Twitch Depression Research Articles

Mechanism of post-tetanic depression of slow muscle fibres.

A brief tetanic stimulation has a very different effect on the subsequent isometric twitch force of fast and slow skeletal muscles. Fast muscle responds with an enhanced twitch force which doubles that of the pre-tetanic value, whereas slow muscle depresses the post-tetanic twitch by about 20%. Twitch potentiation of fast muscle has long been known to be due to myosin light chain 2 phosphorylation. It is proposed that post-tetanic twitch depression in slow muscle is due to the dephosphorylation of the slow isoform of the thick filament protein, myosin-binding protein-C, by Ca2+/calmodulin-activated phosphatase calcineurin, whilst its phosphorylation underlies the force enhancement due to β-adrenergic stimulation in slow and fast muscle.

Sugammadex Is Not a Silver Bullet: Caveats Regarding Unmonitored Reversal.

Sugammadex Is Not a Silver Bullet: Caveats Regarding Unmonitored Reversal.

Influence of the amount of skeletal muscle mass on rocuronium-induced neuromuscular block

ObjectiveTo evaluate the effects of skeletal muscle mass on the rocuronium-induced neuromuscular block. DesignA prospective, double-blinded, observational study. SettingA tertiary care university hospital. PopulationOne hundred one patients aged 18–65 years who were scheduled to undergo major surgery lasting more than 1 h under general anaesthesia. MethodsAll participants underwent body composition analysis before anaesthesia and were allocated into two groups; the muscular and non-muscular group, according to skeletal muscle mass. During anaesthesia induction, rocuronium 1.0 mg kg−1 of total body weight was injected followed by neuromuscular monitoring using train-of-four stimulation every 15 s. Main outcome measuresThe onset time of rocuronium included the elapsed time from the rocuronium injection until 95% depression of first twitch (T1) and the time to no response to TOF stimulation. The duration was evaluated as the elapsed time from the rocuronium injection to 25% recovery of the final T1 (TDUR25), and the time to the reappearance of T1 (TTOF1) and T4 (TTOF4). These pharmacologic data were compared between two groups. ResultsThere was no significant difference in the onset time of rocuronium between the two groups. However, TDUR25 (min) was significantly shorter in the muscular group than in the non-muscular group (p = 0.035 and p = 0.014 in males and females, respectively). TTOF1 and TTOF4 were also shorter in the muscular group than in the non-muscular group. ConclusionsTotal body weight-based dosing of rocuronium might prolong the neuromuscular relaxation effect in patients with a small amount of skeletal muscle.

Effect of the infusion rate of propofol on the onset time of rocuronium.

Administration of propofol, especially rapid administration, decreases patient cardiac output (CO) to various degrees. CO might influence the buildup of an effective drug level within the neuromuscular junction and affect the onset time of neuromuscular blockers. The present study aimed to investigate the effects of different infusion rates of propofol on patient CO and the onset time of rocuronium. A total of 90 patients were randomly assigned to receive propofol (2.5 mg/kg) at an infusion rate of 480 mg/min (group A), 240 mg/min (group B), or 120 mg/min (group C). After the administration of propofol, rocuronium (0.6 mg/kg) was administered to facilitate tracheal intubation. The Finometer monitor was used to obtain the cardiovascular profile during the induction of general anesthesia. Neuromuscular relaxation was monitored by acceleromyography using the ulnar nerve at the wrist surface and electrodes with repeated single twitches. Onset time was defined as the time from the beginning of rocuronium injection until 95% twitch depression. The onset time of rocuronium in the three groups was compared using analysis of variance with the post-hoc Tukey test. A p-value <0.05 was considered statistically significant. After induction, a significant decrease in CO was observed in group A (21.6% ± 4.6%) when compared with the findings in group B (11.6% ± 4.5%) and group C (9.8% ± 4.6%). The onset time of rocuronium was significantly longer in group A (177.7 ± 17.6 seconds) than in group B (121.3 ± 18.3 seconds) and group C (118.3 ± 12.3 seconds). Rapid administration of propofol significantly delays the onset time of rocuronium by altering CO as measured with the Finometer monitor.

