Breathing Circuits Flashcards
Time Constants Equation
vol of circuit (L)/FGF L/min = min, 3 time constants to get to 95% of vaporizer settings
Four Time constants?
98.2%
One Time Constant?
63.2%
Two Time Constants?
86%
What is more important in turbulent flow - gas density or gas viscosity?
Density
Generalized turbulent flow
results when flow of gas through tube exceeds certain value = critical flow rate
Localized turbulent flow
results when gas flow is below critical flow rate but encounters constrictions, curves, valves or other irregularities
Critical Velocity
greatest velocity with which a fluid can flow through a given conduit without becoming turbulent
What constitutes the breathing system?
Essentially FG outflow to scavenging
Function of BC
o Direct oxygen to patient
o Deliver ax gas to patient
o Remove CO2 from inhaled breaths
o Means for controlling ventilation
Insufflation
gases delivered directly to patient airway
Open Breathing Systems
patient inhales only mixture delivered by ax machine, valves direct each exhaled breath into atmosphere
+/- RBB
Rebreathing minimal, no CO2 absorbent
Higher FGF rates
Semi Open Systems
exceeds minute ventilation, generally rebreathing does not occur (depends on FGF), 150mL/kg/min
No chemical absorption of CO2
RBB, unidirectional valves optional
Exhaled gases flow out or mix with FGF
Semi Closed (most machines)
exceeds MVO2, allows partial rebreathing
Part of exhaled gases passes into atmosphere, part mixes with FGF
Chemical absorption of CO2, directional valves, bag
22-44mL/kg/min
Low flow?
10-15mL/kg/min
Closed Systems
Complete rebreathing: 3-10mL/kg/min
Directly matches MVO2
What is rebreathing?
composition of inspired gas mix of fresh gas, rebreathed gas
What is non-rebreathing?
composition of inspired gas same as fresh gas from machine
Rebreathing
inhale previously expired gases from which CO2 may or may not be removed
Possible to have complete rebreathing without increasing CO2
FGF: no RB if vol of fresh gas per minute > P’s minute volume
Mechanical dead space: vol in breathing circuit occupied by gases that are rebreathed without changing in composition
Breathing system components: arrangement can increase or decrease
What happens with excessively high FGFs in RB or NRB systems?
Minimal rebreathing
What happens with low FGF in NRB?
No complete preventing of rebreathing
Causes of increased FiCO2
-Decreased FGF
-Increased dead space
-expired absorbent
-stuck expiratory valve
Resistance
tracheal tube, important factor when determining WOB
Compliance
change in vol over change in pressure, measure of distensibility (mL/cm H2O)
most distensible components = reservoir bag, breathing hoses
Effects of Rebreathing
o Heat, moisture retention
o Altered inspired gas concentrations
Causes of increased inspired vol vs delivered vol
–FGF > than rate at which absorbed by patient or loss through leaks in BS when ventilator in use
–FGF delivered during inspiration added to VT delivered by ventilator
–Increased FGF, IE ratios; decreased RR
Eliminated by modern ventilators
Decreased inspired vol vs delivered vol
Gas compression, distension of components during inspiration = wasted ventilation
–wasted vent increased with increased airway pressure, VT, BS vol, component distensibility
Smaller patients > larger patients
VT decreases DT leaks in BS
–Measure via comparing inspired, expired VT
–Measuring tidal volume at end of expiratory limb will reflect increase caused by FGF, decrease from leaks; will miss decrease from wasted ventilation
Features that Cause Discrepancy btw Inspired, Delivered Oxygen, Ax Gas Concentrations
- Rebreathing
- Air Dilution
- Leaks
- Ax agent uptake by BS components
- Ax agents released from system
Discrepancy btw Inspired, Delivered Oxygen, Ax Gas Concentrations: rebreathing
Depends on volume of rebreather gas, composition
Discrepancy btw Inspired, Delivered Oxygen, Ax Gas Concentrations: air dilution
Negative pressure in breathing system may cause air dilution if leak
* When fresh gas supplied per respiration <VT
Concentration of anesthetic in inspired mixture to decrease, difficult to maintain stable anesthetic state
Discrepancy btw Inspired, Delivered Oxygen, Ax Gas Concentrations: Leaks
Positive pressure in system will force gas out of system
Discrepancy btw Inspired, Delivered Oxygen, Ax Gas Concentrations: Ax agent taken up by breathing system
taken up or adhere to rubber plastics metal and CO2 absorbent
Directly proportional to concentration gradient btw gas/components, partition coefficient, surface area, diffusion coefficient, square root of time
Discrepancy btw Inspired, Delivered Oxygen, Ax Gas Concentrations: Ax agent released by breathing system
Elimination of anesthetic agent from breathing system depends on same factor as uptake functions
Essentially low output vaporizers after vaporizer turned off
Connectors
join two pieces together
o Uses: extend distance btw patient, breathing system; change angle of connection btw patient and breathing system,
o Resistance increases with sharp curves, rough sidewalls
o Add dead space btw patient and breathing system
o increased number of locations disconnection can occur
What are the fittings on the ETT, FG outlet?
