Increase In Cochlear Blood Flow Research Articles

Cochlear Circulation and Labyrinthine Pressure

The effect of glycerol on the cochlear blood flow (CBF) and perilymphatic pressure (PP) was studied in guinea pigs. CBF and PP were measured simultaneously with a laser-doppler flowmeter and a servo-nulling system, respectively. Administration of 12 ml/kg of 50% glycerol or of physiological saline was carried out through a polyethylene feeding tube. Glycerol administration caused a marked increase of CBF and decrease of PP, while physiological saline produced no change. However, some time lag was noted between the changes of CBF and PP. That is, the change of CBF preceded that of PP, and CBF had returned to its normal value when PP reached its maximum Therefore, the increase of CBF seems to be unrelated to an increase in serum glycerol or a decrease of inner ear pressure.

Potential role of angiotensin II in noise-induced increases in inner ear blood flow

Guinea pigs were exposed to 120 dB white noise for 30 min and evidenced a four-fold elevation in plasma concentration of the potent vasoconstricting hormone angiotensin II (AII). Anesthetized animals received intra-arterial injections of AII at doses that approximated the endogenous levels measured following noise exposure. A marked decrease in skin blood flow was observed with a concomitant increase in cochlear blood flow as measured by laser Doppler flowmeters. Increased cochlear blood flow appeared to be secondary to the increases in systemic blood pressure induced by AII. These findings suggest that cochlear blood flow may increase during periods of intense noise exposure.

The effect of CO2-breathing on cochlear blood flow.

The cochlear blood flow was measured with the microsphere methods, before and during CO2-inhalation, in anesthetized rabbits. Different gas mixtures and exposure times were used. The greatest increase of cochlear blood flow was measured after 5 min inhalation of 7% CO2 in air (115%) and the least increase (44%) in animals breathing 7% CO2 in oxygen. The difference is explained as due to a vasoconstriction caused by a high oxygen tension counteracting the vasodilating effect of CO2.

Effects of CO2 inhalation on cochlear blood circulation.

The cochlear blood flow was measured with the microsphere method before, during and after CO2 inhalation in anesthetized rabbits. Different gas mixtures and exposure times were used. The greatest increase of cochlear blood flow was measured after 5 min inhalation of 7% CO2 in air (115%) and the least increase (44%) in animals breathing 7% CO2 in oxygen. The difference is explained as due to a vasoconstriction caused by a high oxygen tension counteracting the vasodilating effect of CO2. The treatment with CO2 of supposed vascular diseases in the cochlea is discussed and the negative effects due to respiratory acidosis are emphasized.

Measurement of Perilymphatic Oxygen Tension for Evaluation of Vasoactive Agents

Perilymphatic oxygen tension in cats was measured following one intravenous administration of vasoactive agents. Intravenous infusion and one drop of the agent on the vessels in the internal acoustic meatus were also administered. Vasoactive agents used were papaverine hydrochloride, dihydroergotamine methanesulfate, histamine phosphate, nicotinic acid, eupaverine, naftidrofuryl, low molecular weight dextran, adrenaline and noradrenaline.Significant discordance of the perilymphatic oxygen tension was detected with the cochlear blood flow as reported by Suga et al3). Namely increase in cochlear blood flow does not always indicate the same trend as seen in the perilymphatic oxygen tension. Dilatation of the vessels in the internal acoustic meatus does elevate the perilymphatic oxygen tension, however effective oxygen supply to the inner ear is also dependent upon other factors influencing the systemic blood circulation. We conclude that vasoactive agents for inner ear diseases should be evaluated not by cochlear blood flow, but by the perilymphatic oxygen tension.

Control of the Microcirculation of the Inner Ear

In general, many similarities are seen between cerebral blood flow and cochlear blood flow in response to vasodilating drugs. Cochlear vessels appear to be weakly controlled by the adrenergic nervous system. Cholinergic agents dilate cochlear vessels, although the vasodilation in the cochlea does not necessarily cause an increase in cochlear blood flow because of the associated hypotension. The effect on the systemic circulation is too profound to make these agents clinically useful. Papaverine, dipyridamole, amyl nitrite, and the plasma kinins produce transient increases in cochlear blood flow. Ten per cent carbon dioxide and 90 per cent oxygen product progressive vasodilation as long as the mixture is inhaled. Hydralazine produces too profound an effect on systemic blood pressure. Histamine and betahistine increase cochlear blood flow but only in doses that produce bronchiospasm in the guinea pig. Nicotinic acid and nicotinyl tartrate have no effect on cochlear blood flow. If one considers safety and efficacy as well as patient tolerance, papaverine appears to be the drug of choice for increasing cochlear blood flow clinically. Histamine and betahistine appear to be acceptable alternatives. Betahistine is no longer available for clinical use because of the failure to demonstrate clinical efficacy of therapy.

COCHLEAR BLOOD FLOW AND FUNCTION: EFFECT OF PRESSOR AGENTS.

Normal values for blood flow velocity in some terminal vascular units of the cochlea (stria vascularis and spiral ligament) and for carotid blood pressure are presented along with the percentage changes produced by administration of epinephrine, norepinephrine, vasopressin and serotonin. Increases in flow velocity in these cochlear vessels can be produced. These are closely related to the measured increases in mean carotid blood pressure. Cochlear function as measured by the microphonic response to a moderate acoustic stimulus is not altered by this increase in cochlear blood flow. Some general relations between flow velocity, oxygen utilization and cell function are discussed.