Depending on the cause, many people have warning signs prior to fainting such as lightheadedness, dizziness, loss of vision (“blacking out”), general weakness, nausea, perspiration, chest pain, shortness of breath or palpitations (the last would be common if the cause was an arrhythmia due to an abnormally rapid heart beat). Bystanders might note that the person who fainted “lost their color” as they appear pale due to reduced blood flow to the head and body.
Syncope can be due to abnormally fast, slow or irregular heart beats (arrhythmia), where the heart is unable to deliver adequate blood flow to the brain. If the heart rate becomes very slow due to degeneration of the intrinsic electrical conduction system of the heart (sick sinus syndrome), one might require a pacemaker. On rare occasions, some medications (such as beta-blockers) can slow the heart rate enough to lead to syncope. Fainting can also occur due to very fast and irregular arrhythmias such as atrial fibrillation or supraventricular tachycardia, both of which are from abnormalities in the upper chambers (atria) of the heart and, by themselves, are not life-threatening. If the heart rate goes fast enough, the heart is unable to pump blood effectively and the blood pressure drops enough to lead to syncope. Some people can be prone to faint due to their heart going too fast at times and too slow at other times, a condition known as tachycardia-bradycardia (“tachy-brady”) syndrome.
Syncope can also be due to ventricular tachycardia which occurs in the lower chambers of the heart (ventricles) and can be due to life-threatening underlying heart disease. It can be a signs of coronary artery disease with inadequate blood flow to the heart muscle including from a heart attackor due to a heart muscle problem itself (cardiomyopathy). It is important to make sure that someone who has fainted is not at risk of ventricular tachycardia because this arrhythmia can lead to ventricular fibrillation and sudden cardiac death.
Structural heart disease
Significant heart valve disease can also lead to syncope, most commonly aortic stenosis. Much less frequent causes in this day and age would include mitral stenosis and also tumors within the heart, even otherwise benign tumors such as atrial myxomas. Blood clots than form in the veins of the legs and travel through the heart to the lungs can cause syncope if large enough (pulmonary embolism).
Primary blood pressure problems
If blood pressure drops enough (especially if it drops rapidly), from whatever the cause, inadequate blood flow to the brain can occur leading to syncope. This is known as hypotension and can be due to significant dehydration, excessive bleeding or too much blood pressure medication. Some people have inadequate “communication” between their blood vessels, heart and brain so that when they stand, an adequate and stable blood pressure is not maintained and the resultant drop in blood pressure leads to fainting. This is due to orthostatic or postural hypotension and is more common in diabetics. As people age or become frail they are more prone to this especially in hot weather or even with mild dehydration (such as due to inadequate fluid intake). Certain medications can also lead to this as can certain neurologic disease affecting the autonomic nervous system (such as Parkinson’s disease).
Whatever the cause, in vasovagal syncope, there is less blood in the upper body (and thus less blood in the brain) when the person is standing. Blood is pooling in the lower body and can be exacerbated if the person is dehydrated or has had some alcohol which tends to dilate blood vessels. The heart responds by more forcefully contracting or beating to make up for the lack of sufficient blood in its own chambers, but this overstimulation of the heart activates the parasympathetic part of the nervous system which is normally responsible for bringing down the heart rate and blood pressure. The parasympathetic nervous system connects to the heart throught the vagus nerve (on the inside of the chest wall) which is the “vagal” in vasovagal.
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Written by and/or reviewed by Mark K. Urman, M.D. and Jeffrey F. Caren, M.D.