190722_Scavenging Capnography and CO2 Absorption Flashcards
Scavenging collection of excess gases
from equipment used in administering anesthesia, or exhaled by patient.
Scavenging removal of excess gases
to an appropriate place of discharge outside the working environment
NIOSH Recommended Levels of Anesthetic Gases in OR: Volatile Halogenated Anesthetic alone
2 ppm
NIOSH Recommended Levels of Anesthetic Gases in OR: Nitrous Oxide
25 ppm
NIOSH Recommended Levels of Anesthetic Gases in OR: Volatile Anesthetic with Nitrous Oxide
0.5 ppm
5 basic components of scavenging system:
- Gas collecting assembly
- Transfer means
- Scavenging interface
- Gas disposal tubing
- Gas disposal assembly
Gas Collecting Assembly
- Captures excess gases at the site of emission (from circuit).
- Delivers them to the transfer means tubing.
- Outlet connection usually 30mm (19mm on older machines) male-fitting.
- Size of connections is important so that it doesn’t connect to other components of breathing system.
- APL Valve
- APL By-pass Valve
- Exhaust Valve
Transfer Means
- Also called exhaust tubing or hose and transfer system.
- Conveys gas from the collecting assembly to the interface.
- Usually a tube with female-fitting connectors on both ends.
- Tubing is short and large diameter, to carry a high flow of gas w/o a significant increase in pressure.
- Must be kink resistant.
- Must be different from breathing tubes
- Color coded yellow and stiffer plastic
Scavenging Interface
- Prevents pressure increases or decreases in the scavenging system from being transmitted to the breathing system.
- Also called the balancing valve, or balancing device.
- Interface limits pressures immediately downstream of the gas-collecting assembly to between -0.5-+3.5cm H2O.
- Inlet should be 30mm male connector.
- Should be situated as close to gas-collecting assembly as possible.
3 basic elements of the Scavenging Interface
- Positive pressure relief-protects patient and equipment in case of occlusion of system.
- Negative pressure relief-limit sub-atmospheric pressure.
- Reservoir capacity-matches the intermittent gas flow from gas collecting assembly to the continuous flow of disposal system.
Scavenging Interface - 2 Types
Open or Closed
Open Scavenging Interface
- No valves - is open to the atmosphere via “relief ports” in reservoir, avoiding buildup of positive or negative pressures.
- Require use of a central vacuum system and a reservoir (open canister –size should allow for high waste gas flows).
- Gas enters the system at the top of the canister and travels through a narrow inner tube to the base.
- Vacuum control valve can be adjusted – varies the level of suction on the canister/reservoir – must be > excess gas flow rate to prevent OR pollution
Closed Scavenging Interface:
POSITIVE-PRESSURE RELIEF ONLY
- Single positive-pressure relief valve opens when a max. pressure is reached
- Passive disposal – no vacuum used, no reservoir bag needed
Closed Scavenging Interface:
POSITIVE-PRESSURE AND NEGATIVE-PRESSURE RELIEF
• Has a positive-pressure relief valve, negative-pressure relief valve, and a reservoir bag.
• Used with an active disposal systems -Vacuum control valve adjusted so that the reservoir bag is NOT over distended or completely deflated
• Gas is vented to the atmosphere if the system pressure exceeds + 0.5 cm H2O
• Room air is entrained if the system pressure is less than -0.5 cm H2O.
- A backup negative-pressure relief valve opens at -1.8 cm H2O if the primary negative-pressure relief valve becomes occluded.
Gas-Disposal Tubing
- Connects the scavenging interface to the disposal assembly.
- Should be different in size and color from the breathing system.
- With a passive system the hose should be short and wide.
- Tubing running overhead ideal to prevent accidental obstruction and kinking
- If connected to an active gas disposal system it must be a DISS connector
Gas-Disposal Assembly
• Consists of components used to remove waste gases from the OR.
Gas-Disposal Assembly - Active
a mechanical flow-inducing device moves the gases (produces negative pressure in disposal tubing; must have negative pressure relief)
• These systems connect the exhaust of the breathing system to the Hospital vacuum system via an interface controlled by a needle valve.
