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Notes: Summary Oxygen partial pressure (pO2) profiles and microcirculatory parameters were recorded in vivo under standardized conditions in the three scalae of the cat's cochlea after unilateral excision of the upper cervical sympathetic nerve. Experiments were performed both under rest conditions and during exposure to loud sounds. pO2 and pH2 were polarographically measured using microcoaxial needle electrodes with sputtered reference elements according to Baumgärtl and Lübbers (1975). The tip diameter of the complete probe was 0.3–0.7 μm. The oxygen pressure field of the three scalae of the basal winding was recorded through the round window membrane during continuous advance of the needle (150 μm/min). Temporal pO2 changes as well as microcirculatory parameters were measured in the scala media after the electrode was fixed in position. Starting out from the air values at the surface of the round window membrane, pO2 decreased continuously from more than 100 Torr during insertion towards the basilar membrane, where it averaged 25 Torr. When the needle had penetrated the basilar membrane and entered the endolymphic space, an increased pO2 of 35–40 Torr was recorded. After penetrating Reissner's membrane, a slightly decreased pO2 was measured in the scala vestib. Aside from the oxygen reaching the cochlea through the round window, the capillaries of the round window membrane were found to contribute to the oxygen saturation of the perilymph near the window. Under rest condition, unilateral upper cervical sympathectomy did not noticeably influence the oxygen pressure field in the three cochlear scalae. After exposure to loud sounds, however, endolymphic pO2 decreased less and recovered quicker on the sympathectomized side than on the intact one. Ventilation of the animals with pure oxygen or gas mixtures of high CO2-O2 concentrations increased the pO2 level within 1 min. The increase was more pronounced in the endolymphic space than in the other cochlear scalae and also the pO2 decrease in the scala media was particularly distinct in the cat. Under exposure to loud sounds the influence of the sympathetic nerve on the intracochlear pO2 was found to depend on the aortic blood pressure. The advantageous effect of sympathectomy was particularly visible at a low aortic pressure but less distinct at a higher one. In some cases, a long-lasting pO2 decrease was noticed in the endolymphic space even after short (1–2 s) exposure to noise. The hydrogen exchange rate recorded in the endolymphic space after exposure to loud sounds (noise of 115 dB) using the hydrogen clearance method was distinctly delayed and, consequently, the capillary blood flow was diminished. Unilateral acute upper cervical sympathectomy favorably influenced the peripheral circulation during exposure to loud sounds by increasing the blood flow (reduced half life period). The experiments showed that unilateral acute upper cervical sympathectomy did not considerably influence the endolymphic pO2 under rest condition, whereas an advantageous effect on the endocochlear pO2 and blood flow was noticed under exposure to loud sounds. Moreover, there was indication of a correlation between sympathetic effect and mean aortal blood pressure.

Notes: Summary The influence of different values of arterial oxygen partial pressure (Po2) on local tissue Po2 was investigated. Tissue Po2 was measured on the surface of the beating hearts of rabbit and cat by the multiwire surface electrode as described byKessler andLübbers (21). In parallel experiments, local hydrogen clearance applying the H2−PH2 probe (33) was used to determine the mean blood flow per area (microflow) at the capillary level. Mean blood flow per area, $$\bar v$$ , was calculated from PH2 clearance curves obtained by local application of rectangular hydrogen pulses. The results are presented as histograms. Under steady state conditions (barbiturate narcosis), tissue Po2 ranged from 5 to 65 Torr (0.67 to 8.67 kPA) with a median of 31 Torr (4.13 kPA) in the cat heart. Mean flow per area covered values between 25 and 11 μm/s with a median of 55 μm/s. For rabbit heart muscle, the median was $$\bar v$$ = 37 μm/s and the range 28 to 46 μm/s. Hyperoxia broadened the range of tissue Po2 and shifted flow per area to lower values. Different degrees of hypoxia shifted the Po2 histogram to the left (median Po2 14 Torr and 4 Torr, respectively; [1.87 kPA and 0.53 kPA]) and the flow histogram to the right.