Neuromuscular blocking effects of cisatracurium and its antagonism with neostigmine in a canine model of autosomal-recessive centronuclear myopathy.

BACKGROUND: Centronuclear myopathy (CNM) is a rare congenital condition associated with skeletal muscle weakness. Patients with CNM may have decreased acetylcholine receptor expression and a reduced number of releasable quanta. Such perturbations could affect the time-course of neuromuscular blocking agents (NMBAs) and their antagonism with cholinesterase inhibitors. As a result of the rarity of CNM, prospective data regarding NMBA use in this subpopulation is scarce. We evaluated the neuromuscular blocking effects of cisatracurium and its antagonism with neostigmine in a canine model of CNM. METHODS: Six dogs with congenital autosomal-recessive CNM and six controls received cisatracurium 0.15 mg kg(-1) i.v. under general anaesthesia and intermittent positive pressure ventilation. Neuromuscular function was monitored with acceleromyography.When the second response (T2) to train-of-four (TOF) stimulation returned, neostigmine 0.04 mg kg(-1) (with glycopyrrolate) were administered i.v. The onset time, time to spontaneous return of T2, and the time to reach a TOF ratio ≥0.9 after neostigmine administration were recorded. RESULTS: Onset time was no different between groups. Median (interquartile range) time to return of T2 was 27 (24-31) min for control dogs and 26 (22-31) min for CNM dogs (P=0.93).After neostigmine administration, a TOF ratio ≥0.9 was reached in 12 (10-15) min and 17 (16-19) min in control and CNM, respectively (P=0.005). CONCLUSIONS: The spontaneous return of T2 was not different between groups. However, neostigmine-facilitated recovery was significantly slower in dogs with CNM. Canine autosomal-recessive CNM does not preclude the use of cisatracurium or its antagonism with neostigmine.

TOF-Watch(R) monitor: failure to calculate the train-of-four ratio in the absence of baseline calibration in anaesthetized dogs.

BACKGROUND: TOF-Watch(®) monitors are designed to display train-of-four (TOF) count when neuromuscular block is intense, and to display TOF ratio when it is less intense. In dogs recovering from non-depolarizing neuromuscular block, when all four twitches are easily visible and apparently of similar magnitude, TOF-Watch(®) monitors often display TOF counts and not TOF ratios, as would be expected. We have never encountered this problem when the monitor was calibrated before neuromuscular blocking agent administration. METHODS: Fourteen healthy female dogs undergoing ovariohysterectomy were investigated. Recovery from neuromuscular block was assessed with a calibrated TOF-Watch SX(®) monitor. When the TOF ratio returned to 90%, the TOF-Watch SX(®) was replaced with an uncalibrated TOF-Watch(®) monitor. The output obtained from the uncalibrated TOF-Watch(®) was compared with that of the calibrated device. RESULTS: The median TOF ratio measured by the calibrated TOF-Watch SX(®) unit at recovery was 91 (86-100)% (n=14). The uncalibrated TOF-Watch(®) monitor displayed TOF counts in six dogs [2 (0, 4)] and TOF ratios in the remaining eight dogs [91 (79, 98)%], that is, the uncalibrated device failed to display appropriately >40% of the time. CONCLUSIONS: TOF-Watch(®) monitors must be calibrated before neuromuscular blocking agents are administered to dogs. When these devices are not so calibrated, they default to a reference value for twitch magnitude that was defined in healthy adult people. Even though neuromuscular transmission was restored in these dogs, we surmise that they did not achieve the default reference value, causing the monitor to display TOF counts rather than TOF ratios.

The train-of-four or double-burst ratios cannot reliably exclude residual neuromuscular block in cats.

This study investigated the relationship between train-of-four (TOF) or double burst (DBS) ratios (T4:T1 or B2:B1) and twitch (T1) or burst (B1) magnitudes during the recovery from rocuronium-induced neuromuscular block in dogs and cats. The main hypothesis was that TOF or DBS ratios recover after the recovery of T1 or B1, and hence high ratio levels are sensitive indicators of restoration of the neuromuscular function. Six anesthetized dogs and six anesthetized cats received 0.5 mg/kg of rocuronium intravenously. The amplitudes of T1 or B1 were measured with mechanomyography during neuromuscular block until the neuromuscular function recovered fully. The TOF or DBS ratio was recorded concurrently. In dogs, recovery of T1 and B1 preceded the recovery of the TOF and DBS ratios, and T1 and B1 were always ≥90% of recovery when the respective ratio reached 0.9. In contrast, T1 was still depressed in 5/6 cats when the TOF ratio reached 0.9. At that moment, T1 was 72.5 ± 19.8% of recovery. Similarly, the DBS ratio returned to 0.9 when B1 was still <90% in 3/6 cats of recovery. The TOF and DBS fade in dogs consistently disappeared after the magnitude of T1 or B1 were restored, and hence, ratios ≥0.9 are a sensitive indicator that the neuromuscular function recovered. Our observation in cats however show that the spontaneous recovery of neither the TOF nor the DBS ratio of 0.9 can reliably exclude residual block, as the magnitude of T1 or B1 was still depressed in several instances.

