In this episode, Tyler interviews Tom Bouthillet and Dr. Stephen Smith on who exactly should get a right-sided ECG.
Imagine you are transporting a patient complaining of chest pain and presenting with this ECG.
You have activated the cath-lab and are running through your ACS guidelines. Is there any reason to perform a right sided ECG? How common is an isolated RV infarct?
In EMT school, I was taught how to assist a patient taking their own nitroglycerin if they developed chest pain. I had to make sure they weren't on any phosphodiesterase inhibitors, grab a blood pressure, and make sure they took the right dose. We would obtain a 12 lead, but I had no clue what I was looking at, and my decision to give nitro was not based on any specific ECG finding.
Fast-forward to paramedic school, and I am taught to ALWAYS perform a 12 lead before giving nitroglycerin. Why? Wellll If they had an inferior wall MI, nitroglycerin was a hard stop. Every time the student would give nitro before obtaining a 12 lead in simulation, their patients would code...Every. Time.
I thought this was weird because patients were prescribed nitroglycerin if they developed chest pain at home. They were certainly not performing a 12 lead on themselves prior to doing this. So what was the fear?
The Fear
EMS is full of cautionary tales (as my buddy Brian Behn points out in this blog). The fear of administering nitroglycerin to a patient with an inferior wall MI is the possibility of plummeting the blood pressure if there is right ventricular (RV) involvement.
Because the RV is preload dependent, dropping preload with nitroglycerin could cause hypotension. This is probably a good place to say that the LV is preload dependent too, but the LV preload is dependent on the RV preload. So if you wipe out the RV, the LV follows.
I believe the fear of nitro is probably healthy, but not for JUST inferior wall MIs. The benefit of sublingual nitro has yet to be proven (as Dr. Smith points out in the interview), and on top of that, a study published in Prehospital Emergency Care in 2015(4) found that hypotension occurred post NTG in 38/466 inferior STEMIs and 30/339 non-inferior STEMIs, 8.2% vs. 8.9%, p = 0.73. That means it makes literally no difference where the MI is when predicting who will become hypotensive after receiving nitroglycerin.
"A drop in systolic blood pressure ≥ 30 mmHg post NTG occurred in 23.4% of inferior STEMIs and 23.9% of non-inferior STEMIs, p= 0.87. There was no statistical difference suggesting inferior wall MI's are more likely to cause hypotension (4)."
Ok, but what if the RV is involved?
While the clinician should suspect right ventricular wall involvement when an inferior wall MI is present, it is definitely not guaranteed. This illustration is looking up from below the heart at the different sources of blood supply.
You probably remember hearing about "dominance" in school. The way dominance is defined is by which artery supplies blood to the posterior descending artery (PDA). It is estimated that close to 80% of the population is right dominant, meaning the PDA is supplied by the RCA (1).
So does this mean that the majority of the time we see an inferior wall MI, the right ventricle is involved? The incidence of right ventricular myocardial infarction co-existing with inferior wall left ventricular dysfunction is said to range from 30-50% (1). However, these are not ISOLATED RV infarcts. A truly isolated RV infarct is extremely rare, but it does exist. However, I have yet to see a case of this where the patient's blood pressure wouldn't already exclude them from receiving nitroglycerin.
I have always found it interesting that most guidelines use systolic blood pressure limitations for giving nitroglycerin when the diastolic pressure is what feeds the coronary arteries. I did an entire blog on this topic here. In this episode, Dr Smith stated if diastolic is < 70 mmHg, nitro may compromise coronary perfusion pressure.
So the only reason left to perform a prehospital ride-sided ECG would be if it helped you detect an OMI and early activation of PCI in a 12 lead with no other PCI qualifying criteria. I asked the EMS folks on Twitter what their experience has been.
I was actually pretty impressed. Out of 54 people, almost half of them have gone straight to the cath lab because of findings from doing a right-sided ECG. Of course, the first question I have is how many of these had no other evidence of OMI on the traditional 12 lead. We know that 30-50% of inferior wall MI's have RV involvement, so if you did a right-sided ECG on your inferior wall MI, that may explain these results.
Here were my mine take-aways.
Do not delay transport to PCI to grab a right-sided ECG.
If you do decide to perform a right-sided ECG, it should not be for the decision on whether or not to give nitro.
If time permits, it may be helpful and confirm your suspicions of RV involvement.
Isolated RV infarcts are extremely rare.
Dr. Smith mentioned if you have S-T elevation in lead V1 (>0.5 mm), and no competing S-T depression in V2, that is very specific but not sensitive for RV involvement. Here is a link to this paper.
We had a lot of fun with this conversation and appreciate Tom Bouthillet and Dr. Stephen Smith for their time and invaluable contributions to FOAMed.
Click on the book below to download Dr. Smith's book "The ECG in Acute MI" for free:
Check out Tom Bouthillet YouTube Channel HERE!
And be sure to check out our EMS and Nursing refresher!
References
(1) Jeffers JL, Boyd KL, Parks LJ. Right Ventricular Myocardial Infarction. [Updated 2020 Aug 16]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK431048/
(2) Saw, J., Davies, C., Fung, A., Spinelli, J. J., & Jue, J. (2001). Value of ST elevation in lead III greater than lead II in inferior wall acute myocardial infarction for predicting in-hospital mortality and diagnosing right ventricular infarction. The American Journal of Cardiology, 87(4), 448–450.doi:10.1016/s0002-9149(00)01401-6
(3) Shahoud JS, Ambalavanan M, Tivakaran VS. Cardiac Dominance. [Updated 2020 Sep 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537207/
(4) Laurie Robichaud, Dave Ross, Marie-Hélène Proulx, Sébastien Légaré, Charlene Vacon, Xiaoqing Xue & Eli Segal(2016)Prehospital Nitroglycerin Safety in Inferior ST Elevation Myocardial Infarction,Prehospital Emergency Care,20:1,76-81,DOI: 10.3109/10903127.2015.1037480
(5) Bischof JE, Worrall CI, Smith SW. In inferior myocardial infarction, neither ST elevation in lead V1 nor ST depression in lead I are reliable findings for the diagnosis of right ventricular infarction. J Electrocardiol.
2018 Nov-Dec;51(6):977-980. doi: 10.1016/j.jelectrocard.2018.08.010. Epub 2018 Aug 10. PMID: 30497759.