The culprit in nearly all cases of syncope is an interruption in blood flow to the brain.
The most common type, accounting for nearly half of all cases, is the vasovagal faint
, during which the vagus nerve
(a cranial nerve that helps regulate heart rate and blood pressure) sends signals that dramatically slow the heart.
The signals can be provoked in susceptible people by distress they feel at the sight of blood or when they experience some other upsetting or threatening event.
When your heart beats too slowly, it can’t sustain the blood pressure needed to feed oxygen to the brain, ....and down you go.
Although we don’t know why some people are more susceptible than others, it occurs more often in females.
Cardiac rhythm disorders
account for another 10 percent of all faints. Originating within the heart itself, unrelated to brain signals or emotional distress, these arrhythmias
can disrupt blood flow. Whether causing the heart to beat much too quickly or far too slowly, the common denominator in these abnormal rhythms is low cardiac output, resulting in insufficient blood flow to the brain.
Sometimes the patient will have a history of heart problems or palpitations (skipping or racing of her heartbeat) prior to fainting.
Yet another 10 percent of faints are caused by orthostatic hypotension, a precipitous drop in blood pressure that occurs when a person stands up. We all experience a mild version of this whenever we rise too quickly and feel slightly woozy. Normally we recover rapidly because evolution has equipped our bodies with a physiological control mechanism that prevents all our blood from draining into our ankles when we are standing. But if this mechanism doesn’t function properly, blood
deserts our heads and follows gravity into our feet. Neurological disorders like Parkinson’s disease—along with certain drugs, including some anti-hypertension medications—can blunt the responsiveness of this system. Orthostatic hypotension also occurs if the circulatory system is not fully expanded, which might happen when you are dehydrated or have had serious blood loss.
Finally, in nearly 2 percent of cases the cause of syncope remains undiagnosed.