Disorders of potassium balance

Zhao Chenghai

Pathophysiology

Outline

• Potassium balance

• Disorders of potassium balance– Hypokalemia– Hyperkalemia

Potassium balance

• Distribution of potassium

• Gains and losses of potassium

• Mechanisms of regulation

• Functions of potassium

Potassium balance

Distribution of potassium

• The intracellular concentration of potassium ranges from 140 to 150 mmol/L.

• The extracellular concentration of potassium (3.5-5.0mmol/L) is considerable less.

Gains and losses

• Food is the main source of potassium intake.

• The kidneys are the main source of potassium elimination.

Mechanisms of regulation

• Renal regulation

• Transcellular shift between the intracellular and extracellular compartments

Renal regulation

• Secretion of potassium by distal and collecting tubules.

• Aldosterone --- a sodium-potassium exchange system.

Sodium is transported back into blood. Potassium is secreted into tubular filtrate. Mineralocorticoid hormone

Plasma potassium levels control aldosterone secretion by adrenal gland.

Transcellular shifts• Sodium-potassium ATPase

– Both insulin and epinephrine increase the activity of sodium-potassium pump.

(An increase in potassium level stimulates insulin release. --- a feedback mechanism)

• Potassium channels – ECF osmolality↑→H2O leaves cell→ ICF K+↑→ K+ mov

es out of cell through K+ channels →ECF K+

– Exercise

• Potassium-hydrogen exchange to maintain electrical neutrality– In acidosis– In alkalosis

Functions of potassium

• Maintain the osmotic integrity of cells– Osmotic pressure in ICF

• Maintain acid-base balance– Through potassium-hydrogen exchange

• Contribute to the reactions that take place in cells– Transform carbohydrates into energy– Convert amino acid to protein– Change glucose into glycogen

• Play a critical role in the excitability of skeletal, cardiac, and smooth muscle.

Resting membrane potential (RMP)

RMP≈-59.5lg[K+]i/[K+]e

Excitability of muscle cells can be affected by the distance between RMP and threshold potential.

Hypokalemia

Hypokalemia refers to a decrease in plasma potassium level below 3.5 mmol/L.

Causes of hypokalemia• Inadequate intake

– inability to obtain or ingest food– Diet deficient in potassium

• Excessive renal, gastrointestinal and skin losses

– Diuretic therapy (thiazide and loop diuretics)– Increased aldosterone level (primary aldosteronism,

stress-cortisol)– burn, sweating increase, vomiting and diarrhea

• Transcellular shift– Administration of insulin (to treat diabetic ketoacido

sis)– β-adrenergic agonist----albuterol (bronchodilator)– Alkalosis

Manifestations of hypokalemia• Neuromuscular manifestations

– Muscle flabbiness, weakness and fatigue– Muscle cramps and tenderness – Paresthesia and paralysis

• Impaired kidney’s ability to concentrate the urine – polyuria, urine with low osmolality, polydipsia (ECF o

smolality↑)

• Gastrointestinal manifestations – Anorexia, nausea, vomitting, – Constipation, abdominal distension, paralytic ileus

• Cardiovascular manifestations– Arrhythmias, increased sensitivity to digitalis toxicity

• Metabolic alkalosis

ECG changes in hypokalemia

• Depression of the ST segment

• Flattening of the T wave

• Appearance of a prominent U wave

• Prolongation of PR interval

Treatment of hypokalemia

• Increasing the intake of foods high in potassium content

• Oral potassium supplements

• Giving potassium intravenously when rapid replacement is needed.– Only if the renal function is adequate

Hyperkalemia

• Hyperkalemia refers to an increase in plasma levels of potassium in excess of 5.0mmol/L.

Causes of hyperkalemia• Decreased renal elimination

– Decreased renal function-renal failure– Treatment with potassium-sparing diuretics – Decreased aldosterone level

• Adrenal insufficiency (addison’s disease)• Treatment with ACEI• Angiotensin II receptor blocker

• Excessively rapid administration

• Movement of potassium from the intracellular to extracellular compartment– Tissue injury such as burns and crushing injuries– Extreme exercise or seizures– Acidosis

Manifestations of hyperkalemia

• Gastrointestinal manifestations – Anorexia, nausea, vomitting, intestinal cra

mps, diarrhea

• Cardiovascular manifestations– Ventricular fibrillation and cardiac arrest

• Neuromuscular manifestations– Paresthesias– Weakness– Muscle cramps

ECG changes in hyperkalemia

• Appearance a peaked T wave

• Widening of the QRS complex

• Prolongation of the PR interval

• Disappearance of the P wave

Treatment of hyperkalemia

• Decreasing intake or absorption of potasssium.

• Using calcium to antagonize the potassium.

• Using insulin and glucose

• Increasing potassium excretion – hemodialysis– peritoneal dialysis

Case1• A 40-year-old man with advanced acquired immu

nodeficiency syndrome (AIDS) presents with an acute chest infection. Investigation confirm a diagnosis of P.carinii pneumonia. Although he is treated appropriately, his serum sodium level is 118mmol/L. Tests of adrenal function are normal.

• What type of disorders happened to this man?• What is the likely cause of this electrolyte distur

bance?

Case 2

• A 70-year-old woman who is taking furosemide (a loop diuretic) for congestive heart failure complains of weakness, fatigue, and cramping of the muscles in her legs. Her serum potassium is 2.0mmol/L, and her serum sodium is 140mmol/L. She also complains that she notices a “strange heart beat” at times.

• What is the likely cause of this woman’s symptoms?

• What would be the treatment for this woman?

Case 3• A 76-year-old woman was brought to the

hospital because she was lethargic and refused to drink fluid. Her blood pressure is 100/60 mmHg. Serum sodium level is 170mmol/L, potassium level is 4.3mmol/L.

• What kind of electrolyte disturbance happened to this woman?

• What is the cause of this kind of disorder?• What is the most severe outcome of this

disorder?