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This page contains information on Isothesia for veterinary use.
The information provided typically includes the following:
  • Isothesia Indications
  • Warnings and cautions for Isothesia
  • Direction and dosage information for Isothesia


This treatment applies to the following species:
Manufacturer: Butler Schein™ Animal Health

Isoflurane, U.S.P. Inhalation Anesthetic

For induction and maintenance of general anesthesia in horses and dogs

Induction is rapid; surgical anesthesia should be produced in 5-10 minutes following label directions

Recovery is rapid and typically uneventful

Active Ingredients

Each mL contains 99.9% isoflurane.

Isothesia Indications

ISOTHESIA (isoflurane, U.S.P.) is used for the induction and maintenance of general anesthesia in horses and dogs.


Isoflurane is a nonflammable, nonexplosive general inhalation anesthetic agent. Its chemical name is 1-chloro-2,2,2-trifluoroethyl difluoromethyl ether.

Some physical constants are:

Molecular weight


Boiling point at 760 mm Hg


Refractive index n20D


Specific gravity 25°/25°C


Vapor pressure in mm Hg**









**Equation for vapor pressure calculation:



A = 8.056

B = -1664.58

T = °C + 273.16

Partition coefficients at 37°C:







Partition coefficients at 25°C - rubber and plastic:

Conductive rubber/gas


Butyl rubber/gas


Polyvinyl chloride/gas








Butyl acetate/gas


Purity by gas chromatography


Lower limit of flammability in oxygen or nitrous oxide
at 9 joules/sec. and 23°C


Lower limit of flammability in oxygen or nitrous oxide
at 900 joules/sec. and 23°C

Greater than useful concentration in anesthesia

MAC (Minimum Alveolar Concentration) is 1.31% in horses1 and 1.28% in dogs.6

Isoflurane is a clear, colorless, stable liquid which does not contain additives or chemical stabilizers. Isoflurane has a mildly pungent, musty, ethereal odor. Samples stored in indirect sunlight in clear, colorless glass for five years, as well as samples directly exposed for 30 hours to a 2 amp, 115 volt, 60 cycle long wave U.V. light were unchanged in composition as determined by gas chromatography. Isoflurane in one normal sodium methoxide-methanol solution, a strong base, for over six months consumed essentially no alkali, indicative of strong base stability. Isoflurane does not decompose in the presence of soda lime, and does not attack aluminum, tin, brass, iron, or copper.

Clinical Pharmacology

ISOTHESIA (isoflurane, USP) is an inhalation anesthetic. Induction and recovery from anesthesia with isoflurane are rapid.2,5 The level of anesthesia may be changed rapidly with isoflurane. Isoflurane is a profound respiratory depressant. RESPIRATION MUST BE MONITORED CLOSELY IN THE HORSE AND DOG AND SUPPORTED WHEN NECESSARY. As anesthetic dose is increased, both tidal volume and respiratory rate decrease.3,6 This depression is partially reversed by surgical stimulation, even at deeper levels of anesthesia.

Blood pressure decreases with the induction of anesthesia but returns toward normal with surgical stimulation. Progressive increases in the depth of anesthesia produce corresponding decreases in blood pressure; however, heart rhythm is stable and cardiac output is maintained with controlled ventilation and normal PaCO2 despite the increasing depth of anesthesia. The hypercapnia which attends spontaneous ventilation during isoflurane anesthesia increases the heart rate and raises cardiac output above levels observed with controlled ventilation.3 Isoflurane does not sensitize the myocardium to exogenously administered epinephrine in the dog.

Muscle relaxation may be adequate for intra-abdominal operations at normal levels of anesthesia. However, if muscle relaxants are used to achieve greater relaxation, it should be noted that; ALL COMMONLY USED MUSCLE RELAXANTS ARE MARKEDLY POTENTIATED WITH ISOFLURANE, THE EFFECT BEING MOST PROFOUND WITH THE NONDEPOLARIZING TYPE. Neostigmine reverses the effect of nondepolarizing muscle relaxants in the presence of isoflurane but does not reverse the direct neuromuscular depression of isoflurane.


ISOTHESIA (isoflurane, U.S.P.) is contraindicated in horses and dogs with known sensitivity to isoflurane or to other halogenated agents.

WARNING(S): Increasing depth of anesthesia with ISOTHESIA (isoflurane U.S.P.) may increase hypotension and respiratory depression. The electro-encephalographic pattern associated with deep anesthesia is characterized by burst suppression, spiking, and isoelectric periods.4

Since levels of anesthesia may be altered easily and rapidly, only vaporizers producing predictable percentage concentrations of isoflurane should be used (SEE DOSAGE AND ADMINISTRATION).

The action of nondepolarizing relaxants is augmented by isoflurane. Less than the usual amounts of these drugs should be used. If the usual amounts of nondepolarizing relaxants are given, the time for recovery from myoneural blockade will be longer in the presence of isoflurane than in the presence of other commonly used anesthetics. Not for use in horses intended for food.

