Breathing Circuits

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Breathing Circuits

Environment created by anesthetist
Fresh gas flow from common gas outlet
Inspiratory side goes to patient
Expiratory side returns exhaled gases
Several different breathing circuits

Anesthesia Machine Designed to Prevent

Hypoxia (O2 pressure, N2O 3:1)
Hypercarbia (absorber)
Barotrauma (regulators, O2 flush)
Foreign matter to airway (filters, check valve)
Anesthetic overdose
Vaporizer (baffles, interlock)
Machine built for optimizing ventilation
Know what is normal for patient
Deliver the safest ventilation

Fresh gas flow or inspired mixture

The gases from the flowmeters
The anesthetic agent from the vaporizer
Should be dry relatively clean mixture

Dead space

Anatomic due to conducting airways
Physiologic due to mismatch in V/Q
Mechanical due to breathing circuit

Alveolar gas or exhaled mixture

Oxygen left over after metabolism
Nitrous oxide not taken up by body
Air mixture left over
Any contaminants not taken up by body (carbon monoxide, ex.)
Agent not taken up by body
What is added from the alveoli
Carbon dioxide
Water vapor
Gases from the body (methane, acetone, N2)

Breathing Circuit and Rebreathing

Should not allow rebreathing of CO2
Should allow rebreathing of
Oxygen not metabolized
Nitrous oxide or air
Anesthetic agent
Design revolves around delivering what is physiologic & 'normal' for patient

Functions of Breathing

Gas exchange
Cellular respiration
aerobic metabolism
anaerobic metabolism
effects of anesthesia on cell metabolism

Control of breathing

central respiratory center
peripheral chemoreceptors
lung receptors (beta2)
effects of anesthesia

Chemical Regulation of Respiration

Maintains proper levels of O2 & CO2
Located in brain stem
Medulla oblongata
Pons

Voluntary Control of Breathing

Cerebral cortex controls
Abolished once patient deeply sedated or anesthetized past Stage II
Compare eye reflexes to laryngeal reflexes

If you rub eyelash and no response, through stage II and should have weakened laryngeal reflexes, can give NMBA now.

Effects of Anesthesia on Breathing

Inhalation agents lower tidal volumes/increase resp. rate
Narcotics lower respiratory rates/increase tidal volume
Deep sedation can lead to apnea
Muscle relaxation leads to apnea
"High" spinal or epidural paralyzes diaphragm & intercostals and lead to apnea

Limbic System Stimulation

Anticipation of activity
Emotional anxiety
Increases rate and depth of ventilation

Temperature

Increased temperature increases respiratory rate
Decreased temperature decreases respiratory rate
Controlled ventilation recommended for febrile patient
Controlled ventilation recommended for cold patient

Pain

Sudden severe pain causes apnea
Prolonged somatic pain increases respiratory rate ( like with contractions in labor)
Visceral pain decreases respiratory rate (like with deep abdominal pain)
Listen and react to respiratory clues about depth

Stretching the Anal Sphincter

Stimulates respiration
Consider intubating these cases
Frequent episode laryngospasm
Watch the surgeon and have patient deep

Airway Irritation

Mechanical irritation
Airways, LMA's, larynoscopes, ETT's
Chemical irritation
Inhalation agents (SEVO LEAST)
Take patient through Stage II gently or IV induction

Blood Pressure

Sudden rise in blood pressure decreases respiration
Sudden drop in blood pressure increases respiration
Small effect you may not appreciate unless patient breathing spontaneously

Responses to O2, CO2 & pH

Since responses impaired, anesthetist must take over ventilation
low ph/high co2 = increase rr
high ph/low co2 = decrease rr
high o2 = decrease o2 flow

Pressure Changes in Respiration

Alveolar pressure changes 758 - 762 mmHg
Pleural cavity pressure always subatmospheric
Normally no communication with 760 mmHg
Pneumothorax caused from trauma from outside or from inside alveolar from ventilation
Diaphragm
Skeletal muscle
Innervated from phrenic nerve C 3-4-5

Minimum Oxygen Requirements

Diffusing capacity for hemoglobin
Volume of a gas that diffused through membrane each minute for a pressure difference of 1 mmHg
Normally 21 ml/min/mmHg (O2 into hgb)
Mean oxygen pressure difference = 11 mmHg (b/w alveoli and hgb)
21 ml/min/mmHg x 11 mmHg = 230 ml/min (Min. O2 requirements)
Explains why we breathe such low TV

Tidal Volume

500ml or 7ml/kg IBW
70% of TV reaches alveoli
30% does not-anatomic dead space
Respiratory rate = 12 breaths per minute

