28 - Potassium and hydrogen homeostasis

What is homeostasis?

Maintaining a stable internal environment

What is a cation?

A positively charged ion

What is the major intracellular cation?

Potassium (K+)

What is the movement in and out of the cell by a Na+/K+ ATPase pump?

3Na+ in and 2K+ out

How does acidosis affect the movement of K+?

K+ leaches out into extracellular fluid causing hyperkalaemia

How does alkalosis affect the movement of K+?

K+ is pumped into cells cause hypokalaemia

How does insulin affect the movement of K+?

Insulin drives K+ into cells by stimulating the Na+/K+ ATPase pump

Give 3 examples of catecholamines that affect the movement of K+?

1) Adrenaline

2) Noradrenaline

3) Dopamine

How do catecholamines affect the movemtent of K+?

1) They trigger beta adrenergic receptors which promote cellular K+ uptake

2) They trigger alpha adrenergic receptors which promote cellular K+ loss

3) This leads to an overall net uptake

Describe how an increase in tonicity affects the movement of K+?

K+ passively leaches out of cells causing hyperkalaemia

Give 3 examples of when extracellular fluid osmolality would be increased:

1) Infusion of hypertonic saline

2) Hyperglycaemia in hyperosmolar hyperglycaemic state (HHS)

3) Mannitol

How does hyperkalaemia affect membrane potential?

Hyperkalaemia raises the resting potential towards the threshold, making is easier for cells to initiate an action potential

How does hypokalaemia affect membrane potential?

Hypokalaemia lowers the resting potential making it harder for cells to initiate an action potential

What is the effect of systemic acidosis on kidney K+ secretion?

Reduced K+ secretion

What is the effect of hyperkalaemia on aldosterone release?

Hyperkalaemia triggers aldosterone release

What is the effect of aldosterone on K+ levels?

Aldosterone causes Na+ retention for K+ excretion as well as H+ secretion from cells in the collecting duct and distal convoluted tubule so that H+ can be excreted with K+

Give 3 key outputs of K+:

1) Urinary output

2) Faecal excretion

3) Skin loss (sweating)

What is the threshold for hyperkalaemia?

Above 5.5 mmol/L

Give 2 common causes of incorrect hyperkalaemia:

1) Leaving a blood sample out at room temperature allowing clotting to occur which increases K+

2) Using an EDTA blood sample bottle (anticoagulant) which already has K+ in it

Give 8 clinical signs of hyperkalaemia:

1) Cardiac arrhythmia

2) Fatigue

3) Vomiting

4) Weakness

5) Sudden death

6) Paraesthesia (tingling)

7) ECG changes

8) Palpitations

Give 4 ECG changes seen in hyperkalaemia:

1) Tall tented T waves

2) Prolonged PR intervals

3) Widened QRS intervals

4) Flattened P waves

Give 8 causes of hyperkalaemia:

1) Hypoaldosteronism (Addison's disease)

2) Tissue damage (Rhabdomyolysis)

3) Fictitious

4) Renal failure

5) K+ sparing diuretics (Spironolactone)

6) Hypoinsulinism (Diabetes)

7) Acidosis

8) Drugs

What is a disease that causes hypoaldosteronism?

Addison's disease

What is Addison's disease?

Under secretion of the adrenal cortex

What is an example of a K+ sparing drug?

Spironolactone

What are the 3 steps to treat hyperkalaemia?

1) Give 10ml of calcium gluconate

2) Give insulin or salbutamol to shift K+ into cells

3) Stop potassium intake (halting K+ rich drugs and food)

What is hypokalaemia?

Potassium level of <3.5mmol/L

Give 8 clinical signs of hypokalaemia:

1) Lethargy

2) Respiratory failure

3) Ventricular arrythmias

4) Decreased tendon reflexes

5) Cardiac arrest

6) Tubular resistance to ADH

7) Weakness and paralysis

8) ECG changes

Give 4 ECG changes seen in hypokalaemia:

1) Flattened T waves

2) ST segment depression

3) Prolonged QT interval

4) Tall U wave

What is the U wave of an ECG?

Repolarisation of the purkinje fibres (usually seen after T wave)

Give 4 causes of hypokalaemia:

1) Insulin therapy

2) Alkalosis

3) Diuretic therapy

4) Vomiting and diarrhoea

When replenishing low K+, what other mineral is important to consider?

Magnesium

What are the 2 steps to treat hypokalaemia?

1) Place patient on a heart monitor

2) Direct K+ replacement therapy (oral or IV if severe)