Spot urinary electrolytes are associated with decongestion and duration of hospitalization in dogs with acute left-sided congestive heart failure secondary to myxomatous mitral valve disease

Objective: To determine whether urinary electrolyte indices measured after IV furosemide administration are associated with clinical outcomes in dogs with left-sided acute congestive heart failure (CHF) secondary to myxomatous mitral valve disease (MMVD). Methods: In this multicenter retrospective study, client-owned dogs with acute left-sided CHF caused by MMVD presenting to 3 referral hospitals between January 2024 and December 2025 were evaluated. All dogs received a single IV bolus of furosemide (2 mg/kg) at presentation. Urine samples were collected from the first spontaneous urination within 2 hours after furosemide administration. Urinary sodium concentration, urinary sodium-to-potassium ratio (uNa:K), and urinary sodium-to-chloride ratio were measured. Associations between urinary electrolyte indices and clinical variables were analyzed. Clinical outcomes included change in radiographic pulmonary congestion score, hospitalization duration, and survival to discharge. Results: 50 dogs met the inclusion criteria. Higher urinary sodium concentration and uNa:K were associated with greater improvement in radiographic pulmonary congestion and shorter hospitalization. Dogs that survived to discharge had a higher uNa:K than those that died during hospitalization. Urinary electrolyte indices were not significantly correlated with echocardiographic or radiographic indices of left atrial size. Conclusions: Urinary electrolyte indices measured after IV furosemide administration were associated with clinically relevant outcomes in dogs hospitalized with acute left-sided CHF secondary to MMVD. Clinical Relevance: Spot urinary electrolyte measurements may provide a simple and noninvasive tool for assessing diuretic response and clinical status in dogs with acute left-sided CHF.