Original Article

Determination of insensible water loss and sodium accumulation behavior from the Medtronic Nautilus Extracorporeal Membrane Oxygenation (ECMO) oxygenator: An in vitro study

¹ Department of Cardiovascular Perfusion, Children’s Wisconsin, Milwaukee, WI, USA
² Electrical, Computer and Biomedical Engineering Department, Milwaukee School of Engineering, Milwaukee, WI, USA

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 19 December 2024
Accepted: 19 May 2025

Abstract

Introduction: Fluid and electrolyte balance is closely managed in extracorporeal membrane oxygenation (ECMO) patients. Neglecting oxygenator-related insensible fluid losses can distort fluid balance and electrolyte levels. While ECMO oxygenator insensible losses are reported, they remain undefined for the Medtronic Nautilus oxygenator. Through in vitro analysis of the Nautilus, we quantified insensible water losses while concurrently observing sodium behavior. Methods: Insensible water losses were determined using a closed circuit. A 12-hour pilot run was conducted to saturate the oxygenator and determine probable water loss rates and sodium accumulation behaviors. Fluid loss and sodium measurements were made at 0, 6, and 12 h. Immediately following the pilot run, three randomly assigned sweep gas rates, 0.5, 1.0, and 1.5 L/min, were evaluated over a 24 h period and replicated in triplicate. The circuit parameters were consistent and controlled for each trial. Data were collected at 0, 12, and 24 h for visualized water loss in the reservoir. After each trial, sterile water was introduced into the circuit via syringe and recorded as replacement volume. Sodium measurements were made for three trials (0.5, 1.0, and 1.5) and collected at 0 and 24 h. Results: Using linear regression analysis, the following insensible water loss equations were produced. Visualized volume: 2.74 mL/h per 1 L/min sweep gas rate or 65.66 mL/day per 1 L/min of sweep gas rate (p < 0.001). Replacement volume: 3.02 mL/h per 1 L/min of sweep gas rate or 72.5 mL/day per 1 L/min of sweep rate (p < 0.001). Sodium accumulation was observed, but not statistically significant due to the small sample size. Conclusion: Insensible water loss in the Nautilus ECMO oxygenator increases linearly with sweep gas rate (p < 0.001), leading to sodium accumulation through evaporation. These losses and the associated risk for hypernatremia should be considered when managing a patient’s fluid and electrolyte balance on ECMO.