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The ins and outs of the anesthesia machine (Proceedings)


The components of an anesthesia machine work together to deliver controlled amounts of oxygen and anesthetic gas.

The anesthesia machine

• Components work together to deliver controlled amounts of oxygen and anesthetic gas

Components of breathing system

• Everything after the common gas outlet

o Anesthetic tubing

o Rebreathing bag

o CO2 absorber

o Scavenge system

Functions of breathing system

• To deliver oxygen and anesthetic gas

• To remove CO2 from exhalation

• To facilitate controlled ventilation

Anesthetic systems/circuits

• Circle system or Rebreathing system

o Commonly used in patients greater than 7 kg

• Utilizes inspiratory and expiratory valves as well as CO2 canister

o Closed or partially closed system

• Also called low-flow anesthesia

• Refers to the oxygen flow rate and position of the pop-off valve

• Non-rebreathing system

o Most often used for patients less than 7 kg

The importance of oxygen

• Anesthesia machines are designed to deliver volatile anesthetics in the presence of oxygen: Oxygen is the carrier gas for anesthetic

o 100% oxygen is commonly used to deliver anesthetics

o This is necessary for the delivery of adequate amounts of O2 to tissues

• Anesthetics reduce tidal volume

• Anesthetics cause some degree of respiratory depression

• It is important to avoid hypoxemia

• Low arterial oxygen concentration

Oxygen cylinders

• Obtained as compressed gas

• Pressurization is necessary to fit a large amount into a relatively small container

• Sizes A-H available, most common are sizes E and H

• Full cylinders contain 2200 psi of gas with varying volumes based on size

o E-cylinders

• Small and easy to carry

• Holds ~ 770 L of oxygen

• Commonly found attached to an anesthetic machine

o H-cylinders

• Huge and heavy

• Holds ~ 7000 L of oxygen

• Commonly found in a closet or chained to a wall

Other medical gases

• All medical gas cylinders are color coded for safety

o Oxygen is green

o Nitrous oxide is blue

o Air is yellow

o Nitrogen is black

• These colors are true in the US only

Pressure regulator

• Provides a safe, constant operating pressure within the machine regardless of the pressure in the tank


• Typically expressed in L/min

• Oxygen enters the bottom of the flowmeter and exits the top

• Allows the anesthetist to adjust the O2 flow rate

• The flow of oxygen can be increased to speed the change of inhalant concentration in the machine

Time constants (TC)

• A time constant represents the volume of the machine in relation to the flow of gases

• It takes approximately 3 time constants to see a 95% change in the concentration within the system

• Machine volume (components of breathing system)

o Sodasord canister ~ 1 L

o Rebreathing bag- depends on size

o Breathing hoses- depends on length and diameter

o Arbitrary example volume 5 L

• Math

o 3 TC X 5 L / 1L/min = 15 min With a flowrate of 1 L/min it takes 15 min to approach a steady state

o 3 TC X 5L / 2L/min = 7.5 min With a flowrate of 2 L/min it takes 7.5 min to approach a steady state

• Factors affecting changes in anesthetic concentration

o Hypoventilation and apnea

• By assisting ventilation you facilitate gas exchange

• This includes oxygen as well as anesthetic


o Increases in flowrates should always be monitored closely

o An increase in system flow also means an increase in flow to patient

• Can be very drying to respiratory tract

• Can cool patients very quickly

o Never close the pop-off valve when you have higher flows

• Pressure builds much quicker so you have less time to react!

o High flows waste anesthetic gas and oxygen and money!


• Convert liquid anesthetic to gas based on vapor pressure

• Adds controlled amounts of anesthetic gas to O2

• Can only release anesthetic in the presence of oxygen (or N2O)

• Each vaporizer is calibrated for a specific agent based on that agents vapor pressure

• Never fill a vaporizer with a different anesthetic

Oxygen flush valve

• Allows fresh oxygen to by-pass vaporizer and enter breathing circuit

• Delivered directly at 30-50 L/min

• Handy but dangerous

• Over pressurization is easy

Flush valve CAUTiON

• Use with extreme care or not at all when the circuit is connected to a patient

• NEVER use with a non-rebreathing system

• NEVER use when pop-off is closed

• DO NOT use in small patients

• The use of this valve decreases anesthetic concentration in the system

Fresh gas outlet and common gas inlet

• Gas mixture (O2 and anesthetic) exits vaporizer and machine at the fresh gas outlet

• The mixture follows a flexible tube and enters the breathing system at the fresh gas inlet

o This is necessary to hook up a bain non-rebreathing circuit

o These hoses are internal on some machines

Flutter valves and Y-pieces

• Fresh gas enters the one-way inspiratory valve on its way to the patient and then is exhaled through the one-way expiratory valve

