The Anesthesia Machine and Breathing Systems

Report
The Anesthesia Machine and
Breathing Systems
Trey Bates, MD
Special Thanks to Judson Mehl
A quick word on medical gas • All those hoses:
• Oxygen
• Air
• Nitrous
• Vacuum
• WAGD
•
Waste Anesthetic Gas
Disposal
2000
PSI
(FULL)
H (7000L)
E (700L)
Pressure Reduction Pathway
• H-Cylinder
2000 psi
• Manifold
55 psi
• Hospital line
55 psi
O2 - 2000 psi, 625 L
N2O - 750 psi, 1590 L
Co2 - 838 piso, 1590 L
Air - 1800 psi, 625 L
Oxygen Failure Protection Device
Flow of nitrous-oxide is
dependent on oxygen pressure.
If oxygen pressure is lost then the
other gases cannot flow past their
regulators
45
psi
2000 psi !!!
Key points for ITE:
• Liquid oxygen must be stored below its critical
temperature of -119 C
• In oxygen tanks, the pressure falls in proportion to
the remaining volume of oxygen
• If a full E-cylinder at 2000 psi contains 700 L O2, then
a half full tank at 1000 psi contains ?
• What about an H-cylinder at 1000 psi?
• What about an E-cylinder at 500 psi?
More math . . . for fun
• ICU transport with
an E-cylinder with
700 psi.
• Need to run a NRB
at 10 lpm.
• Tulane elevator
breaks down. How
much sh*t are we
in?
Nitrous
• Nitrous is NOT an ideal gas. Thus it has several
unique properties:
• Transition between liquid and gas states does not lead to
huge increases in pressure
• It is easy to compress, so the cylinders hold a lot moregas.
• Its critical temp is 36.5 C, so it doesn’t need refrigeration
More on N2O
• N2O is vaporized at the same rate it is utilized
• The pressure in the tank never changes
• You don’t know what you’ve got til its gone (400 L/1600L =
25% remaining))
• The only way to tell how much N2O is left, is to measure
the tank.
• Consult the tare weight on the bottle
• I have never been asked to determine how much N2O was left
based on weight on an ITE.
Breathing Systems
• This stuff matters because:
• Oxygen is pretty important
• Agent delivery is pretty important
• Getting rid of CO2 is pretty important
• And these ALWAYS show up on the ITE. ALWAYS !!
CO2
• Is a . . .
• Cardiac Depressant
• And it . . .
• Increases CBF
• Increases bleeding
• Causes acidosis which . . .
• Shifts the Carboxy-Hgb curve
• Shifts Ca2+ and K+ out of the cell
Anesthesia at
Tulane
Insufflation anesthesia
• Gas delivery under a
drape
• Serious CO2
accumulation without
high gas flow
• Lets be honest,
everybody in the
room is breathing this
stuff
Open Drop Anestheisa
Schimmelbusch mask
Draw-over anesthesia
• Hose serves as
an open-ended
reservoir
• Addition of
oxygen possible
• 1 lpm 30-40%
• 4 lpm 60-80%
• Simple
• Portable
• No scavenging
The Mapleson Circuit
• Ingredients:
• Breathing tube
• Fresh gas inlet
• APL valve
• Reservoir bag
The only real difference is
the order in which the
ingredients occur
Mapleson Circuits
• Important points
• There are NO one-way valves
• There is no CO2 absorber
• Some rebreathing is prevented by venting through the APL
before the next inspiration
Basic Mapleson A
During spontAneous ventilation, the Mapleson A is most
efficient. That long breathing tube full of fresh gas is a
great reservoir for the patients next breath.
Why?
• Giving positive pressure
is going to require me to
partially close that APL
valve. When I ventilate,
half of my FGF is going
to exit the partially open
APL valve.
• During exhalation, all
that exhaled air is going
to fill the breathing tube
because the APL is now
closed and the only way
it is going to vent is if
the gas flows are really
high.
Mapleson D
• The FGF is
happening right at
the patient’s face.
• Now, watch this . .
When I positive pressure
ventilate, I close the APL and
use the old air in the reservoir
to generate the force to blow
the fresh air into the patient.
Anything I lose out of the APL
will be old air.
