Pharmacodynamic behaviour of vecuronium in primary hyperparathyroidism

Effects of anesthetics, sedatives, and opioids on ventilatory control

This article provides a comprehensive, up to date summary of the effects of volatile, gaseous, and intravenous anesthetics and opioid agonists on ventilatory control. Emphasis is placed on data from human studies. Further mechanistic insights are provided by in vivo and in vitro data from other mammalian species. The focus is on the effects of clinically relevant agonist concentrations and studies using pharmacological, that is, supraclinical agonist concentrations are de-emphasized or excluded.

[Effects of changes in ion concentrations, osmolarity, and pH on recovery from atracurium- or vecuronium-induced neuromuscular blockade]

OBJECTIVES

To determine changes in sodium, potassium, calcium, magnesium and chloride ion concentrations in blood, osmolarity, and pH during surgery, and to assess the influence of such changes on atracurium- or vecuronium-induced neuromuscular blockade under inhaled or intravenous anesthesia.

MATERIAL AND METHODS

Prospective study randomizing 119 ASA 1-2 patients; 52.1% of the patients received atracurium (26.8%, with isoflurane; 25.2%, with propofol) and 47.9% received vecuronium (23.5%, with isoflurane; 24.3%, with propofol). The neuromuscular blockade was confirmed by electromyography of the adductor pollicis muscle (stimuli delivered to the cubital nerve). Two venous blood samples were extracted to measure ureic nitrogen, creatinine, glucose, ion concentrations (sodium, chloride, potassium, calcium, and magnesium), and osmolarity. Arterial blood gases and pH were also assessed. The first blood sample was extracted on inserting the venous catheter and the second on recovery of 25% of the first train-of-four twitch.

RESULTS

The onset and duration of action for equipotent doses of atracurium and vecuronium were similar. Likewise, recovery was also similar. Plasma chloride ion and glucose levels tended to rise during surgery, while sodium, potassium and magnesium ion concentrations fell. Both total and effective plasma osmolarities also decreased. Fluid replacement therapy during surgery was at least partly responsible for these changes. Low calcium and magnesium concentrations and alkalosis prolonged some phases of atracurium recovery, while low sodium levels shortened the duration of some atracurium-induced blockade recovery phases. The effect of changes in chloride ion concentrations on recovery was variable. High chloride, low calcium, and especially low sodium ion concentrations shortened some phases of the vecuronium-induced blockade, while alkalosis prolonged its duration.

CONCLUSIONS

Changes in electrolyte concentrations and pH as a result of standard fluid replacement therapy are moderate, well tolerated, and do not exercise a strong effect on the behavior of the neuromuscular blockade.

Association between primary hyperparathyroidism and increased body weight: a meta-analysis

Although primary hyperparathyroidism is frequently asymptomatic, it has been associated with an increased prevalence of hypertension, insulin resistance, dyslipidemia, cardiovascular mortality, and cancer. Previously we reported that patients with primary hyperparathyroidism are heavier than age-matched controls. Increased body weight could contribute to the association between primary hyperparathyroidism and these extraskeletal complications. We searched MEDLINE for English language studies published between 1975 and 2003 that reported body weight or body mass index in subjects with primary hyperparathyroidism and a healthy age- and sex-comparable eucalcemic control group. Seventeen eligible studies were identified. Subjects with primary hyperparathyroidism were 3.34 kg (95% confidence interval, 1.97-4.71; P < 0.00001) heavier than controls in 13 studies reporting body weight. In four studies reporting body mass index, subjects with primary hyperparathyroidism had an increased body mass index of 1.13 kg/m(2) (-0.29 to 2.55; P = 0.12) compared with controls. Standard mean difference analysis showed that subjects with primary hyperparathyroidism had an increased weight or body mass index of 0.3 sd (0.19-0.40; P < 0.00001) compared with controls. We conclude that patients with primary hyperparathyroidism are heavier than their eucalcemic peers, and that increased body weight may contribute to the reported associations between primary hyperparathyroidism and some extraskeletal complications.

