The effect of CO2-breathing on cochlear blood flow

The effect of CO2- and O2-gas mixtures on laser Doppler measured cochlear and skin blood flow in guinea pigs

The effects of carbogen (5% CO2: 95% O2) 10% CO2-in-air and 100% O2 on cochlear blood flow (CBF), skin blood flow (SBP), blood pressure (BP) and arterial blood gases were investigated in the anesthetized, respired or self-respiring guinea pig. In respired animals, CBF and SBF were increased with carbogen and 10% CO2-in-air and decreased with O2. BP was elevated with each gas. In freely breathing animals, only 10% CO2-in-air caused a small increase in CBF; both carbogen and O2 caused CBF to decrease. SPF changes were similar in form, but larger than those seen in respirated subjects. No consistent change in BP was seen during breathing of these mixtures. Arterial PO2 was increased by carbogen and 10% CO2-in-air for both groups. PCO2 increased for both CO2 gas mixtures during forced respiration; but in free-breathing animals PCO2 only increased for 10% CO2-in-air (normal PCO2 values were maintained with carbogen thorough increased breathing rate). The observed changes in CBF were consistent with a balance between a combined vasoconstrictive effect of PO2 and vasodilation effect of PCO2 on cochlear vessels. Analysis of cochlear vascular conductivity (CBF/BP) indicated that vasodilation was significant only with 10% CO2-in-air in respirated animals. In all other conditions the increased CBF apparently reflects the increase profusion pressure associated with respiration of each gas. For clinical purposes, while carbogen does not appear to directly cause vasodilation of cochlear vessels it does lead to an increased oxygenation of the cochlea blood and would appear to avoid the cochlear vasoconstriction caused by 100% O2.

Influence of topically applied adrenergic agents on cochlear blood flow

This study was designed to assess the role of adrenergic receptors in the control of cochlear blood flow. Laser Doppler flowmetry was used to determine the effects of adrenergic drugs topically applied to the round window membrane of the cochlea. The relative influence of the various receptor types (alpha 1, alpha 2, beta 1, and beta 2) was examined by a selection of agonists and antagonists. The agonists norepinephrine and epinephrine, which have mixed alpha- and beta-receptor effects, and phenylephrine, a strong alpha 1-agonist, all induced a dose-dependent reduction in cochlear blood flow. The agonists isoproterenol (beta-active), salbutamol (alpha 2-active) had no effect on cochlear blood flow. Of the antagonists, when tested alone, only the selective alpha 1-antagonist prazosin had a direct effect on cochlear blood flow, demonstrating an increase in cochlear blood flow. The selective alpha 2-antagonist idazoxan, the beta-antagonist propranolol, and the unselective alpha-antagonist phentolamine had no effect on cochlear blood flow. Interaction studies of agonists and antagonists were performed to specifically define the receptor subclasses responsible for the cochlear blood flow increases with norepinephrine and epinephrine. The results are consistent with the presence of an alpha 1-adrenergic sympathetic control of cochlear blood flow.

Measurement of human cochlear blood flow

Cochlear blood flow (CBF) was measured with a laser-Doppler (L-D) flowmeter (Periflux PR2-B) in four unanesthetized human subjects with chronic tympanic membrane perforations and nine anesthetized human subjects undergoing middle ear operations. The L-D recordings were made over the promontory and/or the round window membrane during carbogen breathing and direct electrical stimulation of the cochlea in both groups and with warm water irrigation of the external ear canal in the anesthetized subjects. Carbogen led to little or no change in CBF as monitored with either measurement approach in either subject group. Electrical stimulation yielded an increase (15% to 25%) in CBF as recorded from the promontory in seven of the nine subjects tested. Warm (44 degrees C to 49 degrees C) water irrigation produced changes of 20% to 60% in CBF that were partially recoverable in the 10 minutes available for study. This study demonstrated the feasibility of direct CBF measurement in humans with the L-D method. Moreover, the data indicate that carbogen has little influence on CBF and that electrical stimulation at relatively safe levels and warm water irrigation of the ear canal produce increases in human CBF.

