Perioperative Pharmacologic Considerations in Obesity

Obesity has increased in incidence worldwide. Along with the increased number of obese patients, comorbid conditions are also more prevalent in this population. Obesity leads to changes in the physiology of patients along with an altered response to pharmacologic therapy. Vigilant perioperative physicians must be aware of the unique characteristics of administered agents in order to appropriately provide anesthetic care for obese patients. Because of the variability in tissue content in obese patients and changes in pharmacokinetic modeling, a one-size-fits-all approach is not justified and a more sophisticated and prudent approach is indicated.

Pharmacologic Considerations of Anesthetic Agents in Pediatric Patients: A Comprehensive Review

Acute pain in the pediatric population has important differences in terms of biology, intrapopulation variation, and epidemiology. Discussion as to the pharmacologic considerations of anesthetic agents, such as induction agents, neuromuscular blockers, opioids, local anesthetics, and adjuvant agents, is presented in this article. Special considerations and concerns, such as risk for propofol infusion syndrome and adverse potential side effects of anesthesia agents, are discussed. Anesthesiologists managing pediatric patients need to have a firm understanding of physiologic and pharmacologic differences compared with the adult population. Future studies to improve the understanding of pharmacokinetics in the pediatric population are needed.

[When muscle relaxants have unexpected prolonged duration during anaesthesia]

Muscle relaxation facilitates tracheal intubation and improves surgical conditions during anaesthesia. However, unexpected prolonged muscle relaxation may occur. This article describes important causes of prolonged muscle relaxation and gives suggestions for its prevention. Drug interactions, incomplete reversal, co-morbidity, inaccurate neuromuscular monitoring and critical illness may prolong the effect of muscle relaxants. The anaesthetist must titrate the muscle relaxants using objective neuromuscular monitoring and proper reversal of the blockade when needed.

[Reversion of sedation and general anaesthesia--agonist-antagonist technique]

OBJECTIVE

To study an expediency and efficacy of application of different reverses drugs (naloxone, flumazenil, neostigmine, galantamine, sugammadex) either their separate or combined using.

METHODS

We studied 119 patients underwent endoluminal endoscopic procedures and surgeries on trachea-bronchial tree and intestines under sedation or general anaesthesia.

RESULTS

The article deals with conceptual approaches to the reversal of residual effects of opioids, benzodiazepine sedation and neuromuscular block (the so-called agonist-antagonist technique).

CONCLUSIONS

A reversion of neuromuscular block without using of antagonists' combination does not provide complete recovery of psychomotor and cognitive functions for rapid socialization of patients after anaesthesia.

Comparative pharmacodynamics of pancuronium, cisatracurium, and CW002 in rabbits

Pancuronium is a long-duration neuromuscular blocking drug (NMBD) that has been used in anesthetized rabbits at 0.1 mg/kg. However, there are limited data regarding the time course for recovery from this dose either spontaneously or with pharmacologic reversal. Here we defined the potency, onset, and recovery characteristics for the intermediate-duration NMBD cisatracurium and CW002 (a novel cysteine-inactivated molecule) in the rabbit, and test the hypothesis that these drugs may be alternatives to 0.1 mg/kg pancuronium for survival procedures. New Zealand white rabbits anesthetized with isoflurane were studied in a cross-over design. Potencies of cisatracurium and CW002 were defined as the effective dose for 95% depression of evoked muscle twitch (ED95). Responses to 3×ED95 were used to define onset (time to maximal effect), recovery index (RI; time from 25% to 75% recovery of twitch), and duration (time to complete recovery). Responses to all drugs were determined with and without reversal by neostigmine-glycopyrrolate or L-cysteine. CW002 was 4-fold more potent than was cisatracurium, but their onset, RI, and duration were similar. Pancuronium had similar onset and RI but longer duration, compared with cisatracurium and CW002. Reversal shortened the recovery index and duration for all 3 drugs. At 3×ED95, cisatracurium and CW002 had the same onset as did standard-dose pancuronium, but durations were shorter and more predictable. In addition, CW002 can be reversed without the potential side effects of cholinergic manipulation. We conclude that cisatracurium and CW002 are viable alternatives to pancuronium for survival studies in rabbits.

