Sinus Arrhythmia, Bradycardia, & Tachycardia

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, particularly well-trained athletes

  • 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

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