Failure of critically ill patients to metabolise midazolam

The pharmacokinetics of midazolam and its metabolite, 1-OH midazolam, were studied in six critically ill patients during and after a continuous intravenous infusion of midazolam. Four patients had an increased elimination half-life of midazolam; two were associated with a reduction in plasma clearance with low plasma concentrations of the metabolite, and two with normal metabolite levels and increased volume of distribution. The two patients with reduced clearance suffered from septic shock and were studied over a longer period. Their altered clearance was due to a reduced capability to form the 1-OH metabolite. As their condition improved, plasma concentrations of 1-OH midazolam increased and midazolam clearance returned towards normal. The impaired ability of critically ill patients with septic shock to metabolise midazolam, may be due to reduced organ perfusion and may lead to cumulation of midazolam in these patients.

[Continuous Midazolam infusion for sedation of respirator patients]

Most intensive care (ICU) patients need pharmacological sedation during ventilatory support. The short-acting drug midazolam might be preferable to neuroleptic agents and opiates because of its anxiolytic and sedative properties. The dosage of a drug given in a continuous infusion is based upon knowledge of its clearance and of the function of concentration and effect.

METHOD

A midazolam infusion (7.5 or 15 mg/h) was given to 16 patients receiving ventilatory support for 24 h. Clearance was estimated using the rule Cl = R/C (R = rate of infusion, C = concentration in steady state). To estimate the concentration of midazolam necessary for good sedation, the amount of supplementary injected neuroleptic during midazolam infusion was compared with that of the day before. Plasma cortisol and parameters of energy metabolism, electrolytes, and liver and kidney function were measured.

RESULTS

Patients with disease of abdominal organs showed the lowest values for clearance (1.0-2.92 ml/min/kg). Some of the injured patients showed elevated values for clearance (3.0-21.36 ml/min/kg). Elimination half-life ranged from 1.5-50 h. Changes in intestinal perfusion or cardiac output might be responsible for the wide range. For good sedation, plasma midazolam concentrations had to be above 600 ng/ml. Plasma cortisol levels were not affected by midazolam, nor were metabolism, electrolytes, or liver and kidney function.

CONCLUSION

Continuous infusion of midazolam is a useful way of sedation for ICU patients, but the wide range of clearance values must be considered.

Midazolam sedation following open heart surgery

Midazolam given as hourly intermittent injections was compared with the same dose given by infusion for postoperative sedation in patients after cardiopulmonary bypass. A stable concentration was rapidly attained with the infusion whereas 6-8 h was required to attain stable plasma (trough) concentrations in the intermittent injection group. Plasma concentrations decreased rapidly to low values within 6 h of discontinuation of therapy. High plasma concentrations and a long (16 h) half-life were noted in one patient who may be a slow metabolizer of the drug.

Pharmacokinetics of midazolam in total i.v. anaesthesia

The pharmacokinetics of midazolam during total i.v. anaesthesia were determined in 15 female patients undergoing major surgery. Midazolam was administered together with an analgesic drug and a neuromuscular blocking drug. The dose regimen of midazolam, based on simulations, included two consecutive infusions following a bolus injection at the induction of anaesthesia. Multiple blood samples were drawn and a two-compartment open model was fitted to the measured plasma concentrations. Plasma clearance (483 ml min-1), apparent volume of distribution (1.94 litre kg-1) and terminal half-life (3.1 h) were in agreement with other reports. Thus, there was no obvious evidence that the surgical procedure or the concomitant use of other drugs, or both, influenced the pharmacokinetics of midazolam. The relatively rapid elimination makes midazolam suitable for use by infusion in a total i.v. technique.

Cimetidine and ranitidine increase midazolam bioavailability

Cimetidine has been shown to inhibit the oxidative metabolism of a variety of low- and high-extraction drugs. Despite the findings of initial investigators, there is evidence that ranitidine may exert similar effects. Eight healthy volunteer subjects took part in a within-subject crossover study. They received midazolam, 15 mg, by mouth after pretreatment with cimetidine, ranitidine, or nothing and midazolam, 10 mg, intravenously on separate occasions. Mean absolute bioavailability of midazolam was increased by more than 30% after cimetidine (P less than 0.01) and 26% after ranitidine (P less than 0.05). The data, which agree with a concurrent clinical study indicating greater hypnotic action of midazolam after ranitidine, indicate that this is not a result of enhanced midazolam absorption and that reduced hepatic clearance is the most likely explanation.