Chapter 4 Anesthetic Equipment

Transcription

1 Before the introduction of anesthetic machines, administration of inhalant anesthetics was a hazardous undertaking. Up until well into the twentieth century, liquid anesthetics such as ether or chloroform were administered using open systems ( open cone, open drop, or chamber). The development of modern anesthetic equipment greatly increased the safety and effectiveness of inhalation anesthesia by allowing the administration of precise concentrations of anesthetic and oxygen under controlled conditions. This chapter describes the purpose, function, use, and maintenance of equipment used to administer inhalant anesthetics including machines, vaporizers, breathing circuits, endotracheal tubes, masks, and chambers. As anesthetic accidents and complications are frequently associated with machine and equipment malfunctions and misuse, a comprehensive knowledge of and familiarity with this equipment is essential to the anesthetist s ability to deliver anesthetic gases safely. 1

2 Placement of an endotracheal tube bypasses the oral and nasal cavities, pharynx, and larynx.

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4 Figures A, C, and D are Murphy tubes; Figure B is a Cole tube. Endotracheal tube type, material, and size comparison. A, Cuffed 11-mm, silicone Murphy tube. B, 2.5-mm Cole tube. C, Cuffed 8-mm polyvinyl chloride (PVC) Murphy tube. D, Cuffed 4-mm red rubber Murphy tube. E, Uncuffed 2 mm PVC Murphy tube.

5 Figure 4-2 shows the variety of materials. Figure 4-3 F shows scale. Table 8-2 lists common sizes used in common species.

6 Endotracheal tube parts. A, Valve with syringe attached. B, Pilot balloon. C, Machine end. D, Connector. E, Tie. F, Measurement of length from the patient end (cm). G, Measurement of internal diameter (mm). H, Inflated cuff. I, Patient end. J, Murphy eye. 6

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8 Endotracheal tube parts. A, Valve with syringe attached. B, Pilot balloon. C, Machine end. D, Connector. E, Tie. F, Measurement of length from the patient end (cm). G, Measurement of internal diameter (mm). H, Inflated cuff. I, Patient end. J, Murphy eye. During intubation of mammals, use of a cuffed tube should be used whenever possible for the following reasons: 1.The inflated cuff helps prevent leakage of air and gases around the tube and therefore reduces waste gas pollution in the operating room. 2.Use of a cuffed tube minimizes the risk of aspiration of blood, saliva, vomitus, and other material into the lungs. 3.Cuffed tubes prevent the animal from breathing room air, which may otherwise flow around the outside of the tube and dilute anesthetic gases. Adequate depth of anesthesia is difficult to maintain in animals breathing significant amounts of room air.

9 The placement and management of ET tubes is reviewed in detail in Chapters 9, 10, and 11; however, the following points should be noted: ET tubes should not bind during placement. When correctly used, ET tubes reduce dead space ( Special cautions must be observed when ET tubes are used in conjunction with laser surgery. TECHNICIAN NOTE The ET tube should be no longer than the distance between the most rostral aspect of the mouth and the thoracic inlet. If longer, there is a risk that only one lung will be inflated with oxygen and anesthetic gas or that mechanical dead space will be increased, leading to hypoxemia. 9

10 The size 5 blade for a small animal is large; the size 0 blade is small.

11 Laryngoscope handles and blades. A, Size 4 Miller blade. B, Size 4 McIntosh blade. C, Size 2 Miller blade. D, Size 1 McIntosh blade. E, Laryngoscope handle with size 00 Miller blade in unlocked position. F, Laryngoscope handle with size 3 McIntosh blade in locked position (note that the light turns on when the blade is locked).

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13 The v-gel (docsinnovent) is a veterinary-specific SAD that, at the time of this writing, is available in several sizes for cats and rabbits. The v-gel has an enlarged patient end designed to create a seal around the opening of the larynx and concurrently seal the opening of the esophagus to prevent gastric reflux. The manufacturer states that each v-gel tube can be used 40 times and can be safely sterilized in an autoclave between uses. Detailed information about this device can be found on the manufacturer s website ( Although not in common use, SADs offer an alternative to conventional endotracheal tubes in species such as the cat and rabbit, in which endotracheal intubation is challenging and often results in a higher incidence of complications. 13

14 Masks can be used on a fully conscious patient, a sedated patient, or an anesthetized patient that cannot be intubated. Without an endotracheal tube, brachycephalic breeds are prone to tracheal obstruction during anesthesia. Procedure 9-4 describes mask induction.

