Study of the sensitivity of neonates to digoxin: contribution of erythrocyte 86rubidium uptake test.

In general, there is little agreement how digoxin should be used in newborn, and the results of studies in this field seem contradictory. This study attempts a quantitative assessment of the number and the sensitivity of cellular receptors for digoxin in the organism, by the in vitro measurement of erythrocyte 86Rubidium uptake in neonates compared with adults and old people. Red blood cells are first incubated with differing concentrations of digoxin, and then incubated with 86Rb. The initial level of 86Rb uptake (Rbi) is that observed in the absence of digoxin. The 50% index of captation (IC50) is the digoxin concentration in nanograms per ml at which 86Rb uptake is half Rbi. Three groups of patients were studied: Group I: 12 neonates, less than 5 days old; Group II: 11 adults (26 to 57 years old); Group III: 9 elderly people (71 to 82 years old). Rbi was significantly lower in neonates (Mean +/- SD: 25.8% +/- 3.5, P less than 0.001) and in the elderly (29.9% +/- 3.1) than in adults (36.8% +/- 4.6). IC50 was significantly lower in the elderly (12.1 ng/ml +/- 2.4) than in the adult patients (20.5 ng/ml +/- 5.5, P less than 0.001). In the newborns, values of IC50 were widely scattered (16.2 ng/ml +/- 7.2). The authors suggest that since Rbi reflects Na+, K+-ATPase activity, this activity is diminished in newborn and old people, and indicates that they have fewer cellular receptors for digoxin than adults. In the elderly, the low IC50 would imply increased sensitivity to digoxin. In neonates, the wide range of values for IC50 suggests considerable individual variation in sensitivity to digoxin. The results are consistent with the recently recommended lower dosages of digoxin in neonates.