Between ventilations the new
air is pushing the old air out of
the APL and away from the
patient.
Bain
• The Bain circuit
deposits the FGF
in the same
place as
Mapleson D, but
it traveled
through the
warm, exhaled
air on the way in,
so the FGF is
warmed.
Get it now?
• If not, and you probably wont on the day of the ITE,
then check out this aswesome memory aid. Its
pretty complex:
• Ventilation is most efficient in a
• Mapleson A during spontAneous ventilation
•
•
There is no D in spontaneous
Mapleson
•
D during controleD ventilation
There is no A in controled
The downfalls of the Mapleson
• Lose all the heat and humidity
• High FGF to prevent rebreathing
• All that agent is ventilated out to the room
So, science happened
• And then we added:
• CO2 absorbers
• Unidirectional valves
• Scavenging
• And voila, we have the CIRCLE SYSTEM !!
CO2 absorbers
• How they work:
• So why is it bad that the CO2 absorber “dries out?”
• Well, here is why:
• CO2 + H2O → H2CO3 (this is carbonic acid)
• Then the hydroxide salts in the CO2 absorber do this:
• H2CO3 + 2NaOH → Na2CO3 + 2H2O + heat (this is why they get
warm)
• Then all that Na2CO3 (sodium hydroxide) produced in the first
reaction does this:
•
Na2CO3 + Ca(OH)2 → CaCo3 + 2NaOH (We just regenerated our starting
reagent)
CO2 absorbers
• As the absorbent is used up, it becomes more acidic.
• That purple color change is a pH indicator
• When 50-70% has changed color, its time to change
the absorber.
Granule size is a
trade off:
Larger granules
minimize resistance
to airflow
Smaller granules
maximize surface
area for more
absorption
And what about these unidirectional
• Inspiratory
valves?
• Expiratory
• Valve
incompetence
is usually due
to unseated or
warped disc
• Note what is in
the reservoir
bag
In a closed scavenging system, what
happens to the reservoir bag during
expiration and inspiration?
What does it mean with the opposite
happens?
The reservoir bag expands during expiration
and deflates during inspiration. During
inspiration in MV, the ventilator pressure
relief valve closes, directing ventilator
bellows into patient breathing circuit.
If the PRV is incompetent, there will be a
direct communication between breathing
circuit and scavenging circuitand the
reservoir bag would inflate during
inspiration.
A few questions
• A size E compressed-gas cylinder completely filled
•
•
•
•
•
with N2O contains how many litres?
A. 1160
B. 1470
C.1590
D. 1640
E. 1750
Answer:
• C
• Size E compressed gas cylinders completely filled
contain 1590 L gas
Question
• The pressure gauge on a size E compressed-gas
•
•
•
•
•
cylinder containing O2 reads 1600 psi. How long
could O2 be delivered from this cylinder at 2 LPM?
A. 90 min
B. 140 min
C. 280 min
D. 320 min
E. Cannot be calculated
Answer:
• C
Question
• If the anesthesia machine is discovered Monday
•
•
•
•
•
morning having run with 5L/min of O2 all weekend,
the most reasonable course of action to take before
administering the next anesthetic would be:
A. Turn the machine off for 30 min
B. Place a humidifier in the expiratory limb
C. Avoid the use of Sevoflurane
D. Change the CO2 absorbent
E. Use N2O for the first hour of the case
Answer
• D – of course, but why change it if its not purple?
One last painful question
• A mechanically ventilated patient is transported from the
•
•
•
•
•
OR to the ICU using a portable ventilator that consumes
2L/min of O2 to run the ventilator itself. The patient gets
100% O2 and tidal volumes of 500 ml at a rate of 10/min.
You have an E-cylinder with 2000 psi. The vent will shut
off below 200 psi. How long do you have?
A. 10 min
B. 30 min
C. 60 min
D. 90 min
E. 100 min
The DISS
The PISS
Lets take a quick
look at these socalled “OFPDs”
The OFPD
Lets look again
Lets stop for today
• One thing I want you to note. We have discussed
the HIGH-PRESSURE CIRCUIT to this point.