Gas-Disposal Assembly: Passive
pressure is raised above atmospheric by the patient exhaling, manual squeezing of the reservoir bag or ventilator (needs positive pressure)
The waste gases is directed out of the building via:
• An open window
• A pipe passing through an outside wall
• An extractor fan vented to the outside air
in what type of Gas-Disposal Assembly?
Passive
Gas-Disposal Assembly - Passive
Advantages:
inexpensive to set up
simple to operate
Gas-Disposal Assembly - Passive
Disadvantages:
may be impractical in some buildings
Gas-Disposal Assembly - Active
Advantages:
convenient in large hospitals where many machines are in use in different locations.
Gas-Disposal Assembly - Active
Disadvantages:
vacuum system and pipework is a major expense.
Needle valve may need continual adjustment.
Most common Gas-Disposal Assembly used in hospitals
Active system
Scavenging System Check
- Ensure proper connections between the scavenging system and both APL valve and ventilator relief valve and waste-gas vacuum
- Fully open APL valve and occlude Y- piece
- With min. O2 flow, allow scavenger reservoir bag to collapse completely and verify that pressure gauge reads zero
- With the O2 flush activated, allow scavenger reservoir bag to distend fully, and then verify that pressure gauge reads less 10 cm H2O pressure
GOLD STANDARD to determine if the patient is in fact being ventilated – critical, life-saving monitor is?
Capnography
Capnography
- Used to Confirm ETT and LMA placement
- In general anesthesia without an airway, helps determine if patient is adequately exchanging air/oxygen
- Guide ventilator settings- avoid too much or too little ventilation
- Detect circuit disconnections
- Detect circulatory abnormalities- pulm. embolism, occult hemorrhage, hypotension
- Detect excessive aerobic metabolism: Malignant hyperthermia
- THERE ARE NO CONTRAINDICATIONS
Capnography: Detect circulatory abnormalities
pulm. embolism, occult hemorrhage, hypotension
Capnography: Detect excessive aerobic metabolism
Malignant hyperthermia
Capnography – Clinical Uses
- May be used as estimate of PaCO2 PaCo2>PEtCO2
- Average gradient = 2-5mmHg under GA
- Used as an evaluation of dead space
Colorimetric Measuring of CO2
- Rapid assessment of CO2 presence
- Uses metacresol purple impregnated paper (changes color in presence of acid)
- CO2 combines with H2O—carbonic acid–paper changes color
Infrared Absorption Spectrophotometry Measuring of CO2
-Most common
• Gas mixture analyzed
• A determination of the proportion of its contents
• Each gas in mixture absorbs infrared radiation at different wavelengths
• The amount of CO2 is measured by detecting its absorbance at specific wavelengths and filtering the absorbance related to other gases
Mainstream Capnography
- aka Flow Through
- Heated infrared measuring device placed in circuit
- Potential burns
- Sensor window must be clear of mucous
- Less time delay
- Weight- kinks ETT + increase dead space (Less of an issue with newer technology)
Sidestream Capnograph
• Aspirates fixed amt gas/minute (30-500ml/minute)
- Pediatric sampling- lower Vt = dilution
• Transport expired gas to sampling cell via tubing
• Best location for sampling→ near ETT
• Time delay
• Potential disconnect source
• Water vapor- condensation- traps/filters used
Capnogram Phase I
- An inspiratory baseline
- Should have no CO2 reading
- Inspiration and first part of expiration
- Dead space gas exhaled
Capnogram Phase II
- An expiratory upstroke
- Sharp upstroke represents rising CO2 level in sample
- Slope determined by evenness of alveolar emptying
- Mixture of dead space and alveolar gas
- Steep = even
- Sloped = uneven
Capnogram Phase III
- Alveolar Plateau
- Constant or slight upstroke
- Longest phase
- Alveolar gas sampled
- **Peak at end of plateau is where the reading is taken- End Tidal Partial Pressure of CO2 (PEtCO2)
- Normal Value = 30-40mmHg
- Reflection of PACO2 andPaCO2
Capnogram Phase IV
- Beginning of Inspiration
* CO2 concentration- rapid decline to inspired value
Tracing Interpretation
- Please also review Figure 51-11 in Miller 8th edition and know it well!
