Estimation of body water and salt contents from plasma sodium, protein concentrations, and hematocrit

New formulae for the estimation of change in water (delta W) and salt (delta NaCl) contents of the body from hematocrit, plasma Na, and protein concentrations were proposed based on a compartment model of body fluid regulation: delta W = 0.4(delta Vi/Vi0) + 0.2(delta Ve/Ve0) litres/Kg of body weight; delta NaCl = 1.64[delta Ve/Ve0 + 1)/(delta Vi/Vi0 + 1) - 1)-1] Gms/Kg of body weight; where delta Vi/Vi0, and delta Ve/Ve0 are relative increment of intra and extracellular fluid volume. delta Vi/Vi0 is considered equal to ([Na]0/[Na]-1) under the condition of normal cell membrane, whereas delta Ve/Ve0 may be a nonlinear function of relative increment of plasma volume and hematocrit or sodium concentration. Thus a nomograph was presented to obtain delta W and delta NaCl simultaneously. This nomograph could predict delta W and delta NaCl with satisfactory accuracy in the experiments with the dogs to which fluid of various osmolarity was given intravenously. It was also confirmed in a case of mixed water depletion in which the clinical estimation agreed with that of the present method. The conventional formulae of calculation based on either pure dehydration or purse salt loss proved not to be applicable in the usual cases of body fluid disturbances.