Cardiac & Resuscitation, Columns, Patient Care, Special Topics

Study Assesses Nitroglycerin’s Effect on Hypotension in STEMI Patients

Issue 12 and Volume 40.

THE RESEARCH

Robichaud L, Ross D, Proulx MH, et al. Prehospital nitroglycerin safety in inferior ST elevation myocardial infarction. Prehosp Emerg Care. Aug. 19, 2015. [ePub ahead of print].

THE SCIENCE

This study was designed to determine if nitroglycerin (NTG) administration is more likely to cause hypotension in patients with an inferior wall ST elevation myocardial infarction (STEMI) versus one somewhere else in the heart. The authors defined hypotension as systolic blood pressure < 90 mmHg. They also examined the frequency of systolic blood pressure dropping more than 30 mmHg after NTG administration.

The information was extracted from retrospective chart review of 1,488 EMS reports that had EMS 12-lead ECGs for which the computer interpretation was “acute MI.” They found that hypotension occurred as often in the inferior STEMI (8.2%) as it did in the noninferior STEMI (8.9%) and that systolic blood pressure dropped more than 30 points in 23.4% of the inferior and 23.9% of the noninferior STEMIs.

The authors concluded that there was no difference in NTG-induced hypotension between the inferior and non-inferior STEMIs and that “computer interpretation of the inferior STEMI cannot be used as the sole predictor for patients who may be at a higher risk for hypotension following NTG administration.”

MEDIC WESLEY COMMENTS

Again, another study that disproves what we prehospital providers are currently taught. It’s just been the last few years that the emphasis on right side ECG leads has become the standard to rule out right ventricular (RV) infarct.

I’m not sure that even with the evidence of this study that it will allow us to disregard the concerns of NTG in RV involvement.

Whether a service can interpret, transmit, or use computer-based reading, EMS is expected to make the best judgment for prompt care prior to ED or cath lab care. And if the patient has chest pain and RV involvement, I don’t see how we can disregard the concern of hypotension.

I’ve personally been met at the door by the cardiologist who asks if I did right-sided chest leads before giving nitro. I can’t imagine telling him the American Heart Association (AHA) guidelines and my protocols may not be relevant.

It doesn’t hurt to be cautious when giving nitro to anyone.

And to be honest, it doesn’t hurt to be cautious when giving nitro to anyone. I think it keeps us on our toes. Until I see this change in my protocols and hear it from my medical director, I feel it’s best to keep on keepin’ on with what they know is best practice.

DOC WESLEY COMMENTS

I can’t count the number of times I’ve heard someone say during conference presentation on STEMI, “Don’t give NTG to the inferior MI patient until you’ve done right-sided chest leads to rule out RV infarction.”

It’s become ingrained into many courses and also in the 2010 AHA guidelines, which state: “Administer nitrates with extreme caution, if at all, to patients with inferior STEMI and suspected RV involvement because these patients require adequate RV preload.” But then again, it’s the same AHA guidelines that warn us about giving NTG to patients on erectile dysfunction medication and I know we never forget to ask about that.

But seriously, where did this hysteria over hypotension come from? Let’s look at the percentages. Almost half of all heart attacks occur in the inferior wall that’s supplied by the right coronary artery. Half of inferior MIs involve the right ventricle. Theoretically, if the right ventricle is weakened from an infarct, it won’t function well. Since it functions best when provided sufficient pre-load (blood returning to the heart), it should be more seriously jeopardized by hypotension.

But that’s where the theory doesn’t follow reality. NTG is an equal arterial and venodilator; therefore, it should affect equally those with inferior and non-inferior STEMIs, which is exactly what the authors of this study found. But what about the patients who are already hypotensive?

It stands to reason that if you’re hypotensive, NTG is contraindicated. In fact, in his study almost half the prehospital STEMIs didn’t get NTG. Why not? Perhaps they were already hypotensive or their pain had diminished.

So, chalk up another myth going down in flames.

Learn more from Keith Wesley at the EMS Today Conference & Expo, Feb. 25–27, in Baltimore, Md. EMSToday.com

More Research from JEMS.com.