Introduction: The critically ill patient in cardiorespiratory failure will usually arrive at an ECLS center in a post resuscitation status, with the fluid balance status unknown. Once the patient is placed on bypass, meticulous care is required to titrate the volume of fluids infused, such that an appropriate fluid balance is maintained. The silicone membrane of ECLS is also a source of fluid loss. This “insensible” water that is lost out of the gas port cannot be easily quantified or entered into the total fluid balance equation. With patients remaining on ECLS for longer periods of time, knowledge of the actual amount of water lost from the membrane will enhance fluid calculations and permit more accurate tracking of fluid intake to the patient's needs. We are, for the first time, reporting measured and calculated insensible fluid loss from the silicone membrane of the oxygenator.

Methods: A standard ECLS circuit was connected to a non-compliant reservoir, which was then primed with normal saline. Utilizing two oxygenator membranes (0.4 and 0.8 m2), two gas flow rates (0.5 and 2.0 1/min), and two fluid flow rates (ECFR; 200 and 400 ml/min), inlet/outlet gas temperatures and the temperature of the circuit, pre and postmembrane pressures were monitored. Postmembrane gas was cooled at the outlet port to 0°C to collect the condensed water which was measured. Saline water loss from the system was replaced using a calibrated burette. The temperature of the ECLS fluid was maintained at 37°C. The duration of each experiment was 24 hours. Amounts of water loss and calculated water loss were correlated with fluid flow rate, gas flow rate, and membrane size.

Results: Amount of water loss from the silicone membrane correlated with the gas flow rate (r=0.987; p<0.0001) but was not related to the fluid flow rate, size of the membrane, or inlet gas temperature. The average loss of fluid utilizing the collection,replacement, and calculated methods, from either membrane at each liter/minute of gas flow was 52 ± 3.6, 61.9 ± 5.7, 63.2 ml/day respectively.

Conclusions: By using the water solubility coefficient at 37°C (43.9 mg of water/liter of gas), the calculated maximum amount of fluid loss is 63.2 ml/day at a gas flow rate of 1 1/min [Water loss per day = ECLS gas flow rate (liters/min.) × 63.2 g; {63.2 = 60 × 24 × 43.9/1000}]. Using replaced water loss as the standard, collected and calculated water loss methods are accurate within 15%. Knowledge of accurate fluid loss from the membrane will improve fluid management of critically ill neonates on ECLS.