Comparison of rocuronium-induced neuromuscular blockade in second trimester pregnant women and non-pregnant women

BackgroundThis study set out to compare the onset and duration of rocuronium-induced neuromuscular blockade in second trimester pregnant women and non-pregnant women receiving general anesthesia. MethodsForty-seven pregnant (Group P) and forty-seven non-pregnant (Group C) women were enrolled. Anesthesia was induced with propofol 2.0 mg/kg and rocuronium 0.6 mg/kg, and neuromuscular blockade was assessed with an accelerometric sensor using train-of-four stimulation (TOF-Watch® SX). Tracheal intubation was performed at maximum depression of the first twitch (T1) and anesthesia was maintained with sevoflurane 1.5–2.5% and 50% oxygen in air. We recorded the times to maximum T1 depression and 5% and 25% T1 recovery, as well as the mean arterial pressure and heart rate at baseline, injection of rocuronium, intubation, and 5% and 25% T1 recovery. ResultsThe onset of rocuronium-induced neuromuscular blockade (time to maximum T1 depression) did not differ significantly between the groups. The duration (time to 25% T1 recovery) was significantly longer in Group P than in Group C (45.7 ± 12.9 min vs 40.6 ± 10.4 min, P <0.037). During the recovery period from the blockade, the mean arterial pressure was significantly lower in Group P than in Group C. ConclusionOur data showed that the rocuronium-induced neuromuscular blockade did not significantly differ in onset but lasted significantly longer in second trimester pregnant women compared with non-pregnant women.

Administration of rocuronium based on real body weight versus fat-free mass in patients with lymphedema.

ObjectiveTo compare the clinical pharmacokinetics of rocuronium when applied according to fat-free mass versus real body weight during anesthetic induction of patients with lymphedema.MethodsSixty patients with lymphedema (age, 18–60 years; American Society of Anesthesiologists physical status, I–II) undergoing elective surgery with general anesthesia were randomly divided into two groups. Rocuronium was administered based on the fat-free mass in 30 patients and real body weight in 30 patients. General anesthesia was induced with propofol and remifentanil by target-controlled infusion. Intubation was attempted when the onset time (T1) (time from end of bolus injection to 100% twitch depression) reached maximal inhibition, and respiratory support with mechanical ventilation was then applied. The T1, clinical duration (time from end of bolus injection to recovery of twitch tension to 25% of control), recovery index (time from 25% to 75% of recovery of T1), and dosage were recorded.ResultsComplete data were recorded for 59 patients, and there were no significant differences in the general condition, intubation condition, or median duration of action of rocuronium between the two groups. However, the median T1, recovery index, and dosage of rocuronium were significantly different.ConclusionGood intubation conditions and a shortened clinical duration can be obtained for patients with lymphedema when induction with rocuronium is based on the fat-free mass.

Dose-response and Cardiopulmonary Side Effects of the Novel Neuromuscular-blocking Drug CW002 in Man.

CW002 is a benzylisoquinolinium nondepolarizing neuromuscular-blocking drug found to be inactivated by cysteine in preclinical studies. The current study represents a dose escalation clinical trial designed to describe CW002 potency, duration, cardiopulmonary side effects, and histamine release. Healthy subjects anesthetized with sevoflurane/nitrous oxide were divided into five groups (n = 6), each receiving a fixed CW002 dose (0.02, 0.04, 0.06, 0.08, or 0.10 mg/kg), and one group (n = 4) receiving 0.14 mg/kg. Blood pressure and heart rate were continuously recorded along with airway dynamic compliance. Neuromuscular blockade was assessed with mechanomyography at the adductor pollicis. Arterial blood was obtained before and after CW002 injection for analysis of plasma histamine concentration. Potency was estimated from a baseline sigmoid Emax model. ED50 was found to be 0.036 mg/kg (95% CI, 0.020 to 0.053 mg/kg) and ED95 0.077 mg/kg (95% CI, 0.044 to 0.114 mg/kg). At 0.14 mg/kg (1.8 × ED95), 80% twitch depression occurred in 94 ± 18 s with complete block in 200 ± 87 s. Clinical recovery (25% of maximum twitch) occurred in 34 ± 3.4 min, with a 5 to 95% recovery interval of 35.0 ± 2.7 min. The time to a train-of-four ratio greater than 0.9 ranged from 59 to 86 min. CW002 did not elicit histamine release or significant (greater than 10%) changes in blood pressure, heart rate, or dynamic airway compliance. In healthy subjects receiving sevoflurane/nitrous oxide, CW002 at 1.8 × estimated ED95 produces a clinical duration less than 40 min, elicits no histamine release, and has minimal cardiopulmonary side effects.

