Detection of occult paroxysmal atrial fibrillation

Abstract

This work introduces a novel approach to the detection of brief episodes of paroxysmal atrial fibrillation (PAF). The proposed detector is based on four parameters which characterize RR interval irregularity, P-wave absence, f-wave presence, and noise level, of which the latter three are determined from a signal produced by an echo state network. The parameters are used for fuzzy logic classification where the decisions involve information on prevailing signal quality; no training is required. The performance is evaluated on a large set of test signals with brief episodes of PAF. The results show that episodes with as few as five beats can be reliably detected with an accuracy of 0.88, compared to 0.82 for a detector based on rhythm information only (the coefficient of sample entropy); this difference in accuracy increases when atrial premature beats are present. The results also show that the performance remains essentially unchanged at noise levels up to $100\,\upmu \hbox {V}$ RMS. It is concluded that the combination of information on ventricular activity, atrial activity, and noise leads to substantial improvement when detecting brief episodes of PAF.

Appendix

This appendix describes the steps required for generating test signals with occult PAF; the interested reader is referred to the publications cited below for more information on each of the steps.¹ The ECGs of the PTB database, which served as the starting point for test signal generation, were first subjected to baseline removal and QRST delineation [17].

1.1 Ventricular rhythm

The number of beats in SR and AF episodes was uniformly distributed in the interval [5, 30], unless otherwise stated, and thus the test signals contained about the same number of episodes of SR and AF.

The model in [23] was used to generate RR intervals during SR. The mean heart rate was set to 60 bpm and the standard deviation to 2 bpm, the respiratory rate to 0.25 Hz, and the low-frequency/high-frequency ratio to 1. During AF, an atrioventricular node model was used to generate RR intervals [7]. The mean arrival rate of atrial impulses was set to 6 Hz, the minimal refractory period to 0.25 s, the probability of an impulse to take the slower pathway to 0.6, the maximal refractory period prolongation to 0.1 s (identical for both pathways), and the difference between the two refractory periods to 0.2 s.

1.2 Ventricular morphology

The original T-waves were first resampled to a fixed width, and then, depending on type of rhythm, width-adjusted to match prevailing heart rate. During SR, the T-wave was resampled relative the current RR interval using Bazett’s formula, where the corrected QT interval was set to 420 ms. During AF, the QT interval was shorter than during SR, and set to a fixed value (250 ms). After an AF episode terminated, the T-wave duration was gradually increased over the five next beats so as to produce a smooth transition from AF to SR. When needed, the TQ interval was padded with zeros.

Since APBs occur quite commonly in AF patients [35], a certain percentage of APBs was introduced in the test signal. The occurrence of an APB caused the preceding RR interval to be 25 % shorter and the following 25 % longer.

1.3 P-waves and f-waves

In lead $\hbox {V}_6$, P-waves are usually monophasic in shape and therefore reasonably well modeled by the first Hermite function [13, 31]. The second and third Hermite functions, being biphasic and triphasic, respectively, were added with random weights (normal distribution, zero-mean, variance 0.1) to make the morphology vary over time. Since P-waves in $\hbox {V}_1$ are often biphasic in patients with PAF [15], they were modeled by simply differentiating the corresponding P-wave in $\hbox {V}_6$. The peak-to-peak P-wave amplitude was set to $50\,\upmu \hbox {V}$ in both $\hbox {V}_1$ and $\hbox {V}_6$. The PR interval length was uniformly distributed within the interval [175,185] ms.

The extended f-wave sawtooth model was used together with the parameter values in [26]. The amplitude in $\hbox {V}_{1}$ was taken to be 5 times larger than that in $\hbox {V}_6$ to reflect the fact that f-waves have much larger amplitude in $\hbox {V}_{1}$ than in $\hbox {V}_{6}$. This difference in amplitude was caused by the longer distance from the heart to the electrode site and an electrical vector that is much more scattered during AF.

1.4 Noise

Following summation of ventricular and atrial activities, EMG noise taken from the MIT–BIH Noise Stress Test Database [24] was added to produce the final test signal (the noise first rescaled to the desired RMS value). A number of test signals with different noise levels are displayed in Fig. 8.

Fig. 8

Illustration of test signals in $\hbox {V}_1$ and $\hbox {V}_6$ when the noise level is set to a $20\,\upmu \hbox {V}$, b $50\,\upmu \hbox {V}$, and c $100\,\upmu \hbox {V}$