This page compares a single IV bolus of thiopental to a continuous IV infusion.

Consider a patient given 300 mg of thiopental to induce anesthesia (along with a paralyzing agent), who is rapidly intubated, and started on halothane gas to maintain anesthesia. If the halothane gas was not started, how long before the patient "wakes up"? For the purpose of this case, we can assume that thiopental is adequately modeled using a two compartment pharmacokinetic model (i.e. linear pharmacokinetics with a distribution phase) and that unconsciousness is induced at a plasma concentration > 5 mg/L, although in reality the situation is slightly more complex. The final (total) volume of distribution for thiopental in this patient is 175 L and the elimination clearance is 7 L/hour. Upon IV administration, thiopental immediately distributes into an initial volume that is 20% of the final volume, which subsequently distributes into the total volume of distribution with a half-life of ~0.7 hours.

The below example uses the continuous IV infusion with distribution phase (Model 2) with a 300 mg loading dose and both the infusion rate and infusion interval set to zero. What you should observe is that the patient's drug concentration shoots above 5 mg/L instantaneously, inducing unconsciousness, and subsequently rapidly decreases during distribution phase. After ~30 minutes, the level has fallen below 5 mg/L and the patient will wake up. However, note that nearly all the administered drug is still in the patient's body and will be slowly eliminated with a half-life of around 20 hours. Thus, thiopental is considered to be a short acting agent due to its rapid distribution phase; it is actually eliminated much more slowly.

Compare this to a steady state infusion of thiopental at 60 mg/hour. The below example simply reduces the loading dose to zero and increases the infusion rate to 60 with a 120 hour infusion interval. Infusions like this are done in the ICU to reduce intracranial pressure. The [drug] at steady state is around 8.6 mg/L, which is the same as the peak [drug] in the above example of a single IV bolus to induce anesthesia. However, notice that when the infusion is stopped, around 12 hours are now required for the [drug] to fall below 5 mg/L. During the prolonged infusion, thiopental "saturated" the peripheral compartment, raising its concentration to 8.6 mg/L as well. Hence, after terminating the infusion, the decline in drug concentration becomes dependent on the elimination half-life.