Medical Gases. The Anaesthetic System PRACTICAL PROBLEMS WITH VETERINARY ANAESTHESIA MACHINES

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1 J. vet. Anaesth. Vol. 21 (December 1994) PRACTICAL PROBLEMS WITH VETERINARY ANAESTHESIA MACHINES SANDEE M. HARTSFIELD Department of Veterinary Small Animal Medicine and Surgery, College of Veterina ry Medicine, Texas A&M University, College Station, Texas SUMMARY Many types of anaesthetic machines are available to practicing veterinary surgeons, ranging from the obsolescent to the most modern. To practice anaesthesia safely, veterinary surgeons should understand the functions of the machines, vaporizers, and breathing systems with which they work. In addition, veterinary surgeons should employ procedures for evaluation of their anaesthetic equipment to assure, as much as possible, safety for their patients and the personnel who use the apparatus. INTRODUCTION Although the first anaesthesia machine was developed around 1905 (Dorsch and Dorsch, 1984), modern anaesthetic machines have evolved dramatically over the last 25 years, from simple pneumatic devices to sophisticated, computer-based, fully integrated anesthetic systems (Andrews 1993). The American National Standards Institute (ANSI) published a standard for anaesthetic machines in 1979, with agreement from American anaesthetic machine manufacturers to comply with the standard on machines sold after 1984 (Dorsch and Dorsch, 1984). An updated standard was published in 1989 by the American Society for Testing and Materials (ASTM) (1989). The Canadian Standards Association published similar guidelines in 1979 and 1980 (Dorsch and Dorsch, 1984). Over the last decade, machines for human patients have incorporated numerous mechanisms and monitoring devices to enhance safety. With new machines replacing older models in human hospitals, many used machines designed for use in human beings have become available to veterinary surgeons. This review covers some of the problems, malfunctions, and limitations of anaesthetic machines, vaporizers, and breathing systems which are likely to have application in veterinary anaesthesia. The Anaesthetic System An anaesthetic system has three basic components (the anaesthetic machine per se, the vaporizer(s), and the breathing circuit with or without CO, absorber) which communicate with each other during the administration of inhalation anaesthesia (Andrews, 1993). Simply stated, the anaesthetic machine itself consists of sources of medical gases (O,, with N20 as an option), and regulators and flowmeters for those gases. Generally, one or more vaporizers are located on the machine, outside the breathing circuit (VOC) immediately downstream from the wmeters. In addition, vaporizers may be located in a 86 circle breathing system (VIC) in some veterinary anaesthetic machines and in some obsolete machines intended for human anaesthesia. Medical Gases Anaesthetic machines typically have two sources for each medical gas - a pipeline inlet and a hanger yoke assembly for small (E) cylinders. The pipeline inlets connect to the hospital s central system for 0, and N20; alternatively, the pipeline inlets may link with large gas cylinders (H) that are located in the operating room, especially in veterinary practices. Each pipeline inlet consists of a male DISS (diameter indexed safety system) connector designed to receive a specific medical gas fitting (Dorsch and Dorsch, 1984). Some older machines were fitted with quick connects (eg, Ohio Diamond) instead of DISS connectors. Compressed medical gases, supplied in high pressure steel alloy cylinders, are potential hazards (Webb and Warren, 1982). Potential problems are fire associated with the oxidizing capabilities of 0, and N,O, rapid cylinder decompression due to damaging the valve body or the cylinder itself, and interchange of medical gases. The danger of fire, explosion, or injury to medical personnel is low if the usual guidelines for handling cylinders (Dorsch and Dorsch, 1984) are followed. The primary risk for patients is inadvertent interchange of medical gases leading to delivery of a gas or gas mixture with a low concentration of 0,. In designing modern machines, several strategies have been employed to prevent delivery of an inappropriate gas mixture (Dorsch and Dorsch, 1984): 1. Colour coding and/or labelling of cylinders, high pressure hoses, regulators, and pipeline inlets. 2. Specifying thread types and diameters for regulators and cylinder valve bodies. 3. Using DISS connectors for station outlets and pipeline inlets. 4. Employing the pin indexed safety system for small gas cylinders and yoke assemblies. 5. Colour coding, labelling, and standardizing the location of flowmeters; and using larger, fluted control knobs to distinguish 0, flowmeters (touch coding). 6. Oxygen monitors for the fresh gas supply or for the inspiratory limb of contemporary anesthesic machines (ASTM 1989). Incorporating the pin-index system into the hanger yoke assembly usually prevents attachment of the wrong cylinder (eg, N,O into an 0, yoke), but the safety

2 Figure 1: Hanger yoke for an oxygen cylinder on an anaesthetic machine. The normal configuration for the pins and nipple of the pinindex system are shown for oxygen. The pin-index system can be defeated by damaging or removing the pins or by stacking the pins with washers system can be defeated (Dorsch and Dorsch, 1984). The pins can be removed, bent, broken, or forced deeper into the yoke. The nipple of the yoke can be stacked with enough washers to allow attachment of the wrong cylinder. Yoke blocks accommodate gas supplies other than small cylinders; some yoke blocks do not have pin holes, and some are short and can be attached upside down, both types allowing connection of the wrong gas. Older machines should be inspected thoroughly to assure the integrity of the pin-index system (pins and nipple on the hanger yoke) so that 0, and N,O cylinders cannot be interchanged.