Electrolyte Homeostasis

Question 1

What do U&E’s measure? 6

Answer 1

1) Sodium
2) Potassium
3) Chloride
4) Bicarbonate
5) Urea
6) Creatinine

Question 2

Abnormal electrolytes can be the result of what 3 things?

Answer 2

1) Primary disease state
2) Secondary consequence of a multitude of diseases
3) Iatrogenic causes are common

Question 3

Why are controlled electrolyte levels important? 4

Answer 3

1) Maintenance of cellular homeostasis
2) CV physiology
3) Renal physiology
4) Electrophysiology (heart, CNS)

Question 4

Give 8 diseases that electrolyte imbalances are commonly seen in?

Answer 4

1) Haemorrhage
2) D&V
3) Poor intake - elderly
4) Increased losses -pyrexia
5) Diabetes insipidus
6) Diabetes mellitus
7) Diuretic therapy
8) Endocrine disorders

Question 5

What is diabetes insipidus?

Answer 5

Rare form of diabetes caused by deficiency of the pituitary hormone vasopressin (ADH)

Question 6

What are the 3 important components to think about when considering water and sodium balance?

Answer 6

1) Sodium in, sodium out
2) Water in and water out
3) Redistribution from cells into the extracellular compartment
You are trying to figure out what the intracellular concentrations are likely to be when you are only measuring the blood

Question 7

What is the normal ECF amount and ICF amount?

Answer 7

ECF = plasma + interstitial fluid - 19L

ICF - 23L

Question 8

What is the normal extracellular [Na]?

Answer 8

140mmol/L

Question 9

What will happen if you decrease the bodies total fluid level by 4L?

Answer 9

You will lose some fluid from ECF and some fluid from ICF, this will increase the concentrations of any solute

Question 10

What will happen if you increase the bodies solute excretion to concentrations of electrolytes?

Answer 10

Concentration of all solutes in ECF and ICF will decrease

Question 11

How much of the 19L of ECF is vascular and how much is interstitial?

Answer 11

3L vascular, 16L intersitial

Question 12

What is the bodies total fluid content?

Answer 12

42L

Question 13

What is the normal extracellular [K]?

Answer 13

5mmol/L

Question 14

What is the normal intracellular [Na]?

Answer 14

10mmol/L

Question 15

What is the normal intracellular [K]?

Answer 15

150mmol/L

Question 16

Is the concentration of Na and K the same in plasma as in interstitial fluid, why?

Answer 16

Yes, solutes can move freely between so reach an equilibrium

Question 17

What creates the membrane charges across the cell membrane?

Answer 17

Differences in [Na] and [K] between ECF and ICF

Question 18

What is meant by an isotonic solution?

Answer 18

It has roughly the same conc of solutes as the blood

Question 19

Give an example of loss of isotonic solution?

Answer 19

Haemorrhage

Question 20

What happens to electrolytes and fluid levels in the body with a loss of 2L of isotonic solution?

Answer 20

Loss of 2L of isotonic solution and thus fluid from the ECF and no loss of fluid from the ICF
This is because the loss is isotonic so there is no change to [Na] and thus no fluid redistribution

Question 21

Give an example of loss of hypotonic solution?

Answer 21

Dehydration from vomiting

Question 22

What is meant by hypotonic solution?

Answer 22

Solution with a lower conc of electrolytes

Question 23

What happens to electrolyte and fluid levels in the body with a loss of 3L of hypotonic solution ie. insensible loss?

Answer 23

Loss of fluid from ECF (1L) and ICF (2L) with more being lost from the ICF
Small increase in [Na] in the ECF and thus fluid redistribution between the ECF and ICF

Question 24

Give and example of gain of isotonic solution?

Answer 24

Saline

Question 25

What happens to electrolyte levels and fluid levels when there is a gain of 2L of isotonic solution?

Answer 25

Gain of 2L of fluid to ECF only, no change to ICF No change in [Na] No fluid redistribution (as gain is of isotonic solution)

Question 26

Give an example of gain of hypotonic solution?

