The problems with injection.
One of the main problems with lethal injection is the aqueous pressure in the executee's veins. Veins have an internal pressure (blood pressure) which has to be overcome to allow injection into them. A doctor, when giving a normal intravenous (IV) injection, has to equal and then slightly exceed this pressure. If he produces a pressure that is too great he will rupture the vein. The doctor accomplishes this through his training, fingertip dexterity and experience built up from giving repeated injections. He feels the pressure in the vein against the top of the plunger of the syringe. In the case of a lethal injection the volume of the of fluid required to fill the plumbing of the IV tubes running between the executioner's cubicle and the prisoner means that the syringes must be of 50 - 60 cc capacity. Syringes of this size are too big to give any real feel to the person administering the injection. Thus too much pressure can easily be applied and a rupture of the vein may occur. This can happen in the case of giving a non lethal injection into the good healthy veins in a normal person.

It is not unusual for the condemned prisoner to be a former intravenous drug abuser, with delicate, collapsed veins that can stand far less overpressure. This is also true of people who are insulin injecting diabetics. When a person with normal veins is frightened their veins contract and become hard to find.
Before insertion of the catheters a good vein must be located. It is not unusual for an "cut-down procedure" to have to be performed to find a usable (not necessarily suitable) vein. This entails a minor surgical procedure on the arm, leg or groin carried out under local anaesthetic, using a sub-cutaneously injected dose of Xylocaine or similar local anaesthetic, administered by a medical technician. The public are seldom made aware of this and the witnesses would not typically see any evidence of it during the execution.

To produce a quick and painless death it is vital that the chemicals are injected into a vein, rather of an artery. Veins carry blood to the heart and arteries away from the heart. The path of the chemicals (particularly the potassium chloride) should be via the quickest route to the heart so as not to prolong the execution. If an artery is used by mistake, instead of a vein, the blood carrying the chemicals has to go the "long way round" significantly increasing the time taken to stop the heart. The distinction between a vein and an artery is not an easy one to make. Even a doctor can make a mistake! It is clear from reading reports of executions that the time between commencing the injection and certifying death varies a great deal.