(figure 1) Small cylinders should be carefully attached to the hanger yoke. As stated, the pin-index system can be damaged, but perhaps more importantly, incorrect alignment of the valve body in the hanger yoke can create a hazard. Directing the retaining screw into the valve body s safety relief device instead of the conical depression has caused rapid decompression of a cylinder (Fox and Fox, 1968; Webb and Warren, 1982). In using machines with multiple hanger yokes, each yoke should contain a small cylinder or be fitted with a yoke plug. Replators Regulators reduce and regulate the high pressure from compressed gas cylinders to safer, more functional pressures for the anaesthetic machine. Regulators attach to the valve bodies of large gas cylinders. The threads of J. vet. Anaesth. Vol. 21 [December 1994) 87 the valve outlet mesh with the nut on the inlet (nipple) of the regulator. For large cylinders and regulators, the threads on the nut and valve outlet may be internal or external and right hand or left hand with variable dimensions, creating a safety system to prevent the interchange of gases (Dorsch and Dorsch, 1984). In addition, the nipple of the regulator and the corresponding seat vary in size and shape for different gases. Even though regulators for large cylinders are designed to fit only a cylinder for a specific gas (that is, an 0, regulator fits only an 0, cylinder), the mechanism preventing the interchange of gases can be defeated. Reports exist in both the human and veterinary literature documenting the unintentional administration of hypoxic gases. For example, air instead of 0, was administered to a foal via a jury-rigged regulator (Webb and Warren, 1982). Also, the nipple on a regulator can be changed purposely to allow the regulator s use with another gas or gas mixture; this is a potential hazard if the regulator and nipple are not appropriately identified. Hospital pipeline systems are usually designed to deliver gases to the machine at its normal worlung pressure (50 psig). If the pipeline supply fails, the backup supply is the small cylinder attached at the hanger yoke. Regulators are normally set to deliver 0, and N,O at 45 psig (Andrews, 1990), if the machine has a power outlet for a ventilator. If not, the 0, regulator may be set between 37 and 42 psig (Dorsch and Dorsch, 1984). Even though the pipeline pressures are set higher than pressures from cylinders on the anaesthetic machine, small cylinders should be off when pipeline gases are flowing. This prevents unnoticed depletion of the machine s reserve supply of gases. Also, double yokes which are present on some machines may serve one regulator. If each yoke is not fitted with a check valve, partial transfilling of cylinders can occur. Transfilling of cylinders is potentially dangerous as gas recompresses and generates heat (Dorsch and Dorsch, 1984). Flowmeters The accuracy of flowmeters is important in anaesthesia. Delivery of an appropriate concentration of 0, as well as an adequate quantity of 0, is essential for saturating haemoglobin and for supplying the patient s metabolic needs. Accuracy becomes especially important with closed and low-flow breathing systems and with administration of NzO. The lowest mark on the flowmeter s scale is the lowest accurate flow at which the flowmeter should be used. Extrapolating to lower flow rates is not recommended (Dorsch and Dorsch, 1984). Parts in a flowmeter assembly should not be interchanged since the float (indicator), scale, and glass (Thorpe) tube are calibrated as a unit; when a new part is needed, the entire assembly must be replaced (Dorsch and Dorsch, 1984). The size of the float in relation to the scale influences the accuracy of a flowmeter. Floats may be more than one centimeter long, and reading the scale at the top of the float instead of the bottom may affect the flow rate significantly. Errors in reading the flowmeters for vaporizers such as Copper Kettles and Vernitrols may alter the inspired concentration of anaesthetic profoundly. In

3 J. vet. Anaesth. Vol. 21 (December 1994) general, flow rates are read at the top of the float except for ball-type floats which are read in the center (Dorsch and Dorsch, 1984). New machine standards require that the point of reference for reading the float be indicated on the flowmeter assembly (ASTM 1989). The arrangement of flowmeters is important with multiple gases on a single anaesthetic machine. The standard requires that 0, be delivered downstream of other gases when all gases utilize a common manifold (Dorsch and Dorsch, 1984; Webb and Warren, 1982). If 0, enters the manifold upstream from the other gases, there is a possibility of delivering hypoxic mixtures, and this complication has been reported several times (Dorsch and Dorsch, 1984). Other arrangements of flowmeters may exist in older machines, which should be considered when using both N,O and O,.(Figure 2) The U.S. and Canadian standard is for the 0, flowmeter to be located to the right in a cluster as viewed from the front of the machine. If a flowmeter for a vaporizer is to be placed to the right of the cluster, it must be at least 10 cm from the 0, flowmeter (Dorsch and Dorsch, 1984). This arrangement was intended to standardize the location of control knobs for flowmeters and decrease the likelihood of adjusting the incorrect flowmeter. Standardization of location of the flowmeter for 0, and requiring fluted flow control knobs for 0, flowmeters was intended to reduce errors in adjusting 0, flow rates (Dorsch and Dorsch, 1984). The arrangement of flowmeters on older models of Drager anaesthetic machines allowed delivery of hypoxic mixtures if both O2 and N,O were used and a leak developed at the base of the 0, flowmeter, and this malfunction did result in the death of a horse (Gray et al., 1981). Using a similar machine for a dog (without N,O), I have observed cyanosis and light anaesthesia shortly after intubation and attachment to the machine. After changing to another machine, anaesthesia continued uneventfully. Evaluation of the 0, flowmeter revealed a leak at its base; a faulty seal was created by a folded washer. This leak produced inadequate delivery of O2 to the vaporizer and the patient; subsequently, hypoxaemia