15mm
What are the fittings on everything but the ETT, FG outlet?
22mm
Definition of a rebreathing system
Unidirectional flow of gas, means of absorbing CO2 from expired gases
Full (complete) rebreathing
o Flow rates at or near patient MVO2
o 3-14mL/kg/min
Partial rebreathing
o FGF > MVO2, less than that required to prevent rebreathing
o Low flow = 20-50mL/kg/min
o So-so (medium) flow = 50-100mL/kg/min
o High flow 100-200mL/kg/min
Non (minimal) RB
o >150-200mL/kg/min, may result when circle systems used for small patients (<5kg) with FGF >1L/min
o Most modern vaporizers continue to function optimally down to 200-500mL/min
Pros of Lower FGF rates
More economical in terms of oxygen, inhalant
Less environmental contamination by inhalants (halogenated hydrocarbons)
Improved maintenance of body temp
Cons of Lower FGF
Decreased ax gas delivery
Time required to change ax concentration within circuit significantly increasess
What are the components of a circle system?
fresh gas inlet
inspiratory one way valve, inspiratory/expiratory breathing tubes, expiratory valve
APL valve, reservoir bag
CO2 cannister
What is true about low flows and anesthetic equilibrium?
Takes longer to achieve than high flows
Inspired gas concentrations will not reflect vaporizer settings until nearing equilibrium
Low FGF more commonly used in LA
Fresh Gas Inlet
- Site of gas delivery from common gas outlet of ax machine
- After CO2 absorber, before inspiratory valve
Inspiratory Valve (aka flutter valves)
opens via neg pressure from breath or pos pressure from vent -> gas moves from FGI, reservoir bag to valve in inspiratory limb
Structure of OVW
o Clear dome – direct visualization of valve function
o Cage or guide mechanism (prongs) – prevent disc dislodgement
o Light weight valve with hydrophobic disc – prevent condensation from sticking, which increases resistance to opening
o Valve housing – seat, guides
o Removable cover – access for cleaning, repair, drying
MOA OWV
Gas enters at bottom, raises disc from seat
Gas passes under dome, on through breathing system
Reversing gas flow causes disc to contact seat, preventing retrograde flow
OWV expiration
closed, prevents exhaled gas from entering inspiratory limb -> forces entry into expiratory limb
Positioning of OWV
Veritical or Horizontal
o Vertical valves decrease resistance to gas flow
Matrx circle system
negative pressure relief valve, alternative path of gas flow (room air) to patient should inspiratory valve stuck closed
Movement of valve
does not guarantee valve competence
Incompetent valve = less resistance to gas flow, flow of gas will go more easily through incompetent valve -> rebreathing
Expiratory > inspiratory, exposed to more moisture
Inspiratory Port
Downstream of inspiratory unidirectional valve, 22mm OD male connector
Breathing Hoses
Corrugated plastic or rubber inspiratory, expiratory limbs
Prevents kinking, allows expansion if BC subjected to traction, compression
o 2 x 22mm ID male connector ports for connection to breathing hoses
Connected to ETT via wye piece – 15mm ID female connectors
Dead space extends from wye piece to patient
Does length of tubes affect dead space?
NO!
Length of tubes does not alter amt of dead space or rebreathing DT unidirectional gas flow
Coaxial Systems
Decrease bulk assoc with breathing system, facilitate warming of inspired gases by expiratory gases
o Concentric, side by side
o Universal F circuit, can also be used as NRB
Inner (inspiratory) tube ends just before connection to patient
o Inner tube to patient, exhaled gases flow to absorber assembly via outer corrugated tubing
Disadvantages of coaxial systems
Increased resistance, increased dead space if leak in inner tube/difficult to catch, if gas flow reversed - increased resistance to exhalation
Y Piece
Three-way tubular connector with 2x22mm male ports for connection to breathing tubes, 15mm female patient connector for trach tube or supraglottic airway device
Most often permanently attached to breathing hoses
DEAD SPACE
Expiratory Port
Upstream of expiratory valve, 22mm OD male connector port
What is the most compliant part of a circle system?
Rebreathing Bag (application of LaPlace’s Law)
Expiratory Valve
Closes on inspiration, opens on expiration
Direct gas into expiratory limb of breathing system
Rebreathing Bag
o 22mm female connector at neck
o Tail, +/- loop to hang for drying
RBB Purpose
Compliant reservoir of gas that changes vol with patient’s expiration, inspiration
Allows gas to accumulate during exhalation, provides reservoir of gas for next inspiration
–Rebreathing, economical gas use, prevents air dilution
Assist, control of ventilation
Visual/tactile observation of patient’s spontaneous respirations
Protects from excessive pressures by being source of compliance in the system (LaPlace’s law)
MOA RBB
Addition of vol, pressure increases rapidly to peak then reaches plateau
As bag distends further, pressure falls slightly = max pressure that can develop in breathing system
RBB Size?
