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Lesson

Nitroglycerin Cautions

10 min Meds & Pharma Skip to quiz

Objective: Nitroglycerin lowers preload. It helps some ischemic pain and fluid in the lungs if the pressure allows. It is dangerous in a right-ventricular infarct, low pressure, and recent phosphodiesterase-inhibitor use. The card has the amount.

Why this matters

Nitroglycerin lowers preload. It opens the veins. Less blood returns to the heart.

That helps some ischemic pain. It helps some patients with fluid in the lungs, if the pressure allows. It harms people who need that returning blood. That is a right-ventricular infarct, low pressure, or a patient who is already low on volume.

A recent phosphodiesterase-5 inhibitor (PDE5), used for erectile dysfunction or for some pulmonary hypertension, plus nitroglycerin can cause hypotension that is hard to reverse. Ask. Then check the blood pressure. Then look at the 12-lead. The card has the amount. This lesson does not.

Verbalize
  • The blood pressure is above the protocol minimum. I will recheck it after every dose.
  • I asked about phosphodiesterase-5 inhibitors, including erectile-dysfunction pills, and about pulmonary-hypertension medicines.
  • On the 12-lead, inferior ST elevation means I am thinking about a right-ventricular infarct.
  • The indication matches the protocol: ischemic pain, fluid in the lungs, or both.

Clinical sequence

  1. Chest pain on an ACS pathway: get a 12-lead first if it will not delay care. Check the systolic pressure. Ask specifically about phosphodiesterase-5 inhibitors and pulmonary-hypertension medicines before you give it.
  2. If you suspect an inferior or right-ventricular infarct, nitroglycerin can sharply drop the blood returning to the heart. Follow the protocol. Get a right-sided tracing if the protocol includes it.
  3. Give it only if the protocol and the blood pressure allow it. Recheck the blood pressure after every dose. If the pressure falls, stop. Do not give more nitrate into a falling pressure.
  4. Gastrointestinal bleeding, low pressure, or chest pain after a large blood loss: no nitroglycerin.
  5. Suspected stroke: do not start nitroglycerin, including a patch, to treat the stroke or to lower the blood pressure. The 2026 stroke guideline says prehospital nitroglycerin does not improve outcome and can cause harm. Chest-pain nitroglycerin is a separate decision, and it still needs the usual cautions.

Before You Give Nitro

  • The indication is in your protocol. Systems differ on ischemic pain, pulmonary edema, or both.
  • The blood pressure is above your protocol minimum. Recheck it after every dose.
  • Ask specifically about phosphodiesterase-5 inhibitors and pulmonary-hypertension medicines. Common teaching: sildenafil or vardenafil within about 24 hours, and tadalafil within about 48 hours. Avanafil is shorter. Many EMS protocols use one conservative window. Follow yours. These windows are not a dose.
  • Get a 12-lead when you can, before nitroglycerin, in ACS. Inferior ST elevation should make you think about a right-sided tracing and right-ventricular infarct caution.
  • IV access is useful if the blood pressure falls. Know your local rule.

Key points

  • A headache and a drop in blood pressure are expected effects, not a surprise.
  • This lesson does not authorize a dose, a spray count, or a paste amount.
  • A patient with pulmonary edema and high blood pressure is a different picture from hypotensive cardiogenic shock. Do not treat them the same way.

Common errors

  • Nitroglycerin at a systolic pressure of 80 because chest pain gets nitroglycerin. That number is an error example, not a blood-pressure cutoff. Your protocol sets the cutoff.
  • Never asking about a phosphodiesterase-5 inhibitor.
  • Skipping the 12-lead, then finding inferior ST elevation after the patient is hypotensive.
  • Starting nitroglycerin because a suspected stroke has a high blood pressure.

Practice

One-minute check

Three scenes. Say nitroglycerin go, hold, or ask medical control, and name the caution. (1) Ischemic pain, blood pressure well above the protocol minimum, no phosphodiesterase-5 inhibitor, 12-lead clear so far. (2) Chest pain, systolic pressure under the protocol minimum. (3) Inferior ST elevation. Think about a right-sided tracing and right-ventricular caution before you drop preload.

Related: STEMI, Heart failure and pulmonary edema.

Sources & Further Study

Named guidelines for further study. They are not clinical orders and do not replace your protocol or medical direction.

  • 2025 ACC/AHA/ACEP/NAEMSP/SCAI ACS Guideline — Nitroglycerin cautions in ACS, including hypotension and right-ventricular infarct.
  • NASEMSO National Model EMS Clinical Guidelines (2022) — Model EMS language. Your protocol sets the blood-pressure cutoff.
  • 2026 AHA/ASA acute ischemic stroke guideline — do not start nitroglycerin in the ambulance to treat a suspected stroke. It does not improve outcome and can cause harm.
  • Nitroglycerin is contraindicated with low pressure and recent phosphodiesterase-5 inhibitor use. Common teaching is about 24 hours for sildenafil or vardenafil, and about 48 hours for tadalafil. Use caution in an inferior or right-ventricular infarct. The protocol sets the dose and the blood-pressure cutoff.

Education only. Apply learning within your protocols and medical direction. This is not clinical care guidance.

Check Your Understanding

Questions drawn only from this lesson. After you check, the key is highlighted. Education practice — not a certification exam.

1. Chest pain, SBP under your protocol floor. Partner reaches for nitro because “chest pain gets nitro.” You:
2. Patient admits to a PDE5 inhibitor within your protocol’s window. Nitro risk is:
3. Inferior ST elevation on the 12-lead before the spray. Best framing:
4. After one nitro dose the BP tanks. Next move:
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