It is difficult to understand how an overdose could happen if there was a IV Pump controller in use. The Pump controller could deliver a lethal dose if the setting was wrong, but, if this was the sleep inducer of choice it should have had an defined amount known to be programmed into the electronics of the machine.
Odd, unless there was a malfunction, which happens on a rare occasion. They need to be serviced from time to time, so the most minor of malfunction usually results in the pumps being removed from patient care areas to maintenance.
