Vagal discomfort: why can one suddenly feel unwell, sweat, and lose consciousness during a medical procedure?

First, what is a vasovagal episode?

To understand the vasovagal episode, you need to grasp something very simple: our body continuously regulates our heart rate and blood pressure.

When we are standing, for example, gravity naturally draws part of our blood toward our legs. Our nervous system compensates immediately in order to maintain sufficient blood flow to the brain. Normally, we are not even aware that this regulation is taking place.

But sometimes, a cardiovascular reflex suddenly becomes maladaptive. The heart rate may slow down, the blood vessels may dilate, and blood pressure drops. For a few moments, the brain receives less blood than it needs to maintain normal consciousness.

This is when the episode occurs. And if this reduction is significant enough, the person may lose consciousness for a few seconds: this is what we call a vasovagal reflex syncope.

Why do we talk about the vagus nerve?

The vagus nerve is part of the autonomic nervous system — that part of the nervous system that controls, without us having to think about it, many functions: heart rate, digestion, certain respiratory functions, and numerous cardiovascular reflexes.

During a vasovagal reaction, there is a sudden disruption of this autonomic balance. But contrary to a very widespread belief, the phenomenon does not simply mean 'the vagus nerve has been activated.' It is a more complex cardiovascular reflex, combining to varying degrees a slowing of the heart and, above all, a decrease in the tone of the blood vessels.

The end result is the same: blood pressure drops and blood supply to the brain temporarily decreases.

Why does this often happen during a medical procedure?

This is precisely why I wanted to talk to you about this subject, because in aesthetic medicine we encounter several classic triggers of vasovagal episodes: a needle, an injection, pain, the sight of blood, anticipation of the procedure, fear, stress, being on an empty stomach, heat, prolonged standing, or fatigue. Sometimes even simply the thought of what is about to happen.

And it can happen to someone who is not consciously afraid at all. This is important. I have seen perfectly relaxed patients, chatting normally during a treatment, suddenly turn pale.

Because a vasovagal episode is not necessarily a conscious psychological reaction: it is a physiological reflex. You can be perfectly courageous and still have a vasovagal episode.

What are the signs that should alert you?

A vasovagal episode often has a very useful characteristic: it gives warning before it arrives. This period is called the prodrome. The person may suddenly say: 'I don't feel very well.'

Then certain very characteristic signs appear: a sensation of heat, cold sweats, pallor, nausea, yawning, dizziness, sudden weakness, blurred or tunnel vision that gradually darkens, ringing in the ears, the impression that sounds are fading away, and an overwhelming sense that loss of consciousness is imminent.

When we are performing a medical procedure, recognising these signs quickly is extremely important, because at this stage we can often prevent a fall and sometimes avoid loss of consciousness.

Why do people turn completely white?

This is a sign we observe very frequently: the patient can lose all colour within seconds. This pallor is linked to rapid changes in blood circulation and the autonomic nervous system, and it can be accompanied by profuse sweating.

It is this combination — pallor, sweating, nausea, a sensation of heat and dizziness — that should immediately bring a vasovagal reaction to mind in the appropriate context.

What happens if the person loses consciousness?

In a classic vasovagal syncope, loss of consciousness is generally brief. And something interesting occurs: the person falls or is laid down, meaning they are no longer upright.

The blood no longer has to work as hard against gravity to reach the brain. Cerebral blood flow is restored and the person generally regains consciousness spontaneously. This is why the lying-down position is so important.

What should be done immediately?

When a patient begins to show signs of a vasovagal episode during a procedure, we stop what we are doing. We keep the patient safe to prevent a fall and lay them down, generally on their back. Depending on the situation, we may elevate the legs to encourage blood return to the heart, loosen anything that may be restrictive, and monitor breathing, pulse and, in a medical setting, blood pressure and other vital signs.

And above all: we do not immediately stand the person up the moment they say they feel better. This is a classic mistake. The person opens their eyes, says 'I feel better', wants to get up… and then immediately has another episode.

Why? Because their cardiovascular system has not yet fully recovered. They need a few minutes, and the return to a seated position, then to standing, must be done gradually.

And what if you feel the episode coming on yourself?

The first instinct should be not to try to remain standing. Sit down or, better still when possible, lie down. This avoids two things: a fall, and a further reduction in blood flow to the brain.

In people who are well acquainted with their warning symptoms, certain muscle-tensing manoeuvres can also be helpful: crossing the legs firmly while contracting the muscles, clenching the fists, tensing the arms or leg muscles. Muscle contraction helps to temporarily raise blood pressure and venous return, which can sometimes interrupt the process before loss of consciousness occurs.

Should you give water or sugar?

This is a very common question. Sugar is not the treatment for a vasovagal episode. People often confuse a vasovagal episode with hypoglycaemia: someone turns pale, trembles or feels weak, and immediately the suggestion is 'give them some sugar!' But a vasovagal syncope is not caused by a lack of sugar.

If we have reason to suspect hypoglycaemia — particularly in a person with diabetes or taking certain medications — blood glucose should of course be assessed. But these are two entirely different mechanisms.

As for water, good hydration can be helpful, especially in preventing certain episodes in predisposed individuals. But we obviously never give anything to drink to someone who is unconscious or whose alertness has not fully returned.

Why do some people experience vasovagal episodes so much more easily than others?

There is enormous individual variability. Some people will never experience one in their entire lives; others know the scenario all too well: a blood draw, sweating, pallor, then a faint.

