Hemodynamic collapse following labetalol administration in preeclampsia

On the 3rd day after a cesarean section, because of pre-eclampsia, blood pressure was still high, oral labetalol 100 mg with an 8-hour interval was given, followed by 50 mg i.v. administered over 10 min, twice with a 5-hour interval. The last injection was immediately followed by an atrio-ventricular tachycardia with massive decrease in blood pressure. The reason for this is discussed.