Passing Out (Syncope) Caused by Arrhythmias

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Passing Out (Syncope) Caused by Arrhythmias

Topic Overview

Syncope (say "SING-kuh-pee") refers to a sudden loss of consciousness that soon passes. Syncope may be the first sign that you have an arrhythmia. And it is a very worrisome symptom for several reasons:

  • Passing out can cause a serious injury. For example, passing out while climbing the stairs or while driving can cause serious harm.
  • If you pass out because your brain did not get enough oxygen to function, this may be a warning sign that you have a serious medical condition.

An arrhythmia can cause syncope in the same way that it causes lightheadedness (presyncope). Your heart cannot pump blood effectively during excessively fast (tachycardia) or slow (bradycardia) heart rates, reducing the amount of blood that reaches your brain.

With syncope, though, the arrhythmia causes such a dramatic drop in the blood pressure that the brain does not receive enough blood to keep you conscious, and you lose consciousness as a result. For an arrhythmia to cause syncope, your heart rate must be extremely fast or extremely slow, or you must have some other heart condition in addition to the arrhythmia.

How long does syncope last?

It is important to recognize that syncope is transient, meaning that you wake up soon after passing out. You may wake up because the arrhythmia stops on its own and a normal rhythm and blood pressure return. Even if the arrhythmia persists, you may still regain consciousness. When you have an episode of syncope due to an arrhythmia, it typically happens while you are standing or sitting, and the loss of consciousness causes you to fall to the floor. As soon as you are lying down, blood flows back into your brain, even though your blood pressure may remain low. When enough blood flows back into your brain, you will likely wake up.

What are the risks associated with passing out from an arrhythmia?

Fast or slow arrhythmias may cause you to pass out. Depending on your position and activity at the time of the episode, you may seriously injure yourself. If you are standing up at the time of the arrhythmia, you may pass out and fall. The fall may cause you to injure your head, break an arm or leg, or receive other injuries. If you are driving, you may crash, causing severe injury to yourself and anyone else involved.

Passing out may be a sign that you are at risk for a life-threatening arrhythmia. If you have symptoms of an arrhythmia that may cause you to pass out, do not drive any vehicle until your condition has been evaluated and treated.

When is lightheadedness not caused by bradycardia?

Many of the medicines used to treat heart conditions, such as medicines for high blood pressure or heart failure, can lower the blood pressure excessively and result in lightheadedness. In general, medicine-induced lightheadedness often occurs soon after you stand up (orthostasis) because of the drop in blood pressure that happens when you stand (orthostatic hypotension). In contrast, lightheadedness cause by an arrhythmia can occur even when you are sitting or reclining.

Other causes of lightheadedness include hyperventilation, panic or anxiety attacks, prolonged standing, and excessive fluid loss caused by vomiting, diarrhea, or sweating.


Other Works Consulted

  • Carlson MD, Grubb BP (2011). Diagnosis and management of syncope. In V Fuster et al., eds., Hurst's The Heart, 13th ed., vol. 1, pp. 1125–1138. New York: McGraw-Hill.


ByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerRakesh K. Pai, MD, FACC - Cardiology, Electrophysiology
Last RevisedJune 2, 2011

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