Initially, phenol was injected into a victim's vein, maximizing the medical aura of the entire procedure...Before long, the technique was changed to injecting the phenol directly into the heart. Some witnesses thought that the change was made because the veins were sometimes hard to locate, but the real reason seems to have been the greater killing efficiency of a direct cardiac injection. Patients injected by vein might linger for minutes or even an hour or more...The "concentrated aqueous solution of phenol" that was developed proved "inexpensive, easy to use, and absolutely effective when introduced into the heart ventricle", so that an injection of ten to fifteen millileters into the heart caused death within fifteen seconds.
Phenol injections were given in Block 20:
At that point two Jewish prisoner assistants brought a victim into the room (sometimes victims were brought in two at a time) and positioned him or her on a footstool, usually so that the right arm covered the victim's eyes and the left arm was raised sideways in a horizontal position....The idea was for the victim's chest to be thrust out so that the cardiac area was maximally accessible for the lethal injection, and for him or her to be unable to see what was happening....The person giving the injection--most often the SDG Josef Klehr--filled his syringe from the bottle and then thrust the needle directly into the heart of the seated prisoner and emptied the contents of the syringe.
Thus, an average of two minutes and 22 seconds sufficed to murder one prisoner.