The present study is being designed to study the impact and predictive nature of perioperative hyponatremia on post operative outcomes in patients undergoing Major HPB surgeries for cancers. Major hepatobiliary-pancreatic surgeries will be defined as follows- * Pancreaticoduodenectomy, * Hemihepatectomy or greater with or without caudate lobectomy * Extrahepatic bile duct resection * Extended cholecystectomy * Distal Pancreatectomy with or without splenectomy The primary Objective of the study would be to compare post operative major morbidities and early mortality (\<90 day) in major HPB surgeries done for cancer patients with and without peri-operative hyponatremia. (\<135mmoL/L). Development of systemic complications and long term outcomes (overall survival and disease free survival) will also be evaluated. It will be an observational study consisting of both, a retrospective arm and a prospective arm. All consecutive patients undergoing major HPB surgery from 2010 till 30th June 2024 will be included in the study.