The Anesthesia Machine and Breathing Systems

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