Sir,It is generally said that sudden correction of sodium levels in cases of chronic hyponatremia can cause osmotic demyelination syndrome, and anesthetists bear this in mind during the perioperative period. However, the priming solution used in cardiopulmonary bypass in many cases is saline or a similar solution containing a high level of sodium, and thus potentially causes an unintentional sudden increase in serum sodium at the time of starting the cardiopulmonary bypass.An 87-year-old man (164 cm, 59.5 kg) was diagnosed with a ruptured right iliac artery aneurysm into a right iliac vein and was transported to our hospital by an ambulance for emergency surgery. The patient was lucid at the time of arrival. Although a blood test following anesthesia induction showed that the patient had hyponatremia (sodium level of 124 mmol/L), the time of onset was unknown. Therefore, we purposely used a saline solution and sodium-free 5% dextrose in water for transfusion to avoid causing a sudden increase in serum sodium.Because intraoperative findings also indicated a rupture of the aortic aneurysm into a vein, and the massive hemorrhage was expected, the surgeon quickly decided to repair the blood vessels under cardiopulmonary bypass. The surgeon inserted the arterial cannula into the descending aorta and the venous cannula into the inferior vena cava and started the pump. A blood test immediately after starting the cardiopulmonary bypass showed a sudden increase in serum sodium (137 mmol/L). We quickly switched the transfusion to the 5% dextrose solution. We were not able to test urinary sodium. After that, intraoperative serum sodium levels remained stable at 127–129 mmol/L until the operation was completed. The cardiopulmonary bypass was in use for 145 min, anesthesia time was 320 min, and blood loss was approximately 3400 ml. The patient was discharged from the hospital with no neurological findings.Sudden correction of sodium levels in patients with chronic hyponatremia can cause osmotic demyelination syndrome, and a slow correction of serum sodium (within 12 mmol/L/24 h) is required.[12] However, the cardiopulmonary bypass is commonly primed with a high-sodium solution such as saline (about 1200 ml) and a sodium bicarbonate solution (about 200 ml) in our hospital. When using such priming solutions, it is necessary to watch out for sudden increases in serum sodium levels.[3] Of course, the serum sodium level is influenced by urinary sodium level. In the present case, maybe we should have instructed the perfusionist to change the priming solution. However, it was not possible to give instructions regarding changing the priming solution as this case involved emergency surgery and the unexpected use of cardiopulmonary bypass. We became painfully aware of the need to pay attention to priming solutions when using cardiopulmonary bypass for a patient with hyponatremia.Written consent was obtained from the patient for publication of this report.