Potassium and Magnesium Distribution, ECG Changes, and Ventricular Ectopic Beats During β2-Adrenergic Stimulation With Terbutaline in Healthy Subjects

Abstract

To study the effect of intravenous (i.v.) terbutaline on potassium (K) and magnesium (Mg) distribution, ECG changes, and prevalence of ventricular ectopic beats in healthy subjects. Randomized double-blind, placebo-controlled crossover. Subjects received either placebo or terbutaline (bolus, 0.25 mg; maintenance dose, 5 micrograms/min). University Department of Clinical Chemistry. Ten healthy male volunteers. Mean age was 24.1 (range, 20 to 31) years. Serum potassium and magnesium muscle potassium and magnesium, and muscle sodium-potassium pump number. Urinary excretion of potassium and magnesium. ECG changes (T-wave and QTC interval) and the number of ventricular ectopic beats. Terbutaline produced an immediate decrease in serum potassium level from 4.17 (4.04 to 4.30) mmol/L to a nadir of 3.32 (3.06 to 3.58) mmol/L (p < 0.001). The urinary excretion of potassium decreased from 0.077 mmol/min (0.052 to 0.102) to 0.038 mmol/min (0.025 to 0.051) (p < 0.01). There was an increase in the number of sodium potassium pumps from 1,104.1 nmol/kg dry weight (1,030.6 to 1,177.5) to 1,273.3 nmol/kg dry weight (1,193.5 to 1,353.2) (p < 0.01), but no measurable change in muscle potassium. The QTC interval increased from 395 (385 to 405)ms to 449 (432 to 466) ms (p < 0.003). There was no change in the number of ventricular ectopic beats. Short-term i.v. administration of terbutaline produced hypokalemia partly due to an increase in the number of sodium-potassium pumps. Furthermore, terbutaline induced changes in ECG with a highly significant lengthening of the QTc interval but with an unchanged number of ventricular ectopic beats in healthy subjects.