Hyponatremia Correction Infusate Rate Calculator

Variables considered

In order to determine the infusate rate and the serum Na change per liter, the calculator uses the following variables:

■ Patient weight: to be input in either kg or lbs;

■ Serum Na change per hour: measured in mEq/L;

■ Serum Na: measured in mEq/L and based on laboratory results;

■ Water fraction: dependent on the type of patient for which the correction is calculated. The choice is between child, adult male, female or elderly male or female;

■ IV Na: the concentration of the intravenous Na solution. The choice is between : 5% NaCl, 3% NaCl, 0.9% NaCl or Ringer Lactate solution;

■ IV K: dosed in mEq/L in order to avoid hypokalemia that could exacerbate the Na deficit effect.

■ The measurement unit for the infusate rate: in either mL/hr, L/min, L/sec, mL/min or mL/sec.

Hyponatremia correction formula

The infusate rate formula is the following:

Infusate Rate = (1000 x Serum Na Change Per Hr x ((Water Fract x Weight) + 1)) / (IV Na + IVK – Serum Na)

The change in serum sodium is calculated via:

Serum Na Change Per Liter = (IV Na + IVK – Serum Na) / ((Water Fract x Weight) + 1)

Hyponatremia explained

Decreases in the concentration of serum Na below 136 mEq/L are considered as hyponatremia. Decreases below 125 mEq/L are indicative of severe hyponatremia.

Some of the most common causes of hyponatremia include: vomiting, diarrhea, diuretic use, heart failure, burns, pancreatitis and renal disease.

Congestive heart failure, liver or kidney failure are amongst the conditions associated with low serum Na.

There are three types of hyponatremia:

■ Hypovolemic (decrease in both body water and sodium);

■ Euvolemic (normal body sodium, high body water levels);

■ Hypervolemic (increase in both body water and sodium).

The classification by osmolality is between hypotonic, isotonic, hypertonic.

Symptom severity varies from nausea and general malaise to lethargy and decreased consciousness.

Diagnosis is done by measuring serum Na. in order to near down the cause, analysis of serum and urine electrolytes and osmolality is necessary.

Hyponatremia treatment starts with water intake restriction, the replacement of the Na deficit and initiation of treatment addressing the underlying cause. The rhythm of correction is subject to risk of cerebral edema and brain damage and therefore needs to be closely monitored.

In simple cases, water restriction is enough to balance the serum sodium deficit. Where IV correction is required, the sodium rate should not exceed 8 mEq/L although the initial correction rate can start at 1-2 mEq/L/hr.

References

1. Adrogue, HJ, Madias, NE. Primary Care: Hyponatremia. New England Journal of Medicine 2000; 342(20):1493-1499.

2. M Biswa M, Davies JS. Hyponatraemia in clinical practice. Postgrad Med J. 2007; 83(980): 373–378.