Chloride Dynamics in Heart Failure: The Clinical Implications of Serum Levels and the Emerging Significance of Urinary Chloride

Heart failure (HF) remains a prevalent, high-risk condition particularly common among the elderly population. The impairment of systolic function, secondary dysregulation of maladaptive neurohormonal systems and HF therapies lead to electrolyte disturbances. Hyponatremia is a well-studied, robust marker of adverse prognosis in HF. Serum chloride has emerged as a promising prognostic marker, accurately predicting adverse outcomes in both acute and chronic settings. Hypochloremia reflects a compound of maladaptive neurohormonal, renal and acid–base mechanisms, frequently worsened by diuretic therapy. In the context of HF, low chloride levels are independently associated with increased cardiovascular mortality, HF hospitalization and diuretic resistance. Urinary chloride (uCl−) has recently been shown to serve as a dynamic biomarker of HF severity, Renin–Angiotensin–Aldosterone-system (RAAS) activation, and poor outcomes, offering incremental prognostic value. Our review synthesizes the latest evidence on serum and urinary chloride disturbances in HF, framing key distinctions between acute versus persistent electrolyte disturbances. We also explore the interrelationship between chloride and sodium, the differential impact of hypochloremia in Heart Failure with Preserved Ejection Fraction (HFpEF) versus Heart Failure with Reduced Ejection Fraction (HFrEF), and clinical outcomes stratified by the temporary vs. persistent nature of chloride imbalance. We have also included visual flowcharts for classifying and managing hypochloremia based on the underlying etiology.