PRECAUTIONS: Usage in Pregnancy: Reproduction studies have been performed in mice and rats without evidence of fetal malformation attributable to ISOTHESIA (isoflurane, U.S.P.). Adequate data concerning the safe use of isoflurane in pregnant and breeding horses and dogs have not been obtained.

CAUTION(S): Federal law restricts this drug to use by or on the order of a licensed veterinarian.

Operating rooms should be provided with adequate ventilation to prevent the accumulation of anesthetic vapors.

For veterinary use in Horses and Dogs.

Adverse Reactions

Hypotension, respiratory depression and arrhythmias have been reported.

OVERDOSE: In the event of overdosage, or what may appear to be overdosage, the following action should be taken:

Stop drug administration, establish that the airway is clear and initiate assisted or controlled ventilation with pure oxygen as circumstances dictate.

Isothesia Dosage And Administration:

Premedication: A premedication regimen, which may be employed depending upon the patient status, to avert excitement during induction, might include an anticholinergic, a tranquilizer, a muscle relaxant, and a short-acting barbiturate.

Inspired Concentration: The delivered concentration of Isoflurane should be known. Isoflurane may be vaporized using a flow-through vaporizer specifically calibrated for isoflurane. Vaporizers delivering a saturated vapor which then is diluted (e.g., Vernitrol® vaporizer) also may be used. The delivered concentration from such a vaporizer may be calculated using the formula:

% isoflurane =




where: PA = Pressure of atmosphere, PV = Vapor pressure of isoflurane, FV = Flow of gas through vaporizer (mL/min.), FT = Total gas flow used (mL/min.)

Isoflurane contains no stabilizer. Nothing in the drug product alters calibration or the operation of these vaporizers.


Horses: Inspired concentrations of 3.0 to 5.0% isoflurane alone with oxygen following a barbiturate anesthetic induction are usually employed to induce surgical anesthesia in the horse.

Dogs: Inspired concentrations of 2.0 to 2.5% isoflurane alone with oxygen following a barbiturate anesthetic induction are usually employed to induce surgical anesthesia in the dog.

These concentrations can be expected to produce surgical anesthesia in 5-10 minutes.

Maintenance: The concentration of vapor necessary to maintain anesthesia is much less than that required to induce it.

Horses: Surgical levels of anesthesia in both horses and dogs may be sustained with a 1.5 to 1.8% concentration of isoflurane in oxygen.

Dogs: Surgical levels of anesthesia in both horses and dogs may be sustained with a 1.5 to 1.8% concentration of isoflurane in oxygen.

The level of blood pressure during maintenance is an inverse function of isoflurane concentration in the absence of other complicating problems. Excessive decreases, unless related to hypovolemia, may be due to depth of anesthesia and in such instances may be corrected by lightening the level of anesthesia.

Recovery from isoflurane anesthesia is typically uneventful.2

How Supplied

ISOTHESIA (isoflurane, U.S.P.) is packaged in 100 mL amber-colored bottles.


Store at controlled room temperature 15-30°C (59-85°F).


1. Steffey, E.P., Howland, D. Jr., Giri, S. and Eger, E.I. II.: Enflurane, Halothane and Isoflurane Potency in Horses. Am. J. Vet. Res. 38(7): 1037-1039, 1977.

2. Auer, J.A., Garner, H.E., Amend, J.F., Hutcheson, D.P. and Salem, C.A.: Recovery from Anesthesia in Ponies: A Comparative Study of the Effects of Isoflurane, Enflurane, Methoxyflurane and Halothane. Equine Vet. J. 10(1): 18-23, 1978.

3. Steffey, E.P., and Howland, D. Jr.: Comparison of Circulatory and Respiratory Effects of Isoflurane and Halothane Anesthesia in Horses. Am. J. Vet. Res. 41(5): 821-825, 1980.

4. Auer, J.A., Amend, J.F., Garner, H.E., Hutcheson, D.P. and Salem, C.A.: Electroencephalographic Responses During Volatile Anesthesia in Domestic Ponies: A Comparative Study of Isoflurane, Enflurane, Methoxyflurane, and Halothane. Equine Practice 3: 130-134, 1979.

5. Klide, A.M.: Cardiopulmonary Effects of Enflurane and Isoflurane in the Dog. Am. J. Vet. Res., Vol. 37, No. 2: 127-131, 1976.

6. Steffey, E.P., Howland, D.: Isoflurane Potency in the Dog and Cat. Am. J. Vet. Res., Vol. 38, No. 11: 1833-1836, 1977.

ANADA 200-141, Approved by FDA


NAC No.: 10822741

Telephone:   614-761-9095
Toll-Free:   1-855-724-3461
Every effort has been made to ensure the accuracy of the Isothesia information published above. However, it remains the responsibility of the readers to familiarize themselves with the product information contained on the US product label or package insert.

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