Effects of carbogen on decreases in endocochlear potential and cochlear microcirculation induced by ischemia of the cochlea

Effects of carbogen on cochlear blood flow and hearing function following acute acoustic trauma in guinea pigs

BACKGROUND AND AIMS

Disturbances of microcirculation and hemorheological changes in the inner ear are the results of noise-induced hearing loss (NIHL). Both the disturbances of microcirculation and hemorheological changes are the etiologies of NIHL development, but they are also the results. Although previous reports that inhalation of high concentration of CO(2) may increase cochlear blood flow (CoBF), the effects of carbogen on the cochlear microcirculation and NIHL remain unclear.

METHODS

Changes induced by noise, carbogen and pure oxygen within the cochlear lateral wall microvasculature and in hearing thresholds were observed in guinea pigs using intravital microscopy and the auditory brainstem response. At the same time, arterial oxygen saturation and morphologic changes of cochlear hair cells were observed.

RESULTS

Carbogen inhalation increased vessel diameters and blood flow velocities. Hearing thresholds elevation in the carbogen group was smaller than those in the control and oxygen group (p <0.05). Carbogen inhalation produced a trend toward less threshold shift after noise exposure, which reached statistical significance after day 3 (p <0.01). Respiratory acidosis was not found in our study. The segmented basal membranes of Corti in three groups indicated that no losses or discorders of hair cells were found.

CONCLUSIONS

Carbogen inhalation can preserve hearing in animal models after acute acoustic trauma.

Sensitivity of the endocochlear potential level to cochlear blood flow during hypoventilation

To study the relationship between endocochlear DC potential (EP) and cochlear blood flow (CoBF) under hypoxic conditions, we recorded the EP and CoBF from the basal turn of the cochlea in 21 guinea pigs. Hypoventilation for 10 minutes was induced by reducing the respiratory rate and volume. During hypoventilation, the EP declined in most of the cases to an intermediate level of the positive range in a few minutes. At the midpoint of the 10-minute hypoventilation, angiotensin II (5 microg/kg or 1 mL/kg) was infused for 60 seconds to raise the systemic blood pressure. In this experimental manipulation of systemic blood pressure, the CoBF and EP generally rose transiently. We determined the sensitivity of the EP to a CoBF change (delta) by calculating the deltaEP/deltaCoBF. More specifically, we analyzed the relationship between the deltaEP/deltaCoBF and the EPi (EP level just before angiotensin II infusion). The deltaEP was equal to the maximum EP level after angiotensin II infusion minus the EPi. The deltaCoBF was equal to the maximum CoBF value after angiotensin II infusion minus the CoBF value just before infusion. The deltaEP/deltaCoBF increased most in the range near 70% of the EPi. That is, the deltaEP/deltaCoBF was greater and the EPi was lower in the range above 70% of the EPi. To elucidate this linear correlation in the range above 70% of the EPi, we must consider several factors. In the supplementary experiments for blood gas analysis using 11 guinea pigs, most of the data of the EPi in the range above 70% were found to be obtained under conditions of a PaO2 of more than 12 mm Hg. As to the sensitivity increase of the EP to the deltaCoBF above mentioned, we propose that among several factors in the stria vascularis during hypoxemia, the activation of glycolysis in aerobic metabolism may be involved. As another possible factor, we postulate the increase in the reactive rate of the enzymatic activities that are linked with EP production and respond to the elevated cyclic adenosine monophosphate activity induced by the sympathicotonic state due to hypercapnia.

Sensorineural hearing loss in survivors of neonatal extracorporeal membrane oxygenation

The follow-up information of sensorineural hearing loss (SNHL) in neonatal extracorporeal membrane oxygenation (ECMO) survivors was reviewed by Medline search (1985-1996) using indexing terms; ECMO, follow-up outcome. Only seven reports (from six different centres) provide information on the results of audiologic assessment in 371 ECMO survivors with follow-up duration of 1-10 years. SNHL is a significant neurologic morbidity in neonatal ECMO survivors with a reported frequency of 3-21% (average 7.5%, 28 of 371). From three of the seven reports that provide information of clinical features in the survivors with SNHL, SNHL is often an isolated disability (16 of 19, 84%) and the loss is usually bilateral involving high frequencies. SNHL can be late-onset (7 of 19, 37% diagnosed after infancy) and progressive. Multicentre prospective studies with regular comprehensive audiologic assessment until bilateral thresholds can be obtained are urgently required to define the course of SNHL. Basic research is needed to evaluate the role of various factors (e.g., peroxynitrite and free radicals) in the pathogenesis of SNHL. The information generated will help to develop preventive measures and cost-effective management of these ECMO survivors.

Sensorineural hearing loss in survivors of neonatal extracorporeal membrane oxygenation

From February 1989 to January 1994, nine of 63 (14.3%) survivors of neonatal extracorporeal membrane oxygenation developed bilateral sensorineural hearing loss. Seven of nine children were tested and passed initial or repeat clinical auditory brainstem response evaluation completed before discharge from neonatal intensive care. Hearing loss was suspected and confirmed between 6-36 and 10-48 months of age, respectively. We recommend regular audiologic follow-up for these high-risk infants until bilateral thresholds for hearing can be obtained.