Honest Bill: Part 2
by Dan Schar
William Burroughs never spoke positively of barbiturate abuse, his primary experience involving pentobarbital (Nembutal), yet he travelled its route in search of an opiod cure. Barbiturates are dose dependent central nervous system (brain and spinal cord) depressants. At low doses, relaxation and sedation predominate, at moderate doses, inability to control movement and speech ensue, and still higher doses evoke a state of anesthesia. Burroughs frowned upon the barbiturate addict's staggering, foolish, uncoordinated "spectacle." With regard to addiction and barbiturate abuse, he noted, "The next step down is coal gas and milk, or sniffing ammonia in a bucket."
The barbiturates commonly used in a veterinary context include pentobarbital, phenobarbital and thiopental, each producing similar effects, but differing in duration of action. Thiopental, an ultra-short acting barbiturate, has historically found use in its low dose, hypnotic capacity as truth serum. It is most frequently utilized as an induction anesthetic, capable of rapidly bringing about a transition from consciousness to general anesthesia.
The mechanics of bringing an average size horse, roughly the weight of a compact car, rapidly through deepening planes of anesthesia is not a matter to be taken lightly. Excitation defines the lighter anesthetic planes, and it is important that the anesthetist plows through these early planes at a good clip, safely bringing the patient to a level of unconsciousness while minimizing flailing limbs and other unpredictable events. Thio accomplishes this task consistently, so it is frequently employed in large animal induction. Other agents, including preanesthetic sedatives and muscle relaxants, are used to smooth the transition, but thio drops the animal.
Typically, the induction of a horse into general anesthesia is carried out in a padded stall -- two inches of dense foam line the walls and floor. Numerous induction protocols exist, and it is the anesthetist's role to choose a combination of agents that suit the physical status of the patient, the surgical or medical procedure to be performed, the duration of the procedure and myriad other factors. A common induction protocol favored for elective procedures -- that is, procedures scheduled and not those performed on an emergency basis -- utilizes xylazine, guaifenesin and thiopental. Xylazine, an alpha-2 adrenergic agonist, serves as a preanesthetic sedative. Guaifenesin, employed as a muscle relaxant, finds use in human medicine as an expectorant, facilitating the expulsion of mucous secretions from air passages. It can be found in a host of over the counter cold and flu products. Intravenous administration of these agents is facilitated by preplacing a catheter in the jugular vein.
The anesthetist's goal is a smooth, uneventful induction, which unfolds in the following fashion: xylazine is injected as a bolus, or rapid push, via the catheter. Horses characteristically breathe a heavy sigh and drop their heads, as though they were unbearable weights, once the sedative takes effect. They look, well, sedate. The horse is moved against a wall, and people are called in, placing themselves along the opposite side of the horse to ease its transition to the ground. Whoever is running the drugs calls out "guaifenesin in" and a bag of guaifenesin mixed with saline is run through an extension line into the catheter under high pressure. Guaifenesin is generally run RITE -- rapid infusion to effect -- such that the infusion continues until the horse responds. Most horses sway, shifting their weight on weak legs, and look something like an unsteady drunk; others respond subtlely, relaxing their muscles and taking on a glazed look recognized by an experienced anesthetist. This is a crucial moment, because guaifenesin can drop a horse, but not take it through the transition to general anesthesia -- the barbiturates are employed to that end.
Here, "thio" is called out, and the drug is pushed as a rapid bolus. Ideally, the sway leads to a buckling at the knees, and the horse slowly crumples, guided smoothly to the floor by the assembled crew. A smooth induction is a beautiful sight, but a host of crises can, and have, plagued the procedure. I have seen horses fall unpredictably, pinning people against walls. I have seen thio bolused too early to a massive draft horse, causing it to lurch forward, scattering people as it crashed to the floor. Most tragically, I have seen horses with great athletic potential present for a relatively stable fractured leg repair, only to thrash or misstep during induction, shattering the already fractured bone and ending the horse's career or life.
Pentobarbital and phenobarbital are commonly used to treat seizures in both small and large animals. Phenobarbital's longer duration of action has enabled its use preventing recurrent seizures in epileptic patients. The efficacy of phenobarbital in controlling seizure activity recently became apparent to me in the treatment of an epileptic filly that suffered from an inherited juvenile disease of Arabians. The filly's seizure episodes were preceded by several walking steps on her hind legs, resembling the dance of a circus bear. Immediately following this behavior, the filly would collapse into a state of convulsions, limb paddling and abnormal eye movement usually lasting less than a minute. The mechanics of phenobarbital's distribution in the body require that it initially be given in a loading dose form -- a large dose followed by a tapered dose -- to achieve consistent levels in the blood. Although I had initially controlled the filly's seizures with diazepam (Valium(c)), they continued, becoming seemingly more severe and of longer duration. Once the phenobarbital loading dose was started, seizure activity ceased, and the filly has been seizure free, though, as Burroughs noted, somewhat stuporous, on the drug for several months.
Most of the current production of pentobarbital finds its way into euthanasia solution. At high doses, it is an effective depressant of regions of the brain that control respiration. The trade names of common euthanasia agents hint at the sensitivity with which pharmaceutical companies have marketed these products: Sleepaway(c) (Fort Dodge), Beuthanasia-D Special(c) (Schering), Repose(c) (Syntex), and Fatal Plus-3(c) (Vortech). The latter begs the question: can you ask more than death from a euthanasia product?
About the author:
Dan Schar practices veterinary medicine and is currently an intern in large animal medicine and surgery at the University of Minnesota, Veterinary Teaching Hospital in Saint Paul, MN.