Influence of blood viscosity on cochlear action potentials and oxygenation

Impairment of the cochlear blood supply of guinea pigs was induced in order to study the effects of hypoxia on the cochlear action potentials. The oxygenation of the cochlear structures was decreased by perfusing the ears with polycythemic hyperviscous blood. The validity of using this model of cochlear blood flow was based on the propensity of the blood to flow in a laminar way. Because of the streamlined flow pattern, the blood supplied by the two vertebral arteries does not mix within the common trunk of the basilar artery. The retrograde injection of polycythemic hyperviscous blood into one vertebral artery will affect the ear on the injected side only. The high viscosity of the polycythemic blood decreases the rate of flow of blood through the cochlear vessels; the high oxygen content of this blood, however, avoids hypoxia of the cochlea. Therefore, in order to make the slowdown in the blood flow evident, its oxygen content was reduced to a 'precritical level' before it was infused. Injecting normoviscous blood with a 'precritical level' of oxygen caused a mild reduction in the scala media pO2 of 15.2% for the whole group of twenty animals. The hyperviscous blood with the same level of oxygenation, however, reduced the pO2 in the scala media to 53.3% of normal. These findings explain the difference in the altered click-evoked action potentials in the two groups of animals.