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Key Features

Essentials of Diagnosis

  • Wide variation in sinus rate is common in young, healthy individuals and generally not pathologic

  • Symptomatic bradycardia especially in the elderly or patients with underlying heart disease may require permanent pacemaker implantation

  • Sinus tachycardia usually secondary to another underlying process (ie, fever, pain, anemia, alcohol withdrawal)

  • Sick sinus syndrome manifests as sinus bradycardia, pauses, or inadequate heart rate response to physiologic demands (chronotropic incompetence)

General Considerations

  • Sinus arrhythmia

    • Defined as an irregularity of the normal heart rate defined as variation in the PP interval of more than 120 milliseconds (ms)

    • Occurs commonly in young, healthy people due to changes in vagal influence on the sinus node during respiration (phasic) or independent of respiration (nonphasic)

  • Sinus bradycardia

    • Defined as a heart rate slower than 60 beats/min and may be due to increased vagal influence on the normal sinoatrial pacemaker or organic disease of the sinus node

    • Rates of 50 beats/min or lower (especially during sleep) is a normal finding in healthy individuals

    • However, may be an indication of true sinus node pathology in elderly patients and individuals with heart disease

    • When the sinus rate slows severely, the atrial-nodal junction or the nodal-His bundle junction may assume pacemaker activity for the heart, usually at a rate of 35–60 beats/min

  • Sinus tachycardia

    • Defined as a heart rate faster than 100 beats/min that is caused by rapid impulse formation from the sinoatrial node

    • It is a normal physiologic response to exercise or other conditions in which catecholamine release is increased

    • The rate infrequently exceeds 160 beats/min but may reach 180 beats/min in young persons

    • The onset and termination are usually gradual, in contrast to paroxysmal supraventricular tachycardia due to reentry

    • In rare instances, otherwise healthy individuals may present with "inappropriate" sinus tachycardia where persistently elevated basal heart rates are not in-line with physiologic demands

  • Sick sinus syndrome

    • Defined as a broad diagnosis applied to patients with

      • Sinus arrest

      • Sinoatrial exit block (recognized by a pause equal to a multiple of the underlying PP interval or progressive shortening of the PP interval prior to a pause)

      • Persistent sinus bradycardia

    • May also manifest as chronotropic incompetence

      • Defined as an inappropriate heart rate response to the physiologic demands of exercise or stress

      • An underrecognized cause of poor exercise tolerance

Clinical Findings

  • Sinus arrhythmia (bradycardia or tachycardia): does not cause symptoms in the absence of underlying cardiac disease or other comorbidities

  • When severe sinus bradycardia results in low cardiac output, however, patients may present with

    • Weakness

    • Confusion

    • Syncope, if cerebral perfusion is impaired

  • Atrial, junctional and ventricular ectopic rhythms are more apt to occur with slow sinus rates

  • Sinus tachycardia

    • Most often a normal physiologic response to conditions that require an increase in cardiac output including

      • Fever

      • Pain

      • Anxiety


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