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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EMS 12-Lead

Cardiac Rhythm Analysis, 12-Lead ECG Interpretation, Resuscitation
Comments
Sean V
Rate Related VS. Primary ST-T Changes:
Also forgot to mention decrease the FiO2, 3LPM is getting us a SpO2 of 98%, titrate down so we staying at or above 94%. No need to hyperoxygenate & create all those fun free radicals. I would also include using an EtCO2 nasal cannula, lets get another measure of our cardiac output.
2014-09-20 02:32:20
Sean V
Rate Related VS. Primary ST-T Changes:
Atrial Fibrillation w/ Rapid Ventricular Response. There appears to be possible Delta Waves, the most prominent being in aVL, also leads I, II, and V6. In the EMS 12-Lead there appears to be a fusion beat, 3rd in V2, slurred R-wave appears quite consistent with a Delta wave. I would consider WPW as the primary…
2014-09-20 02:28:16
Brian
Rate Related VS. Primary ST-T Changes:
Afib. There is widespread depression in most leads and aVR has some elevation...but I am skeptical about this ecg. If a quick fluid challenge of 500-1000cc did not slow down the HR I would give him some diltiazem (5mg increments is our protocol or 0.25mg/kg) and slow the rate down a bit and see if…
2014-09-19 21:02:48
Michael Schiavone
Rate Related VS. Primary ST-T Changes:
Isolated ST elevation in AVR with ST depression in several leads. Rapid, irregular rate suggests AFIB with RVR. I would provide entry note with this exact description and leave it to hospital whether or not to activate cath lab. My EMS treatment: IV access, 324 mg. ASA, NTG, Cardizem .25 mg/kg over 2 minutes, consider…
2014-09-19 20:30:35
Dayne
Rate Related VS. Primary ST-T Changes:
AF with RVR @167, LVH and prolonged QT. ST depression to I,II and V3-6 and reciprocal elevation to aVR equal to or >1mm highly suggestive of LMCA or 3-vessel disease. High specificity for proximal occlusion. Aspirin, GTN, IV access, Spo2 >95%, Transport to nearest PCI/Cath Lab facility ASAP
2014-09-19 10:52:36

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