Vol 5-10x VT (10-20mL/kg), ~ patient’s minute volume (VT x RR) -> no consensus
Usually attached via 22mm male bag port
ASTM standards:
o <1.5L bags: pressure shall not be <30, >50cm H2O when bag expanded 4x size
o >1.5L bags: <35, >60
*Latex free bags: allow higher pressures to develop than latex bags
*New bags: greater pressures when overinflated than old bags or if pre stretched
What is another name for APL valve?
Heidbrink Valve
Function of APL
Allows excess gas to escape from patient circuit during expiration
o If functioning properly, should escape of pressure >1-3cm H2O
Anatomy of APL
Control part: controls pressure at which valve opens
–Spring-loaded disc
–Stem, Seat
–Control Knob
–Collection Device, exhaust port
Spring Loaded Disc on APL
disc held onto seat via spring, threaded screw cap over spring allows variation of pressure exerted by spring on disc
How spring loaded disc on APL opens
- increased pressure in BS –> upward force on spring, upward force > downward force of spring –> disc rises, gas flows through valve
- Cap maximally open: little resistance of spring
- Weight, pressure of disc ensures reservoir bag fills before seat rises
- increases pressure downstream –> increases pressure needed to open valve, PEEP
Stem, Seat of APL
Threaded stem, variable contact with seat
Valve opens –> opening of seat comes larger, more gas escapes
Ensures unidirectional gas flow (no backflow of gas from scavenge flowing back into breathing system), supplies slight pressure to keep breathing bag inflated
Control Knob of APL
Rotatory control knob: ASTM -> always closed clockwise, arrow indicator
Collection device, exhaust port
- Excess gases collected, directed to scavenge system via transfer tubing
- Exhaust port: site of discharge, 19 or 30mm male connector
APL Use
Fully open at all times unless PPV (can be isolated from system via bag switch), CPAP
Can be partially closed to prevent collapse of reservoir bag DT negative pressure/vacuum from scavenge system
o Risk of complete closure –> excessive pressure built up in system
o Ensure adequate inspiratory pressure
Bag/Vent Selector Switch
Shift rapidly between manual respiration, automatic ventilation without removing bag or ventilator hose from mount
o Essentially 3 way stopcock
When selector switch in ventilator position, APL valve isolated from circuit –> does not need to be closed
Newer machines: turning ventilator on causes electronically controlled valves to direct gases into proper channels
CO2 Canister Structure
Transparent to monitor color change of absorbent
Screens: hold absorbents in place
Two canisters in series vs one
Smaller canisters allow more frequent changes in [FGF] to be reflected more quickly, improve ventilator performance
Large Canisters
Longer intervals btw absorbent changes
Risk desiccation if in absorbent for long time
Small Canisters
Decreased likelihood of CO, compound A production (fresh absorbent with proper water content)
Decreased interval vol of BS
Must change more frequently
Canister Housing
Space at top, bottom: promotes even distribution of flow through absorber
Space at bottom: accumulation of dust, water
Canister Distribution Pattern
No difference whether gases enter at top or bottom
Start at inlet, progress down sides -> migrates throughout rest of canister
Canister Baffles
annular rings, direct gas flow toward central part of cannister
o Compensate for reduced flow resistance along walls
Side, center tube of canister
conduct gases to/from bottom of canister, return to patient
Bypass of Canister
In older absorbers - allow exhaled gases to completely or partially bypass absorber
o Allow CO2 accumulation
o Canister removed, not replaced - creates bypass
CO2 Absorbent MOA
- Base neutralizing acid –> carbonic acid + CO2 forming water, carbonate; releases heat
o Can predict dryness by measuring % of water in outflow gas
o Very widely inability to absorb nitric oxide, nitrogen dioxide - Monitoring downstream from absorber
High Alkali Absorbents
When desiccated, react with VAs –> CO, CA (sevo)
Do not change color when dry, capacity to absorb CO2 decreased by moisture
Low Alkali Absorbents
reduced NaOH, KOH; smaller amts CO, Cmp A?