and inadequate anaesthesia developed. Because of this flowmeter's design, the float showed a correctly adjusted flow rate for O,, although total gas flow to the vaporizer and circle breathing system was too low. This example illustrates the significant hazard associated with leaks in a flowmeter assembly. Leaks can occur at several locations, including cracks in the glass tube as well as problems with O-rings and gaskets (Andrews, 1990). Leaks in the flowmeter are an important reason for performing leak tests on the low pressure circuit in contemporary machines (Andrews, 1993). When evaluating flowmeters, the float should move freely in the glass tube. Dirt, static electricity, or a damaged float may impair free movement. A sluggish or sticking float indicates the need to clean or replace the assembly. A sticlung float (apparently indicating 0, flow when the 0, cylinder was empty) has been reported as a cause of hypoxia (Dorsch and Dorsch, 1984; Mazzia et al., 1962). When a flowmeter control is not in the "off" position, opening a cylinder valve will cause the float to rise rapidly in the flowmeter tube. This may jam the float at the top of the glass tube where it may not be noticed, or the float may be damaged (Dorsch and Dorsch, 1984). Figure 2: Flowmeter cluster of an older anaesthetic machine with a Copper Kettle vaporizer. Note that the flowmeters for oxygen are not located properly according to newer machine standards. Also, the control knobs for the oxygen flowmeters are not fluted and are similar to all other control knobs. 88 Vaporizers A description of all vaporizers and their perturbations is beyond the scope of this paper. However, some idiosyncrasies and hazards associated with certain vaporizers that are likely to be encountered in veterinary anaesthesia will be included. Tec-type vaporizers, especially the Fluotec Mark I11 and Isotec, are common, and are considered reliable because they are temperature, flow, and back-pressure compensated under reasonable operating conditions. The Tec 111's predecessor, the Fluotec Mark 11, is still sold by suppliers of veterinary anaesthesia equipment, and many are in regular use. Performance data show that the Mark I1 becomes imprecise at flow rates below 4 Llmin, and inaccuracy increases distinctly below 2 Llmin (Dorsch and Dorsch, 1984). At flow rates and dial settings likely to be selected for small veterinary patients, the Mark II's output tends to be lower than control dial settings of less than 2% and higher than dial settings greater than 2%. (Table 1)

4 J. vet. Anaesth. Vol. 21 (December 1994) Dial 1 L/min 2 Llmin 3 L/min 4 Wmin 6 L/min 8 L/min Setting 0.5% 1% 1.5% 2% 2.5% 3% 3.5% 4% Table 1: Approximate output (Volumes %)from a Fluotec Mark II at various flow rates and dial settings. At the very low flow rates required for closed and low-flow maintenance techniques, the Mark II's output may decrease to zero or increase to concentrations much higher than dial settings. Because of its unpredictable output characteristics, the Mark I1 has been categorized as unsuitable for use at low flow rates (Dorsch and Dorsch, 1984; Lin, 1980). Back pressure (eg, positive pressure ventilation) increases the output of the Mark I1 dramatically at flow rates less than 2 L/min (Dorsch and Dorsch, 1984; Hill and Lowe, 1962). A pressurizing valve was developed for the Mark I1 to minimize the effects of back pressure (Dorsch and Dorsch, 1984), but may not be present on all machines with the Mark I1 vaporizer. The operator should fully understand the Mark II's performance characteristics before using the vaporizer clinically, and the output diagram for the Mark I1 should be available for consultation during management of patients. Vernitrols and Copper Kettles are flowmeter-controlled vaporizers which were popular in human anaesthesia for many years. Copper Kettles were the first devices that allowed precision vaporization of liquid anaesthetics (Dorsch and Dorsch, 1984). However, they are not being manufactured in the United States, and are not covered by the ASTM standard of 1989 (Andrews, 1990). Many of these measured-flow (Dorsch and Dorsch, 1984) or saturation vaporizers (Hartsfield, 1986; Lumb and Jones, 1984) are available on used anaesthesia machines, and they are economical to purchase for veterinary practices. The output from a measured-flow vaporizer is gas that is fully saturated with anaesthetic, the amount dictated by the temperature of the liquid anaesthetic and its vapour pressure. Therefore, the saturation vaporizer's output of halothane or isoflurane approaches 32% under typical conditions in operating rooms. Control of the concentration of anaesthetic to the patient is dependent on dilution with gases supplied from the machine; gases from the vaporizer's flowmeter (volatile anaesthetic and 0,) mix with gases from the O2 (and N,O flowmeter if used) flowmeter to produce the desired concentration of volatile anaesthetic. Obviously, it is possible to deliver very high concentrations of highly volatile, highly potent anaesthetics if flowmeters are set carelessly. Use of a relatively simple, hand-held slide rule that accounts for temperature of the liquid anaesthetic, vapour pressure of the specific agent and total fresh gas flow for determination of the flow of gas through the vaporizer is recommended. However, the calculator or slide rule may not be readily available and may be misread (Dorsch and Dorsch, 1984). For standard Copper Kettles and Vernitrols, Table 2 provides flow rates for the vaporizer flowmeter to produce various delivered concentrations at several total fresh gas flow rates at a vaporizer temperature of 21 C. Most of the hazards with saturation vaporizers relate to incorrect use, including errors in calculation of the output of vapour, failure to turn on the vaporizer flowmeter or the vaporizer circuit control valve, and careless handling of the vaporizer during filling and transport. It is possible for liquid anaesthetic to enter the discharge tube of the vaporizer, ultimately delivering very high concentrations of anaesthetic to the breathing system. Overfilling is also possible in older models (Dorsch and Dorsch, 1984). Older saturation vaporizers may not be equipped for back pressure compensation (eg, check valve), and application of intermittent positive pressure ventilation may significantly increase the delivered concentration (Dorsch and Dorsch, 1984; Lowe et, Total 1 Llmin 2 Llmin 3 Wmin 4 Llmin 5 Lfmin Flow 1% Halothane % Halothane % Halothane % Halothane % Halothane ~~~~~ Table 2: Flow rates of 0, (mllmin) from the vaporizer flowmeter of a saturation vaporizer required to deliver 1%, 2%, 3%' 4%' and 5% halothane to the breathing system when the total maintenanceflow rates (0, NzO, and halothane) are 1, 2, 3, 4, or 5 Wmin and the temperature of the liquid halothane is 21 "C

