CASE STUDY OF
USING THE ECG SIGNAL AS A REFERENCE
SIGNAL IN OPTICAL PULSE TRANSIT TIME
MEASUREMENT OF BLOOD FLOW
- THE EFFECT OF DIFFERENT ELECTRODE
PLACEMENTS ON PULSE TRANSIT TIME
Teemu Myllylä, Erkki Vihriälä, Hannu Sorvoja,
University of Oulu, Optoelectronics and Measurement Techniques Laboratory, Finland
Vesa Korhonen,
Oulu University Hospital, Radiology Clinic, Finland
Introduction
Electrocardiography (ECG) is commonly used in pulse transit time (PTT)
measurements to provide a reference signal. This method is based on computing
the temporal difference between the R wave of the QRS complex of the ECG and
the beginning of the following pulse wave measured by photoplethysmography
(PPG) [1,2,3].
However, PTT also relates to non-invasive blood pressure measurements, since
a correlation exists between PTT and blood pressure fluctuations [4,5].
Correlation analysis has proven useful, for example, when compared to BP
measurements by cuff-based methods [6]. Nevertheless, to our knowledge, there
are not many studies assessing the accuracy of the ECG signal itself, especially
in relation to ECG electrode placement, and whether it has an effect on the
accuracy of PTT measurements. This is probably because the time position of the
R wave of the ECG signal with regard to PTT measurements is assumed to be
the same regardless of electrode placement.
Introduction
The following slides present experimental measurements of ECG signals at
various electrode placements. Measurements at a high sampling rate indicate
that the placement of ECG electrodes can have a certain impact on the position
of the R wave in the time domain.
This effect takes the form of small time delays of the R wave pulse between
different ECG measurements, when electrodes are replaced between the
measurements and the placements are not exactly identical. This can be
relevant, particularly when accurate time domain determination of the R wave is
required as a reference in determining the pulse transit time of blood flow.
Additionally, we studied what sensor placements are most suitable for use in
pulse transit time and velocity measurements.
Measurement method
In our experiments, ECG signals were measured simultaneously and independently
by two ECG devices. Both ECG electrodes and their amplifiers were identical.
All in all, the effect of six different electrode placements were measured on three
test persons.
Concurrently with the ECG measurement, heart pulse waves were measured at
various sensor placements by PPG sensors. As a reference measurement, also a
Finapres device was simultaneously measuring PPG and blood pressure from the
finger tip. PPG was measured to estimate the effect of possible R wave delays on
PTT.
Measurement method
To verify the accuracy of our ECG measurements, a verification measurement
was initially conducted on every person. Figure 1 shows the sensor placement
used in this measurement (on the left). This placement provided a reference,
when the sensors of the other ECG device were moved to a different location
for each measurement. Pulse responses of both ECGs were measured
independently and simultaneously. Moreover, the amplifiers of the electrodes
were swapped between measurements 1 and 2.
Figure 1. Verification measurement. Sensors were placed in the positions shown on the
left. As assumed, the recorded pulses are identical.
Measurement method
In subsequent measurements, all sensor placements were measured twice by
swapping the amplifiers of the electrodes (marked between the measurements
n.1 and n.2). This was done to ensure that the amplifiers did not have an effect
on signal response.
The sampling frequency used in the measurements was 40kHz, and each
measurement continued for about one minute. Possible time delays between
the R waves of ECG signals obtained from different sensor placements, were
recorded. Naturally, both ECG signals were measured simultanously by the two
ECG devices.
Measurement method
Figure 2 shows the sensor placements used in our measurements. To ensure
an easy repeatability, only the + electrode of the other ECG was moved to a
different place. Throughout the entire measurement, the other ECG device
continued measuring in the same way using the same sensor placement.
Figure 2. Sensor placements used in the measurements. In each measurement, only one of the
electrodes (+ electrode) was moved to a different position. The placement of the three electrodes
of the other ECG device was always the same, as shown in the figure on the left. Likewise, for
the other ECG, the remaining two electrodes ( + and ref) were always in the same position. In
the placement shown on the right, the + electrode is placed on the person’s back.
