Unit 6 - Anesthesia Machine Flashcards
How does the vaporizer minimize the effect of cooling on vapor pressure and vaporizer output?
temperature compensating valve adjusts ratio of vaporizing chamber flow to bypass flow and guarantees a constant vaporizer output over a wide range of temperatures
How does the vaporizer minimize the effect of cooling on vapor pressure and vaporizer output?
temperature compensating valve adjusts ratio of vaporizing chamber flow to bypass flow and guarantees a constant vaporizer output over a wide range of temperatures
why will a febrile 70 kg adult become hypoxic with an O2 flow at 250 mL/min in a closed circuit system?
VO2 for avg adult is 250 mL/min
conditions like sepsis, fever, thyrotoxicosis increase VO2
2 immediate actions if O2 pipeline supply fails
- turn on O2 cylinder
- disconnect pipeline O2 supply
What 2 functions does drive gas on a pneumatic ventilator serve?
- compresses bellows
- opens and closes spill valve
position of ventilator spill valve during inspiration
closed
ensures that Vt goes to the patient and not to scavenger
minimum expiratory pressure needed to open ventilator spill valve
3 cm H2O
2 complications of overuse of O2 flush valve
barotrauma
awareness
where does the high pressure system begin and end
begins at the cylinder, ends at cylinder regulators
where does the intermediate pressure system begin and end
begins at pipeline, ends at flowmeter valves
components of high pressure system
- hanger yoke
- yoke block with check valves
- cylinder pressure gauge
- cylinder pressure regulators
components of intermediate pressure system
- pipeline inlets
- pressure gauges
- O2 pressure failure device
- O2 second stage regulator
- O2 flush valve
- ventilator power inlet
- flowmeter valves
where does low pressure system begin and end
begins at flowmeter tubes, ends at common gas outlet
components of low pressure system
- flowmeter tubes (Thorpe tubes)
- vaporizers
- check valve, common gas outlet
what does the low-pressure leak test assess
integrity of the low-pressure circuit from flowmeter valves to common gas outlet
how is low pressure leak test performed
by attaching a bulb to the CGO and creating negative pressure (-65 cm H2O)
If bulb reinflates within 10 seconds = fail, leak in system
when is the low pressure leak test performed
before 1st case of day
how to perform high-pressure leak test
Close APL, pressurizing circuit to 30 cm H2O
change in pressure reading = fail, leak
what does a high pressure leak test check in a machine with vs. without a check valve
- Machine with check valve: tests circuit & low-pressure system up to the check valve (does NOT assess for leak between check valve and rest of low-pressure system)
- Machine without check valve: test assesses circuit and entire low-pressure system
where is a check valve usually located
usually downstream from vaporizer, upstream from O2 flush
when is high pressure leak test performed
before the first case of the day
what is the SPDD model
Nomenclature describes location of each gas handling component
Supply, Processing, Delivery, Disposal
What is S in SPDD model & where is it located
- Supply
- How the gases enter the anesthesia machine
- Location: pipeline to back of anesthesia machine
what is P in SPDD model and where is it located
- processing
- How the anesthesia machine prepares gases before delivery to patient
- Location: inside machine up to common gas outlet
what is the first D in SPDD model and where is it located
- delivery
- How the prepared gases are brought to the patient
- Location: breathing circuit
what is the 2nd D in SPDD model and where is it located
- disposal
- How the gases are removed from the breathing circuit
- Location: scavenging system
5 tasks of O2
- O2 pressure failure alarm
- O2 presure failure devices (failsafe)
- O2 flowmeter
- O2 flush valve
- ventilator drive gas
what is the purpose of PISS
pin index safety system
Prevents inadvertent misconnections of gas cylinders
what may allow PISS to be bypassed
Presence of >1 washer between hanger yoke assembly and stem of tank
pin configuration of O2, air, and N2O
O2 = 2,5
air = 1,5
N2O = 3,5
what ensures that gas is preferentially pulled from pipeline if a cylinder is left open
O2 cylinder pressure 1,900 psi drops to ~45 psi upon entry into intermediate system
what is DISS and what is its purpose
diameter index safety system
Prevents inadvertent misconnections of gas hose
what is DISS and what is its purpose
diameter index safety system
Prevents inadvertent misconnections of gas hose
what is the only way to determine cylinder content
the LABEL, not the color
why should O2 cylinder on the back of the machine be OFF when not in use
If you lose pipeline pressure and O2 cylinder is left open, you will use up O2 supply before the failsafe alarm sounds
WHO tank colors
air = black and white
O2 = white
N2O = blue
how many L in O2 tank
660 L
O2 tank psi
1900
which is a liquid in the container: O2, air, N2O
N2O
max L in air cylinder
625 L
max psi of air cylinder
1900
max L in N2O cylinder
1590
max psi of N2O cylinder
745
weight of a N2O container full vs empty
full = 20.7 lbs
empty = 14.1 lb
why does O2 exist as a gas in the tank
