TESTING IN A MODEL
A model has been developed for producing indicator dilution curves in order to test the equations for deriving the areas under the curves. It is important that such a model produces dilution curves which are lognormal in shape (like human indicator dilution curves) with similar values of sigma.
The model consists of a Perspex block drilled to form a series of swirl chambers, each connected to the next by a short length of silicon rubber tubing. The inlet is connected to a header tank which supplies saline at constant pressure.
The fluid enters each chamber tangentially close to the base. This imparts a swirl which is accelerated as the fluid leaves via the conical outlet at the top of each chamber and promotes mixing. The outlet pipe from the block discharges into a collection tank from which the saline is pumped back up to the header tank at a rate high enough to ensure that this is continuously discharging through an overflow pipe and so maintains a constant head. Fluid flowing through the outlet pipe of the Perspex block is sampled via a cannula at 4 ml/min past a lithium sensor. Lithium chloride is injected into the inlet pipe as a bolus. The flow through the circuit can be altered by means of a clamp on the inlet side and can be checked on the outlet side by timed collection into a measuring cylinder. The degree of skewness of the distribution curves can be altered by varying the number of swirl chambers in circuit. The figure shows an example of a curve with a value of sigma of 0.3.
The accuracy with which the method estimates the actual flows in the model is shown in the figure. There were 120 estimations and a flow range of 9:1.
LABORATORY STUDY USING LDCO TO MEASURE FLOWS IN AN ECMO CIRCUIT
In vitro evaluation of a lithium dilution method of measuring cardiac output and shunt fraction in patients undergoing venovenous extracorporeal membrane oxygenation.
Linton R, Turtle M, Band D, O'Brien T, Jonas M. 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 1998; 26: 174-177.
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