5. Scavenging

the basic arrangement comprises collection, transfer, receiving and disposal systems.

: this is usually a shroud that is connected to the APL or expiratory valves of the ventilator via a 30-mm connector (which prevents confusion with components of the breathing system).

Transfer system: this comprises wide-bore tubing to remove the gases. — Receiving system: this is a reservoir system, which is protected against excessive pressures by valves. The positive pressure relief valve is set at 1,000 Pa (1 kPa); the negative pressure relief valve is set at –50 Pa (0.05 kPa).

subatmospheric pressure to eject the gases is generated by a fan which develops a low-pressure high-volume system which can vent 75 l min–1. Other ejector systems include vacuums and venturi devices

Passive systems: the components of the system are as described earlier, and the gases are vented to atmosphere either by the patient’s spontaneous respiratory efforts or by the mechanical ventilator.

: the basic components of the system are again as described , but the vacuum created by a fan or a pump in the disposal system draws the anaesthetic gases through the system. It is important that the negative pressures so generated cannot be transmitted to the patient

: the safe removal of waste theatre gases is a health and safety issue and, since 1989, with the government introduction of Control of Substances Hazardous to Health (COSHH), has been a legal requirement. It is only the local environment that is protected, because the gases are pollutants that are vented unchanged to atmosphere. Most are potent greenhouse gases.

s: some studies have identified increased risks of spontaneous abortion in women exposed to trace concentrations of anaesthetic gases, and also that male anaesthetists were more likely to father daughters than sons. There was, in addition, the suggestion of an increase in haematological malignancies. The association is not strong, because other studies have not replicated these data. Moreover, many millions of general anaesthetics are given annually in the developed world and were this to be a real problem, then its provenance would be a lot more obvious.

Theatre air changes (at least 15 times per hour).
Substitution of total intravenous anaesthesia (TIVA) and regional anaesthesia for inhalational anaesthesia.
Use of low- and ultra-low-flow breathing systems.

Nitrous oxide: 100 parts per million (ppm) Sevoflurane and desflurane: 50 ppm. As yet there are no maximum limits; COSHH states that their similarity to enflurane means that 50 ppm would be appropriate. Halothane: 10 ppm. — USA: in America, the permitted maximum for all halogenated volatiles is 2 ppm.