Vasovagal response

Vasovagal episode
Classification and external resources

Vagus nerve
ICD-10 R55
ICD-9 780.2
DiseasesDB 13777
MeSH D019462

A vasovagal episode or vasovagal response or vasovagal attack[1] (also called neurocardiogenic syncope) is a malaise mediated by the vagus nerve. When it leads to syncope or "fainting", it is called a vasovagal syncope, which is the most common type of fainting[2].

There are a number of different syncope syndromes which all fall under the umbrella of vasovagal syncope. The common element among these conditions is the central mechanism leading to loss of consciousness. The differences among them are in the factors that trigger this mechanism.


Signs and symptoms

Among people with vasovagal episodes, the episodes are typically recurrent, usually happening when the person is exposed to a specific trigger. Prior to losing consciousness, the individual frequently experiences a prodrome of symptoms such as lightheadedness, nausea, the feeling of being extremely hot (accompanied by sweating), ringing in the ears (tinnitus), uncomfortable feeling in the heart, fuzzy thoughts, a slight inability to speak/form words (sometimes combined with mild stuttering), weakness and visual disturbances such as lights seeming too bright, fuzzy or tunnel vision, and sometimes a feeling of nervousness can occur as well. These last for at least a few seconds before consciousness is lost (if it is lost), which typically happens when the person is sitting up or standing. When sufferers pass out, they fall down (unless this is impeded); and when in this position, effective blood flow to the brain is immediately restored, allowing the person to wake up. Short of fainting a person may experience an almost undescribable weak and tired feeling resulting from a lack of oxygen to the brain due to a sudden drop in blood pressure. Taber's Cyclopedic Medical Dictionary.[3] describes this as the "feeling of impending death" caused by expansion of the aorta, drawing blood from the head and upper body.

The autonomic nervous system's physiologic state (see below) leading to loss of consciousness may persist for several minutes, so:

  1. If sufferers try to sit or stand when they wake up, they may pass out again;
  2. The person may be nauseated, pale, and sweaty for several minutes.


Vasovagal syncope occurs in response to a trigger, with a corresponding malfunction in the parts of the nervous system that regulate heart rate and blood pressure. When heart rate slows, blood pressure drops, and the resulting lack of blood to the brain causes fainting.[4]

Typical triggers for vasovagal episodes include:[5]

  • Prolonged standing or upright sitting
  • Standing up very quickly
  • Stress
  • P.O.T.S.(Postural Orthostatic Tachycardia Syndrome) Multiple chronic episodes are experienced daily by many patients diagnosed with this syndrome. Episodes are most commonly manifested upon standing up.
  • Any painful or unpleasant stimuli, such as:
    • Trauma
    • Venipuncture
    • Watching or experiencing medical procedures
    • High pressure on or around the chest area after heavy exercise
  • Arousal or stimulants, e.g. sex
  • Sudden onset of extreme emotions
  • Lack of Sleep
  • Dehydration
  • Urination ('micturition syncope') or defecation, having a bowel movement ('defecation syncope')
  • Random onsets due to nerve malfunctions
  • Pressing upon certain places on the throat, sinuses, and eyes (also known as vagal reflex stimulation when performed clinically)
  • Use of certain drugs that affect blood pressure, such as amphetamine


Regardless of the trigger, the mechanism of syncope is similar in the various vasovagal syncope syndromes. In it, the nucleus tractus solitarius of the brainstem is activated directly or indirectly by the triggering stimulus, resulting in simultaneous enhancement of parasympathetic nervous system (vagal) tone and withdrawal of sympathetic nervous system tone.

This results in a spectrum of hemodynamic responses:

  1. On one end of the spectrum is the cardioinhibitory response, characterized by a drop in heart rate (negative chronotropic effect) and in contractility (negative inotropic effect) leading to a decrease in cardiac output that is significant enough to result in a loss of consciousness. It is thought that this response results primarily from enhancement in parasympathetic tone.
  2. On the other end of the spectrum is the vasodepressor response, caused by a drop in blood pressure (to as low as 80/20) without much change in heart rate. This phenomenon occurs due to vasodilation, probably as a result of withdrawal of sympathetic nervous system tone.
  3. The majority of people with vasovagal syncope have a mixed response somewhere between these two ends of the spectrum.

One account for these physiological responses is the Bezold-Jarisch reflex.