• Gas lines proximal to the flow valves (knobs) are
considered the high-pressure circuit
• Distal to the knobs (eg. In the Thorpe tubes and
onward) you are in the low-pressure circuit.
• To be continued . . .
Flowmeter sequence:
• Oxygen is
universally
on the right
• The knob is
larger and
fluted
• Why?
The less circuit AFTER the O2
joins, the less chance of a leak in
the post-O2 part of the circuit.
It is a safety feature,
but not 100% fool
proof. You can still
make a hypoxic gas
mixture.
This is a Thorpe Tube
** Flow rate across a constriction depends on the
gas’s viscosity at low laminar flows and its density at
high turbulent flows.
• These are
called
“constantpressure
variableorifice”
flowmeters.
• Conductive
coating to
reduce effect
of static
electricity
• Calibrated to
be gasspecific **
Oxygen/Nitrous Oxide ratio
controllers
• Draeger utilizes
this little gem:
• But, Datex-
Ohmeda got it
right
On to vaporizers
• A couple key points on
vaporization
• Anesthetics have a
vapor pressure, which is
the propensity to come
out of solution and form
a . . . Vapor.
• Vapor pressure is temp-
dependent.
• Higher temp =
vapor pressure
The energy required for
vaporization is
manifested as loss of
heat from the anesthetic
solution
As the anesthetic vaporizes, the
solution becomes colder . . . And
when the temp drops, so does the
vapor pressure !!!
Copper kettles
Copper has a high specific
heat
Copper has high thermal
conductivity
Resistant to the temperature
drop from vaporization
Best material to maintain a
constant temperature
Copper Kettle
• Measured-flow
vaporizer
The math gets a little funny here, and they
will throw you a copper kettle equation on
the ITE, so watch this . . .
•
Separate flowmeter
for the gas flowing
through the kettle
•
Gas passing through
the kettle becomes
fully saturated
•
Then you dilute it out
to the proper
percentage with the
other flowmeter
The math of a copper kettle
• Vapor pressure of Halothane (and ISO) is about 243
mmHg at 20 C
• So 243/760 = 32%
So here is what we know
• At atmospheric pressure, if we put 100 ml O2
through the kettle, we will get 150 ml of FGF on the
other side.
• 50 mL of that will be volatile
• If we keep the flow at 5LPM total we can do this
• 100 ml to the kettle → 150 ml, 50 of which is halothane
• 4850 ml to the dilution limb
• 50 ml halothane / 5000 ml total FGF = 1% halothane
For the ITE
• For the ITE, in a copper kettle there should always
be a total of 5L FGF
• Add 50 ml vapor to every 100 ml you put through the
kettle
• Keeping the totals at 5L/min, every extra 100 ml
through the kettle increases the agent concentration
1%
• Eg. 100 → kettle → 150 (50ml agent) + 4850 = 1%
agent
• 200 → kettle → 300 (100ml agent) + 4700 = 2% agent
• 300 → kettle → 450 (150ml agent) + 4550 = 3% agent
Remember, Iso and Halothane have similar vapor
pressures, so this applies to Iso too
Well, that sucked. Lets move on.
• These are
Tec 4
Tec 5
modern
vaporizers
• Tec 4,5,6 all
have similar
mechanisms
• Aladin is very
Tec 6
different.
Aladin Vaporizers
Lets jump back to physics for one
slide
• Recall saturated vapor pressures:
• So, at atmospheric pressure and 20 C, if I let all the
FGF flow through the vaporizer, it would saturate and
produce:
• Halo
243
• Iso
238
• Sevo 157
• Des
672
243/760 = 32%
238/760 = 32%
157/760 = 20%
672/760 = 88%
• These are “slightly” above clinically relevant concentrations
But if we split the FGF between the
vaporizer and a bypass channel . . .
• Well, then
we have a
variable
bypass
vaporizer.
The Datex-Ohmeda version
•
Note the bimetalic strip
•
This serves to
compensate for
temperature
changes
•
Vapor pressure is
temperature
dependent.
•
If its warmer, the
vapor pressure is
higher, we need
to slow the gas
flow through the
chamber.
Tilting hazard !!
• Tilting old vaporizers could flood the bypass area, in which case you
would deliver the full vapor pressure of the agent (Halothane 32%)
The Des Vaporizer -
• The FGF does
not actually flow
through the
sump.
