electrolyte solution

Milliequivalents

Millimoles and Micromoles

Osmolarity

Water and Electrolyte Balance

ELECTROLYTE SOLUTIONS

Preface The molecules of chemical compounds in solution:

may remain intact “nonelectrolytes”, E.g. Urea and dextrose in body water.

or may dissociate into particles known as ions, which carry an electric charge“electrolytes”. E.g. sodium chloride in body fluids.

Sodium chloride in solution provides Na+ and Cl- ions, which carry electric charges.

If electrodes carrying a weak current are placed in the solution, the ions move in a direction opposite to the charges. Na+ ions move to the negative electrode (cathode) and are called cations.

Cl- ions move to the positive electrode (anode) and are called anions.

Preface

Electrolyte ions in the blood plasma include:

the cations Na+, K+, Ca++, and Mg++.

& the anions Cl-, HCO3-, HPO4--, SO4--, organic acids-, and

proteins-.

Electrolytes in body fluids play an important role in:

maintaining the acid-base balance in the body.

controlling body water volumes.

regulating body metabolism.

Applicable Dosage Forms

Electrolyte preparations are used in the treatment of

disturbances of the electrolyte and fluid balance in the body.

In clinical practice, they are provided in the form of:

oral solutions and syrups,

dry granules intended to be dissolved in water or juice to make

an oral solution,

oral tablets and capsules,

intravenous infusions.

Milliequivalents A chemical unit, the milliequivalent (mEq), used to express the

concentration of electrolytes in solution.

It is a unit of measurement of the amount of chemical activity of an electrolyte.

Under normal conditions, blood plasma contains 154 mEq of cationsand an equal number of anions.

The total concentration of cations always equals the total concentration of anions.

However, it should be understood that normal laboratory values of electrolytes vary, albeit within a rather narrow range.

Milliequivalents The concentration of electrolytes in intravenous infusion fluids is

most often stated in mEq/L.

In the International System (SI), molar concentrations [as millimoles per liter (mmol/L) and micromoles per liter (µmol/L)] are used to express most clinical laboratory values, including those of electrolytes.

1 mEq is represented by 1 mg of hydrogen, 23 mg of sodium, 35.5 mg of chlorine, 39 mg of potassium, 20 mg of calcium, and so forth.

How?

Example Calculations of

Milliequivalents

1-What is the concentration, in milligrams per milliliter, of a

solution containing 2 mEq of potassium chloride (KCl) per

milliliter?

2-What is the concentration, in grams per milliliter, of a solution

containing 4 mEq of calcium chloride (CaCl2⋅2H2O) per milliliter?

3-What is the percent (w/v) concentration of a solution containing

100 mEq of ammonium chloride per liter?


Milliequivalents

4- A solution contains 10 mg/100 mL of K+ ions. Express this

concentration in terms of milliequivalents per liter.

5- A solution contains 10 mg/100 mL of Ca++ ions. Express this

concentration in terms of milliequivalents per liter.

6- A magnesium (Mg++) level in blood plasma is determined to be

2.5 mEq/L. Express this concentration in terms of milligrams.


Milliequivalents

7- How many milliequivalents of potassium chloride are represented in a 15-mL dose of a 10% (w/v) potassium chloride elixir?

8- How many milliequivalents of magnesium sulfate are represented in 1 g of anhydrous magnesium sulfate (MgSO4)?

9- How many milliequivalents of Na+ would be contained in a 30-mL dose of the following solution?

Disodium hydrogen phosphate “Na2HPO4.7H2O” 18 g

Sodium biphosphate “NaH2PO4.H2O” 48 g

Purified water ad 100 mL


Milliequivalents

10- A person is to receive 2 mEq of sodium chloride per kilogram of

body weight. If the person weighs 132 lb., how many milliliters of a

0.9% sterile solution of sodium chloride should be administered?


Milliequivalents/Parenteral Nutrition



The formula for a TPN solution calls for the addition of 2.7 mEq of

Ca++ and 20 mEq of K+ per liter. How many milliliters of an

injection containing 20 mg of calcium chloride (CaCl2.2H2O) per

milliliter and how many milliliters of a 15% (w/v) potassium

chloride injection should be used to provide the desired additives?



