Bradycardia & Tachycardia
For patients WITH a pulse, the central question is whether they are stable or unstable — that judgment drives everything from drugs to electricity. This chapter covers the adult bradycardia and tachycardia algorithms.
Symptomatic bradycardia
Bradycardia was historically defined as a rate under 60/min; the 2018 ACC/AHA/HRS guideline uses under 50/min. It becomes an emergency when it causes symptoms — dizziness, acutely altered mental status, ischemic chest discomfort, acute heart failure, hypotension or syncope. Look for causes such as ischemia, hypoxemia, electrolyte problems and drugs. First-line treatment is atropine; if it fails, use transcutaneous pacing or an epinephrine or dopamine infusion, and pace immediately if the patient is in shock and IV/IO access is not yet available. Confirm mechanical capture with a pulse, and plan for transvenous pacing or definitive treatment. An asymptomatic bradycardia generally needs monitoring, not drugs.
Stable vs. unstable tachycardia
The pivotal question is stability, and whether the rhythm is causing it. An unstable tachycardia with a pulse (hypotension, altered mental status, shock, ischemic chest pain or acute heart failure) caused by the rhythm is treated with synchronized cardioversion — timed to the QRS to avoid the vulnerable T wave — and for atrial fibrillation and flutter the 2025 Guidelines recommend starting at 200 J. A stable tachycardia allows a 12-lead ECG and drugs: vagal maneuvers then adenosine for regular narrow-complex tachycardia, and antiarrhythmics such as procainamide, sotalol or amiodarone for stable monomorphic VT with a defibrillator ready. Sustained polymorphic VT is always unstable and gets immediate unsynchronized defibrillation.
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