As requested, during our previous Adenosine discussion, we will briefly review, Parasympathetic stimulation and Atropine pharmacodynamics on the heart.
ACETYLCHOLINE (ACh) is one of the Neurotransmitters, a chemical signal, used by the Central Nervous System, which has many effects on the body, from stimulating muscle contraction, inducing peristalsis (digestion), Bile release by the liver, and as discussed here, decreasing Sinoatrial Node (SAN) and Atrioventricular Node (AVN) stimulation. When the later occurs, often we encounter its effect recorded on the ECG, seen as:
- Sinus Bradycardia
- SA Blocks
- AV Blocks
The most common symptoms of Vagal stimulation include:
- Vasovagal Syncope
- Nausea and vomiting
ACh is released during Vagus Nerve (Cranial Nerve X) stimulation ,which in the heart, binds to M2 Muscarinic Receptors, one of the 5 types of Muscarinic Receptors, which mainly work in CNS and skeletal muscle. Out of all these receptors, binding of ACh to M2 receptors affects the heart and its overall conductivity.
How does this work?
- Decrease Cyclic Adenosine Monophosphate (cAMP) intracellular
- This slows down L-type Calcium Channel opening, leading to decreased automaticity and slightly decreasing contractility
- Potassium (K+) efflux (leaving the cell) is delayed, which prolongs repolarization, delaying the next action potential
The combination of all these actions, hyperpolarize the cells, increasing SA Nodal and AV Nodal threshold, which decreases the overall conduction, mainly through the AVN. This is known as Negative Dromotropic Effect.
Atropine, an antichollinergic, derived from the plant, Atropa Belladonna, or “Deadly Nightshade flower”, blocks ACh binding to M2 receptors, giving it the “Parasympatholytic” property. The goal is not necessarily to increase SAN function, but rather, block the parasympathetic response produced by M2 receptor stimulation, leading to normal SAN and AVN function.
Now that we understand how Vagal Stimulation affects our cardiac function, the use of Atropine makes a bit more sense during suspected bradycardia induced symptoms.