Effects of hypothermic cardiopulmonary bypass on the pharmacodynamics and pharmacokinetics of rocuronium

Comparison of the neuromuscular effects of two infusion rates of rocuronium in anesthetized pigs

BACKGROUND

Neuromuscular blocking agents are frequently administered to pigs used for research. In humans, administration of the drugs is not without risk and may result in accidental awareness under general anaesthesia and postoperative residual neuromuscular blockade that can lead to serious respiratory complications. Despite the extensive administration, the pharmacodynamics of neuromuscular blocking agents are not thoroughly studied in pigs. Therefore, this study investigates the neuromuscular response of two infusion rates of rocuronium, a commonly used non-depolarizing neuromuscular blocking agent. A group of 14 female Danish Landrace-Yorkshire-Duroc pigs used for supervised surgical training, weighing 40.3 ± 2.1 kg (mean ± SD), were included in the study. They received a loading dose of 0.85 mg/kg rocuronium intravenously followed by infusion of either 2.5 mg/kg/hour (L, low dose) or 5 mg/kg/hour (H, high dose) rocuronium for 30 min. Neuromuscular monitoring was performed with acceleromyography using train-of-four (TOF) stimulation. Onset time, time to reappearance of T1, T4, TOF ratio 90% and 100% were recorded.

RESULTS

All pigs in group H experienced loss of T1 throughout rocuronium infusion, whereas six out of seven pigs in group L had reappearance of T1 during rocuronium infusion, with additional reappearance of T4 in three of these pigs. The time to recovery of TOF ratio 90% was 14.0 ± 5.4 (L) and 21.7 ± 6.1 (H) minutes and recovery to TOF ratio 100% was 18.7 ± 6.5 (L) and 27.9 ± 9.2 min (H) (mean ± SD). Substantial inter-animal variation in neuromuscular recovery time was observed.

CONCLUSION

The large inter-animal variation in pharmacodynamic profiles emphasizes that individual neuromuscular monitoring and titration to effect should be used routinely in research protocols that include rocuronium. In addition to other important measures, these actions are key in order to avoid overdosing and limit the risk of residual neuromuscular blockade.

Neuromuscular blockade management in the critically Ill patient

Neuromuscular blocking agents (NMBAs) can be an effective modality to address challenges that arise daily in the intensive care unit (ICU). These medications are often used to optimize mechanical ventilation, facilitate endotracheal intubation, stop overt shivering during therapeutic hypothermia following cardiac arrest, and may have a role in the management of life-threatening conditions such as elevated intracranial pressure and status asthmaticus (when deep sedation fails or is not tolerated). However, current NMBA use has decreased during the last decade due to concerns of potential adverse effects such as venous thrombosis, patient awareness during paralysis, development of critical illness myopathy, autonomic interactions, and even residual paralysis following cessation of NMBA use.

It is therefore essential for clinicians to be familiar with evidence-based practices regarding appropriate NMBA use in order to select appropriate indications for their use and avoid complications. We believe that selecting the right NMBA, administering concomitant sedation and analgesic therapy, and using appropriate monitoring techniques mitigate these risks for critically ill patients. Therefore, we review the indications of NMBA use in the critical care setting and discuss the most appropriate use of NMBAs in the intensive care setting based on their structure, mechanism of action, side effects, and recognized clinical indications. Lastly, we highlight the available pharmacologic antagonists, strategies for sedation, newer neuromuscular monitoring techniques, and potential complications related to the use of NMBAs in the ICU setting.

