"In your AHA Provider manual, you will see it stated in the bradycardia algorithm section that atropine is not effective for Mobitz II and Complete Heart Block. I have had a number of people ask why it is not effective. Here is an explanation:
First, let’s look at atropine and how it works. Atropine increases firing of the sinoatrial node (atria) and conduction through the atrioventricular node (AV) of the heart by blocking the action of the vagus nerve.
With 3rd-degree block, there is a complete block and disassociation of the electrical activity that is occurring in the atria and ventricles. Since atropine’s affect is primarily on the SA node in the atria, a 3rd-degree block would prevent its affect on the SA node from influencing the rate of ventricular contraction which is needed to improve perfusion.
With Mobitz-II, aka, Second-degree AV Block Type II, the situation is similar. There is a partial block in the electrical impulses from the atria (SA) to the ventricles, and thus the affects of atropine would not significantly change the status of the ventricles.
This block can also rapidly progress to 3rd-degree block.
To summarize, Atropine may speed the firing rate of the SA node (atria), but the ventricles are not responding to anything the atria (SA node) puts out. Thus, the heart rates will not increase.
There may be some action at the AV-node with atropine, but the effect will be negligible and typically not therapeutic. Atropine in most cases will not hurt the patient with 3rd-degree block unless they are unstable and you delay pacing to give atropine.
It is important to note that Mobitz II and Complete Heart Block may be associated with acute myocardial ischemia. In this case, if atropine is used and it increases the heart rate there is a high potential for worsening of the myocardial ischemia due to the increased oxygen consumption. The increased heart rate will also reduce the diastolic filling time which may worsen coronary perfusion.
Since new onset mobitz II and Complete Heart Block are commonly associated with myocardial infarction, it would be ideal to keep the HR slow (50-60) to increase diastolic filling time. Anytime you increase HR, the diastolic filling time is what takes the biggest hit.
Transcutaneous Pacing should be the first line in symptomatic Mobitz II and Symptomatic Complete Heart Block. It is very safe & less painful than in previous times due to technology improvements. Research has shown that most individuals can tolerate > 15min of transcutaneous pacing without too much difficulty."
https://acls-algorithms.com/acls-drugs/bradycardia/
