How is "Clinical Accuracy" defined?

Figure 1

• It is well known that blood pressure (BP) varies at different locations due to the reflection at the peripheral vessels and its different local potentiation. Pulse pressure - the difference between systolic and diastolic blood pressure - is smaller in arteries close to the heart like the A. brachialis (1) and increases with distance to the heart (see Figure 1).

• CNAP^®'s finger BP (3) is calibrated to NBP values obtained from the A. brachialis (1)

• IBP is typically measured at the A. radialis (2) or A. femuralis (not shown)

• As can be seen from Figure 1, even with 100% accurate devices (reference IBP, CNAP^® or NBP) the results would differ from each other. Thus, a comparison demonstrates the difference between IBP and NBP - but both are "gold standards".

• The limitations of invasive measurement are best illustrated by citing the FDA standard "ANSI Sp10" [1]:
  • There is almost universal agreement that “true” blood pressure is best determined using a reliable, calibrated transducer directly in an artery. 
  • There is also no disagreement that the direct intra-arterial measurement is fraught with its own set of problems, including variability with radial position, the effects of flow-velocity changes, and the frequency response of the transducer and amplifier.

• NBP is an intermittent, indirect measurement of systolic and diastolic blood pressure. In automated devices, the oscillometric method is the most common technique. Difficulties are well known to all physicians.

Figure 2

Figure 3

Accuracy of absolute values and BP changes

The "ANSI Sp10" standard describes the inherent physiological differences between IBP and NBP and according to a meta-analysis the proposed differences are:

systolic BP < 13.4 mmHg

diastolic BP < 18 mmHg

It is suggested that the beat-to-beat accuracy of systolic, mean and diastolic BP of CNAP^® in comparison to IBP lies within the same range.

As these inherent differences are not a very expressive quality criteria, Ilies et al. have defined a new quality criteria for continuous BP methods - the interchangeability criteria using "percentage error < 18.4 %" [2]

Accuracy of BP rhythms and pulse waves

For the comparison of rhythms and pulse wave other mathematical methods must be performed and and other criteria have to be defined. For clinical evaluation, the comparison of clinical values should be undertaken. Thus, for BP rhythms, the validation of Pulse Pressure Variation (PPV) is the method of choice, whereas for pulse waves Cardiac Output (CO) calculated from Pulse Contour Analysis (PCA) should be used.

Figure 4