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Anesthetic Equipment

The purpose, function, use, and maintenance of machines and equipment used to administer

inhalation anesthetics

Chapter 4

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Endotracheal Tubes (ET Tubes)

Flexible tube placed in the trachea Delivers anesthetic gases directly from the

anesthetic machine to the lungs Advantages

Open airway Less anatomical dead space Precision administration of anesthetic agent Prevents pulmonary aspiration Responds to respiratory emergencies Monitors respirations

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Types of Endotracheal Tubes

Murphy tubes Beveled end and side holes Possible cuff

Cole tubes No side hole or cuff Abrupt decrease in diameter of the tube Used in birds and reptiles

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Types of Endotracheal Tubes (Cont’d)

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Properties of Endotracheal Tubes

Materials Polyvinyl chloride: clear and stiffer Red rubber: flexible and less traumatic, absorbent, and may

kink or collapse Silicone: pliable, strong, less irritating, resist collapse

Length Standard lengths Scale marks distance from patient end (centimeters)

Size Measured by internal diameter (ID) Range from 1 mm to 30 mm

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Parts of the Endotracheal Tube

Patient end Machine end Connector Cuff Pilot balloon and valve

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Parts of the Endotracheal Tube (Cont’d)

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Laryngoscope

Used to increase the visibility of the larynx while placing an ET tube

Parts Handle containing batteries Blade to depress tongue and epiglottis Light source to illuminate the throat

Sizes Small animal 0 to 5; large animal up to 18-inch blade

Types Miller blades McIntosh blades

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Laryngoscopes (Cont’d)

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Masks

Cone-shaped devices used to administer oxygen and anesthetic gases to nonintubated patients

Used for induction and maintenance of anesthesia in very small animals

Plastic or rubber Variety of diameters and lengths Rubber gasket

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Anesthetic Mask

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Anesthetic Chambers

Clear, aquarium-like boxes used to induce general anesthesia

Used in feral, vicious, or intractable animals to reduce stress

Acrylic or Perspex Removable top with two ports Cannot monitor patient closely

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Anesthetic Chamber (Cont’d)

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Anesthetic Machines

Used to deliver precise amounts of oxygen and volatile anesthetic under controlled conditions

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Principles of Operation of Anesthetic Machines

Carrier gas: oxygen or nitrous oxide Liquid inhalant anesthetic: to be vaporized Mixed gases delivered to patient Exhaled gases removed from patient:

scavenging system or recirculated

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Components of the Anesthetic Machine

Compressed gas supply Anesthetic vaporizer (precision or

nonprecision; VOC or VIC) Breathing circuit (rebreathing or

nonrebreathing) Scavenging system

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Components of the Anesthetic Machine (Cont’d)

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Components of the Anesthetic Machine (Cont’d)

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Compressed Gas Supply

Oxygen Used to increase inspired air to at least 30% oxygen Level necessary to maintain cellular metabolism under

anesthesia Used to carry vaporized anesthetic to patient

Cylinders (tanks) Contain large volume of gas under high pressure E tanks (small), attached directly to anesthetic machine H tanks (large), attached remotely to anesthetic machine

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Compressed Gas Supply (Cont’d)

Control valve (outlet port) Located on top of the tank Left loose (open), right tight (closed)

Pressure-reducing valve Reduces outgoing pressure to a usable level

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Compressed Gas Cylinders

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Size H Compressed Gas Cylinder

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Safety Issues with Compressed Gas

Combustibility Yoke attachment High-pressure release Storage Color coding

Oxygen: green (United States) or white (Canada and Europe)

Nitrous oxide: blue Medical air: yellow (United States) or white and

black (Canada and Europe) Carbon dioxide: gray

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Carbon Dioxide and Oxygen Tanks

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Tank Pressure Gauge

Indicates the pressure of gas remaining in a compressed gas cylinder Measured in pounds per square inch (psi) (United

States) or kilopascals (kPa) (Canada and Europe) Determine the number of liters remaining in a

tank Label tanks: full, in service, or empty Keep backup full tank on the machine

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Labeling Cylinders

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Pressure-Reducing Valve (Pressure Regulator)

Reduces gas pressure to a constant 40-50 psi (275-345 kPa)

Color coded

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Line Pressure Gauge

Indicates pressure in the gas line between the pressure-reducing valve and flowmeter

Should read 40-50 psi after the oxygen tank is opened

After turning the tank off, use the oxygen flush valve to evacuate line pressure until the gauge reads 0 psi.

