Cardiac surgery patients at higher risk

June 15, 1999

ANN ARBOR, Mich.-- A new study shows that patients undergoing cardiac surgery were twice as likely to experience certain complications when their pre-surgical potassium levels were below accepted standards. The study is published in the June 16, 1999, issue of The Journal of the American Medical Association.

A group of researchers from leading medical institutions in the United States 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 (CPR) also doubled.

Investigators concluded that screening and replenishing a patient's serum potassium is a low-risk, low-cost intervention that should be considered on a case by case basis for all cardiac surgery cases.

"We hope these results will change the impression that mild decreases in potassium are within normal limits," says Joyce Wahr, M.D., principal investigator and associate professor of anesthesiology in the University of Michigan Health System. "There is a consequence and we should have a heightened awareness that it could be harmful to patients with heart disease."

The large, multi-center study is the first to identify a specific low potassium level prior to surgery that is significantly associated with perioperative arrhythmias---arrhythmias during and after cardiac surgery---and to validate an association between low serum potassium levels and the risk of adverse outcomes in cardiac patients.

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.

Investigators studied 2,402 patients that underwent cardiac surgery at 24 U.S. medical centers from 1991 to 1993. They studied serum potassium levels prior to surgery in relation to perioperative arrhythmias, the need for CPR, cardiac death and death due to any cause prior to discharge.

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 CPR 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.

Wahr says the next critical step is to conduct a clinical trial that studies whether replenishing a person's potassium before cardiac surgery alters their outcome.

The study authors point out that, until there is definitive clinical evidence showing that replenishing potassium actually affects outcomes, doctors should consider this option on an individual case basis. There are other factors to consider. Replenishment generally can take a week and there are often concerns in delaying a needed surgery. The procedure has to be done slowly so as not to disrupt the ratio of potassium outside the cell to that inside---which would alter the cell polarity.

Wahr has this advice in the wake of the study. "Be wary of low potassium levels. They're not as safe as you might think."

The study---Preoperative Serum Potassium Levels and Perioperative Outcomes in Cardiac Surgery Patients---was conducted by a group of researchers comprising the Multicenter Study of Perioperative Ischemia Research Group and was funded by the Ischemia Research and Education Foundation of San Francisco.

University of Michigan

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