ELECTROLYTE SOLUTIONS (Continued)

ELECTROLYTE SOLUTIONS (Continued)

Osmolarity Osmotic pressure is an important biologic parameter which involves diffusion of solutes or the transfer of fluids through semi permeable membranes. Per US Pharmacopeia, knowledge of osmolar concentrations of parenteral fluids is important. Labels of pharmacopeia solutions providing intravenous replenishment of fluids, nutrients, electrolytes and osmotic diuretic mannitol are required to state osmolar concentration. Information provides the doctor, if the solution is, hypoosmotic, iso-osmotic or hyperosmotic with regard to biological fluids and membranes

Osmotic pressure is α (total number of particles in solution) Unit of measurement is milliosmoles For non-electrolytes like dextrose, 1 mmol represents 1 mosmol However, with electrolytes, as total number of particles in solution depends on degree of dissociation of a substance. For e.g. Assuming complete dissociation, 1 mmol NaCl represents 2 mosmol (Na + + Cl - ) of total particles 1 mmol of CaCl2 represents 3 mosmol (Ca ++ + 2Cl - ) 1 mmol of sodium citrate (Na 3 C 6 H 5 O 7 ) represents 4 mosmol (3 Na + + C6H5O7 - ) of total particles

The milliosmolar value of the complete solution is equal to the sum of milliosmolar values of individual ions. U.S. Pharmacopeia lists the following formula for calculation of ideal osmolar concentration: Wt. of substance (g/l) mosmol/l = ------------------------------ No. of Species 1000 Mol. Wt (g) E.g. Calculate of ideal osmolarity for 0.9% Sodium Chloride Solution?

Solution Wt. of substance (g/l) mosmol/l = ------------------------------ No. of Species 1000 Mol. Wt (g) 9 (g/l) mosmol/l = ------------------------------ 2 1000 58.5 (g) = 308 mosmol/l

However, because of the bonding forces, n is slightly less than 2 for NaCl at 0.9% Nacl concentration. Hence, the actual osmolarity of solution is 286 mosmol/l Some Pharmaceutical manufacturers label electrolyte solutions with ideal or stoichiometric osmolarities calculated by the equation just provided, whereas others list experimental or actual osmolarities. Pharmacist should appropriate this distiction.

Osmolarity -- Milliosmoles of solute per liter of solution Osmolality Milliosmoles of solute per kilogram of solvent For dilute aqueous solutions both terms are nearly identical For more concentrated solutions Two values are not identical pharmacist should make distinction between Osmolarity and Osmolality

Contribution of components of normal human serum to the Serum Osmotic Pressure Constituent Mean Conc. (meq/l) Osmotic Pressure (mosmol/kg of water) % of total osmotic pressure Sodium 142 139 48.3% Potassium 5 4.9 1.7 Calcium 2.5 1.2 0.4 Magnesium 2 1 0.3 Chloride 102 99.8 34.7 Bicarbonate 27 26.4 9.2 Proteinate 16 1.0 0.3 Phosphate 2 1.1 0.4 Sulfate 1 0.5 0.2 Organic anions 3.5 3.4 1.2 Urea 30mg/100 ml 5.3 1.8 Glucose 70 mg/100 ml 4.1 1.4 Total 287.7 mosmol/kg 99.9%

Normal serum osmolality - ranges from 275-300 mosmol/kg. Equipment used in laboratories to measure osmolality Osmometers. Electrolyte imbalance Shock Trauma Burns Hyperglycemia Abnormal Blood Osmolality Water intoxication Renal failure

Problems A solution has 5% anhydrous dextrose in water for injection. Represent the concentration in mosmol/lit? Solution: Molecular weight of dextrose 180 As it donot dissociate 1 mmol --- 1 mosmol 5% solution contains ---- 50 g / L From the formula discussed above 50,000 / 180 = 278 mosmol/l

A solution contains 10 mg% of Ca +2 ions. How many milliosmoles are present in 1 lit of solution? Solution 1 mmol of Ca++ (40 mg) --- 1mOsmol 10 mg% of Ca++ = 100 mg / Lit 100 / 40-2.5 mosmol/lit

