Causes of Nocturnal Bradycardia
Nocturnal bradycardia is primarily caused by either physiologic vagal-mediated mechanisms during normal sleep or pathologic obstructive sleep apnea, with the latter being the most important reversible cause to identify in clinical practice. 1
Physiologic Causes
Normal vagal tone during sleep is the most common cause of nocturnal bradycardia in healthy individuals. 1
- Sinus bradycardia is the most common bradyarrhythmia encountered during sleep and represents a physiologic, vagally-mediated response that requires no intervention 1
- These physiologic bradyarrhythmias are particularly common and can be profound in young individuals and conditioned athletes 1
- Various conduction abnormalities can occur physiologically during sleep, including sinus arrest, sinus exit block, all degrees of atrioventricular block, junctional rhythm, and periods of asystole 1
- The frequency of nocturnal bradyarrhythmias appears to decline in middle-aged and older healthy individuals 1
Pathologic Causes
Obstructive Sleep Apnea (Primary Pathologic Cause)
Obstructive sleep apnea is the most clinically significant pathologic cause of nocturnal bradycardia and must be systematically screened for in any patient with documented sleep-related bradycardia. 1
- Sleep apnea demonstrates a prevalence of 24% in men and 9% in women in the United States, with much remaining asymptomatic or unrecognized 1
- The prevalence is substantially higher (47-83%) in populations with cardiovascular diseases 1
- Profound nocturnal sinus bradycardia occurs in 7.2-40% of patients with sleep apnea 1
- Second- or third-degree atrioventricular block occurs in 1.3-13.3% of sleep apnea patients 1
- Sinus pauses occur in 3.3-33% of patients with sleep apnea 1
- The prevalence and severity of these arrhythmias increase with the severity of sleep apnea 1
A stereotypical pattern occurs: progressive bradycardia during apnea/hypopnea episodes (often profound) followed by tachycardia and hypertension during partial arousal, presumably precipitated by hypoxia 1
- This pattern can serve as an electrocardiographic means of indirectly diagnosing sleep apnea 1
- Bradyarrhythmias in sleep apnea patients occur primarily during apneic episodes 1
- Wakeful bradyarrhythmias are uncommon in these patients, and nocturnal arrhythmias are usually asymptomatic 1
- In one study, 88% of patients referred for pacemaker therapy with asymptomatic bradyarrhythmias were documented to have sleep apnea 2
Other Pathologic Causes
Additional pathologic causes of nocturnal bradycardia include: 3
- Sinus node dysfunction
- Medications (particularly beta-blockers, which can cause bradycardia including sinus pause, heart block, and cardiac arrest) 4
- Acute myocardial infarction
- Heart failure
- Exaggerated vagal activity
- Increased intracranial hypertension
- Infection
- Hypothyroidism
- Hypothermia
- Anorexia nervosa
- Prolonged hypoxia
Clinical Approach
When nocturnal bradycardia is identified, screening for sleep apnea symptoms is mandatory (Class I recommendation). 1
Key symptoms to elicit include: 2
- Excessive daytime fatigue
- Snoring
- Witnessed cessation of breathing during sleep
- Frequent nighttime awakenings
If sleep apnea symptoms are present, confirmatory polysomnography testing is indicated. 1
Critical Pitfall to Avoid
Do not proceed with pacemaker implantation in patients with asymptomatic nocturnal bradycardia without first screening for and treating sleep apnea. 1
- Treatment of sleep apnea with continuous positive airway pressure (CPAP) reduces bradyarrhythmic episodes by 72-89% 1
- In patients treated for sleep apnea, 86% remained free of bradyarrhythmia symptoms without requiring pacemaker implantation over 22 months of follow-up 1, 2
- Treating underlying sleep apnea not only alleviates apnea-related symptoms and improves cardiovascular outcomes but also eliminates the need for pacemaker implantation in most patients 1