Sodium Disorder Unit 2

Normal OSMOLALITY SERUM RANGE

275 - 290 mOsm/kg

Estimating Plasma Omsolality: 

Osm = (2xNa) + (BUN/2.8) + (Glucose/18)

Osmolality GAP

(OG): if difference between calculated Osm and measured is > 15

OG calculation example: 

Na - 145 

BUN - 10 

BG - 90 

Osmserum - 322 (measured)

Osmolality = (2 x 145) + (10/2.8) + (90/18) = 299 calculated Osm

IS THERE AN OG? Measured - calculated 

322- 299 = 23 > 15 there is an osmolality gap 

Normal Sodium Serum Range:

135-145 mEq/L

Hypertonic Hyponatremia 

Most frequently seen with elevated BG

• Serum Na falls by 1.6 mEq/L for each 100 mg/dL incremental increase in BG above 100 mg/dL

Will need to correct Na = Na serum + 1.6 [(BG-100)/100]

Hypertonic Hyponatremia corrected Na example

1. Calculate Osm = (2xNa) + (BUN/2.8) + (Glucose/18)

   1. (2x128) + (50/2.8) + (600/18) = 307 mOsm/kg 

   2. Since 307 mOsm > 290 mOsm = HYPERTONIC hyponatremia 

      1. Normal range of Osm 275 - 290

2. Corrected Na = Na serum + 1.6[(BG-100)/100]

   1. 128 + 1.6[(600-100)/100) = 136 mEq/L → is within normal Na limits therefore treat underlying elevated BG

Acute Symptomatic Hypovolemic Hyponatremia Management 

Severe symptoms: altered mental status, seizures, metabolic encephalopathy can develop

Treatment: 3% NaCl at rate of 1 - 2 mEq/L until symptoms resolve  

• Short term goal 120 mEq/L

• Calculate sodium replacement → give 1/2 the deficiency over first 8 hours, then next 1/2 over next 16 hours 

CALCULATING SODIUM REPLACEMENT: 

Calculate Na deficit = TBW x (Na goal - current Na serum)

• Typical goal is 120 mEq

55 kg female patient…

1. TBW = (55 kg)(0.5) = 27.5 L total body water

2. Na deficit?

   1. = TBW x (Na goal - current Na serum)

   2. = 27.5 L x (120 - 100) = 550 mEq Na is the deficit 

3. 1/2 of 550 mEq over the first 8 hours 

   1. 1/2 x 550 = 275 mEq / 8 hours = 34 mEq/hour rate for first 8 hours 

   2. If using 3%...

      1. (34 mEq/hour) x (1000mL/513 mEq) = 66 mL/hour first 8 hours

4. 1/2 over 16 hours 

   1. 1/2 x 550 = 275 mEq/16 hours = 17 mEq/hour for next 16 hours 

   2. If using 3% NS

      1. (17 mEq/hr) x (1000mL/513 mEq) = 33 mL/hour for next 16 hours

Classification of Hyponatremia 

1. Hyponatremia < 135 mEq/L

   1. What is the Osm?

2. Low Osm < 275 → hypotonic hyponatremia 

   1. Volume status

      1. Decreased TBW = hypovolemic hypotonic hyponatremia 

      2. Normal TBW = isovolemic 

      3. Increased TBW (edema) = hypervolemic 

3. Normal Osmo 275 - 290 mEq/L

   1. Pseudo hyponatremia 

4. High Osm > 290

   1. Hypertonic hyponatremia

hyponatremia summary

Free water DEFICIT

 = TBW x [(Na serum/140) - 1]

Provide free water

• Use hypotonic solutions: D5W or 0.45% NaCl

• Give 1/2 total deficit over 24 hours 

• Give remaining 1/2 over next 24- 48 hours as needed

Goal: 0.5 mEq/L/hr decrease in Na serum with max of 12 mEq/L/day

Free Water Example

THIS IS FOR HYPERNATREMIA 

70 kg male patient 

Serum Na = 153 mEq/L 

TBW = (0.6 x 70 kg) = 42 kg

FWD = 42 L x ([153/140]-1) = 3.9 L

• Give 1/2 over 24 hours = 3.9 L/2 = 1.95 L (1950 mL)

• Typically USE D5W for free water deficit replacement 

RATE: 1950 mL/24 hours = D5W at 80 mL/hour for 24 hours CHECK Na EVERY 8 HOURS

Hypernatremia Summary