Autonomic control of vasovagal syncope

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Function

Background:

• The precise interaction between baroreceptor sensitivity (BS), sympathetic withdrawal, and parasympathetic activity in vasovagal syncope remains unclear.
• Altered BS may predispose individuals to abnormal autonomic responses during orthostasis, potentially leading to hypotension and syncope.

Purpose of the Study:

• To investigate the hypothesis that altered baroreceptor sensitivity (BS) contributes to abnormal sympathetic and parasympathetic responses during orthostasis, precipitating vasovagal syncope.

Main Methods:

• Continuous monitoring of blood pressure (BP), heart rate (HR), BS, forearm blood flow, and muscle nerve sympathetic activity (MNSA).
• Measurements were taken in 18 vasovagal syncope patients during head-up tilt, syncope, and recovery, compared to 17 healthy controls.

Main Results:

• During early tilt, BP was maintained by increased HR and MNSA, but BS decreased more in the syncopal group.
• At presyncope, falling BP correlated with increased sympathetic activity, while BS increased to baseline.
• At syncope, BP, HR, and sympathetic activity decreased, with no increase in BS. Vasodilatation and bradycardia were linked to sympathetic withdrawal.

Conclusions:

• The exact mechanism initiating hypotension during presyncope is unknown, but baroreceptor sensitivity (BS) may play a role.
• Vasodilatation and bradycardia in presyncope appear more related to sympathetic withdrawal than increased parasympathetic activity.