Effect of rapid and slow intravenous injection of sodium penicillin on arterial blood pressure in isoflurane-anesthetized horses during surgery.

Objective: To determine the effects of rapid (1 minute) and slow (10 minutes) intravenous (IV) injection of sodium penicillin on arterial blood pressure in anesthetized horses.

Study design: Prospective randomized clinical trial.

Animals: A group of 29 client-owned horses of various breeds, 1-20 years old, with body masses of 360-710 kg.

Methods: General anesthesia was induced with a variety of anesthetic protocols and maintained with isoflurane under mechanical ventilation, with hourly doses of IV lidocaine and an infusion of dexmedetomidine. Horses were administered IV intraoperative penicillin every 2 hours after the preoperative dose, reconstituted with 50 mL of saline (group small dilution, SD) and administered over 1 minute, or with 250 mL of saline (group large dilution, LD) administered over 10 minutes. Systolic, diastolic and mean arterial blood pressures (SAP, DAP, MAP), heart rate, end-tidal isoflurane and carbon dioxide, dobutamine rate and arterial electrolytes were recorded before and for 20 minutes after penicillin. Comparisons between and within groups were with two-way anova.

Results: Dose and time to penicillin delivery during anesthesia were similar between groups. SAP decreased significantly by 4.8-9.6% (p < 0.0001-0.038), DAP by 12.7-25.4% (p = 0.0009-0.016) and MAP by 6.6-18.4% (p = 0.0009-0.028) from injection and for 15-20 minutes in group SD. In group LD, significant decreases in DAP (13.8-18.5%; p < 0.0001-0.005) and MAP (10.1-13.9%; p < 0.0001-0.003) occurred at 3-15 minutes, and DAP (p = 0.013 and 0.008) and MAP (p = 0.016 and 0.007) were higher than for group SD at 1 and 3 minutes. Dobutamine rate and other variables were similar between groups.

Conclusions and clinical relevance: Arterial blood pressure decreased with both SD and LD in anesthetized horses, but to a lesser extent in the slower, more diluted LD group.