[k] Gastroenteritis And Fluid Volume

THE KIDNEY

Gastroenteritis and Fluid Volume

A man weighing 60 kg (132 lbs) has an episode of gastroenteritis with vomiting and diarrhea. Over a 2day period, this individual loses 4 kg (9 lbs) of body weight. Before he became ill, his plasma [Na+] was 140 mEq/L, and it is unchanged by the illness. Assuming the entire loss of body weight represents the loss of fluid, estimate the following: Initial conditions (before gastroenteritis) The initial volumes of the body fluid compartments and the osmoles in these compartments are calculated as follows (osmolality is estimated as 2 x [Na+]): Total body water:

= 0.6 x (60 kg) = 36 L

ICF volume:

= 0.4 x (60 kg) = 24 L

ECF volume:

= 0.2 x (60 kg) = 12 L

Total body osmoles:

= (total body water) (body fluid osmolality) = (36 L) (280 mOsm/kg H2O) = 10,080 mOsm

ICF osmoles:

= (ICF volume) (body fluid osmolality) = (24 L) (280 mOsm/kg H2O) = 6720 mOsm

ECF osmoles:

= total body osmoles - ICF osmoles =10,080 mOsm - 6720 mOsm = 3360 mOsm

New equilibrium conditions (after gastroenteritis) 4 kg of body weight is lost. It is assumed that this entire weight reduction reflects fluids lost through vomiting and diarrhea. Thus, 4 L of fluid is lost. Because the plasma [Na +] is unchanged, a proportional amount of solute was also lost (isotonic loss of fluid). There will be no fluid shifts between the ECF and ICF, because of the absence of an osmotic gradient between these compartments. Thus the ECF loses 4 L of volume and 4 x 280 = 1120 mOsm of solute. Total body water:

= 36 L - 4 L = 32 L

ICF volume:

= 24 L (unchanged)

ECF volume:

= 12 L - 4 L = 8 L

Total body osmoles:

= 10,080 mOsm - 1120 mOsm = 8960 mOsm

ICF osmoles:

= 6720 mOsm (unchanged)

ECF osmoles:

= 3360 mOsm - 1120 mOsm = 2240 mOsm

ICF, Intracellular fluid; ECF, extracellular fluid.