Complex level alterations of the 2f(1)-f(2) distortion product due to hypoxia

For diagnostic purposes and a better understanding of the pathophysiology of inner ear hearing disorders it would be of great interest to have parameters available that indicate inner ear hypoxia. In animal studies typical hypoxia-related alterations of the 2f1-f2 distortion product otoacoustic emissions (DPOAE) such as a reversible level decrease and destabilization could be demonstrated. The goal of this study was to investigate whether these hypoxia-associated alterations can also be observed in humans because this might help develop a new diagnostic tool for patients with inner ear disorders.

In 16 volunteers DPOAE levels were continuously measured at first under normal room air conditions, during and after 8.5h of oxygen deprivation (13% O2) and during re-oxygenation. Saturation of oxygen of arterial blood (SaO2) was monitored.

The mean SaO2 during the hypoxic interval was 78%. A significant decrease in DPOAE level under hypoxia occurred in five different test persons at one or more frequencies (f2=1, 1.5, 2, 3, and 4kHz). A destabilization of the DPOAE level with considerable fluctuations during hypoxia was observed in nine subjects at one or more frequencies. Furthermore, the so called 'post hypoxia effect' could be observed in five participants.

The observations made here have been described similarly in animal studies and seem to be characteristic of metabolic disorders of the cochlea caused by hypoxia. To our knowledge, this is the first study to examine DPOAE level alterations over time in humans under conditions of normobaric hypoxia. If DPOAE destabilization is observed in a clinical setting in patients with certain inner ear hearing disorders hypoxia can be suspected as one underlying pathophysiological cause which might influence treatment decisions.