Context-sensitive recovery of neuromuscular function from vecuronium in dogs: Effects of dose and dosing protocol.

Recovery of neuromuscular function is a gradual phenomenon whereby function progresses from absent to normal. The speed of spontaneous recovery can be used to predict the time when neuromuscular function is expected to be restored. However, the speed of recovery might be affected by the dose of the neuromuscular blocker administered, and by the dosing regimen of that dose. The effects of both factors on the speed of spontaneous recovery from vecuronium were evaluated. Seven dogs were anesthetized three times and the train-of-four (TOF) ratio was measured with acceleromyography. Vecuronium was administered at 0.1 mg/kg, 0.2 mg/kg, or 0.1 mg/kg followed by two doses of 0.05 mg/kg was administered each time. In the divided-dose treatment group, aliquots were administered on return of the first twitch (T1) of the TOF from the previous dose. The duration of surgical block, from injection to return of T1, was longest for the divided-dose protocol, intermediate for 0.2 mg/kg single bolus, and shortest for 0.1 mg/kg (P < 0.0001). The recovery period, from return of T1 to a TOF ratio ≥0.9, was longer for 0.2 mg/kg administered as a single bolus than for the other two groups (P = 0.007). Doubling the dose of a single bolus of vecuronium extended the time of surgical block and prolonged the duration of the recovery period. However, dividing that dose into smaller aliquots extended the period of surgical block while shortening the recovery period. Hence, the spontaneous reappearance of T1 should not be used in isolation to predict the time to complete recovery of neuromuscular function.

Bias, limits of agreement, and percent errors between acceleromyography and mechanomyography in anesthetized dogs.

This study compared measurements of neuromuscular function with mechanomyography (MMG) and acceleromyography (AMG) in nine anesthetized dogs receiving 0.1mg/kg vecuronium intravenously. Train-of-four (TOF) stimulation was applied to each ulnar nerve every 15s. The resulting amplitude of the first twitch (T1) and the TOF ratio were measured with both monitors. The baseline TOF ratio (prior to vecuronium), onset time (time of injection to T1<5%), recovery index (time between T1 values of 25% and 75%) and duration of neuromuscular block (injection to TOF 0.9) were recorded. The MMG TOF ratios when the AMG first reached 0.7 (AMG 0.7) and 0.9 (AMG 0.9) during recovery were also recorded. Values were compared with paired tests and individual errors>25% between monitors were identified for each dog. Bias, limits of agreement (LOA) and percentage error (PE) between methods were calculated from Bland-Altman plots for T1 and TOF ratio for the complete data set, and for TOF≥0.7 during recovery. There were no statistical differences in baseline TOF ratio, onset, recovery index, duration, AMG 0.7 and AMG 0.9. Individual errors>25% were evident in onset, recovery index, AMG 0.7 and AMG 0.9. Overall, T1 and TOF ratio had a small bias, wide LOA and PE>100%. Percent error was reduced to 30% when TOF≥0.7 was analyzed. Although there were no statistical differences between MMG and AMG in any variable of interest, individual discrepancies, wide LOA and high PE suggest that these monitors should not be used interchangeably for serial measurements on the same animals.

Unilateral cervical plexus block for prosthetic laryngoplasty in the standing horse.

BACKGROUND: Locoregional anaesthetic techniques can facilitate certain surgeries being performed under standing procedural sedation. The second and third spinal cervical nerves (C2, C3) are part of the cervical plexus and provide sensory innervation to the peri-laryngeal structures in people; block of these nerves might permit laryngeal lateralisation surgery in horses. OBJECTIVES: To describe the anatomical basis for an ultrasound-guided cervical plexus block in horses. To compare this block with conventional local anaesthetic tissue infiltration in horses undergoing standing prosthetic laryngoplasty. STUDY DESIGN: Cadaveric study followed by a double-blinded prospective clinical trial. METHODS: A fresh equine cadaver was dissected to characterise the distribution of C2 and C3 to the perilaryngeal structures on the left side. A second cadaver was utilised to correlate ultrasound images with the previously identified structures; a tissue marker was injected to confirm the feasibility of an ultrasound-guided approach to the cervical plexus. In the clinical study, horses were assigned to two groups, CP (n = 17; cervical plexus block) and INF (n = 17; conventional tissue infiltration). Data collection and analyses included time to completion of surgical procedure, sedation time, surgical field conditions and surgeon's perception of block quality. RESULTS: We confirmed that C2 and C3 provided innervation to the perilaryngeal structures. The nerve root of C2 was identified ultrasonographically located between the longus capitis and the cleidomastoideus muscles, caudal to the parotid gland. The CP group was deemed to provide better (P<0.0002) surgical conditions with no differences in the other variables measured. MAIN LIMITATIONS: Further studies with larger numbers of horses may be necessary to detect smaller differences in surgical procedure completion time based on the improved surgical filed conditions. CONCLUSIONS: For standing unilateral laryngeal surgery, a cervical plexus block is a viable alternative to tissue infiltration and it improves the surgical field conditions.