Cochlear blood flow: measurement techniques

The measurement of inner ear blood flow and other microvascular variables is subject to unique technical problems which are compounded by methodological limitations. As a result, the interpretation of experimental results is often difficult. This report discusses the most important methods currently available for cochlear blood circulation measurements and the technical problems associated with their use. The use of a combination of measurements to resolve problems of interpretation is stressed. An extensive review of the pertinent literature is provided in relation to each method.

In vivo capillary diameters in the stria vascularis and spiral ligament of the guinea pig cochlea

Blood microvessels in the membraneous lateral wall of the cochlea were examined using intravital microscopic techniques. A video analysis system made serial diameter measurements at 1 micron intervals along the length of selected vessel segments during four experimental conditions. For each vessel segment, the serial measurements were statistically converted into a single diameter estimate, such that the flow resistance in a uniform vessel of this diameter would equal the resistance of the real non-uniform vessel. Nominal vessel diameters found (spiral ligament: 9-12 micron; stria vascularis: 12-16 micron) were nearly double those reported earlier in histological observations (Axelsson, 1968). During stimulation the largest diameter change seen was a 3.7% dilation (about 0.5 micron) in response to breathing 5% CO2 in oxygen. Theoretically, this change could reduce vascular fluid resistance by 16%, nearly enough to explain the observed flow increase of 20%. No diameter changes occurred for 5% CO2 in air despite a 50% flow increase, nor for air pressure pulses applied at the tympanic membrane. Round window electrical stimulation of 50 microA also produced dilation (less than 2.5%), but higher current levels were ineffective. In general, blood flow increases seen in this study could not adequately be attributed to the small lateral wall vessel diameter increases nor systemic causes, suggesting that lateral wall blood flow in these instances is dependent on control within the modiolus.

Cochlear blood flow in the rat. A methodological evaluation of the microsphere method

With the microsphere method it is possible to quantify the blood flow in various organs. The blood flow in the cochlea is only a very small part of the cardiac output and only relatively few microspheres are caught in this organ, which necessitates large groups of animals for such studies. The method has, however, not been fully evaluated for studies of small organs in small animals. In this study, 130 rats of various ages with normal or arterial hypertension were investigated. The blood flows of 97 animals were possible to evaluate. It was found that physiological parameters, such as PCO3, pH, PO2 and mean arterial blood pressure within the rather wide limits usually present in the anesthetized animal did not affect the cochlear blood flow to any great extent and that the method is feasible for studies of the blood flow through the inner ear in small animals.

Drug-induced alterations in cochlear blood flow as recorded by the laser Doppler flowmeter

A number of clinical entities may result from a decrease in inner ear blood flow. Investigators have attempted to determine a relationship between decreased blood flow and such entities. In this article, we shall relate the effects of administration of drugs in animal models on cochlear blood flow, using the laser Doppler flowmeter.

The effect of cervical sympathectomy on cochlear electrophysiology

The purpose of this study was to examine the electrophysiologic responses to sound from guinea pig cochleas, 3--6 days after a unilateral cervical sympathectomy. After recovery from surgery the guinea pigs developed a Horner's syndrome on the sympathectomized side. Some days later their bullas were opened and electrodes were placed bilaterally on the round windows. Most of the sympathectomized cochleas showed signs of decreased sensitivity to sound. They had a smaller dynamic range for the click- and tone-burst evoked compound action potential (CAP) compared with the non-sympathectomized cochleas. The threshold sound levels for the CAPs and the sound levels to produce 1 muV of cochlear microphonic potentials were unaffected. In other animals with chronic implanted electrodes, the same electrical responses were measured with only light sedation before and after sympathectomy and showed the same results. In additional animals the crossed olivocochlear bundle (COCB) was electrically stimulated and a similar inhibition of the CAP was registered on both the intact and the sympathectomized side. The results suggest that the sympathetic nerves to the cochlea, coming from the ipsilateral cervical ganglion and ending near the habenula perforata, may be important for the sound perception as they influence the CAP.