A 28-year-old woman presents with a 5-day history of productive cough, fever (38.5°C), and dyspnea. Chest X-ray shows bilateral interstitial infiltrates. Sputum culture and blood cultures are pending. She is started on azithromycin 500 mg on day 1, then 250 mg daily for 4 days. On day 3 of therapy, she develops palpitations and syncope. ECG shows QTc interval of 520 ms (baseline 380 ms). Which of the following is the most appropriate immediate action?