Transpulmonary lithium dilution technique: time to recalibration and calibration drift

Introduction

We have previously demonstrated that an average of at least two curves is necessary to improve the calibration of the lithium dilution technique of the LiDCO™plus. The precision of the new calibration process is able to detect a least significant change (LSC) of 17% [1]. The primary aim of this study was to evaluate the drift after an initial calibration with two lithium dilution curves. The second aim of the study was to evaluate the relationship between the magnitude of the time to recalibration and the magnitude of the drift.

Methods

Patients requiring monitoring with the LiDCO™plus received an initial calibration plus a second calibration when clinically indicated. Data were downloaded from devices and analysed using the LiDCO™viewPRO v.1 program. Absent, abandoned or rejected calibration curves were excluded. Calibration factors from the first and second calibrations were compared. All recalibrations in which the drift was higher than the LSC (17%) were considered useful calibrations. Regression analysis for the time to recalibration and drift was performed. Receiver operating characteristic curve analysis was performed for the time free of calibration and the usefulness of calibrations (drift >17%).

Results

A total of 45 files from 45 patients with two calibration points were identified. Patient mean age was 62 (39 to 88) years, mean height 1.69 (1.40 to 1.94) metres and mean weight 70.9 (40 to 155) kg. Time to recalibration varied from 5 to 39 hours. In 23 (51%) patients the drift was lower than 17% (unnecessary calibration). In 22 (49%) patients the drift was higher than 17% (necessary calibration).

Regression analysis did not show any significant relationship between the time to recalibration and drift. This was confirmed by receiver operating characteristic curve analysis.

Conclusion

The new implemented calibration process (using the average of two curves) does not show any drift depending on time. Despite this, 51% of the recalibrations are necessary (drift >17%). Further data analysis is necessary to identify when to recalibrate.