HYPONATREMIA: Na < 130mEq

Etiology:

  1. The easiest way to determine the cause is first to calculate the Osmolality.

S & S :

  1. Depends on the rate of onset, on the degree and on  the cause. If onset is fast pt will experience muscle weakness, cramps, AMS, seizures.

DX :

  1. The formula is  2 (Na + K ) + glucose / 18  + BUN / 2.8.  Normal = 280 - 285 mOsm.

If  Osmolality is EQUAL , HIGHER  or LOWER  we deal respectively  with :

                Isotonic Hyponatremia               Hypertonic Hyponatremia
                         (=280-285 mOsm)                           (>285 mOsm)                                  
                                    ¯                                               ¯  
causes:                hyperlipidemia                          hyperglycemia                                      
                          hyperproteinemia                  ( each 100ml/dL increase in
                                                                         glucose above 150 Na
                                                                      decreases by 1.5mEq/L )


                                        Hypotonic Hyponatremia
                   ____________________(<280 mOsm )___________________

                             ¯                                         ¯                                               ¯
                 Hypovolemic                  Hypervolemic                   Normovolemic
                 hypotonic                       hypotonic                          hypotonic
                  hypoNa                          hypoNa                             hypoNa
                            ¯                                         ¯                                               ¯
symptoms:  tachycardic, BP        fluid overload, edema            normotensive, normal pulse 
                            ¯                                         ¯                                               ¯
causes:          Renal loss                 Renal Failure                  Water Intoxication ( urine Na )
                       GI loss                 Heart Failure (CHF)             SIADH (  urine Na & Osm)
                  Third Spacing          Liver Failure (Cirrhosis)                 Hypothyroidism
                  Steroid Deficit                                                                NSAIDS
                     Diuretics                                                               Steroid Deficiency
                      Sweating                                                                Carbamazepine
                                                                                                Thiazide diuretics
                                                                                                Pain, Stress, Nausea
                         ¯                                          ¯                                                      ¯
treatment: - 0.9% NS                    water restrict                     SIADH - water  restrict
                                                         diuretic                       H2O intox - if symptomatic
                                                                                           give 3% saline 25 - 40ml / hr
                                                                                           Treat the underlying cause

General Treatment:

    1. Hyponatremia, if corrected  too fast, the high plasma osmolality will cause the brain cells to dehydrate and produce Central Pontine Myelinosis. To avoid this complication serum Na should not be corrected faster then 1mEq/L/hr.
       
    2. If you are dealing with Na and ECF the use of diuretic is indicated  and the end-goal is serum Na of 125-130mEq/L. Also fluid restriction is indicated to <500 ml/d.
       
    3. To calculate properly the Na deficit,  use the following formula:
       

(BW x TBW) x (Desired -Calculated Na)
         BW (Body Weight) = 80kg
TBW (Total Body Water)  = 60% BW.
Desired Na - Calculated Na = e.g. (130 - 120)
( 70x0.6) x (130-120) = 42x10 = 420mEq Na deficit.

This can be corrected with 0.9%NS (each contains 154mEq/L of Na) or with 3% saline (each contains  513mEq/L of Na and is given 25-40 ml/hr). Half of calculated amount is given in first 8-10 hrs. and the rest over next 14-16 hrs and serum Na must be checked q 2 hrs.

    1. Pt if symptomatic ( seizure or  MS ) is given 3% saline (contains 513mEq Na/L) @ 25-40 ml/hr  until symptoms abate  or serum Na is >125-130mEq/L.
    2. If hyponatremia is severe (<120mEq/L) yet pt is asymptomatic, correction of serum sodium should not exceed the rate of  1mEq/hr. If hyponatremia developed slowly, correction of serum Na should not exceed  0.5mEq/hr.

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