15 Pollution and Scavenging Flashcards
What are the adverse effects of N2O and volatile agents?
There are adverse effects to both the
environment
and
staff (and patients).
Environment:
> Volatile agents and N2O are
both known to damage the ozone layer.
> N2O is also a ‘greenhouse’
gas contributing towards global warming.
> N2O sustains combustion and therefore in the presence of lasers or grease it can become a fire hazard.
Staff (adverse effects are primarily related to the use of N2O):
> Bone marrow toxicity and peripheral neuropathy: N2O inhibits the enzyme methionine synthase,
which is involved in the synthesis
of methionine
(required for myelin formation)
and tetrahydrofolate
(required for DNA synthesis).
It also oxidises the cobalt atom in vitamin B12 rendering it non-functional (vitamin B12 is a cofactor for methionine synthase).
The result is megaloblastic changes in bone marrow, bone marrow suppression, megaloblastic anaemia, impaired spinal cord myelination
(subacute combined degeneration of the cord)
and peripheral neuropathy.
> Teratogenicity:
Exact mechanism is unclear
but is likely to be multifactorial
and involve impaired DNA synthesis,
which can manifest as
neural tube defects.
> Spontaneous miscarriage:
There were reports suggesting
an increased incidence of miscarriages
in dental practice nurses working with N2O.
Although these reports got a lot of
publicity there is still no good level of
evidence to support this observation.
> Substance abuse.
What methods are available to reduce pollution in theatre?
> Air conditioning with
rapid rate of air change (15 times per hour)
> Circle system
> Low gas flows
> Avoid using N2O, use O2 with air mix instead
> Scavenging systems
> Monitoring inspired and expired
N2O and volatile agent concentration
and
adjusting concentration to required clinical effect
> Monitoring theatre pollution levels
> Checking for leaks during daily anaesthetic machine check
> Capping breathing circuits when not in use (there is always a small leak)
> Vaporisers should be filled carefully to ensure no spillage
> Total intravenous anaesthesia technique
> Regional anaesthesia technique
> Rotate staff
> Regular servicing of anaesthetic machinery, gas supply, scavengingsystems and ventilation
> Training and education of staff to be aware of potential hazards and how
to minimise them
How can anaesthetic gases be scavenged?
Scavenging may be classified into
active and passive systems.
Passive scavenging
> Requires no external power.
> Gas movement to the exterior is
due to the pressure generated by the
patient during expiration.
> This is an example of a ventile system
(i.e. wind is used to entrain waste gases).
What are the problems with passive scavenging?
> Passive scavenging simply employs the use of wide-bore tubing to channel expired gases to the exterior and therefore it is not as effective as active methods.
> Excess positive or sub-atmospheric
pressures may be caused by wind or
air movement at the outlet.
> The outlets are above roof level to prevent re-entry of scavenged gas into the building; however, the weight of denser gases such as N2O may exert a back-pressure into the patient’s breathing system.
Active scavenging
> Utilises an external power source such as
vacuum pumps to generate a
negative pressure,
which propels gases to
the external atmosphere.
What are the components of an active scavenging system?
COLLECTING SYSTEM [collection of expired gases from breathing system or ventilator] ↓ TRANSFER SYSTEM [wide-bore 30 mm tubing] ↓ RECEIVING SYSTEM [reservoir with visual flow indicator] ↓ DISPOSAL SYSTEM [air pump or fan generates a vacuum] ↓ EXTERIOR
> Gas from the expiratory valve of the breathing circuit or from the ventilator is collected and channelled via wide-bore, 30 mm diameter transfer tubing to the receiving system.
> The receiving system is usually an open-ended cylinder forming a reservoir for the collection of gases. The cylinder must be open-ended as a safety precaution, ensuring that the patient’s airway cannot be subjected to excess positive or negative pressure.
> The receiving system also has a
flow indicator.
Scavenging flow rate is in
the order of 80 L /min,
which ensures removal of all expired gases.
> Gases in the reservoir are vented to the exterior atmosphere via a disposal system. The disposal system is either an air pump or a fan. It operates within a pressure of −0.5 to +5 cm H2O.
What are the disadvantages of the active system?
> Excessive positive pressure may lead to barotrauma.
> Excessive negative pressure can
deflate the reservoir bag of the breathing
system and lead to rebreathing.
What is the recommended number
of air changes per hour in theatre?
Despite scavenging, there will always be a quantity of gas that escapes into the theatre environment, and therefore the theatre needs adequate ventilation. Theatre ventilation should ensure 15 air changes per hour.
What is COSHH?
The Health and Safety Executive Agency is a government agency with the
role of preventing death,
injury and ill health in Britain’s workplaces. COSHH –
Control of Substances Hazardous to Health – sets safe maximum exposure limits to chemicals and other hazardous substances
What are the maximum
recommended anaesthetic
pollutant levels?
Halo Enf Iso N2O Sevo Des
These levels are based on an 8-hour TWA (time-weighted average).
Halothane 10 ppm
Enflurane 50 ppm
Isoflurane 50 ppm
Nitrous oxide 100 ppm
Sevoflurane 20 ppm (recommended limit by Abbot Laboratories)
Desflurane No data provided
Are there any areas of the hospital
in which long-term exposure limits
may be difficult to achieve?
PACU
> It may be difficult to achieve
acceptable pollution levels in
post anaesthesia care units –
patients waking from anaesthesia with
direct expiration into the environment of
volatiles and possibly N2O.
> Paediatric theatres –
because of the use of non-closed
breathing systems
and high gas flows, e.g. ’Ayre’s T-piece.