A potassium phosphate injection contains a mixture of 224 mg of

monobasic potassium phosphate (KH2PO4) and 236 mg of dibasic

potassium phosphate (K2HPO4) per milliliter. If 10 mL of the

injection are added to 500 mL of D5W (5% dextrose in water for

injection):

(a) how many milliequivalents of K+, and

(b) how many millimoles of total phosphate are represented in the

prepared solution?

Millimoles and Micromoles The SI expresses electrolyte concentrations in millimoles per

liter (mmol/L) in representing the combining power of a chemical species.

For monovalent species, the numeric values of the milliequivalent and millimole are identical.

A mole is the molecular weight of a substance in grams.

A millimole is one thousandth of a mole, and a micromole is one millionth of a mole.

Example Calculations of Millimoles and

Micromoles

1- How many millimoles of monobasic sodium phosphate

NaH2PO4.H2O (m.w. 138) are present in 100 g of the substance?

2- How many milligrams would 1 mmol of monobasic sodium

phosphate weigh?

3-What is the weight, in milligrams, of 1 mmol of HPO4--?

4- Convert blood plasma levels of 0.5 µg/mL and 2 µg/mL of

Tobramycin (m.w. 467.52) to µmol/L.

Osmolarity

Osmotic pressure is important to biologic processes that

involve the diffusion of solutes or the transfer of fluids

through semipermeable membranes.

The United States Pharmacopeia states that knowledge of the

osmolar concentrations of parenteral fluids is important.

This information indicates to the practitioner whether the

solution is hypoosmotic, iso-osmotic, or hyperosmotic with

regard to biologic fluids and membranes.

Osmolarity Osmotic pressure is proportional to the total number of particles in solution.

The unit used to measure osmotic concentration is the milliosmole (mOsmol).

For dextrose, a nonelectrolyte, 1 mmol (1 formula weight in milligrams) represents 1 mOsmol.

This relationship is not the same with electrolytes, however, because the total number of particles in solution depends on the degree of dissociation of the substance in question.

Assuming complete dissociation, 1 mmol of NaCl represents 2 mOsmol (Na+ + Cl-) of total particles, 1 mmol of CaCl2 represents 3 mOsmol (Ca++ + 2Cl-) of total particles, and 1 mmol of sodium citrate (Na3C6H5O7) represents 4 mOsmol(3Na+ + C6H5O7

–3) of total particles.

Osmolarity According to the United States Pharmacopeia, the ideal osmolar

concentration may be calculated according to the equation:

In practice, as the concentration of the solute increases, physicochemical interaction among solute particles increases, and actual osmolar values decrease when compared to ideal values.

Deviation from ideal conditions is usually slight in solution within the physiologic range and for more dilute solutions, but for highly concentrated solutions, the actual osmolarities may be appreciably lower than ideal values.

Osmolarity For example, the ideal osmolarity of 0.9% sodium chloride

injection is:

Because of bonding forces, however, n is slightly less than 2 for solutions of sodium chloride at this concentration, and the actual measured osmolarity of the solution is about 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.

Osmolarity/Osmolality

A distinction should be made between the terms osmolarity

and osmolality.

osmolarity is the milliosmoles of solute per liter of solution,

osmolality is the milliosmoles of solute per kilogram of solvent.

For dilute aqueous solutions, osmolarity and osmolality are

nearly identical.

For more concentrated solutions, however, the two values

may be quite dissimilar.

Osmolarity/Osmolality

Normal serum osmolality is considered to be within the

range of 275 to 300 mOsmol/kg.

Osmometers are commercially available for use in the

laboratory to measure osmolality.

Abnormal blood osmolality that deviates from the normal

range can occur in association with:

shock, trauma, burns, water intoxication (overload), electrolyte

imbalance, hyperglycemia, or renal failure.

Example Calculations of Milliosmoles 1- A solution contains 5% of anhydrous dextrose C6H12O6 in water for

injection. How many milliosmoles per liter are represented by this concentration?

2- A solution contains 156 mg of K+ ions per 100 mL. How many milliosmoles are represented in a liter of the solution?

