Scavenging system, Capnography, and CO2 absorption Flashcards Preview

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Flashcards in Scavenging system, Capnography, and CO2 absorption Deck (55):

Passive gas disposal

  • Pressure is raised above atmospheric by the patient exhaling, manually squeezing of the reservoir bag or ventilator
    • pushes out the gas
  • Waste gases are directed out of the building through an open window, pipe, extractor fan vented to the outside air
  • Advantages: inexpensive, simple to operate
  • disadvantages: may be impractical in some buildings


NIOSH acceptable levels of Volatile halogenated anesthetic alone

2 ppm


  • Phase 1: Inspiratory baseline, no CO2. Inspiratory and first part of expiration (deadspace gas)
  • Phase 2: Expiratory upstroke (mixture of dead space gas and alveolar gas). Slope determined by evenness of alveolar emptyness.
  • Phase 3: All alveolar gas. constant or slight upstroke, reading taken from highest point at end
    • normal value 30-40
  • Phase 4: Beginning of inspiration, rapid decline of CO2


main places where gas needing to be scavenged comes from

  1. APL valve***
  2. pressure relief valve from the ventilator ***
  3. gas analyzers (50-250 ml)
  4. gas that drives the bellows (not always, sometimes goes out into room but that is ok because it is pure oxygen)


*** = most common places scavenged from



  • exhaled gas taking same route through absorber everytime because it is path of least resistance
    • results from loosely packed granules
  • air space occupies 48-55% of the volume of the canister
  • the absorbant along the channels will exhaust before the rest of container and it is difficult to know because they are on the inside
    • use CO2 monitoring to help determine exhaustion


Lithium hydroxide reaction

2LiOH * H2O + CO2 →Li2CO3 + 3H2O - HEAT


Gas disposal tubing

  1. Connects the scavenging interface to the disposal assembly
  2. should be different in size and color from the breathing system
  3. with a passive system the hose should be short and wide
    1. Tubing running overhead ideal to prevent accidental obstruction and kinking


Calcium hydroxide lime ingredients

AKA Amsorb Plus

  • 80% Calcium hydroxide
  • 16% water
  • 1-4% calcium chloride
  • Calcium sulfate and polyvinylpyrrolidine added for hardness

*No activators for quick reaction, only the slow part


esophageal intubation

*may have some CO2 in stomach from insufflation, but it quickly declines.


NIOSH acceptable levels of Nitrous oxide

25 ppm


Baralyme info

  • granules are 4-8 mesh
  • slightly less efficient than soda lime but less likely to dry out.


Gas disposal assembly

components to remove gases from the OR

Two types: Active and passive


Closed Interface

Two types: Positive pressure relief only 


 Positive-pressure and negative-pressure relief


Open interface

  1. NO valves- open to atmosphere via holes on sides, avoids buildup of + or - pressure
  2. REQUIRES use of central vacuum systme and a reservoir
  3. Vacuum control valve can be adjusted. Level of suction must be greater than excess gas flow rate to prevent pollution


Scavenging definition

  1. Collection of excess gases from equipment used in administering anesthesia or exhaled by patients.
  2. Removal of these excess gases to an appropriate place of discharge outside the working environment.


Method of measuring CO2:

Infrared Absorption Spectrophotometry

  • Most common
  • Gas mixture is analyzed and propotion of its contents is determined
  • each gas mixture absorbs infrared radiation at different wavelengths
  • amount of CO2 is measured by detecting its absorbance at specific wavelengths and filtering the absorbance related to the other gases


scavenging interface

AKA balancing valve or balancing device

  1. Prevents pressure increases or decreases in the scavenging system from being transmitted to the breathing system
  2. interface limits pressures immediately downstream of the gas-collecting assembly to between -o.5 to +3.5 cm H20
  3. inlet should be 30 mm male connector
  4. should be as close to gas-collecting assembly as possible


PaCO2 vs ETCO2

  • ETCO2 can be used to estimate PaCO2
    • PaCO2 > ETCO2
    • ETCO2 usually 2-3 mmhg under PaCO2
  • used as an evaluation of dead space





3 basic elements of Scavenging interface

  1. positive pressure relief-protects patient and equipment in case of occlusion of system
  2. negative pressure relief limits subatmospheric pressure
  3. reservoir capacity matches the intermittent gas flow from gas collecting assembly to the continuous flow of disposal system. 


size of absorbent granules

  • 4-8 mesh
  • irregular shape for increased surface area
  • range in size because all too small would increase resistance but still want greatest surface area


Rebreathing capnography

*does not go back down to zero during inhalation

  • Caused by:
    • equipment dead space
    • exhausted CO2 absorber
    • inadequate fresh gas flows (in semi-open system)


Exhalation flow 

  1. gas flows through the mask,
  2. into the rebrathing bag, and out the APL valve.
  3. (simultaneously) fresh gas continues to flow from the common gas outlet of the machine into the common gas inlet of the absorber. 
  4. There is some retrograde flow of the fresh gas down the "return tube" and travels to the bottom of the CO absorber.