Effect of ORM1 genetic polymorphism on time-course of muscle relaxation induced by rocuronium

Objective To evaluate the effect of ORM1 genetic polymorphism on the time-course of muscle relaxation induced by rocuronium. Methods Seventy American Society of Anesthesiologists physical status Ⅰor Ⅱ patients, aged 25-55 yr, scheduled for elective gynecological laparoscopic surgery, with body mass index of 20-25 kg/m2, were enrolled in this study.Anesthesia was induced with iv midazolam and fentanyl and target-controlled infusion of propofol.After the patients lost consciousness, neuromuscular block was assessed with TOF Watch-SX using single stimulation of the ulnar nerve.When the maximal twitch depression was achieved, tracheal intubation was facilitated with rocuronium 0.6 mg/kg.The patients were mechanically ventilated.End-tidal carbon dioxide partial pressure was maintained at 35-45 mmHg.The onset time, nonresponse time, clinical duration, 75% recovery time and recovery index of rocuronium were recorded.Peripheral venous blood samples were collected before surgery for determination of concentrations of plasma alpha1 acid glycoprotein, C-reactive protein(by enzyme-linked immunosorbent assay)and albumin(by biochemical method). ORM1 genotypes were detected by polymerase chain reaction-restriction fragment length polymorphism.The patients were divided into 3 groups according to the ORM1 genotypes: wild homozygote(ORM1*F1/*F1) group (AA group), mutation heterozygote(ORM1*F1/*S) group (AG group)and mutation homozygote(ORM1*S/*S) group (GG group). Results There were 40 cases in group AA, 25 cases in group AG, and 5 cases in group GG.There were no significant differences in plasma alpha1 acid glycoprotein, albumin and C-reactive protein concentrations between the three groups(P>0.05). Compared with group AA, the clinical duration, 75% recovery time and recovery index of rocuronium were significantly prolonged(P 0.05). Conclusion ORM1 genetic polymorphism is one of the genetic factors which affect the time-course of muscle relaxation induced by rocuronium. Key words: Orosomucoid; Polymorphism, single nucleotide; Androstanols; Neuromuscular blockade; Dose-response relationship, drug

Effects of pretreatment with different doses of cisatracurium on succinylcholine-induced fasciculations.

To compare the effects of different doses of cisatracurium pretreatment on succinylcholine-induced fasciculations. 90 patients scheduled for laparoscopic cholecystectomies were equally randomized into three groups to receive pretreatment of 0.005, 0.01, and 0.02 mg/kg cisatracurium, respectively. After the pretreatments, general anesthesia was induced 3.5 minutes later, train-of-four stimulation was monitored 4.5 minutes later, succinylcholine 1.5 mg/kg was injected 5 minutes later, and endotracheal intubation was implemented 6.5 minutes later. Side effects of cisatracurium, intensity of fasciculations, intubating conditions, time and extent to maximal depression of twitch and time for its recovery to 20% of control value, and severity of myalgia at 24 hours postoperatively were recorded. Fasciculations were alleviated significantly after the cisatracurium pretreatment of 0.02 mg/kg, more than with the other two doses (p < 0.01). Intubating conditions, time and extent to maximal depression of twitch, time for its recovery to 20% of the controls, and incidence of myalgia had no significant changes among the three groups (p > 0.05). Transient and tolerable diplopia and difficulty opening eyes emerged after pretreatment of 0.02 mg/kg cisatracurium. The pretreatment of 0.02 mg/kg cisatracurium given 5 minutes before succinylcholine injection could alleviate succinylcholine-induced fasciculations without influence on muscle relaxation effects or endotracheal intubating conditions, but did not affect the occurrence of myalgia, and might produce transient diplopia and difficulty opening eyes.

