Changes In Cochlear Blood Flow Research Articles

Effects of electrical stimulation of the superior cervical ganglion on cochlear blood flow in guinea pig.

It has been proposed that cochlear blood flow (CBF) is controlled in part by the sympathetic nervous system. In the present study the effect of electrical stimulation of the superior cervical ganglion (SCG) on CBF in guinea pigs was investigated using laser Doppler flowmetry (LDF). Animals were anesthetized with diazepam and fentanyl and the SCG was exposed. A custom-designed bipolar cuff electrode was fixed around the ganglion and 1 ms biphasic current pulses were injected at 0.15 mA to 1.5 mA, 6 Hz. Bilateral CBF was monitored, while the ganglion was stimulated for 3 or 5 min before and after the ascending sympathetic trunk and nerve branches from SCG were sectioned. Electrical stimulation of 0.5 mA caused the ipsilateral CBF (CBFi) to decrease 11.7% +/- 1.3 from the baseline (BL), while the contralateral CBF (CBFc) increased slightly due to the change in systemic blood pressure (BP). A linear relation was observed between the level of current stimulation and evoked reduction in CBF. Cervical sympathetic trunk section (between the SCG and the middle cervical ganglion) did not influence the pattern or the amplitude of CBF change in response to electrical stimulation of SCG. Sectioning the efferent fibers of the medial inferior and medial superior branch of the SCG only minimally reduced the amplitude of the CBF decrease evoked by electrical stimulation. However, sectioning the superior lateral branch abolished this decrease.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of loud sound on red blood cell velocity and blood vessel diameter in the cochlea

Using intravital microscopy, we observed both decreases in red blood cell velocity and possible vasoconstriction in stria vascularis capillaries of the rat cochlea in response to loud sound (Quirk el al., 1991). However, our observation of vasoconstriction was subject to error in measurements from the two dimensional images obtained with our silicon intensified (SIT) camera due to the influence of focus causing image blur. The purpose of the current study was to apply an extended focus microscopy technique to obtain quantitative assessment of vessel diameter changes (Avinash et al., 1992), as well as to extend these studies to the guinea pig model. Broad-band sound stimulation at intensities of 84 dB SPL and 110 dB SPL were used. The results show that loud sound induces a sequence of changes in cochlear blood flow. Stimulation with 110 dB SPL resulted in a mean increase (maximum = 27%) in red blood cell velocity for the first 20 min of exposure followed by a gradual decrease below baseline (minimum = −12%) prior to termination of the signal. This velocity decrease and subsequent recovery were associated with significant changes in vessel diameters of selected and measured capillaries. In contrast, the 84 dB SPL stimulus caused an increase in red blood cell velocity (maximum = 20%) and vessel diameter (mean = 7.5) during the stimulation period. No recovery was observed during the 10 min observation period following sound. Several possible mechanisms responsible for these changes are discussed.

Cochlear blood flow in response to dilating agents

Reduced cochlear blood flow (CBF) has been implicated in various pathologies of the inner ear, including sudden deafness, noise-induced hearing loss and Meniere's disease. Thus the aim of some current therapeutic regimens to treat these conditions is to increase CBF and thereby improve oxygenation of the inner ear tissues. Most of the vasodilating agents in clinical use, however, do not have specific experimental evidence to support their effects on CBF. The hypotension which can follow systemic administration may limit their local effectiveness and general utility, just as it complicates the interpretation of the data in animal experiments. In the current study we investigated the effect of six agents, known for their systemic cardiovascular actions, on CBF: hydralazine, sodium nitroprusside, papaverine, nicotinic acid, verapamil and histamine. The effect of these drugs was studied after topical applications on the round window membrane (RWM) and systemic intravenous administrations. CBF was monitored with a laser Doppler flowmeter (LDF). Topical administration of sodium nitroprusside was the most effective in increasing CBF, followed, in order, by hydralazine and histamine. No change in CBF was observed for papaverine, verapamil or nicotinic acid. Systemic administrations of all the agents caused a marked decrease in blood pressure and variable effects on CBF. We discuss the CBF changes in relation to the different pharmacological mechanisms of action of each drug. The study demonstrates the effectiveness of topical application of vasodilating agents in increasing CBF.

The effect of CO 2- and O 2-gas mixtures on laser Doppler measured cochlear and skin blood flow in guinea pigs

The effects of carbogen (5% CO 2:95% O 2) 10% CO 2-in-air and 100% O 2 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% CO 2-in-air and decreased with O 2. BP was elevated with each gas. In freely breathing animals, only 10% CO 2-in-air caused a small increase in CBF; both carbogen and O 2 caused CBF to decrease. SPF changes were similar in form, but larger than those seen in respirated subjects. No consistant change in BP was seen during breathing of these mixtures. Arterial PO 2 was increased by carbogen and 10% CO 2-in-air for both groups. PCO 2 increased for both CO 2 gas mixtures during forced respiration; but in free-breathing animals PCO 2 only increased for 10% CO 2-in-air (normal PCO 2 values were maintained with carbogen thorough increased breathing rate). The observed changes in CBF were consistant with a balance between a combined vasoconstrictive effect of PO 2 and vasodilation effect of PCO 2 on cochlear vessels. Analysis of cochlear vascular conductivity (CBF/BP) indicated that vasodilation was significant only with 10% CO 2-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% O 2.

