What are the physiologic and pathologic causes of tachycardia?

Causes of Tachycardia

Tachycardia (heart rate >100 bpm) stems from either physiological responses to appropriate autonomic demands or pathological conditions requiring identification and treatment of the underlying cause. 1

Physiological Causes

Physiological sinus tachycardia represents a normal compensatory response to:

• Physical activity or exercise
• Emotional stress or anxiety
• Appropriate autonomic influences during normal daily activities

These represent healthy cardiovascular responses and resolve spontaneously when the stimulus is removed. 1

Pathological Causes

Secondary (Reversible) Causes

The most critical clinical task is distinguishing inappropriate sinus tachycardia from secondary causes, as tachycardia should resolve with correction of the underlying condition. 1

Metabolic and Systemic Conditions:

• Hyperthyroidism - screen all patients with unexplained tachyarrhythmias 2
• Anemia - reduces oxygen-carrying capacity, triggering compensatory tachycardia
• Dehydration - decreases preload and cardiac output
• Fever/Infection - increases metabolic demands
• Pain - activates sympathetic nervous system

Cardiovascular Conditions:

• Heart failure - compensatory mechanism for reduced cardiac output
• Structural heart disease (cardiomyopathies) - must be excluded, though tachycardia-induced cardiomyopathy itself is rare 1

Exogenous Substances:

• Caffeine - direct sympathomimetic effects
• Beta-agonist medications (albuterol, salmeterol) - therapeutic agents with tachycardic side effects
• Illicit stimulant drugs (amphetamines, cocaine) - direct cardiac stimulation 1

Primary Arrhythmic Causes

Mechanistically, tachyarrhythmias arise from three fundamental disturbances:

1. Altered automaticity - enhanced firing of normal pacemakers (SA node, AV node) or abnormal automaticity in atrial/ventricular myocardium 3, 4

2. Re-entry circuits - most common mechanism for supraventricular tachycardias, involving anatomically preformed pathways or functional obstacles 4, 5

3. Triggered activity - early or delayed afterdepolarizations causing abnormal impulse generation 3, 4

Specific Arrhythmic Entities:

• Inappropriate Sinus Tachycardia (IST) - unexplained tachycardia at rest or with minimal exertion, related to dysautonomia, neurohormonal dysregulation, or intrinsic sinus node hyperactivity 1

• Supraventricular tachycardias - including AVNRT, AVRT, atrial flutter, atrial fibrillation, and atrial tachycardia 1, 6

• Ventricular tachycardia - requires differentiation from SVT with aberrancy using ECG criteria 1

• Postural Orthostatic Tachycardia Syndrome (POTS) - excessive heart rate increase with postural changes due to hypovolemia, vascular dysfunction, and autonomic dysregulation 7

Critical Clinical Pitfalls

Common diagnostic errors include:

• Premature attribution to anxiety without excluding organic causes - anxiety may coexist with IST but should not be assumed as the primary cause 1

• Failure to recognize tachycardia as an early compensatory signal of life-threatening illness - may be the earliest or only objective sign of critical disease 8

• Underappreciation of persistent tachycardia - associated with adverse outcomes including tachycardia-induced cardiomyopathy 9

The algorithmic approach requires:

1. Determine if tachycardia is sinus or non-sinus rhythm
2. Systematically exclude all secondary causes (metabolic, infectious, cardiovascular, substance-related)
3. If secondary causes excluded and symptoms present, consider primary arrhythmic diagnosis
4. For persistent unexplained tachycardia, pursue electrophysiological evaluation