Defining the Cause of Post-Operative Hyponatremia in the Orthopedic Patient

Background: Post-operative hyponatremia occurs after 30% of orthopedic surgeries, increasing morbidity, mortality and hospital length of stays and hospital costs. The cause of the hyponatremia can be varied, hard to diagnose and impact management. The goal of this study was to determine the causes of post-operative orthopedic hyponatremia and to evaluate the accuracy with which nephrologists and internists interpret the data. Methods: This was a retrospective chart review of patients >21 years old on the adult total joint service who developed postoperative hyponatremia. A hyponatremic order set was developed and patient fluid status was charted by the presence or absence of edema in non-surgical extremities. The patients were treated by their managing physicians. After one year, data on 51 patients were assembled and sent to three nephrologists and three internists to analyze and diagnose the etiology of the hyponatremia. Results: The most common causes of post-operative hyponatremia were hypovolemia (33.7%), the syndrome of inappropriate antidiuretic hormone, SIADH (32.4%), hypotonic fluid (8.2%), acute kidney injury (5.2%) and medications (5.9%). The interrater agreement, measured by kappa coefficient, was moderate (0.43; 95% CI 0.34, 0.53) for the nephrologists and fair (0.38; 95% CI 0.30, 0.46) for the internists. Conclusions: The majority of post-operative hyponatremia following total joint surgery in adults is from hypovolemia and SIADH. The treatment for these is very different: the first requires fluid resuscitation and the latter, free water restriction. Due to an interplay of peri-operative factors, the diagnosis can be difficult for both internists as well as nephrologists.