Pharmacology · Year 2 · from Pharmacology
Case 2: Sildenafil-Nitrate Drug Interaction (Pharmacodynamic Synergism)
Clinical Scenario
A 68-year-old male presents to the emergency department with severe hypotension after taking sildenafil and sublingual nitroglycerin.
Patient Demographics
- Age: 68 years
- Sex: Male
- Weight: 78 kg
Chief Complaint
Dizziness, near-syncope, and severe weakness for 1 hour
History of Present Illness
The patient has a history of stable angina and takes isosorbide mononitrate 30 mg daily. He recently obtained sildenafil from an online pharmacy and took 50 mg approximately 3 hours ago. He subsequently developed chest tightness and used sublingual nitroglycerin (0.4 mg) from his prescription. Within minutes, he became extremely dizzy, diaphoretic, and nearly lost consciousness.
Physical Examination
- Vital Signs: BP 68/40 mmHg, HR 110 bpm, RR 20, T 36.5C, SpO2 96% on RA
- General: Pale, diaphoretic, appears ill
- Cardiovascular: Tachycardic, regular rhythm, no murmurs
- Neurological: Alert but lethargic, no focal deficits
- Skin: Cool, clammy, pale
Workup and Results
| Test | Result | Reference Range |
|---|---|---|
| Troponin I | 0.02 ng/mL | <0.04 ng/mL |
| BNP | 180 pg/mL | <100 pg/mL |
| Creatinine | 1.1 mg/dL | 0.6-1.2 mg/dL |
| Glucose | 145 mg/dL | 70-100 mg/dL |
ECG: Sinus tachycardia, no ST changes, no evidence of ischemia
Diagnosis
Severe hypotension due to pharmacodynamic synergism between phosphodiesterase-5 inhibitor (sildenafil) and nitrates
Pharmacodynamic Principles Illustrated
- Synergistic drug interaction: Sildenafil inhibits PDE5, which breaks down cGMP. Nitrates increase cGMP production. Together, they cause excessive cGMP accumulation and profound vasodilation.
- Same pathway, different targets: Both drugs act on the nitric oxide-cGMP-vasodilation pathway but at different points.
- Contraindicated combination: This is an absolute contraindication due to predictable, severe hypotension.
- Duration of interaction: Sildenafil has a half-life of 4-5 hours, but interaction window extends 24-48 hours.
Treatment
- Place patient in Trendelenburg position
- Aggressive IV fluid resuscitation (normal saline boluses)
- If refractory, consider vasopressor support (phenylephrine preferred - alpha-1 agonist)
- Avoid beta-agonists which may worsen hypotension through beta-2 vasodilation
- Monitor for myocardial ischemia
- Patient education about dangerous drug interaction
- Consultation with cardiology regarding alternative angina management
Clinical Image
Chemical structure of sildenafil, a phosphodiesterase-5 inhibitor that prevents degradation of cyclic GMP, potentiating the vasodilatory effects of nitric oxide and nitrate medications.
Image Source: Wikimedia Commons License: Public Domain URL: https://commons.wikimedia.org/wiki/File:Sildenafil.svg