Chemical structure and metabolism
Unlike endogenous catecholamines (such as adrenaline and noradrenaline), the phenylephrine hydrochloride molecule contains only one hydroxyl group in the aromatic ring. This structural difference is of great practical importance.
The catechol-O-methyltransferase (COMT) enzyme is unable to inactivate the drug. The clinical consequence of this is a longer therapeutic effect compared to typical catecholamines. In addition, resistance to enzymatic degradation allows the drug to maintain efficacy even when taken orally.
Effects on the cardiovascular system
The main mechanism of action of the drug consists in the selective stimulation of postsynaptic $\alpha_1$-adrenoceptors. The response of the circulatory system can be divided into two components:
- Direct effect. Action on vascular wall receptors causes potent vasoconstriction (narrowing of blood vessels). This leads to a pressor effect—a significant increase in blood pressure.
- Reflex reactions. A sharp increase in blood pressure mechanically irritates baroreceptors located in the aortic arch and carotid sinus. The signal is transmitted to the brain, where vagus nerve (n. vagus) tone increases reflexively. Conversely, vasomotor center activity and sympathetic influences are inhibited.
Important conclusion: reflex bradycardia (decreased heart rate) becomes the dominant effect on the heart. Phenylephrine does not inhibit cardiac activity directly; it is a secondary nervous system response to vasoconstriction.
Ophthalmologic effects
In the form of eye drops, the drug affects iris structures. Stimulation of adrenoceptors causes contraction of the radial muscle of the iris, leading to mydriasis—pupil dilation.
Unlike M-cholinoblockers (atropine-like agents), phenylephrine does not affect accommodation. The ciliary muscle, which controls lens curvature, is innervated primarily by the parasympathetic nervous system and does not respond to adrenomimetics. Due to this, the patient retains the ability to see clearly at close range.
Additionally, the drug can reduce intraocular pressure (IOP) in open-angle glaucoma.
Indications for use
The drug is widely used in various fields of medicine:
- Systemic hypotension therapy. The goal is emergency blood pressure elevation. The solution is administered intravenously (duration of effect is about 20 minutes), subcutaneously (40–50 minutes), or intramuscularly.
- Ophthalmology. Used for diagnostic or therapeutic pupil dilation, as well as to reduce IOP in open-angle glaucoma.
- Otolaryngology. Topical application causes vasoconstriction of the mucous membrane, reducing nasal congestion in rhinitis. It is often combined with $H_1$-histamine receptor blockers (e.g., dimetinden as part of the drug Vibrocil).
- Local anesthesia. The addition of phenylephrine to anesthetic solutions causes local vascular spasm. This prolongs the analgesic effect and reduces the resorptive (systemic toxic) effect of the anesthetic on the body.
Side effects and contraindications
Due to severe peripheral vasospasm, tissue ischemia, an excessive spike in blood pressure, and reflex bradycardia may develop. Headache and dizziness are sometimes noted from the central nervous system, and urination disorders from the urinary system.
The drug is contraindicated in:
- Hypertension;
- Predisposition to vascular spasms (including coronary artery spasm);
- Endocrine pathologies (hyperthyroidism);
- Closed-angle glaucoma.
Briefly: answers to topical questions
What is the duration of the pressor effect of phenylephrine via various parenteral routes of administration?
The duration of the pressor effect of phenylephrine directly depends on the chosen parenteral route of administration.
- Intravenous administration — the pressor effect lasts for 20 minutes.
- Subcutaneous administration — the effect persists for 40–50 minutes.
Intramuscular administration of the drug is also permitted in the treatment of hypotensive states, but the exact duration of effect persistence for this route is not specified in the materials.
Direct action — Pressor, Reflex — bradycardia. Remember D-P for the correct interpretation of the cardiovascular effects of the drug.
Class and exam questions
Why does phenylephrine not cause accommodation paralysis?
Its chemical structure has only one hydroxyl group, so the COMT enzyme cannot inactivate it.
Why does phenylephrine not cause accommodation paralysis?
Because the ciliary muscle, which is responsible for eye accommodation, is innervated by the parasympathetic nervous system, which alpha-adrenomimetics do not affect.
What is the mechanism of bradycardia development upon administration of Mesaton?
This is a reflex reaction. A sharp increase in pressure irritates the baroreceptors of the aortic arch and carotid sinus, which leads to an increase in vagus nerve tone and a slowed pulse.
What else to study on this topic
- Mechanisms of reflex regulation of blood circulation by baroreceptors
- Comparative characteristics of alpha-1 and alpha-2 adrenomimetics
- Principles of prolonging local anesthesia with vasoconstrictors
- Differential pharmacotherapy of open-angle and closed-angle glaucoma
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