What happens when someone experiences vasovagal syncope?

What Happens During Vasovagal Syncope

Vasovagal syncope occurs when an inappropriate brain reflex causes your blood vessels to dilate excessively and your heart to slow down or temporarily stop, resulting in a critical drop in blood pressure that starves your brain of oxygen, causing you to lose consciousness. 1

The Physiological Cascade

Initial Reflex Activation

• A triggered brain reflex affects your circulation through two simultaneous mechanisms 1
• Blood vessels throughout your body open too widely, causing blood to pool downward in your body (particularly in the splanchnic/abdominal region), which dramatically drops your blood pressure 1, 2
• Your brain simultaneously sends inappropriate signals to your heart, instructing it to slow down or even temporarily stop beating—this is not heart disease, but a healthy heart receiving the wrong instruction 1

Hemodynamic Collapse

• The combination of vasodilation (reduced systemic vascular resistance) and bradycardia (slowed heart rate) reduces cardiac output and cerebral blood flow below critical levels 2, 3
• Your brain is affected first because it requires substantial blood flow to function and is located at the top of your body, making it the most difficult organ to pump blood to when pressure drops 1, 2
• In younger patients, reduced systemic vascular resistance predominates (75% of cases), while older patients more commonly experience reduced cardiac output 3

The Clinical Sequence

Prodromal Phase (Warning Symptoms)

• Before losing consciousness, you typically experience warning symptoms lasting seconds to minutes 1, 4
• Autonomic activation symptoms include nausea, sweating, pallor/paleness, feeling warm or cold, lightheadedness, dizziness, and visual disturbances (blurred vision, black spots, tunnel vision) 1, 2, 4
• These prodromal symptoms occur because your brain is experiencing marginal perfusion before complete loss of consciousness 2
• The final warning is when everything goes dark and you lose vision—at this point you have only seconds to prevent syncope 1

Loss of Consciousness

• Unconsciousness typically lasts less than one minute (usually no longer than 20 seconds) 1, 5, 4
• You experience flaccid collapse and fall if upright, which can result in injury 1, 4
• Myoclonic jerks (brief muscle twitches) may occur in up to 90% of cases, which can be mistaken for seizure activity 4
• Incontinence (loss of bladder control) can occur during the episode 5
• You will have no memory of what happened during the period of unconsciousness 1

Recovery Phase

• Once horizontal, blood can return to your brain without fighting gravity, and consciousness returns rapidly and completely 1, 2
• After regaining consciousness, you may feel very tired, and children especially may fall asleep 1
• Unlike seizures, there is typically no prolonged confusion after recovery 4

Heart Rate Patterns During Episodes

The European Society of Cardiology has documented specific heart rate patterns during vasovagal syncope 1:

• Progressive bradycardia pattern: Heart rate progressively decreases before loss of consciousness, similar to patterns observed during tilt table testing 1
• Progressive tachycardia pattern: In some patients, heart rate progressively increases (up to 130 bpm) as an insufficient compensatory attempt, indicating sympathetic arousal trying to maintain blood pressure 1
• Minimal variation pattern: Almost no heart rate variation excludes cardiac reflex participation, making reflex syncope less likely 1

Common Triggers

Vasovagal syncope is triggered by specific stimuli that activate the inappropriate reflex 1, 2:

• Emotional triggers: Pain, fear, seeing blood, having blood drawn, emotional distress 1, 4
• Orthostatic stress: Prolonged standing, heat exposure 2, 4
• Situational triggers: Coughing, sneezing, gastrointestinal stimulation, urination, post-exercise, post-meal 2

Critical Clinical Pitfall

Vomiting can occur as both a prodromal symptom and as a trigger for vasovagal syncope 2—however, persistent vomiting causes volume depletion that can exacerbate orthostatic hypotension and increase syncope risk 2. This creates a potential vicious cycle that must be recognized and interrupted with adequate hydration.