Tests
1st tests to order
ECG
Test
A 12-lead ECG should be performed in all patients who present with suspected digoxin toxicity or exposure.
Sinus tachycardia, supraventricular tachycardia, and atrial fibrillation with rapid ventricular response are usually not seen with digoxin toxicity. The ECG may show the characteristic scooped ST segment in therapeutic doses.
Result
premature ventricular contractions, bidirectional ventricular tachycardia, atrial tachycardia with variable or slow ventricular response, accelerated junctional rhythms
digoxin serum concentration
Test
Measured for every patient suspected of digoxin exposure or toxicity, but is only truly reflective after distribution is complete (4-6 hours after the last dose). Levels are drawn immediately on presentation, but often a second level will be required, as levels drawn too early are not reflective of distributed levels.
There is no exact serum digoxin concentration that is predictive of chronic toxicity. There are a number of factors that can affect a patient's susceptibility to digoxin (e.g., hypokalemia, volume status, comorbidities, age, and chronic disease).[22]
Patients not taking digoxin may have measurable digoxin levels that result from high circulating amounts of endogenous digoxin-like substances (EDLS). Conditions such as pregnancy, renal failure, and hypothermia are associated with EDLS; however, levels rarely exceed 0.2 nanograms/mL in these patients.
Also, spironolactone and nondigoxin cardioactive corticosteroids such as digitoxin and oleandrin can cause false-positive elevations of digoxin levels. Additionally, Asian, Siberian, and American ginseng are known to interfere with serum digoxin measurements using fluorescence polarization technology.[25]
Result
over the therapeutic range of 0.5 to 0.9 nanograms/mL in patients taking digoxin for heart failure
serum potassium level
Test
Required for every patient suspected of digoxin toxicity.
Hyperkalemia: serum potassium concentrations are important as a marker for prognosis in acute digoxin toxicity, and also reflect the severity of the toxicity. Patients with hyperkalemia (>5.0 mEq/L) are more at risk of digoxin toxicity and of dysrhythmias. However, acute digoxin toxicity can also cause hyperkalemia by inhibiting the Na+/K+ ATPase pump. Other causes of hyperkalemia include hemolysis, renal failure, and use of ACE inhibitors.
Hypokalemia: chronic digoxin toxicity does not cause hypokalemia, but digoxin toxicity is worsened by hypokalemia. Patients with therapeutic serum concentrations of digoxin can develop symptoms of digoxin toxicity because hypokalemia acts synergistically with digoxin's mechanism of action.[22]
Result
hyperkalemia (>5.0 mEq/L) or hypokalemia (<3.0 to 3.5 mEq/L)
serum magnesium level
Test
Cardiac manifestations of chronic digoxin toxicity are worsened by hypomagnesemia.[22]
Result
may be low
serum creatinine and BUN
Test
Renal function and volume status affect clearance.
Result
may be elevated
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