I perform OHE for spay using midline skin incision and midline celiotomy.

For castration, I use pre-scrotal skin incision and closed-technique.

Pre-op pain med: tramadol 2~4mg/kg IV

Post-op pain-med: meloxicam 0.1~0.2mg/kg IV then TLK CRI

When performing castration, I recommend that you use pre-op intratesticular lidocaine injection and subuctaneous injection at the site you are incising the skin.

Total dose for lidocaine is 2mg/kg divided by the number of site that you are injecting.
(ex: If you are going to inject at both testicles then 1mg/kg on each site, if you are going to inject at both testicles and skin then 0.66mg/kg on each site)

I hope this will help