bc its critical temperature is below room temperature (-119 deg C)
what physics law is used to calculate contents of O2 container
Boyle’s
pressure inside cylinder inversely r/t volume at a constant temp
calculation to determine how much O2 is in cylinder
calculation to determine how long O2 tank will alst
why is N2O a liquid inside the tank
because N2O liquefies under pressure (critical temp is 36.5 C, above room temp of 20 C)
when does N2O psi decrease
only when all liquid is consumed
at this point, tank is more than ¾ empty - approximately 136 L (at 400
when does N2O psi decrease
only when all liquid is consumed
at this point, tank is more than ¾ empty - approximately 136 L (at 400
what is the only reliable way to determine volume of N2O remaining in tank
weigh it
is CO2 a liquid or gas in container
liquid
max volume & service pressure of CO2 cylinder
- Max volume = 1590 L
- Service pressure = 893 psi
Most delicate part of cylinder
cylinder valve
what should be inserted if you dont have a replacement cylinder
yoke plug
if the check valve fails and there’s no plug, gas that should be going to patient will exit machine
what does an MRI-safe cylinder look like
silver, top is the color of the gas it contains
why should a cylinder always be opened slowly
recompression of gas as it travels through outlet can produce significant heat
dust or debris between tank and yoke can serve as fuel (very rare)
how to fix hissing sound from a cylinder leak
- 1st tighten connection
- If that doesn’t work, replace washer between cylinder and hanger yoke assembly
- If that doesn’t work, use a different cylinder
Fire triad
oxidizer, fuel, igniter
what increases risk of fire or explosion with O2 and N2O cylinders
- Temperatures > 130 F or 57 C increases risk of fire or explosion
- Oiling the cylinder increases risk by combining O2 or N2O with oil
safety feature on cylinders in case of fire
If there’s a fire, a safety release device opens
allows cylinder to slowly empty contents
Wood’s metal:
bismuth, lead, tin, and cadmium
what is “cracking” a cylinder
slowly opening the cylinder to flush the valve outlet clean of dust & debris
non-magnetic metal
aluminum
Sets standards for required components of anesthesia machine
American Society for Testing and Materials (ASTM)
Created the 1993 Anesthesia Machine Pre-Use Checkout Procedures
Food and Drug Administration (FDA)
Sets standards for acceptable occupational exposure to volatiles anesthetics
Occupational Safety and Health Administration (OSHA)
Sets the standards for compressed gas cylinders
United States Department of Transportation (DOT)
how are cylinders tested for leaks by DOT
subjected to 1.66 times its service pressure (ex. 1900 psi tested to 3154 psi)
how often should cylinder safety check be performed per DOT
Must be tested every 5 years (10 years with special permit indicated by 5-point star on label)
what’s required on the cylinder label
- Government agency (DOT)
- Type of metal used to construct cylinder
- Maximum filling pressure (psi)
- Serial number
- Manufacturer
- Owner
- Date of last inspection
Main purpose of O2 pressure failure device
monitor for/against low O2 pressure in machine
what does O2 pressure failure device alert
depleted O2 tank, drop in pipeline pressure, or disconnected O2 hose
2 components of O2 pressure failure device
- threshold alarm
- pneumatic device
when does the threshold alarm sound
when O2 pipeline < 28-30 psi
purpose of pneumatic device in O2 pressure failure device
↓ or stops flow of N2O when pressure in O2 < 20 psi
only gas that passes directly from its source to its flow valve at the flowmeter
oxygen
All other gases must encounter fail-safe valve before entering flowmeter
all or nothing response in GE Datex-Ohmeda O2 failsafe device
O2 pressure < 20 psi will completely stop the flow of nitrous oxide
O2 failure protection device in Drager machine
- Decreased pipeline pressure = proportionate decrease in N2O flow
- The flow of nitrous oxide is stopped only when oxygen pressure is extremely low
4 ways to tell if oxygen failure pressure safety device is working
- Turn ON O2 and N2O flow
- Make sure the backup oxygen cylinder is closed & then disconnecting the oxygen pipeline
- As you remove the O2 source, the N2O flow should stop just before the O2 flow stops
- Reintroducing the O2 supply to the anesthesia machine should result in both gases restored to their previous flow rates
what is the gas pressure in the high pressure system
cylinder pressure
what is the gas pressure in the intermediate pressure system
pipeline = 50 psi
tank = 45 psi
3 safety relief devices that prevent a cylinder from exploding when ambient temp increases
- fusible plug made of Wood’s metal (melts at increased temp)
- frangible disk that ruptures under pressure
- valve that opens at elevated pressures
how does oxygen-pipelinen crossover allow a hypoxic mixture
failsafe device responds to pressure (not flow)
if there’s pipeline crossover, the pressure of the 2nd gas will produce pressure to defeat failsafe
4 ways the proportioning device (hypoxia prevention safety device) might allow delivery of hypoxic mixture
- O2 pipeline crossover
- leaks distal to flowmeter valves
- admin of 3rd gas (helium)
