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.