A 48-year-old woman taking sotalol for paroxysmal atrial fibrillation has two brief episodes of collapse. The ECG monitor records recurrent bursts of polymorphic ventricular tachycardia in which the QRS complexes appear to rotate around the baseline; between episodes she is alert and her blood pressure is 112/68 mmHg. Her corrected QT interval is 560 milliseconds. Which one of the following is the most appropriate immediate treatment?