Anaesthetic machine: Pollution and scavenging Flashcards
What are the risks of occupational exposure to anaesthetic waste gases
Recommended frequency of exposure monitoring in the UK
- Un-scavenged operating theatres can show N20 levels of 400-3000 ppm. Long term exposure to N20 can affect the bone marrow.
- General consensus of evidence is no association between occupational exposure to trace levels of waste anaesthetic gases in scavenged operating theatres and adverse health effects. However, desirable that exhaled anaesthetic gases are vented out to maintain a vapour-free theatre environment
- In the UK, routine annual exposure monitoring is recommended
Maximum accepted concentrations for
* Halothane
* Enflurane
* Nitrous oxide
UK vs US
Maximum accepted concentrations are calculated over an 8-hour, time-weighted average
UK:
* Halothane: 10 ppm (parts per million)
* Enflurane: 50 ppm
* Nitrous oxide: 100 ppm
US:
* Maximum accepted concentration of any halogenated agent: 2 ppm. If used in combination with nitrous oxide: 0.5 ppm
* Nitrous oxide (when used as the sole anaesthetic agent): 25 ppm
Methods to decrease polluation in the operating theatre (6)
- Adequete theatre ventilation and air conditioning
- Circle breathing system
- TIVA
- Regional anaesthesia
- Avoidance of spillage during vaporizer filling
- Scavenging
Importance and methods of theatre ventilation.
Rate of air changes required for theatre, labour ward
Theatres that are unventilated are 4x as contaminated with anaesthetic gases and vapours as those with proper ventilation.
Non-recirculating ventilation system usually used to avoid contaminating other parts of the operating theatre suites
Frequent and rapid changes of the circulating air:
* Theatre: 15-20 changes of circulating air per hour
* Labour wards (where anaesthetic agents including entonox are used): minimum 5 air changes per hour
How does the circle breathing system reduce theatre environment contamination
This breathing system recycles the exhaled anaesthetic vapours, absorbing carbon dioxide.
Requires a very low fresh gas flow -> reducing the amount of inhalational agents used.
How do modern vapourizers reduce theatre pollution
- Special agent-specific filling devices reduce spillage and pollution (also act as safety measure)
- Old vaporizers with screw-top inlets increased risk of spillage during vaporizer filling
Scavenging system
Definition, key features (3)
- Scavenging systems collect waste inhalation anaesthetic gases from the breathing system and dispose of them safely
- Scavenging systems should not affect ventilation/oxygenation/dynamics of breathing system
- Should be present in any location in which inhalation anaesthetics are administred
Three main physical components:
* Collecting and transfer component located between the APL valve and the receiving component
* Receiving component which guards against excessive positive (distal obstruction) or negative (increased demand) pressure - therefore maintaining the dynamics of the breathing system
* Disposal component: moves the gases from the receiving component to outside the theatre
Features of an ideal scavenging system (9)
- Is easy and simple to install, and is reliable and cheap to run.
- Does not affect the oxygenation and ventilation of the patient.
- Does not affect the dynamics of the breathing system.
- Is attached with components incompatible with those used in the patient’s breathing system, so avoiding any risk of accidental connection to the patient.
- In case of a fault, prevents adverse events such as barotrauma or, an unacceptable increase in the apparatus dead space. It should also incorporate a mechanism to protect the patient against excessively high or low pressures in the breathing system.
- Is capable of dealing with the various expiratory flow rates generated by patient or ventilator.
- Disposes of waste anaesthetic gases without causing pollution of other parts of the theatre or hospital.
- Performs without being affected by external factors such as the direction of the wind.
- Uses a dedicated vacuum system to dispose of the waste gases that is separate from the suction system used in the hospital.
Passive vs active scavenging systems
Passive:
* Simple to construct, no running costs
* Exhaled gases are driven by either the patient’s respiratory efforts or the ventilator
* Less efficient and reliable than active ones. Rarely used in modern theatres.
Active:
* Require a negative pressure system to function
* Vacuum pump drives the gases through the system.
