Anesthesia Machine and Breathing Systems Flashcards
Basic Machine Schematic
Oxygen cylinder Cylinder pressure gauge pressure reduction valve flowmeter vaporizer fresh gas inlet breathing circuit patient
Compressed gas
Oxygen- absolutely necessary
delivering anesthetic gas in air (21% O2) would lead to hypoxemia due to hypoventilation and V/Q mismatch induced by anesthetics themselves
30-35% O2 minimum acceptable for people and small animals
Metabolic requirement for oxygen
5-10 mL/kg/min
50-100 mL/min in a 10 kg dog
Minimum O2 flow required
Other medical gases
Nitrous oxide
Medical air
Oxygen sources
Cylinder
Liquid (cryogenic oxygen)
Oxygen concentratior
E cylinder
usually single, attached directly to anesthesia machine via yoke
Most common in small animal general practice
Capacity = 660 L
filled to pressure of 2200 psi
boyles law may be used to determine the remaining gas in the tank
H cylinder
Often in banks, supply for central O2
Capacity= 6600 L
filled to pressure of 2200 psi
boyles law may be used to determine the remaining gas in the tank
Compressed gas
Cylinders are color coded Oxygen= green Nitrous oxide = blue Medical air = yellow Other safety mechanisms in place to prevent delivering wrong gas
Tank safety
never leave an unsecured tank sitting upright
E in rack/rolling cage
H anchored to wall or in transport cart with chain
May explode if dropped or falls over-can become projectile
To avoid fire (heat created as gas expands)- clean oils from hands/tank
open valve slowly
open and close valve before attaching to machine to remove dust from connecting port
Pressure gauges
Used to measure cylinder pressures, pipeline pressures, anesthetic machine working pressures, and pressure within breathing system
Cylinder pressure usually in psi
breathing system pressure in cm H2O
Calculate remaining O2
2200/660 = psi left on tank/x L
Minutes = x liters/flow L/min
N2O cylinders
N2O exists in both a gaseous and liquid form in tank - gauge only reads gas pressure
Therefore it is not possible to calculate the amount of gas remaining based upon pressure if liquid N2O remains
Safety systems
Color coded tanks labelling diameter index safety sistem pin index safety system quick connectors 1
Diameter index safety system
non-interchangeable gas-specific threaded connection system
used universally by all equipment and cylinder manufacturers
Pin index safety system
Gas-specific pin patterns that only allow connections between the appropriate cylinder tokes and E tanks
Commonly found on tokes mounted to anesthesia machines, also some cylinder specific regulators/flowmeters
Quick connectors
Manufacturer specific
Facilitate rapid connecting and disconnecting of gas hoses
useful for multipurpose work areas
Regulator
Pressure reducing valve
Decreases tank pressure to a safe working pressure which is supplied to the flowmeter
Prevents pressure fluctuations as the tank empties
Flowmeter
Controls rate of gas flow through the vaporizer (L/min)
Gas enters at bottom at 50 psi and exits at top at 15 psi
Tapered glass tube with moveable float- narrow at bottom, wider at top
Single or double taper- double=more accuracy at lower flow
Calibrated for 760 mmHg and 20C
Reduces gas pressure form 50 psi to 15 psi
gas specific!
