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