A 35-year-old female patient with rheumatic heart disease (RHD) and severe mitral stenosis (MS) presents with sudden palpitations, dyspnea, and coughing up pink frothy sputum. Examination reveals BP 90/70 mmHg, orthopnea, bilateral rales, heart rate 155 bpm, irregular rhythm with variable intensity of first heart sound. Intravenous digoxin 0.4mg is administered. The primary purpose is to: