Hypokalemia is a medical condition characterized by abnormally low levels of potassium in the blood, typically defined as a serum potassium concentration below 3.5 millimoles per liter (mmol/L). Potassium is an electrolyte, a mineral that carries an electrical charge in bodily fluids. It is important for the proper functioning of cells, nerves, and muscles throughout the body, including nerve impulse transmission and heart contractions.
The Body’s Potassium Balance
The human body maintains a tightly regulated balance of potassium. Over 90% resides inside cells, with a smaller portion in extracellular fluid and blood plasma. This uneven distribution creates an electrochemical gradient across cell membranes, maintained by the sodium-potassium ATPase pump. This pump actively transports sodium out and potassium into the cell, maintaining the cell’s resting membrane potential.
The kidneys are the primary organs maintaining the body’s total potassium content by adjusting urinary excretion to match dietary intake. They achieve this through filtration, reabsorption, and secretion within the renal tubules. Most filtered potassium is reabsorbed in the proximal tubule and loop of Henle, while regulated secretion of excess potassium occurs in the distal tubule and collecting duct. Hormones like aldosterone, secreted by the adrenal glands, also influence this balance by stimulating potassium excretion and sodium retention.
Mechanisms Leading to Low Potassium
Hypokalemia can develop through several mechanisms, including inadequate potassium intake, excessive potassium loss, or a shift of potassium from the extracellular space into cells. Inadequate intake, though uncommon as a sole cause due to renal conservation, can contribute, especially with prolonged fasting or a poor diet.
Excessive potassium loss is a common mechanism, occurring through the gastrointestinal tract or kidneys. Gastrointestinal losses often result from chronic diarrhea, excessive laxative use, or protracted vomiting. Vomiting, for instance, leads to volume depletion and metabolic alkalosis, which increase renal potassium excretion as kidneys enhance secretion under secondary hyperaldosteronism.
Renal potassium wasting is another frequent cause, often induced by medications like loop and thiazide diuretics. These diuretics increase sodium delivery to the collecting duct, promoting potassium secretion. Certain kidney disorders, including renal tubular acidosis and magnesium deficiency, can also lead to increased renal potassium excretion. Magnesium deficiency impairs ROMK function, contributing to potassium wasting.
Potassium can also shift from extracellular fluid into cells, causing a temporary reduction in blood potassium levels. This transcellular shift can be influenced by metabolic alkalosis, where hydrogen ions move out of cells in exchange for potassium moving in. Insulin administration can also drive potassium into cells, as can certain medications like beta-2 adrenergic agonists (e.g., albuterol). Rare genetic disorders like hypokalemic periodic paralysis are characterized by episodes of muscle weakness or paralysis due to such intracellular shifts.
How Hypokalemia Impacts Body Functions
Low potassium levels can disrupt normal cellular functions, leading to symptoms across various organ systems. The ratio of potassium inside to outside cells is important for generating action potentials, electrical signals necessary for nerve and muscle function. When extracellular potassium decreases, the cell membrane becomes hyperpolarized, meaning it is more negative inside, requiring a stronger stimulus to generate an action potential.
In the muscular system, hypokalemia commonly manifests as weakness and fatigue, ranging from mild to severe. As potassium levels decrease, muscle cramps, twitches, and even paralysis can occur. Severe hypokalemia can lead to rhabdomyolysis (muscle breakdown) and may affect respiratory muscles, potentially causing respiratory failure.
The cardiovascular system is significantly impacted by low potassium. Hypokalemia can alter the heart’s electrical properties, predisposing individuals to cardiac arrhythmias. ECG changes, such as ST segment depression, T wave flattening or inversion, and prominent U waves, typically appear when serum potassium falls below 3.0 mmol/L. These changes can lead to premature ventricular and atrial contractions, and in severe cases, life-threatening ventricular tachycardia or fibrillation.
In the nervous system, while often asymptomatic in mild cases, severe hypokalemia can contribute to altered mental status, including psychosis, delirium, and hallucinations. Reduced neuronal excitability due to hyperpolarization can impact overall neurological function. The renal system can also be affected, as persistent hypokalemia may impair the kidneys’ ability to concentrate urine, leading to increased urination and excessive thirst. Long-term low potassium can also cause structural and functional kidney changes, including altered sodium reabsorption and increased bicarbonate absorption.