ANESTHETIC
MACHINES
The primary function of any anesthetic
machine is to deliver a precise amount of
oxygen and volatile anesthetic under
controlled conditions to patients
undergoing general anesthesia.
ANESTHETIC MACHINE
1) Oxygen is be delivered at a controlled rate
2) A liquid anesthetic is converted to a gas,
then mixed with oxygen, and delivered to the
patient
3) Gases exhaled from the patient are disposed
of OR re-circulated (after removing the CO2)
REMEMBER: You can also use an anesthesia
machine to deliver oxygen to a patient in
distress. DON’T turn the vaporizer on! You
do not want to anesthetize these patients!
COMPONENTS OF THE
ANESTHESIA SETUP
Can be broken down into 4 parts:
Compressed gas supply

Gas tanks, their pressure gauges, and pressure reducing
valves (may not be visible)
Anesthetic machine

Flowmeter, vaporizer
Breathing circuit

Unidirectional valves, hoses, reservoir bag, pop-off valve,
CO2 canister, O2 flush, pressure manometer, negative
pressure relief valve
Scavenging System

Disposes of excess and waste anesthetic gases
PART 1
Taking a trip with oxygen
GAS CYLINDER
Carrier gas
Gas that is compressed in a metal cylinder and
held under pressure
Available in various sizes

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“E” holds 660 L of oxygen and is attached to the
anesthesia machine
“H” cylinders hold 6900 L of oxygen and stand
separate from the machine
Tanks are delivered and picked up by the
oxygen supply company
Gas Cylinder
Tanks are color coded for safety and recognition
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Oxygen tanks: GREEN (U.S.), white (Canada)
Nitrous oxide tanks: blue (U.S.)
Carbon dioxide tanks: gray (U.S.)
Oxygen cannot flow out of the tank unless the
outlet valve has been turned to the left. (Righty
tighty, lefty loosey)
Small cylinders attach to the anesthetic
machine via a YOKE. Large cylinders are
attached via hoses or pipes. Gas lines may also
be used to bring in the gas from another room.
GAS CYLINDERS
In addition to color-coded tanks, the yokes are
gas-specific. A nitrous oxide tank cannot be
attached to an oxygen yoke. The pin indexing
system that is used to attach it is also
specific to its specific gas.
A full tank of oxygen has a pressure of 18002600 psi. The volume in liters can be
determined by multiplying the psi by 0.3 for an E
tank and 1.7 for an H tank.

0.3 x psi on gauge
L/min to be delivered

This formula will allow you to know how long your tank will last
TANK PRESSURE GAUGE
Display of the pressure of oxygen in the tank.
Reads zero when it is empty, when tank is turned off,
and all gas has been removed from the machine
Actual amount in tank is displayed when the tank is
turned on
Check this gauge BEFORE an anesthetic procedure
(especially the lengthy ones).
TANK PRESSURE GAUGE
Change when the pressure is no lower
than 100 psi (some clinics will require
tanks to be changed much earlier). Refill
line is at 500 psi.
NITROUS OXIDE
A full E cylinder contains 760 psi.
Nitrous oxide is present in liquid and gas
forms in the tank. When the tank is turned on,
liquid evaporates into a gas as other gas
leaves the tank. The pressure of the tank
doesn’t change because of the constant
replacement of the gas until all liquid has been
volitalized. The gauge will not drop until almost
empty.
Anesthetist should change the tank as soon as
500 psi is reached.
PRESSURE REDUCING VALVE
Regulates the pressure of the gas leaving the
tank and going into the anesthesia machine
Allows a constant flow of gas into the machine,
despite pressure changes within the tank
Reduces the pressure of oxygen that leaves
the tank at 2200 psi to a safer 50 psi.
In cases of small tanks and gas lines, the line
pressure is preset at 50 psi. Only the tank
pressure gauge is visible.
PART 2
Now that the gas is in the machine, it’s job is to
mix with the anesthetic and be delivered to the
patient
FLOW METER
Allows the flow rate of oxygen traveling
through the machine to be adjusted by the
anesthetist. Oxygen does not reach the
patient unless this is turned on (neither does
the anesthetic gas.
Calculated in Liters per minute
Separate flow meters for each gas

These knobs can be distinguished from one another
FLOWMETER CONT’D
The dial of the flowmeter is turned on and
either a ball or rotor rises to the selected
flow rate.

