Plasma concentration and local anesthetic activity of procaine hydrochloride following subcutaneous administration to horses.

This research study investigates the local anesthetic effects of procaine hydrochloride, with or without epinephrine, in horses. The findings suggest that small doses of procaine teamed with epinephrine provide long-lasting local pain relief, and present plasma procaine concentrations that aren’t always detectable by the established testing method. The results challenge the use of current regulatory concentration limits for procaine in equine plasma samples taken post-race.

Study Design and Methods

• The study was conducted on six healthy adult horses. The local anesthetic effect was measured by observing the hoof withdrawal reflex latency (HWRL) period before and after administering procaine with and without epinephrine.
• The HWRL period tests how long it takes for the hoof to reflexively pull away from a focused heat lamp. This gives an indication of local anesthetic effectiveness.
• Blood samples were collected before each HWRL measurement to gauge plasma concentrations of procaine. This was done using liquid chromatography-mass spectrometry-mass spectrometry (LC-MS-MS), a widely accepted method for detecting and quantifying molecules in complex samples.

Results

• In the case of procaine alone, only the 10mg dose had a significant effect in prolonging the HWRL period, whereas both the 5mg and 10mg doses of procaine, when given with epinephrine, enhanced the duration of the anesthetic effect.
• Interestingly, plasma procaine concentrations did not always align well with anesthetic activity. Some horses administered the 5-mg dose of procaine and epinephrine showed maximum hoof withdrawal latency, despite procaine concentrations being below the detectable limit of the LC-MS-MS test.

Conclusion

• The study demonstrates that low doses of procaine coadministered with epinephrine can provide long-lasting local analgesia in horses.
• The inconsistent correlation between plasma procaine concentrations and anesthetic effectiveness suggests that current regulatory limits for procaine concentrations in equine plasma samples post-race may need revisiting and reassessment.