Geographical venom variations of the Southeast Asian monocled cobra (Naja kaouthia): venom-induced neuromuscular depression and antivenom neutralization

The Southeast Asian monocled cobras (Naja kaouthia) exhibit geographical variations in their venom proteomes, especially on the composition of neurotoxins. This study compared the neuromuscular depressant activity of the venoms of N. kaouthia from Malaysia (NK-M), Thailand (NK-T) and Vietnam (NK-V), and the neutralization of neurotoxicity by a monospecific antivenom. On chick biventer cervicis nerve-muscle preparation, all venoms abolished the indirect twitches, with NK-T venom being the most potent (shortest t90, time to 90% twitch inhibition), followed by NK-V and NK-M. Acetylcholine and carbachol failed to reverse the blockade, indicating irreversible/pseudo-irreversible post-synaptic neuromuscular blockade. KCl restored the twitches variably (NK-M preparation being the least responsive), consistent with different degree of muscle damage. The findings support that NK-T venom has the most abundant curarimimetic alpha-neurotoxins, while NK-M venom contains more tissue-damaging cytotoxins. Pre-incubation of tissue with N. kaouthia monovalent antivenom (NKMAV) prevented venom-induced twitch depression, with the NK-T preparation needing the largest antivenom dose. NKMAV added after the onset of neuromuscular depression could only halt the inhibitory progression but failed to restore full contraction. The findings highlight the urgency of early antivenom administration to sequester as much circulating neurotoxins as possible, thereby hastening toxin elimination from the circulation. In envenomed mice, NKMAV administered upon the first neurological sign neutralized the neurotoxic effect, with the slowest full recovery noticed in the NK-T group. This is consistent with the high abundance of neurotoxins in the NK-T venom, implying that a larger amount or repeated dosing of NKMAV may be required in NK-T envenomation.

Pharmacological targeting of mitochondrial reactive oxygen species counteracts diaphragm weakness in chronic heart failure.

Diaphragm muscle weakness in chronic heart failure (CHF) is caused by elevated oxidants and exacerbates breathing abnormalities, exercise intolerance, and dyspnea. However, the specific source of oxidants that cause diaphragm weakness is unknown. We examined whether mitochondrial reactive oxygen species (ROS) cause diaphragm weakness in CHF by testing the hypothesis that CHF animals treated with a mitochondria-targeted antioxidant have normal diaphragm function. Rats underwent CHF or sham surgery. Eight weeks after surgeries, we administered a mitochondrial-targeted antioxidant (MitoTEMPO; 1 mg·kg(-1)·day(-1)) or sterile saline (Vehicle). Left ventricular dysfunction (echocardiography) pre- and posttreatment and morphological abnormalities were consistent with the presence of CHF. CHF elicited a threefold (P < 0.05) increase in diaphragm mitochondrial H2O2 emission, decreased diaphragm glutathione content by 23%, and also depressed twitch and maximal tetanic force by ∼20% in Vehicle-treated animals compared with Sham (P < 0.05 for all comparisons). Diaphragm mitochondrial H2O2 emission, glutathione content, and twitch and maximal tetanic force were normal in CHF animals receiving MitoTEMPO. Neither CHF nor MitoTEMPO altered the diaphragm protein levels of antioxidant enzymes: superoxide dismutases (CuZn-SOD or MnSOD), glutathione peroxidase, and catalase. In both Vehicle and MitoTEMPO groups, CHF elicited a ∼30% increase in cytochrome c oxidase activity, whereas there were no changes in citrate synthase activity. Our data suggest that elevated mitochondrial H2O2 emission causes diaphragm weakness in CHF. Moreover, changes in protein levels of antioxidant enzymes or mitochondrial content do not seem to mediate the increase in mitochondria H2O2 emission in CHF and protective effects of MitoTEMPO.

Different magnitude of resistance to non-depolarizing muscle relaxants in dexamethasone-treated rat diaphragm associated with altered acetylcholine receptor expression.

