At no stage has the college ever asked about hypernatremia specifically, but it appears as a garnish to the following SAQs:
- Question 23 from the second paper of 2006 (more about polyuria in brain injury, but lightly touching upon the topic of DI and mannitol diuresis)
- Question 3 from the second paper of 2007 ( also heavily about polyuria in brain injury)
- Question 5.2 from the first paper of 2018 (mainly about the management of DI with virtually zero history or context)
Oh's Manual coverage of hypernatremia is limited to half of page 954 from Finfer and Delaney's fluid and electrolyte chapter, which references only one paper (Adrogué and Madias, 2000). Of this half-page, the majority is taken up by a Blue Box, which classifies causes of hypernatremia into water loss and salt gain. Unfortunately, in the "salt gain" category there is only one entry: "Hypertonic, saline or sodium bicarbonate". The other category is sufficiently diverse to merit a tabulated listing:
Extrarenal water loss
Generally, adverse effects of hypernatremia develop at sodium concentrations in excess of 155-160 mmol/L
- Increased temperature
- Subarachnoid haemorrhage (due to brain shrinkage and vascular rupture)
Rapid rehydration may give rise to cerebral oedema, and this is the main risk in starting any sort of corrective therapy. If the hypernatremia developed over hours, reducing the sodium concentration by 1mmol/hr is appropriate. Just as in the correction of hyponatremia, a daily decrease of 10mmol/L of sodium is a sensible goal.
Calculating the free water deficit
Water is often what's missing from the patient's body fluid compartments, and therefore it is fair to say that at some stage the college may expect their candidates to calculate the water deficit. There are several similar-looking equations available.
A conventional equation quoted by Cheuvront et al (2013) is as follows:
Because the calculation of total body water is usually (body mass × 0.6), the equation is often rewritten as:
Cheuvront et al actually offer seven variations of this equation, which include a proposed correction for unknown body mass, a total body water estimate from body composition, one which uses the actual plasma sodium, one which substitutes plasma osmolality for sodium, and so on. To cut a long story short, they all suck equally, in terms of being unable to accurately predict water losses in dehydrated volunteers.
Adrogué and Madias (2000) quote the same equation, recommend against its use, and suggest a refinement to it:
Thus, a 70kg male with a serum sodium of 160 will enjoy a 3mmol/L decrease in his serum sodium after receiving 1000ml of 5% dextrose with 30mmol of KCl in it.
LITFL rearrange things slightly:
where the total body water is 50%, not 60%. Cadogan did not reference this, but it resembles the equation given by MDCalc:
where % total body water (TBW) is:
- Adult male: 60% (i.e., use 0.6 in the equation)
- Adult female: 50% (0.5)
- Elderly male: 50% (0.5)
- Elderly female: 45% (0.45)
- Child: 60% (0.6)
MDCalc referenced Barsoum & Levine (2002) for this, weirdly selecting only one of the three equations offered by the authors, and not even the one they recommended. Another equation offered by these authors was:
If one were to obsess over these matters, one would enter these equations into a spreadsheet and demonstrate that with identical variables, mathematically the results differ only by 1L of estimated water deficit:
|Author||water fraction||body mass (kg)||serum sodium (mmol/L)||ideal sodium (140 mmol/L)||water deficit (L)|
|Cheuvront et al||0.6||70||160||140||5.25|
|Barsoum & Levine||0.6||70||160||140||-6|
So, in summary, it does not matter which equation you use, and in the grand scheme of things it must be acknowledged that the calculation of a water deficit is a purely self-indulgent exercise in biochemical nerdsmanship, given the well-established inaccuracy of all the equations. In reality, nobody would prescribe 6000ml of 5% dextrose and walk away without any intention of reassessing the sodium or re-examining the patient. Unfortubately, as the college examiners have never used any of these calculations in an SAQ, we do not know what the CICM Officical Scrabble Dictionary free water deficit equation is.