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Nitro for Rate Related Ischemia

Question# 775

The recent direction from base hospital was to administer ASA regardless of the reason behind cardiac ischemia. Example: rate related pain associated with a tachydysrhythmia.

Supposing you arrive to a patient who recently experienced uncontrolled afib, when you arrive they have converted back to a sinus, they still have some chest pain, you administer ASA, should you follow up with nitro as per medical directive?

Supposing you arrived they are still in a arrhythmia, however within your nitro medical directive (HR <150), should you administer nitro to this patient?

Answer:

Cardiac ischemia is decreased blood flow and oxygen to the heart muscle (hypoxia). Ischemia may also cause serious abnormal heart rhythms. Myocardial infarction is when blood flow is completely cut off and leads to necrosis and cellular death. 

"Ischemia results from imbalance between myocardial oxygen demand and supply. An increase in HR will raise both demand and supply. HR is the main determinant of myocardial oxygen or energy demand or both and, by improving myocardial perfusion, controls oxygen supply or energy supply or both." (https://academic.oup.com/eurheartjsupp/article/10/suppl_F/F7/371270). Supply is reduced because coronary artery flow occurs primarily during diastole, but this period is reduced in tachycardia.

Coronary artery diameter and tone also affect myocardial oxygen supply. Coronary artery disease causes atherosclerotic obstructions that reduce the coronary artery diameter. Patients commonly have sufficient oxygen supply until an increased demand is placed, causing chest pain or angina.

"Although atrial fibrillation can cause chest pain and other symptoms that are similar to a heart attack, atrial fibrillation doesn't lead to a heart attack. Instead, a heart attack (myocardial infarction) occurs when the coronary artery, which supplies blood to the heart, becomes blocked, depriving the heart of vital blood and oxygen. Atrial fibrillation doesn't create the conditions that lead to a heart attack. But a heart attack may cause atrial fibrillation. If a coronary artery involved in the heart attack normally supplies blood to the atria, the lack of blood flow may damage the atrial tissue and atrial fibrillation can result." (https://newsnetwork.mayoclinic.org/discussion/atrial-fibrillation-wont-cause-heart-attack-but-can-lead-to-other-serious-complications/#)

"Atrial fibrillation, the most common arrhythmia and chronic coronary syndrome, of one of the forms of coronary ischemia, often coexist as they share many common risk factors. They have a huge influence on each other, promoting and aggravating each other through pathophysiological mechanisms, leading to a challenging vicious circle." (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10305229/#)

So, if someone has chest pain after they have converted back into a normal sinus rhythm, they will likely require follow up to ensure they don't have underlying CAD which might need treatment. If ASA is given because of chest pain, the next steps can be management of the HR and symptoms. As you can appreciate, there is no black/white answer to this. We simply cannot put all arrhythmias in the same bucket as they have different treatment paths. It is also possible to have more than one illness occurring at the same time.

Chest pain/discomfort associated with tachydysrhythmia requires a nuanced approach. The discomfort is often caused by supraphysiologic demand of a disorganized heart, not necessarily ACS/CAD. A focused history should help differentiate these issues.

Generally, ASA could be beneficial and is low risk. Nitro is a medication used to provide symptom relief through vasodilation however, there are also downsides and risks given its ability to produce hypotension, specifically in those who are preload dependent. It may not be helpful in a rate-related ischemic chest pain patient who does not have underlying CAD. There would also be no role for nitro in a pain free patient. Also, it is acceptable to attempt a treatment plan and if it does not work, discontinue and document your reasonings. 

Remember, it’s all about the “why”, as opposed to protocolizing for all scenarios. It is about using good clinical judgement and obtaining a detailed incident history, while maintaining open communication with the patient.

Furthermore, we encourage you to review and use the HEART score tool as it is a reasonable approach and should be documented on ePCR. Although troponin is not available, the other aspects are key to ask and evaluate:

https://www.mdcalc.com/calc/1752/heart-score-major-cardiac-events

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6005932/

To summarize, rate related chest pain is frequently ischemic chest pain, also known as angina.
  • Myocardial ischemia occurs when myocardial oxygen demand exceeds myocardial oxygen supply.
  • One of the factors causing increased oxygen demand is increased heart rate.
  • Increased heart rate also causes a decreased myocardial oxygen supply, as coronary artery flow occurs primarily during diastole, but this period is reduced in tachycardia.
  • Coronary artery diameter and tone also affect myocardial oxygen supply. Coronary artery disease causes atherosclerotic obstructions that reduce the coronary artery diameter.
  • Coronary spasms also reduce the coronary artery diameter and tone.

Medical Directive Category

Cardiac Ischemia

Published

07 February 2024

ALSPCS Version

5.2

Views

105

Please reference the MOST RECENT ALS PCS for updates and changes to these directives.