IN-VITRO EVALUATION OF A NEW LITHIUM DILUTION METHOD OF MEASURING CARDIAC-OUTPUT AND SHUNT FRACTION IN PATIENTS UNDERGOING VENOVENOUS EXTRACORPOREAL MEMBRANE-OXYGENATION

Objective: To evaluate, in vitro, a method of measuring cardiac output and shunt fraction during venovenous extracorporeal membrane oxygenat ion (ECMO). Design: Experimental study using an in vitro model. Settin g: A teaching hospital. Model: An ECMO circuit was set up in parallel with a patient circuit consisting of tubing through which saline was c irculated from a 50-L reservoir by a pump which was set at 3 L/min to represent cardiac output. A second pump in the ECMO circuit drew salin e from the patient circuit and passed it through a membrane oxygenator . The flow from the membrane oxygenator either returned directly to th e patient circuit or was diverted, via a third pump, back into the ECM O circuit, thereby producing a shunt. Interventions: By adjusting the flow rates of the second (ECMO) and third (shunt) pumps, three shunt f ractions of 12%, 25%, and 50% were produced at three different ECMO fl ow rates. Lithium chloride (0.15 mmol) was injected just downstream of the membrane oxygenator; the lithium ion concentration-time curves we re recorded simultaneously in the flow returning to the saline reservo ir and in the flow just upstream of the membrane oxygenator using lith ium selective electrodes. Measurements and Main Results: Nine pairs of curves were recorded, one pair for each combination of ECMO and shunt flow rates. Analysis of these curves allowed shunt flow and ''cardiac output'' to be calculated and compared with the flow rates delivered by the pumps. Mean ''cardiac output'' derived from the lithium dilutio n curves was 2.98 +/- 0.18 (SD) L/min, compared with a delivered pump flow of 3 L/min. Measured shunt flow = 0.008 + 1.09 x actual shunt flo w (R = 0.997). Conclusions: This method would allow cardiac output and shunt flow to be measured in patients undergoing venovenous ECMO. It could result in better patient management and improved cannula design.