* More efficient and reliable than passive (as does not rely on patient respiratory effort or the ventilator to function)
* Usually used in modern operating theatres.
Passive scavenging system: structure and function
Collection and transfer
* a shroud connected to the APL valve (or expiratory valve of ventilator)
* 30mm connector attached to transfer tubing leads to the receiving component (30mm wide bore connector is a safety measure to prevent accidental misconnection to other ports of the breathing system)
Receiving component
* Resevoir bag with two spring-loaded valves that guard against excessive positive (1000 pa) or negative (-50pa) pressures in the scavenging system.
* Without these valves, excessive negative pressure -> collapse of resevoir bag, risk of rebreathing, eccessive positive pressure -> barotrauma.
* Resistance to flow is minimised by mounting the receiving component to the anaesthetic machine to shorten the length of transfer and tubing (important as gases are driven by patient respiratory effort or ventilator)
Disposal component
* Wide bore copper pipe
* Can be connected to external terminal to the atmosphere, exit via the window, or the theatre ventilation -> can be a source of potential problems.
Disposal problems with passive scavenging systems
Connecting passive scavenging system to exit grill of theatre ventilation can present problems
* Recirculation or reversal of the flow: can lead to pollution of other parts of the operating theatre
* Wind and direction cat the external terminal can affect the pressure within the system. Excess positive or negative pressures can be caused at the outlet -> affecting performance of system +/- reversing the flow
* Compressing or occluding the disposal hose can lead to the escape of gases into the operating theatre. Therefore hose should be made of non-compressible material and not placed on the floor.
Picture shows passive scavenging system connected to external terminal (A), exit via window (B) or theatre ventilation (C).
Active scavenging system: structure and function
Collection and transfer (same as passive)
* a shroud connected to the APL valve (or expiratory valve of ventilator)
* 30mm connector attached to transfer tubing leads to the receiving component (30mm wide bore connector is a safety measure to prevent accidental misconnection to other ports of the breathing system)
Receiving component
* Gases flow from collecting component into receiving system
* Usually a valve-less, open-ended resevoir with a visual flow indicator positioned between the receiving and disposal components
* Increased demand (excessive negative pressure) allows ambient air to be entrained - therefore maintaining the pressure. Opposite occurs during excessive positive pressure.
* Therefore can cope with changes in gas flow rates, and passes a uniform gas flow to the disposal system
* Alternative: ressevoir bag with two spring-loaded safety valves can also be used, as with the passive scavenging system.
Disposal component
* Generates vacuum (negative pressure) that is transmitted through pipes
* May be a motorized fan, a pumo or a venturi system
* NOTE an independent vacuum pump should be used for scavenging purposes rather than general suction used as part of the piped medical gases and vacuum. Negative pressures in the PMGV system would be too excessive + might reduce efficiency of suction system in PMGV
Charcoal canisters
Use, function, disadvantages
A simple, compact and mobile passive scavenging system.
- A cannister containing activated charcoal particles connected to the outlet of the breathing system via a length of transfer tubing
- Charcoal particles inside the canister remove the halogenated anaesthetics by filtration
- Usually replaced after 12 hours of use. Increasing weight of the canister is the only indication is is exhausted.
- Heating the canister causes release of the inhalational agents
Disadvantage: do not remove N20, and there is increasing evidence of the potential occupational hazard of N20 -> not widely used
Can a T-piece paediatric breathing system cause theatre pollution?
Yes - due to the open-ended resevoir
A modified version has an APL valve allowing scavenging of the anaesthetic vapours.
The anaesthetist notices the sudden collapse of the reservoir bag of the breathing in a spontaneously breathing anaesthetized patient connected to a scavenging system. He checks the breathing system for any disconnections and finds none. What other possible causes might there be?
3
- Failure of fresh gas flow into the breathing system (rare)
- Excessive negative pressure in a faulty scavenging system
- Ruptured resevoir bag (highly unlikely if routine checks of the anaesthetic machine have been carried out)