• Instead fixed
concentration
Des vapor is
added in
proportion to the
FGF
Why vaporize DES this way?
• Well, the vapor pressure of DES at room temp is 672.
• Problem #1 – The heat loss from that much vaporization
would rapidly cool the vaporizer and end up dropping the
vapor pressure dramatically
• Problem #2 – 672/760 = 88%
• It would take tremendous FGF through the bypass chamber to
dilute that down to a useable level.
• Just one more point on DES which is covered on the ITE – the
old vaporizers for ISO, SEVO automatically compensate for
changes in altitude. But, high altitude DECREASES the partial
pressure for DES, so you will have to manually increase the
concentration of DES at high altitudes.
Why?
• Because it is partial pressure that really matters
• Forget Volume% for a min
• Partial pressure is measured in mmHg
• 2% ISO at 760mmHg = 15.2mmHg (partial pressure)
• So, of the gas coming out of the machine at
760mmHg, 2% of it (or 15.2 of those mmHg) are
Isoflurane.
• We know 2% @ 760 is 15.2 mmHg – but what if we
climb??
• At higher altitude, the decreased Patm will allow more ISO to come out of
solution. So, even if the dial is set at 2%, the actual concentration
coming out is higher – again because the lower Patm lets more ISO
come out of solution.
• But if we went to altitude with half the Patm, we would double the
concentration coming out of the vaporizer, but the partial pressure
remains the same
• 4% @ 380 is 15.2 mmHg – This is how these vaporizers self-equilibrate
• % increases but Patm decreases so overall is same result
• The thing is, DES cant do this, because no matter how high you go in
altitude, the % is exactly what you set on the dial . . . So you have to
deliver higher concentrations to compensate for altitude
The Aladin Cassette vaporizer
•
This diagram
sucks
•
What I want
you to take
from this is that
there is no
bypass channel
in the cassette
itself
•
As such, the
cassette is not
a tipping
hazard.
•
I bet you can
stump the
faculty with this
one
Ye olde breathing
circuit
A couple things to notice
here. This is the FG input.
Note it is on the proximal
side of the inspiratory valve.
This valve would be closed
during exhalation.
This is the old spirometer.
It is just distal to the
expiratory valve. So
when it is taking a
measurement at
expiration, it is getting only
exhaled tidal volume, and
no FGF which would throw
the measurement off.
As proven at lakeside, these spiromed spirometers
really stand the test of time
Designing spiromed
spirometer . . . In
dog costume.
Last few slides on spirometers, I promise
• This is the vane
aneometer.
• vane, like
weather vane. It
spins in the wind.
• It was placed in
line proximal to
the exp. valve
and the TV was
calculated based
on the spin.
Fixed orifice flowmeter
• This is a Pilot
tube. It is used
in aviation.
Here is how it
works:
Pitot tubes are tube shaped and
contain 2 holes. One hole faces
the direction of movement and,
measures the stagnation pressure
of oncoming air. The other hole is
on the side and measures static
pressure. The difference between
these two pressure types allows
for the measurement of dynamic
pressure, which is then used to
calculate the aircraft's airspeed
So how does that help us?
• We can use those same
principles to integrate flow
over time to determine tidal
volumes.
• If you know the airway
pressures you can integrate
that info as well and produce
the flow-volume loops which
can tell you lots about airway
and lung mechanics.
Like this . . .
Ventilators
• Older methods of
external
ventilation relied
on generating
negative
pressure around
the chest wall.
Phases of ventilation
• 4 phases are identified
• Inspiration
• Transition from inspiration to expiration
• Expiration
• Transition from expiration to inspitation
• We classify a vent based on its inspiration and
transition from inspiration to expiration characteristics
Constant pressure
generator
Note they
are based
on the way
they handle
inspiration
Constant
flow
generator
Non-constant
generator
Transition from inspiration to
expiration
• You can terminate inspiration based on one of three
parameters
• Time
• Volume
• Pressure
• I expect most of us understand the volume and pressure
modes.
• Time mode: You set a time allotment for inspiration and you
vary the gas inflow rate during that time allotment until you
reach a tidal volume you are happy with.
