Patients with hyponatremia who undergo overly rapid correction of plasma sodium concentration (>8-12 mmol/L per day) are at risk for osmotic demyelination syndrome (ODS). Some patients, for example those with malnutrition and hypokalemia, are at even greater risk for ODS. An important part of the therapy for severe hyponatremia involves the use of DDAVP to limit a rapid water diuresis as well as the use of appropriate IV solutions, both in an effort to prevent overly rapid correction of plasma sodium.
Patients presenting with hyponatremia frequently require volume expansion and are given large volumes of crystalloid solutions as a result. Very often, normal saline is selected but this solution has a significantly higher tonicity (154 mmol/L) than the plasma of patients with hyponatremia. Inadvertent increases in plasma sodium often result in this context, placing patients at high risk for ODS, especially when factoring in the additional impact of a hypotonic diuresis on the correction of plasma sodium.
If a patient indeed requires volume expansion - and many patients with hyponatremia do not - clinicians should select a fluid that is isotonic to the patient. Below, we provide a table that allows one to mix commonly available solutions, with and without potassium additives as required, to achieve a tonicity that will match that of your patient. An example of how to apply these values follows the table. Recall, potassium will be exchanged with intracellular sodium once it moves inside cells and therefore potassium is counted as a potential sodium when performing tonicity calculations.
For Educational Use Only. Please do not apply these calculations without first verifying they are applicable and accurate for the clinical context.
|Proportion of NS / (0.5NS + KCL)||Concentration with 0 K added to 0.5NS (mmol/L)||Concentration with 20 meq added to 0.5NS (mmol/L)||Concentration with 40 meq added to 0.5NS (mmol/L)|
50 year-old woman with severe nausea and vomiting presents to the ER.
Initial blood pressure was 78/50, HR 110, JVP was flat. Her initial investigations are as follows:
In this example, volume expansion is indeed appropriate using a solution that is isotonic to the patient. Suppose one wishes to add KCL to the initial bolus fluid and provide 1L of isontoic fluid. If one wishes to add 40 meq to the first 1L bolus, then we would look to column 4 (Tonicity with 40 meq added to 0.5NS (mmol/L), and select a value that closely matches the patient's sodium of 120 mmol/L. In this case, 121 mmol/L would suffice and according to the table, one would proportion the bolus 10% NS and 90% 1/2NS plus 40 meq/L KCL, running at a total rate that you wish to administer. The order might appear as:
Infuse 100CC of NS and 900CC of 1/2 NS plus 40 meq/L over 1 hour
Note, you can run the two IV solutions simultaneously through 2 IVs or using a y-connector, an IV pump and a single IV site. Assuming the potassium normalizes, sodium remains 120 mmol/L but she requires ongoing IV boluses or maintenance, then one can simply choose a solution that matches the patients tonicity without containing potassium. For example, a 60:40 split of NS:1/2NS would provide a tonicity (123 mmol/L) closely matching that of the patient.
- Volume expansion is only one part of managing patients with hyponatremia and is often not neccessary.
- If one decides to volume expand a patient with severe hyponatremia, consider using a solution that is isotonic to the patient.
- After the initial volume expansion, management will focus on correction of the plasma sodium concentration, rising at an acceptable rate to avoid ODS.
- When managing cases of severe hyponatremia, correction upwards results from the use of hypertonic crystalloids or oral salt AND diuresis of urine that is hypotonic to the patient.
- Using an isotonic IV solution might help slow upward correction of plasma sodium, but in order to prevent a water diuresis, then clinicians should consider the use of DDAVP.
- Because potassium moves intracellularly in exchange for sodium, when calculating the tonicity of IV solutions, one must count potassium as a sodium osmole.