Anesthesia Systems Dr. Wallace

							ANESTHESIA SYTEMS
The fine art of plumbing and gassing Frank Wallace, MD

Anesthesia Machine

Approach
Anesthesia machines are complex, convoluted packages Over 700 parts, seals, gaskets Chance for trouble anywhere in the system Key to troubleshooting is organizing into main subcomponents

Anesthesia Machine Schematic

Subcomponents
Compressed gas source Flowmeters Vaporizers Breathing systems CO2 elimination system

Compressed gas source
Provides fresh gas flow (FGF) Provides pneumatic power for ventilator Delivered gases  O2  Air  N2O Source  Pipeline system  Cylinder system

Both systems are inline connected Pipeline is primary Cylinder is safety redundant back-up

Pipeline
Gases enter anesthesia system at 50 psi Medical grade, made in the hospital Diameter Index Safety System (DISS) insures against unintentional gas type cross connection Each gas has different diameter on hose end

DISS
• Insures against unintentional cross connection • Specific supply lines for different gases Used for quick-connect fittings and extension hoses May easily be outwitted by installing wrong connector on hose

Cylinder
E-size Gases 1st leave at  Air: 1800 psi  O2: 2200 psi  N20: 750 psi • 1st stage regulator reduces/converts high variable pressures to constant 45 psi to enter system • 45 psi is intentionally < system 50 psi to prevent cylinder silent depletion if left open after its check • Pin Index Safety System (PISS) • Medical grade from certified filling source

PISS
Patterns

Cylinder
Hanger yoke 1. Orients cylinders 2. Provides unidirectional flow 3. Ensures gas-tight seal Check valve 1. Minimizes trans-filling between cylinders 2. Allows cylinder change during use 3. Minimizes atmosphere leak pollution

E cylinders

Critical Temperature
Temperature above which a substance can’t be liquified regardless of pressure O2, Air can only exist as gas since critical temp is -120C and -140C respectively (far below room temp) Conversely, N2O, CO2 only exist as liquid at room temp since critical temp is 36C, 31C respectively

Tank fills

O2 Cylinder

Tank fills

N2O Cylinder

Failsafe valve
1. 2. Exists to prevent N20 flow into system if O2 pressure, not flow, drops <30 psi (preset level); alarm sounds Maintains O2 concentration until flow stops (psi = 0) But….. Does NOT prevent hypoxic mixture Hypoxic guard system further down stream ensures against that (O2 analyzer inspiratory limb, O2 proportioning system in flowmeters)

1. 2.

Pressure Reduction
OHMEDA Has 3 pressure regulators  1st stage: O2 @2200psi45psi N20 @750psi25psi  2nd stage: 45psi14psi  Pressure relief valve: opens when >14psi

Pressure Reduction (cont’d)
DRAGER Has 1st stage regulator, like Ohmeda No 2nd stage regulator Pressure relief valve opens when >14psi

Pneumatic Drive
Bellows are historically either of 2 types Ascending Drive gas pushes bellows down during inspiration Descending = “Hanging Bellows” Drive gas pushes bellows up during inspiration

Pneumatic Drive
Typical Ascending Billows

Flowmeters
Thorpe tube is older term Precisely controls and measure gas flow to common gas inlet Components  Needle valve  Indicator float  Knobs -- flow increase when turned counterclockwise (same as vaporizers)  Valve stops

Types

Indicator Floats

Flowmeters (cont’d)
2 types Constant pressure – a pressure decrease across the float remains constant for all positions in tube Varible orifice – tapered tube, largest diameter at top, smallest at bottom

Flowmeter (cont’d)
Principle
Space between float and tube wall forms a channel or annular opening Flow measurement based on flow past a resistance proportionate to pressure Gas flow increases directly when this opening increases Pressure drop across resulting constriction balanced by float weight Increased gas flow causes increased lifting force on float and reduces constriction area

Flowmeter (cont’d)
Minimum FGF of O2 is 200-300ml/min Flowmeters factory calibrated to lowest accurate point on the visible scale Flows extrapolated <1st scale mark inaccurate Error increases inversely to flow rate Significant at <1 lpm: 70% error of set flow 1-5 lpm: <5% error of set flow Thus, low-flow tubes in series are more accurate Finally, measure [O2] during low flow to avoid hypoxic mixture caused by flowmeter inaccuracies

Flowmeter (cont’d)
Pressure and flow proportionality determined by tube shape (resistance) and physical properties (viscosity vs density) Low flows (float near bottom of tube) Orifice around the float is narrow tubular Poiseuille’s Law operates: flow governed by viscosity High flows (float near top of tube) Orifice around float wide tubular Graham’s Law operates: flow governed by density

Flowmeter (cont’d)
Arrangement 2 tubes for same gas in series for finer control Since gases don’t have same viscosity and density, tubes aren’t interchangeable O2 tube always located to far right Gas flows exit flowmeter into a mixing chamber

