Normal sinus rhythm originates from the sinoatrial node with a frequency of 60 - 100 beats per minute. On electrocardiogram, sinus rhythm is characterized by upright P waves in leads I, II, and aVF, and inverted P waves in lead aVR; the PR interval is 0.12 - 0.20 seconds. Sinus arrhythmia is a type of arrhythmia caused by abnormal impulse frequency from the sinoatrial node or blocked conduction of sinus impulses to the atria. It is classified based on ECG and clinical manifestations into sinus tachycardia, sinus bradycardia, sinus arrest, sinoatrial block, and sick sinus syndrome.
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