Alkali Free Absorbents
–CaOH + small amts other agents to accelerate CO2 absorption, bind water
–No evidence of CO with any VA, little or no CA formation even if desiccated
–Indicator: changes color when dry
Once exhausted, do not revert to original color
o CO2 capacity < high alkali
–Do not deteriorate when moisture lost
Lithium Hydroxide
o Reacts with carbon dioxide to form carbonate
o Does not react with anesthetic agents, even if desiccated
o Expensive, requires special handling - burns to eyes, skin, respiratory tract
Indicators
Acid, base: color = pH-dependent, added to absorbent to signify when ability to absorb CO exhausted
Does not affect absorption
Ethyl violet most common, pH 10.3 -> bright, vivid color change
Phenolphthalein
White –> Pink
Ethyl Violet
White to Purple
Clayton Yellow
Red to yellow
(Clay turns yellow)
Ethyl Orange
Orange to yellow
(sunsets fade)
Mimosa Z
Red to white
Empty the mimosa leaves a clear glass
How granule size measured
Mesh Number
**Higher the mesh number, smaller the particles – most absorbents 4 to 8 mesh
**4 mesh strainer: 4 openings per square inch, 4 mesh will pass through but not through strainer with smaller holes (eg 8 mesh strainer)
**8 mesh strainer: 8 openings per square inch
Pellets, small granules
greater surface area, decreases channeling along low-resistance pathways
Potential for more resistance, caking
How absorbent changes consistency
Fresh granules crumble more easily
Used absorbents become hard - formation of CaCO3
Dust Production of Absorbents
Some granules fragment easily, producing dust
Excessive powder: channeling, resistance to flow, caking, may be blown through system to patient or cause system components to malfunction
Small amounts of hardening agent added to prevent
Sodalime Composition
80% CaOH, 15% H2O, 4% NaOH
How much CO2 can sodalime absorb?
25L CO2/100g sodalime
Baralyme composition
20% BaOH, 80% CaOH +/- KOH
How much CO2 can baralyme absorb?
27L CO2/100g baralyme
FIRE DANGER
Amsorb composition
80% CaOH, 1% Ca sulfate hemihydrate, 3% CaCl, 15% water
How much CO2 can amsorb absorb?
~12.5L CO2/100g
Minimal to no CO formation, minimal CA production, least amt of sevoflurane degradation
What are the human recommendations for absorbents?
only non-desiccated absorbents with no KOH, little to no NaOH
Haloalkene Formation
o Halothane degradation during closed circuit anesthesia
o Produces haloalkene 2-bromo-2-chloro-1, 1-difluoroethene (BCDFE)
–Nephrotoxic in rats
Compound A Formation: Factors that increase Production
Low FGF**
KOH, NaOH absorbents
Higher absorbent temperature** (more exhaled CO2)
Higher concentrations sevoflurane**
Longer duration of ax
Dehydrated absorbents**
Use of barium lime**
Factors that decrease production of compound A
Low alkali, alkali free absorbents
Lower absorbent temperature
(smaller canisters = lower temps)
CO formation
Desflurane, enflurane, isoflurane passed through dry absorbent containing strong alkali
Sevoflurane degraded by absorbent, CO formed if temperature exceeds 80*C
Most to least potential for CO formation?
Des > Enflurane > Iso»_space;> Halothane
Most common situation of CO formation?
Most commonly during first anesthetic of day on Monday, fresh dry gas flowing into the circle system over the weekend causing absorbent to become dehydrated
Or machine don’t use very often
Concentration in BS varies with time, peak in first 60 minutes
How detect CO formation?
Capnograph will appear normal (will not detect)
Pulse oximeter: read carboxyhemoglobin as oxyhemoglobin, unless COHgb levels very high then see slight drop (will not detect)
Not detected by currently used respiratory gas monitors
What might suggest have a problem with CO?
