Cochlear blood flow measured by averaged laser Doppler flowmetry (ALDF)

This report describes a new approach to estimate the hydromechanical properties of a vascular system. Averaged laser Doppler flowmetry (ALDF) was developed by averaging the flux signal of a laser Doppler flowmeter (LDF) synchronized to the heart cycle. The usefulness of this method was verified by manipulation of the cochlear microvasculature. Twelve pigmented guinea pigs under pentobarbital/fentanyl anesthesia were used. The cochlea was surgically exposed and the LDF probe placed on the bony surface of the first turn to monitor cochlear blood flow (CBF). The LDF flux signal (0.2 s time constant) was sampled by an A/D board at 2 kHz for 255 ms and averaged with synchronization to the heart beat. The mean blood flow, peak to peak amplitude, and time (phase) delay of pulsatile flow were measured from the averaged signal. According to a transmission line model of the vascular system, under a given perfusion pressure, mean flow reflects resistance while amplitude and time delay of the pulsatile flow are related to the reactance component of the impedance of the vascular system. During the formation of photochemically-induced thrombosis in the cochlear microvasculature, there was a dramatic mean flux decrease (90.1 ± 3.4% from baseline (BL),N= 6). Additionally, a time-dependent decrease in amplitude and time delay of pulsatile flow were indicated by ALDF. These results suggest a large increase in vascular resistance and significant decrease in compliance. After application of 2% sodium nitroprusside (NP) to the round window membrane, mean flux increased by 78.3 ±17.6% BL; amplitude and time delay of pulsate blood flow increased by 81.0 ± 14.6% (N = 6) and 11.5 ± 3.2 ms (N = 6), respectively. These changes can be interpreted as lowered resistance and increased compliance consistent with vasodilatation by NP. ALDF is a new method which derives additional information from the LDF signal. The new information provided by ALDF can be of importance in assessment of basic cochlear microcirculation and interpretation of peripheral vascular diseases in the cochlea and other organs.