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Flowmeter

Indicates gas flow expressed in liters per minute (L/min)

Reduces pressure of gas to 15 psi (~100 kPa)

Specific for each type of gas Flow rate is controlled by anesthetist

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Flowmeters (Cont’d)

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Oxygen Flush Valve

Delivers a short, large burst of pure oxygen directly into the rebreathing circuit or common gas outlet

Bypasses vaporizer and flowmeter Used to refill breathing bag, to deliver pure

oxygen to a patient, or to dilute the anesthetic gas remaining in the circuit at the end of anesthesia

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Vaporizer Inlet Port

Where carrier gas (usually oxygen) enters a vaporizer from the flowmeter

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Anesthetic Vaporizer

Converts liquid anesthetic agent to a gaseous state

Adds a controlled amount of vaporized agent to the carrier gas

Gas mixture leaves vaporizer through the outlet port

Mixture is known as fresh gas and enters the breathing circuit

Variable-bypass, flow-over vaporizers

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Types of Anesthetic Vaporizers

Nonprecision vaporizer Used to deliver low vapor pressure anesthetics Rarely used

Precision vaporizers Used to deliver a precise amount of anesthetic to

the patient Expressed as a percent of total gases leaving the

vaporizer Used to deliver high-vapor pressure anesthetics Anesthetist controlled

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VOC vs. VIC Vaporizers

VOC = Vaporizer-out-of-circuit Not localized within

the breathing circuit Oxygen from the

flowmeter enters the vaporizer prior to entering the breathing circuit

Precision vaporizers High resistance gas

flow

VIC = Vaporizer-in-circuit Oxygen enters the

breathing circuit from the flowmeter

Exhaled gases pass through the vaporizer

Nonprecision vaporizers

Low-resistance gas flow

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Factors That Affect Vaporizer Output

Vaporizer setting The primary determinant of output in both

compensated and noncompensated vaporizers Controlled by anesthetist

Carrier gas flow influences the concentration of anesthetic in breathing circuit in both compensated and noncompensated vaporizers

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Factors That Affect Vaporizer Output (Cont’d)

Factors that affect output of noncompensated vaporizers Temperature

• Ambient room temperature • Temperature of carrier gas

Carrier gas flow rate Respiratory rate and depth (nonprecision only) Back pressure

• Due to manual ventilation or activation of oxygen flush valve

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Use of Vaporizers

Specific-use vaporizers are color coded Isoflurane = purple Sevoflurane = yellow Halothane = red Desflurane = blue

Induction and maintenance rates Isoflurane = 3-5% induction; 1.5-2.5% maintenance Sevoflurane = 4-6% induction; 2-4.5% maintenance Desflurane = 10-15% induction; 8-12% maintenance

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Precision Vaporizer

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Safety with Vaporizers

Leakage Human exposure After using a non-rebreathing circuit, always

be sure to reattach the connector of the rebreathing circuit to the outlet port or common gas outlet

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Vaporizer Outlet Port and Common Gas Outlet

Vaporizer outlet port Oxygen/anesthetic exits the vaporizer Connected to the common gas outlet or directly

into the breathing circuit Common gas outlet

Fresh gas outlet Connected to the vaporizer outlet port and

breathing circuit

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Fresh Gas Inlet

Where carrier and anesthetic gases enter the breathing circuit

Connected to the vaporizer outlet port or common gas outlet

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Breathing Circuit

Carries anesthetic and oxygen from the fresh gas inlet to the patient

Conveys expired gases away from the patient Rebreathing or non-rebreathing

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Rebreathing System

Circle systems Used on all but very small animals Carbon dioxide removed from exhaled air Exhaled air is inhaled again with added

oxygen and anesthetic

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Rebreathing System (Cont’d)

