electrolytes 2 (Na+, K+, Cl-,HCO3-)

Question 1

major cation ECF

Answer 1

Na+ ( 90%)

• maintains water distribution & osmotic pressure
• intake 130-260 mmol NaCl
• body uses 1-2 mmol/day the rest is excreted

Question 2

ATPase ion Pump

Answer 2

Na+ & K+ move between ICF & ECF by Na+/K+ ATPase ion pump

• 3 Na+ ions move out in exchange for 2 K+ ions as ATP is converted to ADP
• water follows Na+ out of cell which prevents osmotic rupture*

Question 3

3 processes for regulation of sodium

Answer 3

1. intake of H2O in response to thirst
   - stimulated/supppressed by plasma osmolality
2. excretion of H2O
   - in response to AVP ( causes water to be reabsorbed)
3. excretion of Na+
   - involves aldosterone, angiotensin ll & atrial natiuretic peptide (ANP)

Question 4

sodium specimen collection

Answer 4

• serum
• heparinized plasma ( lithium heparin )
• urine ( random or 24hr)
• not significantly affected by hemolysis ( but if grossly hemolyzed Na+ decreased
  bc of dilution)
• refrigerate or freeze for delayed analysis

Question 5

methods of analysis - sodium

Answer 5

chemical methods- outdated
flame emission spectrophometry
atomic absorption spectrophotometry (AAS)

ion- selective electrodes (ISEs)
- most common method in clinical labs

Question 6

Sodium electrode

Answer 6

• a glass ion-exchange membrane is used for Na+ measurement
• Na+ interacts with the tip ( measuring electrode) producing a potential that is dependent on the activity/concentration of Na+
• reference electrode maintains a constant potential
• difference in potential between the reference & measuring electrodes = the activity of Na+

Question 7

direct vs indirect measurement for ISEs

Answer 7

Direct

• undiluted
• elevated lipid/proteins don’t interfere
• more accurate

Indirect

• diluted
• increased lipids & proteins cause electrolyte exclusion effect

electrolyte exclusion effect

• excess lipids or proteins displace plasma water
• less aqueous phase gets added to the diluent
• causes Pseudohyponatremia***- falsely decreased Na+ concentration

Question 8

Sodium electrode -sources of error

Answer 8

protein build up on electrode membranes through continuous use
- causes poor selectively & poor reducibility for results

solution
- routine maintenance to remove protein build up

Question 9

VITROS analyzers slide

Answer 9

use disposable ISE slides

• drop of sample & reference fluid are applied at same time
• dry slide technology

Question 10

Hyponatremia

Answer 10

Na+ level < 135 mmol/L

caused by
- increased Na+ loss ; hypoaldsosteronism, K+ deficiency, diuretics, ketonuria

• Increased H2O retention; renal failure, nephrotic syndrome, CHF
• water imbalance; excessive water intake, pseudohyponatremia

Question 11

hypernatremia

Answer 11

Na+ >145mmol/L

caused by
- increased intake or retention; hyperaldosteronism, excess salt ingestion

• excess H2O loss; diabetes insipidus, profuse sweating
• decreased H2O intake; older ppl or infants, mental impairment

Question 12

Sodium reference ranges

• serum/plasma
• critical

Answer 12

serum/plasma: 135-145 mm/L ( CSF is around the same)

critical <120 or >160 mmol/L

Question 13

major cation in ICF

Answer 13

potassium (K+)

• dietary requirement : 50-150mmol/day
• small amount used, most excreted by kidneys ( not stored well)
• K+ conc regulates neuromuscular excitability, heart contractions
• too low or too high can lead to cardiac arrest

Question 14

regulation of potassium

Answer 14

• nearly all K+ is reabsorbed in the proximal tubules of the kidney
• additional K+ is secreted into urine in exchange for Na+
• any excess K+ consumed in the diet is excreted in the urine; can accumulate to toxic levels in renal failure

Question 15

factors that influence distribution of K+

Answer 15

K+ loss due to Na+, K+ ATPase pump inhibition
- hypoxia, hypomagnesemia, digoxin overdose

Insulin promotes acute entry of K+ into skeletal muscle & liver by increase Na+,K+ATPase activity

Catecholamines like epinephrine ( beta- stimulator) promote entry of K+ into cells

Propanolol ( beta-blocker) impairs entry of K+ into cells

excercise

• K+ released from muscle cells during excercise
• fist pumping during venipuncture can cause falsely high K+ levels

cellular breakdown
- damage to RBCs releases K+ into ECF

Question 16

Specimen collection - potassium

Answer 16

serum

plasma ( heparin preferred)

don’t use EDTA ( K2 EDTA contains potassium )

urine ( random or 24 hr)

