Understanding Atropine

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
  • Dizziness

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

atropine

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.

 

2 Comments

  • James M says:

    I love these drug summaries. Thanks a lot, Ivan, for taking the time to do them. I look forward to seeing more! Perhaps amiodarone?

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Comments
Anne
Adenosine for sinus tachycardia: Try to avoid this!
If it were me, I would seek a second opinion; consult an electrophysiologist. Unlikely that you had sinus tachycardia at that rate.
2015-01-26 09:46:57
John
Adenosine given for a narrow-complex tachycardia over 150
If things such as Sepsis, Drug Use, Medication issues, Anxiety, Pain, etc.. have been ruled out, and the rate is still >160 (or whatever number you use). How else do you determine S-Tach Vs. SVT? Does a 12 Lead Help (QRS Complexes facing different directions due to different pathways etc..) or a simple 4 Lead?…
2015-01-19 19:44:32
Brandon O
Adenosine for sinus tachycardia: Try to avoid this!
She may not have had anxiety, but I bet she was a carrier...
2015-01-18 02:31:08
Brooks Walsh MD
Adenosine for sinus tachycardia: Try to avoid this!
Highly unusual for this to be sinus at that rate!
2015-01-17 19:21:04
Mel
Adenosine for sinus tachycardia: Try to avoid this!
I experience bouts of tachycardia. I had it investigated about 5 years ago after being woken during the night by projectile vomiting and feeling my heart racing/thumping with carotid pulse that I could certainly palpate but had no chance of counting (whether the 2 symptoms were related or coincidental, I don't know). I've had multiple…
2015-01-17 18:47:19

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