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BP+ Repeatability

Three consecutive measurements over a period of approximately 10 minutes were taken in 282 subjects recruited from primary-care facilities. Measurements were taken in the left arm with subjects in a sitting position.

Measurement sets were excluded from analysis if any blood pressure or BP+ measure could not be calculated, if signal-to-noise ratio was less than 0 dB or if augmentation index was more than 300%.

The remaining 254 sets of measurements were then analyzed. Limits of agreement (LoA) were calculated as twice the standard deviation of the difference between the three pairs of each set of three measurements. Intraclass correlation coefficient (ICC) was calculated using a two-way consistency model. A repeatability aspect ratio (AR = LoA / Range) was also calculated.

Parameter

Range

LoA

ICC

AR

Non-invasive blood pressure





Systolic Pressure (mmHg)

87 ~ 177

± 17

0.92

19%

Diastolic Pressure (mmHg)

45 ~ 102

± 9.6

0.92

17%

Mean Pressure (mmHg)

57 ~ 112

± 9.7

0.94

18%

Pulse Rate (bpm)

41 ~ 99

± 10

0.91

17%

BP+ Parameters





Augmentation Index (%)

4.3 ~ 283

± 40

0.90

14%

Central Systolic Pressure (mmHg)

77 ~ 168

± 16

0.93

18%

Central Diastolic Pressure (mmHg)

46 ~ 103

± 9.8

0.92

17%

Central Mean Pressure (mmHg)

58 ~ 126

± 12

0.93

18%


The ICC values indicate that BP+ parameters have excellent repeatability on par with standard oscillometric non-invasive blood pressure parameters.

The variability between the three consecutive recordings may be explained by true physiological changes such as minute-to-minute changes in blood pressure and vasoconstriction. They may also be due to arm motion or other environmental noise. In comparison, Frimodt-Moller et al.2 reports AR = 32% for inter-observer repeatability of tonometric augmentation index.

Augmentation index is calculated as a ratio of two numbers and therefore variability tends to increase with increasing augmentation index. For example, ICC for subjects with a median augmentation index below 150% (still significantly above the expected normal range) is 0.93.

Intra-session Augmentation Index, below 150%.

Intra-session Augmentation Index, below 300%.
Intra-session Augmentation Index, below 300% (top) and 150% (bottom)

Inter-day Repeatability

Corresponding measurements were taken in 20 subjects on two separate occasions an average of 3 days apart under standard conditions. Time of day (morning), fasting, caffeine and room temperature were controlled. Measurements were taken in the left arm with the subject in a sitting position. Data were analyzed in total, and again with 1 outlier removed.


Parameter

Range

LoA

ICC

AR

All data





Augmentation Index (%)

7.8 ~ 93

 ± 34

0.72

40%

Systolic Pressure (mmHg)

101 ~ 154

 ± 26

0.59

50%

Removed 1 outlier





Augmentation Index (%)

7.8 ~ 93

 ± 23

0.87

27%

Systolic Pressure (mmHg)

101 ~ 154

 ± 21

0.73

40%


The ICC and LoA indicate that Augmentation Index has very good day-to-day repeatability and is more repeatable than non-invasive systolic blood pressure.

BP+ measurement of augmentation index is also very favorable compared to published studies of day-to-day repeatability for other arterial stiffness measures. Huck et al.1 give ICC values for pulse wave velocity measurements ranging from 0.56 to 0.62 for various manufacturers' devices. Data from 2-4 are not directly comparable as they use a group mean for inter-day reproducibility.

References

  1. Huck, C.J. et al. Noninvasive measurements of arterial stiffness: Repeatability and interrelationships with endothelial function and arterial morphology measures. Vasc Health Risk Manag 3, 343-349 (2007).
  2. Frimodt-Moller, M., Nielsen, A.H., Kamper, A. & Strandgaard, S. Reproducibility of pulse-wave analysis and pulse-wave velocity determination in chronic kidney disease. Nephrol. Dial. Transplant. gfm470 (2007).doi:10.1093/ndt/gfm470
  3. Wilkinson, I.B. et al. Reproducibility of pulse wave velocity and augmentation index measured by pulse wave analysis. J. Hypertens 16, 2079-2084 (1998).
  4. ter Avest, E., Holewijn, S., Stalenhoef, A.F.H. & de Graaf, J. Variation in non-invasive measurements of vascular function in healthy volunteers during daytime. Clin. Sci 108, 425-431 (2005).