Spot urine sodium as a marker of urine dilution and decongestive abilities in acute heart failure

The decongestion ability in response to diuretic treatment plays a crucial role in the treatment of acute heart failure. This effectiveness is evaluated through the assessment of sodium concentration and urine volume, which are also treatment goals themselves. However, the bidirectional interconnection between these factors remains not fully understood. The objective of this study is to provide mechanistic insights into the correlation between spot urine sodium concentrations (UNa⁺) and urine dilution. This aims to better understand of the decongestive abilities in acute heart failure (AHF). The study was single-center, prospective, conducted on a group of 50 AHF patients. Each participant received a standardized furosemide dose of 1 mg per kg of body weight. Hourly diuresis was measured in the first 6 h of the study, and urine composition was assessed at predefined timepoints. The study group presented the exponential (rather than linear) pattern of relationship between UNa⁺ and 6-h urine volume, whereas relationship between eGFR and 6-h urine volume was linear (r = 0.61, p < 0.001). The relationship between UNa⁺ and all other analyzed indices of urine dilution, including the change from baseline in urine creatinine concentration, urine osmolarity, and urine osmolarity corrected for urine sodium, also exhibited an exponential relationship. Patients who were chronically exposed to furosemide demonstrated a significantly lower urine dilution (1.78 [1.18-3.54] vs 11.58 [3.9-17.88]; p < 0.001) in comparison to naïve individuals. In conclusion, it should be noted that in AHF higher UNa⁺ is associated with disproportionally higher urine dilution, and patients naïve to furosemide have significantly greater ability to dilute urine when compare to chronic furosemide users.