Regional antimicrobial use in horses (Proceedings)

Equine practitioners frequently deal with septic wounds, arthritis, osteomyelitis and tenosynovitis. The primary mechanisms of treatment should always include physical debridement and lavage. Most treatment regiment include systemic antimicrobials and antiinflammatories. A limiting factor in some cases is the ability to obtain effective concentrations of antimicrobials to the sites of infection or contamination.

Antimicrobial use in the horse is not without risks. Renal toxicity and colitis are a couple of the important complications associated with their use. However, less outwardly apparent issues including development of resistant strains of bacteria ultimately challenge the veterinarian to a greater degree.

Regional limb perfusion provides high antimicrobial concentrations in the region of interest and greatly contributes to the elimination of the infection. Antimicrobials and other drugs can be administered via the vasculature in a selected anatomical location that is isolated from systemic circulation by tourniquet(s). With the tourniquet in place a perfusion gradient allows the drug to be moved into extravascular compartments. These are the regions where the antimicrobials need to be to be effective in treating the infection.

The tourniquet must be secured proximal to the desired region to occlude the arterial and venous flow and to keep the perfusate from leaking into systemic circulation. When treating the fetlock and below we use a tourniquet on the mid metacarpus/metatarsus. For carpal/tarsal regions the tourniquet in the mid metacarpus/metatarsus is used in conjunction with one above the carpus/tarsus. We often use a roll of gauze on the medial aspect of the limb to evenly distribute the compression around the limb. We typically use Esmarch bandages for the tourniquet. Exsanguination of the limb will allow a greater volume of perfusate; however we may not do this, particularly in the standing horse.

Some horses show a bit of discomfort after the 20 minute infusion period and removal of the tourniquet. This quickly subsides and seems to resolve quicker if the horse is walked.

The antimicrobial solution can be administered intravenous or intraosseous. Intra-arterial injections are not commonly used because of the potential to inflame and thrombose the artery; however, a single inadvertent IA perfusion does not seem to lead to clinical problems.

With IV perfusion, the vein should be catheterized prior to exsanguination. Short, 1 inch 20- 22 gauge catheter can be used. Alternatively, 19-22 gauge butterfly catheters work well. Care needs to be taken to limit repeat attempts at catheterization because leaking around the injection site into the subcutaneous tissue from multiple attempts can be a problem, particularly if repeated perfusions are going to be required. The objective is to dilate venous capillaries, post capillaries, and lymphatics stretching the junctions between endothelial cells. The gaps allow diffusion without cellular injury. The leakage and high concentration gradient move the antimicrobials into the tissue.

The antimicrobial chosen is best selected by culture and sensitivity; however they are not always available at the time of perfusion. Aminoglycosides are most commonly used. They are water (saline) soluble, concentration dependent, and efficacious against most of the commonly encountered organisms. Cephalosporins and penicillins, macrolides and carbapenems have been used by clinicians. They are time dependent, so high concentrations for short times may not be as valuable as for the aminoglycosides. Enrofloxacin is irritating to vascular lining and should be avoided. Our most common application would be 1 gram of gentamicin (10ml), 10 ml of carbocaine, and 40 ml of saline. We add the carbocaine so that the horse will stand comfortably for 20 minutes with the esmarch on as the sedation wanes. We often give the remainder of the daily systemic dose of gentamicin (6.6 mg/kg SID) at the time of the perfusion. We would use approximately ½ of the above dose and volume in a foal. There are a number of doses that are clinically used and available in the literature. Doses as low as 100 mg have been used successfully. The optimal volume needed is not known. We use up to 100 ml in adult horses for a large region where 10-20 ml is adequate in most perfusions in foals. A general rule is ⅓ of the daily dose of the antimicrobial can be used in the regional perfusion.

Intraosseous infusion is accomplished with the same antimicrobials and volumes as IV. Access to the medullary cavity can be as simple as a 4.0 mm drill hole with insertion of an extension set into the hole or it can be done with an intramedullary infusion device. The big difference with this administration is the pressure required to infuse the solution. Infusion of the medullary cavity can be painful so adequate sedation is important as well as the inclusion of carbocaine in the solution. It can take 5 minutes of consistent firm pressure on a syringe to inject a 60 ml volume. The value of the IO perfusion is the ability to repetitively use an injection port or in horses with swollen limbs where finding a vessel is difficult. Repeated use of veins can lead to thrombosis and difficulty infusing. My most common use of an IO infusion is when I expect I will need to do more than 3-4 perfusions in a single horse.

When done prophylactically or associated with a surgical procedure, the infusion is often started prior to surgery. The tourniquet may be on longer than 20 minutes, often up to an hour, with no notable negative results. Most of the perfusions are done in standing sedated horses. We use a total perfusion time of 20 minutes. We often do it 2-3 days in a row then follow up with another perfusion after skipping a day. This is variable based on the case and clinical response. The treatment protocols are often based on subjective evaluations of response.

Continuous antimicrobial infusion is a different technique, but can be used to maintain sustained levels of antimicrobials in synovial cavities. Use of the Mila continuous infusion devices can deliver a consistent flow from 0.5 ml to 2 ml/hour for up to 4 days. This technique is particularly useful in larger synovial structures where there is room for an indwelling catheter, such as the tarsocrural joint and flexor tendon sheath.

We consistently use regional perfusions to treat infections of the distal limb and prophylactically during orthopedic procedures. It is a procedure done almost daily in the hospital. During this one hour seminar I will present the techniques we routinely use.