Phase 1 · Understand the Body — Why the disease behaves the way it does.
The One-Line Truth
The whole disease in one chain
Everything on this page exists to explain this line.
A heart rate below 60 beats per minute is called a slow heart rate (bradycardia). It becomes an emergency when the slow rate causes symptoms — dizziness, fainting, chest pain, or confusion — because those symptoms mean the body is not getting enough blood. An electrocardiogram (ECG; a tracing of the heart's electrical signals) shows whether the slow rate is coming from the heart's natural pacemaker, from a traffic jam in the heart's wiring, or from a complete electrical block.
The Postal System's Slow Delivery Chain
Think of the heart's electrical system as a postal service with three checkpoints. The SA node is the central sorting facility that decides how many letters (electrical sparks) leave per minute — normally 60 to 100, but in a slow heart rate (bradycardia) it sends out 40 or fewer. The AV node is the inspection station between the mail trucks and the final delivery route — it checks each letter and sometimes delays it, or in block situations, stops it entirely and never passes it on. The bundle of His and bundle branches are the delivery trucks and neighborhood routes — if they are damaged, letters never reach the mailboxes (the ventricles). When the postal system slows down, fewer letters reach houses every minute, so residents (organs) get less mail (oxygen and blood). At first, the postal service tries to compensate by loading bigger stacks onto each truck or speeding up the remaining trucks, but if the bottleneck is at inspection (AV node) or the routes are destroyed (bundle branches), no amount of urgency can fix it — you still get fewer deliveries per minute, and houses start to suffer. That suffering — the dimmed lights (dizziness), the closed businesses (fainting), the empty refrigerators (organ dysfunction) — is all one chain caused by one broken checkpoint upstream.
AHA/ACC/HRS 2018 Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay · American College of Cardiology · American Heart Association · European Society of Cardiology 2021 Pacing Guidelines · reviewed 2026-07-07