ECG Visualization and Arrhythmia Detection — the MIT-BIH Database

Data source — the PhysioNet MIT-BIH Arrhythmia Database

The MIT-BIH Arrhythmia Database is a publicly available collection of electrocardiogram recordings published by PhysioNet at:

https://physionet.org/content/mitdb/1.0.0/

It contains 48 half-hour, two-channel recordings collected from 47 patients at Beth Israel Hospital in Boston between 1975 and 1979. The recordings were manually annotated by at least two independent cardiologists and are widely used in research on automatic arrhythmia detection.

Record 205

The example uses record 205 — a recording from a 59-year-old man treated with Digoxin and Quinaglute. The record contains episodes of ventricular tachycardia (VT) and is often cited in the literature as diagnostically challenging, due to two morphologically distinct forms of premature ventricular contractions (PVC).

Recording parameters:

Raw values are stored as unitless integers (so-called ADC values). Conversion to millivolts:

\[\text{mV} = \frac{\text{ADC} - 1024}{200}\]

The range of actual values in the rec205 file falls between 589–1315 (MLII) and 718–1106 (V1), corresponding to a signal amplitude of roughly ±1.5 mV.

Data preparation

The original recording files (205.hea, 205.dat, 205.atr) are provided in the MIT-BIH format and need to be converted into a binary format recognized by RetractorDB.

The MIT-BIH format 212

The signal in the 205.dat file is packed 12-bit in format 212: every three bytes store two consecutive samples of both channels according to the scheme:

[B0][B1][B2] → MLII = B0  | ((B1 & 0x0F) << 8)
               V1   = B2  | ((B1 >>  4)  << 8)

The values are 12-bit signed (range –2048..2047).

Converting to the RetractorDB format

The script examples/ecg/mitbih2rdb.py reads the 205.hea header, decodes the sample pairs, and writes them as little-endian int32 records into the rec205 file:

650,000 records × 2 fields × 4 bytes = 5,200,000 bytes

At the same time, the script generates an RQL script that plays back the signal (rec205-replay.rql). The descriptor file rec205.desc is created by build.sh.

The whole preparation process is run with a single command from the project’s root directory:

bash examples/ecg/build.sh

The result is three files in the examples/ecg/rec205/ directory:

The RQL query

The file rec205-replay.rql defines two streams:

DECLARE MLII INTEGER, V1 INTEGER STREAM ecg, 1/360 FILE 'rec205'

SELECT ecg.MLII, ecg.V1 STREAM s205out FROM ecg VOLATILE

The STREAM ecg, 1/360 clause sets the time interval of a single sample to 1/360 s, matching the actual sampling rate of 360 Hz. The TYPE DEVICE clause in the descriptor causes the rec205 file to be read sequentially in a loop (after the last sample, reading returns to the beginning), enabling continuous playback of the recording.

The output stream s205out is declared VOLATILE, so it is not written to disk — the data only reaches the consumer process (xqry).

On-screen visualization

The ecg target in the build system is used to display the chart in real time. It runs the script scripts/xplot.sh, which starts xretractor in the background, and then pipes the data stream through xqry into gnuplot.

# from build/Debug
ninja ecg

The invocation CMake expands this into:

scripts/xplot.sh s205out rec205-replay.rql 720,560,1360 --gnuplot-rtl

Meaning of the parameters:

The --gnuplot-rtl option is an xqry parameter that reverses gnuplot’s X axis (set xrange [720:0]). The effect is that the freshest samples appear on the right side of the window, and older ones scroll to the left — similar to the classic ECG printout on paper tape.

The window shown in Fig. 59 displays 720 samples, i.e. exactly 2 seconds of signal at 360 Hz, matching the typical width of a single ECG strip used in diagnostics.

QRS Detection and Arrhythmia Identification

Context — the Pan-Tompkins algorithm

QRS-complex detection is the foundation of automatic ECG analysis. The QRS complex represents ventricular depolarization and corresponds to every heartbeat, visible as a sharp spike in the signal. Knowing the positions of the QRS complexes in time, RR intervals can be computed, and from them, basic rhythm disturbances can be identified:

The Pan-Tompkins algorithm (1985) is a classic, five-stage, pipelined digital-signal-processing algorithm implemented with FIR filters. RetractorDB implements it directly as an RQL query stream, without specialized DSP libraries.