The aim of this study was to investigate the influence of chronic dexamethasone (Dex) administration on rat diaphragm sensitivity to non-depolarizing muscle relaxants (NDMRs) and muscular nicotinic acetylcholine receptor (nAChR) expression, which may help direct future administration of NDMRs. Adult male Sprague-Dawley rats were randomized to receive a daily intraperitoneal injection of Dex (600 μg/kg body mass) or an equivalent volume of saline (N = 20 in each group) for 14 days. We evaluated isometric twitch tensions of nerve-hemidiaphragm preparations elicited by indirect supramaximal stimulation at 0.1 Hz. Real-time quantitative PCR was performed to determine the mRNA expression of two nAChR subunits (ε-subunit and γ-subunit) in the diaphragm. Dex administration markedly (P < 0.01) increased the 50% twitch depression (IC50) of the three NDMRs. The IC50 ratio, which standardized the magnitudes of the resistance, was the largest for atracurium, with the second largest for vecuronium and the smallest for rocuronium (P < 0.01). The ε- and γ-subunit mRNAs were both upregulated with an increased γ/ε ratio in rats exposed to Dex. The results indicated that chronic Dex administration induces hyposensitivity to NDMRs, the degree of which depends on the kind of neuromuscular blocker, and is associated with increased nAChR expression.

Immobilization with Atrophy Induces De Novo Expression of Neuronal Nicotinic α7 Acetylcholine Receptors in Muscle Contributing to Neurotransmission

Mature acetylcholine receptor (AChR) isoform normally mediates muscle contraction. The hypothesis that α7AChRs up-regulate during immobilization and contribute to neurotransmission was tested pharmacologically using specific blockers to mature (waglerin-1), immature (αA-OIVA), and α7AChRs (methyllycaconitine), and nonspecific muscle AChR antagonist, α-bungarotoxin. Mice were immobilized; contralateral limbs were controls. Fourteen days later, anesthetized mice were mechanically ventilated. Nerve-stimulated tibialis muscle contractions on both sides were recorded, and blockers enumerated above sequentially administered via jugular vein. Data are mean ± standard error. Immobilization (N = 7) induced tibialis muscle atrophy (40.6 ± 2.8 vs. 52.1 ± 2.0 mg; P < 0.01) and decrease of twitch tension (34.8 ± 1.1 vs. 42.9 ± 1.5 g; P < 0.01). Waglerin-1 (0.3 ± 0.05 μg/g) significantly (P = 0.001; N = 9) depressed twitch tension on contralateral (≥97%) versus immobilized side (approximately 45%). Additional waglerin-1 (total dose 1.06 ± 0.12 μg/g or approximately 15.0 × ED50 in normals) could not depress twitch of 80% or greater on immobilized side. Immature AChR blocker, αA-OIVA (17.0 ± 0.25 μg/g) did not change tension bilaterally. Administration of α-bungarotoxin (N = 4) or methyllycaconitine (N = 3) caused 96% or greater suppression of the remaining twitch tension on immobilized side. Methyllycaconitine, administered first (N = 3), caused equipotent inhibition by waglerin-1 on both sides. Protein expression of α7AChRs was significantly (N = 3; P < 0.01) increased on the immobilized side. Ineffectiveness of waglerin-1 suggests that the twitch tension during immobilization is maintained by receptors other than mature AChRs. Because αA-OIVA caused no neuromuscular changes, it can be concluded that immature AChRs contribute minimally to neurotransmission. During immobilization approximately 20% of twitch tension is maintained by up-regulation of α-bungarotoxin- and methyllycaconitine-sensitive α7AChRs.

Comparison of four facial muscles, orbicularis oculi, corrugator supercilii, masseter or mylohyoid, as best predictor of good conditions for intubation

Monitoring of facial muscles after neuromuscular blockade can give an early indication of respiratory muscle readiness for tracheal intubation. To assess which facial muscle, the orbicularis oculi, corrugator supercilii, masseter or the mylohyoid, is the best predictor of readiness for intubation after rocuronium. Prospective, randomised, blinded trial. Single centre: Seoul, Korea, from August 2012 to November 2012. Two hundred and eighty-eight patients aged 22 to 64 years were randomised to one of eight study groups: orbicularis oculi, corrugator supercilii, masseter and mylohyoid for rocuronium 0.6 or 1.2 mg kg. The maximum twitch depression at the eyelid (orbicularis oculi), the superciliary arch (corrugator supercilii), the cheek (masseter) and the submental triangle (mylohyoid) was assessed after rocuronium 0.6 and 1.2 mg kg. Endotracheal intubation was performed after maximal neuromuscular blockade, and intubating conditions were appraised. The onset time of rocuronium and the quality of the intubation conditions were assessed. The onset times in the orbicularis oculi, corrugator supercilii and masseter were significantly faster than that in the mylohyoid (P < 0.001). 'Clinically acceptable' intubation conditions were significantly enhanced in the mylohyoid (94%) compared with those in the orbicularis oculi (80%) and masseter (78%) after rocuronium 0.6 mg kg (P < 0.05), and no difference with corrugator supercilii (92%). Despite differences in onset time of orbicularis oculi and masseter compared to mylohyoid (P < 0.05), intubating conditions were similar among the four muscles after rocuronium 1.2 mg kg. Following rocuronium 0.6 mg kg at similar depths of anaesthesia, the monitoring of the corrugator supercilii provided the best balance of a shorter onset time while maintaining 'clinically acceptable' intubation conditions. IRB File No.: HYUH 2012-07-009.

