Inclusion Criteria:  Diagnosis of HFrEF (LVEF≤40%) Implanted cardiac implantable electronic device with at least 3 months of recorded arrhythmic history Patient is scheduled to receive IV ferric carboxymaltose to treat diagnosed iron deficiency  Exclusion Criteria:  Myocardial infarction, acute heart failure or life-threatening arrhythmias in the preceding 15 days Autoimmune disorders, cancer or other diseases other than heart failure that significantly affect patients' life expectancy, appetite and emotional status Known allergic reaction to ferric carboxymaltose.