Nitroglycerin: Major actions and misconceptions
Nitroglycerin or "Nitro" is a commonly encountered medication for EMS, but it may not work the way you think or do what you think it does
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What is nitroglycerin used for?
Nitroglycerin (NTG), also known as glycerine trinitrate (GTN) is a nitrate medication typically administered to relieve anginal chest pain and manage blood pressure through vasodilation. Different preparations of nitroglycerin may also be administered for treatment of renal issues, liver issues and even as an ointment for anal fissures or tears.
Nitroglycerin can be administered through a wide variety of methods, including:
- Sublingual (under the tongue) tablets,
- Sublingual powder,
- Sublingual spray,
- Transdermal (through the skin) paste,
- Transdermal patch,
- Rectal ointment,
- IV infusion, and
- Extended release oral tablets.
Names and dosages vary depending on the route of administration.
This article will concentrate on the forms of nitroglycerin most commonly encountered by EMS; sublingual tablets and sprays, transdermal paste, transdermal patches and IV infusions.
Nitro administration routes
Nitroglycerin dosage, administration routes and trade names include:
- Nitroglycerin sublingual tablets: Nitrostat, individual tablets, 0.3 mg, 0.4 mg or 0.6mg
- Nitroglycerin sublingual spray: Nitrolingual, 1 metered spray, 0.3 mg
- Nitroglycerin transdermal paste: Nitro-Bid, applied with a measuring paper, 7.5mg per half inch
- Nitroglycerin transdermal patch: Nitro-Dur, pre-measured self-adhesive patch, 0.2 mg/hr, 0.4 mg/hr, 0.6 mg/hr, 0.8 mg/hr
- Nitroglycerin IV solution: glyceryl trinitrate IV, pre-mixed solutions, typically 25 mg/250mL, 50mg/250mL, 100 mg/250mL that may be administered as loading boluses of up to 600 mcg followed by maintenance infusions beginning at 5 mcg/min as dictated by patient conditions
The forms of nitroglycerin typically administered by EMS providers of different levels include sublingual tablets or sprays; transdermal paste; and, for some advanced providers, IV nitroglycerin solutions.
The forms of nitroglycerin used by patients typically encountered by EMS providers in the field include sublingual tablets or sprays and transdermal patches.
Nitroglycerin mechanism of action
Nitroglycerin is a vasodilator that has an especially strong effect on vascular smooth muscle found in peripheral veins. Arterial vasodilation may also occur at higher doses of nitroglycerin.
Nitroglycerin forms free radical nitric oxide (NO), which in turn activates chemicals in the blood that cause smooth muscle to relax, resulting in vasodilation. Nitroglycerin is primarily metabolized by liver enzymes.
Sublingual forms of nitroglycerin are absorbed quickly, in one to three minutes, with peak effects in about five to 10 minutes. Sublingual doses of nitroglycerin continue to work for approximately 30 minutes, but with less effect.
Sublingual nitroglycerin is typically administered one tablet or spray every five minutes up to three doses. Some prehospital protocols recommend continuing dosage and even administering higher dose nitroglycerin two to three sublingual doses together in cases of hypertensive congestive heart failure.
Sublingual nitroglycerin is typically discontinued once the desired effect is achieved (typically reduced anginal chest pain or relieved symptoms of congestive heart failure) or if contraindications are encountered.
Transdermal forms of nitroglycerin work more slowly, but maintain therapeutic levels over a much longer period of time, typically 12 to 24 hours. Transdermal nitroglycerin is typically applied by patients to prevent the development of anginal chest pain and by EMS providers to help maintain consistent therapeutic levels of nitroglycerin in the field after administration of sublingual nitroglycerin.
IV solutions of nitroglycerin may be administered for a variety of purposes, but are most commonly used by EMS providers for patients who are intolerant of or unresponsive to sublingual nitroglycerin, or who require higher doses of nitroglycerin than can be provided sublingually or transdermally.
Desired effects and indications of nitroglycerin
Vasodilation decreases myocardial preload and afterload as well as decreasing blood pressure. This decrease in pressure reduces the workload of the heart and helps to relieve heart failure.
For EMS providers, typical nitroglycerin indications include chest pain or discomfort associated with angina pectoris or suspected acute myocardial infarction, as well as pulmonary edema with hypertension.
