Presentation on theme: "The Anesthesia Machine and Breathing Systems"— Presentation transcript:
1The Anesthesia Machine and Breathing Systems Trey Bates, MDSpecial Thanks to Judson Mehl
2A quick word on medical gas - All those hoses:OxygenAirNitrousVacuumWAGDWaste Anesthetic Gas Disposal2000 PSI(FULL)H (7000L)E (700L)
3Pressure Reduction Pathway H-Cylinder psiManifold 55 psiHospital line 55 psiO psi, 625 LN2O psi, 1590 LCo piso, 1590 LAir psi, 625 L
42000 psi !!! 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 regulators45 psi2000 psi !!!
5Key points for ITE:Liquid oxygen must be stored below its critical temperature of -119 CIn oxygen tanks, the pressure falls in proportion to the remaining volume of oxygenIf 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?
6More 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?
7NitrousNitrous is NOT an ideal gas. Thus it has several unique properties:Transition between liquid and gas states does not lead to huge increases in pressureIt is easy to compress, so the cylinders hold a lot moregas.Its critical temp is 36.5 C, so it doesn’t need refrigeration
8More on N2O N2O is vaporized at the same rate it is utilized The pressure in the tank never changesYou 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 bottleI have never been asked to determine how much N2O was left based on weight on an ITE.
9Breathing Systems This stuff matters because: Oxygen is pretty importantAgent delivery is pretty importantGetting rid of CO2 is pretty importantAnd these ALWAYS show up on the ITE. ALWAYS !!
10CO2 Is a . . . Cardiac Depressant And it . . . Increases CBF Increases bleedingCauses acidosis which . . .Shifts the Carboxy-Hgb curveShifts Ca2+ and K+ out of the cell
14Draw-over anesthesia Hose serves as an open-ended reservoir Addition of oxygen possible1 lpm 30-40%4 lpm 60-80%SimplePortableNo scavenging
15The Mapleson Circuit Ingredients: Breathing tube Fresh gas inlet APL valveReservoir bagThe only real difference is the order in which the ingredients occur
16Mapleson Circuits Important points There are NO one-way valves There is no CO2 absorberSome rebreathing is prevented by venting through the APL before the next inspiration
17Basic Mapleson ADuring 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.
18Why?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.
19Mapleson D The FGF is happening right at the patient’s face. Now, watch this . .
20When 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.
21BainThe 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.
22Get 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 aMapleson A during spontAneous ventilationThere is no D in spontaneousMapleson D during controleD ventilationThere is no A in controled
23The downfalls of the Mapleson Lose all the heat and humidityHigh FGF to prevent rebreathingAll that agent is ventilated out to the room
24So, science happened And then we added: CO2 absorbers Unidirectional valvesScavengingAnd voila, we have the CIRCLE SYSTEM !!
25CO2 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)
26CO2 absorbers As the absorbent is used up, it becomes more acidic. That purple color change is a pH indicatorWhen 50-70% has changed color, its time to change the absorber.Granule size is a trade off:Larger granules minimize resistance to airflowSmaller granules maximize surface area for more absorption
27And what about these unidirectional valves? InspiratoryExpiratoryValve incompetence is usually due to unseated or warped discNote what is in the reservoir bag
29In a closed scavenging system, what happens to the reservoir bag during expiration and inspiration? What does it mean with the opposite happens?
30The 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.
31A few questionsA size E compressed-gas cylinder completely filled with N2O contains how many litres?A. 1160B. 1470C.1590D. 1640E. 1750
32Answer:CSize E compressed gas cylinders completely filled contain 1590 L gas
33QuestionThe 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 minB. 140 minC. 280 minD. 320 minE. Cannot be calculated
35QuestionIf 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 minB. Place a humidifier in the expiratory limbC. Avoid the use of SevofluraneD. Change the CO2 absorbentE. Use N2O for the first hour of the case
36AnswerD – of course, but why change it if its not purple?
37One 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 minB. 30 minC. 60 minD. 90 minE. 100 min
43Lets stop for todayOne 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 circuitDistal to the knobs (eg. In the Thorpe tubes and onward) you are in the low-pressure circuit.To be continued . . .
45Flowmeter sequence: Oxygen is universally on the right The knob is larger and flutedWhy?
46The 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.