- defective mcehanic or pneumatic components
2 ways pipeline crossover can cause hypoxic mixture
- O2-pipeline crossover not detected
- flowmeter leak (upstream of flowmeters)
how does an oxygen pressure failure device work
shuts off and/or proportionately reduces N2O flow if O2 pressure < 20 psi
how does an proportioning device work
aka hypoxia prevention safety device
prevents you from setting a hypoxic mixture with flow control valves
limits N2O flow to 3x O2 flow
N2O max with functioning proportioning device
75%
components of the link 25 system
GE Datex-Ohmeda
- O2 & N2O flowmeter valves mechanically linked by a chain (mechanical component)
- Incorporates a second stage regulator for oxygen and nitrous oxide (pneumatic component)
purpose of link 25 system
GE Datex-Ohmeda
If you adjust the flow control valves in such a way that it would create a hypoxic mixture, a gear engages, and oxygen flow is automatically increased
how are O2 and N2O connected in Oxygen Ratio Monitor Controller (Drager)
pneumatically
4 times the proportoning device can’t prevent a hypoxic mixture (won’t alarm)
- Oxygen pipeline crossover
- Leaks distal to the flowmeter valves
- Administration of a 3rd gas (helium)
- Defective mechanic or pneumatic components
what is a Thorpe tube
traditional flowmeter
Controls and measures the FGF that travels towards the vaporizers and CG
what allows fresh gas to enter flowmeter
opening flowmeter valve (flow control valve)
2 opposing forces that determine flowmeter float position
1) FGF pushes up, 2) gravity pushes down
4 types of flowmeter floats
skirted, plumb bob, nonrotating, ball
where is flow measurement taken in the flowmeter
at the widest part of the float
which flowmeter floats are read at the top vs. middle
- Read at the top: skirted, plumb bob, nonrotating
- Read in the middle: ball
what is the annular space
area between float and side wall of the flowmeter
Geometry of annular space affects flow pattern through it
what aspect of a flowmeter provides a constant gas pressure throughout wide range of flow rates
“variable orifice” architecture
laminar flow is dependent on:
gas viscosity
Poiseulle
laminar flow is dependent on:
gas viscosity
Poiseulle
turbulent flow is dependent on:
gas density
Graham’s Law
predicts laminar flow
Re < 2000
predicts turbulent flow
Re > 4000
predicts transitional flow
Re 2000-4000
Reynold’s number calcualtion
(density * diameter * velocity) / viscosity
most delicate part of the machine
flowmeters (glass)
what is the safest flowmeter configuration on anesthesia machine
o2 flowmeter should always be furthest right
how can a leak in the flowmeters cause delivery of a hypoxic mixture
a leak will allow oxygen to escape from low pressure system
why should oxygen flowmeter always be furthest right (closest to manifold outlet)
if a leak develops in any other flowmeters, it won’t reduce the FiO2 delivered
if a leak develops inside O2 flowmeter, can still cause hypoxic mixture
why should oxygen flowmeter always be furthest right (closest to manifold outlet)
if a leak develops in any other flowmeters, it won’t reduce the FiO2 delivered
if a leak develops inside O2 flowmeter, can still cause hypoxic mixture
how do you calculate FiO2 set at flowmeter
max FiO2 delivered with nasal cannula
6 L = 44%
formula for total Vt delivered to patient in machines with fresh gas coupling
= Vt set on ventilator + FGF during inspiration - volume lost to compliance
how is total Vt delivered to patient calculated in machines with fresh gas coupling
- convert FGF from L/min to mL/min
- multiply FGF by I:E ratio
- calculate Vt per breath
- add volume set on ventilator to FGF during inspiration
what 4 things impact Vt delivered to patient in a machine that couples FGF to Vt
- any change in FGF
- bellows height
- RR
- I:E ratio
how does increased FGF impact Vt, Vm, PIP, and EtCO2 in machines that couple FGF to Vt
↑ Vt, Vm, PIP
expect EtCO2 to decrease
how does decreased FGF impact Vt, Vm, PIP, and EtCO2 in machines that couple FGF to Vt
↓ Vt, Vm, PIP
expect EtCO2 to increase
vent changes that increase delivered Vt in machines that couple FGF & Vt
- decreased RR
- increased I:E (ex. 1:2 to 1:1)
- increased FGF
- increased bellows height
vent changes that decrease delivered Vt in machines that couple FGF & Vt
- inc. RR
- dec I:E ratio (ex. 1:2 to 1:3)
- dec FGF
- dec bellows height
what is compliance
a change in volume for a given change in pressure
it is a measure of distensibility
what is compliance
a change in volume for a given change in pressure
it is a measure of distensibility
how does positive pressure inside the breathing circuit affect the circuit
causes it to expand
This quantity of gas does not reach the patient
how does positive pressure inside the breathing circuit affect the circuit
causes it to expand
This quantity of gas does not reach the patient, and therefore does not
volume lost to circuit =
circuit compliance * peak pressure
what is the splitting ratio
when FGF enters vaporizer, it either goes into vaporizing chamber & becomes 100% saturated with anesthetic agent or goes into bypass chamber and does not contact anesthetic agent
what determines the splitting ratio
setting dial on desired concentration
