We will discuss what nitroglycerine does in the body, what it is used for, and what the doses are. To start off, nitroglycerine is a vasodilator which means 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 which 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 with causes the tissue to become oxygenated again, this works the same in a myocardial infarction which is the technical name for heart tissue death. It also works on hypertension because there is less resistance when the vessel diameter is larger so there is less pressure, therefore, the blood pressure is lower. How this affects pulmonary edema is the higher the pressure in the blood vessels in the lungs the higher the fluid pressure has to be in order to pass through the vessel wall and be reabsorbed into the bloodstream. The fluid will follow the path of least resistance. This is also affected by the decreased workload on the heart, if the heart is not having to pump at a higher pressure to circulate the blood then it will also not likely backflow into the lungs causing pulmonary edema-like we see in congestive heart failure.
What are some reasons why we would not give nitroglycerine? We would not give nitroglycerine to a patient that is allergic to it, are bradycardic as it can further drop their pressure if they are already becoming hypotensive, or to patients that have recently taken erectile dysfunction medications as this can also cause a drop in their 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 cause a drop in pre-load to the heart causing a further drop in pressure.
What are side effects of nitroglycerine? Patients can have a headache, dizziness, nausea, hypotension, or flushing with the administration of nitroglycerine. It is important to monitor the patient for these things and to also reassure your patient that these are known side effects and attempt to keep the patient as comfortable as possible. 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 20mcg/min and 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 an 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 or to increase a dose 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, increase or decreases in pain.
In conclusion, nitroglycerine can be an effective medication in controlling pain due to angina or myocardial infarction by causing vasodilation and hopefully allowing some blood flow to the ischemic area of the heart, and also by decreasing workload and oxygen demand of the heart. It can also improve pulmonary edema by decreasing workload on the heart, and it can improve hypertension by vasodilation. Nitroglycerine drips are another tool in our toolbox for managing patients that do not have relief with sublingual nitroglycerine or patches as the dose can be titrated to the symptoms.