After exhaling considerable amt of gas in lung

Forceful exhalation = expiratory reserve volume
Residual volume left over

Functional residual capacity

ERV 1,200 + RV 1,200 = 2,400 ml FRC
Stays in lungs at end expiration
Why we need so little volume with each breath
Each breath brings in 350 ml and out 350 ml
At end-expiration we still have FRC-2,400ml
Amt of alveolar air replace with each breath is 1/7 of the total
Many breaths required to totally exchange alveolar gas
Even after 16 breaths, some of original alveolar air remains

Minute Ventilation

Respiratory rate x tidal volume
12 x 500 ml = 6,000 ml or 6 L
Lower than normal = hypoventilation
Patient disease
Anesthetist must compensate

Alveolar Ventilation

Total volume of new gas entering each minute
AV = RR x (TV - dead space)
Dead space = 150ml for 70kg male
4,200 = 12 x (500 - 150)
Should keep CO2 at normal levels
If you want hyperventilation increase RR or TV
Alveolar gas consists
Fresh gas - metabolized gas
Carbon dioxide
Water vapor

Importance of Hemoglobin

Oxygen solubility low
Needs transport protein in adequate concentration (albumin)
Ensure patient hemoglobin adequate
Perfusion important to deliver hemoglobin
Keep blood pressure within 20% of normal

Gas Exchange and Ventilation

Ventilation not necessary for oxygenation
Ventilation necessary for removal CO2
High solubility (of CO2) necessitates removal
Quantity produced dictates minute ventilation
Cardiopulmonary bypass (Iow CO2 produced, decreased minute ventilation needed)
ECMO only exceptions
Spontaneous breathing keeps PCO2 - 40mmHg in
Absence of disease
High altitude and
Pharmacologic intervention

Carbon Dioxide Production

Resting adult produces .008 gm molecules/min
At STP: 1 mole = 22.4 liters
If 1 mole = 22.4, how much does .008 mole =?
180 ml @ 0 degrees C

1/22.4=0.08/x Solve for x

0 C = 273 K
37 C = 273 + 37 = 310 K
Charles Law

Breathing Circuit Function

Function is to provide final conduit for the delivery of anesthetic gases
link a patient to anesthesia machine
where respiratory exchange takes place
You create the respiratory environment for the patient

Ideal Breathing Circuit

Ventilates efficiently
Easily controls depth of anesthesia
Conserves Heat & humidity
Contributes no resistance to flow
Doesn't cause pollution of OR
Easy to use
Inexpensive

Open (Insufflation)

blowing of anesthetic gases across patient's face
avoids direct contact between patient's airway and breathing circuit
valuable during pediatric anesthesia
no rebreathing of exhaled gases if high flows are used (>10 L/min)
ventilation cannot be controlled

Open-drop anesthesia

not used in modern medicine except developing countries
highly volatile anesthetic (ether or halothane)
Schimmelbusch mask
Why do think the metal mask would get cold? => heat of vaporization!!!
air is carrier gas; supplemental oxygen can be used
rebreathing can be significant => only way to get rid of CO2 is to take mask off

Disadvantages of Open Circuits

poor control of inspired gas concentration and depth of anesthesia
inability to assist or control ventilation
no conservation of exhaled heat or humidity
pollution of OR with large volumes of waste gases

Draw-over anesthesia

nonrebreathing circuits that use ambient air as carrier gas
can use supplemental O2
inspired vapor and O2 concentrations are predictable and controllable
allow IPPV and passive scavenging
allow CPAP and PEEP
low resistance vaporizers
never use N2O
may have low O2 saturation with room air
advantage: simple and portable
disadvantages: depth of TV not well known due to no rebreathing bag; awkward to use for ENT/head surgery due to components located near patient's head

Semi-Open (Mapleson systems)

solve problems of insufflation/draw-over by adding components
breathing tubes, fresh gas inlets, adjustable pressure-limiting valves (APL's), reservoir bags
location of components is basis of Mapleson classification
high FGF's prevent rebreathing of CO2
can use with scavenger system
breathing tubes
fresh gas inlet
APL valve
reservoir bag (breathing bag)
performance characteristics
lightweight, inexpensive and simple
some rebreathing occurs (flow controls amount)
allow quick change in anesthetic concentration due to high FGF

Best Mapleson for Spont Vent and Controlled

Mapleson A best for spontaneous ventilation
Mapleson D best for controlled ventilation
Bain circuit is modification of D
fresh gas inlet tubing inside breathing tube
Spontaneous ventilation (A, D, F, E )
Controlled ventilation (D, F, E, B, C )

Advantages of Mapleson system

Low resistance to breathing made them popular in pediatrics
Less work of breathing
Simple, portable
Easy to assemble
Minimal moving parts
Some retention heat & humidity

