Typically, venous return to the heart decreases which causes a reduced volume of blood with each heartbeat. This causes the sympathetic nervous system to become stimulated which increases the force of the ventricular contractions. In turn, a certain type of receptor in the left ventricle called a mechanoreceptor is hyperstimulated which then causes a paradoxical activation of the parasympathetic nervous system to cause a slowing of the heart rate and dilation of the peripheral veins and arteries. Because the beta-adrenergic receptors are intimately involved with the excessive stimulation of the sympathetic nervous system and the increased contractility of the left ventricle, blocking these receptors with beta blockers effectively blunts the vasovagal syncope response and prevents loss of consciousness. I hope this information helps.
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