Multi wave infrared analyzers may provide warning of isoflurane or desflurane breakdown by displaying wrong agents or mixed agents
* Unusually delayed rise or unexpected decrease in inspired concentration of VA in BS vaporizer setting
* Failed inhalation induction
* Inadequate anesthesia
* Unexpected decrease in inspired concentration
Factors That Increase Production of CO
KOH, NaOH absorbents*
Dry absorbents* – extended period of FGF during non-use
Des > En > Iso»_space;> halothane**
Sevo if cannister >80C
High FGFs: produce more but decreased concentrations bc remove it (also promote decissation)
Factors that decrease production of CO
None if normally hydrated
CO2 absorption
How to Minimize Risk of CO Formation
o Turn off all gas flows after each case, disconnect medical gas pipeline system at the end of the day
o Turn off vaporizers when not in use
o Change absorbent at least once a week, label with filling date, check date as part of daily machine checkout
o Machines not in use should not be filled with absorbent, fill with fresh before each use
o Supplying oxygen to a patient not receiving GA via circle system strongly discouraged
(Ideally FM that connected directly to oxygen pipeline system or auxiliary O2 flow meter on machine)
o Avoid using FG to drive breathing system components
o Negative pressure relief valve on closed scavenging systems should be checked regularly
o Failure of valve to pull room air may result in FGF from machine being drawn through absorbent if APL valve open
o Monitor temperature in the canister change absorbent if excessive heat detected
Barium Hydroxide (BaOH), Baralyme
- interaction with sevoflurane = temperatures of several 100*C
o Reports of fires, melted components of canisters - removed from market
o Soda lime: less elevated temperatures, fires involving desiccated soda lime reported
Storage and Handling of Absorbents
–Supplied in several types of containers
–Once opened resealed ASAP to prevent absorbent reaction with CO2 in air, indicator deactivation, moisture loss
–Temps <32*F: moisture expands granules, fragmentation
–Gentle handling to avoid fragmentation, dust formation -> irritating to eyes, respiratory tract, Caustic to skin particularly when damp
–Important not to overfill, small space at top to promote even gas flow through canister
How, When to Change Absorbent
Most Reliable Method = appearance of CO2 inspired gas
Indicator color change
Heat in canister
Detection of desiccation
Color Change Limitations
Peaking, regeneration seen with absorbance that contains strong bases
* Appears to be reactivated with rest
* Absorbent that shows exhausted color if allowed to at rest often show color reversal – false Impression of usefulness
Absorbents without strong base change color when dried
Channeling: absorbent along channels will become exhausted, CO2 will pass through canister
* Color change may not be visible if on inside of cannister
Absorbent may not contain an indicator
Ethyl Violet
undergoes deactivation even if stored in dark
* Deactivation accelerated in presence of light, especially high intensity or ultraviolet light
Heat in Canister
Changes in absorbent temperature occur earlier than changes in color of the indicator
Some heat production should be apparent unless high FGF being used
If temp of downstream canister exceeds that of upstream canister –> change absorbent of upstream canister
To and Fro Circuits
*Horses
*Waters’ cannister: patient breaths in, out of closed bag connected to ETT/face mask via canister containing absorbent
*FG introduced at patient end of system, APL usually mounted close by
Similar to Mapleson B
Disadvantages of To and Fro
- Absorbent at patient end becomes exhausted first –> increases functional dead space of system
- Cumbersome, risk of inhalation of absorbent dust (resolved with filter)
Logistics of To and Fro
- Part of system btw p, soda lime = dead space –> vol must be kept to minimum
o Canister must be close to patient’s head, technical challenges
o Horizontal position of canister recommended
o Must be well packed to avoid channeling/incomplete absorption
NRB
Absence of unidirectional valves to direct gases, no device for absorbing CO2, FGF must wash CO2 out of circuit
o High FGF rates to flush CO2 from circuit
o Not used for patients exceeding 10kg = less economical DT high FGF
o Recommended FGF: 150-300mL/kg/min
Maintenance of patients on NRB
Traditionally recommended patients <5kg DT lower resistance during breathing, less equipment dead space, smaller total circuit volume
Possible to maintain patients <5kg safely using rebreathing systems provided patients VT adequate to accentuate unidirectional valves
What is also important about NRB circuits?
No clear separation of inspired, expired gases
o Rebreathing when inspiratory flow > FGF
o Best way to determine optimal FGF = monitoring ETCO2
General Principle of NRB
During expiratory pause, high FGF pushes exhaled gas from previous expiration down exhalation conducting tube away from patient toward reservoir bag
Inspiration: inspires gas coming from FGI, exhalation conducting tube
–Under normal circumstances majority of inspired breath comes from FG in exhalation conducting tube
Causes of RB with seemingly sufficient FGF
patient with rapid breathing –> inadequate expiratory pause to wash CO2 distal from patient end especially if large breath taken
Advantages of NRB
–Simple, inexpensive, lightweight, not bulky – no moving parts except APL
–Easy to disinfect/sterilize
–Resistance usually low at clinically used FGFs
–WOB during SpV < circle system
–Variations in minute volume affect ETCO2 less than circle system
–Compression and compliance losses have less effect than compared to circle system
–Changes in FG concentrations = changes in insp gas concentration (no RB)
–No absorbent, no production of toxic compounds
Disadvantages of NRB
–High FGFs: higher costs, increased pollution, difficulty assessing SpV
–Inspired heat and humidity tend to be low
–Can be difficult to determine optimal FGF
–Anything that decreases FGF risks RB
–ABC systems: APL close to patient, inaccesible to provider, SS use awkward
–E, F: difficult to use with SS, air dilution can occur with E
–Not suitable for patients with MH –> not possible to increase FGF enough to remove increased CO2 load
Resp Gas Monitoring with Mapleson Systems
All but A have fresh gas inlet near patient connection port –> difficult to get reliable sample of exhaled gases
Mapleson A System
–FGF away from patient, minimum 50-80mL/min
–Cannot be used with mechanical ventilator that vents excesses gases - entire thing will become dead space
–Most efficient with SpV
Lack Modification of Mapleson A
Added expiratory limb, runs from patient connection to APL valve at machine end of system
Easier to adjust valve, facilitates scavenging excess gases
Slight increased WOB
Coaxial, parallel tube arrangements
How can a Mapleson A system be as efficient as a D during CMV?