Effect of cardiopulmonary bypass on cytochrome P450 enzyme activity: implications for pharmacotherapy

For patients undergoing cardiopulmonary bypass (CPB) during cardiac surgery, there are well-documented changes in the pharmacokinetics (PK) of commonly administered drugs. Although multiple factors potentially underpin these changes, there has been scant research attention on the impact of CPB to alter the activities of cytochrome P450 (CYP) isoenzymes. PK changes during cardiac surgery with CPB have the potential to adversely affect the safety and efficacy of pharmacotherapy and increase the risk of drug-drug interactions. Clinically significant changes in drug PK during CPB are likely to be prominent for drugs where CYP metabolism is a major clearance (CL) mechanism. However, clinical data from patients undergoing CPB surgery in support of this hypothesis are lacking, leaving a significant knowledge gap. In this review, we address the effects of CPB on the release of pro-inflammatory cytokines, in surgeries with and without CPB, both pre and post initiation of surgery. We reviewed literature to explore the relationship between the release of pro-inflammatory cytokines, and the expression and activities of CYP enzymes. Through this approach, we provide new insight on the effects of CPB on the PK of drugs administered to patients in the clinical setting. Future research to address this knowledge gap will have considerable impact to assist clinicians with optimizing pharmacotherapy in this patient population.

The effects of age, isoflurane and sevoflurane on atracurium in lambs

OBJECTIVE

To determine the effects of age, sevoflurane and isoflurane on atracurium-induced neuromuscular blockade in 3-16 week-old lambs.

STUDY DESIGN

Prospective randomized experimental trial.

ANIMALS

Twenty-six Scottish blackface ewe-lambs were anaesthetized for spinal surgery when either 3-6 (mean age 4.6 weeks; n = 18) or 12-16 weeks (mean age 13.7 weeks; n = 15) of age; seven animals were anaesthetized at both ages.

METHODS

After intramuscular injection of medetomidine (10 μg kg(-1)) anaesthesia was induced in the younger lambs either with isoflurane or sevoflurane in oxygen delivered by mask, and in the older lambs with ketamine (4 mg kg(-1)), and midazolam (0.2 mg kg(-1) ) administered intravenously (IV). In both groups anaesthesia was maintained with fixed end-tidal concentrations of either sevoflurane (2.8%) or isoflurane (1.8%) delivered in oxygen. Before surgery meloxicam (0.6 mg kg(-1)), morphine (0.5 mg kg(-1)) and ketamine (1 mg kg(-1) followed by 10 μg kg(-1) minute(-1) ) were administered IV. The lungs were ventilated mechanically to maintain normocapnia. Neuromuscular block was achieved with a loading dose (LD) of atracurium (0.5 mg kg(-1) IV). The peroneal nerve was stimulated (train-of-four every 12 seconds). Evoked responses in the digital extensor muscles were evaluated by palpation and observation. Maintenance doses (MD) of atracurium (0.17 mg kg(-1) IV) were administered when the first twitch (T1) returned. The onset and duration of LD action (T1 absent) and the duration of MD were recorded. Data were analysed using Student's t test, Mann-Whitney U test, repeated-measures anova, Wilcoxon's matched pairs test or Pearson correlation coefficient as relevant (p < 0.05).

RESULTS

Onset of LD action developed significantly (p < 0.05) more rapidly in isoflurane compared with sevoflurane-anaesthetized lambs (55 ± 18 cf. 80 ± 37 seconds). Duration of action of LDs and MDs was longer (p < 0.05) in lambs aged 12-16 than 3-6 weeks (33 ± 5.4 cf. 25 ± 6.4 and 26 ± 4.2 cf. 18 ± 5.5 minutes) but were independent of the anaesthetic used.

CONCLUSIONS AND CLINICAL RELEVANCE

The effect of atracurium is age-dependent in lambs being prolonged in older animals. The onset of neuromuscular blockade is more rapid in isoflurane compared with sevoflurane-anaesthetized lambs.

Cisatracurium pharmacokinetics and pharmacodynamics during hypothermic cardiopulmonary bypass in infants and children

BACKGROUND

Hypothermia potentiates neuromuscular blockade in adults during cardiopulmonary bypass (CPB) but the pediatric literature is sparse. Temperature-dependent Hoffman degradation of cisatracurium may allow reduction in infusion rate (IR) during hypothermia. The effect of hypothermic CPB on the pharmacokinetics (PK) and pharmacodynamics (PD) of cisatracurium has not been described in children.