15 Anesthetic masks. Note the good fit around the patient s muzzle to minimize leakage. 15

16 Chamber induction is described in Procedure 8-5.

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18 Anesthetic machines come in a wide variety of brands, ages, sizes, and sophistication. New machines can cost as little as $2000 to as much as more than $100,000. Underlying similar machine design and principles of operation, is an inherent complexity that must be understood. Comparison of large animal and small animal anesthetic machines. A, Large animal machine with precision isoflurane and sevoflurane vaporizers, a built-in ventilator, and a 30-L rebreathing bag. B, Small animal machine with an isoflurane precision vaporizer and a 5-L rebreathing bag. This picture illustrates differences in size and sophistication among anesthetic machines.

19 Anesthetic machines can also be used to deliver pure oxygen to a critically ill patient. The vaporizer is turned off for this procedure.

20 The most common configuration of an anesthetic machine is one with a precision vaporizer and a rebreathing circuit. Patients under kg use a machine with a precision vaporizer and a nonrebreathing circuit.

21 The anesthetic machine consists of four distinct systems that can best be understood by following the flow of gases from the oxygen source to the vaporizer, through the breathing circuit to the patient, and finally to the scavenging system. Schematic of an anesthetic machine without a common gas outlet, configured with a vaporizer-out-of-circuit (VOC) precision vaporizer. Both rebreathing and non-rebreathing circuits are illustrated. 21

22 Schematic of an anesthetic machine with a common gas outlet, configured with a vaporizer-out-of-circuit (VOC) precision vaporizer. Both rebreathing and nonrebreathing circuits are illustrated. 22

23 The compressed gas supply supplies carrier gases (oxygen and sometimes nitrous oxide) The anesthetic vaporizer vaporizes liquid inhalant anesthetic and mixes it with the carrier gases. Vaporizers are classified as precision or nonprecision, and vaporizer-out-of-circuit (VOC) or vaporizer-in-circuit (VIC) The breathing circuit conveys the carrier gases and inhalant anesthetic to the patient and removes exhaled carbon dioxide. Breathing circuits are classified as rebreathing circuits or non-rebreathing circuits The scavenging system disposes of excess and waste anesthetic gases. Anesthetic machine systems. A, Compressed gas supply: Note the two size E compressed gas oxygen cylinders beside the A s at the bottom of this image. B, Precision anesthetic vaporizer. C, Breathing circuit. Note that the scavenging system (see Figure 5-1) is not visible in this view. 23

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25 In some instances H tanks are used as the primary supply and E tanks are used as a secondary supply. In other cases, E tanks are used for both primary and secondary supply. H tanks are usually stored remotely with tubes connecting them to oxygen supply outlets throughout the hospital/clinic.

26 Compressed gas cylinders may be either small (E tanks) or large (H tanks). E tanks are attached directly to the yoke of an anesthetic machine or may be stored on a cart when not in use. 26

27 H tanks, which are considerably larger than E tanks, are usually stored on a cart or chained to the wall and attached to a machine remotely by a system of intermediate-pressure gas lines that carry gas to quick-release connectors throughout the hospital. Gas lines may take the form of flexible hose, or gas may be carried in pipes mounted within the ceiling or wall. Quick-release connectors join the gas line from the compressed supply to the machine. In many hospitals, H tanks are the primary source of oxygen and E tanks are used as a backup supply. In locations where piped-in gas is unavailable, E tanks are used as both the primary and the secondary supplies. Bank of H tanks used as a primary oxygen supply for the entire hospital via intermediate-pressure gas lines hidden in the ceiling. Ceiling drop with diameter-index safety system (DISS) outlets for oxygen (green), nitrous oxide (blue), medical air (yellow), and vacuum (white). B, Quick-release DISS connectors used to couple intermediate-pressure gas lines to the anesthetic machine. Note that the diameter of each outlet port type is unique to prevent attachment of the wrong connector. 27