Answer 26

Water or dextrose administered

Question 27

What would happen to fluid and electrolyte levels with a gain of 3L of hypotonic solution?

Answer 27

Gain of fluid to ICF and ECF with a greater gain to ECF Small decrease in [Na] in ECF Hence fluid redistribution between ECF and ICF

Question 28

Give 3 examples of physiological compensatory mechanisms for changes to fluid and electrolyte levels?

Answer 28

1) Thirst 2) ADH 3) RAAS

Question 29

Give 3 examples of therapeutic compensatory mechanisms for changes to fluid or electrolyte levels?

Answer 29

1) IV therapy 2) Diuretics 3) Dialysis

Question 30

When and where is ADH produced and what is its function?

Answer 30

Produced by median eminence of pituitary gland and release increases when osmolality rises Decreases renal water loss and increases thirst

Question 31

What are 2 simple tests to ascertain ADH activity?

Answer 31

Measure plasma and urine osmolality If urine>plasma this suggests ADH is active or Measure plasma and urine urea levels (urea is a good indicator of osmolality) if urine>plasma this suggests water retention

Question 32

How is the RAAS activated?

Answer 32

Reduction in intravascular volume and Na depletion

Question 33

What does the RAAS cause?

Answer 33

renal Na retention (and thus water retention)

Question 34

Give a simple test to measure RAAS activity?

Answer 34

Measure plasma and urine Na | if urine

Question 35

What would happen to fluid and electrolyte levels if you replaced a 2L loss of isotonic fluid with 2L of isotonic fluid?

Answer 35

Fluid and electrolyte levels would return to normal | There would be no change in ECF [Na] and thus no fluid redistribution

Question 36

What would happen to fluid and electrolyte levels if you replaced a 2L loss of isotonic fluid with 2L of hypotonic fluid?

Answer 36

ICF fluid levels 1L greater than normal ECF fluid levels 1L less than normal As you would get a reduction in ECF [Na] and thus fluid redistribution

Question 37

What would happen to fluid and electrolyte levels if you replaced a 3L loss of hypotonic fluid with 3L of hypotonic fluid?

Answer 37

Fluid and electrolyte levels would return to normal | [Na] would be restored in ECF and you would get subsequent fluid redistribution

Question 38

What would happen to fluid and electrolyte levels if you replaced a 3L loss of hypotonic fluid with 3L of isotonic fluid?

Answer 38

ICF fluid levels 2L less than normal (unchanged following loss) ECF fluid levels 2L greater than normal (a 3L gain following loss) ECF [Na] would be still be slightly increased compared and you would get no fluid redistribution

Question 39

What is hyponatraemia and what 2 situations can cause this?

Answer 39

Reduced [Na] in ECF Caused by: Too little Na in ECF Too much water in ECF

Question 40

What is hypernatraemia and what 2 situations can cause it?

Answer 40

Increased [Na] in ECF Caused by: Too little water in ECF Too much Na in ECF

Question 41

What can dehydration refer to? 2

Answer 41

1) Water depletion | 2) Fluid (Na and water) depletion

Question 42

What are the 2 differential diagnoses for a patient with hyponatraemia and oedema?

Answer 42

1) CCF Cirrhosis | 2) Nephrosis

Question 43

What would be the diagnosis for hyponatraemia for a euvolaemic patient with a normal plasma osmolality?

Answer 43

Pseudo-hyponatraemia

Question 44

What would the diagnosis for hyponatraemia for a euvolaemic patient with high plasma osmolality?

Answer 44

Hypertonic hyponatraemia

Question 45

What would be the diagnosis for hyponatraemia in a euvolaemic patient with low plasma osmolality?

Answer 45

Water overload - You would then measure the urine sodium to determine the cause of water overload

Question 46

In a patient found to have water overload causing hyponatraemia with a urine sodium of >20mmol/L what are the 3 possible reasons for water overload?

Answer 46

1) SIADH (syndrome of inappropriate ADH) 2) Drugs 3) CRF (chronic renal failure)

Question 47

In a patient found to have water overload causing hyponatraemia with a urine sodium of

Answer 47

1) Stress post surgery | 2) Endocrine: Hypothyroid

Question 48

In a hypovolaemic patient with hyponatraemia and a urine sodium of >20mmol/L what are the 3 possible diagnoses?