Air flow: Inhalation unidirectional valve → Inhalation tube → Animal → Exhalation tube → Exhalation unidirectional valve → Carbon dioxide absorber canister → past reservoir bag → Pop-off valve → Pressure manometer → Inhalation unidirectional valve

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Rebreathing System (Cont’d)

Closed rebreathing system Total system Pop-off valve is nearly or completely closed and

oxygen flow is low Used mostly in large animal anesthesia

Semiclosed rebreathing system Partial system Pop-off valve is open and oxygen flow is high Excess air is released into scavenging system Most common configuration

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Breathing Systems

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Parts of a Rebreathing System

Unidirectional valves Reservoir bag Pop-off (pressure relief) valve Carbon dioxide absorber canister Air intake valve Pressure manometer Corrugated breathing tubes Y-piece

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Parts of a Rebreathing System (Cont’d)

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Unidirectional Valves

Control the direction of gas flow Inspiratory (inhalation) Expiratory (exhalation) Open and close as patient breathes Monitor respiratory rate and depth

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Pop-off Valve

Also known as the exhaust valve, adjustable pressure limiting valve, or overflow valve Allows excess carrier and anesthetic gases to exit

the breathing circuit and enter the scavenging system

Prevents excessive pressure or volume of gases in the circuit

Closed when manually ventilating a patient Controlled by anesthetist

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Pop-off Valve (Cont’d)

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Reservoir Bag (Rebreathing Bag)

Flexible air storage reservoir Indicator of respiratory rate and depth Confirms proper endotracheal tube

placement Allows delivery of anesthetic gases or pure

oxygen to patient Manual ventilation or “bagging”

Various sizes: 500 mL to 30 L Controlled by anesthetist

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Reservoir Bags

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Manual Ventilation (Bagging)

Minimize atelectasis Ventilate every 5-10 minutes

Force fresh gas into alveoli to normalize gas exchange

Normalize respiratory rate

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Carbon Dioxide Absorber Canister

Contains absorbent granules Primary absorbent ingredient: calcium hydroxide Also: water, sodium hydroxide, potassium

hydroxide, calcium chloride, calcium sulfate Granules react with carbon dioxide to form

calcium carbonate Heat and water produced Becomes more acidic with more use Granules must be replaced when depleted

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Carbon Dioxide Absorber Canister (Cont’d)

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Pressure Manometer

Indicates the pressure of gases within the breathing circuit Expressed as centimeters of water (cm H2O),

millimeters of mercury (mm Hg), or kPa Used when manually ventilating (bagging) the

patient to prevent excessive pressure in the lungs

Monitored by the anesthetist

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Pressure Manometer (Cont’d)

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Air Intake Valve

Negative pressure relief valve Admits room air into the circuit if negative

pressure is detected in the breathing circuit May be separate or incorporated into inspiratory

unidirectional valve or pop-off valve Negative pressure is indicated by a collapsed

reservoir bag Patient will develop hypoxemia

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Breathing Tubes and Y-Piece

Breathing tubes Corrugated breathing tubes or inspiratory and

expiratory breathing tubes Carry anesthetic gases to and from the patient Connected to unidirectional valve and Y-piece Three sizes: 50 mm, 22 mm, and 15 mm in

diameter Y-piece

Connects breathing tubes Connects to mask or endotracheal tube

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Breathing Tubes

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Non-rebreathing Systems

Semiopen system Used in very small patients (<2.5 kg) Little exhaled gas is returned to the patient Exhaled gas is evacuated by the scavenging system Fresh gas is routed to the patient directly from the vaporizer No carbon dioxide absorber canister, pressure manometer,

or unidirectional valves Several configurations are available

Components: Endotracheal tube connector, fresh gas inlet, reservoir bag, overflow valve, scavenger tube, and scavenger system

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Configurations of Nonrebreathing Circuits

Bain coaxial circuit (modified Mapleson D system)

