6.8 Scavenging

Question 1

What are the sources of pollution in the operating theatre complex?

Answer 1

• Gas induction
• Spillage
• Leaks from breathing circuit
• Facemasks
• Laryngeal masks
• T-piece or other open circuits
• Post-anaesthetic exhalation of vapour
• Failure to turn off vapour or gas flow after anaesthesia

Question 2

What are the concerns?

Answer 2

• Reports of increased spontaneous abortion rates in female staff
• Increased rates of malignancy
• Decreased fertility
• Mortality
• Staff particularly at risk:
  paediatric anaesthetists,
  bronchoscopists,
  recovery staff
• Pollution and global warming

Question 3

What are the effects of nitrous oxide?

Answer 3

1. Bone marrow toxicity and neurotoxicity
   * Methionine synthetase inhibition can prevent
   production of methionine and tetrahydrofolate.

• Methionine is a precursor of S-adenosyl methionine (SAM),
  which is incorporated into myelin.

Its absence can lead to subacute combined degeneration of the
spinal cord in chronic B12 deficiency and acutely to
dorsal column dysfunction and peripheral neuropathy.

• Tetrahydrofolate is required for nucleotide and DNA synthesis.
• Megaloblastic anaemia can occur in folate and B12 deficiency.

2. Teratogenicity
   * Association is not strong.
3. Spontaneous miscarriage
   * Reports suggesting an increased incidence of miscarriages
   in dental practice nurses working with nitrous oxide.
4. Substance abuse

Question 4

How may anaesthetic gas pollution be reduced or prevented?

Answer 4

• Ensuring operating theatres are efficiently ventilated
  (minimum 15 exchanges/hour)
• Use of closed circuits with soda lime
• Use of TIVA with oxygen/air
• Use of regional anaesthesia
• Consideration when using nitrous oxide and inhalational agents—
  consider low flow
• Careful filling of vaporizers avoiding spillage and using keyed fillers
• Monitoring of theatre pollution levels
• Use of scavenging
• Use of low flow and ultra low flow breathing systems

Question 5

What are the current recommendations for theatre pollution levels?

Answer 5

Control of Substances Hazardous to Health (CoSHH)

sets maximum exposure limits to chemicals and other hazardous substances.

Currently the maximum anaesthetic pollutant levels
(based on 8-hour time weighted average, parts per million)

are as follows:

Halothane 10 ppm

Enflurane 50 ppm

Isoflurane 50 ppm

Nitrous oxide 100 ppm

In the USA, the maximum exposure limit for all halogenated volatiles is 2 ppm.

Question 6

What are the features of a scavenging system?

Passive

Answer 6

Scavenging systems can be classified into passive and active.

Passive:

• Patient dependent.
• No active positive or negative pressure.
• 30 mm connector used between patient and system.
• Gases are vented to atmosphere either by patient’s spontaneous
  respiratory efforts or by mechanical ventilator.
• These are rarely used in modern theatres.
• Cardiff Aldasorber:
  canister containing charcoal particles that absorb
  halogenated volatile agents.
  Absorption does not render agents inert.

Inhalational agents are released into atmosphere when canister is
disposed of by incineration. Device does not absorb nitrous oxide.

Question 7

What are the features of a scavenging system?

Active:

Answer 7

1. Collecting system:
   Collection of expired gases from breathing system or
   ventilator via 30 mm connector
   so that misconnection is prevented.
2. Transfer system:
   Wide-bore 30 mm tubing.
3. Receiving system:
   Reservoir with visual flow indicator.

Consists of two spring-loaded valves
guarding against excessive positive pressure (1000 Pa)
and negative pressure (–50 Pa) developing in the scavenging system.

4. Disposal system:
   Air pump or fan generates a vacuum.
   Connected to the wall and vented outside.
   Able to cope with flows up to 120 L/min.
5. Exterior

Question 8

What are the disadvantages
of an active system?

Answer 8

• Excessive positive pressure may lead to barotrauma.
• Excessive negative pressure can deflate reservoir bag of breathing
  system, leading to rebreathing.

Question 9

What other systems are available to ensure efficient ventilation?

Answer 9

Laminar flow
* Can be used to create an ultra clean environment

• Recommended for prosthetic implant surgery
• Provides over 400 air changes per hour by recirculating air after passing
  it through high-efficiency filters (0.5 μm)

All ventilation systems should provide a positive pressure across any
openings so that when doors are opened, there is less chance of bacterial
escape.

Opening of doors can make system less efficient.

Question 10

What other factors need to be taken into consideration when designing a theatre?

Answer 10

• Temperature control
• Humidity