What causes heart rate fluctuations and how are they treated?

Heart Rate Fluctuations: Causes and Treatment

Primary Causes of Heart Rate Fluctuations

Heart rate fluctuations are primarily caused by variations in autonomic nervous system tone (sympathetic and parasympathetic balance), underlying cardiac arrhythmias, or physiologic responses to systemic stressors. 1

Physiologic Causes

• Autonomic nervous system modulation causes normal heart rate variability through fluctuations in sympathetic and parasympathetic tone during the diurnal cycle and with exercise 1
• Respiratory sinus arrhythmia produces beat-to-beat variations synchronized with breathing, representing normal parasympathetic function 2
• Compensatory responses to fever, anemia, hypotension, shock, or hyperthyroidism produce sinus tachycardia as a physiologic adaptation 1, 3

Pathologic Arrhythmias Causing Rate Fluctuations

• Atrial fibrillation produces absolutely irregular RR intervals due to chaotic atrial activity and variable atrioventricular conduction, with rates fluctuating based on autonomic tone 1
• Premature atrial contractions (PACs) and premature ventricular contractions (PVCs) cause intermittent irregular beats, occurring in nearly all individuals but potentially symptomatic 3
• Supraventricular tachycardia presents with abrupt onset and termination of rapid regular rates, often misdiagnosed as anxiety 4
• Sinus node dysfunction (sick sinus syndrome) causes bradycardia, sinus arrest, or bradycardia-tachycardia syndrome 3

Drug-Induced Causes

• Multiple medications can induce atrial fibrillation, including anthracyclines, ibrutinib, and other chemotherapeutic agents, particularly in elderly patients or those with preexisting cardiovascular disease 1
• Beta-blocker withdrawal in patients with coronary artery disease can precipitate severe tachycardia and ventricular arrhythmias 5

Critical Diagnostic Approach

Immediate Assessment Required

Obtain a 12-lead ECG immediately to document the rhythm, as this is essential to distinguish between primary cardiac arrhythmias and physiologic rate variations. 1, 4

• Attach continuous cardiac monitoring and assess vital signs including blood pressure 1
• Evaluate for hemodynamic instability: acute altered mental status, ischemic chest discomfort, acute heart failure, hypotension, or shock 1
• Check for reversible causes: electrolyte abnormalities (potassium, magnesium), thyroid dysfunction (TSH), anemia (CBC), and medication review 1, 3

Key Diagnostic Distinctions

• Sinus tachycardia has gradual onset/termination with rates typically <150 bpm and upper limit approximating 220 minus patient's age 1, 4
• Supraventricular tachycardia has abrupt onset/termination, often lasting hours, and is frequently misdiagnosed as panic disorder 4
• Atrial fibrillation shows absolutely irregular RR intervals without distinct P waves and atrial cycle length <200 ms 1
• Pre-excitation syndromes (Wolff-Parkinson-White) show short PR interval and delta waves on baseline ECG, requiring specialized management 1, 4

Treatment Algorithms

For Unstable Patients (Rate-Related Cardiovascular Compromise)

Proceed immediately to synchronized cardioversion for unstable tachycardia with signs of shock, acute heart failure, ischemic chest pain, or altered mental status. 1

• Establish IV access and administer sedation if patient is conscious, but do not delay cardioversion if extremely unstable 1
• Use initial biphasic energy of 120-200 J for atrial fibrillation, 50-100 J for atrial flutter/SVT, and 100 J for monomorphic ventricular tachycardia 1
• Increase energy in stepwise fashion if initial shock fails 1

For Stable Atrial Fibrillation with Rapid Ventricular Response

Use IV beta-blockers or non-dihydropyridine calcium channel blockers (diltiazem) as first-line agents for acute rate control in hemodynamically stable patients. 1

• Beta-blockers are preferred in patients with heart failure with reduced ejection fraction due to mortality benefit 1
• Target heart rate of 60-80 bpm at rest and 90-115 bpm during moderate exercise 6
• Avoid verapamil, diltiazem, and digoxin in patients with pre-excitation syndromes, as these can paradoxically accelerate ventricular rates 1
• Digoxin is effective only at rest (not during exercise) and may be used as adjunct therapy in heart failure patients 1

For Sinus Tachycardia

Do not treat sinus tachycardia with rate-controlling medications; instead, identify and treat the underlying physiologic cause (fever, anemia, hypotension, hyperthyroidism). 1

• Recognize that when cardiac function is poor, cardiac output depends on rapid heart rate, and "normalizing" the rate can be detrimental 1, 6
• Rates <150 bpm are unlikely to cause instability unless ventricular dysfunction is present 1, 6

For Premature Contractions (PACs/PVCs)

• Exclude reversible causes: electrolyte abnormalities, hyperthyroidism, stimulant drugs 3
• Beta-blockers are first-line for symptomatic PACs or PVCs 3
• Consider catheter ablation for patients with PVCs and left ventricular dysfunction or for highly symptomatic patients refractory to medical therapy 3

For Drug-Induced Arrhythmias

Immediately identify and discontinue the offending medication as the first step in management. 1

• If discontinuation is not possible (e.g., cancer chemotherapy), reduce dose and monitor closely 1
• Consider alternative agents with lower arrhythmogenic potential (e.g., acalabrutinib instead of ibrutinib, risedronate instead of alendronate) 1
• Follow standard atrial fibrillation management guidelines for rate or rhythm control 1

Specific Medication Considerations

Beta-Blocker Use and Precautions

• Metoprolol can cause bradycardia, heart block, and cardiac arrest; monitor heart rate and rhythm closely 5
• Never abruptly discontinue beta-blockers in patients with coronary artery disease, as this precipitates severe angina, myocardial infarction, and ventricular arrhythmias 5
• Taper gradually over 1-2 weeks when discontinuing chronically administered therapy 5
• Beta-blockers may mask tachycardia from hypoglycemia in diabetic patients, though dizziness and sweating remain 5
• Use with extreme caution in bronchospastic disease; have beta-2 agonists readily available 5

Antiarrhythmic Medications for Ventricular Tachycardia

• Amiodarone (150 mg IV over 10 minutes) is most effective for preventing recurrent monomorphic VT, particularly in patients with coronary disease and poor ventricular function 1
• Sotalol (1.5 mg/kg IV over 5 minutes) is relatively safe and effective but must be avoided in patients with prolonged QT interval 1
• Lidocaine is less effective than procainamide, sotalol, and amiodarone, and should be considered second-line therapy (1-1.5 mg/kg IV bolus, then 1-4 mg/min infusion) 1

Critical Pitfalls to Avoid

• Do not assume rapid heart rate is "just anxiety" without obtaining 12-lead ECG and excluding supraventricular tachycardia, which presents identically to panic attacks 4
• Do not use AV nodal blocking agents (adenosine, calcium channel blockers, digoxin, beta-blockers) in pre-excited atrial fibrillation, as this accelerates ventricular rates and can be life-threatening 1, 4
• Do not treat compensatory sinus tachycardia with rate-controlling drugs when cardiac output depends on elevated heart rate 1, 6
• Do not give adenosine for irregular or polymorphic wide-complex tachycardia 1
• Do not routinely withdraw chronic beta-blocker therapy before major surgery, despite impaired cardiac response to reflex stimuli 5

When to Refer to Cardiology

Refer to cardiac electrophysiology for: documented wide-complex tachycardia, pre-excitation on baseline ECG with paroxysmal palpitations, recurrent episodes of regular paroxysmal palpitations with abrupt onset/termination, or disabling symptoms despite medical therapy. 4