Results
Comparison of two ECG signal responses measured simultaneously using different
sensor placements
Figure 3. Measurements 1.1 and 1.2. The replaced + electrode was located on the left shoulder and the
distance between the + and – electrodes was 7 cm. This caused a delay of approx. 1.5 ms on the R wave,
highlighted on the right showing one R wave.
Results
Comparison of two ECG signal responses measured simultaneously using different
sensor placements
Figure 4. Measurements 2.1 and 2.2. The replaced + electrode was located on the left shoulder (almost
on the neck) and the distance between the + and – electrodes was 12 cm. This caused a delay of approx.
0.3 ms on the R wave, highlighted on the right showing one R wave.
Results
Comparison of two ECG signal responses measured simultaneously using different
sensor placements
Figure 5. Measurements 3.1 and 3.2. The replaced + electrode was located on the right shoulder and the
distance between the + and – electrodes was 29 cm. This caused a delay of approx. 2 ms on the R wave,
highlighted on the right showing one R wave.
Results
Comparison of two ECG signal responses measured simultaneously using different
sensor placements
Figure 6. Measurements 4.1 and 4.2. The replaced + electrode was located on the right side of the
stomach, and the distance between the + and – electrodes was 31 cm. This caused a delay of approx. 4
ms on the R wave, highlighted on the right showing one R wave.
Results
Comparison of two ECG signal responses measured simultaneously using different
sensor placements
Figure 7. Measurements 5.1 and 5.2. The replaced + electrode was located on the right side and the
distance between the + and – electrodes was 50 cm. This caused a delay of approx. 6 ms on the R wave,
highlighted on the right showing one R wave.
Results
Comparison of two ECG signal responses measured simultaneously using different
sensor placements
Figure 8. Measurements 6.1 and 6.2. The replaced + electrode was located on the back and the distance
between the + and – electrodes was 50 cm. This caused a delay of approx. 10 ms on the R wave,
highlighted on the right showing one R wave.
Results
Calculation of the delays between R waves
Two methods were used in signal delay calculations:
1. Searching for maximum values in the time interval
-> Information about delay is the difference between peaks
2. Checking the correlation between signals in the time interval
-> Delay is calculated using the correlation function
The first method finds the maximum value of each signal for each time interval.
Figure 8 illustrates an ECG signal response containing interference. This response
presents 30 time intervals. Additional ‘maximum’ values were cut using thresholds.
In this figure, recorded maximums are marked as dots. Delays between ECG
signals were calculated as a time difference between each peak pair.
In the second method, signals were divided into 24 parts, and the cross-correlation
between signals was checked for each part. The correlation function yielded
information about the delay for each part. Table 1 shows the results of both
methods.
Results
Figure 8. Example of an ECG signal response including some interference, using electrode placement shown
in figure 4. Found maximums are numbered and marked as dots. Time delays of the numbered peaks are
presented in Table 1.
Results
Table 1. Time delays calculated by two methods. The interval numbers refer to Figure 8. Typical time
delays between R waves acquired at different electrode placements ranged approximately from 1 to 15
ms. Wrong results are colored with red
ECG signal 1
Number of
interval
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
ECG signal 2
Maximum
Correlation
Time of maximum [s] Time of maximum [s] Delay [s]
Delay [s]
0,688825
0,69895
0,010125
1,90705
1,9168
0,00975
3,039675
3,049225
0,00955
4,182925
4,193175
0,01025
5,39135
5,4012
0,00985
6,661675
6,671675
0,01
7,969625
7,97935
0,009725
9,208725
9,21865
0,009925
10,459375
10,4696
0,010225
11,6822
11,692325
0,010125
12,893975
12,9033
0,009325
14,1439
14,153475
0,009575
15,306625
14,9143
0,392325
16,846125
16,582175
0,26395
17,797
17,8073
0,0103
19,086725
19,0972
0,010475
20,408875
20,41875
0,009875
21,663925
21,674275
0,01035
22,8977
22,907875
0,010175
24,10155
24,1108
0,00925
25,31665
25,326175
0,009525
26,5444
26,55295
0,00855
27,876075
27,8868
0,010725
29,192325
29,2031
0,010775
0,0132
0,0076
0,011875
0,0169
0,3409
0,01615
0,01095
0,011975
0,016725
0,28345
0,0118
0,016825
0,299725
0,147325
0,016325
0,015825
0,0149
0,011225
0,2453
0,0116
1,093425
0,274575
0,017125
0,01295
Standard deviation
0,09181349
0,236610598
Average
0,03644583
0,121610417
Results
The results for both methods shown in Table 1 are comparable, but the maximum
method seems more accurate. The correlation method cannot properly manage
signals that are very different from each other, especially when a measurement error
occurs (as in interval no. 5).
