Phenylephrine Clonidine

The total surface area of the nasal cavity is about 150 cm2, with the area available for absorption enhanced by the convolutions of the turbinates and the presence of microvilli on the surface of the ciliated and unciliated cells of the respiratory epithelium. The arterial supply of the nose is particularly rich in the respiratory epithelium where the Kiesselbach s plexus lies, an area that is rich in numerous capillary loops. The nasal blood flow has been shown to be sensitive to the action of a variety of inhaled compounds, both locally or systemically acting. Clonidine has been shown to decrease the blood flow whereas histamine and phenylephrine have been shown to induce the converse effect. Such direct changes to blood flow are important in determining the rate and extent of drug absorption from the nasal cavity. 

Inomata S, Nishikawa T, Kihara S, Akiyoshi Y. Enhancement of pressor response to intravenous phenylephrine following oral clonidine medication in awake and anaesthetized patients. Can J Anaesth 1995 42(2) 119-125. 

Phenylephrine Methoxamine Clonidine" Guanabenz Guanfacine Noradrenaline Adrenaline a-Methylnor-adrenaline" Tramazoline Naphazoline Oxymetazoline... 

Experimental studies in patients show that pretreatment with clonidine decreases the blood pressure response to small doses of dopamine does not affect the blood pressure response to noradrenaline (norepinephrine) and can increase the blood pressure responses to dobutamine, ephedrine, isoprenaline (isoproterenol) and phenylephrine. 

An interaction is established, although the exact outcome of the concurrent use of clonidine and these sympathomimetic vasopressors is not clear. It has been suggested that the effects may be different at different doses of dopamine. The authors of the one report, studying phenylephrine and noradrenaline (norepinephrine) with clonidine, suggested that the increase in pressor response was unlikely to be clinically significant. 

The thin, porous and highly vascularised nasal epithelium has a high total blood flow, which facilitates fast absorption of substances. Direct transport to the systemic circulation or the central nervous system makes it possible to obtain a rapid therapeutic effect. The intranasal absorption depends on the mucociliary clearance, pathological conditions such as infections, allergy and obstruction, mucus secretion, moisture content, enzymatic degradation, and blood flow. It should be remembered that the blood flow can be affected by either locally or systemically active substances. These phenomena can determine the nasal absorption of substances. Oxymetazoline and clonidine reduce the blood flow, while phenylephrine and salbutamol raise it. 

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