80. Disorders of potassium balance. Hypo- and hyperkalaemia

Page created on December 17, 2018. Last updated on December 18, 2024 at 16:56

Potassium in the body

98% of all potassium in the body is intracellular. In the intracellular space is the concentration 140 – 160 mM, while in the extracellular space it is just 3.5 – 5.5 mM.

The serum potassium level depends in two things:

• The internal potassium balance, the balance between the intracellular and extracellular compartments
• The external potassium balance, the balance between potassium intake and potassium loss

The internal potassium balance

The internal balance depends on 6 things:

• pH
• Tonicity of the extracellular space
• Insulin
• Catecholamines
• Mineralocorticoids
• Physical activity

pH: When acidosis occurs, the body attempts to buffer the increasing plasma H⁺. One of these is the intracellular buffer, where H⁺ in the plasma is moved inside cells. To maintain electroneutrality, the cells exchange K⁺ to the plasma. The total amount og potassium ion in the body doesn’t change, but a larger fraction of it is moved to the plasma, potentially causing hyperkalaemia. Hyperkalaemia reduces the kidney’s ability to excrete ammonia, which may further worsen the acidosis. For every 0.1 unit reduction in blood pH the plasma potassium concentration increases by approximately 0.2 – 2 mM.

The opposite can occur in case of alkalosis. Hydrogen ions are buffered out of the cells, requiring cells to move K⁺ ions into the cells, potentially causing hypokalaemia.

Reciprocally, an increase in plasma K⁺ concentration (hyperkalaemia) causes cells to buffer this change by moving K⁺ ions into the cells. To maintain electroneutrality, the cells exchange H⁺ to the plasma, potentially causing acidosis. The opposite can occur in case of hypokalaemia.

Tonicity: When the extracellular space is hypertonic (due to increased sodium or glucose concentration for example) will water flow out of cells and into the EC space. Because the K⁺ level inside the cell doesn’t change will the K⁺ concentration increase, as the cells have lost water. This concentration increase causes K⁺ to flow out of the cell, potentially causing hyperkalaemia.

Insulin enhances Na⁺/K⁺ ATPase activity, causing K⁺ to enter the cells. In fact, when there is a hyperkalaemia that should be quickly normalized it is common to give insulin and glucose simultaneously. This doesn’t really fix the elevated K⁺ level in the body though, it just “hides” the potassium inside cells.

Catecholamines also enhances Na⁺/K⁺ ATPase activity, via β₂-receptors. α-receptors decrease the activity.

Mineralocorticoids contribute to the balance. I don’t know how it contributes to the internal balance (book doesn’t explain). Their effect on the external balance is much more important.

Physical activity causes K⁺ outflow from muscle cells.

The external potassium balance

The daily intake of food is ca 40 – 120 mmol K⁺. This extra potassium reaches the extracellular space and not the cells in normal cases. 90% of the lost potassium is excreted by the kidneys, the remaining through the GI tract.

The external balance depends on 5 things:

• K⁺ intake
• Mineralocorticoids
• Filtration
• pH
• GI excretion

K⁺ intake is counter-regulated by kidney and GI excretion, so even a high potassium intake isn’t dangerous (unless it’s intravenous). In end-stage renal failure, when the GFR < 5 mL/min even a few bananas can cause severe hyperkalaemia.

Mineralocorticoids play a much larger role in the external balance than the internal. Aldosterone and some of its weaker precursors enhance Na⁺/K⁺ and Na⁺/H⁺ exchange in the distal tubules and collecting duct. This causes K⁺ and H⁺ loss.

The GFR determines how much K⁺ is filtered. When it’s increased will more water and sodium reach the distal tubules, which increases Na⁺/K⁺ exchange, causing sodium to be reabsorbed and more potassium to be excreted. Increased flow rate, such as in osmotic diuresis, inhibits K⁺ reabsorption. Increased level of anions in the filtrate, like bicarbonate, increases K⁺ excretion because of electroneutrality.

pH also influences the potassium excretion. Renal K⁺ excretion decreases in acidosis and increases in alkalosis.

GI excretion only accounts for 10% of potassium loss in healthy people, but in severe renal failure may this number go up to 50% to compensate for the failing kidneys.

Hypokalaemia

Hypokalaemia is mild when between 3,5 – 3,0 mM, moderate between 2,9 – 2,5 mM, and severe below 2,5 mM.

Most important causes:

• Increased loss of potassium
  • Diuretics (loop diuretics, thiazide diuretics) – most common cause
  • Diarrhoea, vomiting
  • Hyperaldosteronism
  • Hypercortisolism
  • Primary renal tubular disorders (RTA, channelopathies)
• Potassium shift into cells
  • Alkalosis
  • Exogenous insulin
• Decreased potassium intake

Consequences of hypokalaemia:

• Development of metabolic alkalosis
• Hyperpolarized membranes (membrane potential becomes more negative)
  • Muscle weakness
  • Muscle cramps, pain
  • Cardiac arrhythmias
• ECG changes
  • Shallow or inverted T-wave
  • Prominent U-wave
  • ST depression
• Polyuria (hypokalaemia causes nephrogenic diabetes insipidus where the kidney responds less to ADH)

Treatment:

It is treated by treating the underlying cause, as well as supplying potassium orally by diet or supplement, or IV if necessary.

Hyperkalaemia

Hyperkalaemia is mild when between 5,0 – 6,0 mM, moderate between 6,1 – 6,9 mM, and severe above 7,0 mM.

Causes:

• Pseudohyperkalaemia (haemolysis during blood draw)
• Decreased loss
  • Renal insufficiency (CKD or AKI)
  • Addison disease
  • Potassium-sparing diuretics
  • RAAS inhibitors
  • NSAIDs
  • Tubulointerstitial nephritis
• K+ shift out of cells
  • Acidosis
  • Beta blocker
  • Cell lysis (tumour lysis syndrome, rhabdomyolysis, haemolysis)
  • Increased intake (only in case of renal disease)

Consequences:

• Development of metabolic acidosis
• Hypopolarized membranes (membrane potential becomes less negative)
  • Muscle weakness
  • Muscle cramps, pain
  • Cardiac arrhythmias
• ECG changes
  • Peaked T-wave
  • ST elevation
  • Wide QRS

Treatment:

Calcium gluconate infusion is used to stabilize the membrane potential, which prevents development of cardiac arrhythmias, but does not treat the underlying hyperkalaemia. Giving an infusion of insulin + glucose causes an intracellular shift of potassium and can be used in acute cases, as can giving loop diuretics with a fluid infusion. In very severe cases, dialysis is required.

Oral potassium binders are drugs given orally which bind potassium in the GI tract and prevent it from being absorbed or reabsorbed. It has a slow onset of effect and can therefore not be used to treat severe or acute hyperkalaemia, but can be used for chronic or mild/moderate hyperkalaemia.