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Potassium and magnesium distribution, ECG changes, and ventricular ectopic beats during beta 2-adrenergic stimulation
C Tveskov1, M S Djurhuus, N A Klitgaard
1Department of Cardiology, Odense University Hospital, Denmark.
Objective:
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.
Design:
Randomized double-blind, placebo-controlled crossover. Subjects received either placebo or terbutaline (bolus, 0.25 mg; maintenance dose, 5 micrograms/min).
Setting:
University Department of Clinical Chemistry.
Participants:
Ten healthy male volunteers. Mean age was 24.1 (range, 20 to 31) years.
Main Outcome Measures:
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.
Main Results:
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.
Conclusions:
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.