Serum potassium and myocardial infarction

Extracellular (serum) potassium concentration is normally maintained within the approximate reference range of 3.5-5.2 mmol/L; this is important for normal cardiac function. Both reduced serum potassium (hypokalemia) and increased serum potassium (hyperkalemia) can, if sufficiently severe, be associated with potentially lethal cardiac arrhythmia. 

A recent study sought to determine the impact of plasma potassium concentration for patient survival following myocardial infarction. Current guidelines emphasize the importance of avoiding hypokalemia, advising that patients diagnosed with myocardial infarction should be given potassium supplements, if necessary, to maintain serum potassium in the range of 4.0-5.0 mmol/L. 

Some recommend a higher target of 4.5-5.5 mmol/L. The study, which was conceived to test the validity of this expert advice, involved retrieval of the medical records of 38,689 patients admitted with acute myocardial infarction (AMI) to 67 US hospitals for the period 2000-2008. Each of these patients was assigned to one of seven groups, depending on their mean serum potassium concentration for the duration of hospital stay. 

The serum potassium of the seven groups were: <3.0 mmol/L; 3.0 to <3.5 mmol/L; 3.5 to <4.0 mmol/L; 4.0 to <4.5; 4.5 to <5.0 mmol/L; 5.0 to <5.5 mmol/L; and >5.5 mmol/L. Overall, 2265 of the 38,689 (5.8 %) patients did not survive their myocardial infarction and died in hospital. 

A U-shaped relationship between mean serum potassium concentration and mortality rate was observed; highest mortality rates – 46.2 % and 61.4 % – were evident in those with serum potassium <3.0 and those with serum potassium >5.5 mmol/L, respectively. The more significant finding was that lowest mortality (4.9 %) was evident in the group whose serum potassium was in the range of 3.5-4.5 mmol/L. 

This allowed the authors of the study – which is the largest ever to examine the issue – to conclude that the target serum potassium range should be 3.5-4.5 mmol/L rather than the currently recommended 4.0-5.0 mmol/L. Apparently, aggressive use of potassium supplements might do more harm than good for the great majority of patients recovering from myocardial infarction.