28 All compressed gas cylinders have a valve on the top to control the flow of gas. The valve on an H tank has a threaded outlet port through which the gas flows. In contrast, the valve on an E tank has three holes one outlet port, and two pin index safety system holes. Size H compressed gas cylinder with two-stage pressure regulator. A, Threaded outlet port. B, Pressure-reducing valve. C, Tank pressure gauge. D, Line pressure gauge. E, Knurled knob. A, Parts of a size E compressed gas cylinder and yoke. A, Yoke. B, Wing nut. C, Outlet valve. D, Outlet port. E, Pin index safety system holes. F, Nipple of yoke. G, Index pins. H, Nylon washer. B, Opening and closing the outlet valve; loosening and tightening the wing nut.

29 Oxygen flows from the outlet port of the valve when the stem is turned in a counterclockwise direction (i.e., to the left). The stem should be turned slowly until it is fully open. H-tank valve stems often have a knurled knob, that is turned by hand. E-tank valve stems are opened and closed with a special wrench or via a lever that can be turned by hand. 29

30 Oxygen pressure remaining in the intermediate-pressure lines after the valve is closed (called line pressure) should be released by depressing the oxygen flush valve or by turning the flow meter to a high rate of flow until all the gas is vented. This process is referred to as purging the system. Failure to evacuate line pressure may give the anesthetist the false impression that the oxygen is turned on when in fact it is not. 30

31 TECHNICIAN NOTE When handling compressed gas cylinders: Avoid contact with flames, sparks, or other sources of ignition. Turn the tank on only when it is attached to a yoke or pressure regulator. Store tanks only attached to a yoke, secured in a cart designed for this purpose, or chained to the wall. Never attempt to attach a tank to a yoke that does not fit, and never tamper with the safety system on a tank, line, or pressure-reducing valve. 31

32 This figure shows the different yoke attachment holes on the two tanks that will prevent the wrong type of gas cylinder from accidentally being attached to a specific yoke. Outlet port. B, Pin-index safety system holes. Note that the pin holes on the carbon dioxide tank (left) are farther apart than the pin holes on the oxygen tank (right). The unique pin hole position for each gas prevents a tank containing the wrong gas from being attached to the yoke.

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34 A bulk oxygen tank. Used in some large hospitals as a primary oxygen source in place of compressed gas cylinders. JorVet NuVo 10 Oxygen Concentrator. (Photo courtesy Jorgensen Laboratories, Inc.) 34

35 TECHNICIAN NOTE The volume in liters (L) of oxygen present in a compressed gas cylinder can be calculated by multiplying the pressure (in psi) in an E tank by 0.3 or by multiplying the pressure in an H tank by 3. The volume in liters (L) of oxygen present in an E tank can be calculated by multiplying the pressure (in psi) by 0.3 (see Table 4-1). For example, a full E tank, at a pressure of 2200 psi, contains about 660 L of oxygen (i.e., psi). A reading of 1100 psi indicates the tank is approximately half full and therefore contains approximately 330 L of oxygen. The volume in liters for the larger H tank can be calculated by multiplying the pressure (in psi) by 3. For example, a full H tank at a pressure of 2200 psi contains approximately 6600 L of oxygen (i.e., psi). The volume of the oxygen in the tank indicates how much longer the tank can be used. For example, if the anesthetist selects an oxygen flow rate of 1 L per minute (L/min), a full E tank containing 660 L of oxygen will last approximately 11 hours (i.e., 660 minutes), whereas at a flow rate of 2 L/min the same full tank will last approximately 5.5 hours (i.e., 330 minutes). In contrast, a full H tank at a flow rate of 1 L/min contains enough oxygen to last 110 hours (6600 minutes). 35

36 Tanks should be marked full, in service, or empty to indicate their status, and a full backup tank must always be kept on the machine as a spare in case the primary tank runs out. The anesthetist may notice a considerable drop in pressure during a lengthy anesthetic procedure. If not detected in time, oxygen flow to the patient will cease, and this will put the patient in danger because most veterinary anesthetic machines have no alarm to warn of inadequate flow. Therefore the anesthetist must periodically monitor the oxygen tank pressure gauge during each procedure and change the tank when the gauge indicates that the tank is close to empty. Label for compressed gas tanks. Current status of tank is shown by the wording at the bottom of the label. Label shown is from newly acquired tank and reads FULL. When the tank is first opened, the lower portion of the label is removed so that the remaining label reads IN SERVICE. When the tank is empty, the IN SERVICE stub is removed, leaving the label that reads EMPTY. 36

37 See Figure 4-18 B and 4-24 C. The pressure-reducing valve functions passively and therefore requires no action on the part of the anesthetist. The line pressure gauge (if present) should be checked before every procedure, however, to verify correct pressure in the intermediate pressure gas lines (40 to 50 psi).