- Higher concentration directs more fresh gas through vaporizing chamber
- Lower concentration directs more fresh gas through bypass chamber
what determines the final anesthetic concentration exiting the vaporizer
before leaving the vaporizer, the 2 fractions from the splitting ratio mix
purpose of baffles & wicks in vaporizer
increase surface area & turbulence, ensures fresh gas inside chamber becomes 100% saturated with anesthetic agent
what is vapor pressure
pressure exerted by a vapor in equilibrium with its liquid phase inside a closed container
↑ temperature = ↑ vapor pressure
what is latent heat of vaporization
number of calories needed to convert 1g liquid into vapor without causing a change in temperature
what does a temperature compensating valve in the vaporizer do
adjusts ratio of vaporizing chamber flow to bypass chamber flow
Either a bimetallic strip or expansion element
what does a temperature compensating valve in the vaporizer do
adjusts ratio of vaporizing chamber flow to bypass chamber flow
Either a bimetallic strip or expansion element
Guarantees constant vaporizer output over a wide range of temperatures
Temperature compensating valve
how does the temp compensating valve affect bypass chamber flow
- Colder= increases vaporizing chamber flow, ↓ VP, ↓ vaporizer output
- Hotter = increases bypass chamber flow
why are most modern vaporizers out of circuit
isolated from anesthetic breathing system
Can remove vaporizer from anesthesia machine without compromising integrity of breathing system
VP of sevo, des, and iso
- Sevoflurane = 157 mmHg
- Isoflurane = 238 mmHg
- Desflurane = 669 mmHg
most likely cause of vaporizer leak
loose filler cap
most common location for leak in low-pressure system
Internal leak in the vaporizer
Estimated liquid anesthetic usage
liquid anesthetic used per hour (mL) = vol% x FGF (L/min) x 3
what should you do if the vaporizer is tipped
1) drain vaporizer
2) run high FGF through vaporizer for 20-30 min or until all liquid anesthetic is vaporizers
when can a vaporizer be used again after a vaporizer is tipped
When expired gas analysis no longer detects anesthetic agent, can fill and use on a patient
method to eliminate worry of tipping vaporizer
Set to transport mode (T)
what is the pumping effect
can increase concentration of anesthetic leaving vaporizer (vaporizer output)
Can be from anything that causes gas that has already left vaporizer to re-enter vaporizing chamber
what generally causes pumping effect
- PPV
- O2 flush valve
what eliminates concern of pumping effect
Check valve anywhere between vaporizer and CGO
what eliminates concern of pumping effect
Check valve anywhere between vaporizer and CGO
prevents simultaneous administration of different anesthetic gases
interlock system
Makes sure FGF only travels through one vaporizer at a time
prevents simultaneous administration of different anesthetic gases
interlock system
Makes sure FGF only travels through one vaporizer at a time
why is a special vaporizer required for des
- significantly less potent than other volatiles & has VP close to atmospheric pressure
- To achieve the same depth of anesthesia, absolute volume of des that must be vaporized is higher
- Heating and pressurizing Tec 6 to 39 C and 2 atm solves these issues
why can’t a variable bypass vaporizer be used for desflurane
would require bypass flow well beyond the limits of the anesthesia machine to dilute des to clinically useful concentration d/t its high VP
how does ambient pressure affect Tec 6 vaporizer
- Lower ambient pressure (higher altitude) requires higher setting on dial
- Higher ambient pressure (hyperbaric O2 tank) requires lower setting on dial
calculation for required dial setting of desflurane in different pressures
which volatile uses an injector vaporizer
desflurane
method of vaporization for variable bypass vaporizer
flow over
method of vaporization for injector vaporizer
gas/vapor blender
heat creates vapor that is injected into fresh gas
method of vaporization for injector vaporizer
gas/vapor blender
heat creates vapor that is injected into fresh gas
which type of vaporizer compensates for changes in elevation
variable bypass
where is the oxygen analyzer
Resides in inspiratory limb of breathing circuit or component of multigas monitor
what does the oxygen analyzer measure
monitors oxygen concentration (not pressure)
only device downstream of flowmeters that can detect a hypoxic mixture
oxygen analyzer
what can detect hypoxic mixture caused by leak in O2 flowmeter
oxygen analyzer
how does the Galvanic Fuel-Cell work
Increasing oxygen tension generates a current across two electrodes
which is faster - galvanic fuel cell or paramagnetic device
paramagnetic
how does the paramagnetic device work in the oxygen analyzer
increasing oxygen tension creates an increased magnetic attraction
which is self-calibrating - galvanic fuel cell or paramagnetic device
paramagnetic
how can a hypoxic mixture be created in closed-circuit anesthesia
- If consumption increases, but delivery remains constant
- only O2 analyzer will detect
causes of reduced FiO2 during low flow anesthesia in closed circuit
(↑ O2 consumption): sepsis, pain, SNS stimulation, thyrotoxicosis, fever, etc.