Calabadion

To evaluate whether calabadion 1, an acyclic member of the Cucurbit[n]uril family of molecular containers, reverses benzylisoquinoline and steroidal neuromuscular-blocking agent effects. A total of 60 rats were anesthetized, tracheotomized, and instrumented with IV and arterial catheters. Rocuronium (3.5 mg/kg) or cisatracurium (0.6 mg/kg) was administered and neuromuscular transmission quantified by acceleromyography. Calabadion 1 at 30, 60, and 90 mg/kg (for rocuronium) or 90, 120, and 150 mg/kg (for cisatracurium), or neostigmine/glycopyrrolate at 0.06/0.012 mg/kg were administered at maximum twitch depression, and renal calabadion 1 elimination was determined by using a H NMR assay. The authors also measured heart rate, arterial blood gas parameters, and arterial blood pressure. After the administration of rocuronium, resumption of spontaneous breathing and recovery of train-of-four ratio to 0.9 were accelerated from 12.3 ± 1.1 and 16.2 ± 3.3 min with placebo to 4.6 ± 1.8 min with neostigmine/glycopyrrolate to 15 ± 8 and 84 ± 33 s with calabadion 1 (90 mg/kg), respectively. After the administration of cisatracurium, recovery of breathing and train-of-four ratio of 0.9 were accelerated from 8.7 ± 2.8 and 9.9 ± 1.7 min with placebo to 2.8 ± 0.8 and 7.6 ± 2.1 min with neostigmine/glycopyrrolate to 47 ± 13 and 87 ± 16 s with calabadion 1 (150 mg/kg), respectively. Calabadion 1 did not affect heart rate, mean arterial blood pressure, pH, carbon dioxide pressure, and oxygen tension. More than 90% of the IV administered calabadion 1 appeared in the urine within 1 h. Calabadion 1 is a new drug for rapid and complete reversal of the effects of steroidal and benzylisoquinoline neuromuscular-blocking agents.

Train-of-Four and Tetanic Fade Are Not Always a Prejunctional Phenomenon as Evaluated by Toxins Having Highly Specific Pre- and Postjunctional Actions

Nerve-stimulated fade in muscle is generally accepted as a prejunctional phenomenon mediated by block of prejunctional acetylcholine receptors (AChRs) at the nerve terminal, whereas decrease of twitch tension is considered a postjunctional effect due to block of muscle AChRs. Using ligands with specific pre- or postjunctional effects only, we tested the hypothesis that fade is not necessarily a prejunctional phenomenon. Neuromuscular function in rats was evaluated after IM (2.5 U) or IV (12.0 U) injection of botulinum toxin (Botx), or IV (250 μg/kg) α-bungarotoxin (α-BTX) alone. The acute neuromuscular effects of IV 2 mg/kg dihydro-β-erythroidine (DHβE), alone and in combination with α-BTX, were also tested. Botx decreases vesicular release of ACh, and α-BTX binds to postjunctional nicotinic AChRs only, whereas DHβE binds specifically to prejunctional α3β2 AChRs only. In view of the lack of acute effects of Botx even at 2 hours after IV injection, its neuromuscular effects were also evaluated at 24 hours after IM injection (0.6 U) and compared with IM injection of α-BTX (25 μg/kg) or saline also given 24 hours earlier. The sciatic nerve-tibialis muscle preparation, during train-of-four and tetanic stimulation, was used to test neuromuscular effects in vivo. IV and IM Botx had no observable neuromuscular effects at 2 hours. IV α-BTX caused twitch depression within a few minutes, and significant fade (P = 0.002) at 75% of baseline twitch tension; these effects persisted until the end of the observation period of 2 hours. IV DHβE alone caused no significant change in single twitch (P = 0.899) or train-of-four ratio (P = 0.394), but significantly enhanced the fade of IV α-BTX (P = 0.001 at 75% of baseline twitch tension). IM Botx or α-BTX, at 24 hours after their injection, resulted in a significant decrease of single twitch and tetanic tensions (P < 0.0001), but Botx did not cause fade, whereas α-BTX caused significant (P < 0.0001) fade at 24 hours. The tibialis muscle weights and protein expression of α1 subunit of AChR (Western blots) did not differ between Botx, α-BTX and saline-injected groups at 24 hours but increased in denervated muscle (positive control). Botx-induced decreased ACh release in and of itself does not cause fade but does cause decrease of absolute tensions. Decrease of available (functional) postjunctional AChRs by α-BTX did induce fade. The prejunctional fade effects of DHβE on α3β2 AChRs become manifest only when the margin of safety was decreased by concomitant administration of α-BTX. Thus, fade during repetitive stimulation is not always a prejunctional phenomenon and may also reflect the decreased margin of safety of neurotransmission, which can be due to a pure postjunctional AChRs block or to a combination of both pre- and postjunctional AChRs block. Block of prejunctional α3β2 AChRs alone is not necessary and sufficient to cause fade.

