From the Guidelines
Atropine is contraindicated in high-degree (second- or third-degree) atrioventricular (AV) heart blocks because it can worsen the condition rather than improve it. Atropine works by blocking parasympathetic (vagal) influence on the heart, which typically increases heart rate by acting on the sinoatrial (SA) node. However, in high degree heart blocks, the problem lies in the conduction system below the SA node, particularly at the atrioventricular (AV) node or His-Purkinje system. When atropine is administered in these cases, it can increase atrial rate without improving ventricular conduction, potentially worsening the block by increasing the number of non-conducted impulses. This can lead to a dangerous situation where the atria beat faster but the ventricles cannot keep up, potentially causing hemodynamic compromise.
According to the 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1, high-degree AV block is generally considered to be intra- or infra-Hisian and treated with pacing. The use of atropine in patients with infranodal conduction disease or block can be associated with exacerbation of block and is potentially harmful, as stated in the 2018 ACC/AHA/HRS guideline 1.
Instead of atropine, patients with symptomatic high degree heart blocks should receive transcutaneous pacing, dopamine, epinephrine, or isoproterenol as temporary measures until definitive treatment with a permanent pacemaker can be implemented. The American Heart Association guidelines for cardiopulmonary resuscitation and emergency cardiovascular care also recommend avoiding reliance on atropine in type II second-degree or third-degree AV block or in patients with third-degree AV block with a new wide-QRS complex where the location of block is likely to be in non-nodal tissue 1.
Key points to consider in the management of high-degree AV heart blocks include:
- Avoiding the use of atropine due to its potential to worsen the condition
- Using transcutaneous pacing, dopamine, epinephrine, or isoproterenol as temporary measures
- Implementing definitive treatment with a permanent pacemaker
- Carefully evaluating the ECG to diagnose atrioventricular block and determine the level of block.
From the FDA Drug Label
Atropine also may lessen the degree of partial heart block when vagal activity is an etiologic factor In some patients with complete heart block, the idioventricular rate may be accelerated by atropine; in others, the rate is stabilized. Occasionally a large dose may cause atrioventricular (A-V) block and nodal rhythm.
Atropine is contraindicated in high-degree (second- or third-degree) atrioventricular (AV) heart blocks because it can worsen the existing block. Although atropine may lessen the degree of partial heart block when vagal activity is the cause, it can also cause atrioventricular (A-V) block and nodal rhythm, particularly in large doses. This suggests that atropine can potentially exacerbate or worsen high-degree AV blocks. 2
From the Research
Atropine Contraindication in High-Degree AV Blocks
- Atropine is contraindicated in high-degree (second- or third-degree) atrioventricular (AV) heart blocks due to its potential to worsen the condition 3, 4, 5.
- Studies have shown that atropine can paradoxically cause high-degree AV block in patients after cardiac transplantation 3, 4.
- The exact mechanism of this paradoxical response is not clear, but it is thought to be related to the location of the heart block, with patients having infranodal blocks being at higher risk of adverse events 5.
- Atropine can lead to complete atrioventricular block or sinus arrest in some patients, especially those with pre-existing heart block or conduction abnormalities 4, 5.
- The use of atropine in patients with high-degree AV blocks can result in significant morbidity and mortality, and alternative treatments such as pacing or other medications should be considered 4, 6, 7.