If APL doesn’t vent during inspiration
Basic Flow of Gas through a Mapleson A during SpV
Exhalation: GDS+ Galveolar flow toward bag as FGF flows into bag
Bag becomes full, pressure in system increases until APL opens, continuing FGF reverses direction of exhaled flow –> Galv vented first, followed by Gds if FGF high enough
–Lower FGFs: GDS retained within system, rebreathing
–If much lower FGFs: alveolar gas also retained
Mapleson B
FGF 1.5-2.0x minute vent
FGI, APL located near patient end (B for Buddies)
RB away patient end separated from FGI by corrugated tubing
Mapleson B Gas Flow - SPV
APL valve opened completely, excess gas vented through valve during exhalation
Mapleson B Gas Flow - AsV, CMV
Closing APL sufficiently to allow lungs to be inflated, excess gases vented during inspiration
Slightly more efficient than Mapleson A bc fresh gas accumulates at patient end of tubing during exploratory pause
Variable performance during CMV
Mapleson C
B without corrugation
SpV: almost as efficient as A when expiratory pause minimal
Less efficient as expiratory pause increases
FGF 1.5-2x minute ventil
o CMV: FGF 2.5-3x minute ventil
Mapleson D
FGF 1.5/2-3x min ventil
FGF at patient end, bag/APL opposite
Easy gas scavenging
Sensor, gas sampling – btw corrugated tubing/T piece, corrugated tubing/APL, bag/mount, T piece and patient (adults)
Bidirectional PEEP valve – btw APL, corrugated tubing
* Some PEEP valves close with negative pressure application, SpV impossible with that kind of valve in system
T piece
T piece in Mapleson D, E, F
3-way tubular connector with patient connection port, FG port, port for connection to corrugated tubing
Bain Modification
Mapleson D
FGF coaxially inside corrugated tubing, ends at point where FG would enter if classic D
Outer tube clear, allows visual inspection of inner tube
Metal head with holes drilled in, allows for fixed position of reservoir bag, APL
MRI compatible versions: static compliance increased with decreased PIP, VT at same ventilator settings, increased PEEP
Mapleson D SpV
APL open, excess gases vented during expiration
AsV Mapleson D
partial closure of APL, excess vented during inspiration
CMV Mapleson D
connect hose from ventilator in place of reservoir bag, closing APL, excess gases vented through ventilator spill valve
ETCO2 with Mapleson D system
ratio of minute vol to FGF, absolute values
Hazards of a Bain System
entire circuit becomes dead space
Inner tube detached from connections
FG supply tube kinked, twisted
Incorrect system assembly
Mix of FG, exhaled gas if defect in metal head
Leak Test of Mapleson A
occlude patient end, close APL valve, pressurize system
Coaxial lack system: must test integrity of inner tube
* Blow down tube with APL valve closed – leak btw two limbs produces movement of bag
* Occlude both limbs at patient connection with APL open, squeeze bag – leak in inner limb –> bag collapses (gas escapes via APL)
Low FGF with Mapleson D
Upon inspiration, previously expired gas still in tubing – not flushed out by FGF –> rebreathing
Higher humidity, less heat loss, greater fresh gas economy
Elevated ETCO2 will require more work by patient to return to normal levels
How to decrease ETCO2 with low FGF with Mapleson D
high IE time ratio (prolongation of expiratory time allows more time for gas clearance)
slow rise in inspiratory flow rate (pulls FGF)
low flow rate during last part of exhalation
long expiratory pause (allows FGF to push exhaled gases toward APL)
High FGF and Mapleson D System
Little rebreathing bc fresh gas flushes exhaled gas from tubing
Assoc with increased heat, humidity loss
Better in patients with stiff lungs, poor cardiac performance, hypovolemia
Relationship btw ETCO2 and FGF in a Mapleson D System
Low FGF: ETCO2 depends on FGF
High FGF: ETCO2 depends on ventilation
Preuse check: Mapleson D
- Occlude patient and, close APL valve, pressurize system
- APL valve opened dash bag should deflate easily if valve, scavenging system working properly
Mapleson E
o Does not have a bag
o Expiratory port, closed in chamber from which excess gases evacuated
o Numerous modifications of original T piece
o Use: more commonly used to administer oxygen or humidified gases to spontaneously breathing patients
FGF 2-3x minute ventilation
Resp cycle sequence of events similar to Mapleson D
Mapleson E SpV
expiratory limb open to atmosphere
No rebreathing if no exhalation limb
If expiratory limb, FGF needed to prevent rebreathing equal same as Mapleson D
Air dilution if no expiratory limb or volume of limb less than patient’s VT FGF > peak inspiratory flow rate
CMV Mapleson E
intermittent occlusion of expiratory limb, allow FG to inflate lungs
no rebreathing, only FG inflates lungs
No air dilution
Hazards with Mapleson E
intermittent occlusion of the expiratory limb for controlled ventilation arrow overinflation, barotrauma
No bag –> No tactile feel during inflation as with other systems, no pressure-buffering effect
No APL valve to moderate pressure in lungs
What is a unique use of a Mapleson D system?
