Vasovagal collapse is a common reason for unconsciousness. Many of us will experience a vasovagal collapse or syncope once or more during their lifetime. Though mostly innocent, to many of us the experience of a sudden and unwanted loss of consciousness is most disturbing, especially when bystanders witness the attack and call for help of emergency services. People with vasovagal collapse have been resuscitated (sometimes causing multiple rib fractures) and have been brought to hospital by the emergency services. People brought into hospital have often been subjected to all kinds of ancillary investigations such as ECG, EEG, 24-hours ECG or have even received implants for monitoring their heart rhythm over prolonged periods with regular checkups at the cardiologist.

Vasovagal collapse may be accompanied by a complete stop of cardiac activity or asystole. This is caused by excessive inhibition of the heart by overactivity of the vagal nerve. This blockade is always self-limiting and it will stop spontaneously, usually within a minute, allowing the heart to start beating again. Even doctors are not always familiar with the occurrence of asystole in the course of a vasovagal collapse. They tend to order extra investigations to minimise the risk of heart disease. You will understand that undergoing all these extra and unnecessary investigations does not help to comfort someone who has experienced this rather innocent loss of consciousness.

How does a vasovagal collapse occur?

What happens during a vasovagal collapse? For this we turn to a detailed registration of ECG (blue), blood pressure (red) and blood flow velocity to the brain (green) recorded in a patient experiencing vasovagal collapse during a so-called tilt-table test (see below). A tilt-table test is a clinical examination during which a patient is strapped to a table which from a horizontal can be brought to a vertical position. This is a controlled situation allowing all kinds of physiological parameters to be recorded safely.

The onset of a vasovagal collapse can be triggered by standing, by emotional stress or by pain. Presumably, these cause a sudden loss of venous tone in the large capacitance vessels of the body. The blood pools in the lower parts of the body and less and less blood reaches the heart causing cardiac output to decrease (C-D). It is likely that some degree of hyperventilation also plays a role. Hyperventilation causes further relaxation of the capacitance vessels and in addition also causes narrowing of the feeding arteries to the brain. Now cerebral perfusion is even more reduced. The body tries to overcome these difficulties by exaggerating the arterial acceleration (E-I), this is a brief constriction of the arteries during the first phases of each heart beat. This mechanism can temporarily keep some blood flowing to the brain but because of the high pressures it is accompanied by bradycardia: a reflex is activated inhibiting the frequency with which the heart beats (the baroceptor reflex). When strong enough this reflex may even cause the heart to stop beating completely, called asystole. Obviously, blood supply to the brain completely stops and the cerebral mechanisms maintaining consciousness and body posture fail: the person loses consciousness and -in an uncontrolled condition- falls to the ground. Brain activity, recorded by EEG, will stop completely (becomes iso-electric).

The complete loss of brain function explains why the excessive vagal activity is self-limiting: with no blood flowing to the brain the vagal nerve cannot maintain it's activity. The heart is released from excessive vagal inhibition and starts beating again. Cardiac output increases rapidly and the blood flow to the brain is restored (J-L). The person regains consciousness, usually a bit confused but most often no longer than for a period of 1-2 minutes. After a vasovagal collapse subjects may feel weak and nauseous for some time. Usually they maintain a lower heart rate than before the collapse and putting them on their feet too early may evoke a second attack.

Preventing a vasovagal collapse

So what can be done to prevent this unwanted sequence of events? Important is to eat regularly. Many persons experience a collapse after skipping breakfast. Sleep regularly. Avoid situations with long standing and try to find a seat. Many people recognise the feeling of a beginning collapse. Then again, when standing find a place to sit or lie down. Of course, this can be inconvenient or even quite embarrassing but compare this with really losing consciousness and falling to the floor sometimes with loss of urinary or bowel control! In true emergency situations you can try to cross your legs and push them together to prevent blood from pooling in the lower extremities. Then concentrate on your breathing and try to breathe as little as possible. By preventing hyperventilation, which is the normal response to feeling uncomfortable, the blood vessels in the brain will remain open. In this way time is gained allowing the feeling of near fainting to subside.

Witnessing a vasovagal collapse

When witnessing a vasovagal collapse, try to put the person in a supine position with the legs up (causing venous blood to flow to the heart). Wait a while (maximum of 60s) for the heart rhythm to recur spontaneously before starting cardiac resuscitation. When no heart beats can be felt in the person's neck for a period exceeding 60 seconds vasovagal collapse is uncertain and bystanders should be asked to call the emergency services while resuscitation is started.