Cardiac surgery risks higher when potassium levels are low

Joyce Wahr with her research assistant, Dalai Zhou.

Patients undergoing cardiac surgery are twice as likely to experience certain complications when their pre-surgical potassium levels are below accepted standards, according to a study published in the June 16, 1999, issue of The Journal of the American Medical Association.

Researchers from leading medical institutions in the United States, including Joyce Wahr, M.D., associate professor of anesthesiology in the U-M Medical School, examined the potassium levels of more than 2,400 patients undergoing cardiac surgery. They found that arrhythmias, including atrial fibrillation, during and after surgery doubled when a patient’s serum potassium level fell below 3.5 millimoles per liter (mmol/L). At levels below 3.3 mmol/L, the need for cardiopulmonary resuscitation also doubled.

“We hope these results will change the impression that mild decreases in potassium are within normal limits,” says Wahr.

Potassium is essential to maintaining a normal heart rhythm, and is responsible for the conduction of nerve impulses and muscle contraction. The ratio of potassium outside the cell to that inside the cell maintains polarity, allowing an electrical charge to conduct along a row of cells, causing the heart to beat. Hypokalemia results when the level of potassium in a person’s blood becomes too low. Hypokalemia is usually caused by gastrointestinal or renal problems or prolonged treatment with certain prescribed medications. Both low and high potassium levels can cause problems for the heart because the electrical charge is affected.

Wahr and her team found that patients with potassium levels between 3.5 and 5.0 mmol/L showed no association between potassium and the incidence of any perioperative arrhythmias. However, when levels fell below 3.5 mmol/L, the association became progressively stronger and even occurred at levels often considered safe by anesthesiologists, surgeons and other clinicians. When levels dropped below 3.3 mmol/L or rose above 5.2 mmol/L there was an association with the need for cardiopulmonary resuscitation due to cardiac arrest.

Wahr says they don’t know if low potassium levels are a cause of adverse outcomes or a marker, pointing to some other cause. She and colleagues feel, however, that the results of the study provide strong evidence for using a potassium level of 3.5 mmol/L as a standard criterion for preoperative hypokalemia.

The study was funded by the Ischemia Research and Education Foundation of San Francisco. Wahr can be contacted at jwahr@umich.edu