Maximal and submaximal mouth opening with mouth gags in cats: implications for maxillary artery blood flow.

The use of spring-loaded mouth gags in cats can be associated with the development of central neurological deficits, including blindness. In this species, the maxillary arteries are the main source of blood supply to the retinae and brain. Spring-loaded gags generate constant force after placement that could contribute to bulging of the soft tissues between the mandible and the tympanic bulla. Under these circumstances, the maxillary arteries can become compressed as they course between these osseous structures. Smaller gags that might apply less force to the mouth were investigated to determine if they preserved maxillary artery blood flow. Six healthy adult cats were anesthetized. Electroretinography (ERG) and magnetic resonance angiography (MRA) were performed without the use of a mouth gag and during submaximal (plastic mouth gags of 20, 30 and 42 mm in length between canine teeth) and maximal mouth opening. Maximal mouth opening produced alterations in ERG waveforms consistent with circulatory compromise in 1/6 cats and reductions in signal intensity during MRA in 4/6 cats. Placement of a 42 mm plastic gag produced a reduction in MRA signal in 1/6 cats. No changes were observed with smaller gags. The force applied against the mouth was significantly higher with the spring-loaded gag than with any other gags. The use of a smaller mouth gags was associated with fewer alterations of indicators of maxillary artery blood flow. Nevertheless, a 42 mm plastic gag, equivalent to the size of a needle cap, resulted in an abnormal MRA in one cat.

Reversal of moderate and intense neuromuscular block induced by rocuronium with low doses of sugammadex for intraoperative facial nerve monitoring.

We report two cases in which moderate and intense rocuronium-induced neuromuscular block was reversed intraoperatively with low sugammadex doses in order to facilitate electromyographic evaluation of facial nerve function during surgery of the parotid gland and the middle ear. Acceleromyography was used to assess reversal of neuromuscular block before starting electromyography monitoring. Rocuronium-induced neuromuscular block was reversed with sugammadex 0.22mgkg(-1) when the TOF ratio was 0.14 in the first patient, and with sugammadex 2mgkg(-1) during intense block (PTC 0) in the second patient. In each case, appropriate neuromuscular function (TOF ratio≥0.9) was established soon after sugammadex administration, and electromyographic evaluation of facial nerve was successfully conducted. The use of rocuronium and sugammadex, coupled with objective neuromuscular monitoring with acceleromyography, assured complete restoration of neuromuscular function and created the optimal conditions for the surgical team.

Evaluation of maxillary arterial blood flow in anesthetized cats with the mouth closed and open.

The mouth-gag is a common tool used in veterinary medicine during oral and transoral procedures in cats but its use has recently been associated with the development of blindness. The goal of this study was to investigate whether maximal opening of the mouth affects maxillary artery blood flow in six anesthetized cats. To assess blood flow, the electroretinogram (ERG), brainstem auditory evoked response (BAER) and magnetic resonance angiography (MRA) were evaluated qualitatively with the mouth closed and open. During dynamic computer tomography (CT) examinations, detection of contrast medium in the maxillary artery was quantified by measuring the Hounsfield units (HUs). The peak HU, time to peak and mean HU were determined. Changes ⩾10% of these parameters were considered indicative of altered blood flow. ERG and BAER were normal with the mouth closed in all cats, but was abnormal with the mouth opened maximally in two cats and one cat, respectively. During MRA, blood flow was undetected in either maxillary artery in one cat and reduced in the right maxillary artery in two cats, when the mouth was open. During CT, the peak HU decreased ⩾10% in three cats, the time to peak was ⩾10% longer in two cats, and the mean HU was ⩾10% lower in one cat when the mouth was open. No cat developed apparent blindness or deafness. Maximal opening of the mouth caused alterations in several indicators of blood flow in some individual cats.