Hypokalemia

Question 1

Definition

Answer 1

Serum K+ < 3.5 mmol/L

Question 2

Causes

Answer 2

Increased excretion

• Hyperaldosteronism - Conns syndrome = primary hyperaldosteronism due to adrenal adenoma
• Drugs- thiazide diuretics, loop diuretics (inhibit NKCC2 transporter in thick ascending limb of LOH), laxatives
• Cushing’s syndrome (excess cortisol cross-reacts wit mineralocorticoid receptors resulting in excess K+ excretion)
• Renal disease: Nephrotic syndrome (kidneys leak large amounts of protein, K+ is also lost), renal tubular acidosis type 1 & type 2 (kidneys can’t excrete H+ from blood into urine, H+/K+ ATPase activity in apical membrane of alpha intercalated cells decreases)
• GI losses e.g. due to vomiting or diarrhoea

Decreased intake

• dietary deficiency

Shift of K+ from ECF → intracellular fluid

• Insulin therapy - in T1DM & severe T2DM (insulin shoves K+ into cells via Na+/K+ ATPase)
• Beta-2 agonists (Salbutamol= short acting, Salmeterol = long-acting, these also stimulate Na+/K+ pump to shove K+ into cells)
• metabolic alkalosis (body compensates for low serum [H+] by moving H+ from cells into blood in exchange for K+ from blood into cells)

Question 3

Pathophysiology

Answer 3

K+ key for maintaining resting cell membrane potential. Low K+ levels means membrsnes are hyperploarised (inside the cell is much more negative relative to outside) so it is harder for muscles & nerves to reach threshold potential and send an action potential. This can affect smooth, skeletal and cardiac muscles resulting in decreased contraction (excitability of muscles & nerves is decreased).

• Cardiac - arrhythmias and cardiac arrest
• Smooth muscle - constipation
• Skeletal muscle - weakness, cramps and flaccid paralysis
• Respiratory muscles - respiratory depression

Question 4

Presentation

Answer 4

Muscle weakness & hypotonia

Hyporeflexia (sign- muscles less responsive to stimuli)

Constipation

Cramps

Tetany (intermittent muscle spasm)

Palpitations, light headedness, arrythmias

Question 5

Investigations

Answer 5

1st line = U & E - serum K+ < 3.5 mmol/L
ECG changes:
- Prolonged PR interval (>200 ms)
- ST segment depression (everything goes down)
- flattening of T wave (everything goes down)
- Prominent U waves (due to flattening of T-waves, U waves which are positive deflections after the T wave become more prominent)

Question 6

Management

Answer 6

MILD: Usually asymptomatic (3.0-3.4mmol/L)

• Oral K+ replacement e.g. Oral Sando-K

SEVERE: <2.5mmol/L

• IV replacement 40mmol KCL in 1L 0.9% NaCl

Question 7

Complications

Answer 7

• Arrythmias
• Rhabdomyolysis (death of muscle fibres due to direct or indirect injury results in release of intracellular K+ into blood)