47This is a Thorpe TubeThese are called “constant- pressure variable- orifice” flowmeters.Conductive coating to reduce effect of static electricityCalibrated to be gas- specific **** Flow rate across a constriction depends on the gas’s viscosity at low laminar flows and its density at high turbulent flows.
48Oxygen/Nitrous Oxide ratio controllers Draeger utilizes this little gem:
50On 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 pressureThe energy required for vaporization is manifested as loss of heat from the anesthetic solutionAs the anesthetic vaporizes, the solution becomes colder And when the temp drops, so does the vapor pressure !!!
51Copper kettles Copper has a high specific heat Copper has high thermal conductivityResistant to the temperature drop from vaporizationBest material to maintain a constant temperature
52Copper Kettle Measured-flow vaporizer Separate flowmeter for the gas flowing through the kettleGas passing through the kettle becomes fully saturatedThen you dilute it out to the proper percentage with the other flowmeterThe math gets a little funny here, and they will throw you a copper kettle equation on the ITE, so watch this . . .
53The math of a copper kettle Vapor pressure of Halothane (and ISO) is about 243 mmHg at 20 CSo 243/760 = 32%
54So here is what we knowAt 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 volatileIf we keep the flow at 5LPM total we can do this100 ml to the kettle → 150 ml, 50 of which is halothane4850 ml to the dilution limb50 ml halothane / 5000 ml total FGF = 1% halothane
55For the ITEFor the ITE, in a copper kettle there should always be a total of 5L FGFAdd 50 ml vapor to every 100 ml you put through the kettleKeeping the totals at 5L/min, every extra 100 ml through the kettle increases the agent concentration 1%Eg. 100 → kettle → 150 (50ml agent) = 1% agent200 → kettle → 300 (100ml agent) = 2% agent300 → kettle → 450 (150ml agent) = 3% agentRemember, Iso and Halothane have similar vapor pressures, so this applies to Iso too
56Well, that sucked. Lets move on. These are modern vaporizersTec 4,5,6 all have similar mechanismsAladin is very different.Tec 5Tec 4Tec 6Aladin Vaporizers
57Lets 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 /760 = 32%Iso /760 = 32%Sevo /760 = 20%Des /760 = 88%These are “slightly” above clinically relevant concentrations
58But if we split the FGF between the vaporizer and a bypass channel . . . Well, then we have a variable bypass vaporizer.
59The Datex-Ohmeda version Note the bi- metalic stripThis serves to compensate for temperature changesVapor pressure is temperature dependent.If its warmer, the vapor pressure is higher, we need to slow the gas flow through the chamber.
60Tilting hazard !!Tilting old vaporizers could flood the bypass area, in which case you would deliver the full vapor pressure of the agent (Halothane 32%)
61The Des Vaporizer - The FGF does not actually flow through the sump. Instead fixed concentration Des vapor is added in proportion to the FGF
62Why 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 dramaticallyProblem #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.
63Why? Because it is partial pressure that really matters Forget Volume% for a minPartial pressure is measured in mmHg2% 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.
64We 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 same380 is 15.2 mmHg – This is how these vaporizers self-equilibrate% increases but Patm decreases so overall is same resultThe 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
65The Aladin Cassette vaporizer This diagram sucksWhat I want you to take from this is that there is no bypass channel in the cassette itselfAs such, the cassette is not a tipping hazard.I bet you can stump the faculty with this one
66Ye 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.
67As proven at lakeside, these spiromed spirometers really stand the test of time Designing spiromed spirometer In dog costume.
68Last 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.
69Fixed 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
70So 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.
72VentilatorsOlder methods of external ventilation relied on generating negative pressure around the chest wall.
73Phases of ventilation 4 phases are identified Inspiration Transition from inspiration to expirationExpirationTransition from expiration to inspitationWe classify a vent based on its inspiration and transition from inspiration to expiration characteristics
74Note they are based on the way they handle inspiration Constant flow generatorConstant pressure generatorNon-constant generatorNote they are based on the way they handle inspiration
75Transition from inspiration to expiration You can terminate inspiration based on one of three parametersTimeVolumePressureI 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.