Simulation of the Kinetics of Neuromuscular Block

The onset time for paralysis varies 3-fold among nondepolarizing muscle relaxants. Possible explanations include: (a) pharmacokinetic differences among drugs and (b) buffering of drug molecules by acetylcholine receptors as they diffuse into the neuromuscular junction. Although some pharmacokinetic models consider buffered diffusion, these models do not account for either the high density of receptors or synapse geometry. Here, I used computer simulations to calculate the kinetics of buffered diffusion. The goal was to determine the conditions under which buffered diffusion could account for differences in onset time among nondepolarizing muscle relaxants. Monte Carlo simulation was used along with a realistic 3-dimensional model of the rat neuromuscular junction. Simulations determined the time dependence of the number of drug-bound receptors. A 1000-fold range of drug potency was examined. In some simulations, the drug concentration outside the junction was changed instantaneously. In other simulations, the concentration changed according to predictions of pharmacokinetic models assuming time-dependent changes in plasma drug concentration. The rate constant for equilibration of drug between plasma and muscle, keo, was varied between 0.15 and 0.6 min(-1). Twitch amplitude was calculated from receptor occupancy assuming a high safety margin for neuromuscular transmission. Some simulations used a synaptic model with an increased nerve-muscle contact width. Simulations with instantaneous changes in drug concentration at the synapse, indicated that the time to 50% twitch depression (onset time) was 0.1 to 30 seconds and was proportional to drug potency. This corresponds to iontophoretic application of drug to isolated neuromuscular junctions, but is too fast to explain onset times in humans. When pharmacokinetic models were used to calculate the drug concentration outside the synapse, buffered diffusion increased onset times of potent drugs (drugs for which the effective concentration at 50% twitch height is <600 nM). Simulations using keo = 0.6 min(-1) and a model with a 2- to 3-fold wider nerve-muscle contact width indicated that buffered diffusion could account for the differences in clinical onset times among the nondepolarizing muscle relaxants. Monte Carlo simulation provides a biophysically appropriate way to incorporate buffered diffusion into pharmacokinetic modeling. The simulations indicated that buffered diffusion could account for differences in onset time among drugs. However, a better understanding of the geometry of the human neuromuscular junction is needed before the magnitude of the effect of buffered diffusion can be quantified.

The characteristics of the staircase phenomenon during the period of twitch stabilization in infants in TOF mode

