WHICH RESOLUTION FOR RELIABLE ECG P-WAVE

ANALYSIS IN ATRIAL FIBRILLATION?

Federica Censi, Giovanni Calcagnini, Michele Triventi, Eugenio Mattei, Pietro Bartolini

Italian National Institute of Health, Viale Regina Elena 299, 00161 Rome, Italy

Ivan Corazza, Giuseppe Boriani

Policlinico S. Orsola-Malpighi, University of Bologna, Via Massarenti, 9, 40138 Bologna, Italy

Keywords: Atrial fibrillation, ECG analysis, P-wave, Signal resolution.

Abstract: P-wave analysis is becoming more and more used to help indentifying patients at risk for AF. Particularly,

precise measurement of P-wave duration is an important factor in determining the risk of atrial arrhythmias.

However, the methods to extract P-wave duration must be precise and reliable. Automatic analysis of P-

wave must take into account technical aspects, one of those being the bit resolution. The aim of this

manuscript is to investigate the effects of amplitude resolution of ECG acquisition systems on P-wave

analysis. Starting from ECG recorded by an acquisition system with a LSB of 31 nV (24-bit on an input

range of 524mVpp), we reproduced ECG signal as acquired by systems with lower resolution (16, 15, 14,

13 and 12 bit). We found that, when LSB is of the order of 128 µV (12 bit), a single P-wave is not

recognizable on ECG (figure 1, upper panel). However, when averaging is applied, a P-wave template can

be extracted, apparently suitable for P-wave analysis. Results obtained in terms of P-wave duration revealed

that at lowest resolution (from 12 to 14 bit) the error on P-wave duration estimation is important and could

lead to misleading results. However, the resolution used nowadays in modern electrocardiographs (15 and

16 bit) lead to results rather similar to those obtained with higher resolution.

1 INTRODUCTION

Analysis of P-wave is becoming more and more

used for gathering information about the

predisposition of patients to atrial tachycardia and

atrial fibrillation (Censi et al, 2007; Dilaveris et al.,

1998; Hayashida et al., 2005; Ozdemir et al., 2004).

The focus on P–wave is justified by its being

representative of atrial conduction: a longer and

more fragmented P-wave is related to obstacles,

blocks and defects which provoke atrial electrical

path to change and which are considered responsible

for the promotion of atrial tachycardia and

fibrillation.

The interest in the analysis of the P-wave has

increased in the last decades; different methods are

used, from manual analysis based on visual

inspection to automatic processing techniques, by

which reliable, more reproducible and objective

measures can be obtained. Indeed, manual analysis

of P-wave allows to extract quantitative but

operator-dependant parameters such as P-wave

duration and qualitative parameters such as

classification of morphological features

(monophasic, biphasic, etc…). Signal processing

techniques allow to obtain reliable and more

reproducible quantification of P-wave duration,

quantitative measures of P-wave morphology and

quantification of other P-wave features not

distinguishable by visual inspection (e.g. root mean

square of the last 20 ms of the P-wave) However,

even when P-wave is automatically processed, little

care is paid to the technical specifications of the

acquisition system, particularly to its amplitude

resolution, i.e. the value of the less significant bit

(LSB).

The aim of this manuscript is to investigate the

effects of amplitude resolution of ECG acquisition

systems on P-wave analysis. Starting from ECG

recorded by an acquisition system with a LSB of 31

nV (24-bit on an input range of 524mVpp), we

reproduced ECG signal as acquired by systems with

lower resolution (16, 15, 14, 13 and 12 bit).

385

Censi F., Calcagnini G., Triventi M., Mattei E., Bartolini P., Corazza I. and Boriani G..

WHICH RESOLUTION FOR RELIABLE ECG P-WAVE ANALYSIS IN ATRIAL FIBRILLATION?.

DOI: 10.5220/0003091603850388

In Proceedings of the International Conference on Bio-inspired Systems and Signal Processing (BIOSIGNALS-2011), pages 385-388

ISBN: 978-989-8425-35-5

 2011 SCITEPRESS (Science and Technology Publications, Lda.)

2 METHODS AND MATERIALS

2.1 Real High Resolution ECG

We started from ECG recordings obtained by using

the ActiveTwo system by Biosemi. This acquisition

system provides a 24 bit A/D conversion over an

input range of 524 mVpp; thus the LSB is 31 nV. In

this work we analyzed 160 ECG recordings obtained

from 10 patients (16 leads/patient).

2.2 Simulate Lower Resolution ECG

From 24-bit ECG signal, new files containing the

ECG signal at lower resolutions were generated by

rounding the ECG values to the nearest integer

value. Particularly, we reproduced A/D conversion

at 16 bit, 15 bit, 14 bit, 13 bit and 12 bit. This

resolution yields to LSB values as reported in table

1, given the input range of 524mVpp.

Table 1: LSB values obtained using 12,13,14,15 and 16 bit

over the input range of 524mVpp.

Number of bit Value of LSB (mV)

16 8 µV

15 16 µV

14 32 µV

13 64 µV

12 128 µV

2.3 Quantification of P-wave Features

To evaluate the difference in P-wave features, we

automatically quantified P-wave duration and

morphology from P-wave template extracted by

averaging technique using specific algorithms.

