Anesthetic Machines and Breathing Systems



Anesthetic Machines and Breathing Systems




Overview


A variety of equipment is used to deliver inhalant anesthetic drugs. Volatilizing inhalant anesthetics from their liquid form and safely delivering precise concentrations to the animal while minimizing environmental pollution requires the use of relatively sophisticated, expensive, and, at times, cumbersome devices. Regardless of their apparent complexity, most inhalant anesthetic drug-delivery systems use similar, simple designs to deliver oxygen (O2) and anesthetic concentrations and remove carbon dioxide (CO2) and waste gases.





Anesthetic Equipment




I Compressed gases: O2, N2O, and other select gases come in color-coded cylinders of varying size (Tables 12-1 and 12-2). O2 is used as the carrier gas for delivering inhalant anesthetics in most anesthetic circuits





A Cylinders should be handled carefully



B Most veterinary and human anesthetic machines have a hanger yoke for attaching one or more E cylinders; hanger yokes and E cylinders are keyed (coded) with a pin index safety system to prevent inadvertent connection of the wrong cylinder to the wrong gas yoke on the anesthetic machine (Fig. 12-1)


image
FIG. 12-1 The pin index system ensures that the appropriate tank is selected and properly secured to the anesthetic machine.

C Centralized O2 sources usually use G or H cylinders



D O2 and N2O



E Oxygen generators can be installed for central gas supply; they extract O2 from the environment (air: 20% O2) to produce 90% to 95% O2



II Hanger yoke site for E cylinder (see Fig. 12-1)



III Pressure-reducing valves (pressure regulators) are built into most anesthetic machines or can be attached directly to the cylinder



IV O2 “fail-safe” system



V Flowmeters



A A flowmeter controls the rate at which a specific gas (e.g., O2) is delivered (milliliter per minute, liter per minute). The most common gas flowmeter is the rotameter.



B Ideally, the O2 flowmeter should be the last in a series of flowmeters (a hypoxic gas mixture is less likely to develop if a flowmeter tube is cracked)


C Avoid excessive torque (twisting) when closing flowmeters, because the knobs can come to be out of adjustment or be twisted off


D Flowmeter tubes are gas-specific; an N2O flow tube cannot be substituted for an O2 tube


E Individual gas flows are combined downstream from the flowmeters; from there, gases move to an out-of-circuit vaporizer or directly to the anesthetic circuit


F Some machines are equipped with two O2 flowmeters



VI O2 flush valve



VII Common gas outlet: the point-of-gas-exit from the machine for O2, N2O, and vaporized (out-of-circuit) anesthetic gas. It has a 15-mm connector.


VIII Anesthetic vaporizers



A Vaporizers are designed to volatilize liquid inhalant anesthetic drugs and to deliver clinically useful concentrations of anesthetic vapor


B Anesthetic machine vaporizers are located near the flowmeters and can be either out (vaporizer out of the circuit [VOC]) or in (vaporizer in the circuit [VIC]) the anesthetic breathing circuit



1. Precision VOC vaporizers deliver precise anesthetic concentrations (%) of anesthetic vapor that are relatively independent of temperature and fresh gas flow rate; the manufacturer specifies limits of temperature and flow


2. Gas flow within the vaporizer is split between a bypass and vaporization chamber



3. Precision VOCs are agent-specific



4. VICs (also called draw-over vaporizers): the concentration delivered is temperature and flow dependent



a. Draw-over vaporizers are not agent-specific


b. Output depends on the following:



c. Flow-through vaporizer



d. Vaporizer construction



e. Location of vaporizer in the breathing circuit



f. Calibration marks on top of the vaporizer do not indicate the percentage output of VIC vaporizers


g. Types



h. Maintenance: wick should be allowed to dry weekly to rid system of excess water vapor


C General comments




Anesthetic Breathing Systems




I Purpose



II Types of systems



A Open drop or cone system (Fig. 12-2)


image
FIG. 12-2 Open system: the animal is exposed to or breathes room air. Gauze or cotton is saturated with inhalant anesthetic and placed in a cone or chamber from which the animal breathes.


B Nonrebreathing valveless systems (Mapleson classification) rely on relatively high, fresh gas flow rates (Table 12-3) to remove CO2. The following classifications are based on the location of fresh gas inlet and opening (or valve) for exit of exhaled gas. The relative location of the fresh gas inlet and the valve opening determine the efficiency of the circuit.


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Sep 6, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on Anesthetic Machines and Breathing Systems

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