Spectral analysis of fetal sheep EEG during hypoxia.
Abstract Power spectra and coherence functions were obtained on samples of EEG recorded from fetal sheep in utero . Hypoxia was produced by respiring the ewe on gas mixtures containing reduced oxygen pressure. With both halothane and local anesthesia, the EEG changes associated with hypoxia consisted of an early power reduction in higher frequencies followed by a reduction of over-all amplitude leading to the isoelectric stage. The times at which statistically significant changes occurred were not earlier during the progressive fall of fetal arterial pO 2 than visual estiamtes of abnormality made on line from the paper record and were in all cases later than significant cardiovascular changes.
EEG, ECG, and acid-base observations during acute fetal hypoxia.
Abstract The effects of hypoxia on the EEG, heart rate, and acid-base status of fetal sheep were studied in 28 experiments which permitted simultaneous blood determinations on both sides of an intact placenta. Alterations in heart rate and rhythm preceded significant elevations of lactate concentrations and occurred prior to unambiguous changes in the EEG. A bradycardia preceded the onset of the isoelectric stage of the fetal EEG. The consistency of this finding suggested an etiologic relation. The time to recovery of normal EEG rhythms was a function of the length of the isoelectric stage but not of the total period of hypoxia or time to the onset of the isoelectric stage.
Modelling of Mooney viscosity relaxation in natural rubber
Abstract Better assessment of natural rubber (NR) quality and processability, through the identification of new attributes, is still a major challenge. With this scope, 12 model samples of natural polyisoprene and two of synthetic polyisoprene were characterized in terms of Mooney torque relaxation, using the standardized ISO289 method (ML(1+4)100, explained later in the Section 1 ). The main purpose of this study was to identify a model that gave a better fit for the experimental decay of Mooney torque over time (ML(t)=f(t)) than the usual power law model used before (Y=at−b, [G.M. Bristow, A.G. Sears, The use of novel parameters in the assessment of natural rubber processability, NR Technol. 18 (1987); H.G. Burhin, W. Spreutels, J.M. Campbell, MV2000 Mooney viscometer: Mooney relaxation measurements on raw polymers and compounded rubber stocks, Kautsch. Gummi Kunstst. 43 (1990) 431]). The results showed that a generalized Maxwell model, and the Wu–Abbott empirical model ( Y = 1 − a Ln ( t ) − ( b t / ( c + t ) ) ) [T. Wu, J.A. Abbott, Firmness and force relaxation characteristics of tomatoes stored intact or as slices, Postharvest Biol. Technol. 24 (2002) 59–68], recently used for describing stress relaxation of tomato flesh, allowed a better description of the relaxation curve for the samples studied than the power law or stretched exponential traditionally used. Moreover, the total relaxation time (τm) given by the generalized Maxwell model showed good correlation with the total gel content of the samples studied.