Myth or fact?? GTN and inferior MI
In a post on another forum I was asked why GTN shouldn’t be used in Inferior MI patients. In this KYJ I want to explore why you may have learned this relative truism.
To unpack this blanket dogma I’ll say at the outset that there are circumstances where using nitrates in patients is discouraged, but realistically, many of us do and will.
First in our quest is to understand, that more than 70% of blood in your body is currently sitting inside your veins. Like arteries, veins are muscular, and subject to dilation (relaxation), when influenced by drugs like nitrates (Anginine, nitrates, nitro lingual etc).
When taking a nitrate, all blood vessels simultaneously relax, reducing the pressure (blood pressure) inside the vessel. So after a nitrate, blood pressure inside arteries drops, as does venous pressure, resulting in poor venous flow (venous pooling). This results in a reduction of blood returning to the heart (venous return), and subsequent cardiac filling (preload) before the next contraction.
Now heart attacks (Myocardial Infarctions) are, far more often, a left heart injury. Much less frequently, is the Right Ventricle is injured. A right heart MI, is seen concomitantly in about a third of the Inferior MIs. These are injuries to the apex of the left ventricle (the pointy bit at the bottom of a heart), often caused from a blocked artery called the Left Anterior Descending (LAD) artery; that has some distal branches that perfuse portions of the right ventricle. Ok... if you are still with me.....
Glyceryl Trinitrate (GTN) (eg Anginine, nitrolingual etc) are often given first line in patients with chest pain. Frequently, in acute presentations, this is prior to an ECG having been performed yet. In these chest pain patients, the premise is to restore cardiac muscle perfusion by vasodilating the coronary arteries- letting more oxygen in.
GTN is therefore first line and precedes diagnosis of the cause of the chest pain, rendering its use as pre-emptive, or diagnostic rather than curative. That first GTN dose must be evaluated carefully. If it doesn’t have immediate effect (1-5 mins of dramatic improvement) then no more should be used until you can exclude an Inferior ECG change, or Right heart MI. Why?
Remember that GTN vasodilator effect reduces blood flow in veins. So if their right heart (which is responsible for oxygenation of blood) should fail, then the GTN actually worsens the whole heart muscle perfusion by dramatically dropping preload. Now this occurs in about 35% of Inferior MI patients and you’d never see it on an ECG unless you did a right sided ECG, or moved the V4 electrode to the right chest and ran the trace again. This is called a V4R trace.
V4R lead looks at the right ventricle. So should always be performed in Inferior MI (ST elevation in leads II, III, and or AVF). To suspect a right heart MI the V4R lead needs to show 1mm or more elevation.
So knowing NitroG is more venous dilatory than coronary, this vasodilation drops preload, in a struggling right heart which further drops pulmonary blood oxygenation and preload into the left. Worsens cardiac hypoxia.
Rule is - dont use GTN in known right MIs and extreme caution in those with inferior Type 1 MIs (STEMI).
But if you didn’t have an ECG yet, and that is usually the case; then you couldn’t have known. Worsening pain and hypotension after GTN is the Red Flag.