•
You can really play with this mode in patients with terrible lung
compliance and adjust it to try and limit your Ppeak.
Expiratory phase
• The expiratory phase is simple.
• Return the lungs to atmospheric pressure, unless you
have set some PEEP
• A completely passive process
Transition from Exp to Insp
•
It is really this phase on which we base our nomenclature for vent
modes
•
Volume-Control → vent adjusts gas flow rate to deliver set tidal
volume based on set vent rate and I:E ratio
•
Pressure-Control → vent adjusts gas flow rate to deliver a constant
pressure based on set vent rate and I:E ratio
•
You already knew that.
Now its time for some meat and
potatoes
• 2 main types of
Ventilator Circuit
Design:
• Double-Circuit
• Piston-Driven
This is a double circuit ventilator
Remember this guy?
He puts out O2 at
hospital line
pressure (?psi)
This is him too . . . sneaky
So it
makes
sense that
it is
delivering
50 psi of
O2
Remember him, and lets make a few
points
• 1. In a double-circuit system the tidal volume is
delivered from a bellow inside a plastic bucket
• Older vents used hanging bellows that were weighted.
• But when the circuit disconnected the weight would pull the
bellow down and you may think you were still ventilating the
patient.
• This is what they looked like:
Obviously this was not
too safe.
So, we switched to ascending
bellows
• Because for some
reason, we tend to
notice a lot faster
when the bellows
are just laying flat
Before we flip to the next slide, I need a CA-1 to
tell me where the air inside the bellows is
coming from…
So, the vent is really just an automatic
breathing bag
• It’s the
patient’s last
exhaled
breath
• Just like the
breathing
bag, the
bellows is
simply
pushing air
around the
circle system.
This is now the
vent.
This is how they really work:
The inside of
the bellows is
continuous with
the circle
system
The 50 psi O2 from
the oxygen power
outlet enters here and
pressurizes the
bucket. This
squeezes the bellow
Spill valve
• When you are running the vent, the APL is excluded
from the circuit
• But, luckily, the vent has its own APL – the “spill valve”
• Exhaled air beyond the capacity of the bellows and
circle system opens the spill valve just like a pop-off
and gets shunted out the WAGD
Piston Ventilators
• Again, the
drive gas is
completely
isolated from
the circle
system
• Drive
mechanism is
electrically
powered
Why pistons can be better:
• Don’t require much drive gas
• More accurate tidal volumes
• Better for patients with poor lung compliance
• Better for pediatrics s and small patients
• But, pistons have one big downfall:
• During that downward stroke of the expiratory phase they
actually generate negative pressure in the circuit.
• Look again:
So we modified the circle system:
Ever wonder
why the
breathing
bag in room
5 at
children’s
keeps
moving while
you are on
the vent? Its
not to amuse
your during
the 5th
circumcision
of the day . .
.
When the piston
generates that negative
pressure
This valve
closes to
protect the
patients lungs
This valve opens
which allows . ..
Air to be pulled from the breathing bag
So in a piston ventilator the
bag is not excluded from
the circle when on the vent.
Another frequently tested ITE topic
• In the older machines,
there was no mechanism
to compensate for FGF
during vent inspiration.
• So, tidal volumes were
higher than set values
based upon the fresh gas
flows.
Look at this:
There is FGF still
coming in through the
inlet. Any FGF coming
in during inspiration on
the vent will be added
to the tidal volume.
Vent
While the vent is
delivering your 500 ml set
tidal volume . . .
This is how you work through this
problem on the test:
• They will give you a:
• Rate (10 breaths/min)
• TV (1000 ml)
• I:E ratio (1:2)
• FGF rate (6 lpm)
•
With that info you will calculate this:
•
10 breaths / min = 6 second breaths
• I:E of 1:2 means 2 of those seconds spent in inhalation, 10 times per
minute.
•
•
A total of 20 seconds inhalation per min
20 seconds x (6000 ml FGF/ 60 seconds) = 2000 ml added TV over a
minute
• At 10 breaths/ min that is 200 ml added to the TV of each breath
Airway pressures:
•
Peak pressures → highest pressure
during insp. cycle
•
•
Reflects dynamic compliance
Plateau pressure → pressure during
insp. pause
•
Reflects static compliance
•
IN NORMAL HEALTHY LUNGS THESE
TWO NUMBERS SHOULD BE VERY
CLOSE.
