These devices provide oxygen at low cost and eliminate the presence of high-pressure tanks.
Oxygen for the patient breathing system (Fig. 8.2) is delivered either by the flowmeter or the flush valve. The flowmeter delivers oxygen through the vaporizer to the breathing system at a specific rate measured in liters per minute. The flush valve is used to deliver a “burst” of oxygen to the breathing system and should be used only when the patient needs pure oxygen.
Flowmeters are operated with a knob attached to a needle valve. When the needle valve is opened, oxygen flows through a vertical tube that is labeled in milliliters or liters per minute. The flow of oxygen causes an indicator in the tube to rise and the flow rate is read at the appropriate label. The indicators have various shapes, but the most common is a ball. The flow rate is read at the point on the indicator where there is the greatest resistance to flow. If a ball is the indicator, this point would be the center of the ball. When operating the flowmeter, the control knob should not be overtightened. Doing so may result in damage or breakage to the needle valve. Always turn the flowmeter off when oxygen is no longer needed. If the flowmeter is on when the oxygen supply is turned on for the next procedure, the sudden pressure in the flow tube may cause the indicator to lodge at the top of the tube and possibly break the tube.
Caution should be exercised when the flush valve is used to deliver oxygen to the breathing system. Always operate the flush valve with the Adjustable Pressure Limiting (APL) valve or pop-off valve open. Stop the oxygen flush before the rebreathing bag is completely filled. Failure to do so may result in high pressure in the system, which in turn could cause damage to the patient’s airway and lungs. The same is true when using a nonrebreathing system because the oxygen is delivered directly to the patient connection on the system. It should be noted that some anesthetic machines have flush valves that are either restricted or do not have a high-volume flush. This increases the margin of safety when using these flush valves. To assess the volume of oxygen being flushed, occlude the patient connection on the breathing tube, and then operate the flush valve and observe how quickly the rebreathing bag fills.
At the end of the day, the oxygen supply should be shut off or the machine disconnected if a central oxygen system is in use. This will prevent needless oxygen consumption if a flowmeter is not off or if there are small leaks in any of the connections on the machine.
Delivery of a nesthetic
The second basic function is to deliver vaporized anesthetic agent to the breathing system in concentrations that are optimal for the desired effect. Since the liquid agent is in a closed system, a carrier gas must be present to deliver the vaporized agent to the breathing circuit. The gas used to accomplish this is the fresh gas flow of oxygen.
In almost all anesthetic machines used today, the liquid agent is contained in an agent-specific vaporizer that is outside the patient breathing circuit. The fresh gas flows from the flowmeter to the vaporizer. A very precise portion of the oxygen flow is diverted into a vaporization chamber where it is saturated with vaporized anesthetic. It is then mixed with that portion of the flow that bypasses the vaporization chamber and is delivered to the breathing system (Fig. 8.3).
The anesthetic vaporizer also has a thermal compensation device. This is necessary because the liquid anesthetic cools as vaporization occurs and makes vaporization more difficult. This device then diverts more of the flow into the vaporization chamber to compensate for the cooling. Most vaporizers are constructed of brass, which conducts heat from the ambient air to the liquid to minimize the magnitude of the temperature change. This assures that the vaporizer output is constant despite changes in temperature.
Vaporizers have a dial with numbers from 0–5 for isoflurane and 0–8 for sevoflurane. These numbers represent volume percent and indicate the concentration in percent delivered at the output of the vaporizer. If the fresh gas flow is 1 liter per minute and the dial is set at 2%, this means that 0.98 liters per minute of oxygen and 0.02 liters per minute of agent are leaving the vaporizer and are delivered to the breathing system. Doing the math, it is evident that if the fresh gas flow is increased to 2 liters per minute, the amount of agent consumed will be doubled. If the flow rate is 0.5 liters per minute the consumption will be halved. The same relationship is true related to the dial setting on the vaporizer. It is important to remember that the numbers on the dial do not necessarily indicate depth of anesthesia. That is determined by assessing the patient, not the vaporizer setting.
Some vaporizers are filled with liquid by using a pin fill device. An agent-specific spout replaces the cap on the agent bottle. A keyed pin fits into the fill manifold on the vaporizer. It is locked into place, and the vaporizer is filled to the desired level on the window. However, most vaporizers have a funnel fill device, and the liquid agent is simply poured into the vaporizer. Care must be exercised to avoid spillage. Anti-Spil™ adapters are available that replace the cap on the agent bottle and minimize the potential for spillage (Fig. 8.4). The window that indicates the level of liquid in the vaporizer will help prevent overfilling of the vaporizer and will also indicate when the liquid level is low. Operating the vaporizer with the level too low may result in lower than indicated concentrations.