Several factors can increase susceptibility: dehydration, heat, fatigue, prolonged fasting, stress, pain, prolonged standing, alcohol, and sometimes certain medications that lower blood pressure.

In a predisposed individual, several small factors can combine: they arrive stressed, haven't eaten, have had little to drink, the room is warm… then the needle appears, and the threshold is crossed.

Can a vasovagal episode be prevented before an aesthetic procedure?

Often, yes — and this is particularly relevant in aesthetic medicine. If you know that you tend to faint easily during blood draws or injections, please mention it before the procedure. This is absolutely not an insignificant detail: it allows the physician and their team to adapt the conditions of the treatment — for example, by performing certain procedures lying down rather than seated.

Also avoid, unless medically advised otherwise, arriving dehydrated or after an unnecessarily prolonged fast.

And above all, do not try to conceal the first symptoms. Some patients begin to feel unwell but say nothing because they want the physician to finish the injection quickly. That is exactly the opposite of what you should do. Say immediately: 'I'm starting to feel warm,' 'I feel nauseous,' 'my vision seems strange,' 'I feel like I'm going to faint.' At that point, we can intervene before any loss of consciousness occurs.

Is a vasovagal episode dangerous?

In its typical form, in a person with no particular underlying condition, vasovagal syncope is generally benign. However, there are two essential nuances to bear in mind.

The first is the risk of injury from a fall. What can be dangerous is not necessarily the episode itself, but the collapse. Losing consciousness while standing on a staircase, in the street, or while driving clearly does not carry the same consequences as fainting when you are already lying on a medical table.

The second nuance is even more important: not every loss of consciousness is a vasovagal episode. And that is probably the most important medical message in this article.

When should we think about something else?

A loss of consciousness can have various causes. Some are benign, while others can be far more serious. A syncope can in particular be related to a significant drop in blood pressure, certain medications, a cardiac arrhythmia, certain heart conditions, a haemorrhage, or other medical situations. Certain neurological events can also resemble a syncope without actually being one.

This is why we must always look at the context.

What are the signs that should particularly draw our attention?

A loss of consciousness warrants especially careful evaluation when it occurs during physical exertion, when it occurs suddenly with no warning signs whatsoever, or when it is accompanied by chest pain, significant palpitations, or unusual shortness of breath.

Attention is also warranted when it occurs in a person with a known heart condition, when it recurs without an obvious trigger, when it causes significant injury, or when recovery is abnormally slow or incomplete. A family history of sudden death at a young age is also important information to report to the doctor.

In these situations, one should not simply conclude 'I am someone who has vasovagal episodes': the cause must be investigated.

Vasovagal episode or epileptic seizure?

The distinction is not always as straightforward as one might imagine. During a syncope, there can sometimes be a few involuntary muscle movements. Seeing someone lose consciousness and display a few jerking movements does not therefore automatically mean they are having an epileptic seizure.

The context, the duration of the loss of consciousness, the observed manifestations, and above all the recovery after the episode allow the doctor to guide the diagnosis. Prolonged confusion after the episode, for example, is less characteristic of a simple vasovagal episode and warrants evaluation. Once again, a syncope is not diagnosed solely because a person has fainted.

And in aesthetic medicine?

This is precisely where clinical experience becomes important. An injection can trigger a vasovagal episode, but during or after a medical procedure one should never automatically attribute every episode of feeling unwell to the vagus nerve. Depending on the procedure performed and the products used, the doctor must also consider other possible complications.

A severe allergic reaction, for example, can be accompanied by a drop in blood pressure. But the clinical picture can be very different and may involve, depending on the case, difficulty breathing, swelling, hives, wheezing, or other signs. The treatment is then absolutely not the same.

This is why, even when a vasovagal episode seems obvious, a healthcare professional continues to observe the patient and to verify that the course of events truly corresponds to the presumed diagnosis.

The context gives us an enormous amount of information

Imagine two situations. In the first, a patient is apprehensive about needles; during the injection, she says she feels hot, turns pale, starts sweating, feels nauseous, and her vision blurs. She is immediately laid down and recovers quickly. The picture is highly suggestive of a vasovagal reaction.

In the second, a person suddenly loses consciousness during physical exertion, with no warning whatsoever, and describes palpitations just before. That is not the same story.

And that, precisely, is medicine: the same symptom can have completely different meanings depending on the circumstances.

What I want you to take away

A vasovagal episode can be extremely alarming, but in its typical form it is generally a transient reflex of the cardiovascular system: blood pressure drops, sometimes the heart slows down, and for a few moments the brain receives less blood. You then experience heat, nausea, sweating, pallor, dizziness, and blurred vision, followed possibly by a brief loss of consciousness.

During a medical procedure, say immediately that you are starting to feel unwell: do not try to 'hold on for another thirty seconds'. And if you know you have already had vasovagal episodes during injections or blood draws, let us know before the procedure. That allows us to anticipate.

But also remember the other message: not every loss of consciousness is a vasovagal episode. A first unexplained episode, an atypical episode, or a syncope associated with warning signs deserves a proper medical evaluation.

I am Dr Florian Vallecillo, and if I were to leave you with just one idea today, it would be this: a vasovagal episode is often benign, but a loss of consciousness is always a symptom whose context must be understood. When you feel that familiar 'Doctor… I don't think I'm feeling very well…' coming on, say so right away, because a few seconds of anticipation are often enough to turn a very alarming loss of consciousness into a quickly managed episode.