Every lead signal was pre-processed and analysed to

extract the average P-wave characteristic. The first

step is to detect P-waves from the acquired signals

P-waves. Secondly, a beat-by-beat linear piecewise

interpolation was used to remove baseline wander,

on each P-wave. Then, the averaging procedure is

performed to obtain a P-wave template. Ectopic

atrial signals or P-waves with excessive noise were

excluded by conventional template matching of each

P-wave, with an exclusion criterion of cross-

correlation coefficient lower than 0.9 (cross-

correlation threshold). The averaging procedure

went on until 200 beats were included. If the

residual noise level remained at more than 1V

even after averaging of 200 beats, averaging

procedure continued until the noise level reached a

value lower than 1 V. If it was impossible, the lead

was excluded from the study. Residual noise was

measured in the isoelectric segment before the P-

wave (TP track).

For each P-wave template, P-wave duration has

been automatically calculated as the time between

the onset and the offset of the P-wave. P-wave onset

is computed as the first point, among 20 consecutive

points starting from the beginning of the P-wave,

higher than 3 times the residual noise standard

deviation. Offset was analogously defined

considering the first point, among 20 consecutive

samples, starting from the end of the P-wave

window and going backward, where the signal level

rises above the same threshold.

Morphological parameters were extracted

according to the Gaussian function decomposition

described in (Uhley, 2001).

3 RESULTS

Figure 1 shows an example of the same P-wave as

visualized using 12, 13, 14, 15, 16 and 24 bit. While

the signals obtained at 15 and 16 bit are visually

comparable to the highest resolution (24 bit), the

lower resolutions seem not to be suitable for a P-

wave detection and analysis.

[samples]

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0 50 100 150 200 250 300 350 400

-0.2

0.2

0 50 100 150 200 250 300 350 400

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0.2

0 50 100 150 200 250 300 350 400

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Figure 1: An example of the same P-wave as visualized

using 12, 13, 14, 15, 16 and 24 bit.

BIOSIGNALS 2011 - International Conference on Bio-inspired Systems and Signal Processing

386

When averaging is performed, the criterion of

summing up at least 200 beats having a cross-

correlation threshold higher than 0.9 (template

matching) is no longer applicable at all resolutions.

Indeed the resolutions of 14, 13 and 12 bit need a

correlation threshold lower than 0.9 (until 0.5) for a

template to be extracted using no more than 150

beats. Using these less restrictive criteria, a P-wave

template can be obtained at all resolutions

apparently suitable for P-wave analysis.

Figure 2 shows the P-wave templates obtained

for the 12-bit and 13-bit simulated ECG, compared

to the one obtained with the best resolution (24 bit,

dashed line). 12-bit resolution appears to be quite

different from the 24-bit ECG signal, in terms of

onset, offset and fragmentation.

Figure 3 shows the automatic computation of P-

wave duration, according to the algorithm described

in the previous section. Since P-wave template at

lower resolutions are quite more noisy than at higher

resolutions, P-wave duration resulted to be

significantly lower for 14 bit, 13 bit and 12 bit ECG

signals.

Table 2 shows the percentage differences for P-

wave duration measurements, for all bit resolutions

tested respect to the highest resolution of 24 bit.

[µV]

Figure 2: P-wave templates obtained for the 12-bit and 13-

bit simulated ECG, compared to the one obtained with the

best resolution (24 bit, dashed line).

Table 2: Percentage differences for P-wave duration

measurements for all bit resolutions respect to the highest

resolution of 24 bit.

Number of bit Percentage difference

16 0

15 -1.2%

14 -6.4%

13 -15.2%

12 -18.1%

bit

13 bit

bit

Figure 3: Automatic computation of P-wave duration for

all bit resolutions.

WHICH RESOLUTION FOR RELIABLE ECG P-WAVE ANALYSIS IN ATRIAL FIBRILLATION?

387

As far as quantification of morphological features is

concerned, we found that it was not significantly

affected by the ECG resolution.

4 DISCUSSION

P-wave analysis is becoming more and more used to

help indentifying patients at risk for AF.

Particularly, precise measurement of P-wave

duration is an important factor in determining the

risk of atrial arrhythmias.

Some of these analysis is made manually by

expert cardiologist (visual inspection). Automatic

analysis is also performed, using converted digital

ECG signals with variable number of bits.

Commonly used electrocardiographs convert the

signal using 16 or 15 bit, even if some analysis are

made on ECG converted at lower resolution (12 bit).

The LSB of such instruments, given an input

dynamic of some mV, is in the range 5-15 µV.

Given the large number of experimental

evidences on the association between P-wave

prolongation and AF, Uhley suggested to design

ECG machines in order to automatically calculate

and display P-wave duration (Uhley, 2007).

However, the methods to extract P-wave duration

must be precise and reliable. Automatic analysis of

P-wave must take into account technical aspects, one

of those being the bit resolution. In this paper we

investigate the effect of bit resolution of ECG

signals on automatic P-wave analysis. We started

from ECG signals acquired using a high resolution

system having 24 bit resolution (corresponding to a

LSB of 31 nV). We then obtained down-resolved

ECG, simulating ECG acquisition using 16 bit, 15

bit, 14 bit, 13 bit and 12 bit. We found that, when

LSB is higher than 100 µV, a single P-wave is not

recognizable on ECG (figure 1, upper panel).

However, when averaging is applied, a P-wave

template can be extracted, apparently suitable for P-

wave analysis. Results obtained in terms of P-wave

duration revealed that at lowest resolution with LSB

higher than 30 µV, the error on P-wave duration

estimation is important and could lead to misleading

results. In conclusion, the resolution used nowadays

in modern electrocardiographs (about 5 µV) lead to

results rather similar to those obtained with higher

resolution.

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