Eric J. Abrahamsen

Ketamine Stun

The Ketamine Stun is basically the addition of a small dose of ketamine to any injectable chemical restraint technique. I initially developed the Ketamine Stun technique in the early 1990s to cover my limited handling abilities with regard to cats. My first exposure to camelid patients occurred when I left equine practice to teach at Ohio State. Their frequent recalcitrant behavior quickly led to experimentation with low-dose ketamine protocols to improve the level of patient cooperation during diagnostic and therapeutic procedures. Success was immediately evident, and the technique became wildly popular with the food animal clinicians, residents, and students charged with the care of these patients.¹ Because of the success in camelid patients, the Ketamine Stun technique was adjusted for use in ruminants (lower dose of xylazine) and proved to be just as useful.² Equine applications have proven more challenging. Dramatic improvement in cooperation evident a minute or two after an IV bolus of ketamine is administered to patients that were totally uncooperative under the prior detomidine–morphine sedation suggests the potential of the Ketamine Stun technique in horses.^3,4 Unfortunately, the effective chemical restraint levels of ketamine are not far removed from those that produce instability in horses. The Ketamine Stun technique has been shown to reduce stress response to castration and dehorning in calves.^5,6

The term Ketamine Stun (or “Ket Stun”) was derived from the stunned effect the technique produced in patients when administered at doses that produce recumbency. These patients appear to be awake but seem oblivious to their surroundings and the procedure being performed. The IV effect is quite brief (approximately15 minutes), and patients typically stand and appear fairly normal at that time. I initially referred to this state as semianesthetized, but perhaps chemical hypnosis is a more appropriate description.

α₂-adrenergic agonists possess potent sedative and analgesic effects. Opioids are typically thought of as analgesic drugs, but they possess central nervous system (CNS) effects, which, when combined with a tranquilizer or sedative, produce a greater level of mental depression. Ketamine is an N-methyl-D-aspartate (NMDA) receptor antagonist that possesses potent analgesic effects at subanesthetic doses.⁷ Ketamine was initially included in the stun technique for its analgesic properties but likely contributes to the mental aspects of the enhanced cooperation exhibited by patients under the influence of the Ketamine Stun technique. By combining drugs, smaller doses of the individual components can be used while still achieving the desired level of patient control. Dosing must be more conservative when using the ketamine stun technique in standing patients. This limits the degree of systemic analgesia relative to what can be achieved in recumbent patients but still provides improved patient cooperation when compared with more traditional methods of standing chemical restraint in both ruminants and horses.

In ruminants and camelid patients, I use a combination of xylazine, butorphanol, and ketamine. In equine patients, I use detomidine, morphine, and ketamine. Morphine is used to provide analgesic relief in food animal patients and is much less expensive than butorphanol. I have used morphine (0.05–0.06 mg/kg) in ruminant stuns. In standing adult cattle stuns, a similar level of cooperation is achieved with either opioid, but patients appear less obtunded when morphine is used. Some practitioners may find the obtunded appearance useful because it allows them to monitor decay over time in the level of chemical restraint. Deterioration in the level of patient cooperation also may be used to determine when supplemental drug administration would be required. The systemic analgesia provided by the Ketamine Stun technique is not limited to purely chemical restraint applications. Small doses of ketamine (0.22 mg/kg, IV) layered over a mild level of xylazine sedation provide dramatic short-term (15 minutes) relief from moderate colic pain in horses.⁸

Ketamine Stun techniques can be divided into two broad categories: (1) standing and (2) recumbent techniques. The standing Ketamine Stun is used primarily in large ruminants and horses. The recumbent Ketamine Stun is used primarily in small ruminants, camelids, and foals. The level of effect achieved is determined by three variables: (1) dose, (2) route of administration, and (3) initial demeanor of the patient. The stun cocktail may be administered by the IV, IM, or subcutaneous (SQ) route, depending on the systemic analgesia, patient cooperation, and duration desired (Box 45-1).

Box 45-1   Route of Administration Determines the Relative Impact of the Ketamine Stun Technique

Parameter	Relative Ranking
Intensity (analgesia/cooperation)	IV IM > SQ
Onset	IV IM > SQ
Duration of effect	SQ > IM IV

IM, Intramuscular; IV, intravenous; SQ, subcutaneous.

Aggressive dosing increases not only the level of systemic analgesia and patient cooperation but also the risk of instability, unintended recumbency, or the duration of intended recumbency. In patients that must remain standing, balance between the α₂-adrenergic agonist and ketamine is crucial. Greater levels of sedation require lower peak blood levels of ketamine to avoid producing a transient period of instability in equine patients and recumbency in ruminant and camelid patients. Obtaining maximum benefit from the Ketamine Stun technique requires pushing up against the limits of this balancing act. The rapid rise in blood levels produced by IV administration of ketamine presents the greatest challenge. Titrated administration of IV ketamine may be used to minimize risk of untoward responses.

An endless number of permutations are possible when using the Ketamine Stun technique. Many years of experimentation in many different species have provided a great deal of insight into the potential of this technique but have not produced a definitive combination for all situations in each species. The dosage protocols for the Ketamine Stun techniques are deliberately conservative. Results obtained over several years in camelid patients with “mid-range” behavior were used to develop the recommendations to ensure that the most sensitive individuals are not overly affected. IV administration has been used more extensively, so IM dosage recommendations are not as highly refined. I generally use the upper end of the recommended dosage ranges in most normal, healthy patients. Extremely anxious or unruly individuals may require somewhat more aggressive dosing or supplemental administration to achieve the desired effect. I find the lower end of the recommended dosage ranges useful for quick or less invasive procedures. The xylazine component of the Ketamine Stun technique is smaller than the xylazine doses used to produce recumbent sedation in camelid patients, but it is large enough to pose a risk in severely compromised patients; the dosage should therefore be reduced accordingly when using the IV Ketamine Stun technique in these patients. The following examples are provided as a guide, and practitioners are encouraged to experiment with adjustments in doses.

Intravenous Ketamine Stun

The camelid IV Ketamine Stun technique is designed to provide a potent but brief level of chemical restraint. It was designed for short procedures requiring a high level of patient cooperation, systemic analgesia, or both. The enhanced degree of patient cooperation provided by the IV Ketamine Stun technique improves the quality of patient care that can be provided, which has made it very popular with practitioners who have tried it. The thick skin and fiber of the camelid neck may make jugular venipuncture somewhat challenging. The veins on the external surface of ears of some camelids are relatively large and accessible, providing a useful alternative site for IV administration. Most camelid patients require only modest head restraint when a 25-gauge needle and good technique are used. Percutaneous injections via the ear vein should be limited to small volumes.

A combination of xylazine, ketamine, and butorphanol is administered intravenously (Table 45-1). A graceful transition to recumbency occurs approximately 1 minute after IV administration of the Ketamine Stun combination. Patients continue to appear surprisingly “alert” but are typically oblivious to their surroundings and the procedures being performed. Systemic analgesia peaks 1 to 2 minutes after the IV Ketamine Stun administration and diminishes over time. The initial level of systemic analgesia is typically fairly profound. Patients typically are ready to stand and walk with minimal residual effect evident approximately 15 minutes after IV administration of the Ketamine Stun. The clinician should plan ahead and work fast when using the IV Ketamine Stun.