In adults, a sinus rhythm with a frequency below 60 beats per minute is called sinus bradycardia. Sinus bradycardia often occurs with sinus arrhythmia (differences in PP intervals > 0.12 seconds). It is commonly seen in healthy young adults, athletes, and during sleep. Other causes include intracranial diseases, severe hypoxia, hypothermia, hypothyroidism, vasovagal syncope, and the use of cholinergic drugs, amiodarone, β-blockers, non-dihydropyridine calcium channel blockers, or digitalis glycosides. Sinus node dysfunction and acute inferior myocardial infarction can also lead to sinus bradycardia.
Asymptomatic sinus bradycardia usually requires no treatment. If symptoms such as dizziness, amaurosis fugax, and fatigue caused by inadequate cardiac output occur due to a slow heart rate, medications such as atropine or isoproterenol may be used. However, long-term use is often uncertain in effectiveness and can cause serious side effects, so cardiac pacing should be considered.
Figure 1 Sinus bradycardia
In lead II, the P wave is positive, the PR interval is 0.18 seconds, and the heart rate is 48 beats per minute.