Influence of acute normovolemic hemodilution on the pharmacokinetics of Cisatracurium Besylate

BACKGROUND

Acute normovolemic hemodilution (ANH) is an efficacious blood conservation strategy for avoiding or reducing allogeneic blood transfusion. In a previous publication, on a different cohort of patients, we demonstrated that cisatracurium's potency and duration of action were not influenced by ANH, but we could not establish which role, if any, pharmacokinetics played.

METHODS

Forty patients were randomly allocated to the ANH or control groups. Patients received cisatracurium single 100 µg kg-1 bolus dose, serial arterial blood samples were collected and assayed for pharmacokinetic analysis.

RESULTS

Central and steady state apparent volumes of distribution (V1, Vdss) and slope factor (γ) were larger, effect-compartment concentration at 50% neuromuscular block was lower in the ANH (90.8±41.6 mL kg-1, 159.1±39.2 mL kg-1, 6.0±0.9 and 136.4±29.1 ng·mL-1) compared with the control group (65.5±26.1 mL kg-1, 134.8±31.8 mL kg-1, 5.5±0.8 and 158.5±26.0 ng·mL-1) respectively. Elimination half-life (t1/2 β) and mean residence time (MRT) were longer in the ANH (37.2±20.9, 23.5±13.2 min) than the control group (26.8±9.8, 16.9±6.2 min), albeit not statistically significant (P=0.051, P=0.051). There were no significant differences in distribution half-life (t1/2 α), effect-compartment equilibration rate-constant (keo), central and total clearances (Clc, Cl) between the ANH (2.4±1.2 min, 0.070±0.013 min-1, 6.1±1.9 mL kg-1 min-1 and 7.7±2.3 mL kg-1 min-1) and control group (1.9±1.2 min, 0.063±0.008 min-1, 7.0±1.8 mL kg-1 min-1 and 8.5±2.1 mL kg-1 min-1) respectively.

CONCLUSION

ANH altered some pharmacokinetic parameters such as significantly larger volumes of distribution. Other parameters such as elimination half-life were considerably longer albeit not statistically significant.

Myths and facts in neuromuscular pharmacology. New developments in reversing neuromuscular blockade

Pharmacologic reversal of neuromuscular blockade is a topic not very well acknowledged and controversially discussed. Reasons for this are numerous and include missing perception of the potential complications of residual neuromuscular paralysis including an increased morbidity and mortality, as well as low efficacy and numerous unwanted side effects of the available antagonists. Duration of action of muscle relaxants cannot be pharmacologically predicted. Objective neuromuscular monitoring is the only way to detect residual paralysis. This review article would like to discuss in its first part some of the myths which revolve around the use of muscle relaxants, then highlight the problems regarding the use of acetylcholine esterase inhibitors and, in the third part, discuss the steroidal muscle relaxant encapsulator sugammadex.

[New developments in neuromuscular blocking agents]

Curare was clinically used in a patient with acute appendicitis by Griffith and Johnson in January 1942. From the day on, the development of new muscle relaxants went on in the world. Since pancuronium was synthesized by Savage in 1964, it was widely used in clinical cases because it provided sufficient muscle relaxation during operation. At present, pancuronium, vecuronium and rocuronium are routinely used in the world. In the future, we expect development of new muscle relaxants with rapid onset, intermediate duration of action, rapid recovery and without side effects.

Functional recovery after neuromuscular blockade in mechanically ventilated critically ill patients

BACKGROUND

An estimated 24% to 70% of individuals have prolonged paralysis or severe weakness after receiving neuromuscular blocking agents (NMBAs) when therapy is terminated.