5 ~ J. vet. Anaesth. Vol. 21 (December 1994) - al., 1962). Finally, flow rates below the lowest mark on the scale for the saturation vaporizer s flowmeter should not be extrapolated unless instrumental monitoring for inspired or delivered anaesthetic concentration is in use. Some measured-flow vaporizers, including the ones on the Ohio DM 5000 anesthesia machine (Dorsch and Dorsch, 1984) and the Pitman-Moore 980 veterinary anesthesia machine (designed specifically for methoxyflurane) (Operation and Maintenance Manual for the Metomatic Model 980 Veterinary Anesthesia Machine), were calibrated in cubic centimeters (milliliters) of anaesthetic vapour. Applying the common calculations (Lumb and Jones, 1984) or slide rules for Copper Kettles or Vernitrols to determine output for these vaporizers will result in a delivered concentration that is greater than expected. Volatile anaesthetics other than methoxyflurane should not be used in the I M 980 machine (Operation and Maintenance Manual for the Metomatic Model 980 Veterinary Anesthesia Machine). Nonprecision, draw-over vaporizers for in-circuit use (VIC) have been common in veterinary anaesthesia since the introduction of inhalation anaesthetics for veterinary patients. Perhaps, the most used VIC in the U.S. has been the Ohio #8 vaporizer. While the vaporizer is no longer manufactured, it was the basic vaporizer on many of the I itman-moore veterinary anaesthetic machines (Models 960 and 970) for administration of methoxyflurane. Indeed, in a modified form (wick removed), the Ohio #8 vaporizer has been proposed as an inexpensive alternative for administration of the highly volatile, potent anaesthetics, isoflurane (Bednarski et al., 1993) and halothane (Gallagher and Klavano, 1981). Use of Ohio #8 vaporizers for halothane or isoflurane with the wick in place is dangerous due to the high concentration of anaesthetic that may be delivered in the inspired gases. Even with the wick removed, recommendations for using the Ohio #8 vaporizer for isoflurane include familiarity with guidelines for its use and understanding of its limitations (Bednarski, 1993). The Ohio #8 vaporizer also has potential for leaking of anaesthetic into the breathing system when the control lever is in the off position (Dorsch and Dorsch, 1984). With age, the vaporizer s valves may not seat properly, allowing continuous passage of fresh gases through the vaporization chamber and production of anaesthetic-rich gases. (Figure 3) Tipping of most vaporizers containing liquid anaesthetic may cause liquid to enter the bypass channel of the vaporizer outlet (Dorsch and Dorsch, 1984). If this occurs, a very high concentration of anaesthetic vapour may be delivered to the patient; tipping a vaporizer that was not securely attached to an anaesthetic machine has been associated with cardiac arrest in human anaesthesia (Munson, 1965). Also, moving a vaporizer on a mobile machine may alter vaporizer output if the machine is tipped or liquid is sloshed as the machine is moved over doorway thresholds. Generally, vaporizers should be empty if they are transported. Even portable machines should be moved with care. If tipping occurs, Dorsch and Dorsch (1984) recommend running a high flow of 0, through the vaporizer for 20 minutes, with the control dial at a low setting, and they suggest that servicing may be required. 90 Figure 3: The underside of an Ohio #8 vaporizer with the glass jar removed. The two valves and seats in the center of the vaporizer unit open and close to controlflow of gas through the vaporization chamber. The valves may become incontinent over time, leading to flow of gas through the vaporization chamber even when the control arm is off. One anaesthetic machine may be fitted with multiple vaporizers. Newer machines have vaporizers equipped with interlocking mechanisms which do not allow two vaporizers to be on simultaneously (Andrews, 1990). Few veterinary practices have the luxury of using the most modern vaporizers equipped with interloclung devices. In-line, non-interlocked vaporizers offer the possibility of operation with two vaporizers on simultaneously, conceivably resulting in excessive depth of anaesthesia (Dorsch and Dorsch, 1984). The simultaneous use of more than one vaporizer in series also increases the probability of contamination of a vaporizer with an inappropriate agent. If vaporizers are placed in series, the order that has been recommended is methoxyflurane, enflurane, isoflurane, and halothane from upstream to downstream; this reduces the chance of contamination by taking into account both vapour pressure and potency (Dorsch and Dorsch, 1984). Anaesthetic machines with vaporizers in series should be used carefully if an interlocking mechanism is not in place. Occasionally, veterinary surgeons will use a free-standing, calibrated vaporizer that is periodically connected between the common gas outlet and the breathing system or between the outlet of another vaporizer and the breathing system. Such an arrangement provides the opportunity for tipping of the vaporizer, inadvertently reversing the flow through the vaporizer, and forcing O2 through the vaporizer with the flush valve, all of which may significantly increase the output concentration (Andrews, 1990). In general, use of free-standing vaporizers is not the safest approach for access to a second anaesthetic.

