HYPOKALEMIA IN PREGNANT-WOMEN TREATED WITH THE BETA(2)-MIMETIC DRUG FENOTEROL - A CONCENTRATION AND TIME-DEPENDENT EFFECT

The effect of tocolytic treatment with fenoterol on plasma potassium c oncentrations was studied in 83 pregnant women on intravenous tocolyti c therapy. Plasma concentrations of fenoterol and potassium were measu red simultaneously, the time interval between initiation of therapy an d taking the blood sample varying from 2 hours to 100 days. In a subse t of 13 patients this blood sample was taken after two hours of therap y and pretreatment potassium concentrations were measured also. Pretre atment potassium concentrations were normal in these 13 patients and d eclined to 2.88 mmol/L (median) fenoterol concentrations being 320 ng/ L through 1164 ng/L. Potassium concentrations measured later than 24 h ours after initiation of therapy were all in the normal range correspo nding fenoterol concentrations varying from 200 ng/L to 2504 ng/L. The multivariate statistical model for the description of all data showed that the duration of treatment was the only variable which explained the data to a significant extent. This might indicate that tolerance t o the potassium lowering effect of fenoterol had developed within 24 h ours after initiation of therapy. In the subset of 13 patients pretrea tment potassium concentrations were found to be more important in expl aining potassium concentrations than fenoterol concentrations at two h ours. As we did not observe any adverse events in patients with low ce ntrations were normal within 24 hours, we conclude that hypokalaemia d ue to fenoterol in the treatment of premature labor is not of clinical concern.