madison, WI Oct 1988, Ohmeda. The BOC Group, Inc.

10. Richards C: Failure of a nitrous-oxygen proportioning device. Anesthesiology 71:997-999, 1989

l 1. Abraham ZA, Basagoitia J: A potentially lethal anesthesia machine failure. Anesthesiology 66:589-590, 1987

12. Andrews JJ: Understanding your anesthesia machine. ASA Refresher Course Lectures, 242:1-7, 1994

13. Operator's manual. Drager 19.1 Vaporizer. Lubeck, Germany, 1986, Dragerwerk, A.G.

14. Gaughan S, Benumof J: Can an anesthesia machine flush valve provide for effective jet ventilation, Anesthes An- alg 76:800-808, 1993

15. Neufeld PD, Johnson DL, deVeth J: Safety of anaes- thesia breathing circuit connectors. Can Anaesth Soc J 30:646-653, 1983

16. Andrews JJ, Johnston RV, Bee DE, Arens JF: Photo- deactivation of ethyl violet. A potential hazard of sodasorb. Anesthesiology 72:59-64, 1990

17. Fang ZX, Eger EI: Source of toxic CO explained:

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-CHF2 anesthetic + dry absorbent. Anesthesia Patient Safety Foundation Newsletter, 9:, 1994

18. Caplan RA, Posner K, Ward RJ: Adverse respiratory events in anesthesia: A closed claims analysis. Anesthesiol- ogy 72:828-833, 1990

19. Good ML, Gravenstein N: Capnography. In Anesthe- sia Equipment: Principles and Applications. St. Louis, MO, Mosby, 1993, pp. 237-248

20. Good ML: The role of capnography. Problems in An- esthesia 5:230-240, 1991

21. Murphy KG, Segunda JA, Rockoff MA: Severe burns from a pulse oximeter. Anesthesiology 73:350-353, 1990

22. Barker SJ, Tremper KK: Pulse oximetry, in Anesthesia Equipment: Principles and Applications, St. Louis, MO, Mosby, 1993, pp. 249-263

23. Barker SJ, Hyatt J, Shah NK, Gao YJ: The effect of sensor malpositioning on pulse oximeter accuracy during hypoxemia. Anesthesiology 79:248-254, 1993

24. Finley GA, Cohen A J: Perceived urgency and the anesthetist: Responses to common operating room monitors. Carl J Anaesth 38:958-964, 1991

25. Jones D, Lawson A, Holland R: Integrated monitor alarms and alarm overload. Anaesth Intensive Care, 19:101- 102, 1991

26. Buffington CW, Ramanathan S, Turndorf H: Detection of anesthesia machine faults, Anesth Analg 63:79-82, 1984

Equipment complications in anesthetic practice

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