Hyponatremia in a patient presenting with somnolence
Dr. Hilda Fernandez | Morning Report | 06/02/2016
Thiazides are more likely to be the cause of severe hyponatremia than loop diuretics. Possible mechanism is that after thiazide use, patients to have increased thirst mechanism - thus, drink more free water. Thiazides also impair diluting ability by inhibiting electrolyte transport at the site of the distal convoluted tubule. In contrast, loop diuretics rarely cause severe hyponatremia because they reduce osmolality in the renal medulla thereby limiting the kidney's ability to concentrate urine.
Posted 06/02/16 09:42:25 AM by Adam Faye
Caution with overdiuresis - besides impairing concentrating ability in the kidney, it can lower blood pressure (if large diuresis) which will trigger baroreceptors and stimulate ADH release. This can lead to drastic lowering of serum Na levels
Posted 06/02/16 09:45:36 AM by Adam Faye
Tolvaptan- Competitive vasopressin 2 receptor anatagonist. Half-life is ~12 hours, typically see peak diuresis at 4hrs, and is NOT excreted via the kidney (mostly excreted in the stool). Dosing is based upon Urine Osm. If Uosm <300 usually dose 7.5mg, If Uosm >300 would give 15mg dose.
Posted 06/02/16 09:48:25 AM by Adam Faye
If hyponatremia has occurred over a period of >24hrs. or if the duration is unknown assume the hyponatremia is chronic and adjusting the Na levels too quickly can cause osmotic demyelination. The reason for this is that within the first 24hrs. Na+/K+ shift from ECF <-> Cells to keep tonicity the same. It should be noted that if a patient is obtunded or actively seizing there should be rapid correction of Na.
Posted 06/02/16 11:31:19 AM by Adam Faye
One method to assess response to free-water restriction in hyponatremia is to use the equation (Urine Na + Urine K)/(Serum Na) - assuming no diuresis recently that would obscure urine electrolyte values. If this ratio is <.5, a 1L free H20 restriction should help improve hyponatremia. If the ratio is between .5-1 the suggested free water restriction is 500cc, and if the ratio is >1 free water restriction will be unlikely to help improve Na levels.
To conceptualize it, if the U/P ratio is <.5 then there are fewer electrolytes being excreted in the urine than there are in the plasma-this indicates that there is a significant amount of electrolyte free water being excreted in the urine, and thus Na levels should start to rise with a fluid restriction.
Conversly, if there are more electrolytes being excreted in the urine than in the serum, Na levels will continue to decline and fluid-restriction itself may have little utility in improving Na values.
See The Urine/Plasma Electrolyte Ratio: A Predictive Guide to Water Restriction below for further details.
Posted 06/02/16 01:47:45 PM by Adam Faye
Remember when correcting hyponatremia it is less about the hourly correction goal and more about not over correcting within a 24hr. time period.