Generating the signal filters (coef)

The algorithm requires two sets of FIR coefficients, stored as text files (bp_coef.txt, d_coef.txt). They are generated once, with Python scripts, before running detection.

The band-pass filter — gen_bp_coef.py

Step 1 of the algorithm requires a filter that cuts out noise and artifacts outside the QRS band. A passband of 5–15 Hz at fs = 360 Hz produces a response containing the QRS morphology, while attenuating baseline wander (< 5 Hz) and muscle noise (> 15 Hz).

The design method is a windowed sinc:

h_bp[n] = (h_lp2[n] − h_lp1[n]) · w[n]

where:

• h_lp[n] = 2·fc·sinc(2·fc·(n−M)) — the ideal low-pass filter
• w[n] = 0.54 − 0.46·cos(2πn/(N−1)) — the Hamming window, damping Gibbs effects
• M = (N−1)/2 = 12 — the filter’s center point (group delay = 12 samples)

Parameters:

Running the script:

cd examples/ecg/rec205
python3 gen_bp_coef.py
# Saved 25 coefficients to bp_coef.txt
# Coefficients: [-2, -2, -1, 0, 3, 8, 14, 23, 32, 41, 49, 54, 56, ...]
# Sum (DC gain): 5 / 1000 = 0.0050

The coefficients are symmetric about the center (n=12), confirming the filter’s linear phase — an essential property when analyzing ECG, since it guarantees no phase distortion of the QRS morphology.

The differentiating filter — gen_d_coef.py

Step 2 of the algorithm applies a filter that emphasizes the steep edges of the QRS. Pan and Tompkins proposed a 5-point derivative estimator:

y[n] = (1/8T) · (−x[n−4] − 2·x[n−3] + 2·x[n−1] + x[n])

Coefficients (from oldest to newest sample):

h = [−1, −2, 0, 2, 1]

Filter properties:

cd examples/ecg/rec205
python3 gen_d_coef.py
# Saved 5 coefficients to d_coef.txt
# Coefficients: [-1, -2, 0, 2, 1]
# Sum (DC gain): 0  (should be 0)

Implementing the pipeline in RQL — rec205-detect.rql

The file rec205-detect.rql implements the complete five-stage pipeline for both ECG channels (MLII and V1):

DECLARE MLII INTEGER, V1 INTEGER STREAM ecg, 1/360 FILE 'rec205'
DECLARE bp_coef INTEGER[25] STREAM bpf, 1 FILE 'bp_coef.txt'
DECLARE d_coef INTEGER[5]   STREAM df,  1 FILE 'd_coef.txt'

# Extracting the channels
SELECT ecg.MLII STREAM mlii FROM ecg VOLATILE
SELECT ecg.V1   STREAM v1   FROM ecg VOLATILE

# 1. Band-pass filter (5-15 Hz) — 25-tap FIR convolution
SELECT *                        STREAM mlii_win FROM mlii@(1,25)  VOLATILE
SELECT mlii_win[_]*bpf[_]       STREAM bp_acc   FROM mlii_win+bpf VOLATILE
SELECT bp_acc[0]/1000           STREAM bp_out   FROM bp_acc.sumc  VOLATILE

# 2. Differentiation — 5-tap FIR convolution
SELECT *                        STREAM bp_win   FROM bp_out@(1,5) VOLATILE
SELECT bp_win[_]*df[_]          STREAM d_acc    FROM bp_win+df    VOLATILE
SELECT d_acc[0]                 STREAM d_out    FROM d_acc.sumc   VOLATILE

# 3. Squaring (/1000 prevents int32 overflow)
SELECT d_out[0]*d_out[0]/1000   STREAM sq_out   FROM d_out        VOLATILE

# 4. Moving-window integration over 30 samples (~83 ms)
SELECT *                        STREAM mwi_win  FROM sq_out@(1,30) VOLATILE
SELECT mwi_win[0]               STREAM mwi      FROM mwi_win.avg   VOLATILE

