Lithium dilution cardiac output measurement in the critically ill patient: determination of precision of the technique
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KeywordsCardiac Output Central Venous Pressure Pulmonary Artery Catheter Transpulmonary Thermodilution Cardiac Output Measurement
Pulmonary intermittent thermodilution (from the pulmonary artery catheter), transpulmonary thermodilution (PiCCOplus; Pulsion, Munich, Germany) and transpulmonary lithium dilution (LiDCO™plus; LiDCO, Cambridge, UK) are all well-validated techniques in common use in intensive care for cardiac output estimation. The precision has been looked into previously and strategies to improve it have been made (that is, averaging three or four measurements over the respiratory cycle) yet not much is known about the precision of transpulmonary techniques in terms of repeatability. This study aims to look into the coefficient of variation (CV) of the lithium dilution technique in a mixed (medical/surgical) intensive care population and propose a method to improve its precision.
Materials and methods
We performed four consecutive lithium dilution cardiac output determinations on 70 critically ill patients requiring haemodynamic monitoring. The heart rate (HR), central venous pressure (CVP) and mean arterial pressure (mAP) were documented in conjunction with cardiac output estimation. Data were excluded if a ± 5% change in HR, CVP or mAP occurred during the sequential measurements. The CV ((SD/mean cardiac output) × 100) was calculated for single measurements and for the average of repeated measurements. In order to clinically accept the precision of the technique, we aimed to obtain a CV below 10%.
Sixty-five series were suitable for analysis. The CV showed a normal distribution and no correlation with the magnitude of the mean cardiac output. The mean CV for single lithium dilution was 12.3%. The CV for the average of n lithium dilutions was 8.6% for n = 2, 7.1% for n = 3, 6.1% for n = 4.
The CV for one lithium dilution was higher than clinically acceptable (12.3 > 10%). The average of two lithium dilution measurements improves the precision by 30% and shows an excellent CV (that is, 8.6%). When measuring cardiac output with LiDCO an average of two lithium dilution curves provide an excellent precision, and we suggest that in this population (medical/surgical) this approach should always be used when calibrating the pulse pressure algorithm (PulseCO) at the baseline.