Bioreactance and Apelin in the Management of Severe Hyponatremia

Hyponatremia is a severe electrolyte disturbance associated with substantial morbidity and mortality. It often poses a diagnostic and therapeutic challenge. Accurate assessment of patient fluid-volume status is central to effective management. This pilot study aimed to evaluate the usefulness of the Cheetah NICOM bioreactance system and apelin in early differentiation between hypo- and euvolemia in patients with severe hyponatremia. Methods: Patients > 50 years of age with a serum sodium ≤ 125 mmol/L were eligible for inclusion after written informed consent. Blood- and urine analyses of cardiovascular load (NT-proBNP), osmotic stress (copeptin, apelin, osmolality, sodium), mineralocorticoid status (aldosterone, renin) and sympathetic activity (methoxycathecholamines) were analysed at baseline and after isotonic sodium chloride infusion. Bedside bioreactance examination was used to visualise parameters, including stroke volume before and after passive leg raise test. Classification of volume status was made retrospectively blinded for biomarker and bioreactance results. Results: 8 patients (4 hypovolemic and 4 euvolemic), 79 years old, median plasma sodium 120 mmol/L were included. At the Emergency Department volume status was misclassified in all hypo- and in 2 of 4 euvolemic patients. Apelin was significantly higher in hypovolemic patients ((299 vs. 175 ng/ml), p = 0.021). All hypovolemic, but none of the euvolemic, patients had a level above 250 ng/ml. Copeptin did not differ between groups. All patients in the hypovolemic group increased their stroke volume after passive leg raise. Conclusions: Apelin seems to be a promising future biomarker in the early management of severe hyponatremia. Bioreactance measurements may offer a supplement to bedside evaluation of volume status.