Chapter 22

### Sinus Node Dysfunction (“Sick Sinus Syndrome”)

• Sinus bradycardia: Sinus rate of less than 60 bpm.
• Sinoatrial exit block, type I: Progressively shorter P-P intervals, followed by failure of occurrence of a P wave.
• Sinoatrial exit block, type II: Pauses in sinus rhythm that are multiples of basic sinus rate.
• Sinus arrest, sinus pauses: Failure of occurrence of P waves at expected times.

#### Atrioventricular (AV) Block

• First degree: Prolonged PR interval more than 0.2 seconds.
• Second degree
• • Type I: Progressive increase in PR interval, followed by failure of AV conduction and nonoccurrence of a QRS complex
• • Type II: Abrupt failure of AV conduction not preceded by increasing PR intervals.
• High degree: AV conduction ratio 3:1 or greater.
• Complete: Independent atrial and ventricular rhythms, with failure of AV conduction despite temporal opportunity for it to occur.

### General Considerations

The clinical presentation of patients with conduction system disease is determined by the existence of three underlying abnormal conditions: bradycardia, inability to increase the heart rate in response to increases in metabolic needs, and atrioventricular (AV) dyssynchrony (inappropriately timed atrial and ventricular depolarization and contraction sequences).

### Pathophysiology & Etiology

#### Sinus Node Dysfunction

Sinus node dysfunction (“sick sinus syndrome”) is usually due to a degenerative process that involves the sinus node and sinoatrial (SA) area (Table 22–1). Often, the degenerative process and associated fibrosis also involve the AV node and its approaches as well as the intraventricular conduction system; as many as 25–30% of patients with sinus node dysfunction have evidence of AV and bundle branch conduction delay or block.

Table 22–1. Causes of Sinus Node Dysfunction.

Respiratory sinus arrhythmia, in which the sinus rate increases with inspiration and decreases with expiration, is not an abnormal rhythm and is most commonly seen in young healthy persons. Nonrespiratory sinus arrhythmia, in which phasic changes in sinus rate are not due to respiration, may be accentuated by the use of vagal agents, such as digitalis and morphine, and is more likely to be observed in patients who are older and who have underlying cardiac disease, although the arrhythmia is not itself a marker for structural heart disease; its mechanism is unknown. Ventriculophasic sinus arrhythmia is an unusual rhythm that occurs during high-grade or complete AV block; it is characterized by shorter P-P intervals when they enclose QRS complexes. The mechanism is not known with certainty but may be related to the ...

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