- Inspection of whole curve- 5 characteristics: Frequency Rhythm Height Baseline Shape
- Primary use- Verify placement of ETT in the trachea
- Presence of stable CO2 waveforms for 3 breaths > 30 mmHg indicates tracheal intubation
- Does NOT indicate proper position within the trachea- LISTEN to BBS!
Obstructive Lung Disease Pattern: Waveform Changes
COPD, Asthma, Bronchoconstriction, Acute Obstruction
• Slow rate of rise in Phase II
• Steep upslope of Phase III (in extreme cases may not see phase III)
Esophageal Intubation: Waveform Changes
Some / minimal wave that quickly peters out
Rebreathing: Waveform Changes
If value remains above baseline (Zero) at end of phase IV → Rebreathing
Causes of Rebreathing:
equipment dead space
exhausted CO2 absorber
inadequate fresh gas flows
Spontaneous Ventilation/ Recovery from Neuromuscular Blockade: Waveform Changes
Notch in Phase III = tug on bellows
Cardiac Oscillations: Waveform Changes
NOT = spont. vent.
Phase IV Zig zag
Rising CO2 when Ventilation Unchanged
Malignant Hyperthermia
Release of Tourniquet
Release of Aortic/Major Vessel Clamp
IV Bicarb administration
Insufflation of CO2 into peritoneal cavity
Equipment Defects (e.g. expiratory valve stuck, CO2 absorbent exhausted)
Decrease in EtCO2
Hyperventilation- gradual decrease reflects increased minute ventilation
Rapid decrease- PE (thrombus, fat, amniotic fluid, air) V/Q mismatch. Increase in PaCO2-PEtCO2 gradient. Cardiac Arrest
Sampling error- disconnect(s), high sampling rate with elevated fresh gas flow
Carbon Dioxide Absorber
- Chemical neutralization of CO2
- Base neutralizes an acid
- Acid: carbonic acid – formed by reaction of CO2 and H2O
- Base: hydroxide of an alkali or alkaline earth metal
- End product: water, a carbonate, & heat
Classic Absorber
two canisters, a dust/moisture trap at the bottom and a side tube at the right
Issues:
• Couldn’t change during the case because disturbs circle system integrity
• Common source of leaks
Modern single canister models
bypass feature so you can change out during the case
Common Absorbents
Soda Lime (Sodium hydroxide lime)
Amsorb Plus(Calcium Hydroxide lime)
Baralyme
Litholyme (Lithium Hydroxide)
Soda Lime:
- 4% sodium hydroxide
- 1% potassium hydroxide
- 15% H2O
- 0.2% silica
- 80% calcium hydroxide
Soda Lime: Silica
0.2%
added for hardness to prevent dust
Soda Lime: Absorption
• 26 liters of CO2/100g of absorbent granules
• Moisture is essential~Reaction takes place between ions that only exist in presence of water
* a pound of CaOH can absorb 0.59lb of carbon dioxide
Soda Lime: Reaction
Carbon dioxide combines with water to form carbonic acid. Carbonic acid reacts with the hydroxides to form sodium (or potassium) carbonate and water and heat.
