A 50-year-old man with small-bowel fistula has been receiving TPN for the previous 3 weeks through a single-lumen central venous catheter. He is scheduled for exploratory laparotomy and closure of fistula. On the morning of the day of surgery, TPN is discontinued and intra-venous infusion with balanced salt solution (Ringer's lactate) is staed. An hour later, the patient is found to be anxious, sweating, and tachycardic. What is the most likely cause?