7.7 : Depolarizing Blockers: Pharmocokinetics

Depolarizing blockers are administered through intravenous injection. Succinylcholine is the most common choice of depolarizing blockers in emergency clinical practices. Although they have a rapid onset, they readily diffuse away from the motor end plate into the extracellular fluid. They are metabolized by enzymes such as liver butyrylcholinesterase and plasma pseudocholinesterases. This produces a short duration of action, typically 5-10 minutes long, unlike nondepolarizing blockers, which usually persist for at least 20-30 minutes.

Plasma cholinesterase also hydrolyzes some amount of injected succinylcholine, so a tiny amount of the drug can cause blockage. Patient parameters, such as body weight and current physical condition, determine succinylcholine dosage. A higher drug dose that does not pose health risks is often favored to avoid complications arising from ineffective paralysis during a surgical procedure. In addition, the rate of succinylcholine metabolism varies among individuals depending on cholinesterase activity. The presence of an abnormal cholinesterase variant can prolong succinylcholine's action and cause risk to the patient's well-being.

Dibucaine, a pseudocholinesterase inhibitor, is used to test at-risk patients. Dibucaine reduces the activity of wild-type acetylcholinesterase by 80%. However, it can reduce the action of the abnormal variant only by 20%. The dibucaine number defines the percentage inhibition of the enzyme. A dibucaine number above 70 is described as normal, intermediate if it is between 40-70, and atypical if it is below 20. Patients with a dibucaine number below 20 will go into apnea, where respiratory muscles collapse, and the patients stop breathing or remain paralyzed for about 2 hours or longer. In such cases, succinylcholine administration for any surgical process must be avoided.