Effects of Perilymph Volume Adjustments on Cochlear Blood Flow in the Guinea Pig

Previous research suggests a potential relationship between perilymphatic pressure (Pp) and cochlear blood flow (CBF); however, the alterations in Pp necessary to produce changes in CBF have not been adequately described or quantified. The effects of perilymph volume changes on systemic blood pressure (BP) and CBF were presently investigated in the guinea pig cochlea. Five microliters of perilymph were displaced in each of three conditions: viz. evacuation of 5 microliters from the cochlea; replacement of these 5 microliters; and finally the addition of 5 microliters of artificial perilymph into the cochlea. All perilymph volume adjustments were completed in 1-microliter increments during which changes in CBG and BP were recorded. Significant alterations in CBF were observed during 1-microliter perilymph volume adjustments in each condition with no significant changes in systemic BP. The results from this study support our hypothesis that an inverse relationship exists between CBF and Pp in that decreases in perilymph volume yielded elevations in CBF while increases in perilymph volume yielded reductions in CBF.

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.

The olivocochlear efferent bundle and susceptibility of the inner ear to acoustic injury

1. The role of the efferent olivocochlear bundle (OCB) in protecting the inner ear from acoustic injury was studied in the anesthetized cat. Middle-ear muscles (MEM) were cut to eliminate possible effects of this feedback system on the auditory periphery. In each of a series of animals, the OCB was unilaterally transected. The animal was then exposed binaurally to an intense pure tone, and the resultant damage to the two sides compared by measuring threshold shifts in the compound action potential from each ear. Data from each animal provide one control measurement (threshold shift with an intact OCB) and one experimental measurement (threshold shift without a functional OCB). 2. Two experimental series were analyzed. In one the OCB was electrically stimulated, providing maximal firing rates in the efferents projecting to the control ear. In another series the OCB was not electrically stimulated: thus any OCB activity to the control ear was only that evoked by the acoustic stimulation itself. 3. In neither experimental series was there evidence that activity in the OCB provides protection from acoustic injury. These results are in disagreement with conclusions drawn from experiments with acoustic overstimulation of guinea pigs. 4. Interpretations for the discrepancy between the present study and those on guinea pigs include interspecies differences and the possible contribution of the MEM reflex or cochlear blood-flow changes to previously observed effects.

Androgenic effects on angiotensin II-induced blood pressure and cochlear blood flow changes in rats

Ovariectomized, castrated and sham-castrated rats pretreated with oil or testosterone were intra-arterially infused with saline and three doses of angiotensin II while blood pressure and cochlear blood flow were measured. The results indicated a positive dose-response relationship for blood pressure and cochlear blood flow. Sham-castrated males had higher mean blood pressure responses than castrated males, followed by Ovariectomized females. Cochlear blood flow responses were higher in the sham-castrated males than the Ovariectomized females, followed by the castrated males. In comparison to the male groups, the Ovariectomized females evidenced the lowest, middle and highest cochlear blood flow responses to the three increasing doses of angiotensin II. Testosterone pretreatment facilitated angiotensin-induced cochlear blood flow elevations in all three angiotensin doses. These results suggest that endogenous and exogenous androgens may alter blood pressure and/or cochlear blood flow responses to angiotensin II via different mechanisms.

Laser Doppler measurements of cochlear blood flow during loud sound presentation

The laser Doppler flowmeter may give responses to loud sound that reflect the vibration of cochlear structures rather than changes in cochlear blood flow. The present study demonstrates that the positive artifactual response (i.e., increased flow reading) to sound at frequencies above approximately 5 kHz can be eliminated by using flowmeters which have electronic filters at 4 and 12 kHz, limiting the bandwidth of the optical Doppler shifted frequency range to lower frequencies. However, when using the 4 kHz filter a "residual" immediate negative response to loud high-frequency sound (10 kHz tone at 125 dB SPL) is evident at the beginning of the exposure. These findings are discussed with regard to the suitability of the method for investigating the effect of sound/noise on cochlear blood flow.