In Table 1, wrong delay times are coloured with red. The maximum method seems
to always find the narrowest peak and to ignore measurement errors, if they are
wide. Narrow errors, however, the method is not able to deal with (magenta marker
no. 13). It would probably also have problems, if the searched peak were attenuated
by some error.
The Mean Square Error between both methods was calculated by
Results
As the presented ECG signals show, three electrodes, as used in the measurements
here, allowed us to acquire a clear and sharp R wave for PTT measurements. The R
peak was sufficiently good at every electrode placement, but the first three
placements on the left in Figure 2 gave a slightly better signal-to-noise ratio than the
others.
Besides the EEG cap, some EEG devices also include one electrode that can be
placed on back of a patient, as shown in Figure 8. In addition to EEG, this also
allows to measure ECG. However, in our measurements, this placement gave the
lowest quality ECG signal, although it sufficed for PTT measurements. Additionally,
this electrode placement produced more delay in the R peak than the other
placements.
Results
The effect of R wave delays of the ECG on PTT measurements was studied by
simultaneous measurements using three PPG sensors in different places, as shown
in Figure 9. One PPG, with a source detector distance of 3 cm, was attached on the
forehead to obtain pulsations from deeper within tissue. PPG 2 and PPG 3
measured skin blood pulsations.
Figure 9. PPG sensors placed on the forehead, neck and right finger tip.
Results
Figure 10 presents signal pulses in the time domain when sensors are placed as
shown in Figure 9. It can be seen that the effect of the ECG time delay between two
ECGs is rather small in comparison to pulse transit time. This can be noticed
especially when PTT is measured from the finger tip (PPG 3), Table 2.
Figure 10. Comparison of measured signals in the time domain.
Results
Table 2. Comparison of PTT values between ECG1 - PPG3 and ECG2 – PPG 3, placement in Figure 4.
ECG 1
ECG 2
PPG 3
Time of maximum [s]
Time of maximum [s] Time of maximum [s]
Delay ECG 1-ECG2 [s] Delay ECG1-PPG3 [s]
Delay ECG2-PPG3 [s] Approximation error [%]
1
0,298375
0,29825
0,573025
0,000125
0,27465
0,274775
0,045512
2
1,08315
1,083725
1,3603
0,000575
0,27715
0,276575
0,207469
3
1,911325
1,91135
2,1605
2,5E-05
0,249175
0,24915
0,010033
4
2,775825
2,7756
3,0419
0,000225
0,266075
0,2663
0,084563
5
3,69085
3,690725
3,956525
0,000125
0,265675
0,2658
0,04705
6
4,610475
4,609925
4,889825
0,00055
0,27935
0,2799
0,196886
7
5,52075
5,5211
5,795375
0,00035
0,274625
0,274275
0,127447
8
6,4299
6,430475
6,696575
0,000575
0,266675
0,2661
0,215618
9
7,323925
7,32415
7,6047
0,000225
0,280775
0,28055
0,080135
10
8,192775
8,1926
8,46565
0,000175
0,272875
0,27305
0,064132
11
9,069925
9,069725
9,34165
0,0002
0,271725
0,271925
0,073604
12
9,94965
9,949325
10,21583
0,000325
0,266175
0,2665
0,1221
13
10,82463
10,82463
11,10093
0
0,2763
0,2763
0
14
11,658
11,65765
11,93648
0,00035
0,278475
0,278825
0,125685
15
12,47158
12,4712
12,74898
0,000375
0,2774
0,277775
0,135184
16
13,28725
13,28745
13,5566
0,0002
0,26935
0,26915
0,074253
17
14,10833
14,10833
14,37673
0
0,2684
0,2684
0
18
14,9247
14,9245
15,19205
0,0002
0,26735