76Expiratory phase The expiratory phase is simple. Return the lungs to atmospheric pressure, unless you have set some PEEPA completely passive process
77Transition from Exp to Insp It is really this phase on which we base our nomenclature for vent modesVolume-Control → vent adjusts gas flow rate to deliver set tidal volume based on set vent rate and I:E ratioPressure-Control → vent adjusts gas flow rate to deliver a constant pressure based on set vent rate and I:E ratioYou already knew that.
78Now its time for some meat and potatoes 2 main types of Ventilator Circuit Design:Double-CircuitPiston-Driven
80Remember this guy?He puts out O2 at hospital line pressure (?psi)
81This is him too sneakySo it makes sense that it is delivering 50 psi of O2
82Remember him, and lets make a few points 1. In a double-circuit system the tidal volume is delivered from a bellow inside a plastic bucketOlder 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.
83So, we switched to ascending bellows Because for some reason, we tend to notice a lot faster when the bellows are just laying flatBefore we flip to the next slide, I need a CA-1 to tell me where the air inside the bellows is coming from…
84So, the vent is really just an automatic breathing bag It’s the patient’s last exhaled breathJust like the breathing bag, the bellows is simply pushing air around the circle system.This is now the vent.
85This is how they really work: The 50 psi O2 from the oxygen power outlet enters here and pressurizes the bucket. This squeezes the bellowThe inside of the bellows is continuous with the circle system
86Spill valveWhen you are running the vent, the APL is excluded from the circuitBut, 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
87Piston VentilatorsAgain, the drive gas is completely isolated from the circle systemDrive mechanism is electrically powered
88Why pistons can be better: Don’t require much drive gasMore accurate tidal volumesBetter for patients with poor lung complianceBetter for pediatrics s and small patientsBut, pistons have one big downfall:During that downward stroke of the expiratory phase they actually generate negative pressure in the circuit.Look again:
89So 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 . . .
90When the piston generates that negative pressure This valve closes to protect the patients lungsThis valve opens which allows . ..So in a piston ventilator the bag is not excluded from the circle when on the vent.Air to be pulled from the breathing bag
91Another 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.
92Look 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.VentWhile the vent is delivering your 500 ml set tidal volume . . .
93This 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 breathsI:E of 1:2 means 2 of those seconds spent in inhalation, 10 times per minute.A total of 20 seconds inhalation per min20 seconds x (6000 ml FGF/ 60 seconds) = 2000 ml added TV over a minuteAt 10 breaths/ min that is 200 ml added to the TV of each breath
94Airway pressures: Peak pressures → highest pressure during insp. cycle Reflects dynamic compliancePlateau pressure → pressure during insp. pauseReflects static complianceIN NORMAL HEALTHY LUNGS THESE TWO NUMBERS SHOULD BE VERY CLOSE.
96Question time:A SEVO vaporizer will deliver an accurate concentration of an unknown volatile anesthetic if the latter shares which property with sevoflurane?A. Molecular weightB. ViscosityC. Vapor PressureD. Blood gas partition coefficientE. Oil gas partition coefficient
102For any given concentration of volatile anesthetic, the splitting ratio is dependent on which of the following characteristics of the anesthetic?A. Vapor pressureB. Barometric pressureC. Molecular weightD. Specific heatE. MAC at 1 atmosphere
104After 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 tankB. Swith the O2 and N2O lines outC. Disconnect the hospital O2 lineD. Extubate and mask ventilateE. Replace the pulse ox probe
106Pierre Robin Micrognathia Glossoptosis (retraction and posterior displaced tongue)Upper airway obstructionFrequent cleft palate
107Treacher Collins Absent cheekbones Other craniofacial deformities Malformed or absent earsConductive hearing lossMicrognathia
108Goldenhar Anomalous development of first and second branchial arches Hemifacial microsomiaEar, nose soft palate, vertebral abnormalities, often unilateral
109King-Denborough Myopathy similar to central core disease Kids also haveKyphosis/LordosisMicrognathiaCryptorchidismPectus CarinatumOther facial featuresTHESE KIDS ALMOST ALWAYS HAVE MH !!!
110Central Core disease Hypotonia Mild developmental delay Skeletal abnormalities (scoliosis)Absent mitochondria on light microscopyVERY HIGH RISK OF MHCan be autosomal dominant, recessive, but frequently De Novo mutation of RYR1
111Downs Trisomy 21 Hypothyroid Small trachea (reduce tube size) OA instabilityCongenital heart DzNO INCREASED RISK OF MH