OBJECTIVES

The purposes of this study were to (1) evaluate the relationship between recovery of neuromuscular transmission (NMT) and functional muscle activity after NMBA administration; (2) evaluate the relationship between delayed recovery of NMT or muscle activity and functional performance; and (3) determine the predictors of delayed recovery of NMT, muscle activity, and functional performance.

METHODS

This was a multisite study using a prospective, nonexperimental, descriptive design with convenience sampling techniques. Instruments used included a five-point muscle score, Actigraph, and peripheral nerve stimulator.

RESULTS

Key findings were as follows: (1) NMT returned promptly, whereas muscle activity remained severely depressed; (2) only two subjects (5%) recovered functional performance within 24 hours; (3) degree of muscle weakness immediately after neuromuscular blockade was associated with prolonged time to extubation and mobility; and (4) predictors of delayed recovery included cumulative dose of aminosteroid NMBAs, age, and renal function.

CONCLUSION

Prolonged recovery of muscle activity and extreme weakness may occur despite brisk recovery of NMT after neuromuscular blockade.

Randomised controlled trial comparing cisatracurium and vecuronium infusions in a paediatric intensive care unit

OBJECTIVE

To evaluate and compare the efficacy, infusion rate and recovery profile of vecuronium and cisatracurium continuous infusion in critically ill children requiring mechanical ventilation.

DESIGN AND SETTING

Prospective, randomised, double-blind, single-centre study in critically ill children in a paediatric intensive care unit in a tertiary children's hospital.

METHODS

Thirty-seven children from 3 months to 16 years old (median 4.1 year) were randomised to receive either drug; those already receiving more than 6 h of neuromuscular blocking drugs were excluded. The Train-of-Four (TOF) Watch maintained neuromuscular blockade to at least one twitch in the TOF response. Recovery time was measured from cessation of infusion until spontaneous TOF ratio recovery of 70%.

RESULTS

The cisatracurium infusion rate in nineteen children averaged 3.9+/-1.3 microg kg(-1) min(-1) with a median duration of 63 h (IQR 23-88). The vecuronium infusion rate in 18 children averaged mean 2.6+/-1.3 microg kg(-1) min(-1) with a median duration of 40 h (IQR 27-72). Median time to recovery was significantly shorter with cisatracurium (52 min, 35-73) than with vecuronium (123 min, 80-480). Prolonged recovery of neuromuscular function (>24 h) occurred in one child (6%) on vecuronium.

CONCLUSIONS

Recovery of neuromuscular function after discontinuation of neuromuscular blocking drug infusion in children is significantly faster with cisatracurium than vecuronium. Neuromuscular monitoring was not sufficient to eliminate prolonged recovery in children on vecuronium infusions.

Development of ultra short-acting muscle relaxant agents: History, research strategies, and challenges

Author has reviewed the literature and his own work related to the chemistry, pharmacology, and clinical aspects of new muscle relaxants. Emphasis has been placed on the basic science concepts and technologies (e.g. structure-activity relationships, nicotinic receptor pharmacology, and investigation of side effects) behind the development of rapidly and short acting nondepolarizing muscle relaxants.

Physicochemical properties of neuromuscular blocking agents and their impact on the pharmacokinetic-pharmacodynamic relationship

BACKGROUND

Among the factors influencing the onset of action of neuromuscular blocking agents (NMBA), the potency (EC50) and the rate of equilibration between blood and the effect compartment (k(e0)) have been highlighted. Although these descriptors are intrinsically influenced by the physicochemical characteristics of the drug, the impact of lipid solubility, molecular weight and protein binding on pharmacokinetic-pharmacodynamic (PK-PD) descriptors has not been established for most NMBA.

METHODS

The octanol/phosphate buffer distribution coefficients (logD) of various NMBA (vecuronium, rocuronium, mivacurium isomers (cis-cis, cis-trans and trans-trans), doxacurium, cisatracurium, atracurium, succinylcholine) were determined. The free fraction for each drug was measured using an ultrafiltration technique. PK-PD descriptors were obtained from selected clinical studies. Correlations between physicochemical parameters (including molecular weight) and PK-PD descriptors were assessed by linear or multiple linear regression.

