Situation: A 72-year-old woman with heart failure with reduced ejection fraction (left ventricular ejection fraction 30%) is admitted to the medical ward for worsening shortness of breath and leg edema. Her home medications are furosemide 40 mg twice daily, digoxin 0.125 mg daily, carvedilol 6.25 mg twice daily, and spironolactone 25 mg daily. She weighs 64 kg on admission. By day 3 her crackles and leg edema have decreased and she has lost 2.5 kg. She now reports loss of appetite and nausea since yesterday. Her findings today are: Apical pulse: 56/min, irregular Serum potassium: 3.1 mEq/L (3.1 mmol/L) (normal 3.5–5.0) Serum creatinine: 1.1 mg/dL (97 µmol/L) Serum digoxin: 1.4 ng/mL (1.8 nmol/L), drawn 8 hours after the last dose Which conclusion is MOST accurate?

Anorexia, nausea, and a slow irregular pulse are the typical early picture of digoxin toxicity. Her potassium of 3.1 mEq/L, lowered as she diuresed on furosemide, increases the risk of toxicity, so a level below 2.0 ng/mL does not rule it out; the level is also above the 0.5–0.9 ng/mL range commonly targeted in heart failure. Her falling weight, edema, and crackles argue against worsening congestion.

Core conclusion

The most accurate interpretation is digoxin toxicity promoted by hypokalemia. Anorexia and nausea are classic early gastrointestinal signs of digitalis intoxication, and the apical pulse of 56/min, irregular points to slowed, disordered conduction rather than simple beta-blocker effect. The serum digoxin level of 1.4 ng/mL is below the traditional 2.0 ng/mL toxicity cutoff, but that cutoff is not a safe boundary when potassium is low.

Hypokalemia lowers the threshold for digoxin toxicity even when the serum digoxin concentration appears only mildly elevated or “therapeutic.” Digoxin binds to the extracellular portion of the Na⁺/K⁺-ATPase pump. Potassium competes with digoxin for this binding site. When serum potassium falls to 3.1 mEq/L, less potassium is available to occupy the pump, so more digoxin binds, producing a functional toxicity that the serum drug level alone does not capture.

Watch out! A digoxin level below 2.0 ng/mL does not exclude toxicity. Toxicity is a clinical and electrolyte-dependent diagnosis, not a number-dependent one.

Why the other options are less accurate

Option 1: Gut congestion from worsening right-sided heart failure is not supported by the trajectory. The patient lost 2.5 kg, crackles decreased, and leg edema improved. These are signs of effective diuresis and reduced congestion, not worsening right heart failure. Gastrointestinal congestion severe enough to cause anorexia and nausea would typically accompany persistent or worsening edema, weight gain, and jugular venous distension.

Option 2: Expected slowing from carvedilol is only partially true. Carvedilol can slow the heart rate, but a rate of 56/min with new anorexia and nausea in a patient taking digoxin should raise concern for digoxin toxicity rather than be dismissed as expected beta-blocker effect. Beta-blockers and digoxin also have additive effects on AV conduction, which can worsen bradyarrhythmias when digoxin effect is enhanced by hypokalemia.

Option 4: No digoxin toxicity because the level is below 2.0 ng/mL reflects an outdated, oversimplified view. In heart failure, the therapeutic target is much lower than the old toxicity threshold. The DECISION trial used a target serum digoxin concentration of 0.5–0.9 ng/mL in patients with reduced or mildly reduced ejection fraction . A level of 1.4 ng/mL is above that target range, and in the setting of hypokalemia it can be toxic even though it is below 2.0 ng/mL.

Pathophysiology and clinical reasoning

Digoxin exerts its therapeutic and toxic effects primarily through inhibition of the Na⁺/K⁺-ATPase pump on cardiac myocytes. Inhibition raises intracellular sodium, which reduces calcium extrusion through the Na⁺/Ca²⁺ exchanger, increasing intracellular calcium and contractility. However, excessive pump inhibition also raises resting membrane potential toward threshold, increasing automaticity and predisposing to ectopic beats, while simultaneously slowing conduction through the AV node.

Hypokalemia intensifies this effect because potassium and digoxin compete for the same binding site on the Na⁺/K⁺-ATPase pump. When extracellular potassium is low, digoxin binds more avidly, magnifying both the proarrhythmic and conduction-slowing effects. This is why a patient can develop clinical toxicity with a serum digoxin level that would otherwise be considered acceptable.

The patient’s furosemide use explains the potassium loss. Loop diuretics increase sodium delivery to the distal nephron, promoting potassium secretion. As she diuresed and lost 2.5 kg, she also lost potassium, dropping to 3.1 mEq/L. This electrolyte shift occurred while digoxin and carvedilol remained on board, creating the substrate for toxicity.

The clinical picture fits the early toxicity pattern described in case reports. Gastrointestinal symptoms such as nausea and anorexia are frequent early manifestations of digoxin toxicity ^[3]. Bradycardia and conduction abnormalities are also common, and the combination of digoxin with a beta-blocker can produce clinically significant bradyarrhythmias, particularly in older adults ^[1]. In one case, an elderly patient on digoxin and a beta-blocker developed complete AV block, illustrating how the two drug classes can interact to suppress conduction ^[1]. Another report described bradycardia and hypokalemia as key features confirming digoxin toxicity in an older adult ^[2].

Clinical application for the nursing process

Assessment findings that should trigger concern for digoxin toxicity include new anorexia, nausea, vomiting, visual disturbances, and a slow or irregular pulse. The nurse should check the apical pulse for a full minute before administering digoxin and hold the dose for a rate below 60/min unless otherwise ordered. Serum potassium, renal function, and digoxin levels should be reviewed together, not in isolation.

Factor	Finding	Interpretation
Serum digoxin	1.4 ng/mL	Above HF target of 0.5–0.9 ng/mL; not necessarily safe
Serum potassium	3.1 mEq/L	Hypokalemia potentiates digoxin binding and toxicity
Apical pulse	56/min, irregular	Bradyarrhythmia consistent with enhanced digoxin effect
Weight and lung findings	2.5 kg loss, decreased crackles	Improved congestion; argues against worsening right heart failure

Key point! When a patient on digoxin and a loop diuretic develops nausea, anorexia, and bradycardia, evaluate potassium before concluding the digoxin level is safe. Hypokalemia can cause digoxin toxicity at levels below the traditional 2.0 ng/mL threshold.