Disadvantages of Mapleson

Most advantages overshadowed by need for high flow rates
Costs
Dry & cold airway
OR pollution
Difficult to change from spontaneous to controlled ventilation
Has led to many 'modifications' of Mapleson systems for modern use

Additional Components (not in Mapleson, for semi-closed)

CO2 absorber canisters
Unidirectional valves
Inspiratory check valve
Expiratory check valve
reduces need for high FGF
OR pollution reduced by using scavenger system
mandatory oxygen analyzer, airway pressure gauge, respiratory volume monitor

Semi-closed (Circle System)

Most commonly used
Adults and pediatrics
More complex but more efficient than Mapleson
Prevents rebreathing of CO2
Allows use of low flows

Unidirectional Valves

gas vented through APL valve or rebreathed after passing through CO2 absorber
to prevent rebreathing:
valve must be between patient and rebreathing bag on both inspir/expir limbs
FGF cannot enter circuit between expir. valve and patient
APL cannot be located between patient and inspir. valve

Valve ASTM Requirements

Flow direction shall be permanently marked
Functioning of valves should be visible
Resistance of dry & moist valves shall not exceed a pressure drop of 0.15 kPa (1.5cmH2O) at 1L/second flow (60L/min-test flowrate for adults)
Opening pressure of moist valve should not exceed 0.15 kPa (1.5cmH2O)
Reverse flow shall not exceed 60mL/min at any pressure differential to 0.5 kPa (5cmH2O)
Determined to be clinically acceptable & detectable with currently available volume monitors
Valve shall not become dislocated with a reversed pressure differential of 5 kPa (50cmH2O)
Maximum pressure in bag mode of circle system

Testing for Valve Resistance

Discs are light-weight, made of mica (ceramic)
Conditioned with heated & humidified gas flow
Until condensate on valve dome or disc visible (will wear down disc).
Test rig introduces a 1.0L/second flow of gas
Turn flow off and allows valve to close
Adjusts flow of gas to 1.0L/second
Measures the pressure drop across the disc
Shall not exceed 1.5 cmH2O (0.15 kPa)
Limits the work of breathing for spontaneous respiration

Reported Problems with Unidirectional Valves

Malfunction of either valve may allow rebreathing of CO2, resulting in hypercapnia
Faulty valve can lead to increased PIP's or increased ETCO2 with elevated baseline on waveform
Increased resistance

Increased Work of Breathing

Spontaneous ventilation depends on low resistance of light-weight mica discs
Should be no more than 1.5 cmH2O
Increased resistance occurs with
Excessive moisture build-up
Electrostatic build-up
Heavier than normal disc
Taped or glued back together
Replacement with anything other than manufacturer disc
Patient may c/o 'not being able to breathe'

Incompetent Expiratory Valve

Capnograph shows 'elevated baseline'
Baseline should always return to zero if you have no re-breathing
Re-breathing occurring on expiratory limb of waveform
Anatomic dead space exhaled at baseline
Also look at inspired CO2 numeric - FiCO2

Incompetent Inspiratory Valve

Capnograph shows abnormal 'beta angle'
Normal beta angle approximately 90°
Re-breathing occurring during inspiration
Shows on inspiratory side of waveform
Shaded area represents approximately 180° beta

Prevent problems with check valves

Check that valves are present
If too wet, carefully dry and replace
If you drop it & break it, ask for replacement!
Don't leave any foreign matter in dome cup
Watch movement while checking ventilator function
FDA Checkout 12.1:
Check for proper action of the unidirectional valves
Watch valves during case
Be alert to high pressure alarm-switch to bag mode
Recommendation to put circuit on machine (not ports) at end of day to allow evaporation of condensate

Tubes of Breathing Circuit

attached to ports that are connected to unidirectional valves
made of disposable plastic or rubber of various lengths
usually 1 meter in length and has large bore to decrease resistance to gas flow
corrugations present to permit flexibility without kinking
internal volume approx. 400-500ml per meter (.3 meter = 1 foot)
Depends on single or double lumen
Gas flow usually turbulent due to corrugations & angles
Always test circuit before using by determining oxygen flow required to maintain 30 cm H2O pressure in circuit
Length of tubing does not affect mechanical dead space ...it is just empty space
Longer tube...longer diffusion time

Breathing Circuit Dead Space

Dead space is space in circuit occupied by gases that are rebreathed without any change in composition
Part of TV that doesn't undergo alveolar ventilation
Dead space begins at Y piece and extends to any adaptors distal to Y piece (distal limb of Y piece and any ETT or mask between it and patient's airway)
Any increase in dead space should be accompanied by an increase in TV if alveolar ventilation is to remain unchanged (not usually clinically significant)
Always put extension tubings at inspiratory & expiratory check valves
NOT at end of breathing circuit

