Scavenge, Capnography, CO2 Absorber Flashcards
What is scavenging?
- collection of excess gases from administration or exhalation of gas
- removal of excess gas to outside working environment
Who regulates levels of anesthetic gases in the OR?
National Institute of Safety and Health (NIOSH)
What is the recommended levels of gases for:
- Volatile anesthetic
- N2O
- Volatile anesthetic with N2O
- 2 ppm
- 25 ppm
- 0.5 ppm
(ppm = parts per million)
What are the 5 components of the scavenge system?
- gas collecting assembly
- transfer means (tubing to interface)
- scavenging interface (regulates +/- pressure?)
- gas disposal tubing
- gas disposal assembly
What 3 ways does gas get to the scavenging system?
- gas analyzer (occurs right before insp limb)
- ventilator relief valve (vent’s form of APL)
- APL
When does gas enter the scavenging system?
When the ventilator relief valve or APL has too much pressure, the excess is diverted to scavenger.
What does the gas collecting assembly do?
- capture excess gas
2. delivers gas to transfer means tubing
What size and type is the outlet connection for the gas collecting assembly?
What is the size for older machines?
Why is this sizing important?
30 mm, male-fitting
19 mm
so that it isn’t accidentally connected to other parts of the breathing system
What does the transfer means do?
What are 2 other names for the transfer means?
- transfers gas from collecting assembly to interface
- exhaust tubing or hose and transfer system
Describe the size and diameter of the transfer means tubing. Why is this important?
- short and large
- to carry high flows w/o increasing pressure
What color is the transfer means tubing? And what is it made of?
-yellow
-stiffer plastic
(must be kink resistant)
What is the purpose of the scavenging interface?
prevent pressure increases/decreases in scavenging system from being transferred to breathing system
What is another name for the scavenging interface?
balancing valve/device
How much pressure does the scavenging interface limit? Where does it limit the pressure?
- limits to -0.5 to +5 cm H2O
- immediately downstream of gas collecting assembly
What size is the scavenging interface inlet?
-30 mm male connector
How does the scavenging interface regulate pressure and flow? (3)
- positive pressure relief (protects in case of occlusion in system)
- negative pressure relief (limits subatmospheric pressure)
- reservoir capacity (matches intermittent gas flow from gas collecting assembly to continuous flow of disposal system)
What are the 2 types of scavenging interfaces?
- open
2. closed
How does the open scavenging interface work?
- no valves, has holes in reservoir preventing buildup of pressure
- requires central vacuum and reservoir = open canister
- gas enters top of canister thru inner tube to base
- adjustable vacuum control valve - suction on canister/reservoir must be higher or equal gas flow rate to prevent OR pollution
What are the 2 types of closed scavenging interfaces?
- positive-pressure relief only
2. postive and negative-pressure relief
What does the (closed) positive-pressure relief scavenging system consist of? Is disposal active/passive?
- pressure relief valve only - valve opens with max pressure
- passive disposal, no vacuum used, no reservoir bag needed
What does the (closed) positive and negative pressure relief scavenging system consist of?
-has positive pressure relief valve, negative pressure relief valve, reservoir bag
Is the (closed) positive and negative pressure relief scavenging disposal active/passive? What is the max level of positive and negative pressures and how does the system compensate for this?
- active disposal via adjustable vacuum control valve with reservoir bag over distended (for excess positive pressure) or collapsed (for excess negative pressure)
- gas vented to atm if pressure > +5 cm H2O
- room air is sucked in if pressure is less than -0.5 cm
What happens if the primary negative-pressure relief valve in the (closed) positive/negative pressure scavenging system becomes occluded?
A back-up neg press relief valve opens at -1.8 cm H2O
What does the gas disposal tubing do?
connects scavenging interface to disposal assembly
Describe the gas disposal tubing.
Where should it be placed?
- different in size/color from breathing system
- is passive
- hose is short and wide
- overhead to prevent accidental obstruction/kinking
What is the gas disposal assembly?
-components used to remove waste gas from OR
What are the 2 types of gas disposal assembly?
Which is most common
- active (most common)
2. passive
Describe a PASSIVE gas disposal assembly. What type of pressure does it require?
