Phenylephrine, blood flow with

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. 

Phenylephrine is a selective ai adrenergic agonist, with a pharmacological structure similar to norepinephrine (noradrenaline), which causes peripheral vasoconstriction. Stimulation of adrenoceptors in the myocardium has an inotropic effect. However, phenylephrine produced no increase in cardiac output, increased systemic vascular resistance and mean arterial pressure and reduced muscle blood flow in anaesthetized horses (Lee et al 1998). Phenylephrine should be reserved for critical conditions where the perfusion of essential organs is compromised and inotropes are not effectively maintaining organ function. The recommended infusion rates are shown in Table 12.3. 

In 19 healthy parturients undergoing elective cesarean section who were to phenylephrine or ephedrine as a prophylactic infusion supplemented with minor boluses if systolic arterial pressure fell by more than 10 mmHg, both the vasopressors restored maternal arterial pressure effectively [25 J. Ephedrine had no significant effects on Doppler velocimetry but phenylephrine infusion significantly increased the blood flow velocity waveform indices in the uterine and placental arcuate arteries and reduced vascular resistance significantly in the fetal renal arteries. However, healthy... 

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