Answer 48

1) diuretics 2) Addison's 3) Na losing nephritis

Question 49

In a hypovolaemic patient with hyponatraemia and a urine sodium of

Answer 49

1) Vomiting 2) Diarrhoea 3) Skin loss

Question 50

Describe the steps in the cycle of diuretics leading to hyponatraemia?

Answer 50

1) Decreased urine Na reabsorption 2) Leads to Na diuresis, causing increased renal water and Na loss and increased urine [Na] and decreased plasma [Na] 3) This all leads to a decreased intravascular volume (which not only causes increase in plasma [Creatinine] and [urea] due to reduced GFR) also leads to increased release of ADH 4) Increased ADH causes increased water intake which also leads to reduced plasma [Na]

Question 51

Describe the steps in the cycle of SIADH leading to hyponatraemia?

Answer 51

1) Increased ADH leads to decreased urine volume which leads to increased urine [Na] 2) Increased ADH also leads to increased renal water reabsorption which also leads to increased urine osmolality 3) Increased renal water reabsorption also leads to increased IVV which leads to decreased renal Na reabsorption (due to RAAS) which also leads to increased urine [Na] 4) Increased IVV also leads to haemodilution which leads to decreased plasma osmolality, plasma [Na] and plasma [creatinine] and [urea]

Question 52

Other than water retention what else does ADH cause?

Answer 52

Increased thirst

Question 53

Describe the steps in decreased water intake leading to hypernatraemia?

Answer 53

1) Decreased water intake leads to haemoconcentration which causes increased plasma osmolality and increased plasma [Na] (2) Decreased water intake also leads to increased ADH which causes decreased urine volume and increased urine osmolality) 3) Decreased water intake also leads to reduced IVV which reduces GFR which leads to increased plasma [creatinine] and [urea] 4) Decreased IVV also leads to increased Na retention via the RAAS which leads to increased plasma [Na]

Question 54

If a patient is dry and has been treated for chronic cardiac failure, what is the most likely cause for their hyponatreamia?

Answer 54

Diuretics

Question 55

If a patient is thirsty but well hydrated what is the most likely cause for their hyponatraemia?

Answer 55

SIADH (syndrome of inappropriate ADH)

Question 56

If a patient is dry and has trouble swallowing what is the most likely cause of their hypernatraemia?

Answer 56

Decreased water intake

Question 57

If a patient is dry and thirsty with a high blood glucose what is the most likely cause of their hypernatraemia?

Answer 57

Osmotic diuresis

Question 58

Describe the steps in high blood glucose leading to hypernatraemia by osmotic diuresis?

Answer 58

1) Increased plasma [glucose] leads to osmotic diuresis leading to increased water loss (nb. it does lead to some sodium loss which causes increased urine [Na]) leading to haemoconcentration 2) Haemoconcentration leads to increased plasma [Na] and plasma osmolality which leads to increased ADH leading to increased urine osmolality 3) Increased renal loss of water also leads to decreased IVV which leads to decreased GFR leading to increased plasma [creatinine] and [urea]

Question 59

What is the reference range for plasma [K] and what levels are considered dangerous?

Answer 59

3.6-5.0mmol/L | Values 6mmol/L are potentially dangerous

Question 60

What are the potential dangers of high or low plasma [K]?

Answer 60

Cardiac conduction defects | Abnormal neuromuscular excitability

Question 61

Is serum K a good measure of total body plasma?

Answer 61

No, serum plasma does not reflect total body plasma only a small proportion of total potassium is in the plasma Total body potassium is determined by total cell mass

Question 62

How does exchange of ICF -ECF affect plasma K?

Answer 62

Significantly

Question 63

Give an example of 4 conditions/states which can lead to ECF-ICF K exchange?

Answer 63

1) Acidosis 2) Insulin/glucose therapy 3) Adrenaline 4) Rapid cellular incorporation - TPN, leukaemia

Question 64

What is the average potassium intake in a day?

