The LDCO system provides an indicator dilution method of measuring cardiac output. A small dose of lithium chloride is injected via a central venous catheter; the resulting arterial lithium concentration-time curve is recorded by withdrawing blood past a lithium sensor. For an overview of the system see diagram.
The sensor consists of a lithium-selective electrode in a flow-through cell (see - sensor figure). It is disposable, sterilized by gamma irradiation and foil packed. The sensor is connected to a three-way tap on the arterial line and a small peristaltic pump restricts the flow through it to 4 ml/min. The flow-through cell is made of polycarbonate and designed with an eccentric inlet so that the blood swirls past the tip of the electrode. The lithium-selective electrode is made of polyurethane with a central lumen. Silver/silver chloride paint coats the inside and the outside where shown in the diagram. A wick, which is soaked in heparinised saline when the cell is first primed, makes the electrical connection between the blood in the cell in the vicinity of the tip of the electrode and the remote reference. This arrangement ensures adequate constancy of the voltage of the reference which is far enough from the blood to avoid a significant temperature effect. The electrode is filled with a reference material which provides a constant ionic environment and supports the membrane which is dip cast. The membrane is made of polyvinyl chloride and contains a lithium ionophore to make it selectively permeable to lithium ions.
Recording the signal
The voltage across the membrane is related via the Nernst equation to the plasma [Li+]. A correction is applied for plasma sodium concentration because in the absence of lithium the baseline voltage is determined by the sodium concentration. Further details of the effect of sodium on sensor voltage and derivation of plasma lithium concentration from voltage are given (see - details). The voltage is measured using an isolated amplifier, then digitised on-line and analysed. Examples of curves from 2 patients are shown in the figure.
Derivation of cardiac output
Indicator dilution curves recorded in arterial blood consist of primary and secondary curves due to the initial circulation and then recirculation of the indicator. Cardiac output is calculated as:
where LiCl dose is in mmol; Area is the integral of the primary curve (mM.s); PCV is packed cell volume which may be calculated as haemoglobin concentration (g/dl) ÷ 34: this correction is needed because lithium is distributed in the plasma.
For further lognormal mathematics see details.
Advantages of the LDCO method
The advantages of the LDCO method are that it is safe, accurate and simple to use:
Central venous and arterial catheters are usually already in place in patients needing cardiac output measurements. No further catheter is needed, so the method avoids the risks associated with pulmonary artery catheterisation. The method requires withdrawal of approximately 3 ml blood per determination; for an adult this is an insignificant amount. The injectate is an isotonic (150 mM) solution of lithium chloride. The dose needed (0.15 -0.30 mmol for an average adult) is very small and has no known pharmacological effect. The dosage regimen recommended is very conservative, making worst case assumptions including anuria.
Simple to use
Limitations of the LDCO method
The method is contraindicated in patients treated with oral lithium, since the increased background lithium concentration causes the injected lithium to produce a smaller percentage increase in concentration. Cardiac output would therefore be overestimated.
The method should not be used during the first trimester of pregnancy.
As for all indicator dilution methods, abnormal shunts could result in erroneous cardiac output measurements. It is likely that these would be known about; right to left shunts would cause obvious distortion of the initial part of the dilution curve; a left to right shunt would result in the right ventricular output being higher than the flow into the aorta, and it is this higher output which would be measured.
The electrodes drift in the presence of high concentrations of competitive muscle relaxants.