Automatic Blood Pressure Monitor
Calibration
Ross Hamilton
Lei Qu
Hanniff Mohd Nor
David Lee
Introduction
• Dr. Andre Churchwell is our Sponsor
• Learned about the vagaries of
commercial common blood pressure
monitors
• Automatic monitors lost their accuracy
over extended periods of time because
of a lack of calibration
• Results in a repurchase of the
monitor after a certain period of time
and uses
Goals
• Investigate the possible variation from
automatic blood pressure monitor that will
cause diagnosis problems
• Solve the problem of recalibration
• Design a “rezeroing” device such as a
calibration signal circuit to enhance the
accuracy of the monitors
Discrepancies recorded in the
literature
Discrepancies continued
Conclusions from the literature
• Many other literature also indicate a
discrepancies between the mean
measurements of the BP monitors versus
the mercury sphymanometer (ranging from
0.1 to 2.5 mm Hg)
• Thus, we can sufficiently conclude that
there is a discrepancy between the two
measuring devices.
• However, to further complement our
experiment, we have performed the similar
statistic comparison ourselves
Cause of the discrepancy
• Degree of deflation of the arm band
• Affect monitors true zero value
• Degradation of piezoelectric resistor
after usages
Mechanism behind the BP
monitor
• Piezoelectric resistor
• As the arm band inflates, it shuts off the
blood flow in the valves of the patient’s
arm
• As the arm band deflates, the blood flow
will restart, thus there will be pressure from
blood flow to the arm band
• Beginning stage of blood flow is
turbulence flow, and as arm band
releases, flow becomes more laminar
Mechanism Continued
• Pressure goes to piezoelectric resistor
which will act as a transducer and give a
signal readout
• Systolic pressure will be obtained at the
highest variance of the pulse wave
peaks
• the diastolic pressure will be the lowest
variance of the pulse wave peaks
Flow chart
ADC circuit schematic
Microcontroller circuit schematic
Power Source circuit schematic
Operational flow chart
The Gold Standard
• Mercury sphymanometer
• Uses no electronic transducers
• Mercury is extremely pressure-sensitive
• From Watson, et al, the accuracy of
the mercury sphymanometer is
confirmed.
Progress thus far
• The fake arm experiment between the gold
standard and the BP monitor
• Bandpass filter on the breadboard (picture
included)
The fake arm experiment
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Null hypothesis: the mean of the Omron values are equal to the
mean from the sphymanometer
• Assumptions
• The data are obtained from the identical sample
population (generated fr4om the same BP analyzer)
• The gold standard does not have drifting issues
Used the fake arm blood pressure analyzer to generate the BP
20 trials
The paired two-tail t-test was conducted to determine the validity
of the null hypothesis
P value = 6.19 x 10^-15
Null hypothesis rejected
Hypertension case also experimented with a set average BP
(150/100)
P value = 1.15 x 10^-15
Null hypothesis rejected
Conclusion
• The mean of the Omron values are not
equal to the mean of the sphymanometer
• We can infer from this conclusion that
variation resulted from the Omron
• However, the trials are only 20 so this
conclusion is statistically insufficient
Constructing the circuit
• Started constructing the bandpass filter
• The resisters and capacitors will be modified
through trial and error to get a desired filtration of
the analog input
• The circuitry was modified with the help of Dr.
Galloway
• The analog input will have frequency simulating the
actual heart beat (ranging between 1 and 2 Hz)
with a signal intensity of 5 volts
• The signal will be captured using Labview with a
sample rate of 1000 Hz for resolution purposes
Picture of the breadboard
Problems faced
• The ADC element pins are separately too
closely for insertion unto the breadboard
• MCU programming may be complex
• Running out of time so must dedicate
more hours
Future work
• Add a pressure transducer to the input port
• Adapt the ADC unto an artificial chip and
connect it to the filter circuit
• Connect the port to the pressurizing pad
• Construct the MCU unit
• Connect the compartments
• Perform another experiment between the
gold standard and the BP monitor with
greater number of trials (~300 trials)
References
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Discrepancies references
de Greeff A, Shennan A. Blood pressure measuring devices: ubiquitous, essential but
imprecise. Expert Rev Med Devices. 2008 Sep;5(5):573-9. Review
Ellis C, Gamble G, Hamer A, Williams M, Matsis P, Elliott J, Devlin G, Richards M, White H;
New Zealand Acute Coronary Syndromes (NZACS) Audit Group. Patients admitted with an
acute coronary syndrome (ACS) in New Zealand in 2007: results of a second comprehensive
nationwide audit and a comparison with the first audit from 2002.
Heinemann M, Sellick K, Rickard C, Reynolds P, McGrail M. Automated versus manual blood
pressure measurement: a randomized crossover trial. Int J Nurs Pract. 2008 Aug;14(4):296302.
Lamb TS, Thakrar A, Ghosh M, Wilson MP, Wilson TW. Comparison of two oscillometric
blood pressure monitors in subjects with atrial fibrillation. Clin Invest Med. 2010 Feb
1;33(1):E54-62.
Landgraf J, Wishner SH, Kloner RA. Comparison of automated oscillometric versus
auscultatory blood pressure measurement. Am J Cardiol. 2010 Aug 1;106(3):386-8. Epub
2010 May 22. PubMed PMID: 20643251. [
McManus RJ, Mant J, Hull MR, Hobbs FD. Does changing from mercury to electronic blood
pressure measurement influence recorded blood pressure? An observational study. Br J Gen
Pract. 2003 Dec;53(497):953-6.
Misc. resources
http://instruct1.cit.cornell.edu/courses/ee476/FinalProjects/s2005/ww56_ws62/Final%20Proje
ct%20Web/index.html
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1120346/
Watson, S.; Wenzel, R. R.; di Matteo, C.; Meier, B.; and Lüscher, T. F. (1998). "Accuracy of a
new wrist cuff oscillometric blood pressure device". American Journal of Hypertension 11:
1469-1474 (1998).