Hypokalemia , also spelled hypokalaemia , is a low level of potassium (K + ) in the blood serum .  Normal potassium levels are between 3.5 and 5.0 mmol/L (3.5 and 5.0 mEq/L ) with levels below 3.5 mmol/L defined as hypokalemia.   Mildly low levels do not typically cause symptoms.  Symptoms may include feeling tired , leg cramps, weakness, and constipation .  It increases the risk of an abnormal heart rhythm , which is often too slow , and can cause cardiac arrest .