O2 consumption in avg adult
250 mL/min
only device to detect oxygen-pipeline crossover
O2 analyzer
what should you assume when oxygen analyzer alarms
pipeline crossover has occurred until other causes can be ruled out
2 things to do if O2 pipeline supply fails
1) turn ON O2 cylinder, 2) disconnect pipeline O2 supply
what should you do if the oxygen analyzer alarms and O2 concentration in breathing circuit is not increasing with O2 cylinder on
assume machine malfunction and ventilate with Ambu (using different O2 tank or room air)
how to conserve tank O2 in event of oxygen pipeline failure
using low flows
if using machine with O2 as driving pressure to compress bellows, hand ventilate the patient to conserve O2 (if machine uses air to power ventilator or is piston-driven, can remain on ventilator)
provides path for O2 to travel from intermediate pressure system to breathing circuit
O2 flush
flow and pressure O2 flush button exposes circuit to
O2 flow of ~35-75 L/min, O2 pressure of ~50 psi (pipeline pressure)
negative effect of using O2 flush during inspiration
barotrauma
Ventilator spill valve is closed during inspiration
negative effect of using O2 flush during inspiration
barotrauma
Ventilator spill valve is closed during inspiration
2 Functions of Drive Gas on Pneumatic Ventilator
- Compresses bellows
- Opens & closes ventilator spill valve
separate the drive gas circuit from the patient breathing circuit
bellows
2 risks associated with pressing the O2 flush valve
barotrauma and awareness
how does drive gas compress bellows on pneumatic ventilator
- inspiration: drive gas flow increases pressure in ventilator chamber & creates a pressure gradient that pushes fresh gas in pts lungs
- expiration: flow stops and pressure gradient reverses (pressure in chest > inside bellows), pt can exhale Vt
function of spill valve in pneumatic ventilator
- During inspiration, drive gas closes spill valve & ensures that Vt goes to pt and not scavenging
- During exhalation, flow of gas drive stops
function of spill valve in pneumatic ventilator
- During inspiration, drive gas closes spill valve & ensures that Vt goes to pt and not scavenging
- During exhalation, flow of gas drive stops
why is intrinsic PEEP possible in pneumatic ventilators
exhaled Vt first refills bellows & after circuit pressure > ~3 cm H2O, spill valve opens and excess gas exits scavenger
amount of air going to bellows =
Vt + flowmeter flow during exporation
3 things that happen during inspiration in pneumatic ventilators
- drive gas compresses bellows
- drive gas closes spill valve
- fresh gas from ventilator geos to patient
3 things that happen during expiration in pneumatic ventilators
- expired gas refills bellows
- bellows fill completely
- when circuit pressure > 2-4 cm H2O expired gas is directed through spill valve to scavenger
2 methods that reduce barotrauma with O2 flush valve
1.Fresh gas decoupling
2.Inspiratory pressure limiter
what is fresh gas coupling
what is set on ventilator is actually what’s delivered to patient
Vt delivered by ventilator is isolated from flowmeters and O2 flush valve
what is the Inspiratory pressure limiter
can limit breathing circuit pressure during inspiration
when breathing circuit pressure rises above setpoint, excess gas is vented to scavenger (think of as APL valve for ventilator)
how can O2 flush cause awareness
Excessive use adds dilutes partial pressure of volatile, may lead to awareness (gas from flush doesn’t pass through vaporizers & adds gas to circuit that doesn’t have volatile)
gas inside bellows =
exhaled Vt + FGF during expiration
consequences of leak in bellows
- Drive gas mixes with patient circuit
- Increased inspired FiO2 (if drive gas is oxygen)
- Decreased inspired FiO2 (if drive gas is air or air-oxygen mixture)
- Barotrauma (may transmit high gas pressure to breathing circuit)
- Dilution of volatile concentration (can lead to awareness)
movement of ascending bellows
rise during expiration
movement of descending bellows
falls with inspiration
classic problem with descending bellows
may continue to rise & fall with circuit disconnect
which type of bellows is considered safer
ascending
will only partially fill if there’s a leak
which type of bellows is considered safer
ascending
will only partially fill if there’s a leak
mechanism of piston ventilator
Utilize electric motor to compress piston & generate positive pressure
Will not consume tank O2 in event of pipeline failure
mechanism of piston ventilator
Utilize electric motor to compress piston & generate positive pressure
Will not consume tank O2 in event of pipeline failure
what do piston ventilators require for bellow compresion during MV
O2
