Nitroglycerin (NTG) is a powerful medication used primarily to alleviate or prevent angina pectoris, which is chest pain caused by reduced blood flow to the heart muscle. As a potent vasodilator, it works quickly to relax blood vessels, but this carries a risk of toxicity. An excessive dose can cause a serious, life-threatening medical emergency. Understanding the proper therapeutic use and recognizing the signs of an overdose are important for anyone who uses this prescription drug.
How Nitroglycerin Works and Therapeutic Use
Nitroglycerin functions as a prodrug, converting within the body to its active form, nitric oxide (NO). Nitric oxide is a signaling molecule that causes the relaxation of smooth muscles within blood vessel walls, a process known as vasodilation. This widening effect occurs in both arteries and veins, though the most pronounced effect at therapeutic doses is venous dilation.
Venous dilation reduces the amount of blood returning to the heart, lowering the pressure and volume the heart must pump against, which reduces its overall workload. By decreasing the heart’s oxygen demand and increasing blood flow to the coronary arteries, nitroglycerin corrects the imbalance causing anginal chest pain. It is available in various forms, including sublingual tablets, oral sprays, and transdermal patches, designed for immediate relief or sustained prevention.
Recognizing the Signs of Overdose
The signs of a nitroglycerin overdose stem directly from extreme, uncontrolled vasodilation. The most common symptom of toxicity is severe hypotension, or dangerously low blood pressure, which rapidly leads to inadequate blood flow to the body’s organs. This profound drop in pressure often triggers reflex tachycardia, where the heart attempts to compensate by beating rapidly.
Patients experiencing an overdose may suffer from profound dizziness, weakness, and fainting (syncope) due to the lack of blood reaching the brain. A severe, persistent throbbing headache is also common because dilation affects blood vessels in the brain. While mild headaches and flushing are typical side effects, a worsening, unrelenting headache signals potential toxicity.
A specific complication of significant overdose is methemoglobinemia, a blood disorder where hemoglobin iron is oxidized. This chemical change prevents red blood cells from effectively carrying oxygen to the body’s tissues. Symptoms include a bluish discoloration of the skin and lips (cyanosis), confusion, lethargy, and shortness of breath. This condition is associated with high-dose or prolonged exposure.
Emergency Response and Medical Management
If an overdose is suspected, the immediate priority is to contact emergency medical services or a poison control center. While waiting for professional help, the individual should lie down with their feet elevated to help direct blood flow toward the heart and brain. It is important to monitor the person’s responsiveness and breathing until medical personnel take over.
Once in a medical setting, treatment focuses on reversing the severe effects of vasodilation and stabilizing the patient’s circulation. The first line of defense against severe hypotension is the administration of intravenous fluids to increase blood volume. If fluids are insufficient to raise blood pressure, physicians may administer vasopressors, which constrict blood vessels, counteracting the effects of nitroglycerin.
If methemoglobinemia is confirmed through blood tests, a specific antidote called methylene blue may be given intravenously. Methylene blue helps reverse the oxidation of hemoglobin, restoring the blood’s capacity to transport oxygen. Providing medical staff with details about the type of nitroglycerin used, the amount taken, and the time of ingestion allows for accurate and targeted intervention.