0,26755
0,074808
19
15,7485
15,7486
16,0206
1E-04
0,2721
0,272
0,036751
20
16,6089
16,60898
16,8672
7,5E-05
0,2583
0,258225
0,029036
21
17,50643
17,50655
17,77088
0,000125
0,26445
0,264325
0,047268
22
18,4129
18,4127
18,68403
0,0002
0,271125
0,271325
0,073767
23
19,2927
19,29208
19,5716
0,000625
0,2789
0,279525
0,224095
24
20,1621
20,1623
20,4292
0,0002
0,2671
0,2669
0,074878
25
21,04273
21,04305
21,30803
0,000325
0,2653
0,264975
0,122503
26
21,90338
21,90308
22,1775
0,0003
0,274125
0,274425
0,109439
27
22,72163
22,72145
23,01365
0,000175
0,292025
0,2922
0,059926
28
23,52815
23,52835
23,80445
0,0002
0,2763
0,2761
0,072385
29
24,35685
24,35718
24,61708
0,000325
0,260225
0,2599
0,124892
30
25,20358
25,204
25,45558
0,000425
0,252
0,251575
0,168651
31
26,08335
26,08348
26,33258
0,000125
0,249225
0,2491
0,050155
32
26,9658
26,96558
27,21618
0,000225
0,250375
0,2506
0,089865
33
27,88778
27,88798
28,1382
0,0002
0,250425
0,250225
0,079864
34
28,82398
28,82433
29,08825
0,00035
0,264275
0,263925
0,132438
35
29,74188
29,7422
30,01268
0,000325
0,2708
0,270475
0,120015
st dev
0,000159015
0,009974738
0,010068773
average
0,000258333
0,26855
0,268534286
Results
Figures 11 and 12 present ECG cycles measured both with Einthoven and Goldberg
connections by the ECG devices used in our measurements. In figure 11 signals are
obtained with Einthoven connection 1 signals being related to the elektromagnetic
field between left and right arm and in figure 12 using Goldberger connection aVF,
signals relating to the field between left leg and both arms.
Figure 11. ECG cycles measured with an
Einthoven connection 1 (N=30).
Figure 12. ECG cycles measured with a
Goldberger connection aVF (N=30).
Results
Figures 13 and 14 presents vector cardiograms determined with all separate cycles
(Figure13) and with calculated average values (Figure 14). Determining of the vector
cardiogramm was performed by calculating the vector summs of the of the signals in
Fig.11 (y-axis) and 12 (x-axis). These ECG cycles show the position of the R wave
in the time domain with calculated vector sums (the loops pointed by arrows). Some
of in the results presented differences in the ECG signal caused by different ECG
placements can be explained by change of direction of R wave vector.
Figure 13. Determined vector ECG cycles
(N=30) calculated using signals shown in Figs.
11 and 12.
Figure 14. Vector ECG cycle using calculated
average signals in Figs.11 and 12.
Conclusion
The results show that the position of the R peak in the time axis depends on the
placement of the electrodes used in ECG measurements. Although variations in
the time axis are small, they may be in some situations relevant when the R wave
of the ECG is used as a reference in pulse transit time measurements of blood
flow.
Placing the electrode on the back has a larger impact on the time position of the
R wave. Furthermore, it is reasonable to claim that the resolution of the ECG
signal in proportion to the time domain can be partially dependent on electrode
placement.
Acknowledgements
The authors would like to extend their warmest thanks to the following
students for their assistance
Aleksandra Zienkiewicz and Łukasz Surażyński,
Gdansk University of Technology
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