2004 OPEN FORUM Abstracts
CALCULATING BLOOD FLOW THROUGH THE HEMODIALYSIS FILTER DURING EXTRACORPOREAL MEMBRANE OXYGENATION (ECMO).
Joseph G. Dwyer BS, RRT, Gary G.
Oldenburg BA, RRT. The Johns Hopkins Hospital, Baltimore Maryland.
Introduction: The use of continuous arterial to venous hemofiltration (CAVH) has become routine in the treatment of ECMO patients. The placement of the filtration cartridge in the system employed at Johns Hopkins Hospital originates from the outflow side of the membrane oxygenator and inserts on the venous side of the ECMO pump prior to the venous reservoir. This placement creates a shunt of blood flow that is not reflected in the calculated cardiac output support suggested by the pump flow rates displayed on the ECMO pump system. This method evaluation demonstrates that blood flow through the CAVH shunt is directly proportional to the relative pressure generated in the ECMO system (particularly post membrane pressure [PMP]) and can be described in a ratio that allows for correction in the working model for the flow necessary to deliver desired cardiac output ranges to the patient.
Equipment: A Medtronic Cardiovascular ¼” tubing circuit with a .8 meter squared membrane oxygenator was used. This system was placed on a Stockert SIII ECMO delivery system using an occlusive roller head with the Stockert SIII computerized control desk for pressure monitoring. A Transonics Systems HT110 bypass flow meter was placed on the outflow side of the CAVH cartridge to capture measurements as the blood passed through the shunt.
Methods: A Stockert SIII system was assembled with the Medtronic custom tubing pack manufactured for The Johns Hopkins Hospital. The tubing was then primed through the crystalloid phase. The Transonic HT110 flow-measuring device was then applied to the outflow side of the CAVH cartridge to determine measurements flowing through the CAVH shunt. A restrictive devise was applied to the tubing entering the priming bag to simulate changes in patient hemodynamic values. The restrictive device was adjusted to alter the PMP within the system in increments of five (5) mmHg to achieve measurements of flow through the CAVH cartridge. The pump flow ranges reviewed started at 1000 cc/min and decreased to 100 cc/min in increments of 100 cc/min with the restrictive device set to record measurements beginning with the ambient baseline pressure exerted by the set flow, and with increasing restriction resulting in pressure increases of 5, 10, and 15 mmHg in each of the ten prior flow ranges mentioned.
Results: Increases in PMP within the system resulted in proportional increases of flow through the CAVH cartridge. Not only were the CAVH flows consistent within each pump flow range, but the PMPs that exhibited multiple times in other pump flow ranges returned similar CAVH flow values. The values comparing PMP and CAVH flow are linear in nature, and suggest a direct correlation that requires further investigation.
Conclusions: Averaging the values of the pressure measurement within the pump system post membrane (PMP) divided by the measured flow rate of fluid through the CAVH cartridge leads to the following calculation. Further investigation on blood-primed pumps working in vivo is warranted.
PMP / CAVH flow = Filter Correction (FC)
Σ (FC) / n = ratio for flow correction