What are the causes of sinus arrhythmia?

Causes of Sinus Arrhythmia

Physiological (Normal) Respiratory Sinus Arrhythmia

Respiratory sinus arrhythmia is primarily a normal physiological phenomenon caused by respiratory-circulatory interactions, where heart rate increases during inspiration and decreases during expiration due to rhythmic changes in cardiac parasympathetic (vagal) activity. 1, 2

Mechanism of Normal Respiratory Sinus Arrhythmia

• Vagal modulation during breathing is the primary mechanism, with inhibition of cardioinhibitory parasympathetic cardiac vagal neurons during inspiration causing heart rate acceleration 1, 3
• Lung mechanoreceptors detect changes in lung volume and transmit this information to modulate heart rate, while chest wall reflexes play no significant role 4
• Functional benefit: This matching of heart rate to respiratory cycle improves pulmonary gas exchange efficiency by optimizing ventilation-perfusion matching and reducing unnecessary heartbeats during expiration 1

Normal Physiological Contexts

• Universal phenomenon observed in all air-breathing vertebrates, including healthy humans of all ages 1, 2
• Age-related variations occur, with respiratory sinus arrhythmia present throughout the lifespan as a marker of autonomic and cardiac health 2
• Gender differences are minimal, with no significant difference in respiratory sinus arrhythmia between men and women during resting breathing 4

Pathological Causes of Abnormal Sinus Arrhythmia

Primary Sinus Node Disorders

Inappropriate Sinus Tachycardia (IST) represents a pathological form of sinus arrhythmia with two main mechanisms 5, 6:

• Enhanced automaticity of the sinus node causing persistent resting heart rate >100 bpm with mean 24-hour heart rate >90 bpm after excluding secondary causes 5, 6
• Abnormal autonomic regulation with excess sympathetic and reduced parasympathetic tone 5, 6
• Demographics: Predominantly affects females (90%), mean age 38 years, often healthcare professionals 5, 6

Sinus node re-entry tachycardia arises from heterogeneity of conduction within the sinus node, producing paroxysmal bursts of tachycardia with P-waves identical to sinus rhythm 5

Secondary Causes of Sinus Tachycardia

Cardiovascular causes 6:

• Hypovolemia/shock from dehydration, hemorrhage, or fluid losses
• Heart failure with compensatory tachycardia
• Myocardial ischemia
• Pericardial effusion causing tamponade physiology
• Myocarditis

Metabolic and endocrine causes 6:

• Hyperthyroidism causing persistent tachycardia
• Fever/infection driving catecholamine release
• Anemia increasing cardiac output demands
• Acidosis affecting sinus node automaticity

Pharmacologic causes 5, 6:

• Stimulants: caffeine, nicotine, alcohol
• Prescribed medications: salbutamol, aminophylline, atropine, catecholamines
• Recreational drugs: amphetamines, cocaine, ecstasy, cannabis
• Anticancer agents: anthracyclines causing acute cardiotoxicity

Other causes 6:

• Hypoxemia (one of the most common reversible causes)
• Pain
• Anxiety and emotional stress
• Physical deconditioning
• Pulmonary embolism

Diminished or Absent Respiratory Sinus Arrhythmia

Pathological conditions that reduce or eliminate normal respiratory sinus arrhythmia 2:

• Chronic heart failure
• Hypertension
• Conditions affecting cardiac parasympathetic function

Pharmacologic suppression 7, 3:

• General anesthetics (urethane, pentobarbital, ketamine-xylazine) can depress, abolish, or even invert normal respiratory sinus arrhythmia
• Atropine administration may affect respiratory sinus arrhythmia patterns

Critical Diagnostic Distinctions

Postural Orthostatic Tachycardia Syndrome (POTS) must be distinguished from IST before treatment, as it presents with excessive heart rate increase with postural change (>30 bpm or >120 bpm within 10 minutes of standing) 6

Key pitfall: Suppressing sinus rate in POTS causes severe orthostatic hypotension, making this distinction clinically critical before initiating rate control 6