How to Use a Ketamine Constant Rate Infusion in Horses for Analgesia

1. Introduction

Ketamine has been used to produce short-term anesthesia in horses since the mid-1970s [1]. It has also been used as an adjunct to inhalant anesthesia [2], to produce epidural analgesia [3], and more recently, as a peripheral nerve block [4]. However, there is very little information about its use to produce analgesia post-operatively or in intensive care unit (ICU) patients. Ketamine has analgesic actions at central and peripheral sites mediated by multiple receptors including N-methyl-D-aspartate (NMDA) receptors in the spinal cord; it has been used to treat human patients with chronic pain [5,6]. The use of ketamine may allow a reduction in other drugs that depress gastrointestinal motility (such as the opioids) or cause gastrointestinal (GI) ulcers (such as the non-steroidal anti-inflammatory drugs [NSAIDs]). Anecdotal reports of repeated IM administration of ketamine exist; however, ketamine is very painful when given IM, so it is not a practical route. A recent study [a] determined what doses of ketamine could safely be infused into horses for a prolonged period of time. Subsequent to this study, we have used ketamine constant rate infusions (CRIs) in several horses with varying painful conditions that did not respond well to other analgesics. Although difficult to evaluate, apparent analgesia provided by ketamine CRIs varied from mild (when used for septic joints and osteomyelitis) to dramatic (when used in burn patients). Therefore, it may be a useful technique for horses with difficult to manage pain.

2. Materials and Methods

The dose of ketamine that has been safely evaluated by the authors (based on evaluation in six research horses) is 0.4 - 0.8 mg/kg/h. One way to deliver ketamine is to add 3000 mg (30 ml) to a 1-liter bag of saline to create a concentration of 3 mg/ml. The fluid rate can then be adjusted according to the desired dose of ketamine, but this concentration will provide 8 h of ketamine CRI at 0.8 mg/kg/h when given at 133 ml/h. A second method is to add ketamine to the maintenance fluids for a horse as described below. Regardless of the method, the authors strongly recommend the use of a fluid administration pump, because an accidental bolus of fluids containing ketamine could have serious side effects.

A simple way to calculate the mixture is as follows:

1. Calculate the dose of ketamine you want to use per hour. Example: 500 kg horse × 0.4 mg/kg = 200 mg ketamine/h.
2. Decide what fluid administration rate is desired for maintenance. For example, you may wish to run at 1 ml/kg/h or 500 ml/h. In this case, a 5-liter bag of fluids will last for 10 h.
3. Multiply the calculated ketamine dose per hour by the number of hours in the fluid bag. Example: 200 mg × 10 h = 2000 mg or 20 ml ketamine.

This method can easily be adjusted for the higher ketamine dose (example: 0.8 mg × 500 kg = 400 mg × 10 h = 40 ml ketamine added to a 5-liter bag) or for a higher hourly fluid administration rate (example: 0.4 mg/kg × 500 kg = 200 mg ketamine; Lactated Ringer’s Solution (LRS) at 1 liter/h means that a 5-liter bag will last for 5 h; 200 mg × 5 h = 10 ml ketamine added to 1 bag.)

3. Results and Discussion

Although we have not used ketamine infusions in a large number of clinical cases (eight), results have been encouraging enough to recommend that it be tried in practice. Response to ketamine infusion has varied; horses with osteomyelitis or a septic joint showed only a slight increase in weight bearing, but consumption of food and water increased dramatically. Other horses, including two burn patients, appeared to be markedly more comfortable (as judged by a decrease in self-mutilation and heart rate) within 6 - 12 h. Ketamine infusions were used on three horses with colic, but the results were equivocal.

No adverse side effects were seen in any horses maintained on ketamine CRIs for ≤5 days at these dosages; however, all horses maintained on CRIs of ketamine or other drugs need to be monitored frequently. Additional research is needed to determine if CRIs of longer duration may lead to unwanted side effects. Adverse side effects seen with higher infusion rates (≤1.6 mg/kg/h) included ataxia and sensitivity to sound.

4. Conclusion

Ketamine infusions are easy to set up, are inexpensive, and may improve analgesia in painful horses when other analgesic drugs seem to be ineffective.

Footnote

1. Fielding CL, Brumbaugh GW, Matthews NS, et al. Pharmacokinetics and clinical effects of a sub-anesthetic continuous rate infusion of ketamine to awake horses. Unpublished data, 2003.