if there are multiple flowmeters, O2 should be on the far right (downstream) to prevent delivery of a hypoxic gas mixture
Floats
Can be ball or bobbil
Where do you read flow
Middle of ball
Top of bobbin
Quick flush
Delivers O2 from the intermediate pressure area of the machine
Bypasses vaporizer- contains NO anesthetic agent
Delivers gas at rate between 35-75 L/min directly to patient circuit
Appropriate use: quickly decrease anesthetic gas % in the circuit- emergency, recovery
This is pure O2 that has bypassed the vaporizer
Quick flush complication
pneumothorax
small circuit
high pressure
small patient
Anesthetic vaporizers
change liquid anesthetic into vapor
deliver selected % of anesthetic vapor to the fresh (common) gas outlet
-volumes percent
Inhalants- vapor
gaseous state of substance that is liquid at ambient temp and pressure
Halothane, isoflurane, sevoflurane, desflurane
Inhalants-gas
exists in gaseous state at ambient T and P
N2O, Xenon
Vapor pressure
Pressure exerted by vapor molecules when liquid and vapor phases are in equilibrium
Depends on temperature- increases with increasing temperature
Inversely related to boiling point
Saturated vapor pressure
Vapors have a maximum administration percentage
vapor pressure/barometric pressure
ex: iso 32%
vaporizers needed to reduce this to clinically useful doses
Modern vaporizers
agent specific concentration calibrated variable bypass flow over out-of-circuit high resistance compensated for temperature, flow, and back pressure
Anesthetic vaporizers
a specific concentration is created by variable bypass system, where fresh gas flows over a reservoir of liquid anesthetic and mixes with carrier gas
VOC vaporizers-precision
all modern vaporizers are out of circuit (VOC)
carrier gas is from flowmeter
anesthetic % is known = precision vaporizer
VIC vaporizers- non-precision
in the past, vaporizers were in the circuit (VIC)- non precision Glass jar containing wicking material increase surface area for vaporization ensures saturation with anesthetic gas Variable bypass
Carrier gas is patients expired gases
cannot produce a known anesthetic %
not temperature compensated
not currently recommended
Modern vaporizers compensate for
temperature between 15-35 C
flow rate between 0.5 and 10 L/min
Back pressure associated with positive pressure ventilation and use of flush valve
Temperature compensation
Achieved by using materials that are efficient heat conductors
also mechanical thermocompensation
alters the amount of carrier gas directed through the bypass and vaporizing chambers
has a thermal element made of a heat-sensitive metal that reliably expands and contracts based on temperature
Flow rate compensation
achieved by ensuring saturation of gas moving through vaporizing chamber
use of wicks, baffles, and spiral tracks that facilitate vaporization
Back pressure
Can occur during positive pressure ventilation or use of flush valve
may increase vaporizer output if compensation mechanisms not present
modern vaporizers use various mechanisms to prevent this from happening
Vaporizer styles
ohmeda tec 5
drager vapor
penlon sigma
Desflurane vaporizer
Boiling point (23.5C) is close to room temperature
Electric heated vaporizer required
-desflurane maintained in gaseous form
-blends with fresh O2 to achieve vaporizer setting
Common in human med bit not vet bc more expensive
Vaporizers
filled using screw cap port or agent-specific keyed filler port
prevents filling with wrong agent
Require no external power (except desflurane)
Routine maintenance is required and must be performed by a qualified technician
Mounted on a back bar on the machine
cannot be tipped- must be emptied before transporting
What would happen if filled with wrong agent
Depends on vapor pressure and potency of each agent
iso in sevo vaporizer could produce lethal concentration (higher vapor pressure AND higher potency)
Drain and run 1L/min O2 until completely dry
Vaporizer tipped
anesthetic may enter the bypass channel and deliver a high concentration
Run 1L/min O2 through machine with vaporizer off
Common gas outlet
where gas exits the vaporizer
connected by a hose to the fresh gas inlet
hose must be connected so that fresh gas flows to the breathing circuit
connects to either rebreathing or non-rebreathing system
Re-breathing system
rebreathing expired gases after gone through co2 scavanger
Circle
Universal F
Non-rebreathing system
Mapleson A-F
most common= bain (modified mapleson D), mapleson F
Dead space
anatomic/mechanical
breathing tubes in a circle system do not constitute dead space because the flow is unidirectional (no rebreathing)
this is why breathing tubes may be very long without increasing dead space
dead space in non-rebreathing system consists of the space between the fresh gas flow inlet and the patient - differs depending on mapleson type
Anatomic dead space
airway structures that do not participate in gas exchange
oral cavity, larynx, trachea, bronchi