Acceleromyography used to monitor the neuromuscular transmission function is available in infants and children. However, information on the so-called staircase phenomenon during the baseline stabilization period in this population is limited. Our objective was to assess the characteristics of such phenomenon in infants in acceleromyography. Thirty infants were divided into three groups: group A (aged 1-5months, n=10), group B (aged 6-11months, n=10), and group C (aged 12-24months, n=10). Anesthesia was induced and maintained without neuromuscular blocking agents. Patients received a supramaximal stimulus followed by TOF measurements every 15s over the course of 30min with a TOF-Watch(®) SX. All data were collected in a notebook computer using the TOF-Watch(®) SX Monitor software through optical fibers. The evoked T1 responses in younger infants (group A) were significantly lower than those in older infants (groups B and C) from 0-20min. In group A, the signals increased to the maximum value of 121±15% that of the control at 7.25min and decreased toward a plateau of 113±18% at 30min. In group B, the signal heights increased to the maximum value of 143±14% at 9.5min, then decreased slowly to 136±10% at 19.5min, finally reaching 116±12% at 30min. In group C, the mean values of T1 reached the maximum of 139±19% that of control at 9.5min and finally reached 126±22% of control at 30min. The staircase effect presents in a shorter time course and at lower degrees in smaller infants. However, in older infants, staircase effect still presents in a long period and may influence the onset time and duration of twitch depression after muscle relaxants administration.

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

ObjectiveTo evaluate the potency and duration of three subparalyzing doses of vecuronium (VEC) in isoflurane-anesthetized horses. Study designProspective experimental study. AnimalsThirteen healthy adult horses undergoing arthroscopic surgery. MethodsDuring 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. ResultsVecuronium 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. Conclusions and clinical relevanceA 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.

The sensitivity of sheep to vecuronium: an example of the limitations of extrapolation

To the Editor, In the absence of species-specific data and despite the obvious pitfalls, it is common practice for investigators to extrapolate pharmacokinetic and pharmacodynamic information between species. Although sheep are used frequently as a model for research projects, there is a paucity of pharmacological information for many drugs in this species. Our empiric observations suggest that the potency of vecuronium in sheep may be much greater than that in humans. This study measures the potency of vecuronium in sheep under general anesthesia. Following Cornell University Institutional Animal Care and Use Committee approval, fifteen healthy adult female sheep (52–69 kg) were anesthetized for experimental stifle surgery, the results of which are to be published elsewhere. After sedation with intravenous midazolam and morphine, anesthesia was induced with propofol and maintained with isoflurane in oxygen. Neuromuscular monitoring was carried out with acceleromyography (TOF-Watch SX, Organon, Dublin, Ireland), similarly to a previous description in the dog. Train-of-four stimulation was applied to the ulnar nerve every 15 sec (60 mA, 0.2 msec), with the acceleration-sensitive crystal taped to the manus. After baseline stabilization and calibration, a dose of vecuronium (4, 6, or 8 lg kg, randomly allocated) was administered and T1 depression was measured. The averages of three consecutive values at baseline and at maximal depression were used for analysis. The effective doses for 50% and 95% T1 depression (ED50 and ED95) were calculated by the log-dose/logit method as described previously. The mean ED50 (standard deviation [SD]) and mean ED95 (SD) were 3.5 (1.2) lg kg and 13.2 (3.8) lg kg, respectively. The slope of the log-dose/logit linear regression was 2.22. Twitch depression after vecuronium and onset times are presented in the Table. Considering that the ED50 and ED95 of vecuronium in humans are approximately 30 and 45 lg kg, respectively, sheep are substantially more sensitive than humans. Although morphological differences between muscles have been identified to account for unequal responses to vecuronium within a species, further study is needed amongst species to identify the mechanisms responsible for the differences in sensitivity to a given neuromuscular blocking agent (NMBA). There are limitations to this study. First, the study involved a modest number of animals. A recent investigation suggests that at least 24 subjects should be recruited. Second, vecuronium 6 lg kg and 8 lg kg had similar effects (Table). This could be explained by direct muscle stimulation in the presence of complete neuromuscular block; however, this is unlikely considering the pulse duration used (0.2 msec). Furthermore, in pilot studies using this preparation, larger doses ([ 12 lg kg) abolished twitch height completely. It is also possible that the normal variation between subjects combined with a small sample size resulted in the effects of these two doses being indistinguishable. Of importance, the slope for the log-dose/logit relation in these sheep was only 2.22. In humans, the slopes for the log-dose/logit curves of a number of NMBAs approximate 4.5. It is not known whether this is a real difference between species or an artifact created by the small sample size. This question could be answered by collecting data M. Martin-Flores, MV (&) M. D. Pare, DVM L. Campoy, LV R. D. Gleed, BVSc College of Veterinary Medicine, Cornell University, Ithaca, NY, USA e-mail: mm459@cornell.edu