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Accuracy & Precision

Accuracy & Precision

Accuracy

Many publications cite device performance in terms of accuracy. Accuracy in this context is simply the ability to measure the actual Stroke Volume (SV) in millilitres.

The basic measurement of Stroke Distance (SD) by EDM has an accuracy on a single waveform of ±3%. The conversion of this extremely accurate SD uses a nomogram (a constant using age, weight and height) to calculate a more familiar parameter of SV.

Only Doppler has the precision to guide a 10% SV optimization protocol. EDM has the ability to detect change in sequential measurements. In this case the effect of a small fluid challenge on SD or SV. Precision is the ability to measure the same result repeatedly with minimal error.

Precision

EDM’s strength is in its precision, using Doppler to measure flow directly at the source, the decending aorta.

The precision of a technology dictates its ability to guide fluid management. The 10% SV change algorithm used to optimize SV is specific to the EDM and is evidence-based. Other technologies that are less precise may not be as effective in guiding fluid management based on this algorithm.

Prof. Singer established that the error of repeatability of measuring SD for the EDM was 3.8% [1]. For an individual patient the diameter of the aorta will be a constant thus the SV precision will be equal to the known error for SD. [SV = SD x Aortic Root Diameter (from patient nomogram)].

This precision/repeatability error can then be used to determine the least significant change in SD/SV required to ensure confidence in measuring a real hemodynamic change and not just measurement error. With its calculated error of 3.8%, the user can be 99% confident that a measured change in SD/SV of >10% is real (this is based on 99% of normally distributed data points falling within 2.5 standard deviations of the mean).

 

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1Singer, M., J. Clarke, and E.D. Bennett, Continuous hemodynamic monitoring by esophageal Doppler. Crit Care Med, 1989. 17(5): p. 447-52