Mechanical dead space
the portion of the anesthesia circuit where bidirectional flow is occuring (rebreathing of exhaled gases)
if excessive, this may cause an unsafe increase in inspired CO2
face mask
endotracheal tube extending past patients incisors (outside of mouth)
capnograph or other adapters
Y piece
Rebreathing system components
fresh gas input and O2 flush
unidirectional valves (inspiratory and expiratory)
Breathing hoses (circle or universal F)
CO2 absorber
Adjustable pressure limiting valve (pop-off)
reservoir bag
Rebreathing system
one way gas flow (circular)
inspiratory and expiratory breathing limbs
rebreathing is prevented by inspiratory ad expiratory valves
CO2 absorber removes CO2 from expired gases
patient rebreathes gases via the inspiratory limb
Composed of exhaled gases after CO2 removal and fresh gas flow
Rebreathing system advantages
lower fresh gas flow rate required
patient breaths warm, humidified gases (re breaths)
saves money
decreases environmental pollution
Rebreathing system disadvantages
higher resistance to breathing due to valves
changes in anesthetic gas concentration occur slowly d/t lower fresh gas flow
more components -> more potential for leaks
Rebreathing system
One way or unidirectional valves- inspiratory; expiratory
O2 flush valve- bypasses vaporizer, dilutes anesthetic gas in breathing system and reservoir bag, delivers O2 directly to the breathing system at high pressure and flow 35-75 L/min O2
Disconnect patient from circuit before activating to avoid barotrauma
Fresh gas inlet
shared connection between rebreathing and non rebreathing systems
Rebreathing system- cont
adjustable pressure limiting valve or pop-off
limits pressure build up on breathing system
should pop off at 305 cm H2O
should always be open unless- checking machine for leaks before use, administering positive pressure ventilation (manual or mechanical)
Closed APL valve –> increases pressure in breathing system as fresh gas flow continues into circuit with no exhaust -> cardiorespiratory arrest and death
breathing circuit pressure gauge
should be 0 +/- 1 with spontaneous patient breathing
exception- leak check, positive pressure ventilation
re breathing system carbon dioxide absorber
soda lime most commonly used
assembly contains canister to hold soda lime, 2 ports for connecting breathing tubes, fresh gas inlet, +/- unidirectional valve and bag mounts
soda line is calcium hydroxide with small amount of sodium hydroxide
also contains ethyl violet which changes color from white to purple when granules are exhausted
heat and water is produced from reaction between CO2 and soda lime
color change will be seen when active- this does not mean that the absorbent is exhausted
When filling canister do not pack tightly
may cause leaks if present on gaskets- check when machine has an unidentified leak
Signs of CO2 absorbent exhaustion
inspired CO2 is >1-2 mmHg on capnograph (=rebreathing), increased PaCO2 on blood gas
Patient signs
increased RR (attempting to compensate for increased inspired CO2)
increased HR and BP (CO2 -> sympathetic stimulation)
Red mucous membranes (due to CO2 induced vasodilation
Reservoir bag
functions: inspiratory reserve for patient
administering positive pressure ventilation
allows anesthetist to monitor ventilation
Calculation of bag size for small animals
tidal volume (~15 mL/kg) x 6
round up
For horses usually 30L or 20L ventilator capacity
Rebreathing system- oxygen flow rates
many different flow rates can be used
all are safe for patient as long as greater than metabolic O2 requirement (5-10 mL/kg/min)
chosen based on goals and practicality (flow meter is a limitation)
Typical O2 flow rates
Small animals (<50kg) induction and recovery 50-100 mL/kg/min maintenance 20-50 mL/kg/min
Large animals
induction and recovery 20-50 mL/kg/min
maintenance 10-20 mL/kg/min
Non-rebreathing system- components
Fresh gas
non-rebreathing tubes
APL valve (bain) or open/close valve (mapleson F)
Reservoir bag
NO soda lime canister, unidirectional valves
Non-rebreathing system- advantages
light, minimal dead space, minimal resistance to ventilation (use for small patients <3kg)
concentration of anesthetic gas changes rapidly due to high fresh gas flow and small circuit volume
fewer components = fewer potential for leaks
Non-rebreathing system- disadvantages
requires high gas flow rates
patient breaths cold and dry gas d/t lack of rebreathing
more expensive
increases environmental pollution
Non rebreathing system oxygen flow rates
must be high as this is the mechanism for preventing rebreathing of CO2
should be 2-3x minute ventilation
~300 ml/kg/min
Endotrachial tubes and intubation
maintain patient airway
administer O2, deliver inhalant anesthetics
provide positive pressure ventilation
protect airway from foreign material (regurgitation, other fluids and solids)
apply tracheal or bronchial suction
Other: decreases environmental contamination with volatiles anesthetics if cuff is properly inflated