Common (Fresh) Gas Outlet

only one outlet that supplies gas to the circuit
adds new gas of fixed and known composition
has antidisconnect device used to prevent detachment of hose
usually latex-free
oxygen flush valve provides O2 to common gas outlet

Fresh Gas Inlet

gases (anesthetics with oxygen/air/nitrous) from the machine continuously enter the circuit through this inlet
placed between absorber and inspiratory valve
connected with flexible rubber tubing (delivery hose)

Reservoir bag (breathing bag)

attached to bag mount
3 functions
reservoir for anesthetic gases from which patient can inspire
provide visual/tactile means of existence and of volume of ventilation
serve as means for manual ventilation
usually elliptical in shape
nonslippery plastic or latex rubber
sizes from 0.5 to 6L
3 L bag is usually sufficient for most adults
****think of breathing bag as patient's lungs

Adjustable Pressure-Limiting Valve (APL)

pressure relief or pop-off valve
fully open during spontaneous vent. but must be partially closed during manual/assisted vent. (until desired inspiratory press. is achieved)
this allows reservoir bag to fill
valve will open after bag has become distended during expiration
if valve open too much, bag won't fill; if closed too much, rise in pressure could result in barotrauma (pneumothorax)
usually requires fine adjustments
on modern machines, APL valves can never be completely closed; upper limit is 70-80 cm H2O

Pressure Gauge (manometer)

always used to measure circuit pressure between expir./inspir. valves
usually reflects airway pressure if measured close to patient's airway
ASTM standard requires measurement in kPa or cm H2O
rise in pressure may signal worsening pulmonary compliance; increase in TV; obstruction in circuit, tracheal tube or airway
drop in pressure may indicate improvement in compliance; decrease in TV; or leak in circuit

do not give pressure above 20 cmH20 because it will open up esophageal sphincter

Spirometers (respirometers)

used to measure exhaled TV in circuit (usually near exhalation valve)
newer machines measure inspiratory TV near inspiratory valve
flow of gas across vanes within spirometer causes their rotation which is measured electronically
changes in exhaled TV may represent changes in vent settings; circuit leaks; disconnections; or vent malfunction
prone to errors caused by inertia, friction, and water condensation

Disadvantages of circle system

greater size
less portable
increased complexity, resulting in higher risk of disconnect or malfunction
increased resistance, difficulty of predicting inspired gas concentrations during low FGF's

Resuscitation Breathing Systems

AMBU bags or bag-valve-mask units
used for emergency ventilation due to simplicity, portability, ability to deliver almost 100% FiO2
contains a nonrebreathing valve (unlike a Mapleson or circle system)
source of high FGF connected to inlet nipple
patient valve opens during inspiration to allow gas from bag to patient
rebreathing is prevented by venting exhaled gas to atmosphere through exhalation ports
3 sizes : adult, child and infant
adult-TV over 600ml
infant-TV 20-50 ml
components
self-expanding bag, bag refill valve, and nonrebreathing valve
bag is inflated in resting state
intake valve in bag allows PPV because it closes during compression
bag is refilled by flow through fresh gas inlet
has check valve in case FGF is excessive
***may connect a PEEP valve to expiratory port

Functional Analysis of Ambu Bags

minute volume is determined by TV x RR
determined by performance of the resuscitator and the operator
volume varies with size of user's hand (using 2 hands will increase TV)
ASTM standard: capable of delivering at least 40% oxygen when connected to source supplying not more than 15 L/min
delivered O2 concentration is limited by size of reservoir and O2 flow

Disadvantages of Ambu Bags

require high FGF to achieve high FiO2
max. achievable TV's are less than those achieved with a system that uses a 3-L bag
most adult AMBU's have max. TV of 1000ml
moisture in valves can cause them to stick
considerable loss of heat and humidity
bag feels different from other breathing bags
valve is heavy and may cause ETT to be displaced downward
may harbor bacteria-dispose of bags when appear contaminated

FRESH GAS DECOUPLING

modern ventilators compensate delivered TV for the FGF
with traditional ventilators, delivered TV is sum of volume delivered from vent and FGF during inspiratory phase (ex. Narkomeds)
so, TV may change if FGF changes
if FGF increases, TV increases (significant in pediatrics)
if FGF decreases, TV decreases (increases ETCO2)

Drager Julian, Narkomed 600, Fabius GS use fresh gas decoupling

fresh gas is not added to delivered TV
ensures that set and delivered TV's are =
bag inflates during inspiration and deflates during expiration as contents empty into absorber and move toward patient
if a disconnect occurs, breathing bag rapidly deflates

second approach is fresh gas compensation

Aestiva and S/5 ADU
volume and flow sensors allow ventilator to adjust delivered TV so it matches set TV despite FGF

