Auditory nerve-brain stem evoked potentials and EEG during severe hypoglycemia

To investigate the effects of severe hypoglycemia on the ability of the brain to generate electrical activity, the EEG and auditory nerve-brain stem evoked potentials (ABP) were recorded before and during the development of insulin-induced hypoglycemia in rats (ABP only) and cats (ABP and EEG). No significant ABP changes were seen in rats in which the average blood glucose levels had reached 0.38 mmol/l (= 7 mg%). In cats, even though the EEG was depressed, no ABP latency changes and only slightly significant amplitude changes were seen in animals in which the average blood glucose level was 0.8 mmol/l. In both animals blood glucose levels were below 1.66 mmol/l for an average duration of 90 min. Therefore, even though it has been shown that the brain structures involved in generation of the ABP have higher metabolic rates, they seem to be resistant to deviations from homeostasis such as hypoglycemia.     

Influence of experimentally elevated blood viscosity on the auditory nerve-brainstem evoked response and threshold.

Blood viscosity, due to its effect on blood flow, is one of the determinants of oxygen delivery. Therefore the influence of elevated blood viscosity on hearing was studied in rats using the auditory brainstem response (ABR) threshold, wave 1 latency, brainstem transmission time (BTT) and wave 1/4 amplitude ratio. Whole blood viscosity (WBV) was elevated by 15-21% in two different ways: elevating the hematocrit (Polycythemia) by acclimation in a hypobaric chamber, or elevating the plasma viscosity by infusing a solution of Polyvinylpyrrolidone-360 (PVP). ABR was recorded before and 24 h after the blood viscosity was elevated, so that each rat served as its own control. Paired t-tests showed that there was no statistically significant difference in the ABR parameters in each of the groups as a consequence of blood viscosity elevation. In conclusion, the elevation of WBV to this degree for this duration, using two different techniques had no effect either on the function of the auditory nerve and the more peripheral sites, or on the central auditory pathway as studied by ABR.     

Hypoxia induced hearing loss in animal models of the fetus in-utero.

The human fetus in-utero has low arterial oxygen tension. It has, therefore, been suggested that at greater than 28 weeks gestational age, the fetus may have a sensori-neural hearing loss comparable to that seen in adult cats exposed to similar degrees of hypoxia. This is due to hypoxia induced depression of the endocochlear potential. However, fetal blood is provided with compensatory mechanisms (elevated hematocrit and hemoglobin and special fetal hemoglobin) which enable pick up and transport of more oxygen from the placenta than adult blood under the same physiological conditions. Therefore, the hypothesis of a fetal sensori-neural hearing loss due to oxygen lack was tested in the following animal models: a) Adult cats to which feline red blood cells were infused thus causing a polycythemia similar to fetal conditions; b) Adult rats acclimated to altitude in a hypobaric chamber, inducing erythropoiesis with elevated hematocrit and hemoglobin; c) Neonatal guinea pigs and goats studied when they were less than 12 hours old so that the fetal compensatory mechanisms were still present. In each model, hypoxia (PaO2 20-30 mmHg) induced an ABR threshold elevation resembling that obtained in the uncompensated adult animal. Thus these experiments seem to have confirmed the hypothesis of a fetal, hypoxic induced sensori-neural hearing loss even though such experiments have not been conducted directly on fetal animals.     

Survey of attempts to use auditory evoked potentials to obtain an audiogram. Review article

Click stimuli have been used to elicit ABR in auditory diagnosis but the ABR response is dominated by the activity of high-frequency nerve fibres and it is therefore impossible to deduce hearing at lower frequencies from ABR to clicks. Short tone pips or filtered clicks are also inadequate since the response is still dominated by basal turn, high-frequency nerve fibres. In order to attempt to obtain an audiogram by means of ABR, various noise-masking techniques have been used to restrict the response to more specific regions along the basilar membrane. These included presenting click or filtered click stimuli together with either different high-pass noises in order to obtain a derived ABR or notched noise. Though these techniques have been tested on small groups of normal subjects, they have not been adequately clinically validated on sufficient numbers of hearing-loss patients with various audiogram shapes. When tested on patients with low-frequency hearing loss, inappropriate thresholds have been obtained indicating problems with lower frequencies. Alternatively, other types of auditory evoked responses based on less synchronous neural activity have been studied including the SN10 response and the 40 Hz-MLR. These responses have not been adequately clinically tested. In conclusion the evoked potential techniques have not been completely tested in the clinical situation and several studies have indicated problems with the evaluation of lower frequencies.     

