Mouse to elephant: biological scaling and Kt/V.
The construct Kt/V is used by the nephrology community in prescribing dialysis dose. The concerns that have been raised as to what value of V to use in the calculation of Kt/V touch on the more central question of whether filtration rate should be normalized by a parameter other than V. Within the animal kingdom, a number of physiological variables scale to body size according to an equation of the form Y = YoMb, where Yo is a constant, M is body mass, and b is a scaling exponent. Glomerular filtration rate (GFR) in mammals weighing from 30 g to 503 kg scales to body weight with an exponent of 0.77. Hence, GFR per unit body weight (or Kt/V) decreases significantly with increasing body size. Metabolic rate also scales to body size in a wide range of mammals according to the same general equation and with a scaling exponent of 0.75. Because GFR and metabolic rate scale to body mass with virtually the same exponent, a ratio of the two yields a constant independent of body size. We propose that the ratio (filtration rate/metabolic rate) replace Kt/V. Such a ratio would underscore the linkage between filtration rate (and dialysis therapy) and the metabolic demands of the body.
Singer
M A
MA
Department of Medicine, Queen's University, Kingston, Ontario, Canada. armorton@post.queensu.ca
Morton
A R
AR
D000818:Animals; D001835:Body Weight; D004606:Elephants; D005919:Glomerular Filtration Rate; D006801:Humans; D008322:Mammals; D051379:Mice; D006435:Renal Dialysis / Q000706:statistics & numerical data*; D014508:Urea / Q000378:metabolism*
D014508:Urea