In addition to the mechanism described above, a number of other medical conditions may cause syncope. Making the correct diagnosis for loss of consciousness is one of the most difficult challenges that a physician can face. The core of the diagnosis of vasovagal syncope rests upon a clear description by the patient of a typical pattern of triggers, symptoms, and time course. It is also pertinent to differentiate lightheadedness, seizures, vertigo, and hypoglycemia as other causes.

In patients with recurrent vasovagal syncope, or defecation syncope, diagnostic accuracy can often be improved with one of the following diagnostic tests:

  1. A tilt table test
  2. Implantation of an insertable loop recorder
  3. A Holter monitor or event monitor
  4. An echocardiogram
  5. An electrophysiology study


Treatment for vasovagal syncope focuses on avoidance of triggers, restoring blood flow to the brain during an impending episode, and measures that interrupt or prevent the pathophysiologic mechanism described above.

  • The cornerstone of treatment is avoidance of triggers known to cause syncope in that person. However, new development in psychological research has shown that patients show great reductions in vasovagal syncope through exposure-based exercises with therapists.[6]
  • Because vasovagal syncope causes a decrease in blood pressure, relaxing the entire body as a mode of avoidance isn't favorable.[6] A patient can cross his/her legs and tighten leg muscles to keep blood pressure from dropping so drastically before an injection.[7]
  • Before known triggering events, the patient may increase consumption of salt and fluids to increase blood volume. Sports and energy drinks may be particularly helpful.
  • Discontinuation of medications known to lower blood pressure may be helpful, but stopping antihypertensive drugs can also be dangerous. This process should be managed by an expert.
  • Patients should be educated on how to respond to further episodes of syncope, especially if they experience prodromal warning signs: they should lie down and raise their legs; or at least lower their head to increase blood flow to the brain. If the individual has lost consciousness, he or she should be laid down with his or her head turned to the side. Tight clothing should be loosened. If the inciting factor is known, it should be removed if possible (for instance, the cause of pain).
  • Wearing graded compression stockings may be helpful.
  • There are certain orthostatic training exercises which have been proven to improve symptoms in people with recurrent vasovagal syncope. A technique called 'Applied Tension' which involves learning to tense the muscles in your torso, arms, and legs is effective for vasovagal Syncope.
  • Certain medications may also be helpful:
    • Beta blockers (β-adrenergic antagonists) were once the most common medication given; however, they have been shown to be ineffective in a variety of studies and are thus no longer prescribed.[8][9]
    • Other medications which may be effective include: fludrocortisone, midodrine, SSRIs such as paroxetine or sertraline, disopyramide, and, in health-care settings where a syncope is anticipated, atropine.
  • For people with the cardioinhibitory form of vasovagal syncope, implantation of a permanent pacemaker may be beneficial or even curative.[citation needed]


Brief periods of unconsciousness do no harm and are seldom symptoms of disease. The main danger of fainting fits or vasovagal syncope (or dizzy spells from vertigo) is the risk of injury by falling while unconscious.

See also


  1. ^ "vasovagal attack" at Dorland's Medical Dictionary
  2. ^
  3. ^ Thomas, Clayton L. (1993). Taber's Cyclopedic Medical Dictionary (18 ed.). F.A. Davis. ISBN 0-8036-0194-8. 
  4. ^
  5. ^ Vasomotor and vasovagal syncope
  6. ^ a b Durand, VM, and DH Barlow. 2006. Essentials of Abnormal Psychology 4th Edition. pp. 150.
  7. ^ France CR, France JL, Patterson SM (January 2006). "Blood pressure and cerebral oxygenation responses to skeletal muscle tension: a comparison of two physical maneuvers to prevent vasovagal reactions". Clin Physiol Funct Imaging 26 (1): 21–5. doi:10.1111/j.1475-097X.2005.00642.x. PMID 16398666. 
  8. ^ Sheldon R, Connolly S, Rose S et al. (March 2006). "Prevention of Syncope Trial (POST): a randomized, placebo-controlled study of metoprolol in the prevention of vasovagal syncope". Circulation 113 (9): 1164–70. doi:10.1161/CIRCULATIONAHA.105.535161. PMID 16505178. 
  9. ^ Madrid AH, Ortega J, Rebollo JG et al. (February 2001). "Lack of efficacy of atenolol for the prevention of neurally mediated syncope in a highly symptomatic population: a prospective, double-blind, randomized and placebo-controlled study". J. Am. Coll. Cardiol. 37 (2): 554–9. doi:10.1016/S0735-1097(00)01155-4. PMID 11216978. 

External links

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