Extracorporeal Membrane Oxygenation (ECMO) is a vital life-support therapy for critically ill patients, particularly neonates and infants, who are vulnerable to significant fluid and electrolyte shifts. This vulnerability is amplified by their small circulating volumes in relation to ECMO circuit priming volumes. In managing these patients, precise fluid balance monitoring is essential. However, a key factor often overlooked in fluid calculations is the insensible water loss (IWL) from ECMO oxygenators due to sweep gas flow.
The present study by Whittaker Striker and Kong investigates the Medtronic Nautilus ECMO oxygenator to determine the rate of IWL and examine associated sodium accumulation behavior. Conducted in a controlled in vitro environment, the research focused on clinical conditions mimicking neonatal ECMO with sweep gas rates of 0.5, 1.0, and 1.5 L/min and a constant circuit flow rate of 500 mL/min.
Why This Study Matters
Accurate fluid balance assessment can directly impact clinical decisions. Overestimating fluid volume can lead to inappropriate diuretic use or renal replacement therapy, while underestimating it might result in fluid overload—a known contributor to ECMO morbidity and mortality. Moreover, evaporative losses may lead to sodium concentration increases in the circuit, potentially causing hypernatremia with neurologic and renal implications.
Study Design and Methods
A 12-hour pilot study using a 5 L/min sweep gas rate revealed a higher-than-expected fluid loss, saturating the oxygenator prematurely. This confirmed the need for shorter sweep gas trials and initiated the 24-hour main experiments. Nine trials were conducted (three per sweep gas rate), and fluid loss was measured using two methods:
- Visual reservoir level decline
- Syringe-based replacement volume post-trial
Sodium accumulation was analyzed pre- and post-trial in three cases using an ABL90 FLEX PLUS analyzer.
Key Findings
- Linear Relationship: A strong linear correlation was found between sweep gas rate and fluid loss (R² > 97% for both measurement methods).
- Calculated Insensible Water Loss:
- Visual method: ~65.66 mL/day per 1 L/min sweep gas
- Replacement volume method: 72.5 mL/day per 1 L/min sweep gas (statistically significant, p < 0.001)
- Sodium Accumulation:
- All three tested circuits showed elevated sodium concentrations at 24 hours, with one exceeding the analyzer’s upper detection limit (>170 mmol/L).
- Though the sample was too small for statistical analysis, this trend supports the hypothesis that evaporative loss concentrates sodium in the circuit fluid.
Mechanism Behind Water Loss
PMP membranes, like that used in the Nautilus oxygenator, facilitate water vapor loss through activated diffusion. Water from the fluid-side dissolves into a non-porous surface layer, migrates through micropores, and evaporates on the gaseous side. This process is influenced by temperature and sweep gas rate. Fluid loss generates negative pressure in the closed circuit, reinforcing the mechanical impact of evaporation over time.
Clinical Relevance
These findings reinforce the need to incorporate IWL from ECMO oxygenators into routine fluid balance assessments, especially in neonatal and pediatric populations. Given that hypernatremia is linked to neurologic injury, kidney dysfunction, and prolonged ICU stays, tracking both fluid loss and electrolyte changes is essential.
The sweep gas rates used (0.5–1.5 L/min) reflect real-world neonatal ECMO settings, while the circuit flow rate of 500 mL/min aligns with the manufacturer’s minimum recommended blood flow. With insensible losses estimated at ~72 mL/day per 1 L/min sweep gas, a neonate on ECMO with higher sweep requirements could lose over 100 mL/day—a significant volume in a small child.
Limitations
- Small sample size for sodium testing
- Subjective visual volume assessments (despite using graduated reservoirs)
- Use of saline instead of blood in the circuit may affect the generalizability of water and electrolyte loss rates
- No direct evidence confirming that sodium does not cross the membrane (though condensation showed no sodium content)
Conclusions
The study demonstrates that the Medtronic Nautilus ECMO oxygenator has a predictable, linear insensible water loss rate of ~72.5 mL/day per 1 L/min sweep gas flow. In tandem, sodium accumulation due to evaporation presents a risk for hypernatremia. These findings urge clinicians to:
- Account for oxygenator-related IWL when managing ECMO fluid balance
- Consider sodium concentration shifts when determining replacement fluid types
- Avoid unnecessary interventions based on falsely positive fluid balances
In sum, the integration of oxygenator IWL into daily fluid and electrolyte management protocols can enhance care quality and potentially reduce complications in ECMO patients.