6 ~ ~ Veterinary practices often stock more than one volatile anaesthetic. If a vaporizer is inadvertently filled with the wrong agent, it should be decontaminated before it is used in a clinical case. The best plan for all contaminated vaporizers is service by a qualified technician. For Ohio calibrated vaporizers, service is required because the paper wicks must be replaced. For contaminated Tec vaporizers, an option is to drain the vaporizer, flush it with an 0, flow of 5 Llmin for 45 minutes, allow it to stabilize for about two hours (temperature), and refill it with the appropriate anaesthetic. Vaporizers contaminated with a non-volatile contaminant (eg, water or excessive thymol) should be drained and serviced. Common Gas Outlet and Flush Valve The conduits and connections between the vaporizer and the breathing circuit offer several opportunities for leaks, leading to the delivery of inadequate anaesthetic and 0,. A leak at the flush valve in a machine used for small veterinary patient resulted in loss of anaesthetic and 0, at flow rates less than 1 L/min, making it impossible to maintain a surgical plane of anaesthesia at normal fresh gas flow rates for a circle breathing system (Hartsfield and Thurmon, 1978). While hypoxia was not a problem in this case, reduced 0, concentration and hypoxia were reported in human patients due to a similar leak; the cause was an inflowing gas leak due to a faulty O-ring at the base of a side-arm Vernitrol (Mulroy et al., 1976). Later models of the same machine incorporated a check valve to prevent loss of fresh gas flow. Simple disconnections at the common gas outlet of the machine or at the fresh gas inlet to the breathing circuit can cause loss of all gas flow to the circuit and the patient. The standard for contemporary machines requires that the common gas outlet incorporate a retaining device to prevent accidental disconnection (ASTM 1989). Disconnections should be found during the machine check prior to a case, but disconnections can occur during use of the machine if a retaining mechanism is not present. A loose or disconnected hose during an anaesthetic procedure may not be recognized immediately in a spontaneous breathing patient. With a circle system and a VOC, hypoxia is likely to develop, and the patient will get inadequate anaesthetic. Some circle systems incorporate a negative pressure relief valve to allow room air to be entrained when the fresh gas flow is inadequate. With a nonrebreathing system, CO, will be retained, hypoxia is likely, and the plane of anaesthesia will tend to lighten. The use of an 0, analyzer to evaluate inspired gases should allow early detection of this problem. The oxygen flush valve delivers a high flow (35 to 75 L/min) of 0, to the common gas outlet of the anaesthetic machine (Dorsch and Dorsch, 1984; ASTM, 1989). The contemporary machine standard does not allow piping of gas from the flush valve through the vaporizer. The flush valve on older machines, especially those with precision vaporizers added after the machine was manufactured, may direct 0, through the vaporizer with the potential for increased output of anaesthetic agent (Andrews 1990). 1. vet. Anaesth. Vol. 21 (December 1994) - 91 Circle Breathing Systems The importance of resistance to ventilation imposed by the breathing circuit has been debated. In general, breathing systems inducing the least resistance to gas flow should be chosen for spontaneously breathing patients (Dorsch and Dorsch, 1984). Resistance to gas flow through a circle breathing system is influenced primarily by the pop-off (positive pressure relief valve), the unidirectional valves, and the CO, absorbent canister (Dorsch and Dorsch, 1984). The total amount of resistance in a circle breathing system varies with fresh gas flow rate and the type of ventilation. Fresh gas flow rate influences flow through the pop-off valve and therefore resistance to ventilation, and the pattern of ventilation affects the flow rate and therefore the resistance through the soda b e canister and the unidirectional valves (Dorsch and Dorsch, 1984). Resistance to breathing has been cited as a reason for not using adult circle systems for paediatric patients. However, since low flow rates are appropriate for small patients maintained with circle systems, the resistance to breathing is low, and the use of circles in spontaneously breathing paediatric patients may not be contraindicated solely on the basis of resistance (Dorsch and Dorsch, 1984). For veterinary patients, Wagner and Bednarski (1992) concluded that circle breathing systems were appropriate for healthy animals as small as 2.5 kg in body weight, citing two reports in cats to support the argument (Hartsfield and Sawyer, 1976; Suter et al., 1989). Dunlop (1992) recommended adult circle rebreathing systems for healthy patients as small as 3 kg. Hodgson and McMurphy (1993) evaluated the resistance to flow in breathing circuits designed for large animal anaesthesia. They determined total resistance to constant flow in 9 breathing circuits, and measured resistance in certain component parts. Greater resistance occurred with higher flows through all of the breathing circuits. The Drager and Fraser Sweatman circuits were intermediate in total resistance when all 9 circuits were compared, and each circuit had individual parts that contributed significantly to resistance. The inspiratory breathing tube, expiratory breathing tube, soda line canister, inspiratory directional valve, and expiratory directional valve each contributed significantly to resistance in at least one circuit. The authors concluded that low resistance was an advantage for anaesthetized patients and that various parts of the breathing circuits could be redesigned to decrease total resistance. The canister for C02 absorbent, besides being a source of resistance to the flow of gases during ventilation, is an important area for malfunctions in circle breathing circuits. The canister is removed regularly for changing, and failure to adequately create a seal when replacing the canister causes leaks. Normal wear and tear may damage the canister, the caustic effects of soda lime may corrode metal parts, and ageing results in deterioration of gaskets. Simply leaving soda lime granules on the gaskets can make a tight seal impossible. One report described the bypass of soda lime with resultant hypercarbia because of disjunction of the diffuser foot from the conduction tube in the canister on a veterinary machine (Mehusen, 1979). Newer circle sys-

