Blocked atrial premature beats are the most common cause of bradycardia in the critical care setting. They must be distinguished from sinus pauses by searching for
P waves buried within the T wave (Fig 1). The timing of cannon A waves has diagnostic significance. For blocked atrial premature beats, they occur before the first heart sound associated with the pause, and for complete heart block they are unrelated to heart sounds.
Fig. 1 Blocked atrial premature beat. Note the P wave buried in the T wave of the second sinus beat. The third conducted beat follows a shortened PR interval. This occurs because the interval between the atrial premature beat and the next P wave is longer, allowing the PR segment time to shorten.
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