38 Not all machines will have a line pressure gauge. See Figure 4-18 D and 4-24 A

39 Some machines will have two flowmeters for oxygen. One provides a coarse adjustment (>1 L/min) and the other provides a fine adjustment (<1 L/min). At this point there is still no anesthetic agent in the oxygen.

40 If the meter reads zero the patient is not receiving any oxygen. A, Oxygen flow meters with ball indicators. The fine adjustment flow meter on the left is adjusted to 0.5 L/min, and the coarse adjustment flow meter on the right is adjusted to 1.5 L/min for a total oxygen flow of 2 L/min. B, Single oxygen flow meter with both coarse and fine adjustment, adjusted to a flow rate of 2.0 L/min. Inset: Note that below 1 L/min, the scale is marked in 0.2 L increments. At flows higher than 1 L/min, the scale is marked in 0.5 L increments. C, Flow meter with bobbin type indicator adjusted to 2 L/min.

41 TECHNICIAN NOTE When turning off a flow meter, turn clockwise just until the ball or bobbin drops to 0 L/min. Even though the knob can still be turned, do not turn it any further to the right or you will damage the valve! 41

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45 As previously mentioned, the oxygen flush valve should be pressed briefly using only short bursts. Overfilling the bag has the potential of damaging the patient s lungs because of a buildup of pressure. Some anesthetic machines are configured such that the oxygen flush valve discharges into the common gas outlet instead of the rebreathing circuit. In these cases the oxygen flush valve must not be used with a non-rebreathing system attached because a high flow rate of oxygen into this type of circuit can seriously damage the animal s lungs. 45

46 No anesthetic agent can leave the vaporizer unless it is mixed with the carrier gas. If the flowmeter is not turned on, no oxygen will flow and no anesthetic will be delivered to the patient.

47 Precision anesthetic vaporizer for isoflurane set on 2%. A, Inlet port with keyed fitting leading from the flow meters. B, Outlet port with keyed fitting leading to the fresh gas inlet or common gas outlet. C, Safety lock. D, Indicator window. E, Fill port. F, Oxygen flush valve (part of the compressed gas supply). 47

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49 Although nonprecision vaporizers can be used to deliver isoflurane or sevoflurane, as long as specific procedures are carefully followed, this practice is uncommon. In contrast, precision vaporizers are used to deliver all commonly used liquid anesthetics including isoflurane, sevoflurane, and desflurane, and are the main type the technician is likely to encounter in clinical practice. The commonly used liquid anesthetics (isoflurane, sevoflurane, and desflurane) are classified as high vapor pressure liquids. The designation high vapor pressure means that these anesthetics readily evaporate and may reach concentrations of 30% or greater within the anesthetic circuit if the amount of vapor being delivered to the breathing circuit is not controlled. Because the maximum useful concentration for isoflurane and sevoflurane is 5% and 8%, respectively, uncontrolled delivery of anesthetic vapor will result in excessively high levels that could be dangerous for the patient. It is therefore necessary to use a precision vaporizer to deliver these agents, affording the anesthetist more exact control of the anesthetic concentration in the circuit. Precision anesthetic vaporizer for isoflurane set on 2%. A, Inlet port with keyed fitting leading from the flow meters. B, Outlet port with keyed fitting leading to the fresh gas inlet or common gas outlet. C, Safety lock. D, Indicator window. E, Fill port. F, Oxygen flush valve (part of the compressed gas supply). 49

50 Where the vaporizer is placed is determined by the resistance of gas flow through the vaporizer. Gas flow around the breathing circuit is driven by the normal breathing force of the patient.

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53 Most modern precision vaporizers are temperature, flow, and pressure compensated.

