Situation: A nurse on the pediatric ward cares for children with congenital and developmental conditions and supports their families. A 5-month-old with a large ventricular septal defect and heart failure takes digoxin and furosemide. Before the morning digoxin dose, the nurse notes: Apical pulse (1 full minute): 84/min Vomited twice since last night; feeding poorly Serum potassium: 3.1 mmol/L What is the MOST likely explanation?

Vomiting, poor feeding, and bradycardia are early signs of digoxin toxicity in children, and a pulse of 84/min is below the usual infant hold limit of about 90–110/min. Furosemide has lowered the potassium to 3.1 mmol/L, and hypokalemia increases digoxin toxicity. The nurse holds the dose and notifies the physician.

Core mechanism
A large ventricular septal defect creates a left-to-right shunt that increases pulmonary blood flow and left ventricular volume work. When heart failure develops, the infant is commonly treated with digoxin for contractility and furosemide for congestion. The problem is that these two drugs interact in a dangerous way: furosemide promotes renal potassium loss, and the resulting hypokalemia sensitizes the myocardium to digoxin. Digoxin binds and inhibits the Na⁺/K⁺-ATPase pump on the myocyte membrane; when extracellular potassium is already low, less potassium is available to compete with digoxin at that binding site, so digoxin’s effect is amplified even if the serum digoxin level is not dramatically elevated ^[1][2]. This is why a potassium of 3.1 mmol/L is a red flag rather than a minor laboratory finding.

Interpreting the infant’s findings
The apical pulse of 84/min after a full minute is below the usual infant hold threshold of about 90–110/min. In a 5-month-old receiving digoxin, this relative bradycardia is not simply the expected therapeutic slowing of the heart rate. Digoxin does slow the sinus rate through enhanced vagal tone, but a rate below the hold parameter in a symptomatic infant must be read as possible toxicity. The vomiting and poor feeding are also important because gastrointestinal symptoms are among the earliest and most common manifestations of digoxin toxicity in children. When bradycardia, vomiting, and hypokalemia appear together, the clinical picture points strongly toward digoxin toxicity made more likely by low potassium.

Hypokalemia does not merely accompany digoxin toxicity; it actively lowers the threshold at which toxicity occurs. The expert consensus emphasizes that digoxin has a narrow therapeutic window and that nonspecific signs such as nausea, vomiting, and bradycardia create diagnostic uncertainty ^[1]. The case report reinforces that toxicity can develop even when the serum digoxin concentration appears acceptable, especially when a drug interaction or electrolyte disturbance alters digoxin’s pharmacodynamics ^[2]. In this infant, furosemide-induced potassium depletion is the key interaction that shifts the risk.

Why the other options are less likely

Option	Clinical reasoning
1. Viral gastroenteritis unrelated to the medicines	Gastroenteritis can cause vomiting and poor feeding, but it does not explain the bradycardia or the hypokalemia in a child taking furosemide. The combination of findings is better explained by a single drug-related mechanism.
3. Expected slowing of the heart rate by digoxin	Digoxin does slow the heart rate, but a rate below the hold parameter in a symptomatic infant is not an acceptable therapeutic response. The vomiting and low potassium make this interpretation unsafe.
4. Worsening heart failure that needs an extra digoxin dose	Worsening heart failure in an infant typically presents with tachycardia, tachypnea, diaphoresis, and poor feeding, not bradycardia. Giving additional digoxin would increase toxicity risk, especially with hypokalemia.

Watch out! The pulse must be counted for a full minute before digoxin administration in infants. If the apical pulse is below the hold parameter, the dose is withheld and the prescriber is notified. Do not substitute a radial or brachial pulse in an infant; the apical pulse is the standard.

Key point! Furosemide causes potassium loss, and hypokalemia potentiates digoxin toxicity. A low potassium level should be corrected and the digoxin dose held when signs of toxicity are present. The priority nursing action is to hold the digoxin, continue monitoring, and notify the physician rather than giving the scheduled dose or an extra dose.

The safety decision is to withhold the morning digoxin dose because the infant has bradycardia, gastrointestinal symptoms, and hypokalemia—a triad that signals increased digoxin toxicity risk. The potassium level of 3.1 mmol/L is below the normal range and represents a modifiable factor that has made the myocardium more vulnerable to digoxin’s effects ^[1][2].