Aetiologies

• Diabetes insipidus

Clinical Features

• Similar to Hyponatraemia, mostly neurological:
  • Increased temperature
  • Restlessness
  • Irritability
  • Confusion
  • Drowsiness
  • Coma
  • Seizures
  • Subarachnoid haemorrhage (due to brain shrinkage and vascular rupture)

Workup

1. Consider history
   • Loss of solute poor water + primary neurologic disease or lack of access to water
   • As a result most patients are very young or very old
2. Urine osmolality
   • Low urine osmolality (<300 mOsm/kg): Diabetes insipidus
   • High urine osmolality (>600 mOsm/kg): impaired thirst or access to water
3. If Diabetes insipidus, determine type using response to administration of desmopressin

Management

• Chronic hypernatraemia (>48 hours): correction rate ≤10 mmol/L each 24 hours
• Acute hypernatraemia (<48 hours): correction to normal over 24 hour period
• Choice of fluid
  • Normal saline should be used only as initial therapy in patients with end-organ dysfunction secondary to depletion
  • 5% glucose should be used in situations of pure water loss (e.g. Diabetes insipidus)
  • Hypotonic fluid (e.g. half normal saline, quarter normal saline) should be used in situations with hypotonic fluid losses (e.g. diarrhoea)
• Calculate rate

EXAMPLE

An 85 year old woman 70kg with dementia develops diarrhoea while living at a nursing facility. After 4 days, she is unrousable by staff and brought to the emergency department. She appears hypovolaemic on examination and has a serum sodium of 170 mmol/L and a serum potassium of 2.8 mmol/L. What rate of what fluid should be chosen to correct this?

• Hypotonic fluid loss without mention of end-organ dysfunction with hypokalaemia ⇒ 1/2 normal saline + 20 mmol/L $\mathrm{KCl}$
• Rate calculation as follows:

$$\begin{array}{rl}\Delta [\mathrm{Na}{}^{+}]\text{ from 1L of IV fluid}& =\frac{([\mathrm{Na}{}^{+}]{)}_{\text{infusate}}-[\mathrm{K}{}^{+}{]}_{\text{infusate}}-[\mathrm{Na}{}^{+}{]}_{\text{serum}}}{\text{Total body water (L)}+1}\\ & =\frac{(77+20)-170}{70\times 0.45+1}\\ & =-2.2\text{ mmol/L for each 1L of IVF}\end{array}$$

To reduce her sodium by 5 mmol/L over 12 hours, we would therefore need to give $\frac{5}{2.2}\approx 2.3\text{ L}$ of fluid. To account for ongoing losses via diarrhoea, we might want to give another 1 L resulting in a total of 3.3 L over 12 hours giving an infusion rate of 275 mL/hr

Example

A 45 year old 75kg man with diabetes insipidus presents tot he hospital after 4 days of worsening abdominal pain and fever. He is in early shock with HR 115, BP 85/55, creatinine of 140 and undetectable JVP. His serum sodium on presentation is 165 mmol/L. What is the appropriate IV fluid therapy?

• Bolus of isotonic fluid to restore haemodynamics (e.g. MAP > 60 mmHg, urine output > 30 mL/hr) ⇒ 1 L normal saline every 30 minutes x 4 L
• Appropriate fluid in diabetes insipidus is glucose 5%
• Calculate the expected sodium change per litre of glucose 5%:

$$\frac{(0-0)-165}{75\times 0.5+1}=-4.3\text{ mmol/L for each 1L of IVF}$$

To reduce sodium by 5 mmol/L over 12 hours, we therefore need to give $\frac{5}{4.3}=1.2\text{ L}$ of fluid. To account for ongoing losses due to DI, we might want to give another 2L, for a total of 3.2 L over 12 hours which is approximately 260 mL/hr

• We can therefore give both fluids simultaneously or give the boluses of normal saline first then the glucose 5%

• Alternatively and much easily use the free water calculator on MDCalc making sure to plugin in an appropriate desired sodium target (i.e. 10 mmol/24 hours)
  1. Assess volume status and resuscitate appropriately with normal saline
  2. Establish acute vs chronic to determine rate (acute ≈ 1 mmol/L/hr and chronic < 24 mmol/L over 24 hours); when in doubt assume chronic
  3. Set a safe 24-hour target sodium (i.e. in chronic $\text{Target Na}=\text{Current Na}-10$)
  4. Use this as the sodium desired value in the above calculator in MDCalc
  5. Divide this volume by 24 hours to get the mL/hr of pure free water
  6. Add maintenance and ongoing free water losses
     • Insensible losses (skin + respiratory) is roughly 0.5-1 mL/kg/hr and are electrolyte free so also contribute to hypernatraemia
       • Add 10% per °C above 37°C for fever, and more for tachypnoea, burns, open abdomen or unhumidified ventilation
     • Determine the electrolyte free water clearance by the kidney $\text{EFWC}=V_{\text{urine}}\times (1-\frac{\mathrm{Na}{}_{\text{urine}}^{+}+\mathrm{K}{}_{\text{urine}}^{+}}{\mathrm{Na}{}_{\text{serum}}^{+}}$
       • A positive EFWC is the volume of pure water the kidney is dumping per unit time and should be matched by the IVF replacement
     • Do not add isotonic losses to this (e.g. GI output, drain fluid, blood)
  7. Adjust this for the fluid’s actual free-water fraction: 5% dextrose = 1.0 (rate unchanged), 0.45% saline = 0.5 (double it), 0.18–0.225% saline = ~0.75. This gives the volume of that fluid to run
     • $\text{Free Water Fraction}=1-\frac{{\mathrm{Na}{}^{+}}_{\text{fluid}}+{\mathrm{K}{}^{+}}_{\text{fluid}}}{154}$
     • For example, 2.8L of free water over 24h via 0.45% saline is $2.8/0.5=5.6\text{ L}$ or 230 mL/hr
  8. Cross check with formula above
  9. Recheck sodium at 4-6 hours and titrate

Sources

• Youtube Videos
  • Hypernatremia - YouTube
  • Hypernatremia - Examples - YouTube
• Deranged Physiology: Hypernatraemia