SYMPATHETIC-NERVE REGULATION OF COCHLEAR BLOOD-FLOW DURING INCREASES IN BLOOD-PRESSURE IN HUMANS

The purpose of this work was to show that regulation of the blood flow to the cochlea by the sympathetic nervous system occurs in humans at tile level of the cochlear microcirculation during increases in blood pressure and that its involvement depends on the pressure level. Eight anaesthetized patients undergoing tympanoplasty for hearing disease t ook part in a pharmacological protocol of stimulation and inhibition o f the autonomic nervous system (ANS) to provide variations in systolic blood pressure (BPs) and cochlear blood flow (CBF). The CBF was measu red by laser-Doppler flowmetry. Changes in autonomic nerve activity we re brought about by changes in baroreceptor activity (BR) initiated by the injection of an alpha adrenergic agent before and after sympathet ic and parasympathetic blockade, The CBF variations (delta CBF) were p lotted against BPs increases at each stage of the ANS inhibition. The BR diminished significantly after alpha blockade, after alpha and beta blockade, and after alpha and beta blockade and atropine, by 50% (P < 0.01), 29% (P < 0.05): and 95% (P < 0.001) respectively. The BPs incr eased significantly (P < 0.01) by 36 (SD 9)%, 47 (SD 1)%,, and 67 (SD 16)% respectively. The CBF response to an increase in BPs exhibited tw o opposing variations in the patients: CBF decreased significantly in one group, and increased significantly in the other group. In both gro ups, delta CBF decrease and delta CBF increase, respectively, were sig nificant after ANS blockade; even so the decrease and increase, respec tively, levelled off at BPs around 160 mmHE before ANS blockade. For B Ps below 160 mmHg, correlations between delta CBF and BPs were signifi cant before inhibition and after inhibition of ANS. For BPs above 160 mmHg, BPs and delta CBF were not correlated before inhibition of ANS, and were significantly correlated after inhibition of ANS. For BPs bel ow 160 mmHg, CBF response to the BPs increase was the same before and after ANS blockade, i.e. ANS control did not predominate; even so, for BFs above 160 mmHg, the CBF response to BPs increase was different be fore and after ANS blockade: CBF varied significantly after ANS blocka de as it varied for BPs below 160 mmHg, while it remained constant bef ore ANS blockade that elicited ANS control of CBF. In conclusion, symp athetic nerve regulation via its vasomotor lone at the level of cochle ar microcirculation occurred markedly when the blood pressure was abov e 160 mmHg; the autonomic nervous system would appear to control the c ochlear blood flow against large variations in blood flow in response to hypertensive phenomena.