A study done on a small population of AMI patients by researchers points to the fact that Serum potassium at 3.5 and 4.5 mEq/L was better than the recommended
4.0 and 5.0 mEq/L.

Team@CMHF

Research suggests that the serum potassium levels in patients admitted to hospital with acute myocardial infarction (AMI) should be maintained within a lower range than is currently recommended by current guidelines.
Generally, the normal range for serum (blood) potassium is considered to be 3.5-5.0 mEq/L. However, in patients with AMI, clinical practice guidelines recommend maintaining a potassium value greater than 4.0 mEq/L (usually 4.0-5.0 mEq/L) in an effort to avoid ventricular arrhythmias

The findings suggest that AMI in patients who have a post-admission serum potassium level between 3.5 and 4.5 mEq/L are less likely to die than those with higher or lower potassium levels. However, current guidelines endorse maintaining serum potassium levels between 4.0 and 5.0 mEq/L or, in some cases, 4.5 and 5.5 mEq/L, The Emory (Emory University School of Medicine) study suggests that hospital mortality rates are twice as high in AMI patients with potassium levels of 4.5-5.0 mEq/L (the “upper third” of what is considered a “normal” potassium range of 3.5-5.0 mEq/L) compared to patients with potassium levels of 3.5 to <4.5 mEq/L (“lower and middle thirds” of the “normal” potassium range). At the same time, ventricular fibrillation / cardiac arrest rates did not increase significantly until potassium levels decreased below 3.0 mEq/L.

They conducted a retrospective cohort study of 38,689 patients with biomarker-confirmed AMI who were admitted to hospital between January 2000 and December 2008. In-hospital serum potassium measurements were taken for all patients.

The risk for mortality doubled for patients with post-admission potassium of 4.5 to less than 5.0 mEq/L, at 10%, and was even greater at higher potassium levels
Of note, mortality was also significantly higher among patients with a potassium level of less than 3.5 mEq/L, at 11%, than the reference group.

Of the 38,689 patients with AMI, 1707 (4.4%) experienced an episode of ventricular fibrillation, ventricular flutter, or cardiac arrest during hospitalization.

“Maintaining serum potassium levels between 3.5 and 4.5 mEq/L may be more advisable than the 4.0 to 5.0 mEq/L range currently recommended by practice guidelines in patients with AMI,” write the authors.
Mr Goyal from Emory Study“Although our findings are observational, they suggest that it might be reasonable to amend the guidelines to recommend maintaining a potassium range between 3.5-4.5 mEq/L in most AMI patients.”

“These [current] guidelines are based on small, older studies that focused only on ventricular arrhythmias (and not mortality) and were conducted before the routine use of beta blockers, reperfusion therapy, and early invasive management in AMI patients.”

The authors point out that their findings apply solely to patients with AMI and cannot be extrapolated to patients with other cardiac conditions, including heart failure..

In a related commentary, editorialists Benjamin Scirica and David Morrow from the Harvard Medical School in Boston write: “It remains clinically reasonable to avoid significant hypokalemia (<3.5 mEq/L) in patients post-AMI, particularly with significant, sustained, ventricular ectopy or other high-risk features.”

Ref: http://www.incirculation.net/NewsItem/Serum-potassium-should-be-lower-than-current-guide.aspx
http://shared.web.emory.edu/whsc/news/releases/2012/01/guidelines-for-potassium-levels-after-heart-attack-re-evaluated-in-emory-study.html