Syncope is defined as a sudden loss of consciousness and postural tone, followed by spontaneous recovery. Almost a third of the cases reported never find an explanation. Beyond this, there are various causes. Some we have already discussed include seizures, transcient ischemic attacks, strokes, and heart disease (eg, cardiac ischemia and cardiac arrhythmias).
This leaves us with two remaining important causes, vasovagal syncope (neurocardiogenic syncope) and postural hypotension (orthostatic hypotension). These are the primary subjects of this page.
Vasovagal syncope occurs through abnormal reflexes regulating the heart and blood vessels. The fainting is due to abrupt bradycardia and vasodilation under circumstances in which, if anything, the opposite should be occurring.
There may be no obvious precipitating circumstances, although in many cases certain situations may trigger the syncope. For example, prolonged standing is frequently involved, or perhaps something such as eating a large meal in a warm restaurant . But it never occurs when the patient is lying down. There is no generally agreed upon explanation for this failure in the mechanisms regulating arterial pressure.
Sometimes healthy runners will faint for unknown reasons at the end of a race. This too is typically put in the category of vasovagal syncope.
In the second to minutes leading up to the fainting, the patient may feel weakness, nausea, pallor, and sweating.
One particular situation that is often, but not always, included under vasovagal syncope is a specific response to a stressful or painful event. First, the patient shows increased sympathetic effects, such as increased heart rate and arterial pressure. Then, immediately following the stressful occurrence, sympathetic effects abruptly are withdrawn, causing a sudden decrease in heart rate and arterial pressure. The low blood flow to the brain causes the patient to lose consciousness and fall to the ground. After a couple minutes on the floor, the patient revives with no ensuing symptoms. Notably, there are no arrhythmias, sustained "palpitations", or signs of a seizure.
To avoid this problem, health care practicioners are advised to have patients, family and friends seated for bad news, injections or sutures. But should someone faint in this way, it is important that well-meaning bystanders do not immediately jerk the patient upright, since the arterial pressure likely will fall again. Also, the patient should not be traumatized with slapping, dousing with cold water, ammonia capsules, etc. Finally, fainting cannot immediately be assumed to be to vasovagal syncope, since there are numerous causes for loss of consciousness.
Hysteria and hyperventilation do not fall in this category because they cause "light-headedness" through respiratory alkalosis brought on by blowing off carbon dioxide too fast. Another type of fainting not included in this category is that due to "psychogenic" causes. Certain patients with emotional problems may faint several times a day, although this is not associated with any abnormal cardiovascular responses.
A tilt table test is now the established means of diagnosing vasovagal syncope. Heart rate and arterial pressure are measured throughout the test. The test begins with the patient in the horizontal position in a warm room. The patient is then tilted to about 70 degrees or more and left there for about one half hour, or until symptoms develop. In a typical case of vasovagal syncope, nothing unexpected occurs for a while. But after perhaps fifteen or twenty minutes there is an, abrupt, pronounced decreased heart rate and fall in arterial pressure. Other typical symptoms also appear at this time, such as sweating, pallor, and nausea. Returning the patient to the horizontal position stops the response. By contrast, as you observe in lab, the heart rate of normal subjects does not suddenly start to fall in the upright position.
Often, to make the test more sensitive, the patient is given a drug to cause vasodilation if the first attempt does not elicit the response.
In orthostatic hypotension (postural hypotension), patients tend to faint on standing. One typical cause is depleted extracellular fluid volume, perhaps due to use of a diuretic. Another is autonomic dysfunction, such as sometimes occurs in diabetic neuropathy. In general, the carotid baroreceptor reflex does not respond strongly enough to compensate for the pooling of blood in the leg veins.
The tilt table test gives a different response here. Upon tilting to the upright position the arterial pressure begins drifting down. It is not associated with an abrupt drop in the heart rate and arterial pressure. Indeed, the heart rate increases slowly as the arterial pressure falls.