Filters, humidifiers and scavenging systems

Question 1

Which microbe has been implicated in transmission by breathing equipment?

Answer 1

Hepatitis C

Question 2

What size of particles can breathing system filters remove?

Answer 2

> 0.2 μm.

Question 3

Where should a filter be positioned

Answer 3

Anywhere but most commonly between the patient and the breathing circuit

Question 4

What are the two main types of filter?

Answer 4

Electrostatic
Hydrophobic (mechanical)

Question 5

How does an electrostatic filter work

Answer 5

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

Question 6

How does a hydrophobic filter work?

Answer 6

A hydrophobic pleated membrane filters airborne and liquid contaminants

Question 7

What problems are associated with filters?

Answer 7

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

Question 8

Name the five types of filtration devices available

Answer 8

•
heat and moisture exchanger with no filter (HME)

•
electrostatic filter only

•
pleated filter only

•
electrostatic filter with HME

•
pleated filter with HME.

Question 9

Why are the terms ‘electrostatic’ and ‘pleated’ used to distinguish the two common types of filter not ideal?

Answer 9

Both types rely to some extent on electrostatic charge to hold particles within the filter material and both types of material could be pleated.

Question 10

What is the main difference between ‘electrostatic’ and ‘pleated’ filters

Answer 10

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 (

Question 11

What problems does increasing fiber density have on the breathing circuit and how is this problem mitigated?

Answer 11

Increases resistance to gas flow

Pleating the material increases the surface area and mitigates the problem

Question 12

What is the meaning of ‘hydrophobic’ in terms of the hydrophobic pleated filter?

Answer 12

the surface of the filter material repels water

Question 13

What temperature and humidity should HME achieve for inspired air

Answer 13

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)

Question 14

Describe the hazards caused by inadequate humidity

Answer 14

1. 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

Question 15

Why are humidity requirement during anaesthesia different to intensive care? What are the humidity requirements of anaesthesia compared to ICU

Answer 15

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

Question 16

What level of humidity does a circle circuit provide and where does this water come from?

Answer 16

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

Question 17

Why are electrostatic filters not recommended for use in anaesthesia?

Answer 17

Circle systems can contain condensation –> this liquid can pass through low density electrostatic filters under pressures typically encountered in anaesthesia.

Question 18

What is essentially the only appropriate type and position of filter during anaesthesia with a circle breathing system and why

Answer 18

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.

Question 19

Why are medical gases dry?

Answer 19

To protect against corrosion and condensation inside cylinders and pipes

Question 20

Why do inspired gases need heating

Answer 20

To assist in providing the latent heat of vaporization required for saturation of gases with water to prevent drying out and ulceration

Question 21

Name and describe the four types of humidifiers

Answer 21

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

Question 22

Define absolute humidity

Answer 22

The mass of water vapour present in a unit volume of air

g/m^3 = mg/L

Question 23

Define relative humidity

Answer 23

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
(%)

Question 24

Define humidity at saturation

Answer 24

maximum mass of water that can be carried in a unit volume of air per unit time AT A PARTICULAR TEMPERATURE

Question 25

What were the alleged risks associated with long term exposure to anaesthetic gases?

Answer 25

? Increased incidence of lymphoid tumors

? Miscarriage

this was never proven but resulted in the Control of Substances Hazardous to Health (COSHH) regulations

Question 26

What are the acceptable levels for the commonly used volatile anaesthetics introduced by the Control of Substances Hazardous to Health regulations in 1989?

Answer 26

N2O - 100ppm
Sevoflurane - 60 ppm
Enflurane - 50 ppm
Isoflurane - 50 ppm
Halothane - 10 ppm

Question 27

Differentiate and define absorption and adsorption

Answer 27

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

Question 28

What are Aldasorbers

Answer 28

These are charcoal cannistors that can effectively adsorb volatile agents

Question 29

What is the limitation of the charcoal canisters?

Answer 29

They cannot remove N2O –> therefore they are not used but can still be found in the developing world

Question 30

How do passive scavengers systems work?

Answer 30

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

Question 31

What problems may arise with passive scavenger systems?

Answer 31

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

Question 32

What are differences and similarities between active and passive scavenger systems

Answer 32

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

Question 33

What properties should the active scavenger system disposal system have

Answer 33

Low Pressure and high flow –> capable of removing 75L/min

Question 34

Is the piped vacuum system adequate for the purposes of anaesthetic gas scavenging?

Answer 34

No. It uses lower flows and may be located in an area where staff members work

Question 35

Why is the scavenger receiving system required

Answer 35

To prevent the active disposal system from sucking gases directly from the patient. Includes wide bore tubing and reservoir bag

Question 36

How long do HMEs take to achieve steady state and become effective

Answer 36

5 to 20 minutes

Question 37

What relative humidity can HMEs achieve?

Answer 37

70%

Question 38

To what temperatures can inspired gases be warmed to by HMEs

Answer 38

29 - 34 ºC

Question 39

Do high flows affect HME efficiency?

Answer 39

No

Question 40

Do high tidal volumes affect HME efficiency

Answer 40

Yes

Question 41

Can HMEs be used satisfactorily long-term in the ICE

Answer 41

NO. They become inadequate after 24 hours of use

Question 42

Why can’t an ordinary vacuum pump be used for active scavenging?

Answer 42

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