A 25 year old man was brought to the emergency room with a broken leg. His medic

Question

A 25 year old man was brought to the emergency room with a broken leg. His medical history was unremarkable, although he reveals that his mother had died of some “complication” following routine surgery. The patient was taken to surgery for repair of his fractured left distal tibia. He was premedicated with, among other drugs, succinylcholine (a muscle relaxant) and was placed on a ventilator. It was noted that vigorous muscle fasciculations (twitches) began almost immediately. Nevertheless, general anesthesia was begun with halothane.

1. Succinylcholine binds to the ACh receptors at the motor end plate. Explain the mechanism by which it could produce muscle relaxation and why muscle twitches could occur after the administration of succinylcholine. Muscle twitches could occur after administration of succinylocholine

2. There was an immediate tachycardia, and it was noted by the surgeon that the patient became “as stiff as a board” and the patient’s core temperature increased significantly. What could account for these two symptoms?

3. The halothane anesthesia was terminated and the patient was cooled with ice. Dantrolene was administered intravenously and oxygen was given. Dantrolene inhibits the release of Ca++ from the SR. Explain how this would reverse the patient’s hyperthermia.

Explanation / Answer

1. Succinylcholine binds to acetylcholine receptors at the motor end plate. This causes a disorganized depolarization of the motor end plate and calcium is released. As succinylcholine is not degraded by acetylcholinesterase, it stays for a longer time when compared to acetylcholine. By maintaining the membrane potential above threshold, it does not allow the repolarization of the membrane. Calcium is then taken up by the sarcoplasmic reticulum. This causes the relaxation of the muscle. Muscle twitches after the administration of succinylcholine is a side-effect of the drug. It is caused by the depolarizing phase of the drug. It can be prevented by giving a small dose of a nondepolarizing muscle blocker prior to the administration of succinylcholine. ----------------------------- 2. The state of the patient becoming as stiff as a board is due to the paralysis of muscles by the drug. The drug may facilitate the release of histamines which causes tachycardia. The increase in core temperature is a rare reaction common to anaesthetic drugs like halothane and paralytic agents like succinylcholine.  These drugs cause an uncontrolled increase in skeletal muscle oxidative metabolism which overwhelms the body's capacity to regulate temperature and supply oxygen. -------------------------- 3. By reducing intracellular concentrations of calcium and by inhibiting the release of calcium ions from the sarcoplasmic reticulum, the drug dantrolene inhibits the contraction of the muscles. By removing the aggravating drugs and giving dantrolene we can prevent the death of a person. When calcium ions are inhibited, it prevents the release of the vesicles containing the neurotransmitter. In this way it prevents the signal from reaching the muscle. This ensures that the muscles do not contract but relax. The oxygen is given as the body is overwhelmed by a sudden increase in metabolism. By removing the halothane, and injection of dantrolene, the drug-induced hyperthermia can be reversed.

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