What do U&E’s measure? 6
1) Sodium
2) Potassium
3) Chloride
4) Bicarbonate
5) Urea
6) Creatinine
Abnormal electrolytes can be the result of what 3 things?
1) Primary disease state
2) Secondary consequence of a multitude of diseases
3) Iatrogenic causes are common
Why are controlled electrolyte levels important? 4
1) Maintenance of cellular homeostasis
2) CV physiology
3) Renal physiology
4) Electrophysiology (heart, CNS)
Give 8 diseases that electrolyte imbalances are commonly seen in?
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
What is diabetes insipidus?
Rare form of diabetes caused by deficiency of the pituitary hormone vasopressin (ADH)
What are the 3 important components to think about when considering water and sodium balance?
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
What is the normal ECF amount and ICF amount?
ECF = plasma + interstitial fluid - 19L
ICF - 23L
What is the normal extracellular [Na]?
140mmol/L
What will happen if you decrease the bodies total fluid level by 4L?
You will lose some fluid from ECF and some fluid from ICF, this will increase the concentrations of any solute
What will happen if you increase the bodies solute excretion to concentrations of electrolytes?
Concentration of all solutes in ECF and ICF will decrease
How much of the 19L of ECF is vascular and how much is interstitial?
3L vascular, 16L intersitial
What is the bodies total fluid content?
42L
What is the normal extracellular [K]?
5mmol/L
What is the normal intracellular [Na]?
10mmol/L
What is the normal intracellular [K]?
150mmol/L
Is the concentration of Na and K the same in plasma as in interstitial fluid, why?
Yes, solutes can move freely between so reach an equilibrium
What creates the membrane charges across the cell membrane?
Differences in [Na] and [K] between ECF and ICF
What is meant by an isotonic solution?
It has roughly the same conc of solutes as the blood
Give an example of loss of isotonic solution?
Haemorrhage
What happens to electrolytes and fluid levels in the body with a loss of 2L of isotonic solution?
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
Give an example of loss of hypotonic solution?
Dehydration from vomiting
What is meant by hypotonic solution?
Solution with a lower conc of electrolytes
What happens to electrolyte and fluid levels in the body with a loss of 3L of hypotonic solution ie. insensible loss?
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
Give and example of gain of isotonic solution?
Saline
What happens to electrolyte levels and fluid levels when there is a gain of 2L of isotonic solution?
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)
Give an example of gain of hypotonic solution?
Water or dextrose administered
What would happen to fluid and electrolyte levels with a gain of 3L of hypotonic solution?
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
Give 3 examples of physiological compensatory mechanisms for changes to fluid and electrolyte levels?
1) Thirst
2) ADH
3) RAAS
Give 3 examples of therapeutic compensatory mechanisms for changes to fluid or electrolyte levels?
1) IV therapy
2) Diuretics
3) Dialysis
When and where is ADH produced and what is its function?
Produced by median eminence of pituitary gland and release increases when osmolality rises
Decreases renal water loss and increases thirst
What are 2 simple tests to ascertain ADH activity?
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
How is the RAAS activated?
Reduction in intravascular volume and Na depletion
What does the RAAS cause?
renal Na retention (and thus water retention)
Give a simple test to measure RAAS activity?
Measure plasma and urine Na
if urine
What would happen to fluid and electrolyte levels if you replaced a 2L loss of isotonic fluid with 2L of isotonic fluid?
Fluid and electrolyte levels would return to normal
There would be no change in ECF [Na] and thus no fluid redistribution
What would happen to fluid and electrolyte levels if you replaced a 2L loss of isotonic fluid with 2L of hypotonic fluid?
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
What would happen to fluid and electrolyte levels if you replaced a 3L loss of hypotonic fluid with 3L of hypotonic fluid?
Fluid and electrolyte levels would return to normal
[Na] would be restored in ECF and you would get subsequent fluid redistribution
What would happen to fluid and electrolyte levels if you replaced a 3L loss of hypotonic fluid with 3L of isotonic fluid?
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
What is hyponatraemia and what 2 situations can cause this?
Reduced [Na] in ECF
Caused by:
Too little Na in ECF
Too much water in ECF
What is hypernatraemia and what 2 situations can cause it?
Increased [Na] in ECF
Caused by:
Too little water in ECF
Too much Na in ECF
What can dehydration refer to? 2
1) Water depletion
2) Fluid (Na and water) depletion
What are the 2 differential diagnoses for a patient with hyponatraemia and oedema?
