Isoflurane: Product Information

ISOFLURANE — isoflurane liquid
ASPEN VETERINARY RESOURCES LTD

SPL UNCLASSIFIED SECTION

ISOFLURANE, USP
INHALATION ANESTHETIC

Nonflammable, nonexplosive.

For Veterinary Use in Horses and Dogs.

WARNING

Not for use in horses intended for food.

Caution

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.

DESCRIPTION

Isoflurane, USP is a nonflammable, nonexplosive general inhalation anesthetic agent. Its chemical name is 1-chloro-2,2,2-trifluoroethyl difluoromethyl ether, and its structural formula is:

isoflurane structure

Each mL contains 99.9% isoflurane.

Some physical constants are:

Molecular weight184.5
Boiling point at 760 mm Hg48.5° C (uncorr.)
Refractive index n 20 D1.2990 to 1.3005D
Specific gravity 25°/25° C1.496
Vapor pressure in mm Hg**20° C238
25° C295
30° C367
35° C450

**Equation for vapor pressure calculation:

log10Pvap= A + B/T where: A = 8.056

B = -1664.58

T = °C + 273.16 (Kelvin)

Partition coefficients at 37° C:
Water/gas0.61
Blood/gas1.43
Oil/gas90.8
Partition coefficients at 25° C — rubber and plastic
Conductive rubber/gas62.0
Butyl rubber/gas75.0
Polyvinyl chloride/gas110.0
Polyethylene/gas~2.0
Polyurethane/gas~1.4
Polyolefin/gas~1.1
Butyl acetate/gas~2.5
Purity by gas chromatography>99.9%
Lower limit of flammability in oxygen or nitrous oxide at 9 joules/sec. and 23° CNone
Lower limit of flammability in oxygen or nitrous oxide at 900 joules/sec. and 23° CGreater 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 containing no 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 (at normal operating temperatures), and does not attack aluminum, tin, brass, iron or copper.

CLINICAL PHARMACOLOGY

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 induction of anesthesia but returns toward normal with surgical stimulation. Progressive increases in 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 increasing depth of anesthesia. The hypercapnia which attends spontaneous ventilation during isoflurane anesthesia increases 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 intraabdominal 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.

INDICATIONS & USAGE

Isoflurane, USP is used for induction and maintenance of general anesthesia in horses and dogs.

CONTRAINDICATIONS

Isoflurane, USP is contraindicated in horses and dogs with known sensitivity to isoflurane or to other halogenated agents.

WARNINGS

Increasing depth of anesthesia with isoflurane may increase hypotension and respiratory depression. The electroencephalographic 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.

Keep out of reach of children.

PRECAUTIONS

Isoflurane, USP, like other inhalational anesthetics, can react with desiccated carbon dioxide (CO2 ) absorbents to produce carbon monoxide which may result in elevated carboxyhemoglobin levels in some patients. Case reports suggest that barium hydroxide lime and soda lime become desiccated when fresh gases are passed through the CO2 absorber canister at high flow rates over many hours or days. When a clinician suspects that CO2 absorbent may be desiccated, it should be replaced before the administration of isoflurane, USP.

PREGNANCY

Usage in pregnancy : Reproduction studies have been performed in mice and rats with no evidence of fetal malformation attributable to isoflurane. Adequate data concerning the safe use of isoflurane in pregnant and breeding horses and dogs have not been obtained.

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