Sorting the Wheat from the Chaff: Monitoring Serum Levels after Fab Treatment in a Newborn with PJRT and Digoxin Intoxication

Objectives: We report on a male newborn with supraventricular tachycardia manifesting in gestational week 34 with hemodynamic compromise. He was delivered by Caesarean section in week 35 and remained in tachyarrhythmia (heart rates of 200–250 bpm). Chemical (adenosine) and subsequent electrical cardioversion were without lasting effect, propafenone and esmolol were equally insufficient. At this point, the patient was transferred to our unit, where ECG showed PJRT. Esmolol had to be discontinued due to hypotension and amiodarone was introduced. Still reduced myocardial contractility was then met with the addition of low-dose digoxin (DGN), aware of the known drug interaction (3 µg/kg/d). The rhythm converted to predominantly sinus rhythm with only short episodes of “breakthrough” tachycardia, susceptible to vagal maneuvers or adenosine. After several days, already on oral amiodarone, the patient developed progressive sinus bradycardia and emesis. While there were no typical ECG changes, toxicological analyses confirmed the suspicion of digoxin intoxication with serum levels reaching 15.7 nmol/L. As heart rates were undercutting 75 bpm, digoxin fragmented antibodies (FAB) were administered in a dose of 4 mg (calculated from the trough level of 4 nmol/L × kg × 0.3) and well tolerated with quick regression of symptoms. Serum DGN levels only gradually decreased and were difficult to interpret, as standard tests cannot differentiate between the active and the antibody-bound substance.

Conclusion: Intrigued by the discrepancy between the lack of typical ECG changes and symptoms suspicious for DGN intoxication, we turned to reviewing the literature. Several publications describe elevated serum DGN levels without prior digoxin treatment in neonates. This phenomenon has been attributed to the presence of Digoxin-like immunoreactive substances DLIS which are able to interfere with routine DGN level measurements and can presumably reach therapeutic ranges. DLIS and FAB may leave us in uncertainty of pharmacologically active DGN and subsequently in a therapeutic dilemma. This case report aims to highlight the intricacy of interpreting standard laboratory measurements of DGN levels in certain constellations and to stress the benefit of using free fractions for monitoring of digoxin levels in some patients and especially after FAB are administered.

Publication History

Article published online:
21 February 2021