WATER and Electrolyte Balance Clinical Consideration Good Homeostasis -- Maintaining body water and electrolyte balance is component of good health Dietary Input OutPut Kidneys, Lungs, GI tract, Skin Endocrine Process Maintain Optimum Osmolality of Body Fluids

Fluid / Electrolyte therapy is provided to rectify any imbalance of osmolality of body fluids Treatment can be Customized based on patient needs Patient on Diuretic Therapy A K+ supplement along with adequate water intake. Total body water in adults 55-60% of the body weight Increase in body fat --- Lesser proportion of water Infants have 75% of body water For adults - A minimum of 2.5 L of daily water intake (Ingested liquids, food and oxidative metabolism) are required for maintaining Homeostasis.

General terms.. 1500 ml of water / Sq.meter of body surface might be employed for calculating daily requirements of adults. On weight basis. 32 ml/kg Adults and 100-150 ml/kg Infants --- Daily requirements for healthy intake Careful estimation should be made in human subjects with any disorders affecting homeostasis. Also, composition is described in literature with respect to body compartments --- Intracellular (within cells), Intravascular (In blood plasma), Interstitial (Between cells in tissue). Intervascular and Interstitial --- Together called -- Extracellular

Sodium and Chloride have principle affect on Extracellular Fluid Potassium and Phosphate --- on --- Intracellular fluids Cell membranes are freely permeable to water Hence, Osmalility of extracellular fluid (290 mosmol/kg) is approximately equal to intracellular fluid. Therefore, osmolality of the plasma is accurate guide to estimate osmolality of intracellular fluid.

The following formula may be employed to estimate plasma osmolality Plasma Osmolality (mosmol/kg) = 2([Na] + [K]) plasma + BUN/2.8 + Glucose / 18 Where Na and K are in meq/l, BUN and Glucose are in mg/dl Various Clinical calculations Determination of body water requirement, Plasma osmolality estimation, Osmolality and Milliequivalent contents of physiologic electrolyte solutions.

Probelms Based on the discussion above, Calculate the daily water requirement of a healthy human adult with a body surface area of 1.8 m 2? Solution: Water Requirement -- 1500 ml/m 2 Hence 1.8 m 2 contains --- 2700 ml

Estimate the plasma osmolality of the given data: Na 2+ -- 135 meq/l, K+ -- 4.5 meq/l, BUN 14 mg/dl, Glucose 90 mg/dl Solution: Plasma Osmolality (mosmol/kg) = 2[(Na) + (K)] Plasma + BUN/2.8 + Glucose / 18 Where Na and K are in meq/l, BUN and Glucose are in mg/dl = 2(135+4.5) + 14/2.8 + 90/18 = 289

Calculate the meq of Na+, K+ and Cl-, millimoles of dextrose and osmolality of following parentaral fluid? Dextrose anhydrous 50g NaCl 4.5g KCl 1.49g Water for Injection, ad 1000mL

Solution NaCl: 1 EqWt of NaCl - 58.5 g 1 meq of NaCl - 58.5 mg 4500 mg of NaCl 76.9 meq of Na+ and 76.9 meq of Cl- Similarly for KCl we can calculate, 20 meq of K+ and Cl- For dextrose - Mol. Wt 180 1 mmol of anhydrous dextrose 180 mg 50 gms of anhydrous dextrose 50,000 mg 50,000 mg represent -- 277.7 mmol

Hence Osmolarity is calculated as Dextrose anhydrous: 278 mmol * 1 particle/mmol = 278 mosmol NaCl: 77 meq * 2 particles / meq (or mmol) = 154 mosmol KCl : 20 meq * 2 particles / meq (or mmol) = 40 mosmol Total = 278 + 154 + 40 = 472 mosmol

Reference Lecture material taken from the book Pharmaceutical Calculations, 13 th Edition by Howard C. Ansel. Lecture material from Dr. Karla s lecture notes.