- CO2 + H2O H2CO3
- H2CO3 + 2NaOH (KOH) Na2CO3 (K2CO3) + 2H2O + HEAT
- Na2CO3 (K2CO3) + Ca(OH)2 (quick reaction) CaCO3 + 2NaOH (KOH) + HEAT
Some CO2 may react directly with Ca(OH)2, but this reaction is MUCH slower CaCO3 + H2O + HEAT
Calcium Hydroxide Lime
- Aka: Amsorb Plus
- 80 % calcium hydroxide
- 16% water
- 1-4% calcium chloride
Calcium Hydroxide Lime:
Calcium sulfate and polyvinlypyrrolidine
added for hardness
Calcium Hydroxide Lime: Absorption
• 10 liters of CO2/100g of absorbent granules
Calcium Hydroxide Lime: Reaction
- CO2 + H2O H2CO3
2. H2CO3 + Ca(OH)2 CaCO3 + 2 H2O + HEAT
Baralyme: Barium Hydroxide Lime
- 20% BaOH and 80% CaOH
- Small amounts of NaOH and KOH may be added
- Granules are 4-8 mesh
- No hardening agent is needed
- slightly less efficient than soda lime but less likely to dry out
- No water
Baralyme: Absorption
similar to soda lime
26 liters of CO2 per 100 grams granules
Chemical Reaction of Barium Hydroxide lime
- Ba(OH) + 2(8H2O) + CO2 -> BaCO3 + 9H2O + Heat
- 9H2O + 9CO2 -> 9H2CO3
- 9H2CO3 + 9Ca(OH)2 -> 9CaCO3 + 18H2O + Heat
Litholyme: Lithium Hydroxide Monohydrate
note: there is also an anhydrous formulation
- 75% lithium hydroxide (LiOH)
- 12-19% H2O
- <3% lithium chloride (LiCl)
Litholyme: Reaction
2 LiOH * H2O + CO2 Li2CO2 + 3H2O – HEAT
Litholyme: Absorption
1 pound of LiOH absorbs 0.91 lb of carbon dioxide
Indicators
- An acid or base whose color depends on pH
- Color conversion signals absorber exhaustion
- Color reverts back with rest (especially in NaOH containing formulations)
- Replace absorbent with 50-70% color change
- Ethyl violet – most common—critical pH = 10.3
Ethyl Violet: Color when fresh
White
Ethyl Violet: Color when exhausted
Purple
Size of Absorbent Granules
- 4-8 mesh (granule size= number of openings per inch in a sieve through which particles can make it through)
- Irregular shape – increased surface area
- Small granules increase resistance
- Provide greater surface area
- Blend of large & small minimize resistance with little sacrifice in absorbent capacity
Size of Absorbent Granules - proportional to
resistance & absorbance
smaller granular size = more absorbent & higher resistance
Granule Hardness
- Excessive powder →channeling resistance & caking
- Soda Lime – silica added to increase hardness
- Tested with steel ball bearings & screen pan
- % of original remaining = hardness number
- Hardness number should be > 75
Channeling
- Preferential passage of exhaled gas flow through absorber via pathways of low resistance
- Results from loosely packed granules
- Air space occupies 48-55% of the volume of the canister
- Absorbent along channels may exhaust
- CO2 may filter through channels not visible
- CO2 monitoring
- Some manufacturers now use a polymer to bind the granules in pre-formed channels to prevent channeling
CO2 granules degrade volatile anesthetics agents to some extent, especially…..
sevoflurane
When degraded by a strong base in carbon dioxide absorbents (containing KOH and to a lesser extend NaOH), sevoflurane forms……
Compound A
compound A has been demonstrated to be nephrotoxic in rats, but….
• Does not appear to be an issue with absorbents with no……
KOH/NaOH
Manufacturer recommends not more than______hours at flow rates of 1 to <2 L/min.
Most practitioners use at least__liters of fresh gas flow with sevo – although this practice is____evidenced based.
2 MAC hours
2 liters
Not
___________________has been known to accumulate in desiccated (dry) NaOH and KOH containing absorbents when they are not used for 24-48 hours, and can result in critically high levels of ____________________ in exposed patients.
Carbon monoxide
carboxyhemoglobin
_____________is associates with the highest accumulation of carbon monoxide.
Desflurane
High flow through a system for prolonged time (such as if one forgets to turn down the O2 flow over the weekend) does what to the adsorbent?
What is the result?
Dries it out!
With dried out absorbent, a slow reaction occurs with the volatile agents and absorbents that produces CO!!
Fire hazard
Baralyme (now withdrawn from the market)
Anesthesia Safety Foundation Recommendation on Safe Use of Carbon Dioxide Absorbents
Turn off all gas flow when the machine is not in use
Change absorbent regularly
Change absorbent whenever the color change indicates exhaustion
Change all absorbent, not just one canister
Change absorbent when uncertain of the state of hydration, such as if FGF
Low flows preserve humidity in granules
Inhalation and the absorber
Rebreathing bag > absorbent > return tube > mixed w/ fresh gas (common gas inlet) > inhalation check valve > inspiratory hose of circuit
Exhalation and the absorber
exhaled gases flow through the mask, into the rebreathing bag, and out the APL valve.
Fresh gas continues to flow from the common gas outlet at the machine into the common gas inlet at the absorber.