3- A solution contains 10 mg% of Ca++ ions. How many milliosmoles are represented in 1 liter of the solution?

4- How many milliosmoles are represented in a liter of a 0.9% sodium chloride solution?

Clinical Considerations of Water and

Electrolyte Balance Maintaining body water and electrolyte balance is an essential

component of good health.

Water provides the environment in which cells live and is the primary medium for the ingestion of nutrients and the excretion of metabolic waste products.

Normally, the osmolality of body fluid is maintained within narrow limits through: dietary input, regulatory endocrine processes, balanced output via the kidneys, lungs, skin, and the gastrointestinal

system.


Electrolyte Balance In clinical practice, fluid and electrolyte therapy is undertaken

either to: provide maintenance requirements, replace serious losses or deficits.

Body losses of water and/or electrolytes can result from a number of causes, including: vomiting, diarrhea, profuse sweating, fever, chronic renal failure,

diuretic therapy, surgery, and others.

The type of therapy undertaken (i.e., oral or parenteral) and the content of the fluid administered depend on a patient’s specific requirements.


Electrolyte Balance

Examples:

A patient taking potassium wasting diuretics may simply require a

daily oral potassium supplement along with adequate intake of water.

An athlete may require rehydration with or without added

electrolytes.

Hospitalized patients commonly receive parenteral maintenance

therapy of fluids and electrolytes to support ordinary metabolic

function.

In severe cases of deficit, a patient may require the prompt and

substantial intravenous replacement of fluids and electrolytes to

restore acute volume losses resulting from surgery, trauma, burns, or

shock.


Electrolyte Balance

Total body water in adult males normally ranges between 55% and 65% of body weight depending on the proportion of body fat.

The greater the proportion of fat, the lesser the proportion of water.

Values for adult women are about 10% less than those for men.

Newborn infants have approximately 75% body water, which decreases with growth and increases in body fat.


Electrolyte Balance

Of the adult body’s water content, up to two thirds is intracellular and one third is extracellular.

The proportion of extracellular body water, as a fraction of total body weight, decreases in infants in the first year from approximately 45% to 30% while the intracellular portion increases.

For an adult, approximately 2500 mL of daily water intake (from ingested liquids and foods and from oxidative metabolism) are needed to balance the daily water output.


Electrolyte Balance

In general terms, 1500 mL of water per square meter of body surface area may be used to calculate the daily requirement for adults.

On a weight basis, estimates of 32 mL/kg for adults and 100 to 150 mL/kg for infants have been cited as average daily requirements of water intake for healthy individuals.

These estimated requirements differ greatly in persons with clinical disorders affecting water and electrolyte homeostasis and in conditions of acute deficit.


Electrolyte Balance The composition of body fluids generally is described with regard

to body compartments: intracellular (within cells), intravascular (blood plasma), interstitial (between cells in the tissue).

Intravascular and interstitial fluids commonly are grouped together and termed extracellular fluid.

Although all electrolytes and nonelectrolytes in body fluids contribute to osmotic activity: sodium and chloride exert the principal effect in extracellular fluid, potassium and phosphate predominate in intracellular fluid.


Electrolyte Balance

Since cell membranes generally are freely permeable to water, the osmolality of the extracellular fluid (about 290 mOsm/kg water) is about equal to that of the intracellular fluid.

Therefore, the plasma osmolality is a convenient and accurate guide to intracellular osmolality, and may be approximated by the formula:

where: sodium (Na) and potassium (K) are in mEq/L, and blood urea nitrogen (BUN) and glucose concentrations are in mg/100 mL (mg/dL).

Example Calculations of Water

Requirements and Electrolytes in Parenteral

Fluids 1- Calculate the estimated daily water requirement for a healthy

adult with a body surface area of 1.8 m2.

2- Estimate the plasma osmolality from the following data: sodium,

135 mEq/L; potassium, 4.5 mEq/L; blood urea nitrogen, 14 mg/dL;

and glucose, 90 mg/dL.

3-

Example Calculations of Water

Requirements and Electrolytes in

Parenteral Fluids

electrolyte solution

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