The fill cap on funnel fill vaporizers has an O-ring that seals against the funnel surface. Each time the cap is removed the O-ring and funnel surface should be wiped with a 4 × 4 gauze pad moistened with alcohol (Fig. 8.5). This prevents the buildup of dirt particles that could cause the O-ring to leak and allow anesthetic vapor into the work environment.
All vaporizers have a drain plug to allow the vaporizer to be drained. This plug is usually in the center of the funnel fill device. Continuous removing of the cap to fill the vaporizer may cause the drain plug to become loose and may result in agent leaking from the drain. The drain plug should be checked periodically to ensure that it is tight.
No vaporizer should be subjected to “blunt force trauma” as a result of moving the anesthetic machine or from other portable equipment being moved in the vicinity of the anesthetic machine. Not only can this result in external damage, but internal damage to the thermal compensating device may also occur, resulting in poor performance.
The external surface of the vaporizer should be cleaned periodically using a 4 × 4 gauze pad moistened with alcohol. No liquid should be poured or sprayed on the vaporizer for the purpose of cleaning.
The calibration of the vaporizer should be checked periodically, and if necessary, the vaporizer should be serviced. Most vaporizers will provide many years of reliable service with proper care and maintenance.
Removal of carbon dioxide
The third function of the anesthetic machine is to remove carbon dioxide. The following sections discuss the components necessary in a rebreathing system to remove the carbon dioxide.
Canister for carbon dioxide absorber
A container for baralyme or sodalime granules is necessary to chemically remove the carbon dioxide from the circuit. Failure to remove exhaled carbon dioxide from the breathing circuit results in increased carbon dioxide in the blood. This produces a respiratory acidosis and an increase in respiratory rate. Both conditions are detrimental to a successful anesthetic procedure.
This process utilizes the principle of a base neutralizing an acid. The expired carbon dioxide reacts with water to form carbonic acid, which in turn is neutralized by the alkaline chemicals in the absorbent. The end products of this reaction are water and a carbonate.
The most common absorbent is sodalime. By weight it is 4% sodium hydroxide, 1% potassium hydroxide, 14–19% water and enough calcium hydroxide to make 100% (Dorsch and Dorsch 1999). A small amount of pH sensitive dye is also added. As the absorbent reacts with carbon dioxide, the color changes to purple indicating that the absorbent is exhausted.
The water in the absorbent is necessary for the chemical reaction that takes place between the carbon dioxide and the absorbent. The high-moisture absorbents (14–19%) have a slower rate of absorption and do not exhaust as rapidly as low-moisture absorbents. The humidity of the breathed gases does not affect the capacity of the sodalime to absorb carbon dioxide (Dorsch and Dorsch 1999).
This chemical reaction is exothermic, meaning that heat is released as the reaction takes place. This heat can be detected on the outside of the canister. If heat is not present, it may mean that absorption is not taking place. The amount of carbon dioxide absorption is about 26 liters/100 grams of absorbent. Efficiency may vary depending on the design of the canister and the method of packing (Hartsfield 1996). If the absorbent is working properly there should be a negligible amount of carbon dioxide being rebreathed by the patient.
One-way valves serve as another functional component necessary to remove carbon dioxide. These are check valves that allow the gas in the circuit to move in only one direction. Therefore, fresh gas is always being moved toward the patient and expired gas away from the patient. Most valves are horizontal discs that rest on top of open tubes. They open and close individually based on patient inspiration and expiration. On most machines, the discs are enclosed in a clear chamber that allows visual verification of proper function. The one-way valves, in effect, minimize the amount of mechanical dead space that is created by the machine. In a properly functioning rebreathing circuit, this space is where the inspiratory and expiratory tubes join to where the endotracheal tube attaches.
The final component on the machine that removes carbon dioxide is the adjustable pressure limiting (APL) valve or pop-off valve. Since the flow of fresh gas into the system is greater than what the patient removes, there must be a way for the excess gas to be removed. With a high fresh gas flow, more carbon dioxide will be removed through the APL valve and vice versa. This valve is usually located near the expiratory valve or the rebreathing bag. It is very important to keep the valve open unless positive pressure is being used to inflate the lungs. If the valve remains closed, pressures that will be fatal to the patient will be created in the system.