7 tems are constructed with materials less vulnerable to the effects of soda lime. Some circle breathing systems designed for human use included a mechanism for purposely bypassing the soda lime canister. The bypass was intended for use during the changing of soda lime and for intentional elevation of the inspired concentration of C02 (Dorsch and Dorsch, 1984). If these systems are used in veterinary practice, the operator must understand the function of the bypass, should notice the position of the bypass control before each case, and should not operate the circle in the bypass mode. One hazard associated with the soda lime canister is the possible inhalation of alkaline dust. With the reservoir bag on the exhalation side of a circle breathing system, squeezing the bag to deliver a breathe can force dust into the inhalation side of the circle (Dorsch and Dorsch, 1984). Other problems with circle systems include failure to replace unidirectional valve discs after cleaning, leaving the canister s drain open, and deteriorated rubber or plastic reservoir bags and breathing hoses. All circle breathing systems should be operated with a gas-collecting assembly (scavenging pop-off valve). Older circle systems without scavenging capabilities which are still in use should be fitted with new pop-off valves and scavenging systems. The machine standard for the outlet of the pop-off valve is 19 mm 0.d. to fit corrugated tubing of the same internal diameter. When scavenging pop-off valves were first introduced, some had outlets sized at 22 mm 0.d.; this created a site for interchange of a breathing tube and scavenger tubing, with the potential for disastrous results if the system is used with a patient (Dorsch and Dorsch, 1984). Non-rebreathing Systems Resistance to ventilation in the Mapleson systems is minimal (Dorsch and Dorsch, 1984), and this may be advantageous for small patients. The advantages of modern non-rebreathing systems for very small patients include improved ventilation, better gas exchange, greater control of the depth of anaesthesia, and fewer mechanical problems (Hodgson, 1992). Hazards of non-rebreathing systems relate primarily to outflow occlusion and damage to the lungs, including development of pneumothorax. Care in positioning the non-rebreathing systems and judicious use of relief valves during positive pressure ventilation are important considerations. Activation of the flush valve when a nonrebreathing system is being used can result in over pressurizing the respiratory system and pneumothorax. Therefore, the machine s flush valve should never be used when a nonrebreathing system is attached to a patient. Nonrebreathing mounting systems are available commercially and these systems may help to prevent accidental outflow occlusion, especially if they incorporate a safety release system. It is important for the operator to know the individual characteristics of the particular mounting system in use. Anaesthesia Apparatus Checkout Recommendations Regular evaluation of anaesthetic apparatus is important to ensure safety for both personnel and patients. For J. vet. Anaesth. Vol. 21 (December 1994) 92 the patient, delivery of appropriate concentrations and amounts of O2 and anaesthetics is essential. For personnel, assurance of an environment that is free of excessive waste gases is necessary. Evaluation of an anaesthetic machine includes the high pressure, intermediate, and low pressure areas (Dorsch and Dorsch, 1984; ASTM, 1989). The high pressure area includes the gas cylinders, hanger yokes, yoke blocks if any, high pressure hoses, pressure gauges, and regulators. These components are exposed to variable pressures as the gas cylinders empty (up to 2200 psig for 0, and 745 psig for N20). Tests for leaks should include inspection for loose connections and audible leakage, pressure checks (loss of pressure when cylinder valves are open and flowmeters are off), and use of soapy water solutions to snoop for leaks (creation of bubbles) especially at joints. The intermediate pressure area (usually 45 to 50 psig) includes pipeline inlets, conduits from pipeline inlets to flowmeters and conduits from regulators to flowmeters, the flowmeter assembly, and the O2 flush apparatus. Tests include visual inspection, listening for leaks, and use of soapy water solutions. The low pressure area includes the VOC vaporizers(s), conduits from the flowmeters to the vaporizer, conduit from the VOC to the common gas outlet, and conduit from the common gas outlet to the breathing system. Pressures are usually slightly above atmospheric pressure. Routine tests include visual inspection and pressure checks with the breathing system; in most machines, pressure applied to the breathing system affects the low pressure area of the machine. A universal negative-pressure leak test has been proposed for contemporary anaesthetic machines to evaluate the low pressure area; the test requires a simple suction bulb (Andrews, 1993). The flowmeters and vaporizers are off during the test. The suction bulb is attached at the common gas outlet and squeezed until it fully collapses, creating a vacuum in the low-pressure area. If the bulb reinflates in less than 10 seconds, a significant leak is present. The test is repeated with the control dial of the vaporizer on to detect any internal leaks that might not be found with the vaporizer off. This test allows differentiation between leaks in the low pressure area of the machine and the breathing system. The test is capable of detecting leaks as small as 30 ml/min and has been described as extremely reliable. To date, there are no reports regarding the utility of negative-pressure leak testing on veterinary anaesthetic machines and in a typical veterinary setting. A check of anaesthetic equipment should be made each day before anaesthetizing the first patient, and the breathing system should be evaluated before each patient. The operation manual for individual machines give specific guidelines for evaluation and checkout, and machines with special features require individualized attention. Ventilators on anaesthetic machines should be evaluated, but will not be included in this discussion. The following procedures are modified from the Anesthesia Apparatus Checkout Recommendations from the FDA s Center for Devices and Radiological Health (ASTM, 1989) and are appropriate for evaluation