Assessment and management of patients with abnormal calcium

Calcium is essential for homeostasis, and normocalcemia should be maintained strictly during the perioperative period. The biochemistry of calcium equilibrium results from the calcium-sensing receptors on the parathyroid glands, which detect changes in calcium concentrations and initiate the proper response. Asymptomatic hypercalcemia is a common metabolic derangement that is often discovered on routine serum screening. The most common etiologies are primary hyperparathyroidism and cancer. Increasingly, parathyroidectomy is the preferred therapy for primary hyperparathyroidism. Severe hypercalcemia ("hypercalcemic crisis") should be managed aggressively with a combination of intravenous fluids, steroids, bisphosphonates, and calcitonin. Some of these patients may require an urgent parathyroidectomy for calcium control. Hypocalcemia needs to be verified, as many cases of hypocalcemia are the artifact of hypoalbuminemia. Severe hypocalcemia occurs after subtotal or total parathyroidectomy with auto transplantation as well as after massive resuscitation or blood transfusion. Strategies aimed at correcting calcium concentrations depend on the severity of symptomatology. If symptoms are mild, oral calcium supplementation can be given; otherwise, intravenous calcium should be administered.

Reduced duration of muscle relaxation with rocuronium in a normocalcemic hyperparathyroid patient

PURPOSE

To report a case of reduced duration of action of rocuronium in a patient with normocalcemic hyperparathyroidism (HPT).

CLINICAL FEATURES

A 56-yr-old patient with primary HPT, who had had surgical resection of three and a half parathyroid glands nine months previously, was referred to our institution for further investigation of a persistent increase in parathyroid hormone. Preoperatively, the patient had a normal serum ionized and total calcium. The patient was diagnosed with a persistent parathyroid adenoma and was scheduled for an elective parathyroidectomy. General anesthesia was induced with iv propofol, fentanyl and succinylcholine. Intraoperatively, anesthesia was maintained with nitrous oxide in oxygen, and isoflurane. Neuromuscular blockade was attained using incremental doses of rocuronium. The average duration of 0.15 mg x kg(-1) incremental doses of rocuronium was 5.9 min (expected: 13-18 min), and that of 0.2 mg x kg(-1) was ten minutes (expected: 19-23 min).

CONCLUSION

Primary HPT even in the absence of hypercalcemia may result in resistance to competitive blockade by rocuronium. It suggests that primary HPT may cause acetylcholine receptor up-regulation resulting in hyposensitivity to non-depolarizing muscle relaxants.

Population pharmacodynamics: strategies for concentration-and effect-controlled clinical trials

OBJECTIVE

To explore and evaluate various strategies for drug concentration-and effect-controlled clinical trials, respectively, in the context of studies of population pharmacodynamics (concentration-effect relationships).

METHODS

The relative utility of drug concentration- and pharmacologic effect-controlled, randomized clinical trials with two or three concentration-effect measurements for each subject has been explored by computer simulation. The basis for these simulations was a sigmoid-Emax (maximum effect) pharmacodynamic model with Emax = 100%, EC50 (drug concentrations required to produce an effective intensity of 50%) = 10 concentration units, gamma = 2, and no hysteresis. Emax and gamma were held constant whereas EC50 was assumed to be log-normally distributed with a 26% coefficient of variation of the natural lognormalized data. A smaller random variability and variability due to measurement error also were incorporated in the simulations. To explore the implications of variable and unknown Emax and gamma values, the suitability of linear and log-linear interpolation procedures for two-point concentration-effect data in different regions of the sigmoid-Emax curve was compared.

RESULTS

Pharmacologic effect-controlled clinical trials with 300 hypothetical subjects and targeted effect intensities of 25% and 75% yielded very good estimates of drug concentrations required to produce effect intensities of 35%, 50%, and 65%, whereas concentration-controlled trials yielded much poorer estimates. Moreover, the concentration-controlled trials, despite optimum choice of targeted concentrations, yielded a large number of data points with poor information content (effect intensities of < 15% or > 85%). Determinations based on targeted effect intensities of 25% and 75% yielded better estimates of individual EC50 values than those targeted for 25% and 50% or 50% and 75% effect intensity. Results were not significantly improved by adding a third measurement (targeted to 50% effect) to the 25% and 75% effect design. Estimations of drug concentrations required to produce an effect intensity of 50%, based on log-linear interpolation of exact concentration-effect data at 25% and 75%, yielded exact results independent of gamma value (0.5-8.0) whereas linear interpolation produced large overestimates at gamma = 0.5 or 1.0 but satisfactory estimates at gamma > or = 2.0. Similar calculations for an effect intensity of 15% based on exact concentration-effect data at 5% and 25% yielded reasonably good estimates by both methods of interpolation over a wide range of gamma values. A review of the clinical literature showed that gamma values are usually 2 or higher.

CONCLUSIONS

Population pharmacodynamic studies of reversibly acting drugs without pharmacodynamic hysteresis or time dependency (e.g., tolerance) can be successfully conducted using a pharmacologic effect-controlled randomized clinical trial design with only two properly selected target effect intensities per subject.