What are the causes of isolated tachycardia?

Causes of Isolated Tachycardia

Isolated tachycardia results from either physiological responses to identifiable stressors or inappropriate sinus node activity when no physiological trigger exists, requiring systematic evaluation to distinguish reversible secondary causes from primary cardiac rhythm disorders. 1, 2

Physiological (Secondary) Causes

The ACC/AHA guidelines emphasize that physiological sinus tachycardia occurs when the heart rate appropriately responds to underlying conditions and should resolve with correction of the trigger. 1

Metabolic and Systemic Conditions

• Fever and infection - among the most common triggers of appropriate tachycardia 1, 2
• Dehydration - causes compensatory increase in heart rate 1, 2
• Anemia - reduces oxygen-carrying capacity, requiring increased cardiac output 1, 2
• Hyperthyroidism - increases metabolic rate and sympathetic tone 1, 2
• Heart failure - compensatory mechanism for reduced cardiac output 1, 2
• Pain - activates sympathetic nervous system 1, 2
• Acid-base disturbances - metabolic derangements trigger tachycardia 2

Exogenous Substances

• Caffeine - direct sympathomimetic effect 1, 2
• Beta-agonist medications (albuterol, salmeterol) - stimulate beta-adrenergic receptors 1, 2
• Illicit stimulants (amphetamines, cocaine) - potent sympathetic activation 1, 2
• Alcohol and nicotine - trigger sympathetic response 2
• Anticancer treatments (particularly anthracyclines) - cardiotoxic effects 2
• Other medications: aminophylline, atropine, catecholamines, cannabis 2

Psychological Factors

• Anxiety disorders - the ACC/AHA guidelines specifically identify anxiety as an important trigger, with patients often having associated anxiety disorders 1, 3
• Emotional stress - physiological response expected to resolve when stressor is addressed 3

Inappropriate Sinus Tachycardia (IST)

IST is a diagnosis of exclusion defined as sinus tachycardia unexplained by physiological demands, with resting heart rates >100 bpm and 24-hour average rates >90 bpm. 1, 4

Proposed Mechanisms

The ACC/AHA guidelines note the cause remains unclear, with proposed mechanisms including: 1, 4

• Dysautonomia - autonomic nervous system dysfunction 1, 4
• Neurohormonal dysregulation - abnormal hormonal control 1
• Intrinsic sinus node hyperactivity - primary sinus node abnormality 1, 4

Critical Distinctions

IST must be differentiated from: 1

• Atrial tachycardia arising from the superior crista terminalis 1
• Sinus node reentrant tachycardia 1
• Postural orthostatic tachycardia syndrome (POTS) - though overlap may exist; POTS patients have predominant postural symptoms and treating the sinus rate may cause severe orthostatic hypotension 1

Structural and Arrhythmic Causes

Supraventricular Tachycardias

• Atrioventricular nodal re-entrant tachycardia (AVNRT) 2, 5
• Atrioventricular re-entrant tachycardia (AVRT) 2, 5
• Atrial tachycardia 2, 5
• Atrial flutter 2
• Sinus node reentry tachycardia 2

Structural Heart Disease

• Cardiomyopathies - can precipitate tachyarrhythmias 2

Common Diagnostic Pitfall

The most critical error is misdiagnosing anxiety or panic disorder when organic causes exist. 6 The ACC/AHA guidelines mandate a Class I recommendation to evaluate for and treat reversible causes in all patients with suspected IST before making this diagnosis of exclusion. 1 Look specifically for: thyroid function abnormalities, complete blood count for anemia, medication and substance use history, and structural heart disease on echocardiography. 1, 2

Clinical Approach Algorithm

1. First, rule out all secondary causes listed above through targeted history, physical examination, and laboratory testing 1, 2
2. If anxiety-related, distinguish whether tachycardia is physiological response to anxiety (appropriate) versus IST with comorbid anxiety 1, 3
3. If no secondary cause identified, confirm sinus mechanism on ECG (P waves upright in leads I, II, aVF; biphasic in V1) with rates >100 bpm at rest and >90 bpm average over 24 hours 1
4. Exclude other arrhythmias through ECG during tachycardia, comparing with sinus rhythm 2, 5
5. Only then diagnose IST as the remaining possibility 1