Ayres T-Piece (Mapleson E system) Magill circuit (Mapleson A system) Lack circuit (modified Mapleson A system) Jackson-Rees circuit (Mapleson F system) Norman mask elbow (Mapleson F system)

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Operation of an Anesthetic Machine

Daily inspection Oxygen and liquid anesthetic levels Leaks Pop-off valve or overflow valve

Machine choice is based on patient body weight Small animal machine <150 kg Large animal machine 150 kg

Choose rebreathing system

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Choice of Breathing System

Primarily based on patient size Also based on

Cost Control of anesthetic depth Conservation of heat and moisture Production of waste gas

Choice of breathing system will determine Type of equipment required Position of pop-off valve Carrier gas flow rates

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Carrier Gas Flow Rates

Calculating gas flow rate Patient body weight Tidal volume (VT) 10 mL/kg/min Respiratory minute volume (RMV) = VT ×

respiratory rate (~20 bpm) Type of breathing system Expected period of anesthesia

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Mask or Chamber Induction Flow Rates

High flow rates required Mask: ~30 times VT for dogs, cats, neonate

large animals, pigs (1-5 L/min) Chamber: 5 L/min for small animals

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Flow Rates in a Semiclosed Rebreathing System

After induction with injectable agent: 50-100 mL/kg/min (SA machine) and 8-10 L/min (LA machine)

When making changes in anesthetic depth: 50-100 mL/kg/min (SA machine) and 8-10 L/min (LA machine)

During maintenance: 20-40 mL/kg/min (SA machine) and 3-5 L/min (LA machine)

During recovery: 50-100 mL/kg/min (SA machine) and 8-10 L/min (LA machine)

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Flow Rates in a Closed Rebreathing System

Normally used during maintenance only Oxygen flow must equal oxygen requirements

of the patient Minimum requirement = 5-10 mL/kg/min

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Safety Concerns with a Closed Rebreathing System

Carbon dioxide accumulation Increased pressure in anesthetic circuit

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Flow Rates in a Non-rebreathing System

Require high flow rates per unit body weight during all periods

Rates are based on patient body weight and Mapleson classification of circuit

Usually used on patients weighing <7 kg

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Care and Maintenance of Anesthetic Equipment

Compressed gas cylinders Inspected and maintained by company that owns

them Silicone or Teflon-based lubricants safe for difficult

tank valves Tank and line pressure gauges, pressure

manometer, and oxygen flush valve Require no regular maintenance

Pressure-reducing valve adjusted to 40-50 psi Flowmeters require no regular maintenance

Check accuracy occasionally

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Care and Maintenance of Anesthetic Equipment (Cont’d)

Vaporizer Serviced and maintained by manufacturer or

service professional Vaporizer inlet port, outlet port, common gas

outlet, and fresh gas inlet Check and replace hoses as necessary Routine low-pressure leak tests

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Care and Maintenance of Anesthetic Equipment (Cont’d)

Unidirectional valves Disassemble, clean, inspect Prevent water vapor, mucus, and dust buildup Check integrity of the valves

Pop-off valve Check for proper operation and adjust as

necessary Daily and during an anesthetic procedure

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Care and Maintenance of Anesthetic Equipment (Cont’d)

Reservoir bag, breathing tubes, and Y-piece Remove and clean after each procedure Prevents patient-to-patient transfer Hang to dry Check integrity of each part before use

Carbon dioxide absorber canister Change granules and clean canister as per

guidelines Wear gloves and a mask when handling granules Check integrity of each part before use

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Disinfecting Anesthetic Equipment

Endotracheal tubes, laryngoscope blades, face masks

To prevent spread of disease from patient to patient

Wash with disinfectant, rinse, dry, reassemble Check integrity of each part before use

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Disinfecting Anesthetic Equipment (Cont’d)

Disinfectants Chlorhexidine gluconate: not 100% effective Glutaraldehyde solutions (2%): short shelf life,

toxic, absorbed Ethylene oxide gas: special equipment needed,

toxic, absorbed Steam under pressure (autoclave): damages

rubber surfaces Discard damaged equipment