Filters, humidifiers and scavenging systems Flashcards
Which microbe has been implicated in transmission by breathing equipment?
Hepatitis C
What size of particles can breathing system filters remove?
> 0.2 μm.
Where should a filter be positioned
Anywhere but most commonly between the patient and the breathing circuit
What are the two main types of filter?
Electrostatic Hydrophobic (mechanical)
How does an electrostatic filter work
When air flows through an electrostatic filter, and electrostatic charge is created which filters out electrically charged molecules in the gas flow.
They are much LESS effective when wet
How does a hydrophobic filter work?
A hydrophobic pleated membrane filters airborne and liquid contaminants
What problems are associated with filters?
An increase in dead-space, especially in children
An increased resistance to flow, especially if wet
In severe cases the filter may block with secretions, blocking the anaesthetic circuit
Name the five types of filtration devices available
•
heat and moisture exchanger with no filter (HME)
•
electrostatic filter only
•
pleated filter only
•
electrostatic filter with HME
•
pleated filter with HME.
Why are the terms ‘electrostatic’ and ‘pleated’ used to distinguish the two common types of filter not ideal?
Both types rely to some extent on electrostatic charge to hold particles within the filter material and both types of material could be pleated.
What is the main difference between ‘electrostatic’ and ‘pleated’ filters
The density of the fibers
“Electrostatic” - relatively low fiber density with relatively high electrostatic charge
“pleated” - Relatively high density of fibers with relatively low electrostatic charge (
What problems does increasing fiber density have on the breathing circuit and how is this problem mitigated?
Increases resistance to gas flow
Pleating the material increases the surface area and mitigates the problem
What is the meaning of ‘hydrophobic’ in terms of the hydrophobic pleated filter?
the surface of the filter material repels water
What temperature and humidity should HME achieve for inspired air
30 ºC and 30 g.m^-3
These are the numbers during body temperature and pressure saturated (BTPS) conditions (i.e. 37 °C and 44 g m−3)
Describe the hazards caused by inadequate humidity
- Drying, inflammation and ulceration of mucosa
- -> cell death, partial shedding or even complete denudation of the basement membrane. The epithelium must repair and regenerate to restore its functions – dysfunction of mucociliary elevator
Dysfunction of the mucociliary elevator –> cell damage –> decreased compliance, decreased FRC, atalectasis and shunt
Why are humidity requirement during anaesthesia different to intensive care? What are the humidity requirements of anaesthesia compared to ICU
In anaesthesia, the time duration of the bypass of the upper airway is much shorter.
Anaesthesia (up to 10 hours) –> 20 g.m^-3 required
ICU (usually > 10 hours) –> 30 g.m^3 required
What level of humidity does a circle circuit provide and where does this water come from?
The reaction of exhaled CO2 with soda lime generates water vapour that can adequately humidify inspired gases, particularly if low fresh gas flows are employed –> therefore the additional use of an HMEF is probably not required. –> however it can still be used and will contribute to increased humidification
Why are electrostatic filters not recommended for use in anaesthesia?
Circle systems can contain condensation –> this liquid can pass through low density electrostatic filters under pressures typically encountered in anaesthesia.
What is essentially the only appropriate type and position of filter during anaesthesia with a circle breathing system and why
Hydrophobic pleated filters to protect the breathing circuit are appropriate during anaesthesia. The reaction of CO2 with soda lime tends to create a humidity of approximately 20 g.m^-3 which is sufficient for anaesthesia < 10 hours. However, liquid-borne transmission of microbes is a prevalent risk which is not mitigated by the use of low density electrostatic filters.
Essentially: a barrier rather than a humidifier is required –> hydrophobic pleated membrane filters are high enough density to prevent liquid-borne transmission of microbes at pressures generated during anaesthesia.
Why are medical gases dry?
To protect against corrosion and condensation inside cylinders and pipes
Why do inspired gases need heating
To assist in providing the latent heat of vaporization required for saturation of gases with water to prevent drying out and ulceration
Name and describe the four types of humidifiers
Cold water - gas is bubbled through sterile water at room temperature –> unless there is a VERY LOW gas flow –> relative humidity achieved is minimal
Hot water - Provides greater relative humidity by electrically heating the water but associated with risks including scalding and infection
Nebulizers - Supplies water in droplet form: The size of the droplet determines where it falls out of the mist: Too large (>5um) –> will deposit in the breathing system, too small (<1um) will reach the alveoli –> water intoxication
HMEF - aims to conserve the patient’s own heat and water without the need for external power or water source. Compact, cheap and effective but efficiency decreases over time
Define absolute humidity
The mass of water vapour present in a unit volume of air
g/m^3 = mg/L
Define relative humidity
Ratio between the absolute humidity and the humidity at saturation at the same temperature
At a specific temperature:
Relative humidity = Absolute humidity/Humidity at saturation
(%)
Define humidity at saturation
maximum mass of water that can be carried in a unit volume of air per unit time AT A PARTICULAR TEMPERATURE
What were the alleged risks associated with long term exposure to anaesthetic gases?
? Increased incidence of lymphoid tumors
? Miscarriage
this was never proven but resulted in the Control of Substances Hazardous to Health (COSHH) regulations
What are the acceptable levels for the commonly used volatile anaesthetics introduced by the Control of Substances Hazardous to Health regulations in 1989?
N2O - 100ppm Sevoflurane - 60 ppm Enflurane - 50 ppm Isoflurane - 50 ppm Halothane - 10 ppm
Differentiate and define absorption and adsorption
Adsorption: accumulation of a substance at the surface of a solid or a liquid
Absorption: assimilation of a substance within the bulk of a solid or liquid
What are Aldasorbers
These are charcoal cannistors that can effectively adsorb volatile agents
What is the limitation of the charcoal canisters?
They cannot remove N2O –> therefore they are not used but can still be found in the developing world
How do passive scavengers systems work?
Collect, transfer and receive gases in a similar way to active systems. However, gases are vented passively to the outside atmosphere, driven by the patients respiratory effort of ventilator and entrained by the wind
What problems may arise with passive scavenger systems?
The direction/strength of the wind affect performance - can even reverse gas flow
Water vapor may run back into the system
Waste transfer from one theatre to another
What are differences and similarities between active and passive scavenger systems
Similarities –> collecting and transfer systems are similar
Difference –> Active scavenging systems have an ACTIVE DISPOSAL system consisting of a fan or pump used to generate a vacuum to drive the waste gases through the system
What properties should the active scavenger system disposal system have
Low Pressure and high flow –> capable of removing 75L/min
Is the piped vacuum system adequate for the purposes of anaesthetic gas scavenging?
No. It uses lower flows and may be located in an area where staff members work
Why is the scavenger receiving system required
To prevent the active disposal system from sucking gases directly from the patient. Includes wide bore tubing and reservoir bag
How long do HMEs take to achieve steady state and become effective
5 to 20 minutes
What relative humidity can HMEs achieve?
70%
To what temperatures can inspired gases be warmed to by HMEs
29 - 34 ºC
Do high flows affect HME efficiency?
No
Do high tidal volumes affect HME efficiency
Yes
Can HMEs be used satisfactorily long-term in the ICE
NO. They become inadequate after 24 hours of use
Why can’t an ordinary vacuum pump be used for active scavenging?
Because of the nature of the expired gas flow, scavenging systems must be able to dispose of both high and variable gas flows (high flow, low pressure). An ordinary hospital piped vacuum (low flow and high pressure) may be unable to withstand these demands and so a specific AGSSis used
