Prognostic utility of serum potassium in chronic digoxin toxicity: a case-control study.

OBJECTIVE: In contrast to patients with acute digoxin overdose, the prognostic utility of the serum potassium concentration for patients with chronic digoxin toxicity is unclear. In such patients, we aimed to evaluate the relationship between pre-treatment serum potassium and survival.
METHODS: This was a case-control study at an urban Poison Control Center affiliated with a large urban medical center. We compared the serum potassium concentration between patients with chronic digoxin toxicity resulting in fatality (cases) over a 7-year period (2000-2006) versus survivors (controls) over a 1-year period (2007-2008).
RESULTS: During the study period, there were 13 fatalities (cases) and 13 survivors (controls), of whom seven cases and five controls received appropriately dosed digoxin-specific antibody Fab fragments (Fab). There were no statistically significant differences between cases and controls with respect to serum digoxin concentration, creatinine, age, or sex. Serum potassium elevation pre-Fab was significantly associated with fatality both in mean difference (p < 0.03) and using a dichotomous cutoff of 5.0 mEq/L (p < 0.001), which performed with 92% sensitivity (95% CI 67, 99). In 86% of deaths despite appropriate Fab administration, the clinical presentation included the combination of bradycardia plus hyperkalemia.
CONCLUSION: In these patients with chronic digoxin toxicity, elevated serum potassium was associated with fatality. The combination of bradycardia and hyperkalemia strongly predicted fatality even in cases with appropriate Fab administration.