SINUS DYSRHYTHMIA Some variation in the sinus node discharge rate is common, but if the variation exceeds 0.12 s between the longest and shortest intervals, sinus dysrhythmia is present. The ECG characteristics of sinus dysrhythmia are (1) normal sinus P waves and PR intervals, (2) 1:1 AV conduction, and (3) variation of at least 0.12 s between the shortest and longest P-P interval ( Fig 24-4).
Clinical Significance Sinus dysrhythmia is frequently a normal finding in children and young adults; it tends to disappear with advancing age. Sinus dysrhythmia is most commonly a phasic (respiratory variation) variety and less commonly a nonphasic variety. In the phasic variety, the sinus node rate accelerates during inspiration and decelerates during expiration because of changes in vagal tone occurring with respiration (Bainbridge reflex). The irregularity in either the phasic or nonphasic varieties can be exaggerated by conditions that increase vagal tone. During long intervals of sinus dysrhythmia, junctional escape beats may occur.
SINUS BRADYCARDIA Sinus bradycardia occurs when the sinus node rate falls below 60. The ECG characteristics of sinus bradycardia are (1) normal sinus P waves and PR intervals, (2) 1:1 AV conduction, and (3) atrial rate below 60 ( Fig.24-5).
Clinical Significance Sinus bradycardia represents a suppression of the sinus node discharge rate. Sinus bradycardia can be (1) physiologic (in well-conditioned athletes, during sleep, or with vagal stimulation), (2) pharmacologic (digoxin, narcotics, reserpine, b-adrenergic antagonists, calcium channel blockers, quinidine), or (3) pathologic (acute inferior myocardial infarction, increased intracranial pressure, carotid sinus hypersensitivity, hypothyroidism).
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