Arrhythmias

I. Introduction to Arrhythmias

Cardiac arrhythmias are disruptions in the heart's rhythm or rate, manifesting in seven basic patterns: early beats, bigeminal beats, grouped beats, pauses, bradycardia, tachycardia, and chaotic rhythms. This document will detail the diagnosis and management of specific rhythm disorders.

II. Approach to the Patient With Bradycardia

Bradycardia is defined as a heart rate below 50 beats per minute (<50/min). While it can be a normal finding in trained athletes or during sleep, it can also be symptomatic, presenting as "light-headedness, syncope, exertional intolerance, dyspnea, or fatigue."

Clinical Presentation and Evaluation

Diagnosis involves a "thorough history, physical examination, focused laboratory testing (electrolyte levels, thyroid function testing), and resting 12-lead ECG." It's crucial to identify severe or unstable conduction abnormalities requiring urgent intervention and to investigate "extrinsic and reversible causes," such as ischemia, hypothyroidism, infections, electrolyte imbalances, and medication use (e.g., AV nodal blockers). Echocardiography, exercise stress testing, and ambulatory ECG monitoring may also be helpful. Sleep apnea should be considered for nocturnal bradycardia.

Types of Bradycardia

• Sinus Bradycardia: Sinus rhythm with a heart rate below 50/min. Pathologic causes often include "sinus node dysfunction due to age-related myocardial fibrosis." Extrinsic causes commonly involve "medication use (β-blockers, donepezil, neostigmine, pyridostigmine)."
• Atrioventricular (AV) Block: Classified into three degrees:
• First-degree AV Block: "Delay in AV conduction (PR interval >200 ms)." Often benign.
• Second-degree AV Block:Mobitz type 1 (Wenckebach): Progressive PR prolongation until a QRS complex is dropped.
• Mobitz type 2: Intermittent nonconducted P waves with unchanging PR intervals. This type "usually occurs below the AV node and has a higher risk for progression to complete heart block."
• Third-degree AV Block (Complete Heart Block): "No P waves conduct to the ventricles. AV dissociation is observed on the ECG."

Management of Bradycardia

• Acute Management: For hemodynamically unstable bradycardia, "intravenous atropine should be administered." If ineffective, "chronotropic drug infusions (e.g., dopamine or epinephrine)" can be used until temporary pacing is implemented. "Temporary pacing is indicated for transient conditions causing hemodynamically unstable bradycardia or asystole."
• Long-Term Management (Permanent Pacing):Reversible and extrinsic causes should always be addressed first.
• Common indications for permanent pacing include:
• "Symptomatic bradycardia without reversible cause"
• "Permanent AF and symptomatic bradycardia"
• "Alternating bundle branch block"
• "Complete heart block, high-degree AV block, or Mobitz type 2 second-degree AV block, regardless of symptoms."
• Patients with stable left or right bundle branch block without prolonged PR interval generally do not require pacing due to low risk of progression to complete heart block (1%-3% per year).
• Cardiac Implantable Electronic Devices (CIEDs): Various devices offer functions beyond pacing, including antitachycardia pacing and defibrillation. These include Transvenous pacemakers, Leadless pacemakers, Implantable cardioverter-defibrillators (ICDs), Subcutaneous ICDs, Cardiac resynchronization therapy–pacing (CRT-P), and Cardiac resynchronization therapy–defibrillator (CRT-D).

III. Approach to the Patient With Tachycardia

Tachycardia is defined as a heart rate above 100 beats per minute (>100/min). Symptoms range from "tachypalpitations" to "syncope," or it may be discovered incidentally.

Clinical Presentation and Evaluation

"Documentation of tachycardia on ECG and correlation with symptoms is the key component of the diagnostic evaluation." Tachyarrhythmias are broadly categorized as supraventricular (originating above the AV node, normal QRS) or ventricular (originating below the AV node, widened QRS). History, medication review, physical exam, thyroid function testing, and echocardiography are part of the evaluation.

Antiarrhythmic Drugs

Antiarrhythmic agents are often classified by the Vaughan-Williams system (Classes I-IV), though many have multiple mechanisms. "Class I and class III agents are the most effective antiarrhythmic drugs; however, due to their membrane-active effects, they carry some paradoxical risk of inducing arrhythmia."

• Class I (Sodium Channel Blockers):Class IB (Lidocaine, Mexiletine): Primarily for ventricular arrhythmias.
• Class IC (Flecainide, Propafenone): Used for atrial fibrillation and SVT. "Contraindicated in patients with ischemic or structural heart disease because of the risk for promoting ventricular arrhythmias and death."
• Class II (β-Adrenergic Blockers): (e.g., Metoprolol, Propranolol) Decrease heart rate, prolong PR interval. Used for rate control in atrial arrhythmias and SVT.
• Class III (Potassium Channel Blockers): (e.g., Sotalol, Dofetilide, Amiodarone, Dronedarone) Prolong QT interval. Used for atrial and ventricular arrhythmias. "Class III antiarrhythmic therapy typically is initiated in an inpatient setting" with QTc monitoring. Amiodarone is widely used but has significant toxicities (thyroid, liver, lung, eye) requiring regular monitoring.
• Class IV (Calcium Channel Blockers - nondihydropyridines): (e.g., Verapamil, Diltiazem) Decrease heart rate, prolong PR interval. Used for SVT and rate control of atrial arrhythmias.
• Other Agents:Adenosine: An A1-receptor agonist causing brief AV block, used for "termination of SVT."
• Digoxin: Increases vagal activity, slowing AV node conduction, used for "rate control of atrial fibrillation."

Types of Tachycardia

• Sinus Tachycardia: Elevated heart rate due to "physiologic demand or distress," such as exercise, pain, or anxiety.
• Inappropriate Sinus Tachycardia (IST): Elevated resting heart rate with exaggerated increases during activity, often in women aged 20-40. Diagnosis requires exclusion of secondary causes.
• Postural Orthostatic Tachycardia Syndrome (POTS): A dysautonomia with orthostatic intolerance and excessive tachycardia upon standing (increase of ≥30/min or to >120/min within 10 minutes).

Supraventricular Tachycardias (SVTs)

Rapid heart rhythms originating from the atrium or requiring AV node conduction. They are common, often in younger patients and women, and typically occur without structural heart disease. ECG usually shows a narrow-complex tachycardia.

• Acute Management of SVT: "Vagal maneuvers, including the Valsalva maneuver or carotid sinus massage, are first-line therapy." "Adenosine can be used to terminate SVT and simultaneously help diagnose its mechanism."
• Atrioventricular Nodal Reentrant Tachycardia (AVNRT): Accounts for two-thirds of SVT cases (excluding AF/flutter). Caused by a reentrant circuit within the AV node. "AV nodal blockers (β-blockers or calcium channel blockers) are used to prevent recurrent AVNRT." "Catheter ablation of AVNRT has a high success rate."
• Atrioventricular Reciprocating Tachycardia (AVRT): Accessory p...