• The Y-piece helps facilitate the one-way gas flow

• Only present in the circle system

• Allows for re-breathing of gases

Re-breathing bags

• Should hold a minimum of 60 ml/kg of the patients body weight

o 5-6 X tidal volume (10-15 mL/kg)

o Always round up

• Allows patient to take a large breath

• Allows anesthetist to observe breathing

• Allows anesthetist to breathe for the patient

• Size of the bag does matter

o If bag is too big

• Impairs monitoring of breathing rate

• Adds volume to machine

• Slows changes in inspiratory anesthetic concentrations when settings are altered

o If bag is too small

• Animal is unable to take an adequate breath

• Bag collapses on inspiration


• Transports anesthetic gas from machine to patient

• At least two different diameter hoses available for small animals

o 22 mm for larger dogs

o 15 mm for small dogs and cats

• Different lengths also available

Endotracheal tubes

• Many are one-time use only in humans

o Cuffs tend to wear out over time

• All ET tubes should be checked for leaks prior to use

o Make sure cuff inflates

o Can place inflated tube in a bowl of water and look for leaks/bubbles

o Discards any leaky tubes

Pop-off valve/APL

• Prevents build-up of pressure or volume within the circuit

• Vents pressure at 2 cm of water

• Pressure build-up may impede venous return or cause barotrauma

o The distention and possible rupture of alveoli from excessive pressure which may lead to pneumothorax and/or subQ emphysema

• Always keep a hand on the pop-off valve (when you close it) to make sure that it gets re-opened

Carbon dioxide absorber

• Contains absorbent granules that remove CO2 from gas

• Any gas that gets returned to the patient passes through here on its way back

CO2 absorber perks

• It lowers fresh gas flows

• Reduces waste of anesthetic and oxygen

• Lowers the overall cost of anesthesia

• Allows for re-breathing of gases

Carbon dioxide absorbent

• Calcium hydroxide or barium hydroxide

• Exothermic reaction

o Produces both heat and water

• Must be changed when exhausted

o Won't absorb CO2 if spent

o May lead to patient hypercarbia if spent

o Has a pH color indicator to tell when to change it

• White to lavender or blue

• Short-lived chemical reaction; will change back to white even if spent

o Fresh granules can be easily chipped and crumbled

o Saturated granules are hard and brittle


• Measures the pressure of gases within the anesthetic circuit and patient

o Useful when breathing for a patient or setting up a ventilator

• Pressure should not exceed 15-20 cm H2O or 10-14 mmHg


• Collects waste gas and disposes of it

o Passive disposal

• Non-circulating ventilation systems

• Activated charcoal

o Active disposal

Non-rebreathing systems

• No exhaled gas is returned to patient

o Adequate oxygen flows required to prevent rebreathing of gases

o 250-500 ml/kg/min

• Evacuated by scavenge based on O2 flows

• No CO2 absorber

• When to use depends on patient size

• Many types, two common are both modified Mapleson D

o Ayers T

o Bain

Circle system Vs. Non-rebreathing system

• Circle system

o Low flows

o Rebreating of gases

o More economical

o Recycled air is warmer and humidified than fresh gas

o May be cumbersome

o More resistance to breathing for small patients

• Non-rebreathing system

o Little resistance to breathing

o Does not require a CO2 absorber

o Allows inspired concentrations of anesthetic to be changed rapidly

o Promotes hypothermia and drying of respiratory tract

o Can be wasteful if used in larger patients (high flows)

Mechanical dead space

• The area where bi-directional flow takes place

o The endotracheal tube

o Anything between the Y-piece and the ET tube

• CO2 monitor adapter, swivel adapter, elbow

• Excessive pieces should be kept to a minimum in small patients

o Hose length does not contribute to dead space but can increase the resistance to breathing

Troubleshooting machine leaks

• Areas prone to leaks

o Around the sodasorb canister, around valve caps, re-breathing bag, hoses, any joint that gas passes through

• If you suspect a leak...

o Go through and tighten all joints and connections

o Leak check again

• If still leaking keep O2 on and spray a dilute soapy liquid around all joints in the breathing circuit

o The gas sneaking out of the system will cause bubble to form so keep your eyes open for them!


Dorsch JA, & Dorsch SE. Understanding Anesthesia Equipment. Baltimore: Williams & Wilkins, 1999

Hartsfield, SM. Anesthetic Machines and Breathing Systems. In: Lumb & Jones Veterinary Anesthesia and Analgesia. Iowa: Blackwell, 2007. pg. 453-494

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