A Comparison of Laser Doppler and Intravital Microscopic Measures of Cochlear Blood Flow

Many inner ear disorders may be caused by alterations in cochlear blood flow (CBF). However, each measurement technique used to monitor CBF has limitations in examining the relationship between otopathologic states and blood flow. This study investigates laser Doppler flowmetry (LDF) and its fundamental drawback: the unknown relationship of LDF output to actual CBF. LDF readings are directly compared with concurrent intravital microscopy (IVM) measures of erythrocyte velocity in the lateral wall of the guinea pig cochlea. Positive end expiratory pressure, spontaneous respiration of 5% and 10% carbon dioxide, phenylephrine, and direct electrical stimulation of the cochlea were used to manipulate CBF. High, positive correlations were found between simultaneous LDF and IVM measurements of CBF. In addition, the study demonstrated that current microdissection techniques used to perform IVM do not cause changes in CBF. IVM measurements of CBF are a more sensitive indicator of CBF changes than are LDF measures. Despite the high correlation between measurement techniques within a single manipulation, simultaneous LDF and IVM measurements differed between manipulations. This may reflect regional changes in CBF affected by these manipulations and differences in the sampled vascular beds contributing to these two measures. It is unlikely that a single calibration factor can be defined that would allow the conversion of LDF output to actual units of blood flow across different manipulations used to alter CBF.

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.

Cochlear blood flow increases after systemic hemodilution: Comparison of simultaneous laser doppler flowmetry and radioactive microsphere measurements

Guinea pig cochlear blood flow was measured before and after systemic normovolemic hemodilution with high molecular weight dextran. Absolute determinations of blood flow (in the cochlea, brain, kidney and lung) were accomplished by use of radioactive-labeled (85Sr or 141Ce) microspheres. Relative measurements of the cochlear blood flow changes were made simultaneously by the use of a laser Doppler flowmeter. The flowmeter probe was placed on the first cochlear turn. Hemodilution to an average systemic hematocrit of 20% increased cochlear blood flow by 250% as measured with microspheres. The laser Doppler instrument significantly underestimated the actual flow increase giving an indication of 148%. Furthermore, the data, when analyzed on an individual trial basis, showed a very poor correlation between the two methods. The theoretical basis for these findings in relation to the use of the laser Doppler instrument is discussed.

Angiotensin II-induced changes in cochlear blood flow and blood pressure in normotensive and spontaneously hypertensive rats

Previous investigations in our laboratory have measured significant increases in the circulating levels of the potent vasoconstric-tive hormone, angiotensin II (AII; 26 and 64 pg/100 μl plasma, normal and noise exposed, respectively), during and following noise exposure in the alert rat (Wright et al., 1981). In the present study, these levels were approximated through intra-arterial infusion in the anesthetized spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto (WKY) rat. Laser Doppler flowmeter measurements of cochlear blood flow (CBF) indicated that despite equivalent All-induced elevations in systemic blood pressure, CBF in the SHR did not increase to the levels measured in the WKY. Pretreatment with the specific angiotensin receptor antagonist sarile, (Sar 1,Ile 8-AII), reduced All-induced elevations in systemic blood pressure in members of both strains, but did not change the overall pattern of CBF. These results indicate that SHRs may have a compromised cochlear circulation that is refractory to increases in systemic blood pressure.

Changes in cochlear blood flow during acoustic stimulation as determined by 14C-iodoantipyrine autoradiography.

Local blood flow was measured in the tissues of the cochlea using the [14C]iodoantipyrine autoradiographic technique. Flow observed without acoustic stimulation was compared with that seen during exposure to wide-band noise at 85 or 105 dB SPL. Compared with the unexposed cochlea, substantial increases in blood flow were observed during exposure to 85 dB SPL noise in the spiral ganglion, VIII nerve and spiral lamina. Little or no change was noted in external wall structures. These results are consistent with changes in cochlear metabolism which have been reported previously using similar techniques, suggesting that increases in blood flow may be linked to increases in local metabolism. No changes in blood flow were measured during exposure to 105 dB SPL noise. This result is similar to those of other investigators using potentially damaging intensities of acoustic stimulation.

Measuring cochlear blood flow by laser Doppler spectroscopy.

Cochlear blood flow (CBF) was studied with a commercially available laser Doppler system in 20 guinea pigs. The cochlea was exposed to permit placement of the laser Doppler probe over the intact lateral wall of the basal turn. Ketamine and xylazine were used for anesthesia, and blood pressure was monitored from the femoral artery. In some cases, skin blood flow was monitored with a second laser Doppler system, and cardiac output was monitored with an ultrasonic Doppler system placed over the right brachiocephalic artery. We found that the laser Doppler signal is composed primarily of blood flow supplied by the internal auditory artery. Local pressure on the contents of the internal auditory canal after occipital craniotomy was found to reduce CBF to 15% of its original value in a reversible fashion. There was no change in CBF after bilateral occlusion of the common carotid arteries. There appears to be a mechanism governing CBF that stabilizes its value in the face of changes in blood pressure and cardiac output. This is similar to the vascular behavior of the central nervous system. Through the use of positive airway pressure and blood removal at different rates, cardiac output could be depressed to varying degrees. The magnitude of decrease in CBF was clearly related to the rate at which cardiac output and blood pressure dropped. This was confirmed when intravenous phenylephrine was given in sequential and increasing doses. CBF increased as blood viscosity decreased, as expected according to the vascular behavior of the central nervous system.(ABSTRACT TRUNCATED AT 250 WORDS)