Further clarification is needed on the acronyms PAC's and PVC's as each has multiple definitions. If you meant current heart issues and/or potential heart issues, the FDA warns against taking Metformin as it could lead to fatal congestive heart failure.
I reread my post and it isn't clear. The FDA warning not to take Metformin is for someone who currently has heart issues. No evidence a healthy person would develop heart issues by taking Metformin.