Closed System

extremely low flow anesthesia and completely closed circuit (less than 500ml)
maintenance of constant anesthetic state by addition of gases and inhalation agent at same rate that body stores or eliminates them (do calculations for MV and CO2 production)
once stable level is established using high flows, APL is closed completely and FGF rate is reduced to levels equal to patient's metabolic need
induction is difficult
hypoxia or recall if nitrous is administered (b/c low flows)
unpredictability of dosage of anesthesia for each patient
induction may be prolonged when using low flows
once adequate depth is established, there is little need for agent or nitrous oxide as they are rebreathed (O2 needs replenished due to its metabolism)
open circuit every 1-3 hours and run at higher flows for 5-10 min to washout nitrogen eliminate harmful substances/metabolic gases
not used frequently due to empirical calculations and fear of morbidity and mortality

CO2 Absorbers

Difference in circle system from Mapleson (in Mapleson, increase FGF to blow off/dilute CO2)
Inclusion of absorber canisters beneficial
Not necessary to use high flowrate
More economical to recycle mixed gases
Retention of heat and humidity
Patient will be warmer
Airways better humidified
Location between bag/APL or vent & fresh gas inlet sends mixed gases via canisters where carbon dioxide absorbed

Granules Chemically Neutralize CO2

Chemical reaction between acid & base
Weak acid reacts with strong base
Products are weak base & byproducts
CO2 + H2O  H2CO3
Carbonic acid neutralized by base
Sodium hydroxide
Potassium hydroxide
Calcium hydroxide
Products: carbonates, water & heat

Water

H2O composed of 2 H atoms & 1 O atom
When water dissociates or separates it forms H+ and OH-.
This process is known as ionization
The hydrogen ion, H+, has a positive charge because it lost an electron
The hydroxide ion, OH-, has a negative charge due to its gaining an electron
When another substance that ionizes is added, acids and bases are formed
An acid is created when excess hydrogen ions are present
A base is formed due to extra hydronium/hydroxide ions
To determine if a substance is acidic or basic, the pH should be determined.

Acids

Substances produce H+ in aqueous solution
Proton donor
Add hydrogen ions to a solvent
Strong acids dissociate 100% in water
Every molecule breaks apart
pH very low (0 - 3)
Stomach-hydrochloric acid, car battery-sulfuric acid
Weak acids dissociates at lower %
Not every molecule breaks apart
pH closer to 7 (3 -6)
Citric acid in lemons, acetic acid in vinegar

Bases or Alkali

Substance produces OH- in aqueous solution
Able to accept an H+ (proton acceptor)
Strong base dissociates 100% in water
Every molecule breaks apart
High pH (10 -14)
Na, K & Ca hydroxides reactive & caustic to skin
Weak base dissociates to lesser degree
Not every molecule breaks apart
pH closer to 7 (8 - 10)
Baking soda
Minerals react w/ acid to form water & salt
Oxides, hydroxides & carbonates of metals

Neutralization Reaction

Acid + Base => H2O + Salt
Always exothermic (produces heat)
57.7 kj per mole H+
Granules (metal hydroxides) are the base
Carbon dioxide + water is carbonic acid
Products are heat and water
Adds "heat and humidity"
Chemically absorbs carbon dioxide

Water ON & IN Granules

Required for chemical reaction
Patient's carbon dioxide
Water in granules
Carbonic acid is acid for base to absorb
Prevents agent absorption into granule
If dry, pores open & more agent absorbed
Percent water depends on formula
USP requires 14-19%
Baralyme (octahydrate) performs worse than Soda sorb when dry

Requirements for CO2 Absorbers

Should not be toxic itself or when mixed with inhalation agents
All agents have been shown to degrade to some degree by absorbent granules
Low resistance to airflow
100% efficiency
All of the carbon dioxide that enters the absorber canister should be absorbed

CARBON DIOXIDE ABSORPTION

CO2 absorption makes rebreathing of exhalations possible
conserves agent and gases while preventing respiratory acidosis/hypercarbia
rebreathing gas conserves heat & humidity
CO2 must be eliminated to prevent hypercapnia
Gas flows determine amount of rebreathing in circle system (increased FGF = decreased rebreath and vice versa)
FGF's 0.3-0.5 L/min provide near-total rebreathing with full reliance on CO2 absorption
FGF's greater than 4-5 L/min have little reliance on absorbent

CO2 Absorbents Equation

CO2 chemically combines with H2O to form carbonic acid
CO2 absorbents contain hydroxide salts that neutralize carbonic acid
end products include heat, water, and calcium carbonate
CO2 + H2O = H2CO3
H2CO3 + 2NaOH =Na2CO3 + 2H2O + heat (fast)
Na2CO3 + Ca(OH)2 = CaCO3 + 2NaOH (slow)
note: water and sodium hydroxide are regenerated

Dessication

is the physical break down of the granules, this can lead to increased agent absorption and CO poision.

