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Hypokalemia refers to the condition in which the concentration of potassium in the blood is low. The prefix hypo- means low (contrast with hyper-, meaning high). Kal refers to kalium, the Neo-Latin for potassium, and -emia means “in the blood.”
Normal serum potassium levels are between 3.5 to 5.0 mEq[1]; at least 95% of the body’s potassium is found inside cells, with the remainder in the blood. This concentration gradient is maintained principally by the Na+/K+-ATPase pump.
Potassium is essential for many body functions, including muscle and nerve activity. The electrochemical gradient of potassium between the intracellular and extracellular space is essential for nerve function; in particular, potassium is needed to repolarize the cell membrane to a resting state after an action potential has passed. Decreased potassium levels in the extracellular space will cause hyperpolarization of the resting membrane potential. This hyperpolarization is caused by the effect of the altered potassium gradient on resting membrane potential as defined by the Goldman equation. As a result, a greater than normal stimulus is required for depolarization of the membrane in order to initiate an action potential.
In certain conditions, this will make cells less excitable. However, in the heart, it causes myocytes to become hyperexcitable. Lower membrane potentials in the atrium may cause arrhythmias because of more complete recovery from sodium-channel inactivation, making the triggering of an action potential more likely. In addition, the reduced extracellular potassium (paradoxically) inhibits the activity of the IKr potassium current[2] and delays ventricular repolarization. This delayed repolarization may promote reentrant arrythmias.
Hypokalemia can result from one or more of the following medical conditions:
Mild hypokalemia is often without symptoms, although it may cause a small elevation of blood pressure,[4] and can occasionally provoke cardiac arrhythmias. Moderate hypokalemia, with serum potassium concentrations of 2.5-3 mEq/L, may cause muscular weakness, myalgia, and muscle cramps (owing to disturbed function of the skeletal muscles), and constipation (from disturbed function of smooth muscles). With more severe hypokalemia, flaccid paralysis, hyporeflexia, and tetany may result. There are reports of rhabdomyolysis occurring with profound hypokalemia with serum potassium levels less than 2 mEq/L. Respiratory depression from severe impairment of skeletal muscle function is not uncommon.
Some electrocardiographic (ECG) findings associated with hypokalemia are flattened T waves, increased U waves, ST segment depression, and prolongation of the QT interval. The prolonged QT interval may lead to arrhythmias.
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