Sildenafil

Mechanism of hazardous interactions

Sildenafil, along with other PDE-5 inhibitors (e.g., tadalafil), has a pronounced effect on vascular tone. The main problem when prescribing it in clinical practice is the unpredictable and sharp enhancement of the hypotensive effect when taken concurrently with a number of cardiac medications. Combined use can provoke severe arterial hypotension requiring immediate medical intervention. Patients taking antihypertensive or antianginal agents require careful monitoring of blood pressure (BP).

Combination with organic nitrates

Organic nitrates (nitroglycerin, isosorbide dinitrate, isosorbide mononitrate) are used as antianginal agents for the relief and prevention of angina pectoris attacks.

Concomitant use of sildenafil and organic nitrates is strictly contraindicated. This combination causes synergistic relaxation of vascular smooth muscle, leading to critical hypotension.

Nitrates themselves also have a number of strict contraindications associated with hemodynamic disorders, in which their administration (especially against the background of sildenafil) is unacceptable:

• Acute circulatory failure (shock, collapse).
• Severe arterial hypotension and hypovolemia.
• Elevated intracranial pressure, cerebral hemorrhage.
• Angle-closure glaucoma.

Interaction with alpha-adrenoreceptor antagonists

Alpha-adrenoreceptor antagonists are another group of drugs with which sildenafil enters into a dangerous interaction, significantly enhancing their hypotensive effect. When prescribing sildenafil, it is necessary to consider the specific side effects of α-blockers, which can be multifold aggravated:

1. "First-dose effect": a sharp drop in blood pressure upon the first intake. To minimize the risk, treatment with selective α₁-blockers is started at low doses, and the first dose is taken at bedtime in the supine position.
2. Orthostatic hypotension: patients should avoid abruptly transitioning to an upright position. The drugs are used with extreme caution in elderly patients due to an increased risk of falls.

The dosing regimen for uroselective drugs (tamsulosin, alfuzosin) involves oral administration in the morning after a meal. Pharmacokinetics of non-selective agents varies: from short-acting (phentolamine intravenously — several minutes) to prolonged hypotensive effects (phenoxybenzamine — 48 hours or more due to irreversible receptor blockade).

Sildenafil in the context of complex cardiodynamic therapy

When assessing the safety of sildenafil use, it is important to consider the entire spectrum of therapy the patient is receiving. Many drugs have their own hypotensive action or affect compensatory mechanisms of the heart:

• Calcium channel blockers (CCBs): enhance the hypotensive effect when combined with α-blockers. Marked hypotension is a contraindication for both non-dihydropyridine (verapamil, diltiazem) and dihydropyridine (nifedipine, amlodipine) CCBs. Short-acting nifedipine is also contraindicated in acute myocardial infarction and unstable angina.
• β-adrenoreceptor antagonists: the combination of selective α₁-blockers with β-blockers can neutralize reflex tachycardia. However, they are absolutely contraindicated in bradycardia (HR < 50–60 bpm), second- to third-degree AV block, cardiogenic shock, and acute decompensated heart failure. Non-selective forms must not be used in exacerbated bronchial asthma and COPD.
• ACE inhibitors and angiotensin II receptor blockers: their combination with selective α₁-blockers counteracts RAAS activation, which additionally lowers blood pressure.
• Ivabradine: contraindicated in severe arterial hypotension, resting HR < 60 bpm, acute myocardial infarction, and unstable angina.

Class and exam questions

What else to study on this topic

• Organic nitrates: classification, pharmacodynamics, and indications for use
• Alpha-adrenoreceptor antagonists: uroselective and non-selective drugs, "first-dose effect"
• Absolute and relative contraindications to prescribing β-adrenoreceptor antagonists
• Differences in contraindications between dihydropyridine and non-dihydropyridine calcium channel blockers
• Ivabradine: mechanism of action and specific contraindications in bradycardia

Other topics in the "Additional Topics" section

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