Modified Mapleson D system for one lung ventilation to apply CPAP to non dependent lung
Mapleson F
Bag with mechanism for venting excess gases
–Hole in tail, side of bag that occluded by using finger to provide pressure
–Ventilator may be used in place of bag
APL valve placed near patient connection to provide protection from high pressure
FGF 2-3x minute vent
SS: enclose bag in chamber from which waste gas suctioned or attaching devices to relief mechanism in bag
Modifications of Mapleson F System
o Ayre’s T piece: no scavenging
o Jackson Rees modification of Ayre’s Y piece: attached RBB
Mapleson F SpV
Relief mechanism left open
Mapleson F AsV
relief mechanism occluded sufficiently to distend bag, then squeeze bag
Mapleson F CMV
Bag replaced by hose from ventilator
Hazards of Mapleson D
Same as for E
Bag in system - excessive pressure less likely to develop
Use of ventilator with ram of oxygen to produce inspiration with T piece system: disconnection at common gas outlet may not be detected by airway pressure monitor DT high resistance of fresh gas tubing
Humphrey’s
Convert Mapleson A system to D/E to utilize most efficient modes
o A: spontaneous, DEF: CMV
Humphrey’s Level up in A mode
RBB on ADE block connected to inspiratory pathway (Map A)
Breathing hose connecting block to patient = insp limb
Expired gas carried back along other limb, vented at APL
Minimizes any potential mixing of inspired, expired gas
APL away from patient end of system = decrease Apparatus dead space
Humphrey’s Lever Down
o RBB, APL isolated from BS
o Inspiratory limb in A now delievers gas to patient end as a T piece
o Breathing hose returning gas to ADE block now functions as reservoir limb a T piece
o No RBB = Map E
PEEP Valves
Electronically controlled, disposable, manually controlled
Add peak end expiratory pressure to system
Fixed pressure PEEP valves
indicate amount of peep provide, >1 = additive effect
Variable PEEP valves
adjust amount of peep using scale or manometer
Unidirectional vs bidirectional PEEP valves
Bidirectional: second flow channel with own one way valve, recommended
–Incorrect orientation = no application of PEEP
Unidirectional peep valve incorporated incorrectly against flow of gas in inspiratory, expiratory limb –> block gas flow
When not see PEEP indicated on pressure gauge?
- Incorrect application of bidirectional valve
- Pressure gauge on absorber side of expiratory unidirectional valve
Filters
Protect patient from microorganisms, airborne particulate matter
Protect anesthesia equipment, environment from exhaled contaminants
ASA, CDC recommendations: no recommendation for placing in BS unless patient has infectious pulmonary disease
Consequences of filters in pediatric systems
Increase resistance, relative inefficiency
Mechanical Filters
(pleated hydrophobic): physically prevent microorganisms, particles from passing
Electrostatic Filters
rely on electrostatic forces to hold organisms within loosely woven charged filter element, less effective than mechanical filters
HEPA Grade Filters
High Efficiency Particulate Aerosol
trap >99.7% particles with diameter 0.3micrometers
Problems with Filters
o Should not be placed downstream of humidifier, nebulizer - less efficient when wet
o increased resistance, dead space esp if condensation
o Obstruction (increased PIP)
o Leak
o Erroneous entitle gas concentrations, poor CO2 waveforms
Administration of Bronchodilators
- Shake well prior to administration, discharge maximal when canister upright
- Slow deep inspiration followed by pause of 2-3s before exhalation - enhanced amount of medication deposited into airway
o 30 to 60 seconds between puffs
Humidification and Bronchodilators
high humidification causes aerosol droplets to increase in size –> rain out
Goals of a circle system?
- Minimize absorbent decussation
- Maximum inclusion of FG in inspired mixture, maximum venting of alveolar gas
- Minimal consumption of absorbent – use absorbent as little as possible
- Accurate readings from respirometer placed in system
- Maximal humidification of inspired gases
- Minimum dead space
- Low resistance
- Minimal pull on airway equipment
- Convenience
What are the three requirements of any circle system configuration?
