The Anesthesia Machine and Breathing Systems

1. The Anesthesia Machine and Breathing Systems Trey Bates, MD Special Thanks to Judson Mehl

2. 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)

3. Pressure Reduction Pathway • H-Cylinder2000 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

4. 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 !!!

5. 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?

6. 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?

7. 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

8. 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.

9. 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 !!

10. 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

11. Anesthesia at Tulane

12. 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

13. Open Drop Anestheisa Schimmelbusch mask

14. Draw-over anesthesia • Hose serves as an open-ended reservoir • Addition of oxygen possible • 1 lpm 30-40% • 4 lpm60-80% • Simple • Portable • No scavenging

15. 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

16. 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

17. Basic Mapleson A During spontAneousventilation, the Mapleson A is most efficient. That long breathing tube full of fresh gas is a great reservoir for the patients next breath.

18. 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.

19. Mapleson D • The FGF is happening right at the patient’s face. • Now, watch this . .

20. 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.

21. 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.

22. 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

23. 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

24. So, science happened • And then we added: • CO2 absorbers • Unidirectional valves • Scavenging • And voila, we have the CIRCLE SYSTEM !!

25. 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)

26. 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

27. And what about these unidirectional valves? • Inspiratory • Expiratory • Valve incompetence is usually due to unseated or warped disc • Note what is in the reservoir bag

29. In a closed scavenging system, what happens to the reservoir bag during expiration and inspiration? What does it mean with the opposite happens?

30. 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 circuitandthe reservoir bag would inflate during inspiration.

31. 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

32. Answer: • C • Size E compressed gas cylinders completely filled contain 1590 L gas

33. 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

34. Answer: • C

35. 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

36. Answer • D – of course, but why change it if its not purple?

37. 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

38. The DISS The PISS

40. Lets take a quick look at these so-called “OFPDs”

41. The OFPD

42. Lets look again

43. 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 . . .

45. Flowmeter sequence: • Oxygen is universally on the right • The knob is larger and fluted • Why?

46. 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.

47. This is a Thorpe Tube • These are called “constant-pressure variable-orifice” flowmeters. • Conductive coating to reduce effect of static electricity • Calibrated 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.

48. Oxygen/Nitrous Oxide ratio controllers • Draeger utilizes this little gem:

49. But, Datex-Ohmeda got it right

50. 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 !!!