CHAPTER 117 Regional Limb Perfusion with Antimicrobials
The aim in treating orthopedic infections and infected limb wounds in horses is eradication of bacterial load, removal of foreign material, elimination of inflammatory mediators and free radicals, pain relief, and restoration of the normal environment to promote tissue healing. The goal of antimicrobial therapy is to achieve antimicrobial concentrations above the minimum inhibitory concentration (MIC) in the infected tissue without inducing toxic effects in the animal. This can be accomplished by systemic and local administration of antimicrobial agents.
In the last decade, multiple studies in horses have revealed beneficial effects of local antimicrobial therapy; for example, intra-articular administration, regional intravenous (IV) or intraosseous administration, and injection of repositol materials can be excellent adjunctive treatments for limb infections caused by infected wounds, septic arthritis, septic tenosynovitis, and osteomyelitis. The ease and safety of the procedure have made the use of regional limb perfusion of antimicrobials popular with many equine clinicians.
Regional perfusion delivers high local concentrations of antimicrobials to a selected region of the limb. The antimicrobial is injected either IV into a superficial vein proximal to the site of infection or via the intraosseous route into the medullary cavity of a bone in proximity to the infected site. For both techniques a tourniquet is placed proximal and distal to the site of infection to occlude the superficial venous system. As the perfusate is infused, high concentrations and pressure gradients between the intravascular and extravascular compartments are created, allowing the antimicrobial to diffuse into the surrounding tissues. Local tissue antimicrobial concentrations can reach 25 to 100 times the MIC for most common equine pathogens. Therefore, the greatest bactericidal effect results from administration of concentration-dependent antimicrobials such as the aminoglycosides. A high peak concentration-to-MIC ratio (Cmax: MIC ratio) is associated with greater bactericidal effect and longer post-antimicrobial effect. Use of time-dependent antimicrobials such as ceftiofur can also be justified because with this technique the antimicrobial tissue half-life is likely to be longer after perfusion of the region than after parenteral administration of the same antimicrobial.
The most common antimicrobials administered by regional perfusion are amikacin and gentamicin because they are concentration dependent and have good efficacy against most pathogens that cause orthopedic infections in horses. Other antimicrobials that have been used include potassium penicillin G, ampicillin, cefotaxime, ceftiofur, vancomycin, and enrofloxacin (Box 117-1). The antimicrobial dosage varies among agents and is approximately 150 mg to 1 g of antimicrobial diluted with physiologic (0.9%) saline solution to a total volume of 35 to 60 mL of solution. Additional pharmacokinetic studies are necessary to evaluate effective dosages with these techniques.
