Diagnostic Chemistry Lecture 3 Major Electrolytes

Define Potentiometry:

Measurement of an electrical potential difference between two electrodes in an electrochemical cell when the cell current is zero

-We use Ion-selective electrodes

A potentiometry system contains which two types of electrodes (Half-cells)?

1. Reference electrode

-Provides stable, known electrical potential

2. Indicator electrode

-Its electrical potential changes in response to measured ion (Ion-selective electrode)

Each electrode (Half cell) contains what?

An internal Ag/Agcl electrode which is in contact with an internal electrolyte

Cell potential is measured with?

A voltmeter

Potentiometric ISE directly measure?

Voltage difference between ISE and reference electrode

Ecell = E indicator - E reference

What is the Nernst equation?

Describes how an electrode's electrical potential changes with the activity of an ion

What is the simple Nernst Equation?

C sample = C calibraor x 10 ^ DeltaE/S

Describe what Ion-selective membrane electrodes are:

1. Respond preferentially to selected ions

2. Membrane type determines which ion its selective to

3. Interfering ions is described by selectivity coefficient

4. The measured potential is proportional to the logarithm of the ions activity

What are glass membrane electrodes used to measure?

pH and Na+

-Also as an internal H+ selective transducer for pCO2 sensors in blood gases

-Made from melts of silicon and/or aluminum oxide mixed with oxidies of alkali metal

What are Polymer membrane electrodes used to measure?

pH and all electrolytes

*** Predominant class of ISE in clinical lab use

-Response mechanisms of ISE type categorized in 3 ways

= Charged, dissociated ion exchanger

= Charged associated carrier

=Neutral ion carrier

Describe Charged, dissociated ion exchanger:

-Routinely used for Cl- measurement

-Uses a charged, lipid soluble membrane component to extract oppositely charged ions from the sample

-Selectivity depends mainly on how readily each ion enters the membrane

Describe charged, associated carriers:

Interact specifically with the target ion through ion exchange or complex formation

-PVC membranes are more durable and easier to use than original liquid-membrane

What is the most common type of polymer membrane electrode?

Neutral ion carriers

Describe Neutral ion carriers (ionophores) PME:

-Ionophore reversibly binds or complexes with the ion of interest in the sample

-Selectivity is based o how well the ions size/shape fits into a cavity within ionophpre

-Ionophore carries selected ion into the membrane - a membrane potential related to ions activity

Which electrode is used for routine K+ analysis, High potassium selectivity?

Valinomycin Potassium electrode

Which electrode is selective for carbonate?

Carbonate ionophore electrode

Define Electrolytes:

Ionic elements that carry a (+) or (-) charge when dissolved in water

What are cathodes and anodes?

Cathodes = Negative electrode

-Attracts Cations (+)

Anodes = Positive electrode

-Attracts anions (-)

What are the major functions of of electrolytes?

1. Water homeostasis

2. Acid-Base balance

3. Heart and muscle function

4. Enzyme cofactors

What are the 4 major electrolytes?

1. Sodium

2. Potassium

3. Chloride

4. Bicarbonate (HCO3)

What is the major extracellular fluid cation?

Sodium (Na+)

Sodium makes up what percent of inorganic cations per liter of plasma?

90%

What is the central role of sodium as an electrolyte?

Central role in ECFs water distribution and osmotic pressure

-Where sodium goes water follows

-Na+ homeostasis disorders occur due to excessive loss or gain

What is the reference range for sodium?

135-145 mmol/L

What is the main specimen type for sodium?

Serum or plasma

What are the common interferences for sodium?

*NOT affected by hemolysis

-ECF

* Lipemia

What is the most common method testing sodium?

Ion-selective electrode (indirect ISE)

Define Hyponatremia:

Low sodium (

What are the three types of Hyponatremia?

1. Hypo-osmotic (Most common)

-Decrease solute and Na+ in plasma

2. Hyperosmotic

-Increase solute in plasma; Na+ shifts into cells

3. Iso-osmotic

-Decrease Na+ in plasma but normal amount of solute

*To distinguish the 3 you need plasma osmolality

What is the electrolyte exclusion effect?

Happens with indirect ISE because it assumes that all plasma samples are 93% water

-Proteins or lipids increase can disrupt this proportion

What causes hypo-osmotic depletional hyponatremia?

Sodium loss is greater than water loss

-Diuretics

-Burns

What causes hypo-osmotic Dilutional hyponatremia?

Excess water retention

-Edema

-Kidney failure, cirrhosis and kidney failure

What is the most common cause of hyperosmotic hyponatremia?

Increased blood sugar (hyperglycemia)

-Decreases 2mmol for every 100mg/dL increase in glucose

What is the most common cause of Iso-osmotic hyponatremia?

Pseudohyponatremia caused by electrolyte exclusion effect

Define Hypernatremia:

High sodium (>150mmol/L)

-Always hyperosmolar

-Symptoms mainly neurological (neuronal loss of h20 to ecf)

Hypernatremia is rare in patients with?

