Skeletal Muscle Relaxants
Drugs that affect skeletal muscle function include two different therapeutic groups: those used during surgical procedures and in the intensive care unit (ICU) to produce muscle paralysis (ie, neuromuscular blockers), and those used to reduce spasticity in a variety of painful conditions (ie, spasmolytics). Neuromuscular blocking drugs interfere with transmission at the neuromuscular end plate and lack central nervous system activity. These compounds are used primarily as adjuncts during general anesthesia to facilitate endotracheal intubation and optimize surgical condi-tions while ensuring adequate ventilation. Drugs in the spasmo-lytic group have traditionally been called “centrally acting” muscle relaxants and are used primarily to treat chronic back pain and painful fibromyalgic conditions. Dantrolene, a spasmolytic agent that has no significant central effects and is used primarily to treat a rare anesthetic-related complication, malignant hyperthermia.
30-year-old woman is rushed to the emergency depart-ment at a major trauma center after a motor vehicle acci-dent. Although in significant pain, she is awake, alert, and able to give a brief history. She states that she was the driver and was wearing a seatbelt. There were no passengers in the car. Her past medical history is significant only for asthma, for which she has been intubated once in the past. She has no allergies to medications. There are multiple lacerations on her face and extremities and a large open fracture of her right femur. An orthopedic surgeon has scheduled immedi-ate operative repair of the femur fracture, and the plastic surgeon wants to suture the facial lacerations at the same time. You decide to intubate the patient for the procedure. What muscle relaxant would you choose? Would you choose the same agent if she had experienced a 30% total body burn in a fire at the time of the accident? What if the past medical history included right-sided hemiparesis of 10 years’ duration?
Because of trauma and associated pain, it is assumed that gastric emptying will be significantly delayed. To avoid pos-sible aspiration at the time of intubation, a very rapid-acting muscle relaxant should be used so the airway can be secured with an endotracheal tube. Therefore, succinylcholine is the agent of choice in this case. Despite its side effects, succinyl-choline has the fastest mechanism of action of any currently available skeletal muscle relaxant. An alternative to succinyl-choline is high-dose (up to 1.2 mg/kg) rocuronium, a nonde-polarizing muscle relaxant. At this dose, rocuronium has a very rapid onset, which approaches but does not quite equal that of succinylcholine.Both burns and neurologic injuries result in the expression of extrajunctional acetylcholine receptors. In patients with recent burns, succinylcholine use can lead to life-threatening hyperkalemia. Although the drug would not result in danger-ous hyperkalemia if given immediately after a severe neuro-logic injury, in a patient with a chronic paralysis, its use may lead to hyperkalemia. Therefore, succinylcholine would also be contraindicated in a patient with long-standing hemiparesis.