The post‐tetanic count during vecuronium‐induced neuromuscular blockade in halothane‐anaesthetized dogs

Abstract

ObjectiveTo evaluate the post‐tetanic count (PTC) for predicting the return of reversible neuromuscular blockade at the n. facialis–m. nasolabialis (nF–mNL) and n. ulnaris–mm. carpi flexorii (nU–mCF) nerve‐muscle units (NMUs) during profound vecuronium neuromuscular blockade in halothane‐anaesthetized dogs. Study designRandomized, prospective, experimental study. AnimalsTwenty‐five dogs (seven male 18 female) undergoing surgery; mean age: 4.8 years; mean body weight 22 kg. MethodsThirty minutes after acepromazine (0.05 mg kg−1) and morphine (0.5 mg kg−1) pre‐medication, anaesthesia was induced with intravenous (IV) thiopental and maintained with halothane, N2O and O2. The lungs were mechanically ventilated and end‐tidal halothane concentration (Fe′HAL) maintained at 1.04%. Neuromuscular transmission was monitored using the train‐of‐four count (TOFC) at one nF–mNL and both nU–mCF units. Vecuronium (50 µg kg−1 IV) was injected after 15 minutes constant Fe′HAL. When the first twitch (T1) at both nU–mCF units had disappeared (t = 0) one (randomly allocated) ulnar nerve was stimulated every 5 minutes using PTC; TOF stimulation continued at the other sites. The PTC was plotted against the interval between recording time and T1's reappearance at the other NMUs. ResultsAt t = 0, the mean PTC in the contralateral nU–mCF unit was 18 (range 0–20). Mean PTC was a minimum at t = 5, rising to the maximum (20) at 25 minutes. Six dogs were vecuronium‐resistant as monitored by PTC. Excluding data from these revealed a strong negative relationship between ulnar PTC and the time taken for T1's return at the facial (r = −0.7018; p < 0.00001) and contralateral ulnar (r = −0.8409; p < 0.00001) NMUs. Conclusion and clinical relevancePost‐tetanic count monitoring beginning >5 minutes after the TOFC at nU–mCF = 0 provided a reliable estimate of T1's return at ulnar and facial NMUs.