Effect of delayed refrigeration on plasma factors in whole blood collected in CPDA-2

Extension of the time within which whole blood may be separated into components offers logistic advantages for the operation of remote mobile drawing teams. We evaluated the effect of an 8-hour hold of whole blood at room temperature before preparation of components. Plasma coagulation activity and opsonic factor content were studied in 14 units drawn into the anticoagulant-preservative solution citrate-phosphate-dextrose-adenine (CPDA-2). At the time of collection, an additional 7-ml aliquot was drawn into 1 ml of CPDA-2, the plasma separated and frozen immediately. Components were prepared from whole blood units allowed to rest undisturbed at 22 +/- 1 degrees C for 8 hours. After 8 hours, a significant decrement of about 10 percent was found in the concentration of fibrinogen, plasminogen, fibronectin, and activity of Factor V. Factor VIII activities (VIIIAHF and VIIIAGN) were not significantly different after 8 hours. Our results indicate that room temperature storage for 8 hours before component processing has minimal effects on potentially labile plasma protein factors using CPDA-2 anticoagulant-preservative solution.     

Platelet counts during rapid massive transfusion

The effect of rapid massive transfusion upon platelet counts has been studied retrospectively in 24 patients treated for severe trauma. Multiple linear regression analysis showed that a platelet dilution model with an exponential form described the quantitative relationship between the pre- and posttransfusion platelet counts and the volumes of blood products administered. Intravenous salt solutions had very little effect upon the platelet count even when large volumes were infused. Red blood cell transfusions decreased the platelet count to an extent two and one-half times greater than that of colloid infusions. The differences in the magnitudes of the dilution effects are attributable to the different physiologic volumes of distribution of the various classes of infusates. If only red blood cells (RBC) and fresh frozen plasma (FFP) are transfused, two useful equations obtain. First, the current platelet count (Pt) can be predicted knowing the initial platelet count (Po) and the number (#) of units of RBC and FFP administered: Pt = Po (0.634) e-0.046 [#RBC + #FFP/8]. Second, the point during massive transfusion when platelet transfusions will be necessary can be predicted knowing the initial platelet count.     

Reflections on the role of a traveling wave along the basilar membrane in view of clinical and experimental findings

European Archives of Oto-Rhino-Laryngology - Air conduction (AC) is accompanied by displacements of the two cochlear windows, bulk fluid flow between them, a pressure difference across the basilar...     

The cochlear microphonic potential does not reflect the passive basilar membrane traveling wave

OBJECTIVE: Understanding the cochlear microphonic potential (CM) can contribute to its clinical usefulness, e.g. in auditory neuropathy where the CM is present, while auditory evoked neural potentials are absent. The space-time pattern of the basilar membrane traveling wave in the cochlea is thought to be responsible for generation of the CM and its pattern along the cochlea. This relationship was studied in two experiments. EXPERIMENT I: METHODS: The threshold of the CM was measured in animals before and after drilling a hole in the wall of the inner ear vestibule. Such a hole likely reduces the magnitude of the basilar membrane traveling wave. RESULTS: The hole did not cause a change in CM threshold. CONCLUSION: This is evidence that the passive basilar membrane traveling wave described by von Bekesy in cadavers in response to high intensity stimulation may not be the trigger for cochlear activation and CM generation at low intensities. EXPERIMENT II: METHODS: A saline filled tube provided fluid coupling between the cochlea of one animal and that of a second through the perforated round windows of their cochleae. RESULTS: In response to sound stimulation of animal 1, CM (and ABR) could be recorded in animal 2, even when animal 1 was no longer living. CONCLUSION: It is highly unlikely that a basilar membrane traveling wave could have been induced in cochlea 2 in these conditions. SIGNIFICANCE: It is therefore suggested that at low sound intensities, the initial event activating the cochlea in general and the CM in particular is the fluid pressures (condensations/rarefactions) induced in the cochlear fluids by sound-induced stapes footplate vibrations.     

Development of inner ear (cochlear and vestibular) function in the fetus-neonate.

The development of function in the various receptors in the inner ear was studied in the neonatal rat, which is altricious with respect to hearing, using short latency evoked potentials, both auditory (ABR) and vestibular (VsEP). It was found that VsEPs could be recorded from all the vestibular end-organs by post natal day (PND) 8, whilst ABR could only be recorded from all animals on PND 14, showing the earlier onset of vestibular function in the inner ear. These results are discussed with relation to onset of inner ear function in the human fetus.