Influence of different representations of the oscillometric index on automatic

determination of the systolic and diastolic blood pressures

Vojko Jazbinšek, Janko Lužnik, Zvonko TronteljInstitute of Mathematics, Physics and Mechanics

Jadranska 19, 1000 Ljubljana, Sloveniavojko.jazbinsek@imfm.uni-lj.si

Introduction• Most of automated non-invasive blood pressure – (NIBP) devices use

oscillometric technique based on some empirically derived criteria applied to the so-called oscillometric index [1], which is defined as a certain characteristic physical property of the measured arterial pressure pulses.

• The recorded data in a typical NIBP device contain the arterial pressure pulses in the cuff, superimposed on the cuff deflation.

• Some NIBP devices have also implanted microphone inside the cuff, which enables measurements of Korotkoff sounds [2], which are otherwise listened by a stethoscope in a conventional BP measurement in the office.

• In this study we use different physical properties of recorded data to form different representations of oscillometric index.

• The aim of this study is to find out how these different representations influence on systolic (SP) and diastolic (DP) pressures determined by known algorithms, such as height (HB) and slope based (SB) methods.

1 K-G. Ng, CF .Small. Survey of automated non-invasive blood pressure monitors, Journal of Clinical Engineering; 19:452-475, 1994.

2 N.C. Korotkoff. On the subject of methods of determining blood pressure. Bull. Imperial Mil. Med. Acad. (St. Petersburg), 11:365-367, 1905.

Measurements• EU-project “Simulator for NIBP” • LODE (Groningen, NL)• Compressor for the cuff inflation

and a pressure sensor built in a personal computer

• Upper arm cuff (Accoson, UK) with implanted microphone

• Simultaneous measurements with commercial automated NIBP device OSZ4 (Welch Alyn)

Measured data – microphone

Audible part (Korotkoff) [10-40] Hz

Low frequency part [0.3-10] Hz

Measured data – pressure

Pressure pulses - filter[0.3-40] Hz

Time derivative of pressure data

Overview of data obtained with NIBP device

Filtered pulses [0.3-40] Hz

Time derivative of pressure data

Audible part of microphone data

NIBP data Oscillometric indexFiltered pulses [0.3-40] Hz

Time derivative of pressure data

Audible part of pressure data

Filtered pulses [0.3-40] Hz

Time derivative of pressure data

Audible part of pressure data

Characterization of oscillometric index Exy

Algorithms for SP and DP

• Height based (HB) methoduses characteristic height ratios (SP - 0.45, DP – 0.7)

• Slope based (BS) methoduses maximum slope of the curve

• Non-monotonic curve – constraints in SB method

Evaluation of NIBP devicesTwo standard protocols for evaluation of NIBP devices:1. British Hypertension Society (BHS) – at least grade B

2. American Association for the Advancement of Medical Instrumentation (AAMI):

• Average absolute difference: I∆p| ≤ 5 mm Hg• Standard deviation: SD ≤ 8 mm Hg

Absolute difference between standard and test device

Grade ≤ 5 mm Hg ≤ 10 mm Hg ≤ 15 mm Hg

A 60 % 85 % 95 %

B 50 % 75 % 90 %

C 40 % 65 % 85 %

D otherwise

Evaluation protocol• Modified version of AAMI and BHS protocols

min( , ) 4 max( , )round .5

SP DP SP DPSP DP

V V V VV

• Similar to BHS protocol, results were classified into grades (6)

• For every SP and DP, we found classification values VSP and VDP, and calculated combined value VSP+DP (biased to the worse of the VSP and VDP)

• Like in AAMI protocol, we calculated I∆p|±SD (average absolute difference).• In addition, we have also calculated ∆p ±SD (average difference), • linear regression correlation coefficient r,• and maximum difference ∆pm

Height based method (HB) - Average height ratios HSxy in HDxy for 92 recordings

Filtered pulses [0.3-40] Hz

Time derivative of pressure data

Audible part of pressure data

Results for height based method (HB)

Type of data:

a) p – pressure pulses(92 recordings)

b) d – derivatives

(92 recordings)

c) k - Korotkoff(32 recordings)

Type of envelope: min-max (p) min-cog (c) cog-max (d)

Tip of data:a) p – pressure pulsesSB with constraints(92 recordings)

b) p – pressure pulses SB without constraints(92 recordings)

c) d - derivativeSB with constraints(92 recordings)

c) k - KorotkoffSB with constraints (32 meritev)

Type of envelope: min-max (p) min-cog (c) cog-max (d)

Summary of evaluation results for SB method

Filtered pulses [0.3-40] Hz

Filtered pulses without constraints

Audible part of pressure data

Summary of evaluation results for HB method

Filtered pulses [0.3-40] Hz

Time derivative of pressure data

Audible part of pressure data

Comparison of HB and SB evaluation results for Epp (peak-to-peak pressure pulses) envelopes

HB

SB

Linear regression Bland-Altman plot

• Study of 92 recordings performed on 23 healthy volunteers.

• Evaluation of two known algorithms, height based (HB) and slope based (SB) methods, applied to different representations of oscillometric index Exy.

• We used modified combination of standard AAMI in BHS evaluation protocols.

• We found that HB method can be applied to envelopes that exhibit a rapid change of amplitude near DP and SP.

• Same behaviour is needed also for SB method, but additional constraints should be applied due to non-monotonic shape of envelopes (especially for SP).

• For both methods we got best results for oscillometric index formed from peak-to-peak values of pressure pulses.

Conclusions