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Guaifenesin is a centrally acting muscle relaxant which is used as a part of many anesthetic protocols in the horse. Its mechanism of action is not precisely known, but the relaxation of skeletal muscles, mild analgesic, and mild sedative properties allows for lower doses of other sedatives and anesthetic agents. Guaifenesin is used for induction before inhalation anesthesia, during inhalation anesthesia, and with total intravenous anesthesia (TIVA) for short procedures. Guaifenesin has minimal effects on diaphragmatic function and produces relatively little respiratory depression at normal doses. It is primarily used in horses although it has been studied and is used in other domestic species.
Guaifenesin was also called GG, glyceryl guaiacolate,
and a number of other similar chemical names. Guaifenesin is compatible
with D-5-W and sterile water. It is usually reconstituted before each
use. Warming and agitation will help resolubilize the drug, particularly
in cold temperatures.
Guaifenesin is used in veterinary hospitals as a part of the induction protocol for inhalation anesthesia. It is commonly used after sedation with an alpha agonist drug, followed by ketamine. Other drugs including benzodiazepines, butorphanol, thiopental, and other barbiturate drugs may be added at the anesthesiologist's discretion. Induction and recovery are generally smooth.
Guaifenesin is also used in the field or in veterinary hospitals for TIVA for procedures lasting less than an hour. Anesthesia is induced using an alpha agonist such as xylazine, detomidine or romifidine followed by ketamine. Guaifenesin (5%) solution is combined with additional alpha agonist and ketamine to create what is commonly called "Triple Drip." This combination is used to prolong anesthesia up to an hour. The degree of muscle relaxation and quality of recovery is generally good with the Triple Drip protocols. Further information regarding drug concentrations and dose rates may be found in veterinary anesthesia textbooks.
Guaifenesin within a Triple Drip combination is also
administered with inhalant anesthesia as a means of reducing the total
amount of inhalant used in a given case. This combination of intravenous
and inhalant anesthesia is referred to as "balanced anesthesia."
Balanced anesthesia has the advantages of less cardiovascular depression,
diminished need for additional drugs to support hemodynamics, and improved
recoveries.
• Guaifenesin may cause a mild decrease in blood pressure, and an increase in heart rate.
• There are conflicting reports regarding hemolysis,
and reactions of the vascular endothelium with higher concentration solutions
of guaifenesin.
• ARCI class 4 drug.
• Guaifenesin has been shown to cross the placenta. Neonatal levels
immediately after delivery were about 30% of the level in the mare. TIVA
with Triple Drip has been used successfully in field situations for dystocia.
Appropriate ventilation and respiratory support for the foal may be necessary.
• Donkeys and to a lesser extent mules may be more sensitive to guaifenesin.
Mules are reported to be intermediate between horses and donkeys.
• Perivascular administration will cause tissue reaction. Guaifenesin
should be administered using an intravenous catheter.
• Anticholinesterase drugs such as physostigmine are contraindicated.
(Plumb 2005)
• Guaifenesin is relatively safe. The margin of safety is reported to be 3 times the normal dose and cardiovascular side effects are rare. Signs of overdose include apneustic breathing, nystagmus, hypotension, increased muscle rigidity. Although there is no specific antidote, Guaifenesin has a relatively short half life (60 to 85 minutes). (Plumb 2005) Supportive treatment should be instituted while the drug is being cleared.
Dr.
Barbara Forney is a veterinary practitioner in Chester County, Pennsylvania.
She has a master's degree in animal science from the University of Delaware
and graduated from the University of Pennsylvania School of Veterinary Medicine
in 1982.
She began to develop her interest in client education and medical writing 1997. Recent publications include portions of The Pill Book Guide to Medication for Your Dog and Cat, and most recently Understanding Equine Medications published by the Bloodhorse.
Dr. Forney is an FEI veterinarian and an active member of the AAEP, AVMA, and AMWA.
You can purchase books by Dr. Forney at www.exclusivelyequine.com
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