How to Attain Effective and Consistent Sedation for Standing Procedures in the Horse Using Constant Rate Infusion

1. Introduction

For several decades, alpha-2 agonists have been used to provide effective sedation and analgesia to horses undergoing standing procedures. In the past several years, more potent alpha-2 agonists such as detomidine, medetomidine, and romifidine have been developed and are widely used in equine practice [1]. Currently, only detomidine is approved for horses in the United States.

Alpha-2 agonists bind to and stimulate alpha-2 adrenergic receptors in the central and peripheral nervous systems [1,2]. The binding of these drugs results in sedation, analgesia, and anesthetic-sparing effects [1,3]. Investigations have determined that detomidine has greater potency than medetomidine and romifidine, is longer lasting than the alpha-2 agonist xylazine [4-6], and results in profound standing sedation and analgesia when combined with butorphanol [7]. Furthermore, constant rate infusion with butorphanol has been found to provide excellent analgesia without inducing ataxia, decreased borborygmi, and defecation [8].

The use of detomidine and detomidine/butorphanol combinations to facilitate standing surgery in equine practice is common. Often, procedures are short, and a single bolus of drug combination is adequate to complete procedures such as standing castration, wound suturing, or biopsy. Unfortunately, significant side effects, such as cardiopulmonary depression, alar and laryngeal relaxation with subsequent stridor, respiratory depression, and decreased intestinal motility, are associated with alpha-2 agonist administration [1,9]. These side effects become problematic when additional boluses are required for long-standing procedures such as laparoscopy, laparotomy, upper airway procedures, and large wound repairs. The level of ataxia increases with each additional injection, and large fluctuations of sedation make longer procedures difficult to complete.

Providing sedation by constant rate infusion is one method of overcoming the negative effects of alpha-2 agonist administration during standing surgery [1,10,11]. This presentation describes a method of drug administration that initiates and maintains continuous and steady sedation; this facilitates the execution of longer, and perhaps more painful, standing surgical procedures requiring more than one drug bolus. This method may also decrease unwanted side effects.

2. Materials and Methods

The materials needed include detomidine [a], torbugesic [b], a 14-gauge IV catheter, IV tubing, and a constant rate infusion (CRI) syringe pump [c] (Fig. 1).

Figure 1. A CRI syringe pump with syringe attached. Desired doses can be pre-programmed into the pump.

The horse should be placed in stocks, if used, or led to the area where the procedure will take place. An IV catheter is placed into a jugular vein and sutured to the skin (Fig. 2). The detomidine is diluted to 1 mg/ml. The syringe pump is pre-programmed to administer the following dosages. A loading dose of detomidine is administered as an 8.4 μg/kg IV bolus, followed by 0.5 μg/kg/min for 15 min, 0.3 μg/kg/min for 15 min, and 0.15 μg/kg/min thereafter until a few minutes before the procedure is complete. If an infusion pump cannot be obtained, a detomidine CRI can be delivered using a 60-drop/ml microdrip administration set and a 500-ml bag of 0.9% sodium chloride injection using this method. Remove 5 ml of sodium chloride from the 500-ml bag and add 5 ml of 10 mg/ml detomidine. The resulting solution will be 100 μg/ml of detomidine. After administering the bolus injection of 8.4 μg/kg IV, use the drip set to administer the following rates: 0.005 drops/kg/s for 15 min, 0.003 drops/kg/s for 15 min, and 0.0015 drops/kg/s thereafter. The drip rate can be adjusted to the desired level of sedation as the procedure progresses.

Figure 2. The IV catheter is placed and securely sutured to the skin.

If further sedation and analgesia is desired, another CRI syringe pump can be used to deliver butorphanol. This infusion line can be piggybacked into the main infusion line supplying the detomidine. The loading dose of butorphanol should be administered at 17.8 μg/kg, and thereafter, a 0.38 μg/kg/min constant rate of infusion should be used.

Most horses can return to the stall without significant levels of ataxia ~5 - 10 min after the drugs are discontinued. Food should be withheld for at least 4 h after operation. If stocks are used, the halter should have foam padding where the side ropes attach (Fig. 3), and ideally, the horse’s head should be supported (Fig. 4). If stocks are not available, an object that readily supports the horse’s head should be used.

Figure 3. The head should be adequately padded to avoid iatrogenic facial nerve paralysis.

Figure 4. The head should also be supported under the mandible with heavy foam padding.

3. Results

At Cornell, this procedure has been used for various standing procedures including laser cordectomies, laparotomies, laparoscopies, sinusotomies, large wound repairs, rectovaginal reconstruction, short arthroscopic procedures, plate removal, and dental procedures. The authors estimate that CRI of alpha-2 agonists, with or without butorphanol to supply sedation and analgesia, facilitates ~20 - 30 standing surgical procedures. This technique provides outstanding sedation characteristics and eliminates the added risk and expense of general anesthesia, which also allows for the performance of more procedures. Complications encountered include temporary facial nerve paralysis from inadequate padding of the halter, frequent urination, and cardiovascular depression.

4. Discussion

This method of providing sedation and analgesia to horses undergoing standing surgical procedures has proven invaluable in our hospital. The syringe pump should be used to achieve an optimum level of continuous sedation. Alternatively, diluting the detomidine in a bag of saline and infusing by gravity while adjusting the drip rate can be used. However, this method has a number of inconvenient features, including a rate variance that depends on head droop, an inaccurate drip set, and an inconsistent delivery of the drug. Furthermore, a second person is required to monitor and adjust the drip rate. The syringe infusion pump can be purchased from many medical equipment supply companies at costs ranging from $200 to $6000.

It is important to support and adequately pad the head of the horse at the beginning of the procedure. The sedation level is usually so consistent that the surgeon can focus on the operative procedure with little concern for inadequate head support resulting in neuropraxia of the facial nerves. When adequately supported, the horse can maintain this head position for the entire procedure.

Detomidine is an alpha-2 agonist that causes frequent urination when administered intravenously. We believe, however, that it is easier for the horse to urinate using CRI, because large fluctuations in ataxia are not occurring. If available, an echocardiogram (ECG) should be used to monitor for cardiac arrhythmias as a result of cardiopulmonary depression.

Overall, delivering sedation and analgesia to horses undergoing standing procedures by CRI results in constant and consistent levels of sedation and anti-nociception, alleviates associated stress for the surgical staff, and causes minimal stress for the horse without the associated large fluctuations in sedation that occur with other methods.

Footnotes

1. Detomidine, Pfizer Animal Health, West Chester, PA 19380.
2. Torbugesic, Fort Dodge Animal Health, Fort Dodge, IA 50501.
3. Protege, MedEX, Carlsbad, CA 92008.