**note: exhaled gas is NOT traveling through the CO2 absorber


Soda Lime ingredients

  • 4% sodium hydroxide
  • 1% potassium hydroxide
  • 15% H2O
  • O.2% Silica
  • 80% calcium hydroxide


  • Churare cleft
    • recovery from neuromuscular blockade
      • spontaneous movement of diaphragm


Transfer means 

AKA exhaust tubing or hose 


transfer system

  1. conveys gas from the collecting assembly to the interface
  2. usually a tube with female-fitting connectors on both ends
  3. tubing is short and large diameter, to carry a high flow of gas q/o a significant increase in pressure
  4. must be kink resistant
  5. must be different from breathing tubes (often yellow and stiffer plastic)


Baralyme reaction

Ba(OH)2 + 8H2O + CO2 → BaCO3 + 9H2O + heat

9H2O + 9CO2 → 9H2CO3

9H2CO+ 9Ca(OH)2 → 9CaCO3 + 18H2O + HEAT


*releases a lot of water


Cardiac oscillations

  • reflection of the cardiac impulses through trachea
  • totally normal, can't do anything about it!


5 components of scavenging system

  1. Gas collecting assembly
  2. transfer means
  3. scavenging interface
  4. gas disposal tubing
  5. gas disposal assembly


Beta angle

  • should be 90°
  • if angle increases, there is rebreathing or airtrapping


Compound A

  • Formed from degraded Sevoflurane
  • Has toxic renal and pulmonary effects
  • Must use at least 2L FGF.


Ethyl Violet

  • color when fresh: white
  • color when exhausted: purple
  • critical pH = 10.3
  • **most common


CO2 absorber indicators

  • made of an acid or base whose color depends on pH
  • color conversion signals absorber exhaustion
    • color can revert back after rest, but it is still exhausted
  • replace absorbent with 50-70% color change


Sidestream Capnography

  • gas is diverted and anazlyzed (50-250 ml)
    • can measure O2, CO2, N2O, and anasthetics
  • Uses infrared analysis, compares sample to a known quantity, requires calibration
  • time delay
  • potential disconnect source
  • reading affected by distance from pt and condensation


Recommendation on safe use of Carbon dioxide absorbents

  1.  Turn off all gas flow when the machine is not in use.
  2. change absorbent regularly
  3. change absorbent whenever the color indicates exhaustion (50-70% color change)
  4. change all absorbent, not just one canister
  5. change absorben when uncertain of the state of hydration, such as if FGF is on overnight or running high.
  6. low flows preserve humidity in granules


Gas collecting Assembly

  1. captures excess gases at the site of emission
  2. delivers them to the transfer means tubing
  3. outlet connection usually 30 mm (19 mm in old machines) and male fitting
    1. size is important so that it doesnt connect to other components of breathing system


Lithium Hydroxide ingredients

AKA Litholyme

  • 75% lithium hydroxide
  • 12-19% water
  • <3% lithium chloride


Soda lime info

  • capable of absorbing 26 liters of CO2 per 100g of granules
  • water is present as thin film on granule surface
  • moisture is essential- Reaction takes place between ions that only exist in presence of water


Barium Hydroxide Lime ingredients

AKA Baralyme

  • 20% Barium hydroxide and water
  • 80% Calcium hydroxide
  • small amounts of NaOH and KOH may be added
  • No hardening agent needed


Scavenging system check

  1. Ensure connections are good between scavenging system and the APL, ventilator relief valve, and vaste-gas vacuum.
  2. Fully open APL valve and occlude Y-piece
  3. With min O2 flow, allow scavenger reservoir bag to collapse completely and verify that the pressure gauge reads 0
  4. Push O2 flush, make sure scavenger bag gets bigger and pressure gauge doesnt go above 10 cm H2O


Calcium Hydroxide Lime reaction

CO2 + H2O → H2CO3

H2CO3 + Ca(OH)2 → CaCO3 + 2H2O + HEAT


Granule hardness

  • important because too much powder build up would increase resistance and cause caking
  • soda lime: silica added to increase hardness
  • tested with steel ball bearings and screen pan
    • % of original remaining = the hardness number
    • hardness should be >75


Positive-pressure relief only

  1. Single positive-pressure relief valve opens when a max. pressure is reached
  2. passive disposal, no vacuum used, no reservoir bag needed.