For patients at home, indications typically include prevention or treatment of chest pain, or discomfort associated with angina pectoris or suspected acute myocardial infarction.
Contraindications of nitroglycerin
ABSOLUTE: Discontinue nitroglycerin administration or do not administer nitroglycerin to patients who:
- Have a known sensitivity to nitrate medications
- Have taken erectile dysfunction medications within the past 24 hours, such as Viagra, Cialis, Levitra, Stendra, Staxyn, sildenafil, avanafil, tadalafil or vardenafil.
- Are hypotensive (typically <90 SBP or <65 MAP)
- Are children under 12 years old
- Are experiencing increased intracranial pressure
- Have severe anemia
RELATIVE: Nitroglycerin administration may be considered with caution to patients who:
- Have a known or suspected right ventricular infarct and are thus heavily dependent on maintaining pre-load (although recent studies have found few complications with administration of nitro in RV infarcts)
- Are bradycardic
- Are tachycardic
- Are currently taking antihypertensive medications, including beta blockers, phenothiazines or other nitrates
- Are under the effects of alcohol ingestion or intoxication
Nitroglycerin side effects include:
- Reflex tachycardia
- Proper storage of nitroglycerin is essential. Nitroglycerin is rapidly inactivated by light, heat, air contact and moisture.
- Patients should not chew or swallow nitroglycerin tablets.
- Alcohol consumption prior to or concurrent with nitroglycerin use may increase the vasodilating and hypotensive effects of nitroglycerin.
- Patients may develop a tolerance or resistance to nitroglycerin over time and may require increased dosages for therapeutic effect.
- Excessive nitrate administration may cause methemoglobinemia, a condition that prevents red blood cells from releasing oxygen to the tissues.
- When administering the spray form of nitroglycerin, beware that no providers accidentally inhale the medication due to close proximity with the patient or a poorly directed spray.
- Nitroglycerin from transdermal patches or paste may be absorbed through any contact with the skin. Always wear gloves when handling, applying or removing. When removing, clean the area by wiping with gauze and disposing of carefully.
Nitroglycerin myths and misconceptions
Like many commonly encountered prehospital treatments, some of the common knowledge surrounding nitroglycerin contains more myths and misconceptions than evidence-based facts. Here are some of the top myths and misconceptions about nitroglycerin treatment:
- If the patient does not react to nitroglycerin it can’t be cardiac pain. MYTH: While nitroglycerin is an effective prophylactic against and treatment for anginal chest discomfort, nitroglycerin will not relieve all cardiac signs and symptoms.
- Nitroglycerin is a pain-killer. MISCONCEPTION: While nitroglycerin can lessen anginal chest pain by helping to decrease myocardial workload (and therefore, oxygen consumption), it is not, itself, an analgesic (pain-relieving) medication.
- If the patient is having chest pain, nitroglycerin will improve their outcome. MYTH: While nitroglycerin may help lessen the pain from angina pectoris and even myocardial infarction, it is not associated with improved outcomes. In addition, prolonged exposure to nitroglycerin may inhibit aldehyde dehydrogenase (ALDH2), an enzyme that may have a significant cardioprotective role during cardiac events.
- Administration of nitroglycerin to a patient with right sided heart failure will kill the patient. MISCONCEPTION: While patients suffering from right-sided heart failure are particularly dependent on their preload, which administration of nitroglycerin reduces, it is not typically considered an absolute contraindication in these patients. Any patient administered nitroglycerin who develops hypotension should receive a trial administration of IV fluids. Normal precautions for administration of fluids to patients with any type of heart failure, including monitoring of blood pressure and lung sounds apply.
- You have to start an IV before administering nitroglycerin. MYTH: While intravenous access is routine for any patient with suspected acute coronary syndromes, patients often take multiple doses of nitroglycerin at home with no IV access in place. As with any medication, nitroglycerin administration should be guided by local protocols and medical direction.
- If a patient responds to nitroglycerin, they must be having a cardiac event. MISCONCEPTION: As a smooth muscle agent, nitroglycerin will often relieve symptoms associated with gall bladder obstruction or spasm, as well as esophageal irritation and spasm.
Take the opportunity to share this article with your fellow providers. Powerful treatments like nitroglycerin demand well-informed responders.