what is fresh gas decoupling in a piston ventilator
Deliver constant Vt regardless of changes to FGF, RR, I:E
function of positive pressure relief valve in piston ventilator
opens at 75 +/- 5 cm H2O
prevents excessive pressure in circuit
function of Negative pressure relief valve in piston ventilator
opens at -8 cm H2O
entrains room air, protects patient against negative end-expiratory pre
function of Negative pressure relief valve in piston ventilator
opens at -8 cm H2O
entrains room air, protects patient against negative end-expiratory pre
do piston ventilators add PEEP like gas-driven bellows
Nope
why do gas-driven bellows add PEEP
automatically add 2-3 cm H2O d/t design of ventilator spill valve
movement of breathing bag during mechanical ventilation with piston ventilator
Bag inflates during inspiration, deflates during expiration
Bag won’t move when pt initiates spontaneous breaths while on ventilator
movement of breathing bag during mechanical ventilation with piston entilator
Bag inflates during inspiration, deflates during expiration
Bag won’t move when pt initiates spontaneous breaths while on ventilator
Delivers a preset Vt over predetermined time
volume controlled ventilation
Delivers a preset inspiratory pressure over predetermined time
pressure controlled ventilation
what causes PIP to increase in VCV
↑ airway resistance or ↓ lung compliance
variable factors in VCV
Inspiratory pressure varies with compliance
variable factors with PCV
Vt and inspiratory flow vary with compliance
variable factors with PCV
Vt and inspiratory flow vary with compliance
fixed settings in VCV
Vt, inspiratory flow rate, inspiratory time
fixed settings in PCV
PIP, inspiratory time
variable settings in PCV
Vt, inspiratory flow
inspiratory flow in VCV
held constant during inspiration
inspiratory flow in PCV
typically uses decelerating pattern – begins high to achieve inflation pressure, then slows to maintain constant inflation pressure
advantages of PCV
- Delivers larger Vt for given inspiratory airway pressure
- Inspiratory flow pattern may improve gas exchange
- Reduces risk of ventilator-assoc. lung injury (VALI)
- Useful if pt has low compliance or to compensate for leaks
PCV Disadvantages
- Increased airway resistance or decreased lung compliance = ↓ Vt
- Extra attention to conditions that alter compliance or resistance (cause Vt to change)
things that can cause decreased Vt with PCV
decreased compliance (pneumoperitoneum, Trendelenburg), increased resistance (bronchospasm, kinked ETT)
things that cause increased Vt with PCV
increased compliance (pneumoperitoneum released, Tburg to supine), decreased resistance (bronchodilators, suctioning airway secretions)
3 situations when PCV is better than VCV
- Patient has low compliance: pregnancy, obesity, laparoscopy, ARDS
- High PIP would be dangerous: LMA, neonate, emphysema
- Need to compensate for leak: LMA, uncuffed ETT
what does a negative deflection just before breath indicates
patient-triggered breath
which breath is assisted and which is spontaneous
green = assisted
blue = spontaneous
controlled mandatory ventilation
Machine initiated breath delivers preset Vt & RR on fixed schedule
why is CMV assoc. with risk of patient-ventilator asynchrony
Doesn’t compensate for pt initiated breaths
best vent mode for apneic patients
CMV
assist control mode
- Machine initiated breath delivers preset Vt & RR
- Spontaneous breaths receive full preset Vt
risk of overbreathing with assist control
hyperventilation & respiratory alkalosis
risk of overbreathing with assist control
hyperventilation & respiratory alkalosis
vent mode that allows the patient to breath spontaneously between machine initiated breaths
SIMV
how does SIMV guarantee a set minute ventilation
the more the patient works, the less assistance the ventilator provides and vice versa
PCV-VG
Benefits of pressure control ventilation, but also guarantees a predetermined Vt while applying the minimum pressure required to achieve it
PSVPro
- “Pro” = protect
- Spontaneously breathing patient receives PSV
- If pt becomes apneic, ventilator converts to pressure-control ventilation (PCV)
- When pt resumes spontaneous ventilation, ventilator goes back to PSV
2 benefits of CPAP
augments the pt’s spontaneous breath, reduces airway collapse during expiration
difference in CPAP and PSV
PSV only applies pressure to the circuit when the patient initiates a breath (there is nothing during expiration)
P1 and P2 in BiPAP
P = pressure
- P1 = inspiratory positive airway pressure (think pressure support for a spontaneous breath)
- P2 = Expiratory positive airway pressure (think CPAP during exhalation)
when is BiPAP useful
patient with COPD or when CPAP isn’t quite enough
what is APRV
