The effect of the organophosphate trichlorfon on the neuromuscular blocking activity of atracurium in halothane‐anesthetized horses

To determine whether cholinesterase inhibition by an organophosphate would influence atracurium's neuromuscular blockade, six horses were anesthetized and paralyzed with atracurium (total of five injections per horse) on experimental Day 1, then were given trichlorfon (64 mg/kg per os) 6 days later. On Day 7, horses were anesthetized and paralyzed in the same manner as on experimental Day 1. Blood was taken to measure serum cholinesterase activity prior to anesthesia on Days 1 and 7. No significant difference was noted in atracurium's neuromuscular blocking activity between the 2 experimental days (P < 0.05), despite Day 7 cholinesterase activity that was 16% of pre‐trichlorfon values. For atracurium Injections 1 and 2–5, 85 and 43 μg/kg of atracurium, respectively, were required to produce a 95–99% reduction in hoof twitch. The time from injection to maximum twitch reduction was approximately 9 min after Injection 1 and 5 min after subsequent injections. Time from injection to maximum twitch reduction was significantly longer for Injection 1 than Injections 2–5 on both experimental days. The time from maximum twitch reduction until 10% recovery was approximately 8 min, with no significant difference between experimental days. The time for twitch recovery from 10 to 75% was approximately 17 min for all injections. Antagonism of atracurium with edrophonium caused the twitch height to return to pre‐atracurium strength in approximately 7 min. Edrophonium caused a significant increase in arterial blood pressure. Heart rate change was variable after edrophonium. Recovery to standing was uneventful for five of the six horses on Day 1, and for all six horses on Day 7. It was concluded that atracurium's neuromuscular blockade was not prolonged by trichlorfon exposure.