Osmolality/Osmolarity ===================== þ Osmolarity: - defined as solute per unit of _solvent_ - Effective Osmolality is decreased to about 93% of calculated in blood due to intermolecular attraction; normal is calculated at about 304 for plasma but corrects to 283 with this fudge factor. - standard formula for estimating osmolarity: BUN (mg/dL) Glu (mg/dL) Ethanol 2[Na+] + ----------- + ----------- + ------- = about 275 to 295 3 or 2.8 20 or 18 5 or 4.6 Glucose, BUN, and ethanol are in mg/dL. Ref: Tintinalli, 3rd. ed. p572. þ Correcting for effects of severe hyperglycemia - standard formulas for quick evaluation to see if hyponatremia is maybe just pseudohyponatremia due to hyperglycemia: Na+ decreases by 1 for each increase of 60 in the glucose, or Na+ decreases by 1.6 for each increase of 100 in the glucose. See Also þ Osmolality: - defined as solute/unit of _solution_ þ Tonicity: - "effective osmotic pressure" (Dorland's) - defined by cell shrinking or swelling (Guyton) - concentration of _cell impermeant_ solutes þ Oncotic Pressure: - osmotic pressure due to colloids. (Dorland's) - Capillary Oncotic Pressure = oncotic pressure due to high Na+ permeability (unlike cell membrane) (Guyton) - Donnan Effect increases oncotic pressure about 50%, normal capillary oncotic pressure is 19 mm Hg based on colloids, but Donnan effect brings this up to about 28 mm Hg þ Body Water - Total Body Water (TBW) is about 60% of total body weight (~40kg) - Intracellular is about 2/3 of TBW - Interstitial Fluid is about 1/3 of TBW þ Plasma - everthing in blood except the cells - 93% water - 7% solids - to correct for hypertriglyceridemia, multiply by 0.002, and to correct for protein over 8 g/dL, multiply by 0.25; this is due to increased viscosity resulting in less plasma delivered to the test chamber by the pump. þ Osmoregulation and ADH þ SIADH (Syndrome of Inappropriate ADH secretion) þ Diabetes Insipidus (DI) þ Osmolal Gap: - Per Fleischer and Ludwig, 2E, p 576: Each 100 mg/dl of EtOH raises the osmolal gap by 22-25 mOsm/kg. "I wanted to draw the group's attention to an excellent article from Denver which has led me to put much less credence in use of the osmolal gap for diagnosis of exotic alcohol ingestions. I've been meaning to get this out for some time. Here is the reference": Glaser DS. Utility of the serum osmol gap in the diagnosis of methanol or ethylene glycol ingestion. Ann Emerg Med 1996;27:343-346. ABSTRACT: Ingestion of methanol or ethylene glycol is a toxicologic emergency. The osmolal gap has been widely advocated as a screen for serum methanol or ethylene glycol. Unfortunately, for several reasons the osmolal gap fails in this capacity. First, an accurate serum osmolality can often not be obtained. Second, the calculated serum osmolarity will vary greatly, depending on the formula used to estimate it. Third, ethylene glycol has such a large molecular weight that even toxic amounts may contribute minimally to a patient's overall osmolality. Finally, because of metabolism, little ethylene glycol or methanol may be present when a patient presents with toxicity. These limitations invalidate the osmolal gap as a screen for ethylene glycol or methanol ingestion. --James Li, MD