•
Increased peak and
plateau pressures:
• Increased TV
• Worsening overall
compliance
• T-berg
• Pulm edema
• Insufflation
• Tension
Pneumo
• Mainstem
•
Isolated increased
peak pressures
• Increased insp flow
rate
• Increased Airway
resistance
• Bronchospasm
•
Tube kink
• Secretions
• Aw compression
Question time:
• A SEVO vaporizer will deliver an accurate
concentration of an unknown volatile anesthetic if the
latter shares which property with sevoflurane?
• A. Molecular weight
• B. Viscosity
• C. Vapor Pressure
• D. Blood gas partition coefficient
• E. Oil gas partition coefficient
Answer:
• C
• Which of the following valves prevents transfilling
between compressed gas cylinders?
• A. Fail safe valve
• B. Pop off valve
• C. Pressure sensor shutoff valve
• D. APL valve
• E. Check valve
Answer
• E
• When the pressure gauge on a size E N20 cylinder
begins to fall from 750 psi, how many liters of gas
remain in the cylinder?
• A. 200
• B. 400
• C. 600
• D. 800
• E. Cannot be calculated
Answer
• B
• For any given concentration of volatile anesthetic,
the splitting ratio is dependent on which of the
following characteristics of the anesthetic?
• A. Vapor pressure
• B. Barometric pressure
• C. Molecular weight
• D. Specific heat
• E. MAC at 1 atmosphere
• Answer : A
• After induction and intubation with confirmation of
•
•
•
•
•
tracheal placement the O2 sat begins to fall. The
analyzer as well as the mass spectrometer show
inspired O2 concentration of 4%. O2 line pressure is
55 psi and there is a E cylinder with 1600 psi. What
do you do?
A. Exchange the tank
B. Swith the O2 and N2O lines out
C. Disconnect the hospital O2 line
D. Extubate and mask ventilate
E. Replace the pulse ox probe
Answer:
• C
Pierre Robin
• Micrognathia
• Glossoptosis
(retraction and
posterior displaced
tongue)
• Upper airway
obstruction
• Frequent cleft
palate
Treacher Collins
• Absent
cheekbones
• Other craniofacial
deformities
• Malformed or
absent ears
• Conductive
hearing loss
• Micrognathia
Goldenhar
• Anomalous
development of first
and second
branchial arches
• Hemifacial
microsomia
• Ear, nose soft
palate, vertebral
abnormalities, often
unilateral
King-Denborough
• Myopathy similar to central
core disease
• Kids also have
•
•
•
•
•
Kyphosis/Lordosis
Micrognathia
Cryptorchidism
Pectus Carinatum
Other facial features
• THESE KIDS ALMOST
ALWAYS HAVE MH !!!
Central Core
disease
• Hypotonia
• Mild developmental
delay
• Skeletal abnormalities
(scoliosis)
• Absent mitochondria
on light microscopy
• VERY HIGH RISK OF
MH
• Can be autosomal
dominant, recessive,
but frequently De
Novo mutation of
RYR1
Downs
• Trisomy 21
• Hypothyroid
• Small trachea
(reduce tube
size)
• OA instability
• Congenital heart
Dz
• NO INCREASED
RISK OF MH
Gastroschisis
Omphalocoele
TEG
Lithotomy
• Common peroneal
nerve injury
• Foot drop
• Loss of eversion
• Loss of toe extension
• May also cause
stretch on sciatic,
causing sciatica
Do not confuse with
excessive hip flexion
• Which causes
Meralgia
paraesthetica
• Lateral Femoral
Cutaneous Nerve
• A SENSORY
ONLY
NEUROPATHY
• Excessive retraction in lower
abdominal surgery
• Femoral nerve
• Quads weakness (decreased
knee extension)
• Numbness anterior thigh,
medial leg
Difficult Forceps Delivery
• Obturator nerve
• Decreased leg
adduction
• Numbness at the
medial thigh
Thank you all for a great year !
AND GOOD
LUCK
SATURDAY

similar documents