The vagal cardiac branches are parasympathetic fibres from the vagus nerve (CN X) descending to the heart via superior and inferior cervical cardiac branches and thoracic cardiac branches. They synapse in intrinsic cardiac ganglia (primarily in the epicardium at the SAN and AVN) to slow the heart rate and reduce AV conduction velocity. The left vagus predominantly affects the AVN; the right vagus predominantly affects the SAN.
The vagal cardiac branches are the pathway for the cardioinhibitory component of the vasovagal syncope reflex, where excessive vagal tone slows the heart rate (bradycardia or asystole) and reduces blood pressure. Vagal manoeuvres (Valsalva, carotid sinus massage) terminate supraventricular tachycardias by transiently increasing vagal tone at the AVN. Atropine blocks vagal cardiac effects to treat symptomatic bradycardia. Cardiac vagal denervation occurs in heart transplant recipients, eliminating the resting vagal tone and producing the characteristic higher resting heart rate of the transplanted heart.
Orthotopic heart transplantation surgically denervates the donor heart, eliminating all vagal cardiac branch innervation and the resting vagal tone; the transplanted heart has a higher resting rate (90-100 bpm) due to unopposed intrinsic SAN automaticity, does not respond to vagal manoeuvres, and recovers slowly after exercise cessation due to the absence of vagally-mediated deceleration.
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