Monitoring partial pressure or concentration of CO2 in respiratory gases.

  1. Gold standard for ETT placement (3 waves over 30)
  2. Determine if pt is actually being ventilated.
  3. Guide ventilator settings (too much or too little)
  4. Detect abnormalities- Pulm embolism, disconnect, obstructive airway, hypotension, Malignant hyperthermia


Calcium Hydroxide lime info

  • Capable of absorbing 10 liters of CO2 per 100g of granules


Mainstream capnography

  • AKA flow-through
  • only measures exhaled, no scavenge needed
  • heated infrared measuring device placed in circuit
  • less time delay
  • potential burns
  • adds weight to circuit, could kink ETT


Alpha angle

  • normal 100-110°
    • bigger angle shows some kind of obstruction
    • COPD, Asthma, Bronchoconstriction, Acute obstruction


Method of measuring CO2:


  • rapid assessment- quickly changes color
  • uses metacresol purple impregnated paper which changes color in presence of an acid
    • CO2 combines with H2O to make carbonic acid and paper changes color
  • Only tells expired CO2, not inspired


Positive-pressure and Negative-Pressure relief

  1. has a positive-pressure relief valve, negative-pressure relief valve, and a reservoir bag
  2. used with an active disposal system- vacuum control valve adjusted so that the reservoir bag is over distended or completely deflated.
  3. Gas is vented to the atmosphere if the system pressure exceeds +3.5 cm H2O
  4. Room air is entrained if the system pressure is less than -0.5 cm H2O
    1. a backup negative-pressure relief valve opens at -1.8 cm H2O if the primary negative-pressure relief valve becomes occluded


Causes of rising CO2 when ventilation is unchanged

  1. hypoventilation (increase MV)
  2. malignant hyperthermia
  3. release of turniquit (pooled venous blood and lactic acid are now circulating)
  4. release of aortic/major vessel clamp
  5. IV bicarb administration
  6. insufflation of CO2 into peritoneal cavity (for laproscopic procedures)
  7. equipment defects (expiratory valve stuck, CO2 absorber exhausted)


Causes of decrease in ETCO2

  1. Hyperventilation (gradual decrease reflects increased minute ventilation)
  2. Rapid decrease- PE, V/Q mismatch
  3. Cardiac arrest
  4. sampling error (disconnections, high sampling rate with elevated fresh gas flow)


Soda lime reaction

CO2 + H2O →H2CO3

H2CO3 + 2NaOH (KOH) → Na2CO3 (K2CO3) + 2H2O + HEAT

Na2CO3 (K2CO3) + Ca(OH)2 → CaCO3 +2NaOH (KOH) + HEAT

*some CO2 may react directly with the Ca(OH)2 but this reaction is much slower


Inhalation flow through absorber

  1. Gas is pulled from the rebreathing bag and travels through CO2 absorber (from top to bottom), 
  2. then travels up the "return tube",
  3. where it meets the fresh gas from the common gas inlet.
  4. It then passes the inspiratory check valve and goes through the inspiratory tubing to the face mask.

*during inhalation is the only time expired gas actually goes through the CO2 canister (stops at the rebreathing bag on exhalation..)


NIOSH acceptable levels of volatile anesthetic with Nitrous oxide

0.5 ppm


Problems with absorbents

  1. Soda lime degrades volatile anesthetics
  2. 4x as much sevoflurane breaks down in Baralyme than soda lime
  3. Sevo degrades to form compound A
  4. CO can accumulate in absorbers not used in 24-48 hours b/c of slow reaction with the volatile agents and absorbers. (flush machine at beginning of day with FGF.
  5. Desflurane has highest accumulation of CO--flush system with 100% O2 for 15 min before use


Active gas disposal

  • a mechanical flow-inducing device moves the gases (hospital vacuum)
    • produces negative pressure in disposal tubing
    • must have negative pressure relief
  • Advantages: convenient in large hospitals where machines are used in different locations
  • disadvantages: vacuum system and pipework is a major expense, needle valve may need continual adjustment