METHODS AND MATERIALS

Using neuromuscular monitoring with a Datex Relaxograph, cisatracurium IR was adjusted to obtain a pseudo-steady state during each phase of surgery (pre-CPB, CPB, post-CPB). Paired samples were taken at each phase. Cisatracurium plasma concentrations (Cpss) were determined by HPLC. Core and skin temperatures were recorded.

RESULTS

Data from ten infants were analyzed: Group 1: mean 33.6°C; Group 2: mean 21.9°C. To maintain T1% between 5% and 10% in Group 2, the IR was decreased by a mean of 89% (P < 0.001). IR was not significantly different in Group 1. Post-CPB IR approximated pre-CPB rates in both groups. During CPB, Cpss fell by 27% in Group 1 and by 50% in Group 2 (P = 0.039). Post-CPB Cpss was not significantly different to pre-CPB in either group. Clearance did not change significantly in Group 1 but fell significantly in Group 2 during CPB (P = 0.002). Clearance post-CPB was unchanged from pre-CPB.

CONCLUSIONS

Cisatracurium IR may be decreased by around 60% during CPB with moderate hypothermia but can be maintained at baseline during mild hypothermia.

Perioperative complications of hypothermia

Perioperative hypothermia is a common and serious complication of anesthesia and surgery and is associated with many adverse perioperative outcomes. It prolongs the duration of action of inhaled and intravenous anesthetics as well as the duration of action of neuromuscular drugs. Mild core hypothermia increases thermal discomfort, and is associated with delayed post anaesthetic recovery. Mild hypothermia significantly increases perioperative blood loss and augments allogeneic transfusion requirement. Only 1.9 degrees C core hypothermia triples the incidence of surgical wound infection following colon resection and increases the duration of hospitalization by 20%. Hypothermia adversely affects antibody- and cell-mediated immune defences, as well as the oxygen availability in the peripheral wound tissues. Furthermore mild hypothermia triples the incidence of postoperative adverse myocardial events. Thus, even mild hypothermia contributes significantly to patient care costs and needs to be avoided.

Consequences of inadvertent perioperative hypothermia

Perioperative hypothermia triples the incidence of adverse myocardial outcomes in high-risk patients. Mild hypothermia significantly increases blood loss and augments allogeneic transfusion requirement, but the molecular pathophysiology of this effect remains to be elucidated. Only 1.9 degrees C core hypothermia triples the incidence of surgical wound infection following colon resection and increases the duration of hospitalization by 20%. Hypothermia adversely affects antibody- and cell-mediated immune defences, as well as the oxygen availability in the peripheral wound tissues. Mild perioperative hypothermia changes the kinetics and action of various anaesthetic and paralysing agents, increases thermal discomfort, and is associated with delayed post-anaesthetic recovery. Finally, mild core hypothermia influences pulse oximetry monitoring and various electrophysiological indices of the nervous system, with questionable clinical significance, as yet.

Vecuronium pharmacokinetics and pharmacodynamics during hypothermic cardiopulmonary bypass in infants and children

PURPOSE

To determine the effect of moderate and deep hypothermic cardiopulmonary bypass (CPB) on the pharmacokinetic and pharmacodynamic behaviour of vecuronium in infants and children.

METHODS

We studied 12 patients undergoing surgery for congenital heart disease under narcotic-nitrous oxide anesthesia. Neuromuscular blockade was maintained constant (TI 4-10% by Datex electromyograph) by adjusting a vecuronium infusion. Plasma vecuronium concentrations (Cpss) were analysed by HPLC to describe a pseudosteady-state during each of the pre-CPB, CPB and post-CPB phases. Paired arterial blood samples were taken 20 min apart after at least 20 min of constant infusion.

