Final answer:
The most likely cause of the patient's symptoms is pneumothorax.
Step-by-step explanation:
The most likely cause of the patient's difficulty breathing and hypoxia immediately after undergoing a sympathectomy for pancreatic cancer is Pneumothorax. Pneumothorax occurs when air leaks into the space between the lung and chest wall, causing the lung to collapse partially or completely. This can result in symptoms such as shortness of breath, chest pain, and hypoxia.