Fainting, or syncope is a temporary loss of consciousness when the brain doesn’t get enough oxygen. Many things, including dehydration, low blood sugar, or anxiety, can cause it. As all doctors know, a common trigger for fainting is seeing blood or a needle.

Emotional fainting is very common in humans, especially in younger people, but it is extremely rare in non-human vertebrates. Fainting may be a response that Homo sapiens has acquired in addition to the other self-preservation responses observed in non-human vertebrates.

The body’s sympathetic nervous system is activated when a person sees blood.

In the face of perceived or actual danger, and to increase the chance of self-preservation, fear responses in animals escalate from “freeze” to “fright” as a function of the proximity of the danger; intermediate responses are termed “flight” or “fight.”

All of these responses are mediated by the autonomic nervous system, but our understanding of what action is triggered by what element of the system is far from complete.

For example, an increase in the parasympathetic nervous system (PNS) activity can have exactly the same effect as a decrease in sympathetic nervous system (SNS) activity.

The end result is the net result of the balance between the system’s two branches.

Freeze. The first step in the self-preservation sequence is to “stop, watch and listen.” A state of alarm is triggered. The patient becomes hypervigilant and evaluates the impending danger. The “freeze” response likely provides a better chance of survival because most mammalian predators’ visual cortex and retina have evolved primarily to detect moving objects.

Flight or fight. Fight or flight is the most common response of animals and humans faced with imminent danger. Both responses are characterized by activation of the sympathetic nervous system, resulting in tachycardia, hypertension, and dilation of skeletal muscle vessels.

Drop. Syncope, fainting, or vasovagal syncope occurs when the body overreacts to a stimulus, causing a sudden drop in blood pressure. This drop in blood pressure can cause a decrease in blood flow to the brain, leading to a complete loss of consciousness.

Why does fainting occur in humans but not in animals?

Upright posture and bipedalism. Because most humans walk upright, the brain is located 30 to 50 cm above the heart. Therefore, the systolic pressure must be strong enough to overcome 30 mmHG of hydrostatic pressure. In addition, humans have a very large capacitance vascular bed in the lower body (abdomen and legs). If the volume of blood and oxygen below the diaphragm is suddenly excessive, there is an increased risk of oxygen deficiency in the brain. This explains why most fainting occurs when the patient is sitting or standing. In humans, blood flow to the brain is about 20 percent of cardiac output. Giraffes don’t seem to have this problem because they walk on four feet and have a much smaller brain (with less oxygen demand) than humans.

The power of emotion. The human brain is more sophisticated than that of other mammals. Eighty percent of brain matter is dedicated to the neo-cortex, especially the prefrontal areas, considered the site of thought and emotion. Emotions are reported to trigger half of all syncopal episodes.

Intense pain (or injury) is a common cause of syncope. Some areas, such as the abdomen, seem particularly prone to syncope. Intense joy, fear, or disgust can cause vaso-vagal syncope in humans.

Blood phobia. Three percent of the U.S. population is affected by severe blood phobia. It seems to be more common in young women. A Paleolithic threat explanation has been proposed. Fainting at the sight of blood or a sharp object (weapon) could lead a predator or attacker to believe that the victim is dead or incapacitated, which could cause the attack to stop. This could be advantageous to the victim.

Animals don’t faint at the sight of blood. Only significant blood loss (more than 30 percent of their total blood volume) can cause loss of consciousness.

Another way to prevent vasovagal syncope is to stay hydrated and maintain healthy blood glucose levels. Dehydration and low blood sugar can both cause fainting, so it’s important to stay hydrated and eat regularly.

Needle phobia. At least 10 percent of American adults have a fear of needles. According to  James G Hamilton, the author of the pioneering paper on needle phobia, needle phobia could be genetic and have some basis in evolution. Avoiding stab wounds and sharp objects could obviously provide a survival benefit. Patients who suffer from vasovagal needle phobia fear the thought, sight, or feeling of needles. Interestingly many patients report not a fear of the procedure itself but rather of the vaso vagal episode itself.

The phobia can also be associated with a traumatic event, such as an excruciating procedure or witnessing one.

Potential benefits of fainting

Loss of consciousness provides a temporary escape from an unpleasant or unbearable situation. Fear of pain, pain itself, or any other traumatic situation can trigger syncope, resulting in loss of consciousness, and the person no longer experiences the unwanted event.

Loss of postural tone (due to bradycardia and vasodilation) can benefit cerebral perfusion. Lying down restores oxygenation to this major organ and may be a self-preservation mechanism against hemorrhage.

To prevent fainting during blood sampling for procedures involving a needle:

• Ensure that the patient is comfortable and relaxed.
• Ask the patient to lie down or sit comfortably, providing pillows if necessary.
• Encourage them to take slow, deep breaths and to focus on something pleasant in the room.
• Keep the patient hydrated, as dehydration can increase the risk of fainting. Make sure the patient is well-hydrated before the procedure. If possible, offer water or juice.
• Using a smaller needle can help reduce pain and discomfort during the procedure, which may help prevent fainting.
• Use vibration (sensory noise) in the area to be injected to minimize the local perception of the needle.
• Use a cold spray like ethyl chloride to control superficial pain at the puncture site.
• Applying a numbing cream, such as EMLA,  to the area before the procedure can also help reduce pain and discomfort, which may also help prevent fainting.
• To take their mind off the procedure, offer the patient a distraction, such as music, a video, or a conversation.
• During the procedure, monitor the patient’s vital signs, such as heart rate, breathing rate, and overall nervousness. If you notice any signs of fainting, such as dizziness or lightheadedness, stop the procedure immediately, help the patient lie down, and elevate his or her legs.
• After the procedure, offer the patient a drink and a snack to replenish fluids and blood sugar. Also, ask the patient to sit or lie down for a few minutes before standing up to prevent fainting.

Jean Paul Brutus is a hand surgeon.