Criteria of individual and population-wide resistance to acute hypoxia

Abstract

It is shown that the geometric mean of survival time and the deviation of ~ from x are measures of a population's resistance to acute hypoxia; that a measure of the individual resistance of an animal to such hypoxia is Student's parameter for that animal; that the distribution of Student's parameter in a population is a lognormal distribution under any conditions; and that the number of animals with low resistance is always equal to the number of highly resistant animals. The procedure described makes it possible to limit the number of animals used in an experiment by recording with the requisite accuracy the proposed measures of resistance to acute hypoxia at the population and individual levels. Key Words: hypoxia; resistance; individual resistanceKnowledge of individual variations in physiological parameters is necessary for defining the limits of age-and sex-specific variabilities in these parameters, the normal ranges of various responses, and the reserves of adaptive/compensatory mechanisms. The course of any disease is largely determined by individual responsiveness, which is based on congenital and acquired traits. As found in studies of responses to oxygen deficiency, there are individuals in any population of animals that survive when most of the test animals (even 95%) have died [2,3]. Animals differing in susceptibility to hypoxia exhibit substantial differences in behavioral, physiological, and metabolic variables [4,5].Although several methods are now available for quantifying the results of experiments on animals with acute hypoxia [1-3], these methods are not without their shortcomings. Linear estimates are valid only when variations in the recorded parameters are small. Different procedures may yield discrepant resuits depending on the empirical coefficients used. Department of Normal Physiology, Russian University of Peoples' Friendship, MoscowA relatively accurate method of linear analysis was proposed by Berezovskii [3] for rats separated, according to survival times at an altitude of 11,000 m, into groups with low, medium, and high resistance to hypoxia, using 30% deviations from the arithmetic mean of survival times calculated for all the rats tested over the same period as cutoff points for their separation into these groups. The cutoff points were set at equal intervals from the mean survival time and were shifted accordingly when its arithmetic mean changed. Berezovskii's method yields normal results if the deviations of survival times from the arithmetic mean are not large, whereas in the case of a nonlinear distribution where the minimal value differs from the maximal by one order of magnitude, this method produces a considerable deviation of the mean value from the center of the distribution curve.The aim of the present study was to identify, by means of a strict mathematical analysis, criteria by which the resistance of an animal population to acute hypoxia can be characterized as completely as possible.

Changes in hypoxia resistance in rats during daytime

“…Because of asymmetry, the center of distribution was most precisely characterized by the median [5]. The mean suggested tbr this purpose in [4] was higher than the median, while the geometric mean proposed in [1 ] was lower. The variations in the group size during the day (HR group was always the smallest) was probably due to the difference between theoretical and empirical distributions.…”

“…These variations is probably due to differences between empirical and theoretical samples, which is evidenced by its highest value at the beginning of the day when the sample sizes were not as large as later on. Thus, dally variations of SP values in Wistar rats, like dally and seasonal variations in outbred rats [l ], were described by a log-normal distribution [1]. The log-normal distribution was typical not only of large (n=104-139), but also of relatively small (n=11-63) samples.…”

“…Reportedly, rats can be classified as low-resistant (LR), moderate-resistant (MR), and high-resistant (HR), when their resistance to hypoxia is measured by the survival period (SP) under acute hypoxic conditions [1,3,4,8,10]. It has been proposed to calculate the mean SP value as an arithmetic mean for the total population and to determine the boundaries between MR and other groups as symmetrical points around the mean (for instance, _+30% of its value; this range is allowed to be voluntarily extended or narrowed [4]).…”

“…It has been shown that there can be significant differences (up to 30%) between SP values computed as an arithmetic and a geometric mean [1]. It is known that SP undergoes seasonal and circadian variations [2].…”

Changes in hemodynamics and respiration in animals with different resistance to acute hypoxia

“…Special normobaric and hypobaric tests make it possible to classify the animals as high-(HR), medium-(MR) and low-resistant (LR) to hypoxia [1][2][3]6,9]. Biochemical studies showed that animals with different resistance to hypoxia differ not only in parameters of energy, lipid, and carbohydrate metabolism [2,6], but also in the functional characteristics of hemodynamic regulation.…”