Removing Waste Anesthetic Gas
Since anesthetic gas is removed through the APL valve, it also performs the final function of the anesthetic machine—to remove waste anesthetic gas (WAG) from the work environment. All APL valves are designed to receive a 19 mm diameter evacuation tube that carries the WAG to the evacuation device.
The evacuation tube can be connected to a canister containing activated charcoal, which absorbs the anesthetic gas. This passive method is effective only if the canister is replaced according to the directions. Most often the canister must be replaced when it has absorbed 50 grams of agent.
If relatively high flow rates are used, the canister will expire sooner than if a lower flow rate is used. As more of the canister becomes expired, higher flow rates might move some of the WAG through the canister without being captured by the charcoal.
This tube can also be directed into a nonrecirculating ventilation exhaust duct. The exhausted air will move the WAG from the tube to the outside. This method is preferred over the charcoal canister because the WAG is being moved to the outside. It must be emphasized that the WAG is vented into an exhaust duct and that the air in the duct is not being recirculated to other areas in the hospital but is going directly to the outside.
In this system, the WAG is moved passively from the APL valve to a duct system that removes the gas through a vacuum or other device that draws the WAG to the outside. This system requires the use of an atmospheric interface that allows the system to draw room air and prevent the system from actively removing gas from the APL, which results in the collapse of the rebreathing bag (Fig. 8.6).
These interfaces are different depending upon the evacuation system in use. The interface has a connection to receive the 19 mm tube from the APL and another connection to receive a tube that removes the gas to the evacuation device, which may be a vacuum pump or an enclosed fan that is vented to the outside. If a vacuum pump is used, there must be a way to adjust the vacuum so the atmospheric interface will function properly.
Troubleshooting before a Procedure
The assessment of the machine includes the oxygen supply system, the flowmeter, the flush valve, the breathing system, the WAG removal system, and all hose and tubing connections on the machine.
The O2 regulator on the tank should be set at 40–60 pounds per square inch (psi), and all connections should be free of leaks. A large leak can usually be heard and should be corrected to prevent loss of oxygen. Small leaks can be detected by applying a solution containing surfactant and watching for bubbles. Many older machines have solid plumbing from the O2supply connection to the flowmeter. It is difficult to assess the presence of leaks in that situation, but if a leak is discovered it may require the attention of a service representative. The connections to the flush valve should be secure and free from leaks.
The breathing system
This portion of the machine needs to be assessed very carefully because any leaks present will likely include anesthetic vapor. This part of the machine is most susceptible to problems, but they are usually easy to correct. The process of discovering these problems utilizes visual and auditory inspection and a pressure test. The assessment should focus on the one-way valves, sodalime canister, breathing bag and circuit, and WAG connections and tubing.
Valves that can be dissembled will have either a rubber O-ring or gasket that forms a seal between the dome and the valve body. If this O-ring or gasket is torn, deformed or missing, it should be replaced. The plastic dome should be examined for chips or cracks and if any are found, the dome should be replaced. The threaded rings that hold the dome against the O-ring or gasket can be easily cross-threaded when assembling the one-way valve. When the ring becomes difficult to turn, the operator thinks it is tight and stops tightening the ring. This results in failure to seal the dome and can be a major leak in the system. If enough force is applied, the threads can be permanently damaged and new parts are required.
On some machines, it is possible to impinge the one-way disc between the dome and the valve body in such a way that it remains open all the time or it fails to open when the patient breathes. The one-way valves can be tested by using a surgeon’s mask or by curling one’s hand around the Y-piece, breathing into the circuit, and observing that the one-way valves move in the proper sequence.
There are many different styles of canisters, but functionally there are only two types. If the gases go only one way through the canister, the canister will be open on top and will have a perforated bottom with a gasket or O-ring that seals both the top and bottom surfaces of the canister. If the canister contains a baffle or a return tube in the center of the canister so that gas comes in the top and then flows to the bottom, around the baffle, and back to the top, there will be a seal only on the top surface. These two types of canisters can be described either as one-way or two-way canisters.
Since the one-way canister has a seal on both the top and bottom surfaces there is a greater potential for leaks. Soda sorb granules and dust can accumulate on the lower gasket or sealing surface and prevent the canister from forming a tight seal. This is one of the most common causes of leaks in an anesthetic circuit, and care must be taken to assure that all surfaces are clean and free of soda sorb dust and granules.
