As an Anesthetist for far too many years, let me tell you why succinylcholine is such a perfect murder weapon.
The best poisons usually have three things in common: small effective dose, also called Median Lethal Dose (or LD50), ease of administration, and rapid and definitive action. The fourth characteristic, the difficulty in detection by a forensics team is a big premium that most poisons don’t posses. Most poisons, that is, except succinylcholine and maybe a few others. Forensic scientists please correct me if I’m wrong.
A quick review of pharmacology shows succinylcholine is a muscle relaxant. Anesthesiologists and Nurse Anesthetists (CRNA’s) call it “sux”. Sux is commonly used before intubations, as it completely relaxes patients’ muscles and facilitates intubation of the trachea with an endotracheal tube. Sux is a rapidly acting depolarizer that can be given intravenously (IV) or intramuscularly (IM). Once administered, succinylcholine circulates in the blood, reaches nicotinic receptors on the surface of muscle cells, and there it imitates the action of acetylcholine, a neurotransmitter that our nerves naturally release to make our muscles move. When succinylcholine is given, seconds later the patient fasciculates, and all muscles in his body become depolarized. In essence, sux makes every muscle twitch to the point that it becomes unresponsive to any subsequent stimulation: you can’t breathe, you can’t even blink.
Sux is highly effective. In IV form, 100 mg of sux will depolarize every muscle in the body of a 70kg man in about 30-45 seconds. And the patient will not be able to take another breath for at least 5 minutes. So without assisted ventilation, he is dead as quickly as three minutes. The IM dose of sux is not much different, but takes two to four times the IV dose and takes 3-5 minutes to begin working.
So there you have it: succinylcholine is an easy to inject poison, it is highly effective, and is guaranteed-to-work quickly to kill.
The fourth characteristic of succinylcholine is sux is almost impossible to detect because its metabolites are all naturally occurring molecules. It works this way. Most molecules of succinylcholine break down in blood into succinylmonocholine and choline, thanks to a circulating enzyme called pseudocholinesterase. The process is so efficient only a small fraction of sux molecules given actually reach neuromuscular junctions. Succinylmonocholine is then hydrolyzed into succinic acid, or succinate, a naturally occurring substance.
Succinate is famous because it is an important player in TCA (Krebs) cycle, a series of chemical reactions that powers all living cells that use oxygen.The entire human complex pinnacles on the oxygen molecule. Take it away and our body chokes up and dies.
Ruby Johnson, CRNA,Phd