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Patient
45-year-old woman with no prior history, asymptomatic, evidence of bradycardia on auscultation; recording of a 24-hour Holter-ECG;
Trace
A sinus rhythm of 56 beats/minute is initially found with normal atrioventricular conduction (normal PR interval); there is a subsequent slowing of the heart rate with a sinus bradycardia of 28 beats/minute; possible second-degree sinoatrial block with PP intervals during the pause corresponding to twice the normal PP interval;
Exergue
The first-degree sinoatrial block does not induce bradycardia; the second-degree sinoatrial block type 2 is identified by the presence of sinus pauses of duration equal to a multiple of the baseline cycle; in the third-degree sinoatrial block, there is no identifiable sinus activity.
It is often difficult to differentiate on a surface electrocardiogram 1) a depression in automaticity and 2) an abnormality in impulse conduction (sinoatrial block). Only the actual depolarization of the atria (P wave) is recorded while the sinus activity is not.