Exhaustion

This is when less CO2 is being absorbed and the absorbent is almost worn out by all the chemical reactions, when the carbonates outweigh the hydroxides. Will see increased FiCO2, ETCO2 and white to purple.

Soda Lime

Soda Lime
most common absorbent
can absorb 23-26 L per 100g of absorbent
hardeners (silica and kieselguhr) added
minimize formation of dust/harder to break down
water content (15%)
calcium hydroxide is main ingredient (80%)
sodium hydroxide and potassium hydroxide (5%)
size 4 to 8 mesh
exhausted when all hydroxides have become carbonates
freshness determined by feel, taste and appearance
half of volume of canister is gas
regeneration or peaking seen with soda lime
soda lime appears to be reactivated with rest
color will revert back to white but absorptive capacity will be low and purple color will reappear quickly after brief exposure to CO2
***there is no true regeneration of activity occurring (just a dye reaction).

Baralyme

similar to soda lime
activator is barium hydroxide (20%)
calcium hydroxide (80%)
small amount of water present
no hardeners needed (less likely to produce dust)
slightly less efficient than soda lime but less likely to dry out if stored under poor conditions
also contains an indicator for color change
may undergo some false regeneration
size 4 to 8 mesh

Indicators

acid or base whose color depends on pH
as carbonate is formed from hydroxide, pH becomes less alkaline and granules change color (becomes ACIDIC)
from white to violet/blue
added to absorbent to indicate when exhaustion has occurred
does not affect absorption
ethyl violet is most common due to vivid color change
undergoes deactivation in high intensity light (tinted cover)
due to regeneration, may not be able to rely on color change solely for granule exhaustion
***use of capnometry to detect rising inspired CO2 is most important

High COHgb in Swine Study

48 hour drying time @ 10 LPM
H2O: from 12% to 3%
Reservoir bag removed
3 pigs died due to 80% COHgb after 20 minutes
24 hour drying time @ 10 LPM
Bag removed: H2O dec. and inc. temp and inc. CO
CO peaked at 8,800 to 13,600 ppm
Baralyme (73%)
Sodalime (53%)
Bag removed @ 5 LPM: Not dry enough to produce CO2
Bag ON: Not dry enough to produce CO

High COHgb in Swine Study Conclusions

Oxygen @ 10 LPM for 24 hours (with bag OFF) can lead to CO poisoning with Desflurane in pigs
Oxygen @ 10 LPM for 24 hours (with bag ON) insufficient to dry granules
Oxygen @ 5 LPM for 24-48 hours (with bag ON) insufficient to dry granules
If you walk in and see 5-10 LPM flowing with no bag what should you assume?
If you walk in and see 5-10 LPM flowing with bag ON what should you assume?
Would you change canisters?

When to Change the Canisters

1990 FDA/CDC recommended Q 24 hours
Few departments complied due to $
Most follow manufacturer recommendation
Change when they become exhausted
Color change means exhaustion
No indication when dessication occurs
NO indication if flow left on
Aline patients COHgb
Vigilance to turning OFF flows
Technicians help ensure flows OFF
At end of case
At end of day
Change every 30 days now
Cannot use with flammable anesthetics

Consult WR Grace

H2O content stable for one month after plastic wrapping removed
Chemical reaction product = H2O
If no flows run without patient on circuit, H2O should remain stable
Vigilance to flows with techs and staff
OK to leave on machine for 1 month
Change if we suspect flows left ON

Halothane + Soda Lime

Metabolite forms
BCDFE
2 bromo, 2 chloro, 1, 1 difluoethane
Specific nephrotoxin in rats
No nephrotoxicity in humans
Metabolite found in human urine after Halothane

Degradation of Sevoflurane

CH4 + O2 → CH3OH → CH2O → HCOOH → CO2
Methane → Methanol → Formaldehyde → Formic acid → Carbon Dioxide
Products of this pathway (methanol, formaldehyde, formate or formic acid)
Each of the compounds are quite toxic in relatively low concentrations.