Airway Pressure Release Ventilation
- Like BiPAP, but there is a high level of CPAP throughout most of the respiratory cycle
- The high level of pressure is released at preset intervals to facilitate exhalation
patient requirements for IRV
Requires a paralyzed and sedated patient (no spontaneous ventilation)
when is IRV useful
patient with a small FRC or in the patient with ARDS
risk with IRV (inverse ratio ventilation)
dynamic hyperinflation (auto-peep or breath stacking)
what is IRV
IRV reverses typical I:E ratio by allocating more time to inspiration
gas transport in conventional modes of ventilation
gas transport occurs by convection (large airways) and convection + molecular diffusion (small airways and alveoli)
what is HFV
High Frequency Ventilation
HFV delivers a Vt below anatomic dead space in conjunction with a very high respiratory rate
how does gas transport occur in HFV
ombination of molecular diffusion, coaxial flow, and high velocity flow
how do carbon dioxide absorbents work
remove exhaled carbon dioxide from the breathing circuit
a base neutralizes an acid
what should you do with exhausted soda lime in the middle of a surgical procedure
increase FGF to convert circle system into semi-open configuration
best balance of mesh size in soda lime
4-8 mesh granules used
* Small granule = high surface area with high resistance
* Large granule
best balance of mesh size in soda lime
4-8 mesh granules used
* Small granule = high surface area with high resistance
* Large granule
how big is each granule of mesh in soda lime
Each granule is between 1/8 and 1/4 inch diameter will pass through mesh with 4-8 holes per square inch
This size provides the best combination of absorptive capacity and airflow resistance
2 Major Problems with Soda Lime
1) exhausted absorbent (no longer able to neutralize CO2)
2) desiccated absorbent (too dry)
what makes soda lime a strong base
Sodium hydroxide
when does soda lime turn blue-purple
As pH falls < 10.3, indicator dye such as ethyl violet will change to blue-purple
As CO2 consumes basic substrates, pH of absorbent ↓
when does soda lime turn blue-purple
As pH falls < 10.3, indicator dye such as ethyl violet will change to blue-purple
As CO2 consumes basic substrates, pH of absorbent ↓
Best indicator of expired soda lime:
presence of inspired CO2 in breathing circuit
when can ethyl violet in soda lime revert to a colorless state
when machine not in use but quickly returns to blue-purple in presence of CO2 if exhausted
___ is required to facilitate reaction of CO2 with CO2 absorbent
water
when is absorbent dessicated
when its devoid of water
water content of absorbent granules
Granules hydrated to 13-20% by weight
adverse effect of desiccated absorbent
Increases production of carbon monoxide des > iso»_space;> sevo) & compound A with sevo
most unstable volatile in soda lime
sevo
methods to ↓ Risk Carbon Monoxide & Compound A
- Utilize low FGF to preserve water content
- Turn off FGF in between cases
- Change all absorbent at one time (not a single canister in dual canister set up)
- Change canisters when ethyl violet signifies exhaustion
- Change canisters if unsure about level of hydration (Ex. FGF left on overnight)
why is aborbent strongly irritating to skin and mucous membranes
Sodium hydroxide is strongly alkaline
purpose of silica in absorbent
provides hardness & minimizes dust production
absorbent that doesn’t contain strong bases
Calcium Hydroxide Lime (Amsorb Plus)
no NaOH, KOH
absorbent that doesn’t contain strong bases
Calcium Hydroxide Lime (Amsorb Plus)
no NaOH, KOH
additive in calcium hydroxide lime that acts as a humectant
CaCl
purpose of CaCl additive in calcium hydroxide lime
humectant (opposite of dessicant)
keeps absorbent moist
purpose of CaCl additive in calcium hydroxide lime
humectant (opposite of dessicant)
keeps absorbent moist
added to Calcium Hydroxide Lime to increase hardiness
calcium sulfate & polyvinylpyrrolidone
Amsorb Plus Benefits
- No carbon monoxide production
- Very little or no compound A production
- Lower risk of fire compared to soda lime
Amsorb Plus Drawbacks
- Soda lime can absorb 26 L CO2 per 100 g absorbent; calcium hydroxide can only absorb 20.6 L of CO2 per 100 g absorbent
- Absorptive capacity is less than soda lime (requires more frequent replacement)
- More expensive
function of scavenger system
remove excess gas from anesthesia circuit & minimize environmental exposure to waste anesthetic gas
how is constant pressure maintained in circuit with scavenging system
must only remove an amount of gas equal to FGF minus volume of gas lost due to patient’s O2 consumption
most critical component of scavenging system
interface