- positive pressure
- pressure is raised above atm by pt exhalation, squeezing reservoir bag or from ventilator
- waste gas goes out of building via window, pipes to an outside wall, extractor fan vented to outside air
What are the advantages and disadvantages of the PASSIVE gas disposal assembly?
- adv: inexpensive, simple
- disadv: impractical in some buildings
Describe the active system of the gas disposal assembly.
- mechanical flow device moves gases and causes negative pressure in disposal tubing
- must have negative pressure relief valve
- connects exhaust of breathing system to hospital vacuum via interface controlled by a needle valve
What are the advantages and disadvantages of the ACTIVE gas disposal assembly?
- adv: convenient in large hospitals
- disadv: expensive vacuum and pipework, needle valve may need continual adjustment
What are you checking with the scavenging system check?
How do you perform this check?
- connections between scavenging system and APL valve, ventilator relief valve and waste-gas vacuum
1. fully open APL valve and occlude y-piece
2. allow scavenger reservoir bag to collapse and pressure gauge to be at zero
3. activate O2 flush to distend reservoir bag, pressure gauge should read < 10 cm H2O
How is capnography useful?
- ETT placement (gold standard)
- determine pt ventilation
- guide vent settings (too much/too little)
- detect abormalities i.e. PE, MH, disconnect, obstr a/w
What are the contraindications to capnography?
there are NONE
What clinical info can capnography provide?
- estimate PaCO2
(PaCO2 > PEtCO2 with a gradient of 2-5 mmHg under general anesthesia i.e. PEtCO2 = 38, then PaCO2 around 40-45) - evaluates dead space (increasing dead space, widens gradient)
What are the 2 types of methods in measuring CO2 in expired gas?
Which is most common?
- colorimetric
2. infrared absorption spectrophotometry (most common)
How does the colorimetric method of measuring CO2 work?
- rapid (i.e. CO2 detector in ICU)
- uses metacresol purple impregnated paper that changes color in presence of acid (carbonic acid formed by CO2 + H2O)
How does the infrared absorption spectrophotometry work?
- gas mixture is analyzed
- CO2 is measured by detecting absorbance at specific wavelengths and filtering absorbance related to other gases
What are the 2 types of measurement techniques used in capnography?
- mainstream
2. sidestream
Describe mainstream capnography.
What are advantages/disadvantages?
- aka flow thru
- heated infrared measuring device placed in circuit
- sensor must be clear of mucous
- less time delay
- potential burns, weight > kinks ETT
Describe sidestream capnography.
Where is it sampled?
- aspirates fixed amt gas/min (50-500 ml)
- transport expired gas to sampling cell via tubing
- infrared analysis by comparing sample to a parameter that is calibrated to 5% or 35 mmHg
What are advantages/disadvantages of sidestream capnography?
What effect on pediatric sampling?
- sampling is near ETT = best location
- time delay, potential disconnect, condensation from exhalation
- lower tidal volume = dilution
What occurs in Phase I of the capnogram?
- inspiration to first part of expiration
- inspiratory baseline
- no CO2
- dead space exhaled
What occurs in Phase II of the capnogram?
- expiratory upstroke representing rising CO2 level
- slope is determined by evenness of alveolar emptying
- is a mixture of dead space and alveolar gas
What occurs in Phase III of the capnogram?
- alveolar plateau causing constant/slight upstroke
- LONGEST phase
- peak at end of plateau = where PEtCO2 reading is sampled
- reflects PACO2 (alveolar) and PaCO2 (arterial)
What is the normal range of PEtCO2?
30-40 mmHg
What occurs in Phase IV of the capnogram?
- beginning of inspiration
- CO2 concentration has a rapid decline to inspired value
What are 5 characteristics of a capnogram tracing?
- frequency
- rhythm
- height
- baseline
- shape
How do you assess capnography for ETT placement?
stable CO2 waveform for 3 breaths
What does capnography NOT tell you about ETT placement?
- Does not indicate proper position in trachea
- Must listen to bilat breath sounds!!
What does capnography tracing tell you?
