[k] effects of mannitol infusion
Post on 13-Nov-2014
123 Views
Preview:
TRANSCRIPT
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)
1
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
2
top related