Kinetics and dynamics of lorazepam during and after continuous intravenous infusion

Objective 

To evaluate the kinetics and dynamics of lorazepam during administration as a bolus plus an infusion, using electroencephalography as a pharmacodynamic end point.

Methods 

Nine volunteers received a 2-mg bolus loading dose of lorazepam, coincident with the start of a 2 μg/kg/hr zero-order infusion. The infusion was stopped after 4 hrs. Plasma lorazepam concentrations and electroencephalographic activity in the 13- to 30-Hz range were monitored for 24 hrs.

Results 

The bolus-plus-infusion scheme rapidly produced plasma lorazepam concentrations that were close to those predicted to be achieved at true steady state. Mean kinetic values for lorazepam were as follows: volume of distribution, 126 L; elimination half-life, 13.8 hrs; and clearance, 109 mL/min. Electroencephalographic effects were maximal 0.5 hr after the loading dose, were maintained essentially constant during infusion, and then declined in parallel with plasma concentrations after the infusion was terminated. There was no evidence of tolerance. Plots of pharmacodynamic electroencephalographic effect vs. plasma lorazepam concentration demonstrated counterclockwise hysteresis, consistent with an effect-site equilibration delay. This was incorporated into a kinetic-dynamic model in which hypothetical effect-site concentration was related to pharmacodynamic electroencephalographic effect via the sigmoid E_max model. The analysis yielded the following mean estimates: maximum electroencephalographic effect, 12.7% over baseline; 50% effective concentration, 13.1 ng/mL; and effect-site equilibration half-life, 8.8 mins.

Conclusion 

Despite the delay in effect onset, continuous infusion of lorazepam, preceded by a bolus loading dose, produces a relatively constant sedative effect on the central nervous system, which can be utilized in the context of critical care medicine.