8 J. vet. Anaesth. Vol. 21 (December 1994) of machines and breathing systems before the first case of the day: 13. Assure that the pop-off valve provides relief of pressure when the flush valve is activated Check central 0, and N20 supplies for adequate quantities of gases and pipeline pressures. Inspect the flowmeters, vaporizers, gauges, and supply hoses. Assure correct mounting of cylinders in the hanger yokes; the presence of a wrench for the cylinder valve; and a complete, undamaged breathing system with adequate absorbent for CO,. Assure that the waste scavenging system is connected to the pop-off valve and is working properly. If a charcoal canister is being used, confirm that it is not exhausted. Turn the flow-control valves off. Assure that the vaporizer is properly filled with the filler cap sealed and the control dial off. Check 0, cylinders on the machine. With the pipeline supply disconnected, 0, cylinder valve off, and pressure gauge at zero, slowly open the valve to check the pressure (> 500 psig) and determine the presence of leaks (a slow drop in pressure on the gauge). With multiple 0, cylinders, each cylinder should be checked. Check the N20 supply (if present) as in #6. Test the flowmeters for each gas. With the flow-control valve off, the float should rest at the bottom of the glass tube. Adjust flow through the full range to assure proper function (no sticking or erratic movements). With the vaporizer off, no anaesthetic odour should be present when the 0, flowmeter is on. For a circle system, test the function of the unidirectional valves. Wearing a surgical mask, exhale through the exhalation limb to check the exhalation valve, and compress the reservoir bag (pop-off valve closed and Y-piece open) to check the inhalation valve. Valve discs should be present and should rise and fall appropriately. Test for leaks in the circle breathing system and the anaesthetic machine. Close the pop-off valve, occlude the Y-piece, fill the system with O,, and turn the 0, flow to 5 L/min. As the pressure in the system reaches 20 cm of H,O, reduce the flow until the pressure in the system (manometer) no longer rises. The 0, flow should be negligible; a high leakage rate is unacceptable. Squeeze the reservoir bag to create a relatively high pressure (40 to 50 cm of H,O), and assure a tight system. In checking the circle system for leaks, one recommendation is to fill the closed circle to a pressure of 30 em of H,O and assure that the leak rate is less than 250 ml/min (Bednarski, 1991) or that the pressure drop is less than 5 cm of H,O in 30 seconds. Open the pop-off valve slowly, and observe the release of pressure. Occlude the Y-piece, and verify that only a negligible positive or negative pressure develops with an 0, flow rate of zero or 5 L/min. 93 Nonrebreathing systems should also be tested before use. For a complete system check of a Bain system, the patient port should be occluded, the relief valve should be closed, and the reservoir bag should be distended. The reservoir bag should remain fully distended and pressure within the system should not decrease. The complete system check does not assure a leak-free inner tube of the coaxial system. Therefore, the inner tube is evaluated by temporarily occluding the inner tube at the patient end with the flowmeter flowing at approximately 1 L/min. During a short period of occlusion, the float in the 0, flowmeter should fall (Dorsch and Dorsch, 1984). The complete system check will usually suffice for other nonrebreathing systems (eg, Norman mask elbow and Ayre's T-piece systems). Conclusion Morbidity and mortality data relating to the malfunction and misuse of anaesthetic equipment in veterinary patients is sorely lacking. While some significant, even lethal, problems with anaesthetic machines have been reported, no conscientious effort has been made to determine the incidence of serious injury or death associated with veterinary anaesthesia equipment. Neither the mortality rate nor the incidence of near accidents (eg, temporarily closed pop-off valve leading to short-term pressure build-up in the lungs) with anaesthetic machines has been documented in veterinary anaesthesia for large groups of patients. Perhaps the upshot of this discussion of problems with veterinary anaesthetic machines should include efforts to promote increased awareness of the hazards of anaesthetic equipment, to determine the true morbidity and mortality associated with their use, and to educate veterinary students and general practitioners about these risks. REFERENCES American Society for Testing and Materials. (1989). Minimum performance and safety requirements for components and systems of anesthesia gas machines (ASTM F ). American Society for Testing and Materials, Philadelphia. Andrews JJ (1990). Inhaled anesthetic delivery systems. In Miller RD, ed. Anesthesia 3rd ed, Churchill-Livingstone, New York Andrews JJ (1993). Understanding your anesthesia machine. In Annual Refresher Course Lectures, American Society of Anesthesiologists, Washington, D.C. 163,l-7. Bednarski RM (1991). Anesthetic equipment. In Muir WW 111 & Hubbell JAE, Eds. Equine Anesthesia, Monitoring and Emergency Therapy. Mosby Year Book, St. Louis. Bednarski RM, Gaynor JS & Muir WW 111. (1993). Vaporizer in circle for delivery of isoflurane to dogs. Journal of the American Veterinary Medical Association 202, Dorsch JA & Dorsch SE (1984). Understanding Anesthesia Equipment. 2nd ed. Williams and Wilkins, Baltimore. Dunlop CI The case for rebreathing circuits for very small animals. The Veterinary Clinics of North America: Small Animal Practice 22,

