Sinus bradycardia is an arrhythmia that originates in the SA node, and is defined by a sinus rate less than 60 bpm. 2
Many individuals, particularly those who partake in regular vigorous exercise, have resting heart rates less than 60 bpm. For those individuals, sinus bradycardia is normal and healthy, and does not require evaluation or treatment. However, some individuals develop symptomatic sinus-node dysfunction. In the absence of correctable underlying causes, idiopathic sinus-node dysfunction is referred to as sick sinus syn-12
drome, and occurs with greater frequency in association with advancing age. The prevalence of sick sinus syndrome is approximately 1 in 600 individuals over the age 12
of 65 years.
Clinical Presentation and Diagnosis of Sinus Bradycardia
• Many patients are asymptomatic, particularly those with normal resting heart rates less than 60 bpm as a result of physical fitness due to regular vigorous exercise
• Susceptible patients may develop symptoms, depending on the degree of heart rate lowering
• Symptoms of bradyarrhythmias include dizziness, fatigue, light-headedness, syncope, chest pain (in patients with underlying CAD), and shortness of breath and other symptoms of heart failure (in patients with underlying left ventricular dysfunction)
• Cannot be made on the basis of symptoms alone, as the symptoms of all bradyar-rhythmias are similar
• History of present illness, presenting symptoms, and 12-lead ECGthat reveals sinus bradycardia
• Assess possible correctable etiologies, including myocardial ischemia, serum potassium concentration (for hyperkalemia), thyroid function tests (for hypothyroid-ism)
• Determine whether patient is taking any drugs known to cause sinus bradycardia. If the patient is currently taking digoxin, determine the serum digoxin concentration and ascertain whether it is supratherapeutic (less than 2 ng/mL [2.6 nmol/L])
Sick sinus syndrome leading to sinus bradycardia may be caused by degenerative changes in the sinus node that occur with advancing age. However, there are other
possible etiologies of sinus bradycardia including drugs (Table 9-2). Pathophysiology
Sick sinus syndrome leading to sinus bradycardia occurs as a result of fibrotic tissue
in the SA node, which replaces normal SA node tissue.
The desired outcomes of treatment are to restore normal heart rate and alleviate patient symptoms.
Treatment of sinus bradycardia is only necessary in patients who become symptomatic. If the patient is taking any medication(s) that may cause sinus bradycardia, the drug(s) should be discontinued whenever possible. If the patient remains in sinus bradycardia after discontinuation of the drug(s) and after five half-lives of the drug(s) have elapsed, then the drugs(s) can usually be excluded as the etiology of the arrhythmia. In certain circumstances, however, discontinuation of the medication(s) may be undesirable, even if it may be the cause of symptomatic sinus bradycardia. For example, if the patient has a history of myocardial infarction or heart failure, discontinuation of a P-blocker is undesirable, because P-blockers have been shown to reduce mortality and prolong life in patients with those diseases, and the benefits of therapy with P-blockers outweigh the risks associated with sinus bradycardia. In these patients, clinicians and patients may elect to implant a permanent pacemaker in order to allow the patient to continue therapy with P-blockers.
Table 9-2 Etiologies of Sinus Bradycardia
Idiopathic ("sick sinus syndrome") Myocardial ischemia Carotid-sinus hypersensitivity NeLrocardiac syncope
Electrolyte abnormalities: hypokalemia or hyperkalemia
Systemic lupus erythematosus Scleroderma Sleep apnea Drugs: Adenosine Arniodarone ß-Blockers Cisplatin Citalopram
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