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veterinary
farriery
2023
Cohort Study

Potassium penicillin and gentamicin pharmacokinetics in healthy conscious and anesthetized horses.

Authors: Wilson Katherine E, Bogers Sophie H, Council-Troche R McAlister, Davis Jennifer L

Journal: Veterinary surgery : VS

Summary

General anaesthesia significantly alters how horses eliminate potassium penicillin and gentamicin, with both drugs showing reduced clearance rates, prolonged half-lives, and elevated minimum plasma concentrations during unconsciousness compared to the awake state. In this crossover study of six Thoroughbreds, penicillin concentrations remained above therapeutic minimum inhibitory concentration (MIC) for substantially longer during anaesthesia (332 minutes versus 199 minutes), whilst gentamicin clearance decreased from 1.48 to 1.18 mL/kg/min—findings with direct implications for surgical prophylaxis protocols. However, the practical benefit of extended plasma half-lives is offset by a critical limitation: synovial fluid penetration was markedly inferior in anaesthetized horses at both early (30 minutes) and later timepoints (6 hours), meaning joint and tendon sheath infections may receive inadequate drug exposure despite acceptable systemic concentrations. These results suggest that whilst redosing penicillin within 4–5 hours of anaesthesia initiation is unnecessary from a pharmacokinetic standpoint, practitioners should reconsider dosing regimens when treating suspected intra-articular infections in anaesthetised horses, as standard IV dosing may fail to achieve concentrations sufficient to combat common pathogens. The findings underscore the importance of species-specific and state-specific antimicrobial dosing strategies, particularly when anaesthesia-induced physiological changes compromise drug distribution to critical anatomical sites.

Read the full abstract on PubMed

Practical Takeaways

  • When treating anesthetized horses, do not redose penicillin/gentamicin within 4-5 hours as plasma concentrations are maintained longer than in conscious horses due to reduced clearance.
  • Be aware that despite adequate plasma concentrations, intrasynovial drug delivery is significantly impaired during anesthesia—consider alternative antimicrobial strategies or dosing adjustments if treating suspected joint infections in anesthetized horses.
  • Antimicrobial dosing protocols may need adjustment for anesthetized horses to balance safety (avoid accumulation) with efficacy (ensure adequate tissue penetration), particularly for joint-related procedures.

Key Findings

  • Anesthetized horses had significantly higher minimum plasma concentrations and longer half-lives for both penicillin (0.44 vs 0.11 μg/mL; 71 vs 59 min) and gentamicin (3.0 vs 1.9 μg/mL; 149 vs 109 min) compared to conscious horses.
  • Penicillin remained above therapeutic MIC for 332 minutes in anesthetized versus 199 minutes in conscious horses, suggesting reduced elimination during anesthesia.
  • Synovial fluid concentrations were paradoxically lower in anesthetized horses at both 30 minutes and 6 hours for both antibiotics, despite higher plasma levels.
  • Redosing penicillin/gentamicin in anesthetized horses is not necessary before 4-5 hours, but intrasynovial penetration may be inadequate for treating joint infections.

Conditions Studied

pharmacokinetics during general anesthesiaantimicrobial efficacy in anesthetized horses