Routes of intubation
Oral
Nasal
Tracheal
Pharyngotomy
Types of ETT
PVC, rubber, silicone Cuffed/uncuffed Murphy Cole Wire-reinforced
Cuffed ETT
protects airway and environment, but may not be indicated for certain patients (v small, birds)
Cuff must be inflated carefully to avoid tracheal trauma
Cuff types
high volume- low pressure (preferred for tracheal protection)
high pressure-low volume
Murphy ETT
can be cuffed or uncuffed
has a murphy eye that allows gas flow if end of tube is obstructed
most common ETT in veterinary anesthesia
Pilot balloon and valve
size marker in mm (ID=internal diameter)
cm marks to determine length of tube in patient
Cole ETT
uncuffed
used commonly in avian patients, has a shoulder that seals against the glottis
Wire reinforced (armored) ETT
used to prevent collapse of tube lumen when patients are placed in extreme flexion (usually ophtho procedures)
cannot use for MRI (contains metal)
ETT sizes
tubes with larger radius and shorter length will have less resistance to gas flow
Radius has the larges effect (poiseuille’s law)
Preparing to intubate
Determine size you think +/- 1 size (based on weight, breed, species, trachial palpation)
inflate cuff to check for leaks
ensure ETT is clean and dry
Cuff syringe
Tube tie (tie around tube, then above muzzle or behind ears)
+/- special supplies (stylet, mouth gag, capnograph, etc)
Laryngoscope
miller/macintosh
Laryngoscope
Makes intubation safer and easier
allows visualization of airway
apply light pressure to base of tongue, rostral to epiglottis
do not place the blade of it on epiglottis- could cause damage
ETT cuff inflation
procedure requires 2 people
connect patient to circuit with O2 flowing
close the adjustable pressure limiting (APL) valve using the safety system (push rather than screw closed)
squeeze the reservoir bag to a total pressure of 20 cm H2O
listen at the patients mouth for ai escaping the trachea. If you do not hear a leak at the initial squeeze then no air is needed in the cuff
If a leak is heard at 20 cm H2O add air to the ETT cuff just until no leak is heard
Open the APL valve
ETT cuff hints
do not inflate cuff without first checking to see if there is a leak
exception ruminants- air should be added before any movement d/t high risk of regurgitation
Caution when moving patients with inflated ETT cuff
Disconnect from circuit before moving
Tracheal tears are not uncommon in cats due tp moving patient with cuff inflated and breathing tubes connected
Complications of intubation- laryngeal damage
laryngospasm, inflammation, edema, hemorrhage
Complications of intubation- tracheal damage
over-inflated cuff
moving or twisting patient with inflated cuff
may lead to mucosal damage, tracheal rupture (-> SQ emphysema, pneumomediastinum, etc), persistent tracheal membrane (avian)
Complications of intubation- ETT obstruction
secretions (mucus most common), cuff over inflation
Complications of intubation- endobronchial intubation
ETT to far into airway
must measure tube at time of intubation, ETT should not extend past thoracic inlet
leads to hypoxemia +/- hypercapnia
Complications of intubation- ETT inhalation or ingestion
if patient chews tube (usually upon recovery)
do not wait to long to extubate
scavenging waste gases
necessary due to detrimental effects of excess waste gas (volatile anesthetics and N2O) on personnel
reproducing effect most serious
also headaches, nausea
involves collecting and transporting waste gases from the anesthetic machine to a safe disposal area
active or passive
Waste gases
Exposure to volatile anesthetic agents should be < 2 ppm
100%=1mil ppm
1%= 10k ppm
must be >125 ppm to smell it `
Minimize exposure
scavenge at all times- keep patient attached to machine during recovery so they are not breathing agent into room air ensure that the machine has no leaks- leak test before use use ETT with properly inflated cuff avoid mask or chamber inductions check for tight fittings use low O2 flows maintain appropriate room ventilation use keyed systems for filling vaporizers
scavenging waste gases- passive
no vacuum
exhaust directly to atmosphere (via window or hole in wall)
F air canister
absorbs halogenated agents (anesthetic vapors)
does not scavenge N2O
scavenging waste gases- active
piped vacuum (white quick connector)
most common
central vacuum capable of handling high flows
F air canister- advantages
absorbs anesthetic vapors
does not release to atmosphere
portable
F air canister- disadvantages
does not absorb N2O
flow limited
added resistance
Must be discarded when anister has gained 50g
Other courses of pollution
capnograph (removes sample from breathing circuit) needs to be scavenged
Face mask and chamber inductions
Recovery areas
esp large animals
horses exhale a lot of volatile agent in recovery stall
Volatile agent spills
clean immediately, place contaminated material in a well ventilated area for disposal
