The evidence for clinical practice guidelines which recommend maintaining serum potassium levels between 4.0 and 5.0 mEq/L (mmol/L in UK) in patients with acute myocardial infarction (AMI) are based on small studies that associated low potassium levels with ventricular arrhythmias in the pre−beta-blocker and prereperfusion era.

This retrospective cohort study evaluated the relationship between serum potassium levels and in-hospital mortality in AMI patients in the era of beta-blocker and reperfusion therapy (since beta-blockers reduce the incidence of postinfarction mortality and sudden cardiac death, raise serum potassium levels and suppress hypokalaemia-mediated ventricular arrhythmias).

The cohort included 38 689 patients who were admitted to 67 US hospitals between January 2000, and December 2008. All patients had in-hospital serum potassium measurements and were categorized by mean post admission serum potassium level (<3.0, 3.0-<3.5, 3.5-<4.0, 4.0-<4.5, 4.5-<5.0, 5.0-<5.5, and ≥5.5 mEq/L).
The main outcome measure was all-cause in-hospital mortality and the composite of ventricular fibrillation or cardiac arrest.

The researchers reported the following results:

• There was a U-shaped relationship between mean post admission serum potassium level and in-hospital mortality that persisted after multivariable adjustment. Compared with the reference group of 3.5 to less than 4.0 mEq/L (mortality rate, 4.8%; 95% CI, 4.4%-5.2%), mortality was comparable for mean post admission potassium of 4.0 to less than 4.5 mEq/L (5.0%; 95% CI, 4.7%-5.3%), multivariable-adjusted odds ratio (OR), 1.19 (95% CI, 1.04-1.36).

• Mortality was twice as great for potassium of 4.5 to less than 5.0 mEq/L (10.0%; 95% CI, 9.1%-10.9%; multivariable-adjusted OR, 1.99; 95% CI, 1.68-2.36), and even greater for higher potassium strata.

• Similarly, mortality rates were higher for potassium levels of less than 3.5 mEq/L.

• In contrast, rates of ventricular fibrillation or cardiac arrest were higher only among patients with potassium levels of less than 3.0 mEq/L and at levels of 5.0 mEq/L or greater.

The researchers concluded that in patients with AMI, the lowest mortality was observed in those with post admission serum potassium levels between 3.5 and <4.5 mEq/L compared with those who had higher or lower potassium levels.

They note that their findings challenge current clinical practice guidelines that endorse maintaining serum potassium levels between 4.0 and 5.0 mEq/L and suggest that the optimal range of serum potassium levels in AMI patients may be between 3.5 and 4.5 mEq/L and that potassium levels of greater than 4.5 mEq/L are associated with increased mortality and should probably be avoided.