Predictors of hyponatremia following elective primary unilateral knee arthroplasty at a tertiary centre: A retrospective observational cohort and predictive model.

Introduction: Hyponatremia is a common electrolyte disorder. This can be associated with nausea, disorientation and in more serious cases a decreased level of consciousness or neurological deficits. These symptoms may lead to increases in the cost of hospital care and significant morbidity. The purpose of this retrospective, observational cohort study is to investigate the impact of hyponatremia on patient and systems specific outcome measures in those undergoing elective, unilateral total knee arthroplasty (TKA) at two hospitals in Auckland, New Zealand over a twelve-month period.

Materials And Methods: Patients were stratified into two groups based on the presence or absence of post-operative hyponatremia (defined as a blood sodium of <135 mmol/L with a concurrent decrease of ≥5 mmol/L between the pre- and post-operative recordings). Outcomes collected included Quality of Recovery - 15 (QOR) scores, time to assisted mobilisation, discharge ICD-10 complication codes and hospital length of stay.

Results: During the study period 236 patients underwent surgery. Eighty-six (36.4%) patients met criteria for post-operative hyponatremia. This finding was associated with prolongation of the hospital length of stay (4.17 (3.26-5.18) versus 4.28 (3.31-5.45) days, p = 0.031) and a reduction in the QOR score on the second post-operative day (113.0 (99.5-126.5) versus 105.0 (94.0-118.0), p = 0.039). There was no difference in the time to assisted mobilisation.

Conclusions: Hyponatremia is a common finding following TKA. This abnormality is associated with small changes in patient specific outcome measures. These implications of these findings may become more significant in settings where same day or rapid discharge from hospital is targeted.