Anesthesia and Sedation of Miniature Horses

CHAPTER 8 Anesthesia and Sedation of Miniature Horses



Nora S. Matthews, Carin Ponder


American Miniature horses measure no taller than 38 inches (95 cm) at the withers. According to the breed associations (American Miniature Horse Association, Alvarado, Texas, www.amha.org; and American Miniature Horse Registry, Morton, Illinois, www.shetlandminiature.com), breed standards require that miniature horses possess the proper conformation characteristics that are common to most of the larger breeds (Figure 8-1). The breed originated from English and Dutch mine horses brought to the United States and used in Appalachian coal mines as late as 1950. The Shetland Pony has also influenced the breed.


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Figure 8-1 Example of good conformation in a miniature horse.


Veterinarians should be aware of some of the genetic defects in miniature horses, some of which may have an impact on anesthesia. Collapsing trachea, apparent hypoplastic trachea, narcolepsy, myopathy, sinus osteoma, patellar luxation, angular limb deformities, complete ulnas and fibulas, occipitoatlantoaxial malformation, cardiac anomalies, and ataxia have all been reported in miniature horses. Many of these abnormalities seem to be associated with dwarf-like (achondroplastic) conformation (Figure 8-2). Thorough preoperative examination is important for establishing an awareness of preexisting conditions.


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Figure 8-2 Example of dwarf features in a miniature horse.



PREOPERATIVE CONSIDERATIONS


Adult miniature horses may be the size of a large dog (ranging in weight from 45 to 136 kg [20.5 to 62 lbs]), whereas foals may be very small (7 to 15 kg [3.2 to6.8 lbs]); therefore, use of appropriate-size equipment is vital (Figure 8-3). It can be a considerable adjustment for equine practitioners to adapt to working with such small horses because the equipment needed, including endotracheal tubes, breathing hoses, catheters, and syringes and needles, are the same as those used in small animals. Appropriate endotracheal tubes range from 5 to 18 mm in internal diameter, and 16- to 22-gauge catheters (3 to 5 mm long), 20-gauge needles, and small syringes are typically used in miniature horses. In some instances, working with a small-animal practitioner who can provide suitable equipment may be in the best interest of the patient.


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Figure 8-3 Miniature horse foal anesthetized with small animal equipment.


Preoperative evaluation should be thorough and must include weighing the horse, performing a complete physical examination, and obtaining laboratory tests as indicated. It may be difficult for equine practitioners to estimate the weight of miniature horses accurately because they are so small. Use of a weight tape may help to improve accuracy, but in one study in which weight obtained by use of a tape was compared with that obtained with a scale, the tape overestimated body weight by as much as 16%. For this reason, a scale should be used to obtain body weight whenever possible.


There are some important differences between miniature and full-sized horses with regard to hematologic and serum biochemical variables. Potassium and phosphorus concentrations are higher, whereas magnesium and thyroid hormone concentrations tend to be lower than those in full-sized horses. Erythrocyte counts are lower, total leukocyte counts tend to be higher, and the neutrophil-to-lymphocyte ratio is nearly the reverse of that in full-sized horses. Miniature horses also appear to be susceptible to hyperlipidemia, which should be treated before anesthesia and surgery.



Restraint


Although small, miniature horses may be temperamental and difficult to restrain for examination and venipuncture. A typical horse twitch is usually too big for the miniature horse’s muzzle and will not stay in position. Alternatively, a vise grip from a hardware store can be used, or the horse may respond better to a so-called shoulder twitch (grasping the skin overlying the shoulder). It should be remembered that humans are larger than a miniature horse. Some miniature horses may be more aggressive toward larger people, especially if the body language of the person is dominant. Approaching miniature horses slowly and with submissive body language may be helpful in preventing aggressive actions such as kicking.



Sedation

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May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Anesthesia and Sedation of Miniature Horses

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