Approach to hypokalemia: Clinical sciences

Hypokalemia is defined as a low serum potassium level, usually below 3.5 milliequivalents per liter. Mild hypokalemia can be asymptomatic, but more severe hypokalemia can cause life-threatening symptoms like paralysis and cardiac arrhythmias. Some common causes of hypokalemia include hypomagnesemia, low potassium intake, conditions associated with transcellular shift of potassium, as well as extrarenal potassium wasting, and increased mineralocorticoid activity. Finally, keep in mind that hypokalemia can also occur due to acid base disorders.

Now, if you suspect hypokalemia, you should first perform an ABCDE assessment to determine if your patient is unstable or stable. If the patient is unstable, stabilize the airway, breathing, and circulation. Next, obtain IV access and put your patient on cardiac telemetry. This is important because extreme drops in serum potassium can lead to dangerous cardiac arrhythmias, such as Torsades de Pointes and ventricular fibrillation. You should also monitor vital signs and provide supplemental oxygen, if needed.

Now that we're done with unstable patients, let’s go back to the ABCDE assessment and discuss the stable ones. If your patient is stable, first obtain a focused history and physical examination. Also obtain labs, including a comprehensive metabolic panel, and check a 12 lead ECG. History typically reveals weakness, muscle cramps, or in extreme cases, even ascending paralysis. Also, there might be a history of diuretic or laxative use, while others may report severe diarrhea. On the other hand, physical examination typically reveals an irregular pulse and decreased deep tendon reflexes; while lab results show a serum potassium level below 3.5 milliequivalents per liter.

Here’s a clinical pearl to keep in mind! Pseudohypokalemia refers to the falsely low potassium level, often caused by a blood sample being left in a warm environment several hours before processing. This causes the potassium in the sample to shift from the extracellular space into the intracellular space, which gives a false impression of decreased potassium levels. Additionally, leukemia can cause pseudohypokalemia due to excessive potassium uptake by cancerous cells. In addition to lab tests, check the ECG, which may reveal broad flat T waves, ST segment depression, and U waves. Other findings include a prolonged PR interval or a prolonged QT interval. At this point, you can diagnose hypokalemia!

Now, here’s a high-yield fact! If severe hypokalemia results in ECG changes or cardiac arrhythmia, immediately administer an IV infusion of potassium to normalize the potassium level and stabilize the heart rhythm. However, if Torsades de Pointes is present, give IV magnesium first, as this rhythm has a high likelihood of developing into ventricular fibrillation.

Ok, once you confirm that your patient has hypokalemia, your next step is to order labs to check serum magnesium levels. If it’s below reference range, diagnose hypokalemia due to hypomagnesemia.

Here’s another clinical pearl! Hypomagnesemia can exacerbate hypokalemia by accelerating renal potassium wasting and impairing its reuptake. So, to restore potassium levels effectively, start with magnesium replenishment.

On the other hand, if the serum magnesium is not below the reference range, consider decreased potassium intake as the cause of hypokalemia. Low dietary intake of potassium can occur with profound malnutrition, alcohol overuse, or pica, which is the ingestion of inorganic material such as clay. If the history reveals low potassium intake, you can diagnose hypokalemia due to low potassium intake. In the general population, hypokalemia from low dietary intake of potassium is uncommon, but in hospitalized patients who can't eat, it’s fairly common and can develop quickly, so watch out for this!

Now, let’s say your patient’s history reveals a normal potassium intake, then consider conditions associated with transcellular shift of potassium, which actually refers to the movement of potassium from the extracellular fluid into intracellular space! Certain medications are known to cause a transcellular shift of potassium, such as beta agonists, theophylline, and insulin. So if your patient is taking any of these drugs, you can diagnose medication-induced hypokalemia.

On the flip side, another cause of transcellular potassium shift is hyperthyroidism. Thyroid hormones increase sodium-potassium pump activity, causing an increase in cellular potassium uptake and subsequent hypokalemia. So, if your patient has a known history or symptoms suggestive of hyperthyroidism, such as weight loss, heat intolerance, or palpitations, diagnose hypokalemia due to hyperthyroidism.