What is the differential diagnosis for sinus tachycardia?

Differential Diagnosis of Sinus Tachycardia

Sinus tachycardia represents a heterogeneous group of conditions that must be systematically categorized into physiological/secondary causes, pathological causes, and primary sinus tachycardia syndromes—each requiring distinct diagnostic and management approaches. 1, 2

Physiological and Secondary Causes

The most common category involves appropriate sinus tachycardia responding to identifiable stressors:

Cardiovascular and Hemodynamic

• Hypovolemia/shock from dehydration, hemorrhage, or fluid losses 1, 2
• Hypotension requiring compensatory heart rate increase 2
• Anemia causing increased cardiac output demands 1, 2
• Heart failure with compensatory tachycardia 1, 2
• Myocardial ischemia or infarction 2
• Pulmonary embolism 2

Metabolic and Systemic

• Hypoxemia from any cause—one of the most common reversible triggers 2
• Fever/infection driving catecholamine release 1, 2
• Hyperthyroidism causing persistent tachycardia 1, 2
• Pheochromocytoma with episodic catecholamine surges 2
• Acidosis affecting sinus node automaticity 1

Pharmacological and Substance-Related

• Stimulants: caffeine, alcohol, nicotine 1, 2
• Prescribed medications: salbutamol, aminophylline, atropine, catecholamines 1
• Recreational/illicit drugs: amphetamines, cocaine, ecstasy, cannabis 1
• Anticancer agents: anthracyclines (doxorubicin, daunorubicin) causing acute cardiotoxicity 1

Physiological States

• Pain of any etiology 2
• Anxiety and emotional stress 1, 2
• Physical deconditioning 1

Primary Sinus Tachycardia Syndromes

These represent intrinsic sinus node or autonomic dysfunction requiring different management:

Inappropriate Sinus Tachycardia (IST)

• Persistent resting heart rate >100 bpm with mean 24-hour heart rate >90 bpm after excluding all secondary causes 1, 2
• Excessive heart rate increase with minimal activity 1
• Non-paroxysmal pattern distinguishing it from reentrant arrhythmias 1
• Demographics: 90% female, mean age 38 years, often healthcare professionals 1, 3
• Mechanism: Enhanced sinus node automaticity or abnormal autonomic regulation with excess sympathetic/reduced parasympathetic tone 1

Postural Orthostatic Tachycardia Syndrome (POTS)

• Excessive heart rate increase with postural change (>30 bpm or >120 bpm within 10 minutes of standing) 1, 2
• Must be distinguished from IST before treatment, as rate control in POTS causes severe orthostatic hypotension 2
• Symptoms include dizziness, presyncope, and orthostatic intolerance 1

Sinus Node Reentry Tachycardia

• Paroxysmal episodes distinguishing it from other sinus tachycardias 1
• P-wave morphology identical to sinus rhythm 1, 3
• Endocardial activation pattern: superior-to-inferior and right-to-left 1
• Reentrant mechanism involving the sinus node 1

Other Arrhythmias Mimicking Sinus Tachycardia

Critical to exclude these from the differential:

Supraventricular Tachycardias

• Atrial tachycardia: P waves in second half of cycle, rate 100-250 bpm, may be associated with digitalis toxicity 3
• AVNRT: Absent or barely visible P waves, pseudo r' in V1, pseudo S in inferior leads, rate 140-250 bpm 4, 3
• AVRT: Accessory pathway involvement, may show pre-excitation on baseline ECG 3

Wide Complex Tachycardia

• Sinus tachycardia with bundle branch block can masquerade as ventricular tachycardia 5
• Requires careful ECG analysis showing P waves with normal morphology preceding each QRS 5

Diagnostic Algorithm

Initial ECG Assessment

• Confirm P-wave morphology: positive in leads I, II, aVF; negative in aVR; axis 0-90 degrees 1
• Verify non-paroxysmal pattern to distinguish from reentrant mechanisms 1
• Assess QRS width and regularity to exclude other arrhythmias 3

Systematic Exclusion of Secondary Causes

• Complete blood count for anemia or infection 2
• Thyroid function tests (TSH, free T4) for hyperthyroidism 2
• Medication and substance review including over-the-counter and illicit drugs 1, 2
• Echocardiogram if myocarditis, structural heart disease, or heart failure suspected 2
• 24-hour Holter monitoring if IST suspected, documenting persistent daytime tachycardia with nocturnal normalization 1, 2

Critical Pitfall

Never attempt rate control in compensatory sinus tachycardia (hypovolemia, hypoxemia, heart failure) as cardiac output depends on elevated heart rate and suppression can be catastrophic 2. Always identify and treat the underlying cause first 1, 2.