removal of too much gas could create negative pressure in circuit, removal of too little increases risk of barotrauma
how is FGF to scavenger controlled
what determines amount of gas remaining in circuit & amount released
- Spontaneous ventilation determined by APL valve
- Mechanical ventilation determined by ventilator spill valve
how is FGF to scavenger controlled
what determines amount of gas remaining in circuit & amount released
- Spontaneous ventilation determined by APL valve
- Mechanical ventilation determined by ventilator spill valve
5 Components of scavenging system
- Gas Collecting Assembly
- Transfer Tubing
- Scavenging interface
a. Open System
b. Closed System - Gas Disposal Assembly Tubing
- Gas Disposal Assembly
Collects waste gas from breathing circuit
Gas Collecting Assembly
location of Gas Collecting Assembly
at APL & ventilator spill valve
directs collected gas to interface (connects APL to scavenging interface)
Transfer tubing
what type of scavenging can only be used with active system
open system interface
benefits of open system scavenging interface
Removes risk barotrauma or removal of FGF from circuit
risk of too much suction with open scavenging interface
entrains air in scavenger
how do open vs closed scavenging interfaces connect to atmosphere
- open = open to atmosphere
- closed = communicates to atmosphere with pressure valves
required for closed system scavenging interface with passive system
positive pressure relief
required for closed system scavenging interface with active system
positive and negative pressure relief
required for closed system scavenging interface with active system
positive and negative pressure relief
required for closed system scavenging interface with active system
positive and negative pressure relief
when does positive pressure relief valve pop off in closed interface
+5 cm H2O
when does negative pressure relief valve entrain room air in closed interface active system
-0.5 cm H2O
Removes gas from scavenging interface
Gas Disposal Assembly Tubing
Directs gas from scavenger to hospital suction and ultimately atmosphere
Gas Disposal Assembly
how does passive scavenging disposal remove gas
relies on positive pressure of fresh gas leaving interface
how does active scavenging disposal remove gas
suction
Most common source of preventable equipment-related complications
circuit disconnect
Most common site of circuit disconnect
Y-piece
2nd most common: CO2 absorbent canister
Most common site of circuit disconnect
Y-piece
2nd most common: CO2 absorbent canister
Most common site of circuit disconnect
Y-piece
2nd most common: CO2 absorbent canister
Most common site of circuit disconnect
Y-piece
2nd most common: CO2 absorbent canister
Sources of Low Pressure in Circuit
- Circuit disconnect
- Leaks in circuit
- Malfunction of bag/ventilator selector switch
- Moisture buildup in flow sensors preventing proper function of ventilator
- Incompetent ventilator spill valve (Vt directed to scavenger)
- Defective CO2 absorbent canister
- Leaks around CO2 absorbent (common after granules changed)
4 ways to monitor for circuit disconnect
pressure, volume, EtCO2, vigilance
monitors concentration of O2 in circuit
Oxygen analyzer
what should you do if unable to ventilate due to low pressure
ventilate with bag-valve-mask and O2 tank while providing TIVA
Consequences of ↑ circuit pressure
- subcutaneous emphysema
- ↓ venous return
- ↓ CO
- CV collapse
- hypotension
- PTX
- barotrauma
- high PEEP
what should you do first if PIP increases enough to trigger high PIP alarm
irst rule out patient related causes (#1 – bronchospasm) and attempt to manually ventilate
what should you assume if high PIP and when pt removed from ventilator (APL open) PIP returns to baseline
most likely explanation is failed ventilator spill valve
what should you assume if high PIP and when pt removed from ventilator (APL open) circuit pressure ↑
most likely cause is that scavenger is occluded or positive pressure relief valve failed
what should you do if PIPs are increased d/t occluded scavenging or positive relief valve failure
remove scavenger tubing from APL or remove pt from breathing circuit and ventilate with Ambu bag/begin TIVA
Developed guidelines for max exposure to inhalational anesthetics in the OR
OSHA
OSHA max exposure to inhalational anesthetics in the OR
- Halogenated agents alone should be < 2 ppm
- N2O alone should be < 25 ppm
- Halogenated agents + N2O should be < 0.5 ppm and 25 ppm respectively
Determinants of Exposure to Waste Gases
- Amount of OR ventilation and air turnover
- Functional status of anesthesia equipment
- Your practice as a CRNA
how much negative pressure should be created for a proper negative pressure test
-65 cm H2O
