Response of the Committee for Anaesthesia,Analgesia and Pain Prophylaxis of the Society for Laboratory Animal Science to ‘Temporary inhalation anaesthesia in experimental pigs’

In Sasaki et al.'s article, Letter to the Editor: Temporary inhalation anaesthesia in experimental pigs (Lab Anim 2010;44:69–70), a device for short-term inhalation anaesthesia with sevoflurane in ketamine/metedomidine-sedated mini-pigs was presented. The device consists of a 500 mL beaker (standard laboratory glassware), three pieces of gauze (30 × 30 cm) and a cut-off surgical glove. The pieces of gauze were soaked with 5 mL of sevoflurane. The device covered the pig's maxilla, nose and mandible. The device was fitted closely to the animal's nose by means of the surgical glove to prevent leakage of sevoflurane. The animals were removed from the device when the respiratory rate decreased and palpebral reflex disappeared.

The Committee for Anaesthesia, Analgesia and Pain Prophylaxis of the Society for Laboratory Animal Science comments on this publication as follows:

The anaesthesia procedure using this device appears easy to perform, cheap and practical but in our opinion it is potentially harmful to both animals and man for the reasons detailed below.

The animal is exposed to extremely high concentrations of anaesthetic gas resulting from uncontrolled vaporization of sevoflurane. The vapour pressure of sevoflurane (760 mmHg, 20°C) is 160 mmHg. Therefore, the maximum concentration of sevoflurane under the conditions given is ∼21%. This is equal to ∼10 times the minimum alveolar concentration (MAC) of sevoflurane in pigs, which is 2–2.5%, and is around 2.5 times the concentration used for mask induction in unsedated pigs (7–8%). Thus, the unusual method of producing the anaesthetic gas by uncontrolled vaporization of sevoflurane entails a considerably increased risk of anaesthetic-related problems (e.g. cardiac or respiratory arrest).

The device described cannot be sealed tightly to the pig's nose as claimed by the authors because in this case normal breathing would not be possible. In the case of a very small amount of leakage, breathing could be hindered, and increased breathing effort, hypoxia and hypercapnia might result. Even if there were enough leakage to facilitate normal breathing, rebreathing is very likely to occur. Additionally, waste anaesthetic gases will pollute the proximate environment, which would be harmful to investigators.

Since volatile anaesthetics depress respiratory function, all potent volatile anaesthetics (e.g. sevoflurane, isoflurane and desflurane) have to be administered in an oxygen-enriched gas mixture with a minimum oxygen concentration of 30% (FiO₂ ≥ 0.3).

The anaesthesia device presented (a beaker, used as simple face mask) does not allow sufficient fresh gas supply, administration of additional oxygen or elimination of waste gas, in particular CO₂. Consequently, the pig has to rebreath the CO₂-enriched and oxygen-reduced gas in the beaker. Thus, hypoxia and hypercapnia might occur, which additionally increases the risk of anaesthesia-related problems. Furthermore, the amount of oxygen inspired cannot be calculated exactly. Therefore, measuring oxygen saturation by pulsoxymetry is strongly recommended.

(3) It should be pointed out that people working with the described mask system are not protected from inhaling sevoflurane because of the inherent leakiness of the system and the lack of a waste gas scavenging unit. Additionally, handling the device, i.e. soaking the gauze with sevoflurane and empting/cleaning the beaker, will be harmful to the health of investigators. In addition, the danger of injuries to personnel and animals from accidentally broken glass beakers has to be considered. Thus, due to exposure of personnel to volatile anaesthetics and other hazards, employee protection would need to be taken into consideration with this device.

In summary, the inhalation anaesthesia methodology presented increases the risk of anaesthetic-associated death because of exposure of the animal to extremely high concentrations of sevoflurane combined with worsening of respiratory depression due to the lack of both sufficient fresh gas supply and elimination of waste gas. The method does not represent current best practice in veterinary or laboratory animal anaesthesia and may also contravene health and safety legislation (i.e. current personnel protection regulations).

In conclusion, the device described is potentially harmful in several respects, and thus its use cannot be recommended.

The disadvantages of the device presented could be circumvented by the use of an anaesthetic machine equipped with an oxygen supply, a vaporizer and waste gas scavenging unit. In this way, sevoflurane can be delivered in a controlled and reproducible way, fresh air with high oxygen content can be provided (e.g. 100% oxygen with mask induction), and waste gasses, particularly CO₂, are eliminated. Using such up-to-date methodology reduces substantially the risk of anaesthesia-related problems and death, and additionally protects personnel from being exposed unnecessarily to anaesthetic gas. As an alternative to the use of volatile anaesthetics, injection anaesthesia protocols can be tailored to the needs of the specific research model.