In this post, we will discuss what nitroglycerine does in the body, what it is used for, and appropriate dosing.
Nitroglycerine is a vasodilator, meaning it causes smooth muscle in the vasculature to relax thereby decreasing blood pressure and decreasing workload on the heart. It can be used to treat angina, myocardial infarctions, hypertension, and acute pulmonary edema.
How does nitroglycerine work? Nitroglycerine is a nitrate that causes systemic vasodilation. This means it works more on veins than arteries. The nitroglycerine is converted into nitric oxide in the body which in turn causes vasodilation, relaxes smooth muscle, and decreases myocardial oxygen demand. It can also improve coronary collateral circulation by vasodilation so that even if there is plaque or an embolism causing an occlusion the increased diameter of the vessel due to vasodilation may allow blood through to the ischemic area thereby decreasing tissue death. Nitroglycerine has a short half-life of 1-4 minutes which is why it can be given every 3-5 minutes sublingually. How does this affect angina, myocardial infarctions, hypertension, and pulmonary edema? Angina is what we call the “heart pain” that patients experience with tissue ischemia to the heart. So if there is an occlusion of a coronary vessel the tissue beyond that vessel is not getting oxygen and begins to die. When the vessel is relaxed it may allow blood through, allowing the oxygenation of tissues once again. For this reason, nitroglycerine can be extremely beneficial in myocardial infarction (the technical name for tissue death). Due to increased vessel size, the pressure within the vessels is decreased, making nitroglycerine an effective means of treating acute hypertension. It may also be utilized in pulmonary edema. As pressure within the blood vessels of the lungs is decreased, fluid accumulating in the lungs can more easily pass into pulmonary circulation. Additionally, the decreased workload on the heart means blood is less likely to backflow into the lungs, mitigating the likelihood of pulmonary edema (as we see in congestive heart failure).
What are some reasons we may not give nitroglycerine? We would not give nitroglycerine to a patient that is allergic to it, is bradycardic - as it can further drop their pressure if they are already becoming hypotensive - or to patients that have recently taken erectile dysfunction medications (these medications also cause vasodilation, and the compounded effect of these medications may cause a severe drop in blood pressure). It should also be used with caution in patients that have a suspected or known right-sided infarct as the drop in blood pressure can reduce preload, resulting in a further drop in pressure.
What are the side effects of nitroglycerine? Headache, dizziness, nausea, hypotension, or flushing may all occur with the administration of nitroglycerine. It is important to monitor the patient and to also reassure your patient that these are known side effects. If the patient already has a prescription for nitroglycerine they may already be familiar with these side effects.
So we’ve decided that our patient would benefit from a nitroglycerine drip, but now what do we do? Nitroglycerine drips are usually started at 5 mcg/min and titrated up from there. It is initially titrated up by 5mcg/min until you have reached 20mcg/min. Dosing is then increased by 10mcg and then 20mcg up to 200 mcg/min. Of course, start with a lower dose and titrate up from there until you get relief from symptoms while maintaining adequate blood pressure. Refer to your local protocols regarding maximum doses that can be administered. Common infusion concentrations that are carried on ambulances are 25, 50, or 100 mg/250 ml. So if you have a 25mg/250ml solution you have 0.1 mg/ml or 100 mcg/ml. You would use a microdrip set (60gtt/ml). You need to administer 0.05 ml so 3 drops per minute or one drop every 20 seconds for a dose of 5 mcg/min. You can also use this does to titrate up by 5 mcg/min. Once you have your drip running, continue to monitor your patient for a drop in blood pressure, signs of worsened or improved perfusion, as well as increases or decreases in pain.
In conclusion, nitroglycerine can be an effective medication in controlling pain resulting from angina or myocardial infarction by causing vasodilation. Vasodilation will hopefully allow some blood flow to the ischemic area of the heart. Nitroglycerine can also help in decreasing the workload and oxygen demand of the heart, which may improve pulmonary edema and hypertension. Thus, nitroglycerine drips are another tool in our toolbox for managing patients not experiencing relief with other routes of nitroglycerine administration.