54 Start with the lower end of the range if the patient has been premedicated. Each precision vaporizer is designed to be used with a specific inhalant anesthetic such as sevoflurane, isoflurane, halothane, or desflurane and is colorcoded (purple, isoflurane; yellow, sevoflurane; red, halothane; and blue, desflurane). The vaporizer is turned on by depressing the safety lock and turning the dial to the desired level. The safety lock is present as a lever on some vaporizers and is incorporated into the dial on others (Figure 4-32). Inhalant anesthetic levels are measured in percent concentration. For common domestic species, the induction rate of isoflurane is approximately 3% to 5%. The maintenance rate is about 1.5% to 2.5%. The induction rate of sevoflurane is approximately 4% to 6%, and the maintenance rate is about 2.5% to 4%. The induction rate of desflurane is approximately 10% to 15%. The maintenance rate is about 8% to 12%. Patients should receive rates in the lower end of these ranges if preanesthetic medications have been administered. The dial of a precision vaporizer (see Figure 4-30) is graduated in percent concentration (e.g., 1%, 2%). Throughout any anesthetic procedure, the anesthetist must control the amount of inhalant anesthetic delivered to the patient by periodically adjusting the vaporizer dial. Appropriate and safe use of the vaporizer requires knowledge, experience, and detailed observation. Specifics regarding safe use of the vaporizer are discussed in detail in Chapters 9, 10, and 11.

55 Vaporizers must be checked before each procedure to make sure that enough anesthetic remains in the vaporization chamber. Most vaporizers have an indicator window at their base that allows the technician to inspect the amount of liquid anesthetic remaining. In order for the vaporizer to function properly, the liquid anesthetic level must be between the upper and lower lines of the window (Figure 4-33). The vaporizer should be refilled as needed but should be kept at least half full at all times. Overfilling a vaporizer will result in anesthetic overdose, and underfilling will result in an inability to keep the patient anesthetized. 54

56 Each precision vaporizer is designed to be used with a specific inhalant anesthetic such as sevoflurane, isoflurane, halothane, or desflurane and is colorcoded (purple, isoflurane; yellow, sevoflurane; red, halothane; and blue, desflurane). The vaporizer is turned on by depressing the safety lock and turning the dial to the desired level. For common domestic species, the induction rate of isoflurane is approximately 3% to 5%. The maintenance rate is about 1.5% to 2.5%. The induction rate of sevoflurane is approximately 4% to 6%, and the maintenance rate is about 2.5% to 4%. The induction rate of desflurane is approximately 10% to 15%. The maintenance rate is about 8% to 12%. Patients should receive rates in the lower end of these ranges if preanesthetic medications have been administered. The dial of a precision vaporizer is graduated in percent concentration (e.g., 1%, 2%). Throughout any anesthetic procedure, the anesthetist must control the amount of inhalant anesthetic delivered to the patient by periodically adjusting the vaporizer dial. Vaporizers must be checked before each procedure to make sure that enough anesthetic remains in the vaporization chamber. Most vaporizers have an indicator window at their base that allows the technician to inspect the amount of liquid anesthetic remaining. In order for the vaporizer to function properly, the liquid anesthetic level must be between the upper and lower lines of the window. The vaporizer should be refilled as needed but should be kept at 55

57 least half full at all times. Overfilling a vaporizer will result in anesthetic overdose, and underfilling will result in an inability to keep the patient anesthetized 55

58 In order for the vaporizer to function properly, the liquid anesthetic level must be between the upper and lower lines of the window. The vaporizer should be refilled as needed but should be kept at least half full at all times. Overfilling a vaporizer will result in anesthetic overdose, and underfilling will result in an inability to keep the patient anesthetized. TECHNICIAN NOTE Vaporizers must be checked before each procedure to make sure the liquid anesthetic level is between the upper and lower lines of the indicator window. The vaporizer should be refilled as needed but should be kept at least half full at all times. The indicator window of an anesthetic vaporizer. The level of the liquid anesthetic must be kept between the upper and lower lines 56

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61 The vaporizer outlet port is connected to the common gas outlet or directly to the breathing circuit by a hose with a male keyed connector. This keyed connector prevents the operator from inadvertently attaching the inlet hose to the outlet port. It is important to check that this fitting is securely attached to the vaporizer before the commencement of any procedure. 59

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