Flowmeter (cont’d)
Leak = break in any gas tube or hose If O2 not in furthest downstream position, escaping O2 could result in hypoxic mixture to patient Hypoxic mixture is less likely, but not impossible Know low pressure circuit leak test procedure for Drager vs Ohmeda

Flowmeter (cont’d)

Vaporizers
Types Measured flow aka…  Bubble-through or Copper Kettle



Vaporizers
Varible-bypass (2 paths) aka..  Direct reading  Concentration calibrated 1 sat’d w/volatile via close contact w/surface 1 bypass Bimetallic temperature strip regulates paths Anesthetic concentration via flow ratio between the 2

Each is calibrated for a specific agent under standard conditions of 20C, 760T Volatile delivery independent of total gas flow up to 15 lpm and of ambient pressure Older models can allow intermittent back pressure during low concentrations
Called “pumping effect” when additional

Vaporizers

Vaporizers
ISOFLURANE Ohmeda vs Drager

Vaporizers
DESFLURANE

Breathing systems
4 Types Open Closed Semiopen Semiclosed

Breathing systems
OPEN: Drip can ether Advantages 1. No artificial airway resistance or dead space 2. Easy to use in open field conditions Disadvantages 1. Heat and humidity loss 2. Ventilation can’t be controlled 3. Nonexistent scavenging - - exhaled gases vented to common atmosphere 4. Unstable anesthetic levels - - when pt light, VT increase, entraining RA, further dec’s anesthetic

Breathing systems (cont’d)
CLOSED Lost art Ultra low flow FGF state FGF of O2 metered to MVO2, namely 3ml/kg/min Partial rebreathing of volatiles allowed Advantages High conservation of both volatiles and O2 Low environmental pollution Max conservation of heat and humidity

Breathing systems (cont’d)
Disadvantages 1. Volatile and O2 can’t be rapidly changed in response to stimuli 2. O2 uptake not changed, but N2O and volatile uptake is dec’d = diffusion hypoxia

Breathing systems (cont’d)
Disadvantages 3. Delivered volatile concentrations uncertain as exhaled gases depend on tissue uptake 4. High inflows needed initially, but demand dec’s over time 5. Initial FGF 3 lpm x 15mins limits this

Breathing systems (cont’d)
SEMICLOSED aka Circle system Circle System

Advantages 1. Heat and humidity conservation 2. Proper scavenging, economical agent use
Disadvantages 1. Inc’d resistance to SV, thus inc’d Vd 2. Bulky, dec’d portability, complex design

Breathing systems (cont’d)
3. Valve and connection malfxn, misconnects vs disconnects at any of 10 locations 4. Obstructions and leaks account for 1/3 of malpractice cases in closed claims analysis project for unrecognized hypoxic injury

Breathing systems (cont’d)
SEMIOPEN aka Mapleson systems 6 types: A through F Points: Rebreathing CO2 retention possible for all Rebreathing potential fxn of SV vs CV To limit rebreathing, FGF x >2 pt’s Ve for both SV, CV for practical use

Mapleson systems
Order of increasing rebreathing SV: A<D<C<B Mneumonic: “All Dogs Can Bite” CV: D<B<C<A Mneumonic: “Dog Bites Can Ache”

Mapleson systems
1st gets the APL APL Before FGF Cut off “B”
(designed short by Dr Waters for CO2 scrubber)

APL Distal to FGF E = D without bag
F = J-R modifi’d. D, E Bain = modifi’d D

CO2 Absorption
2 kinds of absorbers Soda Lime Has silica to give hardness to granules, minimize alkaline dust forming Baralyme No silica since is inherently harder

CO2 Absorption
SODA LIME CO2 + H2O <- -> H2CO3 H2CO3 + 2NaOH (KOH) < - -> Na2CO3(K2CO3)
+ H2O + heat
Na2CO3(K2CO3) + Ca(OH)2 < - -> CaCO3

+ 2NaOH (KOH)

CO2 Absorption
BARALYME Ba(OH).8H2O + CO2 < -- > BaCO3 + 9H2O + heat 9H2O + 9CO2 < - - > 9H2CO3 9H2CO3 + 9Ca(OH) < -- > CaCO3 + 18H2O + heat

CO2 Absorption
POINTS: Ideal H2O content needed for optimal CO2 scrub Too much reduces surface area Too little reduces H2CO3 formation (esply with soda lime) Both scrubbers have a pH-sensitive dye, ethyl violet, that changes color as granules exhaust d/t H2CO3 accumulation Sevo unstable in soda lime – Cmpd A produced Heat and humidity added to insp’d gases, a good thing

CO2 Absorption
Max efficiency is 26L CO2/100gm absorbent Actual efficiency less d/t gas flow, canister design, channeling, moisture content, end point to detect granule exhaustion Canister design Single chamber: 10-15L CO2/100gm Dual chamber: 18-20L CO2/100gm

CO2 Absorption
Dual Chamber Canister

Finally….
THE END