A 28-year-old woman is admitted following a stab wound to the left lower chest. On examination, she is alert, blood pressure 94/60 mmHg, heart rate 115 bpm, respiratory rate 22/min, and oxygen saturation 96% on room air. Breath sounds are present bilaterally. FAST shows no free fluid in the abdomen or pericardium. A chest X-ray reveals a small left hemothorax. After establishing IV access and initiating fluid resuscitation, what is the most appropriate next step?