Cardiac arrest is a sudden, life-threatening emergency where the heart stops beating effectively, ceasing blood flow to the brain and other vital organs. This halt can lead to rapid loss of consciousness, disability, or death if not addressed immediately. Prompt medical intervention is necessary to improve survival and minimize complications.
What Defines a Shockable Rhythm?
A “shockable rhythm” refers to specific abnormal electrical activities in the heart that can be corrected by an electrical shock, known as defibrillation. The two primary shockable rhythms are ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT). In ventricular fibrillation, the heart’s lower chambers, the ventricles, quiver chaotically instead of contracting to pump blood. This disorganized electrical activity prevents effective blood circulation.
Pulseless ventricular tachycardia occurs when the ventricles beat very rapidly, often exceeding 100 beats per minute. These contractions are too fast for the chambers to fill with blood, resulting in no discernible pulse and cessation of blood flow. These rhythms are “shockable” because defibrillation, which delivers a controlled electric current, can depolarize the heart muscle and reset its electrical system. This allows the heart’s natural pacemaker to re-establish a normal rhythm. Defibrillation is the immediate and primary treatment for these rhythms during cardiac arrest.
Primary Medications for Shockable Rhythms
When initial defibrillation attempts are unsuccessful in converting a shockable rhythm, specific medications are administered as part of the Advanced Cardiovascular Life Support (ACLS) protocol to support resuscitation efforts.
Epinephrine
Epinephrine, also known as adrenaline, is a commonly used medication in cardiac arrest. It primarily binds to alpha-1 adrenergic receptors in peripheral blood vessels, causing vasoconstriction. This increases aortic diastolic pressure and improves coronary and cerebral perfusion during cardiopulmonary resuscitation (CPR). Epinephrine also stimulates beta-1 receptors in the heart, potentially increasing heart rate and contractility. The standard adult dose is 1 milligram (mg) administered intravenously (IV) or intraosseously (IO) every 3 to 5 minutes, typically after initial unsuccessful defibrillation attempts.
Amiodarone
Amiodarone is an antiarrhythmic medication used for shock-refractory ventricular fibrillation or pulseless ventricular tachycardia, meaning these rhythms persist or recur after defibrillation. It works by blocking potassium ion channels, prolonging the myocardial cell action potential duration and refractory period. This stabilizes the heart’s electrical activity and reduces excitability, preventing rapid and irregular firing. Amiodarone also affects sodium channels, calcium channels, and alpha- and beta-adrenergic receptors. The initial adult dose is typically 300 mg IV or IO, with a second dose of 150 mg if the arrhythmia persists.
Lidocaine
Lidocaine is another antiarrhythmic medication, an alternative to amiodarone for shock-refractory ventricular fibrillation or pulseless ventricular tachycardia. It primarily blocks voltage- and pH-dependent sodium channels in the heart. By slowing sodium ion influx into injured myocardial cells, lidocaine raises the cells’ electrical stimulation threshold, suppressing automaticity and helping restore control to the heart’s natural pacemaker. The initial adult dose is usually 1 to 1.5 mg/kg IV or IO, with additional doses of 0.5 to 0.75 mg/kg every 5 to 10 minutes, up to a maximum of 3 mg/kg. These medications are typically administered after initial defibrillation and epinephrine.
How Medications Support Resuscitation
The medications used for shockable rhythms during cardiac arrest play a supportive role in the overall resuscitation effort by addressing specific physiological challenges. Epinephrine’s primary contribution is to enhance coronary and cerebral perfusion during cardiopulmonary resuscitation (CPR). By causing peripheral vasoconstriction, epinephrine redirects blood flow to the heart and brain, increasing the likelihood of successful defibrillation and return of spontaneous circulation (ROSC).
Antiarrhythmic medications like amiodarone and lidocaine primarily increase the likelihood of successful defibrillation and help maintain a stable rhythm post-conversion. These drugs stabilize the erratic electrical activity of ventricular fibrillation and pulseless ventricular tachycardia, making the heart more receptive to defibrillation. By regulating electrical impulses, they help prevent immediate recurrence of life-threatening arrhythmias after a successful shock. While no medication has definitively shown to improve long-term survival after cardiac arrest, their role in achieving short-term outcomes like ROSC and hospital admission is recognized.