Compound A
Fluoromethyl-2-,2-diflouro-1-triflouromethyl vinyl ether
Nephrotoxic in rats
Proximal tubule lesions
Controversial renal effect in humans
Increase levels for 1-2 hours
Level plateaus, then declines
HIGHEST DURING LOW FLOW (less than 1L)

Compound A

Sevoflurane reacts with CO2 granules to produce compound A
lethal at 130-340 ppm; renal injury at 25-50 ppm
nephrotoxic in rats; NO HUMAN CASES REPORTED
blood levels increase for 1-2 hrs after administration, plateau then decline
recommend using higher FGF's with Sevo (2-5 L/min) to flush absorber of toxic compounds
***product insert does not recommend sevoflurane at total FGF's of less than 1L/min for more than 2 Mac-hours
-higher FGF dilute and decrease risk of Comp A and renal injury

Trichloroethylene (Trilene)

Used in 1970s for analgesia for trigeminal neuralgia and dentistry
In alkali and heat, it degrades into toxins
Dichloroacetylene (cranial nerve lesions, encephalitis)
Phosgene -COCl2 (pulmonary edema & ARDS)
CO (phosgene + water product)
If patient had in past 24-36 hours, do not use breathing circuit with CO2 absorber
Use regional if possible and/or Mapleson system

Trichloromethane (Chloroform)

Phosgene formation
Toxicity arise from inhalation
Reacts with water, forms hydrochloric acid
Symptoms appear 2-24 hours later
Pulmonary edema, pneumonia, abscess, death
Break down product of chloroform
.1% ethanol acts as preservative
Carbon monoxide formed from reaction

Fires with Sevoflurane

Rare (4 in 1 year) fires in Baralyme
Baralyme dessicated
One airway fire in St. Louis
One explosion of expiratory valve through ceiling tile at CHMC induction room
Details of others unknown
Baralyme taken OFF market
Recommendations issued

Recommendations from Abbott

Replace canisters if possibly desiccated
Walk into room and flowmeters ON
Walk into room and vent still cycling
Turn OFF flows and machine at end of day
Turn OFF vaporizers when not in use
Check for proper plastic packaging when replacing canisters
Periodically monitor temperature
How are we supposed to do this?
Replace canisters routinely (every 30 days or less)

New Absorbents

strong bases (NaOH, KOH) implicated in CO and compound A
eliminating these activators produces absorbent that has similar physical characteristics and CO2 absorption efficiency (controversial) as compared with soda lime
Amsorb is now widely available...CHMC
lithium hydroxide is also effective but not available
goal is to maintain efficiency while lessening production of byproducts
Dragersorb 800 and Medisorb are used now (contain less NaOH and no KOH)

Absorber Toxicity

resistance of filled canisters is low
inhaled dust is caustic and is an irritant
may lead to bronchospasm, laryngospasm and pneumonia
trap for water and dust that prevents passage of dust toward patient is incorporated beneath the lower canister (empty as needed)
FDA recommends: when circuit is pressurized, release pressure through APL valve and not through elbow near patient's face (in case of dust)
handle absorbents gently avoid fragmentation and dust formation (always wear gloves)

Low Resistance to Airflow in Absorbers

Inside canister, there are granules and...
Empty space for gases to flow through
Tightly packed granules adds resistance
Specifically sized to minimize resistance
Compromise between resistance and absorptive capabilities of granules
Mesh size screens control size of granules

USP Mesh Standards

'85% 4 to 8 mesh'
'7% oversized'
'7% undersized'

Air Space in Canister

Goal is 65% of canister as airspace
Best compromise between
Maximum absorptive capacity
Minimal resistance to airflow
Size of absorber must match patient TV
Estimate with 50% of total capacity
Precise at 65% of total capacity
Similar to Mapleson length of tubing

Match TV to Absorber Capacity

If absorber capacity = 1400 ml
Airspace is 50-65% of 1400
Airspace = 700 - 910 ml
Appropriate for patient up to 90 Kg
Assuming 10 ml/kg for TV
What happens if airspace was 350 ml?
---up to about 45kg, so only can be used on small pt.
TV should NOT exceed airspace

Wall Effect

More resistance inner diameter of canister
Less resistance near wall due to more air space distribution
Gas flows here preferentially

Channeling

Gas flows in characteristic pattern through absorber..path of least resistance
Top center
Down the sides
To the bottom
When we change, we'll get rid of top canister and let bottom canister go to top (b/c top wears out first and then bottom).

Efficiency, Break Point, and Exhaustion

100% efficient if 100% of carbon dioxide absorbed in the canister
Amount of time 'time efficiency'
Breakpoint - inspired CO2 = .1%
Exhaustion - inspired CO2 = .5%
Rebreathing - if inspired CO2 goes higher

Efficiency Varies with Hardness

Porous granules absorb more
Hard granules absorb less
100% hard rock absorbs nothing
O% hard sponge absorbs everything
Compromise between absorption and dust
Additives to decrease dust
Silica added to dec. dust-clogs the granules
Kieselguhr (diatomaceous earth) hardens

How to Measure Hardness

Start with 50 gram sample
Subject to 45 psi pressure for 1 minute or
Put in pan with steel ball bearings
Agitate granules and ball bearings
Put what remains into 10 mesh screen
Agitate again and weigh what remains
Remains must weigh 40 grams
Sift through 8 mesh screen
75-80% of granules should remain

