Hypernatraemia

Hypernatraemia is a not a common presentation in ED, as intense thirst often prevents significant hypernatraemia in neurologically intact individuals. So… Mortality rates are high (20-70%) and the severity of hypernatraemia has been shown be an independent predictor of mortality.

However, there is little good data on hypernatremia to base guidance on, and definitions vary within the literature

Severity:

  • Mild: [Na] 145-149mmol/l
  • Moderate: [Na] 150-159mmol/l
  • Severe: [Na] 160-189mmol/l
  • Extreme: [Na] ≥190mmol/l

Presentation:

  • Early Non-Specific: anorexia, nausea, vomiting, muscle weakness, lethargy
  • Neurological: Altered level of consciousness ranging from irritable to coma, due to brain shrinkage (which may precipitate intracranial bleeding), demyelination is rare.
  • Hypovolaemia: dry mucosa to to mottled skin and shock

Causes:

  • Water Losses:
    • Diabetes Insipidus: include the following
      • Drugs: lithium, penicillins, aminoglicosides, phenytoin, ethanol, etc.
      • Renal: sickle cell, amyloidosis, obstructive uropathy
      • Brain injury: tumour, vascular, infection, trauma, surgery
      • Congenital: majority X-linked
    • Renal: renal failure, osmotic diuresis, (DM, Mannatol), Loop diuretics
    • GI losses: severe prolonged D+V, fistulae
    • Sweating
  • Water Intake Deficit:
    • Impaired thirst: brain disease
    • Restricted intake/access: often dementia patients
  • Sodium Overload:
    • Ingestion: excess salt, household bleach, sea water
    • Iatrogenic fluid: Hypertonic Saline, Sodium Bicarbonate
    • Mineralocorticoid excess: Cushings, primary aldosteronism

Investigation:

  • Blood: U&E, Glucose, Osmolality
  • Urine: Electrolytes, Osmolality
  • Fluid: Input-Output

Management:

  • Mild: [Na] 145-149mmol/l – likely suitable for admission to ward
  • Moderate: [Na] 150-159mmol/l – Involve Paed/Medical Senior
  • Severe/Extreme: [Na] ≥160mmol/l – Involve Paed/Medical Senior + Involve ICU if appropriate

1.0 Treat the Underlying Cause

Try to identify the precipitant, which is often in the history and treat if possible

2.0 Treat the Hypernatraemia

2.1 Hypovolaemia.(patient displaying signs of shock)

  • Use 0.9% NaCl as initial resuscitation fluid
    • in this cohort 0.9% NaCl is often hypotonic incomarison to the patient
  • However, if the patient is not displaying signs of shock proceed to fluid replacement.

2.2 Fluid replacement (involve Paeds/Medical/ICU teams at an early stage)

  1. Calculate: Total Body water (TBW)
    • TBW = Patients body weigh (kg) x Y
      • Y=0.6 (Young Male/Child)
      • Y=0.5 (Female/Older Male)
      • Y=0.4 (Dehydrated)
    • E.G 80kg dehydrated patient male
      • TBW = 80kg x 0.4 = 32 litres
  2. Calculate: Free Water Deficit (FWD)
      • FWD = TBW X ((Lab [Na]- 140)/140)
      • E.G. Above patient with lab [Na] of 174mmol/l
        • FWD = 32 x ((174-140)/140)
          • FWD = 32 x (34/140) = 7.8 litres deficit
  3. Determine a suitable sodium correction rate.
    • Neurological symptoms: require more rapid treatment correction up to 2mmol/l/hr
    • Others: common practice is to aim for 0.5mmol/l/hr(12mmol/l/day) reduction OR correction over 48/hrs
        • However data suggests faster corrections are safe, and mortality may be higher if we correct at a slower rate
    • Reason why correction is relatively slow – is potential for cerebral oedema

2.21 Replacement Fluid

  • Clinical Hypovolaemia – replace with 0.9% NaCl IV First
    • Then recheck [Na] and recalculate FWD
  • Correction of Sodium
    • Ideally Oral/NG
    • If IV required – 5% Dextrose

2.22 Targeting a specific reduction

  • 48 hr correction
    • Calculated FWD, replace 1/2 in 1st 24hr and 1/2 in 2nd 24hr
  • Specific Reduction – i.e how much will 1 litre of IV fluid reduce serum[Na]
    • Expected Change in [Na] per litre of fluid = (infused [Na] – serum [Na])/(TBW+1)
      • infusion [Na]:0.9% NaCl = 154mmol/l, 5% Dextrose = 0mmol/l, Hartmanns = 131mmol/l
      • E.G. 5% Dextrose(0mmol/l), serum [Na] 164mmol/l, TBW 32l
        • Expected Change in [Na] per litre of fluid = (0-164)/(32+1)
        • Expected Change in [Na] per litre of fluid = -164/33
        • Expected Change in [Na] per litre of fluid = -4.97mmol/l
    • Rate of fluid (1000ml bag) = (-)Expected Change in [Na]/Target reduction rate
      • E.G. 5% Dextrose, Expected Change -4.97mmol/l, Target reduction 0.5mmol/l/hr
        • Rate of fluid (1000ml bag) = 4.97/0.5
        • Rate of fluid (1000ml bag) = 9.94hr (10hrs)

2.23 Na re-testing

  • Hypovolaemia correction required – Retest after correction as a minimum
  • Mild: [Na] 145-149mmol/l – 8hrly
  • Moderate: [Na] 150-159mmol/l – 4-6hr
  • Severe/Extreme: [Na] ≥160mmol/l – Close monitoring of fluid balance an [Na]

2.24 Calculating Ongoing fluid loss

Ideally this is not something for ED as the patient will be on the ward (but who knows!!), If they remain in ED for a prolonged period it is worth calculating their further water losses, but this will require measuring urine concentration of sodium.

  • Electrolyte Free Water Excretion per Hour = Urine flow rate x (1 – ((Urine[Na] + Urine[K])/Serum[Na]))
  • This would need adding into replacement regime

 

References

 

 

 

 

 

 

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