Pharmacology

Nitroglycerin

Nitroglycerin (NTG) is short-acting venodilator (vasodilator at higher doses) which, like all nitrates, is converted to nitric oxide to exert its effects. In medicine, NTG is routinely used to improve the mismatch between cardiac oxygen supply and demand by dilating the coronary arteries as well as the large venous capacitance vessels which return blood back to the heart. Less preload = less stretch = less contractility = less demand for oxygen.

A special note should be made regarding right ventricular (RV) dysfunction (either pre-existing, ischemic, etc.) As an intensivist and cardiothoracic anesthesiologist, depressed RV function is always challenging. I have a low threshold to perform bedside echocardiography (or TEE if warranted) with a standard/right-sided EKG looking for new right bundle branch blocks or ST segment changes in leads mapped to the coronary vessel(s) that likely supply the RV.

Patients with RV dysfunction are very sensitive to changes in preload – too much, and the RV can over-distend, fall of its Starling curve, and fail. Too little, and the left heart won’t receive adequate preload to provide adequate systemic perfusion. NTG should, therefore, be used cautiously (or avoided entirely) to avoid a precipitous drop in RV filling pressure.

Continued and prolonged use of NTG can lead to methemoglobinemia, hemolysis in patients with G6PD deficiency, as well as short-term drug tolerance (tachyphylaxis).

Interestingly, NTG is a fairly unstable explosive when subjected to external pressures like heat. In the 1880s, Alfred Nobel (where the “Nobel Prize” gets its name) combined NTG with diatomaceous earth (the same coagulation cascade activator used in my ACT tubes) to create the more stable compound he called dynamite.

Drop me a comment with questions below! 🙂

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