Vet Tips

Techniques that Facilitate Equilibration with Inhalant Anesthesia
11/18/2008

nhalant Anesthesia: The time required for an animal to equilibrate with the inhalant anesthetic can be shortened, if the inhalant anesthetic machine has been equilibrated ("primed") with oxygen and the inhalant anesthetic (i.e. isoflurane, sevoflurane) prior to connecting the rebreathing circuit (Y-piece) to the animal's endotracheal tube. Priming the anesthetic circuit with oxygen and inhalant anesthesia before connecting the Y piece to the patient hastens the transition to the inhalant anesthetic, since the anesthetic circuit (generally 5-7 L of gas) is generally filled with room air before beginning anesthesia. The animal breathes primarily room air when connected to an anesthetic circuit that is not primed and does not begin to breath adequate concentrations of inhalant anesthetic until the oxygen flow and vaporizer have been turned on for several minutes, thereby diluting or flushing out the room air (O2 + inhalant anesthetic replace the room air).

How to facilitate equilibration with Inhalant Anesthesia
a. Close the pressure relief ("pop-off") valve
b. Turn the oxygen flow to 1 L/minute
c. Turn the inhalant anesthetic vaporizer to the maintenance concentration for the inhalant anesthetic used. (ex. 2% for isoflurane and 3% for sevoflurane)
d. Put a neoprene or plastic stopper ("cork") in the Y-piece and let the anesthetic circuit fill with oxygen + inhalant anesthetic until the rebreathing bag (2-3L) is half to three-quarters full.
e. Once the animal has been induced with injectables, remove the stopper and connect the Y-piece to the animal's endotracheal tube.
f. While the animal receives the injectables and is entubated, you can turn the oxygen flow off with the stopper in place, for increased handler safety and as a cost cutting measure.
g. DO NOT FORGET TO ADJUST (OPEN SLIGHTLY) THE "POP-OFF VALVE!

How to Save Money with Inhalant Anesthetics
Inhalant anesthetic costs can be reduced by reducing the oxygen (fresh gas) flow rate. The animal's metabolic rate determines its O2 requirements and is generally less that 5 ml/kg during anesthesia. This predicts that you need 50 ml of O2 flow for every 10 kg of body weight. Even a 50 kg (110 lbs) dog is supplied with adequate O2 with a flow rate of 500 ml of oxygen. Therefore, standard 1 L/min O2 flow rates, although helping to maintain the delivery of O2 and inhalant anesthetic to the animal, are extremely wasteful and unnecessarily pollute the environment and are expensive. Cutting the maintenance O2 flow rate to 500 ml can cut inhalant anesthetic costs in half if you are currently using a standard 1 L/min flow rate. The only time that high O2 flow rates are needed alone, (activating the "flush valve") are during the deliberate dilution of the circuit inhalant anesthetic in the circuit as in recovery or in the case of an emergency. Minimum O2 flow rates during the delivery of inhalant anesthetic (isoflurane, sevoflurane) should probably be kept above 200 ml/minute regardless of how small the patient is, in order to insure proper function of most vaporizers.

Web Surfer Tip: refer to the "Handbook of Veterinary Anesthesia 4th Edition" by Dr. William Muir for more information which is listed in the NEW PRODUCTS section.

Submitted by:
William Muir, DVM,PhD, ACVA, ACVECC, VCPCS
338 West 7th Avenue
Columbus, Ohio 43201