What are the causes of sinus tachycardia?

Causes of Sinus Tachycardia

Sinus tachycardia results from either appropriate physiological responses to increased metabolic demands or from pathological conditions that inappropriately elevate the sinus rate, and identifying the underlying cause is essential because treatment targets the etiology rather than the rate itself. 1

Physiological (Appropriate) Causes

Physiological sinus tachycardia represents a normal compensatory response where the sinus node appropriately increases heart rate in response to increased sympathetic tone or metabolic demands 1:

• Exercise and physical activity - the most common physiological trigger 1
• Emotional stress and anxiety - catecholamine-mediated response 1, 2
• Pain - sympathetic activation 1, 2
• Fever/infection - each 1°C elevation increases heart rate by approximately 10 bpm 1, 2

Pathological (Secondary) Causes

Cardiovascular Conditions

• Hypovolemia/shock - from dehydration, hemorrhage, or fluid losses requiring compensatory tachycardia to maintain cardiac output 1, 2
• Heart failure - compensatory mechanism to maintain cardiac output in reduced ejection fraction 1, 2
• Myocardial ischemia - sympathetic activation and reduced cardiac function 2
• Pulmonary embolism - hypoxemia and right heart strain 2, 3

Metabolic and Endocrine Disorders

• Hyperthyroidism - excess thyroid hormone directly increases sinus node automaticity 1, 2
• Anemia - compensatory increase in cardiac output to maintain oxygen delivery 1, 2
• Hypoxemia - direct effect on sinus node and sympathetic activation 1, 2
• Acidosis - affects sinus node automaticity 1

Pharmacological and Substance-Related

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

Primary Sinus Tachycardia Syndromes

These represent intrinsic sinus node dysfunction rather than appropriate responses 4:

Inappropriate Sinus Tachycardia (IST)

• Definition: Resting heart rate >100 bpm with mean 24-hour heart rate >90 bpm after excluding all secondary causes 1
• Mechanism: Enhanced sinus node automaticity or abnormal autonomic regulation with excess sympathetic/reduced parasympathetic tone 2, 5
• Demographics: 90% female, mean age 38 years, often healthcare professionals 2
• Key feature: Excessive heart rate increase with minimal activity 2

Postural Orthostatic Tachycardia Syndrome (POTS)

• Definition: Excessive heart rate increase (>30 bpm or >120 bpm) within 10 minutes of standing 2
• Critical distinction: Must be differentiated from IST before treatment, as rate control in POTS causes severe orthostatic hypotension 2, 6
• Mechanism: Form of dysautonomia improved by fludrocortisone 1

Sinus Node Reentry Tachycardia

• Mechanism: Microreentry within the sinus node complex 1
• Key feature: Paroxysmal episodes with abrupt onset and termination, P-wave morphology indistinguishable from sinus rhythm 1
• Distinguishing characteristic: Can be induced and terminated by programmed stimulation, cycle lengths 350-550 ms 1

Diagnostic Algorithm

Step 1: Confirm Sinus Origin

• 12-lead ECG during tachycardia: P waves positive in leads I, II, aVF and negative in aVR, with normal P-wave morphology 1, 2
• Verify non-paroxysmal pattern: Sinus tachycardia is non-paroxysmal, distinguishing it from reentrant mechanisms 1

Step 2: Assess Hemodynamic Stability

• Immediate evaluation: Check for acute altered mental status, ischemic chest pain, acute heart failure, hypotension, or shock 2
• Oxygenation: Evaluate for hypoxemia, tachypnea, retractions, and check pulse oximetry 2

Step 3: Systematic Exclusion of Secondary Causes

• Complete blood count: Rule out anemia or infection 2
• Thyroid function tests: TSH and free T4 to exclude hyperthyroidism 2
• Medication and substance review: Comprehensive assessment of all potential pharmacological triggers 1, 2
• Echocardiogram: If myocarditis suspected or structural heart disease concern 2
• 24-hour Holter monitoring: If IST suspected to document mean 24-hour heart rate 2

Critical Pitfalls to Avoid

• Never attempt to normalize heart rate in compensatory tachycardia - cardiac output depends on elevated heart rate, and lowering it can precipitate cardiovascular collapse 2
• Always distinguish IST from POTS before initiating rate control - suppressing sinus rate in POTS causes severe orthostatic hypotension 2, 6
• Do not diagnose IST without excluding all secondary causes - hyperthyroidism, anemia, and other reversible causes must be ruled out first 2, 6
• Avoid coding sinus tachycardia as supraventricular tachycardia (I47.1) - this code is reserved for paroxysmal SVT with abrupt onset/termination, not sinus tachycardia 7

Management Principles

Treatment targets the underlying cause rather than the rate itself 1, 2:

• Physiological sinus tachycardia: No specific drug treatment required; address the trigger 2
• Secondary causes: Treat the underlying condition (e.g., thyroid replacement for hypothyroidism, antibiotics for infection, volume resuscitation for hypovolemia) 2, 6
• Rate control indications: Symptomatic physiological sinus tachycardia related to anxiety/stress, post-MI patients for prognostic benefit, heart failure patients, or symptomatic hyperthyroidism 2
• First-line rate control: Beta-blockers 2
• Alternative agents: Non-dihydropyridine calcium channel blockers (diltiazem or verapamil) if beta-blockers contraindicated 2