Background and Aims PECS block is an established regional block for patients undergoing mastectomy(1).

Methods A 38 year-old woman was scheduled for total mastectomy and sentinel lymph node biopsy for breast cancer and was consented for a PECS II block and a general anaesthetic. She was on ramipril and bisoprolol for a decline in LV function following chemotherapy. She reported no known allergies.

Results Anaesthesia was induced with fentanyl 100mcg, propofol 200mg and rocuronium 30 mg iv for facilitation of endotracheal intubation. Following induction an ultrasound-guided PECS II block was carried out with no complications. Thirty mls of l-bupivacaine were used in total. Shortly after amoxicillin/clavulanic acid was given iv and the surgeon injected blue dye. Five minutes after skin incision the patient’s blood pressure dropped to 50/20 mmHg. No rush was noted. 100% oxygen was given. Allergy to the antibiotic or the local anaesthetic was suspected and the patient was treated as such. After a total of 450mcg of adrenaline the patient was stabilized. A decision was made to proceed with the procedure. At the end she was transferred to ICU and extubated the same night. She attended an allergy clinic two weeks later where allergy to the blue dye was diagnosed.

Conclusions Blue dye allergy is a rare but potentially devastating adverse effect of blue dye injection for breast surgery and can present without the typical signs of oedema, urticaria and bronchospasm(2). Even though intra-operative allergic reactions are caused mostly by muscle relaxants (70%), latex(10%) and antibiotics, blue dye should always be considered(2,3).