From the Guidelines
The parasympathetic nervous system (PNS) activation indeed induces hypotension and causes vasodilation, primarily through decreasing cardiac output rather than through widespread vasodilation. The PNS promotes a "rest and digest" state in the body, which includes lowering blood pressure and causing vasodilation in specific areas like the gastrointestinal tract, genitals, and some blood vessels in the face 1. When the parasympathetic system is stimulated, the vagus nerve releases acetylcholine, which slows heart rate and reduces the force of heart contractions. This system also decreases respiratory rate, increases digestive activities, stimulates salivation, promotes urination, and facilitates sexual arousal.
The balance between the parasympathetic and sympathetic nervous systems is crucial in maintaining blood pressure and vascular tone. While the parasympathetic system has a blood pressure-lowering effect, the sympathetic system has more extensive control over blood vessel diameter throughout the body. The use of certain drugs, such as non-selective beta-blockers, can affect this balance and reduce portal pressure by decreasing splanchnic blood flow 1. However, the primary mechanism of PNS-induced hypotension is through decreased cardiac output, rather than widespread vasodilation.
In clinical practice, understanding the role of the PNS in regulating blood pressure and vascular tone is essential in managing conditions such as orthostatic hypotension and vasovagal syncope. The use of volume expanders, moderate exercise training, and tilt-training may be beneficial in reducing syncope recurrence 1. Additionally, avoiding volume depletion and prolonged exposure to upright posture and/or hot confining environments can help prevent syncope.
The evidence from recent studies, such as the 2017 practice guidance by the American Association for the Study of Liver Diseases, highlights the importance of understanding the complex interactions between the parasympathetic and sympathetic nervous systems in maintaining blood pressure and vascular tone 1. By prioritizing the most recent and highest-quality studies, clinicians can make informed decisions about the management of conditions related to PNS activation and hypotension.
From the Research
Parasympathetic Nervous System Activation
- The parasympathetic nervous system (PNS) plays a role in regulating blood pressure and vascular tone, with studies suggesting that PNS activation can induce hypotension and cause vasodilation 2, 3.
- A study published in the Journal of Hypertension found that long-term activation of vagal parasympathetic pathways reduced blood pressure in spontaneously hypertensive rats 2.
- Another study published in Federation Proceedings found that electrical stimulation of the cardiac end of the cut vagus nerve produced a cholinergic coronary vasodilation that was blocked by atropine, suggesting that PNS activation can cause vasodilation 3.
Mechanisms of Vasodilation
- The mechanisms of PNS-induced vasodilation are complex and involve the interaction of sympathetic and parasympathetic nerve terminals in the adventitia of cerebral arteries 4.
- A study published in the American Journal of Physiology found that sympathetic activation increased basilar arterial blood flow in normotensive but not hypertensive rats, suggesting that PNS activation may play a role in regulating cerebral blood flow 4.
- Another study published in the Journal of the Autonomic Nervous System found that activation of perivascular parasympathetic nerves enhanced cerebral blood flow, but did not facilitate the normalization of cerebral blood flow reduced by sympathetic stimulation 5.
Clinical Implications
- The clinical implications of PNS activation and vasodilation are significant, particularly in the management of hypertension 6.
- A study published in the Journal of Cardiovascular Pharmacology found that vasodilating beta-blockers, such as carvedilol and nebivolol, may have a more favorable effect on the vasculature compared to traditional beta-blockers, allowing for better tolerability and adherence 6.
- Overall, the evidence suggests that PNS activation can induce hypotension and cause vasodilation, and may have important clinical implications for the management of hypertension and other cardiovascular diseases 2, 3, 4, 5, 6.