1) CCF Cirrhosis
2) Nephrosis
What would be the diagnosis for hyponatraemia for a euvolaemic patient with a normal plasma osmolality?
Pseudo-hyponatraemia
What would the diagnosis for hyponatraemia for a euvolaemic patient with high plasma osmolality?
Hypertonic hyponatraemia
What would be the diagnosis for hyponatraemia in a euvolaemic patient with low plasma osmolality?
Water overload - You would then measure the urine sodium to determine the cause of water overload
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?
1) SIADH (syndrome of inappropriate ADH)
2) Drugs
3) CRF (chronic renal failure)
In a patient found to have water overload causing hyponatraemia with a urine sodium of
1) Stress post surgery
2) Endocrine: Hypothyroid
In a hypovolaemic patient with hyponatraemia and a urine sodium of >20mmol/L what are the 3 possible diagnoses?
1) diuretics
2) Addison’s
3) Na losing nephritis
In a hypovolaemic patient with hyponatraemia and a urine sodium of
1) Vomiting
2) Diarrhoea
3) Skin loss
Describe the steps in the cycle of diuretics leading to hyponatraemia?
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]
Describe the steps in the cycle of SIADH leading to hyponatraemia?
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]
Other than water retention what else does ADH cause?
Increased thirst
Describe the steps in decreased water intake leading to hypernatraemia?
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]
If a patient is dry and has been treated for chronic cardiac failure, what is the most likely cause for their hyponatreamia?
Diuretics
If a patient is thirsty but well hydrated what is the most likely cause for their hyponatraemia?
SIADH (syndrome of inappropriate ADH)
If a patient is dry and has trouble swallowing what is the most likely cause of their hypernatraemia?
Decreased water intake
If a patient is dry and thirsty with a high blood glucose what is the most likely cause of their hypernatraemia?
Osmotic diuresis
Describe the steps in high blood glucose leading to hypernatraemia by osmotic diuresis?
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]
What is the reference range for plasma [K] and what levels are considered dangerous?
3.6-5.0mmol/L
Values 6mmol/L are potentially dangerous
What are the potential dangers of high or low plasma [K]?
Cardiac conduction defects
Abnormal neuromuscular excitability
Is serum K a good measure of total body plasma?
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
How does exchange of ICF -ECF affect plasma K?
Significantly
Give an example of 4 conditions/states which can lead to ECF-ICF K exchange?
1) Acidosis
2) Insulin/glucose therapy
3) Adrenaline
4) Rapid cellular incorporation - TPN, leukaemia
What is the average potassium intake in a day?
60-200mmol/day
Why does ICF-ECF exchange of K have such a significant effect on plasma [K]?
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
Why can changes in pH lead to changes in plasma [K]?
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
Does acidosis lead to hyper- or hypokalaemia?
Causes potassium to move out of the cells leading to hyperkalaemia
Does alkalosis lead to hyper- or hypokalaemia?
Causes potassium to move into the cells leading to hypokalaemia
What is a normal glucose reference range?
3.5-5.5mmol/L
Give the 5 main causes of hyperkalaemia?
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
What are the 5 possible treatments for hyperkalaemia?
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
What are the 3 causes of potassium depletion?
1) Low intake
2) Increased urine loss (diuretics/osmotic diuresis, tubular dysfunction, mineralocorticoid excess)
3) GIT losses (vomiting, diarrhoea/laxatives, fistulae)
Give 2 causes of hypokalaemia without potassium depletion?
1) Alkalosis
2) Insulin/glucose therapy
With potassium depletion to
Affects the neuromuscular system - lethargy, muscle weakness and cardiac arrhythmias
With potassium depletion to
1) Neuromuscular - lethargy, muscle weakness, heart arrhythmias
2) Kidneys - polyuria, alkalosis - increased renal HCO3 production
3) Vascular
4) Gut
What common things may be found in a history suggesting potassium depletion? 5
1) Diarrhoea
2) Vomiting
3) drugs (diuretics, digoxin)
4) Symptoms of lethargy/weakness
5) Cardiac arrhythmias
What 2 findings are likely to be made in electrolyte investigation in a person with potassium depletion?
1) Hypokalaemia
2) Alkalosis - raised HCO3-
What are the 3 main treatments for potassium depletion?
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
In a replacement of potassium deficit how much should be given orally and how much IV?
1) Oral = 48mmol/day +diet
2) IV =
Describe the steps in aldosterone leading to hypernatraemia?
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]
