Muscle Relaxants

Muscle paralysis is a necessary component of cardiac anesthesia. There are currently nine relaxants which are nondepolarizers (i.e., they block acethycholine release at the neuromuscular junction) and one which is a depolarizer (i.e., depolarizes the neuromuscular junction).

Several nondepolarizers are used for cardiac anesthesia. Many have systemic effects which could be either detrimental or helpful for patients undergoing open heart surgery. The older nondepolarizers include tubocurarine, metocurine, and pancuronium. These relaxants are long acting. Tubocurarine and metocurine block autonomic ganglia and cause histamine release. Pancuronium blocks parasympa-thetic receptors and has some sympathogonic activity, actions which may be useful in patients who present with significant bradycardia due to beta or calcium channel blockers.

Newer nondepolarizers have been developed which have little or no hemodynamic effects. Atracurium and vecuronium have an intermediate duration of action.

Atracurium is degraded through the disruption of this chemical structure (Hofman elimination pathway) and ester hydrolysis in plasma, which makes this relaxant useful in patients with kidney or liver failure. However, atracurium does induce histamine release. Cisatracurium is an isomer of atracurium. It appears to cause less histamine release than does atracurium.

Vecuronium has no hemodynamic effects, but it is metabolized by the liver and is excreted by the kidney and bile.

Mivacurium is a short acting nondepolarizer with virtually no hemodynamic effects. It is useful for induction of anesthesia.

Rocuronium is a long acting nondepolarizer which, at large doses , has a rapid onset of action. At a dose of 1.2 mg/kg Rocuronium can provide paralysis sufficient for intubation in less than 90 seconds.

The depolarizer, succinylcholine, is very short acting and is generally used only in emergency situations where the airway must be immediately secured by intu bation or in cases where preintubation ventilation during induction of anesthesia is very difficult because of an abnormal airway or chest wall rigidity due to large doses of fentanyl or sufenta. Muscle fasciculations occur after injection which reflect the depolarizing activity of succinylcholine. Postoperative muscle pain may result from fasciculation. Administration of a small dose of a nondepolarizer prior to the administration of succinylcholine may prevent these fasciculations.

Succinylcholine stimulates parasympathetic receptors and autonomic ganglia and can therefore cause arrhythmias. It may also induce histamine release. Other complications due to succinylcholine include hyperkalemia and malignant hy-perthermia in susceptible patients.

Table 4.1. Cardiac Drugs—Antiarrhytmic Drug Therapy

Class I A

Mechanism of Action Blocks sodium channels Blocks different potassium channels

Blocks sodium channels Indirectly blocks opening of the calcium channel by blunting sympathetic activation Blocks potassium channels Ibutilide

Directly blocks calcium channels

Common Drugs Used in the OR Procainamide

Lidocaine

Beta blockers (Esmolol)

Amiodarone, Bretyllium,

Verapamil, Diltiazam, Nicardipine

Amiodarone is now the first line drug for the treatment of recurrent and destabilizing ventricular tachycardia. It lowers the threshold for defibrillation (lidocaine increases this threshold). Like any drug that blocks any part of the potassium channel (and thus prolongs the refractory period), amiodarone can cause atypical ventricular tachycardia (torsade de pointe).

Adenosine is the treatment of choice for supraventricular tachycardia. It binds to purinergic receptors which interact with the potassium channels, slowing conduction time through the AV node. It also interrupts reentry pathways through the AV node. Adenosine is taken up by erythrocytes and vascular endothelial cells. Its activity is potentiated by nucleoside transport inhibitors such as dipyrdamole and inhibited by adenosine receptor antagonists such as the methylxanthines.

Cardioversion is the treatment of choice for atrial fibrillation. Ibutilide is an alternative pharmacological treatment for atrial fibrillation; however, torsade de pointe is a frequent complication of Ilbutilide.

Automatic Implantable Cardioverter Defibrillators (AICD) are now commonly implanted in patients with unsustained, spontaneous ventricular tachycardia with left ejection fractions of between 20 and 40%.

Table 4.2. Cardiac Drugs—Antiarrhytmic Drug Dosages

Adenosine 6mg IVP, 12 mg IVP in 1-2 min.: Peds 50|ig/kg IVP

Amiodarone Loading dose: 150 mg over 10 minutes; then 360 mg over the next 6 hours; then 540 mg over the next 18 hours

Bretylium Loading 5-10mg over 8-10 min, maint. 5-10mg q6h

Ibutilide .5-1mg infusion over 10 min (.005-.01mg/kg if patient weighs Less than 60 kg Lidocaine Loading 1-1.5mg/kg, maint. .1mg/kg/hr

Procainamide 1 gram over 30 minutes; maint. 1-6 mg/min: Peds 3-6mg/

kg over 5 minutes (max 100mg/dose) Verapamil 5-10mg over 2-3 minutes, may repeat dose 15-30 min.

Peds. .1-.2mg/kg over 2 minutes

Table 4.3. Cardiac Drugs—Inotropic Drugs

Sympathomimetic Drugs

Dobutamine (Dobutrex) 250mg/250ml(1000|ig/ml) 2-20|ig/kg/min,

Dopamine (Intropin) 400mg/250ml(1600|ig/ml) 3-10|ig/kg/min,

Ephedrine 5-25mg IVP

Epinephrine 2mg/250ml(8|ig/ml) 1-8|ig/min,

1 |ig/min=7.5ml/hr Isoproterenol (Isuprel) 1mg/250ml(4|ig/ml).05-.1|ig/kg/min,

1|ig/min=15ml/hr Norepinephrine (Levophed) 8mg/100ml(80|i/ml).05-.3|i/kg/min

Phosphodiesterase Type III Inhibitors

Amnirone (Inocor) 100mg/250ml(400|ig/ml)1-1.5/kg loading,

Milrinone (Primacor) 100mg/500ml D5(200|ig/ml)50m/kg loading over

10 min., .375-.75|ig/kg/min maintenance The phosphodiesterase inhibitors should be used in chronic congestive heart failure or other conditions when the ß receptors are down regulated.

Other Inotropes

Calcium Chloride 10-20mg/kg, 250-1000mg IV

Calcium Gluconate 30-60mg/kg IV

Digoxin (Lanoxin) 1-1.25mg(.25-.5mg increments) or 10-15|ig/kg loading dose, .125-.25mg/day

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