- UNIDIRECTIONAL VALVES MUST GO BTW RBB AND PATIENT
- FGF CANNOT ENTER BTW EXPIRATORY VALVE AND P - WOULD NEVER REACH PATIENT
- APL VALVE CANNOT BE BTW PATIENT AND INSP VALVE - WOULD GET VENTED OUT BEFORE REACHING PATIENT
Anestar: differences btw classic circle system
RBB near FGI, ventilator upstream to inspiratory valve; FG decoupling valve downstream of RBB
CMV: fresh gas decoupling, no spill valve
Ohmeda ADU differences with classic circle system
o FGF enters system btw inspiratory unidirectional valve, Y piece
o RBB, APL, ventilator (spill valve) = classic circle system position
with continuous FGF, DT location of inspiratory valve, no retrograde FGF through absorbent
Drager 6400 vs classic circle system
RBB near FGI, ventilator (piston) = exhalation side of absorber downstream of expiratory unidirectional valve
APL valve, mechanical exhaust valve = exhalation side of absorbent
FG control valve
Mechanical exhaust valve
Fabius GS, Apollo BS
FG decoupling valve
Ventilator btw FG decoupling valve, inspiratory unidirectional valve
RBB btw expiratory unidirectional valve, absorber
Dead Space of the Circle System
Patient port of Y piece to partition between inspiratory, expiratory tubing
When exhalation or inhalation starts, gases and breathing tubes move in opposite direction from usual flow until stopped by closure of one of unidirectional valves = backlash
o clinically insignificant if unidirectional valves competent
o Causes slight increase in dead space
Heat, Humidity
Moisture: from exhaled gases, absorbent, water liberated from neutralization of carbon dioxide
Gases and inspiratory limb of circle system near room temperature
o Even at low FGFs, only 1-3*C above room temp
How to Increase Humidity in a circle system
lower FGF
increasing ventilation
fresh gas upstream of absorber
wetting inspiratory tubing
humidifier smaller
canisters
coaxial breathing tubes
What is the most important determinant of the internal vol in the circle system?
Canister size
Relationship btw Inspired, Delivered Concentrations
No rebreathing: concentration of gases, vapors and inspired mixture will be close to those in fresh gas –> everything being delivered to patient will be newly added to system
Larger BS internal volume, greater the difference btw inspired, delivered concentrations
Nitrogen
Before any FG delivered, concentration of nitrogen in BS ~80%
Enters via exhaled gases, leaves via APL valve, ventilator spill valve, leaks
Hinders establishing high [N2O] may cause low inspired oxygen concentrations
Denitrogenation
using FGF for few minutes to eliminate most of nitrogen in system
After denitrogenation, elimination by patient will proceed at slower rate
Side Stream Analyzers and Nitrogen
If side stream gas monitor directs gases back to anesthesia circuit, nitrogen concentration may increase –> many analyzers entrain air as reference gas
leak in sampling line can also entrain air
carbon dioxide: with absorbent
should be near 0
Unless failure of one or more unidirectional valves
High FGF: limits increase in FiCO2
CO2 without absorbent
depends on FGF, arrangement of components in circle system, ventilation
Oxygen
affected by:
– rate of oxygen uptake by patient
–uptake/elimination of other gases by patient
–arrangement of components
–ventilation
–FGF, volume of BS
–concentration of oxygen in fresh gas
Anesthetic Agents
Removed via absorption, degradation –> slower inductions, exposure of subsequent patient to VAs in system
Dry absorbent removes more than wet
Things that affect relationship btw inspired, delivered concentrations of inhalants
Uptake by patient
Uptake by components of the system
Arrangement of system components
Uptake, elimination of other gases by patient
Volume of system
Concentration in FGF
Degradation by absorbent
FGF
Low flow with circle system
technique in which a circle system with absorbent used at FG inflow of less than patients alveolar minute volume
Closed system anesthesia: form of low flow anesthesia in which FGF = uptake of ax gases, oxygen by patient, system and gas sampling – no venting by APL
Requirements of Low Flow Ax with Circle System
Requires flow meter that provide low flows, vaporizers calibrated to be accurate at low FGF or VIC
Must continuously measure oxygen concentration
Must increase FGF to compensate for gases removed by monitor with side stream gas analyzer
If closed circuit used, constant circuit volume achieved by:
RBB: increased size, increased FGF; increased size, decreased FGF
Ventilator with ascending bellows: adjust FGF so bellows below top of housing at end of exhalation
Ventilator with descending bellows: adjust FHF so bellows just reaches bottom of housing at end of exhalation
Closed system ax: high flows be used for 1-2’ at least 1x/hr to eliminate gases that have accumulated in system (Nitrogen, CO2)
o must increase FGF if rapid change in any component of inspired mixture desired
Advantages of Low Flow with Circle System
o Significant savings, esp with VAs
o Less personnel exposure: decreased VA in OR, vaporizers filled less (less exposed during filling)
o Better environmental impact
o Lower FGF, longer takes for change in FGF to cause comparable change in inspired concentrations
o Inspired humidity increases, rate of fall of body temp decreases
o Dangerously high pressures in BS take longer to develop
