Atrial Fibrillation (AFib) is the most common sustained heart rhythm disorder, characterized by chaotic electrical signals in the heart’s upper chambers, the atria. Syncope, or fainting, is a sudden, transient loss of consciousness followed by spontaneous recovery, occurring when the brain is temporarily deprived of adequate blood flow. Yes, AFib can cause syncope, and when it does, it signifies a serious disruption in heart function requiring immediate medical evaluation. A syncopal episode indicates the heart rhythm is severely compromising circulation.
The Immediate Cause of Fainting
The physiological basis for any syncopal event is cerebral hypoperfusion, meaning insufficient blood flow to the brain. In AFib, this often occurs when irregular electrical activity is conducted too rapidly to the ventricles, resulting in a Rapid Ventricular Rate (RVR). When the ventricles beat excessively fast, they lose the necessary time to fully relax and fill with blood between contractions. This period of relaxation and filling is known as diastole.
Inadequate diastolic filling time severely limits the amount of blood the heart can pump out with each beat (stroke volume). This causes a precipitous drop in cardiac output, the total volume of blood pumped per minute. This profound reduction in blood circulated to the body, particularly the brain, is the direct mechanism triggering the transient loss of consciousness. The heart may be beating quickly, but the contractions are ineffective for maintaining systemic circulation.
Specific AFib Rhythms That Cause Syncope
AFib-related syncope is typically linked to two opposite extremes of heart rhythm: the rate being too fast or the rate being too slow. The most common scenario is AFib with a poorly controlled RVR, where the heart rate can exceed 150 beats per minute. The exceedingly rapid and chaotic ventricular contractions lead to hemodynamic collapse, as the heart simply cannot fill fast enough to sustain adequate cardiac output. This is especially dangerous for individuals who already have underlying heart disease.
The other significant rhythm issue is post-conversion bradycardia or a prolonged pause, which occurs when AFib terminates spontaneously or following medical intervention. During the AFib episode, the heart’s natural pacemaker, the sinus node, is suppressed by the rapid atrial activity. When the chaotic rhythm suddenly stops, the sinus node may take a dangerously long time to “wake up” and resume a normal pacing rhythm, unmasking underlying Sick Sinus Syndrome. These pauses can last for several seconds, resulting in a period of asystole that causes syncope.
Clinical Assessment and Stabilization
Following a syncopal episode, immediate medical attention is necessary to determine if AFib was the cause and to assess the patient’s hemodynamic stability. The initial clinical assessment involves a thorough history and physical examination, followed by a standard 12-lead electrocardiogram (EKG) to document the current heart rhythm. If the AFib is paroxysmal, meaning it comes and goes, continuous monitoring is often required to capture the rhythm disturbance at the time of the event.
Diagnostic Tools
Diagnostic tools are used to correlate symptoms with the actual heart rhythm. In cases of recurrent, unexplained syncope, an implantable loop recorder (ILR) may be placed under the skin for monitoring over several years.
- Holter monitors record the heart rhythm over 24 to 48 hours.
- Longer-term cardiac event recorders provide extended monitoring.
- The ILR allows physicians to detect brief but dangerous pauses or episodes of RVR.
Treatment is focused on stabilizing the underlying AFib rhythm and preventing future syncopal events. For syncope caused by RVR, stabilization involves rate control using medications such as beta-blockers or calcium channel blockers to slow the ventricular response to the atrial chaos. If syncope is caused by dangerous bradycardia or a prolonged pause, the definitive treatment is often the implantation of a permanent pacemaker. This small device ensures that the heart maintains a minimum, life-sustaining heart rate, providing a reliable electrical backup to prevent further syncopal episodes.