Minimize hemolysis ( hemolysis falsely increases K+ results )

separate plasma/ serum from cells ASAP or may give false increased K+

Question 17

methods of measurement for Potassium

Answer 17

flame photometry
-older method q

ion-selective electrodes (ISEs)
-most routinely used

Question 18

Potassium electrode

Answer 18

ion-exchange electrode

solid plastic membrane contianing valinomycin
- an antibiotic that will selectively bind with K+

KCL is the inner electrolyte solution

Can be direct or indirect mode
- difference between the two method is only slight compared to Na+

Question 19

clinical significance of potassium

Answer 19

increased or decreased concentration of K+ will affect muscle irritability = cardiac arrest

Question 20

K+ reference ranges
serum
critical

Answer 20

serum : 3.5-5.1 mmol/L

critical: < 2.8 or >6.0
- always check for hemolysis first

Question 21

Major anion in ECF

Answer 21

Chloride

- major contributor to osmolality ( with Na+ )

Question 22

Regulation of Chloride

Answer 22

Cl- usually shifts due to changes in Na+ or HCO3-

Cl- in the diet

• absorbed by intestinal tract
• then filtered by glomerulus
• then passively reabsorbed ( with Na+) by proximal tubules

excess Cl- is ecreted in urine & sweat

excessive sweating stimulates aldosterone to act on the sweat glands to conserve Na+ & Cl-

Question 23

How Chloride maintains Electroneutrality

Answer 23

1. Na+ is reabsorbed along with Cl- in proximal tubules
   - Na+ reabsorption is limited by amount of Cl- available
2. Chloride shift ( hamburger phenomenon)
   - CO2 from cellular metabolism in tissues moves into the plasma & the red cells
   - in RBCs, CO2 form carbonic acid which splits into H+ & HCO3-
   - Deoxyhemoglobin (HHb) buffers H+
   - HCO3- moves diffuses into plasma
   - Cl- dissuses into RBC & electric balance is maintained

Question 24

Chloride specimen collection

Answer 24

serum/plasma ( separate from cells)

lithium heparin ( best anticoagulant)

urine ( 24hr is best )

CSF ( decreased CL in bacterial meningitis ; glucose also decreased in bacterial meningitis)

Sweat ( screen for cystic fibrosis )
- >60 mmol/L is positive for CF

Question 25

Chloride methods of analysis

Answer 25

- amperometric -coulometric titration - mercurimetric titration - colorimetry - ion -selective electrode (ISE)

Question 26

Amperometric - coulometric titration analysis of chloride

Answer 26

• Silver ions are generated coulometrically from a silver electrode and combine with chloride ions in the patient sample. • Ag+ + Cl- → AgCl • When all of the Cl- in the sample has bound to Ag+, the free Ag+ indicates the endpoint. • The coulometric generator and timer shut off and the amount of time elapsed is used to calculate the Cl- in the sample. • An example of this is the Cotlove Chloridometer.

Question 26

Amperometric - coulometric titration analysis of chloride

Answer 26

• Silver ions are generated coulometrically from a silver electrode and combine with chloride ions in the patient sample. • Ag+ + Cl- → AgCl • When all of the Cl- in the sample has bound to Ag+, the free Ag+ indicates the endpoint. • The coulometric generator and timer shut off and the amount of time elapsed is used to calculate the Cl- in the sample. • An example of this is the Cotlove Chloridometer.

Question 27

Ion selective electrode for chloride

Answer 27

• Most common method • Uses an ion-exchange membrane to selectively bind Cl- via a redox reaction • Used in clinical chemistry analyzers

Question 28

Chloride clinical significance

Answer 28

Cl- passively follows Na+ therefore Cl- disorders often have the same causes as Na+ disorders causes of Hypochloremia - increased Cl- ( vomiting, diabetic ketoacidosis, aldosterone deficiency,Pyelonephritis) - high serum HCO3- ( respiratory acidosis, metabolic alkalosis) causes of hyperchloremia - increased intake - excess loss of HCO3- ( renal tubular acidosis, metabolic acidosis, through GI tract)

Question 29

Chloride references ranges

Answer 29

serum/ plasma 98-107mmol/L

Question 30

Bicarbonate (HCO3-) or total CO2 (TCO2)