RESULTS

A wide range of log D (-4.15 for succinylcholine to 0.75 for vecuronium) and free fraction (from 31% for vecuronium to 80% for succinylcholine) is observed for NMBA. Molecular weight combined with either lipid solubility (r2=0.70; P=0.001) or free fraction (r2=0.84; P<0.001) were highly correlated with potency, while for k(e0) a greater degree of correlation was obtained when both lipid solubility and free fraction (r2=0.74; P=0.002) were included.

CONCLUSIONS

The basic characteristics of NMBAs, namely, molecular weight, lipid solubility and protein binding, are strongly associated with the kinetics of the drug response.

[Anesthesia with propofol, remifentanil and cisatracurium in renal transplantation]

A 41-year-old woman with end-stage renal insufficiency in peritoneal dialysis for 3 years received a kidney transplant under anesthesia with remifentanil, propofol, and cisatracurium. She had a history of hypertension and was being treated with enalapril, metoprolol and erythropoietin. After anesthetic induction, blood pressure fell significantly and surgery was performed in a context of hemodynamic stability. The postoperative course was good, with a functional graft and adequate diuresis from the start. Anesthetics with minimal residual effects and as little renal toxicity as possible are ideal for use in kidney transplantation. The drugs used in this case had pharmacokinetic and pharmacodynamic properties that make them particularly appropriate for such patients.

Selecting neuromuscular-blocking drugs for elderly patients

The physiological changes that occur with increasing age can have significant effects on the pharmacokinetics of neuromuscular-blocking drugs. Changes in cardiac output can affect drug distribution and therefore the speed of onset of neuromuscular block. A decrease in muscle mass and increase in body fat with age can also affect their distribution. The deterioration in renal and hepatic function associated with aging affects the clearance and elimination of many neuromuscular-blocking drugs. The effects of these physiological changes on the pharmacokinetics of neuromuscular-blocking agents may not become apparent clinically in healthy individuals until the age of at least 75 years. There is very little evidence to suggest any alteration in the sensitivity of the neuromuscular junction to neuromuscular-blocking drugs with increasing age. Neuromuscular-blocking drugs that undergo a significant degree of organ-dependent elimination, such as pancuronium bromide, vecuronium bromide, rocuronium bromide and doxacurium chloride, may have a significantly prolonged duration of action in elderly patients. These drugs can be used safely in elderly patients if the anaesthetist is aware of their altered pharmacokinetics in this patient group. Appropriate changes must be made to drug dosage and dose intervals. As the pharmacokinetic changes can be unpredictable, monitoring of neuromuscular block is strongly advised when using these drugs in such patients. The risk of residual block occurring postoperatively after the use of pancuronium bromide increases with age. The duration of action of mivacurium chloride may also be prolonged in the elderly; this change has not been demonstrated to be a result of an alteration in plasma cholinesterase activity. In contrast, there is no evidence of an alteration in the action of suxamethonium chloride (succinylcholine chloride) with increasing age. Atracurium besilate and cisatracurium besilate undergo predominantly organ-independent elimination. Onset of block with these two drugs may be prolonged in the elderly, but their clinical duration of action does not alter significantly with age, making them particularly suitable for use in this patient group. Although atracurium besilate may cause histamine release, there is little evidence of it producing haemodynamic changes in the elderly. Its (1R,1R')-isomer, cisatracurium besilate, has very little direct or indirect cardiovascular effect and is, therefore, the most suitable nondepolarising agent to use in elderly patients.