9 ~ I. vet. Anaesth. Vol. 21 (December 1994) Fox JWC & Fox EJ (1968). An unusual occurrence with a cyclopropane cylinder. Anesthesia and Analgesia 47, Gallagher LV & Klavano PA (1981). Scavenging waste anesthetic gases from obsolescent anesthetic machines. Journal of the American Veterinary Medical Association 179, Gray PR, Pascoe PF, Dohoo S & McDonell WD (1981). Anesthetic machine leak (letter). Journal of the American Veterina y Medical Association 179, Hartsfield SM (1986). Anesthetic machines and breathing systems. In Short C. E. Ed. Principles and Practice of Veterina y Anesthesia Hartsfield SM & Sawyer DC (1976). Cardiopulmonary effects of rebreathing and nonrebreathing systems during halothane anesthesia in the cat. American Iournal of Veterinary Research 37, Hartsfield SM & Thurmon JC (1978). Reduced anesthetic vapor concentration in a breathing circuit related to a leak in the oxygen flush apparatus. Veterina y Anesthesia 5, Hill DW & Lowe HJ (1962). Comparison of concentration of halothane in closed and semiclosed circuits during controlled ventilation. Anesthesiology 23, Hodgson DS (1992). The case for nonrebreathing circuits for very small animals. 172e Veterinary Clinics of North America: Small Animal Practice Hodgson DS & McMurphy RM (1993). Resistance to flow in large animal anesthetic machine breathing circuits. Proceedings of the Annual Meeting of the American College of Veterinary Anesthesiologists. Lin C (1980). Assessment of vaporizer performance in low-flow and closed-circuit anesthesia. Anesthesia and Analgesia 59, Lowe HJ, Beckham LM, Han YH & Evers JL (1962). Vaporizer performance: closed circuit fluothane anesthesia. Anesthesia and Analgesia 41, Lumb WV & Jones EW (1984): Veterinary Anesthesia. 2nd ed. Lea and Febiger, Philadelphia. Mazzia VDB, Mark LC, Binder LS, Crawford EJ, Gade H, Henry EL, Marx GF & Schrier Rl (1962). Oxygen and the anesthesia machine, Nm York State Journal of Medicine 62, Menhusen MJ (1979). Anesthetic machine malfunction resulting in soda lime bypass and hypercarbia. Journal of the American Animal Hospital Association 15, Mulroy M, Ham J & Eger EI 11. (1976). Inflowing gas leak, a potential source of hypoxia. Anesthesiology 45, Munson WM Cardiac arrest: hazard of tipping a vaporizer. Anesthesiology 26,235. Operation and Maintenance Manual for the Metomatic Model 980 Veterinary Anesthesia Machine, Ohio Medical Products, Madison Wisconsin. Suter CM, Pascoe PJ, McDonell WN & Wilson B (1989). Resistance and work of breathing in the anesthetized cat: comparison of a circle breathing circuit and a coaxial breathing system. Proceedings of the Annual Meeting of the American College of Veterina y Anesthesiologists. Wagner AE & Bednarski RM (1992). Use of low-flow and closed-system anesthesia. Journal of the American Veterina y Medical Association 200, Webb A1 & Warren RG (1982). Hazards and precautions associated with the use of compressed gases. Journal of the American Veterinary Medical Association 181, THE RELIABILITY OF MODERN MONITORING IN VETERINARY ANAESTHESIA YVES P.S. MOENS Anaesthesia Section, Department of General and Large Animal Surge y, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 12,3508 TD Ufrecht, The Netherlands. INTRODUCTION General clinical assessment of a subject or an animal under anaesthesia always was, and still is, based primarly on close observation by the anaesthetist of vital signs and levels of consciousness. Following several studies on anaesthetic mishaps and their possible prevention in human anaesthesia (Cooper et al., 1978; Cooper et al., 1984), a plea was made for the routine use of monitors to assist the anaesthetist during anaesthesia. Although in human anaesthesia the electrocardiogram (ECG) and noninvasive blood pressure measurement were already routinely performed, the monitors considered to be most useful in mishap prevention were the pulse oximeter and the capnograph. Applied together, these two technologies were considered potentially preventive in 93% of the preventable mishaps in human anaesthesia (Tinker et al., 1989). Inevitably these monitors are finding their way to the veterinary operating room. However, if these devices are to be useful to a veterinary anaesthetist, the latter must be able to appreciate the reliability of the provided data. False information may mislead the anaesthetist and may result in an incorrect diagnosis and inappropriate therapy. Capnometry and Capnograph Capnometry is the measurement of the partial pressure or concentration of CO, in the patient airway dur- 94 ing the entire ventilatory cycle. In the first place it provides the means to assess alveolar ventilation for ordinarly the peak CO, value reflects end-tidal (ETCO,) or alveolar CO,. However, a calibrated display of the CO, waveform -the capnogram- is essential for correct interpretation of the data. Furthermore capnography gives information about the integrity of the airway, the functioning of the breathing circuit and patient cardiopulmonary function (Moens and De Moor, 1981; Moens and Verstraeten, 1981). Most capnographs use an infrared sensor to which sampling gases are carried by tubing (sidestream analysis) from a point usually close to the distal end of the endotracheal tube. With this technique an adequate watertrap in the sampling line is essential to prevent erroneous readings when aspiration of moisture contaminates the sensor. Sometimes peak CO, is much lower than ETCO, and this situation is often suggested by a lack or by an abnormal shape of the alveolar plateau in the capnogram. It can be due to sampling errors, to inadequate tidal volumes or to a rapid respiratory rate. Peak CO, will be lower than ETCO, when the gas sample is contaminated with room air through leaks in the sampling line itself or its connections with the capnograph or the endotracheal tube. Too low values for ETCO, are also reported with the use of Mapleson D and Bain circuits because of the dilution of the gas sample by fresh gas. Because of the