Since the late 1960s there has been speculation that trace anaesthetic gases/vapours may have a harmful effect on operating theatre personnel. It has been concluded from currently available studies that there is no association between occupational exposure to trace levels of waste anaesthetic vapours in scavenged operating theatres and adverse health effects. However, it is desirable to vent out the exhaled anaesthetic vapours and maintain a vapour-free theatre environment. A prudent plan for minimizing exposure includes maintaining equipment, training personnel and monitoring exposure routinely. Although not universally agreed upon, the recommended maximum accepted concentrations in the UK (issued in 1996), over an (see Table 3.1 for main causes) 8-hour time-weighted average, are as follows:
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100 particles per million (ppm) for nitrous oxide
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50 ppm for enflurane
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50 ppm for isoflurane
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10 ppm for halothane
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20 ppm for sevoflurane (recommended by Abbot Laboratories)
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no limit set for desflurane, although a 50 ppm target is advisable due to its similarity to enflurane.
| Anaesthetic techniques | Incomplete scavenging of the gases from ventilator and/or APL valve |
| Poorly fitting face mask | |
| Paediatric breathing systems, e.g. T-piece | |
| Failure to turn off fresh gas and/or vaporizer at the end of an anaesthetic | |
| Uncuffed tracheal tubes | |
| Filling of the vaporizers | |
| Exhalation of the gases/vapours during recovery | |
| Anaesthetic machine | Leaks from the various connections used; e.g. ‘O’ rings, soda lime canister |
| Others | Cryosurgery units and cardiopulmonary bypass circuit if a vapour is used |
These levels were chosen because they are well below the levels at which any significant adverse effects occurred in animals and represent levels at which there is no evidence to suggest human health would be affected.
In the USA, the maximum accepted concentrations of any halogenated agent should be less than 2 ppm. When such agents are used in combination with nitrous oxide, levels of less than 0.5 ppm should be achieved. Nitrous oxide, when used as the sole anaesthetic agent, at 8-hour time-weighted average concentrations should be less than 25 ppm during the administration of an anaesthetic.
The Netherlands has a limit of 25 ppm for nitrous oxide, whereas Italy, Sweden, Norway and Denmark set 100 ppm as their limit for exposure to nitrous oxide. The differences illustrate the difficulty in setting standards without adequate data.
Methods used to decrease theatre pollution are listed as follows.
- 1.
The facility should have adequate theatre ventilation and air conditioning, with frequent and rapid changing of the circulating air (15–20 times/h). This is one of the most important factors in reducing pollution. Unventilated theatres are four times as contaminated with anaesthetic gases and vapours compared to those with proper ventilation. A non-recirculating ventilation system is usually used. A recirculating ventilation system is not recommended. In labour wards, where anaesthetic agents including Entonox are used, rooms should be well ventilated with a minimum of 5 air changes/hr.
- 2.
The circle breathing system recycles the exhaled anaesthetic vapours, absorbing CO 2 . It requires a very low fresh gas flow, so reducing the amount of inhalational agents used. This decreases the level of theatre environment contamination.
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Another alternative is total intravenous anaesthesia.
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Regional anaesthesia is another option.
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Avoid spillage and use fume cupboards during vaporizer filling. This used to be a significant contributor to the hazard of pollution in the operating theatre. Modern vaporizers use special agent-specific filling devices as a safety feature and to reduce spillage and pollution.
- 6.
Scavenging can also be used.
Sampling procedures for evaluating waste anaesthetic vapour concentrations in air should be conducted for nitrous oxide and halogenated agents on a yearly basis in the UK and on a quarterly basis in the United States in each location where anaesthesia is administered. Monitoring should include:
- a.
leak testing of equipment
- b.
sampling air in the theatre personnel breathing zone.
Anaesthetic equipment, gas scavenging, gas supply, flowmeters and ventilation systems must be subject to a planned preventative maintenance (PPM) programme. At least once annually, the general ventilation system and the scavenging equipment should be examined and tested by a responsible person.
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In scavenged areas, there is no association between occupational exposure to anaesthetic agents trace levels and adverse health effects.
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There are no agreed international standards of the maximum accepted concentrations of agents in the theatre environment.
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Routine monitoring and testing (PPM) are mandatory.
Exam tip: It is important to know the causes of pollution in the operating theatre, the methods used to reduce the pollution and the acceptable maximum concentrations (in ppm) of vapours in the operating theatre.
Scavenging
In any location where inhalation anaesthetics are administered, an adequate and reliable system should be used for scavenging waste anaesthetic gases. A scavenging system is capable of collecting the waste anaesthetic gases from the breathing system and discarding them safely. Unscavenged operating theatres can show N 2 O levels of 400–3000 ppm.
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Should not affect the ventilation and oxygenation of the patient.
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Should not affect the dynamics of the breathing system.
A well-designed scavenging system should consist of a collecting device for gases from the breathing system/ventilator at the site of overflow, a ventilation system to carry waste anaesthetic gases from the operating theatre and a method for limiting both positive and negative pressure variations in the breathing system.
The performance of the scavenging system should be part of the anaesthetic machine check.
Scavenging systems can be divided into passive and active systems.
Passive system
The passive system is simple to construct with zero running cost.
Components
- 1.
The collecting and transfer system consists of a shroud connected to the adjustable pressure limiting (APL) valve (or expiratory valve of the ventilator). A 30-mm connector attached to transfer tubing leads to a receiving system ( Fig. 3.1 ). The 30-mm wide-bore connector is designed as a safety measure in order to prevent accidental misconnection to other ports of the breathing system ( Fig. 3.2 ).
Fig. 3.1
Diagram of a passive scavenging system.
(With permission from Aitkenhead, R., Smith, G., 1996. Textbook of Anaesthesia, 3rd ed. Churchill Livingstone.)
Fig. 3.2
Attaching a 30-mm connecter to the APL valve of the breathing system. The 30-mm wide bore is designed as a safety measure.
- 2.
A receiving system (reservoir bag) can be used. Two spring-loaded valves guard against excessive positive (1000 Pa) in case of a distal obstruction or negative (–50 Pa) pressures in case of increased demand in the scavenging system. Without these valves, excessive positive pressure increases the risk of barotrauma should there be an obstruction beyond the receiving system. Excessive negative pressure could lead to the collapse of the reservoir bag of the breathing system and the risk of rebreathing.
- 3.
The disposal system is a wide-bore copper pipe leading to the atmosphere directly or via the theatre ventilation system.
Mechanism of action
- 1.
The exhaled gases are driven by either the patienťs respiratory efforts or the ventilator.
- 2.
The receiving system should be mounted on the anaesthetic machine to minimize the length of transfer tubing, therefore minimizing resistance to flow.
Problems in practice and safety features
- 1.
Connecting the scavenging system to the exit grille of the theatre ventilation is possible. Recirculation or reversing of the flow is a problem in this situation.
- 2.
Excess positive or negative pressures caused by the wind at the outlet might affect the performance and even reverse the flow.
- 3.
The outlet should be fitted with a wire mesh to protect against insects.
- 4.
Compressing or occluding the passive hose may lead to the escape of gases/vapours into the operating theatre and thereby polluting it. The disposal hose should be made of non-compressible materials and not placed on the floor.
Active system
Components
- 1.
The collecting and transfer system is similar to that of the passive system ( Fig. 3.3 ).