Answer 30

bicarbonate is the 2nd most abundant anion in ECF total CO2 is comprised of - dissolved CO2 - carbonic acid ( H2CO3) - bicarbonate bicarbonate accounts for 90% of total CO2

Question 31

Bicarbonate importance in the body

Answer 31

HCO3- is a major component of buffer systems in blood. • NaHCO3-/H2CO3 (sodium bicarbonate/carbonic acid) ``` Chloride Shift in the tissues and lungs • This maintains electroneutrality (pluses = minuses on either side of the cell membrane) • If HCO3- → into cell Then out ← Cl- And vice versa ```

Question 32

Regulation of Bicarbonate (HCO3-)

Answer 32

• 85% of HCO3- is reabsorbed by proximal tubules in kidneys; 15% by distal tubules. ``` • Tubules are only slightly permeable to bicarb, so usually reabsorbed as CO2. - HCO3- combines with H+ to form H2CO3 - H2CO3 dissociates into H2O and CO2 - CO2 diffuses back into ECF ``` • Excess HCO3- flows into the urine * In alkalosis: * Kidneys increase excretion of HCO3- in urine to correct pH • In acidosis: • HCO3- is reabsorbed by the proximal and distal tubules to correct pH.

Question 33

Bicarbonate Specimen Collection

Answer 33

* Serum and lithium heparin plasma• (arterial and whole blood measurements were discussed last chapter) * Analyze ASAP * Keep anaerobic * i.e. Don’t decap specimen * Exposure to air causes a loss of CO2 = ↓TCO2 * Can ↓ by 6 mmol/L per hour

Question 34

TCO2 Methods of Analysis

Answer 34

* ISE Method (pCO2) * Combination pH electrode * Silicon rubber membrane which is permeable to CO2 gas * pH sensitive glass electrode detects a change in pH (↓ pH is measured) OR • Gas-sensing electrode

Question 35

PCO2 (Bicarbonate) pH Glass Electrode

Answer 35

The sample is acidified first to convert all froms of CO2 to CO2 gas ↓ in pH measured i.e. H+ ions that are produced

Question 36

Gas-Sensing CO2 Electrode

Answer 36

same equation as ISE ``` The membrane is in contact with a solution that is permeable only to CO2. The change in pH of the HCO3- is detected by a pH electrode. TCO2 ```

Question 37

TCO2 enzyme method

Answer 37

• Sample is alkalinized to convert all forms of CO2 to HCO3- Phosphoenolpyruvate + HCO3- -------> Oxaloacetate + H2PO4- Oxaloacetate + NADH + H+ ---------> Malate + NAD+ The rate of change in absorbance of NADH ~ [HCO3-]

Question 38

TCO2 /HCO3- - Clinical Significance

Answer 38

``` Causes of Decreased HCO3- Metabolic acidosis Renal Tubular Failure with Hyperchloremia Compensated Respiratory Alkalosis Severe Diarrhea ``` ``` Causes of Increased HCO3- Metabolic Alkalosis - Severe vomiting - Excessive intake of alkali Compensated Respiratory Acidosis ```

Question 39

CO2 reference ranges

Answer 39

Plasma / serum TCO2 (venous) 23-29 mmol/L | Plasma / serum HCO3- 21-28 mmol/L

Question 40

Anion Gap

Answer 40

* A number that represents the concentration of unmeasured anions (proteins, phosphates, sulfates and organic acids) * Not a real Gap in anions and cations * Created by concentration differences between commonly measured cations (Na+ + K+) and anions (Cl- + HCO3-) • Useful for: - Indicating an increase in one or more unmeasured anions in serum (Metabolic Disorders)** if an increased anion gap usually indicates a metabolic disorder - Quality control for analyzer used to measure electrolytes** should never be a negative number if negative indicates a problem with the analyzer

Question 41

Anion gap calculation

Answer 41

Calculation: measured cations – measured anions | Na+) - (Cl- + HCO3-) OR (Na+ + K+ ) - (Cl- + HCO3-

Question 42

Anion Gap refernece range

Answer 42

Anion Gap (AGap) should be positive • 7 - 16 mmol/L or • 10 - 20 mmol/L if K+ included

Question 43

Abnormal anion gap

Answer 43

Low(rare) - multiple myeloma - analyzer error ``` High • Methanol • Uremia • Diabetic ketoacidosis • Paraldehyde • Iron, inhalants, ibuprofen • Lactic acidosis • Ethylene glycol, ethanol ketoacidosis • Salicylates, starvation ketoacidosis ```