IN-VITRO EVALUATION OF A NEW LITHIUM DILUTION METHOD OF MEASURING CARDIAC-OUTPUT AND SHUNT FRACTION IN PATIENTS UNDERGOING VENOVENOUS EXTRACORPOREAL MEMBRANE-OXYGENATION
R. Linton et al., IN-VITRO EVALUATION OF A NEW LITHIUM DILUTION METHOD OF MEASURING CARDIAC-OUTPUT AND SHUNT FRACTION IN PATIENTS UNDERGOING VENOVENOUS EXTRACORPOREAL MEMBRANE-OXYGENATION, Critical care medicine, 26(1), 1998, pp. 174-177
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.