Observations of the potency and duration of vecuronium in isoflurane-anesthetized horses

To evaluate the potency and duration of three subparalyzing doses of vecuronium (VEC) in isoflurane-anesthetized horses.

Prospective experimental study.

Thirteen healthy adult horses undergoing arthroscopic surgery.

During isoflurane anesthesia, horses received one of three doses of vecuronium (25, 50, or 100 μg kg(-1)). Neuromuscular transmission was monitored with acceleromyography (AMG) with train-of-four (TOF) stimulation of the radial nerve. Maximal depression of the first twitch (T1), and onset time were recorded for each dose. Recovery time to a TOF ratio >90% was also evaluated.

Vecuronium 25 μg kg(-1) produced no observable T1 depression in four horses. VEC 50 μg kg(-1) (n = 5) produced a maximal T1 depression of [median (min, max)] 41 (20, 71) % in four horses, and no neuromuscular block was seen in the fifth. VEC 100 μg kg(-1) was given to four horses and produced a T1 depression of 73 (64, 78) %. Of the four horses in which VEC 50 μg kg(-1) produced a measurable neuromuscular block, three recovered spontaneously 43 (40, 52) minutes after VEC administration; a fourth subject received edrophonium to reverse residual block at the end of the surgery. Spontaneous recovery after VEC 100 μg kg(-1) occurred by 112 minutes in one horse, and had to be facilitated by edrophonium in the remaining three horses, more than 2 hours after VEC had been given.

A dose of 100 μg kg(-1) VEC in isoflurane anesthetized horses failed to produce complete paralysis. The partial neuromuscular block lasted at least 2 hours after this dose had been administered. Edrophonium was required to reverse the neuromuscular block in three of four horses. It is likely that more than 100 μg kg(-1) VEC would be necessary for complete neuromuscular blockade in horses, and that this dose will last >2 hours.