Nitroglycerin (NTG)

Category: Pharmacology Sources: raw/protocols/nm-sop-guidelines-treatment-2022.pdf; UCLA EMT Ultimate Study Guide (2026) Last updated: 2026-05-06

Overview

Nitroglycerin (NTG) is a nitrate vasodilator used to relieve ischemic chest pain in suspected ACS. It dilates coronary arteries (increasing myocardial oxygen delivery) and reduces preload by dilating venous capacitance vessels (reducing the workload the heart must pump against). In the prehospital setting, EMT-B assists patients with their own prescribed NTG or — in some systems — administers from agency stock under on-line medical control. NTG has more contraindications than any other EMT-B medication and is one of the most tested drug topics on the NREMT cognitive exam.

Key Points

• Class: Nitrate vasodilator
• Forms: Sublingual tablet (0.4 mg), sublingual spray (0.4 mg per spray), patch (not used prehospital for acute relief)
• Dose: 0.4 mg sublingual — one tablet or one spray under the tongue; allow to dissolve, do not swallow
• Repeat: May repeat every 5 minutes × 3 total doses if pain persists and BP remains adequate (SBP ≥90 mmHg before each dose)
• Onset: 1–3 minutes sublingual
• Duration: 20–30 minutes
• Authorization: On-line medical control required in most systems — NTG is not a standing-order drug in most EMT-B protocols due to the contraindication burden
• EMT-B role: Assist patient with their own prescribed NTG, OR administer agency-stocked NTG if authorized by medical control

Mechanism

NTG releases nitric oxide → smooth muscle relaxation → venodilation (decreases preload → decreases cardiac work) + coronary vasodilation (increases myocardial oxygen supply). Net effect: more O₂ delivered to ischemic myocardium, less work required to pump.

The drop in preload also drops blood pressure. This is the mechanism behind the most dangerous contraindication: if preload is already critically low (right ventricular MI, hypovolemia) or if the patient is on a drug that also drops BP (PDE-5 inhibitors), NTG can cause severe refractory hypotension.

Contraindications — The Critical List

NTG has the most tested contraindication profile in EMT-B pharmacology. Know all of these:

Contraindication	Reason	What happens if you give it
SBP <90 mmHg	NTG drops BP further; cannot tolerate additional preload reduction	Cardiovascular collapse
PDE-5 inhibitor use within 24–48 hours	Both drugs drop BP; combination = severe refractory hypotension	Potentially fatal hypotension
Suspected right ventricular (RV) MI	RV depends on preload to function; NTG reduces preload	RV failure → cardiovascular collapse
HR <50 or >100 bpm	Extreme bradycardia or tachycardia indicates compromised hemodynamics	Relative contraindication — contact medical control
Head injury / increased ICP	NTG causes cerebral vasodilation → increases ICP	Worsens intracranial hypertension
Patient has already taken 3 doses	No additional benefit; hypotension risk increases with each dose	Hypotension

KEY DIFFERENTIATOR — PDE-5 Inhibitors: Sildenafil (Viagra), tadalafil (Cialis), vardenafil (Levitra), and avanafil (Stendra) are PDE-5 inhibitors prescribed for erectile dysfunction and pulmonary hypertension. They cause vasodilation on their own. Combined with NTG, the result is profound, potentially irreversible hypotension. Always ask before NTG: "Have you taken Viagra, Cialis, or any medication for erectile dysfunction in the last 24–48 hours?" Tadalafil (Cialis) lasts up to 48 hours — a longer window than sildenafil (24 hours).

KEY DIFFERENTIATOR — Right Ventricular MI: Inferior MI (ST elevation in leads II, III, aVF) frequently involves the right ventricle. The RV is a thin-walled, low-pressure pump that depends on adequate preload (filling pressure) to function. NTG drops preload → RV cannot generate output → cardiovascular collapse. Prehospital clue: inferior MI + hypotension + JVD + clear lung sounds = suspect RV involvement. Do NOT give NTG without on-line medical control guidance in inferior MI.

Indications

• Chest pain consistent with ACS (pressure, tightness, squeezing, radiation to arm/jaw) in a patient with known cardiac history or strong ACS presentation
• Patient has their own prescribed NTG (confirms pre-existing cardiac diagnosis)
• All contraindications have been screened and cleared
• On-line medical control has authorized administration

Procedures

1. Obtain on-line medical control authorization
2. Check BP — must be ≥90 mmHg systolic
3. Screen all contraindications (see table above) — ask specifically about PDE-5 inhibitors
4. Position patient seated or supine (NTG causes hypotension; standing = syncope risk)
5. Tablet: place under the tongue; patient allows to dissolve completely; do not swallow
6. Spray: spray once under the tongue; patient closes mouth; do not inhale
7. Reassess BP and pain at 5 minutes
8. If pain persists and SBP ≥90 mmHg: may repeat up to 3 total doses every 5 minutes with medical control authorization
9. Document: time, dose, BP before each dose, pain response, number of doses given
10. Notify receiving hospital — "patient received [X] doses of NTG, last dose at [time], BP [X] pre-dose, pain [improved/unchanged]"

After NTG administration: Keep patient supine or seated. Monitor BP closely — NTG-induced hypotension can develop rapidly. Have the patient report any headache (common, from cerebral vasodilation), dizziness, or worsening symptoms.

What to Do if BP Drops After NTG

If SBP falls below 90 mmHg after NTG administration: 1. Place patient supine with legs slightly elevated (improves venous return) 2. Administer high-flow O₂ 3. Contact medical control 4. Do NOT give additional NTG 5. Transport priority — the patient may have underlying RV involvement or severe hypovolemia

NM Protocol Notes

In most NM EMS systems, NTG is NOT a standing-order medication at EMT-B level due to the contraindication burden. Contact on-line medical control before administration.

When assisting with patient's own prescribed NTG: - The prescription confirms a prior cardiac diagnosis - Still requires confirmation of contraindications before assisting - Still requires medical control in most NM protocols

Dose in NM protocols: 0.4 mg sublingual; repeat every 5 minutes × 3 total doses as authorized; check BP before each dose.

NREMT Relevance

• Authorization: on-line medical control required (not a standing-order drug in most systems)
• Dose: 0.4 mg sublingual; may repeat × 3 total, every 5 minutes, with BP check before each
• Absolute contraindications (the tested list): SBP <90, PDE-5 inhibitor use within 24–48 hrs, suspected RV MI, head injury/increased ICP
• PDE-5 inhibitors: Viagra/Cialis/Levitra — always ask; Cialis lasts 48 hours
• Right ventricular MI: inferior MI + hypotension + JVD + clear lungs = suspect RV; no NTG
• Position patient seated or supine before giving — NTG drops BP, standing = syncope risk
• Headache after NTG is expected (cerebral vasodilation) — not a sign to stop treatment
• Do NOT give NTG for aortic dissection — tearing pain, unequal arm BP = dissection, not ACS

Related

• acs-chest-pain — full ACS assessment and management; NTG in context of ACS treatment sequence
• aspirin — co-administered with NTG in ACS; no contraindications in aspirin allergy for NTG
• medication-orders — on-line medical control process; repeat-back requirement
• six-rights — medication check before every NTG dose; check BP (right patient condition) each time
• shock — NTG-induced hypotension management; preload physiology
• nremt-master-differentiator — AMI vs. Aortic Dissection; NTG contraindicated in dissection

Sources

• raw/protocols/nm-sop-guidelines-treatment-2022.pdf — ACS / Chest Pain protocol; NTG administration
• UCLA EMT Ultimate Study Guide (2026) — Pharmacology / Nitrates; ACS Chapter