Insights from Corrected Chloride Calculation in Dogs with Congestive Heart Failure

Hypochloremia associated with congestive heart failure (CHF) in dogs is likely multifactorial. Loop diuresis causes 1:2 sodium (Na+):chloride (Cl-) loss, whereas free water retention (FWR) causes 1:1 Na+:Cl- dilution. We hypothesized that serum Cl- corrected for Na+ would approximate measured Cl- (mCl-) in most CHF dogs because of loop diuresis, but that Cl- would normalize after correction in some CHF dogs, indicating FWR. Dogs with preclinical heart disease served as controls (no diuretics, no FWR expected) for comparison to CHF dogs. The corrected Cl- (cCl-), mCl-, and their difference were compared between 71 preclinical and 100 CHF dogs, and between Stage C (76) and Stage D dogs (24), using the formula: cCl-= (mid-reference range Na+/measured Na+) x mCl-. Both mCl- and cCl- were lower in CHF vs. preclinical dogs, and Stage D vs. Stage C dogs (both p< 0.0001). The cCl- was higher than mCl- in CHF dogs (p=0.014), but not preclinical dogs (p=0.2). Median (10th-90th percentile) difference between cCl- and mCl- was 0.69mmol/L (-2.16 to 3.4) for CHF dogs. This difference was greater for Stage D vs. Stage C dogs (p=0.0006). A diuretic resistance indicator (uNa:uK< 1.0) was found in 5 CHF dogs with >90th percentile increase in Cl- after correction. No hypochloremic CHF dogs had normal cCl-. Serum Cl- remained low after correction in hypochloremic CHF dogs, consistent with loop diuretic effects on electrolytes; however some showed notable increases in Cl- after correction, supportive of concurrent FWR. Future study correlating cCl- to serum antidiuretic hormone concentrations is warranted.