RESULTS

Nine cases were analysed, mean age 20 mo, mean weight 9 kg. Three patients had deep and six moderate hypothermia. In the pre-CPB phase Cpss fell into two groups (mean +/- SD: 330 +/- 42 ng x ml(-1); 127 +/- 27 ng x ml(-1); P < 0.001); similarly the clearances showed a bimodal distribution (mean +/- SD: 5.08 +/- 0.94; 11.51 +/- 0.2 ml x min(-1) x kg(-1) P < 0.001), although in different patients. During CPB this bimodal distribution disappeared. Vecuronium infusion rate (VIR) decreased by 84% and 92% from pre-CPB to CPB phase in deep and moderate hypothermia groups respectively (P < 0.05), paralleled by decreases in Cpss of 36% (P > 0.05) and 52% (P < 0.05).

CONCLUSION

Changes in vecuronium requirements and plasma concentrations during CPB demonstrate that vecuronium pharmacokinetics and pharmacodynamics are both affected by hypothermic CPB in infants. The finding of bimodal distributions for plasma vecuronium and vecuronium clearance highlights the need for individual monitoring of neuromuscular blockade in this age group.

The pharmacokinetics of anesthetic drugs and adjuvants during cardiopulmonary bypass

The institution of cardiopulmonary bypass during cardiac surgery has profound effects on the plasma concentration of drugs and thus their therapeutic effectiveness. These changes occur through acute hemodilution, altered plasma protein binding, hypotension, as well as the use of hypothermia and heparin administration. Isolation of the lungs from the circulation and the possible sequestration of drugs in the bypass circuit also affect drug plasma concentrations on bypass. The individual characteristics of the drug in question are also important in determining the final plasma concentration: Lipid soluble drugs with a high volume of distribution may be more readily taken up by bypass equipment, but the initial fall in concentration at the start of cardiopulmonary bypass may be more readily counteracted by back diffusion into plasma, if large tissue stores have accumulated. The extent of the drug's plasma protein binding is of importance as the effective free fraction in plasma for highly bound drugs will be sensitive to changes in plasma protein binding brought on by factors such as hemodilution, heparin administration as well as alpha, acid-glycoprotein binding. Clearly the fate of drugs administered before or on bypass is complex and can only be accurately determined by specific studies evaluating drug plasma concentrations. This review updates the available data on anesthetics and drugs used during cardiac surgery in order that anesthetists may predict better the likely effect of drugs administered before or during cardiopulmonary bypass.

Residual neuromuscular blockade after cardiac surgery: pancuronium vs rocuronium

PURPOSE

To determine the incidence of residual neuromuscular blockade after cardiac surgery in patients receiving either rocuronium or pancuronium for muscle relaxation.

METHODS

In a prospective, controlled, double-blind study, 20 patients undergoing coronary artery bypass were randomized to receive either rocuronium (n = 10) or pancuronium (n = 10) during surgery. Anaesthesia was induced with sufentanil, benzodiazepine and propofol or ketamine, and maintained with air/O2/sufentanil/isoflurane. Neuromuscular blockade was induced with 0.1 ml.kg-1 from blinded syringes containing recuronium (6 mg.ml-1) (Group R) or pancuronium (1 mg.ml-1) (Group P). Relaxants were administered according to clinical criteria and reversal agents were not given. After surgery, neuromuscular transmission was assessed by train-of-four stimulation of the ulnar nerve/adductor pollicis EMG (Datex Relaxograph). Mean values from three trains of stimuli were recorded and repeated 30 min later if TOF ratio was < 0.7. Time to extubation was recorded.

RESULTS

On arrival in the ICU, nine of 10 patients in group R but only three of 10 patients in group P demonstrated four visible responses (P < 0.05). Mean TOF ratio in group P, 0.03 +/- 0.05, was less than in group R, 0.68 +/- 0.34 (P < 0.001). All patients in group P and 4 of 10 patients in group R had TOF ratio < 0.7 (P = 0.01). Time to extubation in group P (median 18, range 6-48 hr) was not statistically different from that in group R (14, 5-44 hr).

CONCLUSION

Residual neuromuscular block, TOF ratio < 0.7, is common after cardiac surgery but the incidence is less when pancuronium is replaced by rocuronium.