Read at the center of the ball or top of the
rotor
Gas travels through the flowmeter
VAPORIZER
Converts the liquid anesthetic agent
(usually isofluorane or sevofluorane) into
a vapor.
Adds controlled amount of these vapors
to the carrier gas
Next stop as oxygen travels from the
flowmeter
The flowmeter must be on to deliver
anesthesia to the patient as it needs to
be mixed with a carrier gas.
VAPORIZER
Similarly, the vaporizer must be on to deliver
any anesthesia to the patient. Otherwise, only
the carrier gas will be delivered
The mixture of the anesthetic gas and carrier
gas is known as FRESH GAS.
Once mixed, fresh gas cannot return to the
vaporizer.
Use the correct anesthetic with the correct
vaporizer! (Don’t put sevo in the iso machine)
VAPORIZER
Amount of anesthetic liquid left in
vaporizer is visible in the indicator
window. Check BEFORE your surgery to
see if it needs refilling!
If for some reason the vaporizer
is tipped over (usually the
whole machine), turn the
vaporizer off and run oxygen
only through the machine for
15 minutes to flush it out.
PRECISION VS. NONPRECISION
VAPORIZERS
Precision vaporizers delivers a precise,
controlled amount of anesthetic to the patient
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Expressed as a % which is chosen based on an
anesthetic’s MAC and the patient’s requirements
$
Commonly used anesthetics can reach
concentrations as high as 30% + if they are not
controlled
PRECISION VS. NONPRECISION
VAPORIZERS
Nonprecision vaporizers are simple,
cheaper, and are typically used for
anesthetics with low vapor pressure such
as methoxyfluorane
PART 3
BREATHING CIRCUIT
The system that brings the fresh gas
from the vaporizer to the patient and
takes the expired gases from the
patient
May contain unidirectional valves,
reservoir bag, pop off valve, CO2 canister,
O2 flush, negative pressure relief valve,
pressure manometer.
UNIDIRECTIONAL VALVES
One-way valves that allow the flow of fresh gas to
enter the inhalation valve and exit the exhalation
valve.
Valve is either a rigid disk or a flap that flutters as gas
flows past it
Inhalation valve opens as patient inhales, anesthetic
enters the hose, then the endotracheal tube and the
patient
CO2 and anesthetic gases are then exhaled, travel
down the hose and through the unidirectional
exhalation valve. This valve prevents the expired
gases from traveling back to the patient before the
CO2 is removed
RESERVOIR BAG
Also called a rebreathing bag
Fills as gases enter the circuit or patient
exhales, deflates as patient inhales
FUNCTIONS
1) Stores gas
2) Helps in determining correct endotracheal
tube placement. Movement of bag with
breaths = tube in trachea
3) Allows assessment of respiratory rate and
depth
4) Allows “bagging” of the patient
- reverse atelectasis if present
-removal of CO2 and anesthetic that builds
up when respirations have decreased in
volume
-assist or control ventilation – esp if in
respiratory arrest
Volume of bag should be minimally
60mL/kg (round up) See pg 119 for
reference chart
Sizes range from ½ L to 30 L (1,2,3L
commonly used on small animals)
SIGNS THAT YOUR RESERVOIR
BAG IS NOT THE RIGHT SIZE:
Overinflates rapidly if too small. If
overinflated, animal will have problems
exhaling
Bags should consistently be ~ ¾ full
Very little movement of bag with breaths if
too big
POP-OFF VALVE
AKA pressure relief valve
Allows excess gas to leave the breathing
circuit and be scavenged
Prevents the build-up of excess gas or
pressure within the circuit. If the pressure
were allowed to build up (forgot to open
pop off valve), the alveoli in the lungs
could rupture
KEEP the pop-off valve OPEN unless
you are bagging the patient (or if you
have a low-flow/closed system)
POP OFF VALVE
In some instances, the degree that the pop
off valve is opened changes with the flow
rate and how full the reservoir bag is
CO2 ABSORBER
Gases that don’t exit the machine via the pop off valve go
into the CO2 canister and are then returned to the patient
CO2 canister usually contains soda lime that removes CO2
from other gases breathed out.
“Exhaused” soda lime granules no longer absorb CO2.
HOW DO I KNOW WHEN THE GRANULES ARE EXHAUSTED?
Color change to violet, off-white or pink depending on the
brand. Based on pH.
CO2 saturated granules are hard and brittle, new ones can be
chipped and crumbled
Once color becomes abnormal, it is possible that it changes
back to normal within hours
CO2 CANISTER WITH SODA
LIME
O2 flush
When activated, O2 bypasses the
flowmeter and vaporizer and enters the
circuit
High flow rate (up to 75L/min!)
Never use with a non-rebreathing system
Can use to fill the bag, help a critical
patient, dilute the anesthetic
PRESSURE
MANOMETER/GAUGE
Measures pressures of gas within the
circuit in cm H20
Eyes on this while bagging your patient
DO NOT GO ABOVE 20 CM H20!
NEGATIVE PRESSURE RELIEF
VALVE
When an active scavenging system is
utilized, if negative pressure is detected in
the circuit, this valve opens and allows
room air in.
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Particularly in instances when there is
excessive suction
Also in instances where the O2 flow rate is too
low or the tank runs out of oxygen
Better for the patient to breathe room air than no
air