A pharmacokinetic-pharmacodynamic model for neuromuscular blocking agents to predict train-of-four twitches

The train-of-four (TOF) stimulation pattern consists of 4 stimuli (T1, T2, T3, and T4) at 2 Hz, and is used in daily anesthesiological practice to determine the degree of relaxation caused by muscle relaxants. At a surgical levels of relaxation the degree of relaxation can be estimated by counting the number of "measurable" or "visible" muscular reactions to the 4 stimuli in the TOF stimulation pattern (TOF count). During recovery relaxation can be estimated by calculating the TOF ratio (T4/T1). Bartkowski and Epstein described a pharmacokinetic-pharmacodynamic (PK-PD) model to predict TOF ratio by modifying and extending the PK-PD model as described by Sheiner to use a hypothetical distributed effect compartment described by a median equilibration rate constant and a dispersion parameter. We extended the Bartkowski and Epstein PK-PD model to simulate all four TOF twitches by including EC50 terms for T2 and T3. We fit this model to data from the pig and compared the results to fitted models using separate PD models for each TOF twitch (extended Sheiner model). The extended Bartkowski and Epstein model fit the twitch height data from all four TOF twitches better than the extended Sheiner model and has fewer parameters.

Synthesis and structure-activity relationships of neuromuscular blocking agents

The first use of neuromuscular blocking agents (muscle relaxants) in clinical practice (1942) revolutionised the practice of anaesthesia and started the modern era of surgery. Since 1942 introduction of tubocurarine (18) neuromuscular blocking agents have been used routinely to provide skeletal muscle relaxation during surgical procedures allowing access to body cavities without hindrance from voluntary or reflex muscle movement. After the introduction of tubocurarine and the depolarizing suxamethonium chloride (4) (1949) several nondepolarizing steroidal and nonsteroidal neuromuscular blocking agents with different onset time and duration of effect were introduced e.g. gallamine triethiodide (1) (1949), methocurine (2) (1949), alcuronium chloride (3) (1963), pancuronium bromide (9) (1968), vecuronium bromide (11) (1982), pipecuronium bromide (10) (1982), atracurium besylate (5) (1982), doxacurium chloride (6) (1991), mivacurium chloride (8) (1992), rocuronium bromide (12) (1994) cisatracurium besylate (7) (1996), and rapacuronium bromide (13) (2000). SZ 1677 (14) a steroid type nondepolarizing neuromuscular blocking agent under development (preclinical phase). This review article deals with a comprehensive survey of the progress in chemical, pharmacological and, in some respects, of clinical studies of neuromuscular blocking agents used in the clinical practice and under development, including the synthesis, structure elucidation, pharmacological actions, structure activity relationships studies of steroidal and nonsteroidal derivatives.

Interactions of neuromuscular blocking drugs

Many drugs interact with neuromuscular blocking drugs and often enhance the induced block; this is of clinical importance for volatile anaesthetics, antimicrobials, magnesium and some more specific drugs. Difficulty in reversing the block occurs with calcium-channel blockers and polymyxin. Phenytoin, carbamazepine and other anticonvulsants may cause resistance to neuromuscular blocking drugs. Moreover, clinically important interactions are found between individual neuromuscular blockers. Giving succinylcholine after a non-depolarizing neuromuscular blocking drug prolongs the onset of succinylcholine; when non-depolarizing drugs are administered after succinylcholine their effects are prolonged. The succinylcholine block is prolonged when the drug is administered during recovery from pancuronium or following neostigmine reversal. Drugs or diseases that decrease the activity of plasma cholinesterase may prolong a succinylcholine-induced block. Finally, liver dysfunction, renal failure, disturbances of acid-base balance, change in temperature and neurological diseases all have an effect on the profile of the neuromuscular blocking drugs; the response to an induced block may be altered in patients under intensive care and those with cancer. Although knowledge of the most important theoretical interactions of neuromuscular blocking drugs is favourable, the anaesthetist should be aware that pharmacological interactions can lead to an unpredictable induced neuromuscular block in many cases in daily clinical practice. Therefore anaesthetists should become familiar with the use of neuromuscular transmission monitoring in order to manage the block correctly.