Use this online calculator to compute infusion rates for hypertonic and normal saline solutions, and estimated body water compartment volumes, for mild to severe hyponatremia.
The Hypertonic and Normal Saline Infusion Calculator from GlobalRPH allows users to enter a patient’s weight (in lbs or kg), gender (male, female, elderly male, elderly female), and current sodium level (expressed as meq/L).
The calculator then automatically computes the infusion rate for hypertonic saline (3%), which is “usually reserved for severe hyponatremia (sodium < 115 meq/L),” in order to reach a target level of 120 meq/L or 125 meq/L. The calculator “determines the rate necessary to increase the serum sodium at 0.5 meq/L/hr” until the specified target sodium level is reached.
The calculator also computes the infusion rate for normal saline (0.9%), which is preferred in cases of “mild to moderate hyponatremia (serum sodium: 125-134 meq/L).” Users can set a target sodium level of 135 meq/L or 140 meq/L, and set a maximum rate of increase of 0.5-2.0 meq/L/hour (an important factor to consider because, as noted by the authors of this resource, “Hyponatremia of unknown duration - maximum recommended rate of infusion is 0.5 meq/L/hr. Rapid correction of low sodium can lead to osmotic demyelination of the brain / brainstem. The maximum sodium increase within the first 24 hours should not exceed 10 to 12 meq/liter or <20-25 meq/L over 48 hours. However, some sources state that faster infusion rates may be used in known acute conditions such as hyponatremia that has persisted for less than 24-48 hours.”).
Using this patient data, the calculator also computes estimated body water compartment volumes. It allows users to determine “the approximate compartmental volume increases utilizing various maintenance solutions” based on the type of solution used (normal saline, D5W, 0.45NS, or D5 0.45NS) and the volume of solution used (expressed in ml).
For a 220 lb male with a current sodium level of 108 meq/L, the calculator returns the following values:
Hypertonic Saline (3%) infusion calculator
To reach a target sodium level of 120 meq/L (at a serum sodium rate of increase of 0.5 meq/L/hr), the calculator recommends infusing 3% hypertonic saline at 58.5 ml/hr for 24.0 hours or less. It also reminds users that “sources vary as to the maximum recommended serum sodium increase in a 24hr period - range: 8 to 12 meq/L/24hr.” According to the authors, “this should produce the target level requested.” Users are reminded:
Discontinue hypertonic saline at this time and continue depending on the clinical situation water restriction or normal saline infusion
The maximum sodium increase within the first 24 hours should not exceed 10 to 12 meq/L or <20-25 meq/L over 48 hours
Frequent monitoring of the serum sodium level is necessary during the infusion
Normal Saline (0.9%) infusion calculator
Using the same patient data as above, to reach a target sodium level of 140 meq/L (at a serum sodium rate of increase of 0.5 meq/L/hr), the calculator recommends infusing normal saline (0.9%) at 195 ml/hr for 64 hours or less. It offers the same disclaimers and warnings as above, and advises users to “discontinue normal saline at this time and continue depending on the clinical situation discontinuation of fluids or a maintenance fluid such as D5W, 1/2NS, etc.” It also reminds users that, in severe cases of hyponatremia, “the initial infusion rate may be increased to 390 ml/hr (equivalent to 1 meq/L/hr serum Na+ increase) x 2-3 hours, and then decreased to keep the maximum increase in serum sodium to 0.5 meq/L/hr.”
Estimated Body Water Compartment Volumes
The calculator estimates that total body water (TBW) is 0.6 x body weight (in kg) for non-elderly adult males. For the patient data used above, it estimates TBW at 60 liters, two-thirds of which (40 liters) is intracellular fluid and one-third of which (20 liters) is extracellular fluid. IT informs users that 75% of this extracellular fluid (15 liters) is interstitial fluid, and 25% (5 liters) is intravascular volume (ie, plasma). Users can “determine the approximate compartmental volume increases” by selecting various maintenance solutions and volume.