The use of lithium dilution for measuring cardiac output and shunt fraction in patients during venovenous extracorporeal membrane oxygenation: a feasibility study in a flow model

Background

Increasing numbers of adult and pediatric patients are being treated with venovenous ECMO using double lumen catheters. One of the major limitations of these catheters is the recirculation of arterialised blood back into the ECMO circuit, rather than passing through the right heart and pulmonary circulation. This shunt fraction may amount to 50% of the total flow of the ECMO circuit and the management of these patients has been hindered by the absence of a simple clinical method for measuring cardiac output and this shunt fraction. We have previously described a method of measuring cardiac output in which lithium chloride is injected intravenously and its plasma concentration-time curve measured in arterial blood using a lithium-selective electrode [1].

Methods

The patient circulation and ECMO circuit were represented by a flow model (Fig. 1). Pump C was set at 3 1/min throughout. Pump E (ECMO) and S (shunt) were varied to provide 3 shunt fractions, (approximately 12, 25, 50%) at each of 3 ECMO flow rates (approximately 0.5, 1.0 and 2.0 l/min). Following a bolus injection of lithium chloride (0.15 mmol), its dilution curves were recorded simultaneously by sensors A and B (see Fig. 1). The flows through pump S and pump C were calculated from the 9 pairs of lithium dilution curves and compared to the actual flows delivered.

Results

This method would be suitable for adult and pediatric patients (in whom a smaller dose of lithium would be given). Although cardiac output could be measured during ECMO if the lithium was injected into the pulmonary artery, the proposed method avoids pulmonary artery catheterisation and allows shunt flows to be calculated.

Figure 1

Diagram of the flow model. Pump C representing the cardiac output was set up to draw 3 l/min from the 50 l bucket of saline. Some of this flow passed via the ECMO pump (E) through the membrane oxygenator (M) and then either recirculated through the ECMO circuit via the shunt pump (S) or returned to the bucket. LiCI injections were made just downstream of the oxygenator and dilution curves were recorded simultaneously by the sensors A and B.

Table Results for the 9 injections (l/min)