This technique (Union, §t..aL 1997) employs the dye dilution principle using lithium chloride as the indicator. This is injected as a 0.15- to 0.3-ml bolus into a central vein. Arterial plasma [Li+] levels are measured by a lithium-selective electrode in a flow-through cell connected by a three-way tap to a standard arterial cannula. The blood sampling rate is 4 ml/min. The voltage across the electrode membrane is related to plasma [Li +]. A correction is applied for plasma sodium concentration as this can cause interference. Cardiac output is computed from the lithium concentration-time curve using the following equation:
where the LiCl dose is in millimoles and Area is the integral of the primary curve (mmol/s). The packed cell volume (PCV) is a necessary correction as lithium is only distributed in plasma and not in erythrocytes. The primary curve is due to the initial circulation of lithium following injection. The integral of the primary curve is obtained from an equation deriving the integral of a lognormal from the first part of the curve. The secondary curve, which is due to recirculation of the indicator, is ignored. Early studies have shown good agreement with cardiac output measured by both continuous and bolus thermodilution techniques.
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