- adequacy of exhalation (i.e. asthma, COPD, etc)
- disconnect
- quality of CO2 absorption
- changes in perfusion/dead space
*REVIEW VARIOUS ABNORMAL WAVEFORMS
*REVIEW VARIOUS ABNORMAL WAVEFORMS
What are reasons why you may see rising CO2 when ventilation is unchanged? (6)
- malignant hyperthermia
- release of tourniquet (build up of lactic acid and CO2)
- release of aortic/major vessel clamp
- Bicarb IV
- insufflation (pumping) of CO2 into peritoneal cavity
- equipment defects (i.e. exp valve stuck, CO2 absorber exhausted)
Why might you see a decrease in EtCO2?
- hyperventilation (decrease CO2 = increased minute vent)
- PE (rapid decrease) = VQ mismatch = increase in PaCO2 and PEtCO2 gradient
- cardiac arrest
- sample error (i.e. disconnect, high sample rate with high FGF)
How does the CO2 absorber work?
What is the end product?
- causes chemical neutralization of CO2
- hydroxide of an alkali or alkaline metal (base) neutralizes carbonic acid
- water, carbonate, heat
What is the path of CO2 as it flows thru the absorber?
top to bottom
What are 2 common absorbents (bases)?
- soda lime (sodium hydroxide)
2. amsorb plus (calcium hydroxide lime)
What is soda lime made of?
- 4% sodium hydroxide
- 1% potassium hydroxid
- 15% H2O (to react with CO2 = H2CO3)
- 0.2% silica (hardens particles to prevent dust)
- 80% calcium hydroxide
How many liters can a soda lime CO2 absorber hold?
26 liters of CO2/100 g of absorbent granules
How does soda lime absorb CO2? (chemical reaction)
SODA LIME
- CO2 + H2O <> H2CO3 (carbonic acid)
- H2CO3 reacts with hydroxides to form sodium (or potassium) carbonate and water and heat
=H2CO3 + 2NaOH (KOH) <> Na2CO3 (K2CO3) + 2 H2O + HEAT
=Na2CO3 (K2CO3) + Ca(OH)2 <> CaCO3 + 2NaOH (KOH)
(some CO2 may react directly with Ca(OH)2 [calcium hydroxide] but is much slower)
What is calcium hydroxide lime made of?
-aka Amsorb PLus
-80% calcium hydroxide
16% water (creates carbonic acid)
1-4 % calcium chloride
-calcium sulfate and polyvinlypyrrolidine (adds hardness)
How many liters can calcium hydroxide lime CO2 absorber hold?
10 liters of CO2/100 g of absorbent granules
How does calcium hydroxide absorb CO2? (chemical reaction)
CO2 + H2O <> H2CO3
=H2CO3 + Ca(OH)2 <> CaCO3 + 2 H2O + heat
What signals absorber exhaustion?
What happens to the color when absorber rests?
- color conversion dependent on pH of acid or base
- color reverts back with rest
When do you replace absorbent?
What is the most common color change?
- 50-70% color change
- ethyl violet > from white to purple
What are the sizes of absorbent granules?
What shape are the granules and why?
Why is there a blend of sizes?
- # 4 (small) to #8 (large) mesh (mixture)
- irregular to increase surface area
- large and small mixture minimize resistance and can maintain absorbent capacity
Why do absorbent granules have excessive powder?
channeling resistance and caking
What is the hardness number of absorbent granules?
What should it be?
Why is silica added to soda lime?
- % of what is remaining
- > 75
- to increase hardness
What is channeling of CO2 absorbent granules?
Why is channeling a concern?
- channels formed from exhaled gas through granules of low resistance (loosely packed granules)
- air space occupied 50% of volume in cannister
- absorbent along channels may exhaust and CO2 may filter thru channels that are not visible
What effect does soda lime have if in contact with anesthetic gases? (3)
- dry soda lime may degrade sevo, iso, enflurane, desflurane to carbon monoxide
- degrades sevoflurane and halothene to unsaturated nephrotoxic compounds (compound A)
- fire
What are 5 recommendations on safe use of CO2 absorbents?
- turn off all gas flow when machine not used
- change absorber regularly or when color changes
- change whole absorbent (not just one)
- change when uncertain of state of hydration (i.e. FGF)
- low flows preserve humidity in granules
