Misplaced concreteness

Nephrology, more than other specialties is plagued by misplaced concreteness. We get false senses of precision because of the myriad of equations that spit out results to the milliliter. All of those equations from Kt/V, to water deficits, to IVF brain teasers depend on an estimate of total body water.

Everyone knows the rule of thumb that young males are 60% water, young females are 50% water and the percent body water falls as people age or get fatter.

Going beyond these rules of thumb, how is total body water measured empirically? The gold standard is heavy water dilution.

This works by giving a sample of heavy water and then waiting for it to equilibrate. Then the investigators measure the heavy water content of exhaled water vapor or a blood sample, the fraction of the water that is heavy water will be equivalent to the fraction of total water which is heavy water. Then since one knows the amount of heavy water given to the patient, one can calculate total body water.

When this is done, or when one reviews the primary literature, as was done in this study the numbers are a little different.

The drop in total body water (in red) in men never gets down to 50% as predicted for the elderly, and in women almost all of the numbers are below 50% and the trend to lower percentages through aging holds if you ignore the 9 women over the age of 70. Of note, these patients are not that obese, see BMIs in blue.
The above study lead to the development of the Watson equation to determine total body water. The Watson equation uses age, weight and height for men and height and weight for women:
  • Males: TBW (in liters) = 2.447 + (0.09156 × age) + (0.1074 × height) + (0.3362 × weight)
  • Females: TBW (in liters) = –2.097 + (0.1069 × height) + (0.3362 × weight)

This study in peritoneal dialysis patients (peritoneum empty) showed surprisingly close relationship (R=0.92) between deuterium dilution and the Watson equation:

Another study of PD patients demonstrated one of my pet peeves, the major effect of obesity has on total body water.

The chart is a bit difficult to understand. The Y axis shows the Watson calculation of total body water minus heavy water dilution. So negative numbers indicates cases where the Watson method underestimates TBW. When the Y-axis is positive the Watson calculation overestimates TBW. The X-axis expresses obesity as body fat over body water. Really? fat over water. You couldn’t just graph this versus BMI?

IV Fluid Brain Teaser: Salt versus Saline

Everyone knows that if you give a liter of saline, all of it remains in the extracellular compartment.

But what if you give a patient just the salt from the saline and none of the water? How much does the solute contribute to the increase in the extracellular volume? How does 154 mmols of NaCl affect the size of the extracellular and intracellular compartments?

Assume the patient is a 70 kg lean young male with a serum osmolality of 280 mOsm/kgH2O. Ignore any renal losses during the process.

For full credit fill out the following:

Total body water:
Size of the extracellular compartment:
Size of the intracellular compartment:


Step one calculate the total number of osmoles the patient has:

70 kg lean young male means 60% total body water or 42 liters
42 liters times 280 mOsm/Kg = 11,760 osmoles in the body

Giving 308 mosm of solute will increase that to 12,068. There is no additional water so dividing that by 42 liters gives us a new osmolality of 287 mOsm/Kg water.

Remember that even though the solute is trapped in the extracellular compartment, the osmolality is the same across all body compartments since water can flow from compartment to compartment.

Now we need to find out how much the extracellular compartment expands in osmoles.

Before the addition of solute the extracellular compartment should be one third of total body water, so 14 liters times osmoality of 280 is 3920 mOsmoles. Add 308 and then divide that by the new osmolality to give you the new volume:

That increased volume of course comes from the intracellular compartment, so it goes down by 0.7 liters. You can also get there by taking the original volume of 28 liters multiplying by 280 mOsm to get 7840 miliosmoles and divide that by the new osmolality of 287:
So the addition of 308 miliosmoles from the bag of saline will increase the extracellular compartment by 0.7 liters. Only 0.3 liters less than the increase you would get with a liter of 0.9 NS. 
It’s all about the salt.

Total body water: 42 liters
Size of the extracellular compartment: 14.7 liters
Size of the intracellular compartment: 27.3 liters