Electrolyte homeostasis

Question 1

What is measured by U&Es?

Answer 1

Sodium

Potassium

(Chloride)

(Bicarbonate)

Urea

Creatinine

Question 2

Why might a person have abnormal electrolytes?

Answer 2

Primary disease state

Secondary consequence of a multitude of diseases

Iatrogenic problems are very common

Question 3

What is important in the correction of abnormal electrolytes?

Answer 3

Maintenance of cellular homeostasis

Cardiovascular physiology - BP

Renal physiology - GFR

Electrophysiology - heart, CNS

Question 4

What are the physiological water electrolyte homeostasis mechanisms?

Answer 4

Thirst

ADH

Renin / Angiotensin system

Question 5

What therapeutic interventions are available?

Answer 5

Intravenous therapy

Diuretics

Dialysis

Question 6

Where is ADH produced?

Answer 6

Produced by median eminence and release increases when plasma solute concentration rises.

Question 7

What are the actions of ADH?

Answer 7

Decreases renal water loss

Increases thirst

Question 8

What tests may be used to ascertain ADH status?

Answer 8

Measure plasma & urine solute concentration

• urine > plasma suggests ADH is active

Measure plasma & urine urea

• urine >> plasma suggests water retention (ADH active)

Question 9

What are possible triggers for the renin-angiotensin system?

Answer 9

Na depletion

Haemorrhage

Question 10

What are the actions of the renin-angiotensin system?

Answer 10

Renal Na retention

Question 11

What test can be used to ascertain renin-angiotensin status?

Answer 11

Measure plasma & urine Na

• If urine < 10 mmol/L suggests R/A/A active

Question 12

What is hyponatremia?

Answer 12

Too little Na in ECF

Excess water in ECF

Question 13

What is hypernatremia?

Answer 13

Too little water in ECF

Too much Na in ECF

Question 14

What is dehydration?

Answer 14

Water deficiency

Fluid (Na and water) depletion

Question 15

What potassium serum values are dangerous?

Answer 15

Values 6.0 mmol/L are potentially dangerous

• Cardiac conduction defects
• Abnormal neuromuscular excitability

Question 16

What is the relationship between potassium and hydrogen ions?

Answer 16

Changes in pH cause shifts in the equilibrium

• acidosis - potassium moves out of cells -> hyperkalaemia
• alkalosis - potassium moves into cells -> hypokalaemia

Question 17

What are the possible causes of hyperkalaemia?

Answer 17

Artefactual

• Delay in sample analysis
• Haemolysis
• Drug therapy
• Excess intake

Renal

• Acute Renal Failure
• Chronic Renal Failure Acidosis (intracellular exchange)

Mineralocorticoid Dysfunction

• Adrenocortical failure
• Mineralocorticoid resistance - eg spironolactone

Cell Death

• Cytoxic therapy

Question 18

What treatment is given for hyperkalaemia?

Answer 18

Correct acidosis if this is cause

Stop unnecessary supplements / intake

Give glucose & insulin - Drives potassium into cells

Ion exchange resins - GIT potassium binding

Dialysis - short and long-term

Question 19

What are the possible causes of potassium depletion?

Answer 19

Low intake Increased urine loss

• diuretics / osmotic diuresis
• tubular dysfunction
• mineralocorticoid excess

GIT losses

• vomiting/diarrhoea/laxatives
• fistulae Hypokalaemia without depletion
• alkalosis
• insulin / glucose therapy.

Question 20

What is the effect of acute potassium depletion?

Answer 20

Acute changes in ICF/ECF ratios

Neuromuscular:

• lethargy, muscle weakness, heart arrhythmias

Question 21

What is the effect of chronic potassium depletion?

Answer 21

Chronic losses from the ICF:

Neuromuscular:

• lethargy, muscle weakness, heart arrhythmias

Kidney:

• polyuria
• alkalosis
• increase renal HCO3 production

Question 22

What is the treatment for potassium depletion?

Answer 22

Prevention

• Adequate supplementation

Replacement of deficit

• oral - 48 mmol/day + diet
• IV - < 20 mmol/L

Question 23

When is is appropriate to monitor potassium levels?

Answer 23

Diuretic therapy

Digoxin use

Compromised renal function

In support of IV resuscitation (eg DM Ketacidosis)