THE KIDNEY

Effects of Mannitol Infusion

A woman weighing 50 kg (110 lbs) and with a plasma [Na+] of 145 mEq/L is infused with 5 g/kg (2.3 g/lb)

of mannitol (molecular weight of mannitol is 182 g/mole).

Estimate the following equilibrium values. Assume that mannitol is restricted to the ECF compartment, that

no excretion occurs, and that the infusion volume of the mannitol solution is negligible (total body water

unchanged):

Initial conditions (before mannitol infusion)

Total body water: _______________L

ICF volume: _______________L

ECF volume: _______________L

Total body osmoles: _______________mOsm

ICF osmoles: _______________mOsm

ECF osmoles: _______________mOsm

The initial volumes of the body fluid compartments and the osmoles in these compartments are calculated

as for problem I:

Initial total body water = 0.6 x (50 kg) = 30 L

Initial ICF volume = 0.4 x (50 kg) = 20 L

Initial ECF volume = 0.2 x (50 kg) = 10 L

Initial total body osmoles = (total body water) (body fluid osmolality)

= (30 L) (290 mOsm/kg H2O) = 8700 mOsm

Initial ICF osmoles = (ICF volume) (body fluid osmolality)

= (20 L) (290 mOsm/kg H2O) = 5800 mOsm

Initial ECF osmoles = total body osmoles - ICF osmoles

= 8700 mOsm - 5800 mOsm = 2900 mOsm

To determine the effect of the mannitol, the total amount infused must first be calculated. At 5 g/kg, a

total of 250 g was infused (1.374 moles of mannitol). Because mannitol is a single particle in solution, this

adds 1,374 mOsm to the ECF. The mannitol will raise ECF osmolality and result in the shift of fluid from the

ICF into the ECF.

New total body water = 30 L (unchanged)

New total body osmoles = 8700 mOsm + 1374 mOsm = 10,074 mOsm

New ICF osmoles = 5800 mOsm (unchanged)

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New ECF osmoles = 2900 mOsm + 1374 mOsm = 4274 mOsm

New plasma osmolality = new total osmoles/total body water-

10£¬074mOsm/30L = 336 mOsm/kg H2O

New ICF volume = ICF osmoles/New Posm = 5800mOsm/£¨336mOsm/kgH2O£© = 17.30 L

New ECF volume = total body water - ICF volume

= 30 L - 17.3 = 12.7 L

Because mannitol increases the osmolality of the ECF, 2.7 L of fluid shifts from the ICF into the ECF. To

calculate the new plasma [Na+], it is assumed that the amount of Na+ in the ECF is unchanged after

mannitol infusion. Originally, there were 2900 mOsm attributed to Na+ (2 x [Na+] x ECF volume) in the ECF.

Because the Na+ osmoles are unchanged but now present in a larger volume, the new plasma [Na+] is

calculated as follows:

New plasma Na+ osmoles = 2900mOsm due to Na+/12.7 L = 228 mOsm/L

New plasma [Na+] =£¨Na+osmoles£©/2 = 228mOsm/2 = 114 mEq/L

New equilibrium conditions (after gastroenteritis)

Total body water: _______________L

ICF volume: _______________L

ECF volume: _______________L

Total body osmoles: _______________mOsm

ICF osmoles: _______________mOsm

ECF osmoles: _______________mOsm

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