Answer 64

60-200mmol/day

Question 65

Why does ICF-ECF exchange of K have such a significant effect on plasma [K]?

Answer 65

The ICF [K] is 150mmol/L This means the total K in cells is 3400mmol The ECF [K] is 5mmol/L This means the total K in ECF is 70mmol A 1% shift of K from the ICF into the ECF causes a 34mmol increase in ECF taking it from 70mmol to 104mmol - a significant increase

Question 66

Why can changes in pH lead to changes in plasma [K]?

Answer 66

K+ and H+ exchange across membranes They both bind to negatively charged proteins (eg Hb) Changes in pH cause shifts in the equilibrium between K+ and H+ Acidosis causes potassium to move out of the cells leading to hyperkalaemia Alkalosis causes potassium to move into the cells leading to hypokalaemia Conversely K depletion and excess can affect acid-base status

Question 67

Does acidosis lead to hyper- or hypokalaemia?

Answer 67

Causes potassium to move out of the cells leading to hyperkalaemia

Question 68

Does alkalosis lead to hyper- or hypokalaemia?

Answer 68

Causes potassium to move into the cells leading to hypokalaemia

Question 69

What is a normal glucose reference range?

Answer 69

3.5-5.5mmol/L

Question 70

Give the 5 main causes of hyperkalaemia?

Answer 70

1) Artefactual (delay in sample analysis, haemolysis, drug therapy - excess intake) 2) Renal (acute or chronic renal failure) 3) Acidosis 4) Mineralocorticoid dysfunction (not producing angiotensin) (adrenocortical failure, mineralocorticoid resistance eg. sprionalactone) 5) Cell death - cytotoxic therapy

Question 71

What are the 5 possible treatments for hyperkalaemia?

Answer 71

1) Correct acidosis if this is the cause 2) Stop unecessary supplements/intake 3) Give glucose and insulin - drives potassium into cells 4) Ion exchange resins - GIT potassium binding 5) Dialysis - short and long-term

Question 72

What are the 3 causes of potassium depletion?

Answer 72

1) Low intake 2) Increased urine loss (diuretics/osmotic diuresis, tubular dysfunction, mineralocorticoid excess) 3) GIT losses (vomiting, diarrhoea/laxatives, fistulae)

Question 73

Give 2 causes of hypokalaemia without potassium depletion?

Answer 73

1) Alkalosis | 2) Insulin/glucose therapy

Question 74

With potassium depletion to

Answer 74

Affects the neuromuscular system - lethargy, muscle weakness and cardiac arrhythmias

Question 75

With potassium depletion to

Answer 75

1) Neuromuscular - lethargy, muscle weakness, heart arrhythmias 2) Kidneys - polyuria, alkalosis - increased renal HCO3 production 3) Vascular 4) Gut

Question 76

What common things may be found in a history suggesting potassium depletion? 5

Answer 76

1) Diarrhoea 2) Vomiting 3) drugs (diuretics, digoxin) 4) Symptoms of lethargy/weakness 5) Cardiac arrhythmias

Question 77

What 2 findings are likely to be made in electrolyte investigation in a person with potassium depletion?

Answer 77

1) Hypokalaemia | 2) Alkalosis - raised HCO3-

Question 78

What are the 3 main treatments for potassium depletion?

Answer 78

1) Prevention - adequate supplementation 2) Replacement of deficit 3) Monitor plasma potassium regularly especially if taking diuretics, digoxin, compromised renal function and in support of IV resuscitation

Question 79

In a replacement of potassium deficit how much should be given orally and how much IV?

Answer 79

1) Oral = 48mmol/day +diet | 2) IV =

Question 80

Describe the steps in aldosterone leading to hypernatraemia?

Answer 80

1) Aldosterone leads to increased urine Na reabsorption and decreased urine K reabsorption, which leads to decreased renal loss of water and increased urine [K] and decreased urine [Na] 2) Decreased renal loss of water causes increased IVV which increases GFR leading to decreased plasma [creatinine] and [urea] 3) Increased IVV also leads to decreased ADH release, which leads to increased water loss which also leads to increased plasma [Na]