What abnormality indicates a sick sinus syndrome on an EKG?

The presence of alternating periods of bradycardia and tachycardia on an EKG is indicative of sick sinus syndrome. This condition is characterized by the sinus node's inability to maintain a consistent heart rhythm, often resulting in fluctuations between very slow heart rates (bradycardia) and very fast heart rates (tachycardia). These irregularities can manifest in episodes where the heart rate can abruptly decrease, followed by an increase, leading to symptomatic experiences like dizziness or palpitations.

Sick sinus syndrome is primarily due to dysfunction of the sinus node, which is responsible for initiating the heartbeat. This dysfunction can lead to the heart not pacing properly, causing the alternation of bradycardia and tachycardia. This distinctive pattern on the EKG is a crucial clinical clue for identifying the syndrome and ensuring proper diagnosis and subsequent management, such as monitoring or potential pacemaker placement.

The other abnormalities listed, while significant, do not specifically characterize sick sinus syndrome. For instance, consistent tachycardia may suggest different arrhythmias, shortened PR intervals might indicate conditions like pre-excitation syndromes, and complete heart block reflects a different type of conduction disturbance.