Variables that Affect Efficiency

Packing of canister
Channeling
If airspace = patient TV
Granules'characteristics
Hardness
Size
Water content
Flowrate used through granules
If exceed pt minute ventilation, residence time low
Absorption capacity reduced
Ideal flowrate = minute ventilation or less

Void Space in Canister

Efficiency decreases when empty space fills up with products of reaction
As carbonates and water are produced, they have to be somewhere
They fill up interstitial (void) space
As void space dec., less airspace to accommodate patient's tidal volume
Lose 60cc/hour per 1000 cc Sodasorb (may see CO2 rebreath before large color change, may need to increase TV to compensate for lost space)
TV >or equal to available void space = inefficiency

Exhaustion

breakpoint is defined as time at which the first trace of unabsorbed CO2 is detected in inspiratory port of the absorber
approx 0.1%
exhaustion is the time at which the CO2 level at the inspiratory port reaches 0.5%

Replace Canisters When:

when inspired CO2 is > 3-5 mmHg
when ETCO2 is increased
when 50-75% color is changed
when temperature of canister is cool (not reliable)

Bases Caustic to Skin

Sodalime more caustic to Baralyme
Led many departments to use Baralyme
Along with high water content in product
Dust implicated in patient injury
Facial burns
Bronchospasm
Irritation to mucous membrane
Direction of gas flow dec. dust expulsion
Able to drain out via absorber drain
Prevention of foreign matter to airway

Use Redundancy in Monitoring

Rely on monitors that measure CO2
Look at expiratory number...chart it
Keep your eye on inspiratory number
If rebreathing occurs, look at canisters
If dye added, granules should be purple
Change canisters when necesssary
Don't forget clinical signs
Hyperpnea...deep breathes
Hypertension - esp. with long laproscopic procedure.

Absorber

granules of absorbent are contained within 1-2 canisters that fit snugly between a head and base plate
this is called an absorber
usually double canisters because they permit more CO2 absorption, less frequent absorbent changes, and lower resistance
absorber includes 2 ports for connection of breathing tubes, fresh gas inlet, inspiratory and expiratory unidirectional valves, an APL valve and bag mount

Canisters

hold absorbent
side walls are transparent
with fresh absorbent in each canister, CO2 is absorbed mostly in upstream chamber then as that becomes exhausted, it enters downstream chamber
disposable canisters available

Housing (canister support)

head and base usually made of metal
gaskets at top and bottom fit against canisters
can raise base of housing so canisters seal against gaskets
lowering base creates gap between canister and gasket (a leak)
absorbers have actuated cam to raise and lower base
there are spaces at top and bottom of absorber for incoming gases to disperse before passing through absorbent or for outgoing gases to collect before passing on through circle (this promotes even distribution of flow)

Baffles

annular rings that serve to direct gas flow toward central part of canister
placed at top and bottom of absorber
increase path of travel for gases along sides and help compensate for wall effect (or else sides would get worn out first)

Side Tube

external to canisters
conducts gases either to or from bottom of absorber
main flow of gases passing through absorber will be opposite to gases passing through side tube

Exhaustion of Absorbent

means 'drying out'
clinical signs
increased ETCO2; increased FiCO2
increased HR and BP
hyperventilation
respiratory acidosis
arrythmia
SNS activation
color of granules at end of case
***capnography and indicator color change are primary indications of exhaustion

if inspired CO2 is more than 3-5 mm Hg, FGF should be increased to 5-8 L/min
this converts system to semi-open where rebreathing of exhaled gases is minimized

only 2 reasons for increase in inspired CO2:

absorbent granules are exhausted
unidirectional valves are faulty

Replacement

do not change old-style canisters or loose fill in middle of case (may delay ventilation if you can't get canister to seal)
may change canister in newer machines without interrupting ventilation
if granules do become exhausted, and it's not safe to change in middle of case, change FGF to 1-2 times MV
this will ensure inspired CO2 is reduced to acceptable levels
replace granules after end of case
wear gloves
in Drager Narkomed, lower base of absorber housing with cam
remove canisters
discard upper canister and move lower canister to upper position
insert new canister in lower position
always date and time canister
canister position is reversed when you are finished!!

Replacement Recommendations by Company

Sodasorb manufacturer recommends changing the absorbent if left in machine for > 48 hrs.
Drager recommends their absorbent in Fabius GS be changed if machine has been idle for 48 hrs. or at least every Monday morning
2 reasons for these cautious guidelines:
gas flows may be left on overnight or all weekend which dries out granules (they do not regenerate)
ethyl violet indicator may be inactivated by intense light
***University Anesthesia Techs change soda lime when 75% exhausted OR 30 days from date

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