# 5. Adaptive threshold — 2x moving average over 180 samples (0.5 s)
SELECT *                        STREAM mwi_long FROM mwi@(1,180)  VOLATILE
SELECT mwi_long[0]              STREAM mwi_thr  FROM mwi_long.avg VOLATILE

# Output: MLII centered, V1 centered, detection signal ×5
SELECT mlii[0]-900, v1[0]-900, (mwi[0]-mwi_thr[0]*2)*5 \
STREAM detect_out FROM mlii+v1+mwi+mwi_thr VOLATILE

Rationale for the parameters

The @(1,25) operator creates a sliding window of 25 samples, while [_] and sumc implement discrete convolution — see the chapter Underscore Symbol Processing.

The /1000 division in step 3 compensates for the integer scale of the coefficients — without it, the product d_out × d_out would exceed the int32 range (2,147,483,647) for typical ECG amplitudes.

The output expression (mwi[0]-mwi_thr[0]*2)*5 implements the adaptive threshold: the value is positive only when the MWI envelope exceeds twice the current moving average — indicating a detected QRS. The ×5 multiplier scales the detection signal to a range visually comparable to the raw ECG on the chart.

Running it — ninja ecg-detect-qrs

The process is started with a single command from the build/Debug directory:

cd build/Debug
ninja ecg-detect-qrs

CMake expands this target into the command:

scripts/xplot.sh detect_out rec205-detect.rql 720,-400,400 --gnuplot-rtl

Meaning of the parameters:

The xplot.sh script starts xretractor in the background (compiling and executing the queries), then pipes the detect_out stream through xqry into gnuplot in continuous mode. The gnuplot window refreshes with every new batch of samples.

Figure description — the gnuplot window

In Fig. 60, three signals are visible, corresponding to the three fields of the detect_out stream:

[detect-out-0] red line — MLII centered (mlii − 900)

The raw ECG signal from lead MLII, shifted by the 900 ADC baseline point so that the zero axis corresponds to the isoline. Two sharp spikes (amplitude ≈ 280 ADC ≈ 1.4 mV) around samples 520 and 350 from the right edge represent two consecutive QRS complexes. The clear QRS morphology, with a dominant R peak, confirms the band-pass filter is working correctly — noise has been suppressed while the peak retained its amplitude.

[detect-out-1] blue line — V1 centered (v1 − 900)

The signal from lead V1 of the same recording. QRS morphology in V1 is, as a rule, less pronounced than in MLII, which is visible in the figure — the blue signal shows a smaller R-peak amplitude at similar QRS time positions. Having both channels available at once allows distinguishing supraventricular contractions (APC) from ventricular ones (PVC), since ventricular QRS complexes show a distinct morphology in V1.

[detect-out-2] green line — the QRS detection signal ((mwi − 2·mwi_thr) × 5)

The algorithm’s output signal. A positive value means a detected QRS complex — the moving-window-integration envelope exceeded twice the adaptive threshold. In the figure, two clear positive pulses are visible, coinciding in time with the QRS peaks on the MLII channel. Between beats, the line stays close to zero or slightly below — confirming the detector’s specificity.

The gap between the two visible QRS complexes is approximately 170 samples, which at 360 Hz gives:

RR ≈ 170 / 360 ≈ 0.47 s  →  HR ≈ 127 bpm

This value falls within the range of ventricular tachycardia (VT, 79–216 bpm) recorded in record 205, suggesting that the visualized fragment of the recording comes from one of the 6 VT episodes noted by the MIT-BIH cardiologists.

Process flow diagram

The diagram below (Fig. 61) shows the complete data flow from the raw MIT-BIH recording to arrhythmia identification, indicating where RetractorDB carries out the Pan-Tompkins algorithm, and its relationship to classic arrhythmia-recognition methods:

The right branch of the diagram — Arrhythmia identification — represents classic post-QRS-detection analysis methods, which can be built as further RQL queries layered on top of the detect_out stream:

RetractorDB provides RULE operators and windowed aggregates (.avg, .sumc), which make it possible to implement the methods above in the same RQL query language, without leaving the system’s environment. QRS detection is the first and necessary step in this hierarchy.