Normal thirst response and water access

Describe Hypovolemic hypernatremia:

Excess h20 loss

-Osmotic diuresis concern

-Can be distinguished renal and extrarenal ny measuring Na+ concentration in urine

*** Na+ = >20 = renal

***Na+ =

Describe Hypervolemic Hypernatremia:

Excess water retention

-Net gain of water and Na+

-Most common in hypertonic saline and sodium bicarbonate given to hospitalized patients

-Less common =cushing syndrome

Describe Normovolemic Hypernatremia:

Normal H20

-Normal volume

Happens before hypovolemic

-Lack of thirst mechanism

-diabetes insipidus

What is the major Intracellular fluid cation?

Potassium (k+)

What is the primary organ for potassium regulation?

Kidneys

-Excrete excess

What is the reference range for Potassium?

1. Adult serum

-3.5-5.1 mmol/L

Plasma potassium can be lower or higher than serum potassium?

Lower 0.1-0.7

What are the the specimen types for potassium?

Serum or plasma (prefered)

urine

Whole blood in blood gas settings

What are the main interferences in testing potassium?

1. Extremely sensitive to hemolysis

-False increase

2. Increased WBC can increase potassium value

3. Aged uncentrifuged samples

-K+ leaks into cells causing false negative

4. Skeletal muscle activity can increase value

-Leaving a tourniquet on too long

What is the main method for testing potassium?

Indirect ISE

Define Hypokalemia:

Low potassium (

What are the main causes for hypokalemia?

1. Redistribution of extracellular K+ into cells

-Common after insulin administration

-Feature of alkalosis

=ECF K+ trades places with ICF H+

-Catecholamine production prompted by stress or trauma

What is a True K+ deficit?

1. Renal losses (>25 mmol/day in urine)

2. Nonrenal losses (

A decrease in potassium in ECF is associated with?

1. Muscle weakness

2. Irritability

3. Paralysis

Sever hypokalemia (

Neuromuscular symptoms

-Critically low: tachycardia, cardia arrest

Define Hyperkalemia:

High potassium (>5.0)

What causes hyperkalemia?

1. Metabolic acidosis

-ECF H+ trades places with ICF K+

2. Increased intake

3. Increased retention

*** True, prolonged hyperkalemia is caused by moderate to severe kidney disease with anuria

What are the symptoms of Hyperkalemia?

-Disorientation

-Weakness

-Tingling

-Paralysis

-Cardiac conduction defects

What is the result of Severe hyperkalemia (>7.0)

Peripheral vascular colapse

-Cardiac arrest

* K+ greater than 10.0 is FATAL

What are the functions of Chlroide?

Like Na+ helps maintain:

-water distribution

-Osmotic pressure

-ECF anion-cation balance

=Travels with Na+ and K+ to balance their (+) charges

-Differentiates acid-base disturbances

What is the reference range for Chloride?

98-107 mmol/L (serum/plasma)

What are the specimens for testing chloride?

-Whole blood

-Plasma

-sweat

-fecal

What are the two common methods for testing chloride?

1. ISE (blood)

2. Coulometric amperometric titration (Sweat)

* not impacted by hemolysis

How does coulometric amperometric titration work and how does it correlate with cystic fibrosis?

1. Depends on reaction between Ag + Cl- =AgCl

2. Sweat Cl- confirms CF

-Collection through pilocarpine iontophoresis (BOC QUESTION)

-Ensure evaporation doesnt occur

What are the reference ranges for cystic fibrosis in sweat?

What is Hypochloremia and the associated diseases?

Low chloride (

What is Hyperchloremia and the associated causes?

High chloride (>107 mmol/L)

-Dehydration

-Prolonged diarrhea with loss of bicarbonate

*decreased bicarbonate = Cl- retention

-Overtreatment with IV saline

-Respiratory alkalosis

What is chloride shift?

When there is metabloic or respiratory alkalosis: HCO3 moves into RBC and Cl- out into plasma

-When there is acidosis: HCO3 moves out into the plasma and Cl- moves into RBC

Define Bicarbonate (HCO3):

Carbon dioxide

-90-95% of total CO2 is HCO3

-2nd largest plasma anion fraction

-Bodys primary chemical pH buffer

What is the reference range for Bicarbonate?

22-28 mmol/L

What is the Main Caution when testing bicarbonate/

UNCAPPED TUBES

-they are susceptible to CO2 loss about 2-3mmol every hour

What is the common method for testing HCO3?

Enzymatic

-Specimen is alklalinized to convert CO2 to CO3

-HCO3 goes to oxaloacetate - NAD+

-Decrease in absorbance at 340nm is proportional to total CO2

What diseases cause a decrease and increase in Bicarbonate?

1. Decrease

-Diarrhea

-CKD

-Shock

-Diabetic ketoacidosis

2. Increase

-Vomitting

-DIueretics

-Antacid overuse

Define anion gap:

Difference between serum Na+ concentration and sum of the serum and bicarb concentrations

-Gap represents negatively charged substances not routinely measured (albumin or phopshate)

What is the anion gap formula and reference range?

AG = [Na+} - [Cl- + HCO3-]

- 6-14 mmol/L