The two-way canister has less potential for leaks because it seals only at the top of the canister. However, since there is usually a tube down the center of the canister, the tube should be covered or plugged when being filled to prevent spilling of granules into the tube. Failure to remove the plug before attaching the canister to the machine will result in very high resistance to breathing. Some two-way canisters that have a baffle in the center have a threaded insert that fastens the canister to the machine. This insert must be covered when filling the canister to prevent soda sorb from getting into the threads. It is not necessary to fill soda sorb canisters to the very top. This can result in granules falling off or rubbing against the surface of the absorber gasket as the canister is placed into position on the absorber assembly. A 1/2–3/4 inch space should be left at the top of the canister.
Breathing bags and circuit
The major area of concern is where the tubes and bag connect to the machine. The stress created by repeatedly connecting and disconnecting the parts will cause breaks or cracks that will eventually cause leaks. These are usually visible by careful inspection prior to using the machine. To discover a hole in the bag, it will be necessary to do a pressure test so that the bag becomes distended and the hole is visible or audible. A hole in the breathing tubes can also be discovered during the pressure test, but it sometimes requires the use of a leak test solution to disclose the hole.
WAG tubing and connections
The most common problem is a poor connection to the APL valve on the machine. All connections should be made with the correct tubing size and with the proper adapters. The evacuation tubing should also be inspected for holes or tears and replaced if necessary. If the tubing lies on the floor and the anesthetic machine is moved frequently, the wheels may run over the tubing causing damage. One of the most common problems with the WAG tubing is the presence of very small holes, probably caused by cats that play with the tubing. In this case the tubing should be replaced at least once a year.
Many of these problems can be discovered by visual examination. Components that are broken, installed improperly, or disconnected can be discovered if the technician is familiar with the machine and understands the flow of fresh and breathed gases, as well as the correct placement and orientation of all components.
Performing a Pressure Test
A pressure test is performed by attaching the breathing circuit and bag, closing the pop-off valve, and occluding the patient end of the breathing circuit. Pressurize the circuit to 30 cm H2O, setting the flowmeter to 200 cc/minute, and observe the manometer for decreasing pressure. If the pressure holds or rises very slowly there is no leak, but if the pressure drops there is a leak greater than 200 cc/minute and it should be isolated.
To isolate the leak by auditory inspection, pressurize the circuit to 60 cm H2O and set the flowmeter to maintain the pressure. Put one ear close to any point on the circuit where there is a joint or connection and listen for the leak. It is possible that the leak cannot be discovered in this manner because either the leak does not create any sound or it is in a place that is difficult to get close enough to hear.
The leak can also be isolated by using a solution-containing surfactant. It is sprayed on the machine at any point where a leak is possible. In the presence of a leak the solution will bubble. Sometimes there is a delay of several seconds between the time the solution makes contact and the appearance of bubbles. Once the leak is discovered the problem can be corrected.
At this point the vaporizer can be tested for external leaks. Once the pressure is holding and stabilized, turn the vaporizer dial to 0.5%. This will pressurize the vaporization chamber and will create a slight drop in pressure on the manometer. The pressure should stabilize again. If the pressure, however, continues to drop, this is an indication of a leak in the vaporizer. This leak most likely will be in the fill spout and is easily corrected by cleaning or replacing the O-ring on the fill spout cap.
Following the pressure test of the breathing circuit, the evacuation tubing should also be tested for leaks. To check 22 mm tubing simply connect each end of the tubing to the inspiratory and expiratory valve connections and then pressurize the circuit to 30 cm H2O as before. If the tubing is 19 mm, appropriate adapters can be used to connect the tubing in the same way as the 22 mm tubing; then perform the test. Before testing the evacuation tubing, verify that the breathing circuit does not have leaks.
Troubleshooting during a Procedure
Sometimes problems may not be discovered before a procedure and are discovered during the procedure by observing the patient. Therefore it is necessary to ask several relevant questions related to the patient and potential problems with the machine, and not problems related to technique—i.e., improper intubation.
Frequently it is difficult to keep patients anesthetized. There are several possible explanations for this. The most obvious is that there is a leak somewhere in the breathing circuit that is allowing room air to enter and dilute the concentration of anesthesia.
There may also be a one-way valve that is not functioning properly and is allowing the patient to rebreathe expired gases, resulting in lower anesthetic concentration and higher carbon dioxide levels. This is a persistent problem in some machines that have one-way valves that are vertically oriented and become warped due to the effect of gravity. The valve, therefore, doesn’t close properly. Occasionally, horizontal one-way valves will become warped or curled and will not seat properly (Fig 8.7). This problem must be discovered visually unless inspired CO2 levels are monitored.