ECG and Cardiac Signal Processing

Overview

Process electrocardiogram (ECG) and photoplethysmography (PPG) signals for cardiovascular analysis. This module provides comprehensive tools for R-peak detection, waveform delineation, quality assessment, and heart rate analysis.

Main Processing Pipeline

ecg_process()

Complete automated ECG processing pipeline that orchestrates multiple steps.

signals, info = nk.ecg_process(ecg_signal, sampling_rate=1000, method='neurokit')

Pipeline steps:

1. Signal cleaning (noise removal)
2. R-peak detection
3. Heart rate calculation
4. Quality assessment
5. QRS delineation (P, Q, S, T waves)
6. Cardiac phase determination

Returns:

• signals: DataFrame with cleaned ECG, peaks, rate, quality, cardiac phases
• info: Dictionary with R-peak locations and processing parameters

Common methods:

• 'neurokit' (default): Comprehensive NeuroKit2 pipeline
• 'biosppy': BioSPPy-based processing
• 'pantompkins1985': Pan-Tompkins algorithm
• 'hamilton2002', 'elgendi2010', 'engzeemod2012': Alternative methods

Preprocessing Functions

ecg_clean()

Remove noise from raw ECG signals using method-specific filtering.

cleaned_ecg = nk.ecg_clean(ecg_signal, sampling_rate=1000, method='neurokit')

Methods:

• 'neurokit': High-pass Butterworth filter (0.5 Hz) + powerline filtering
• 'biosppy': FIR filtering between 0.67-45 Hz
• 'pantompkins1985': Band-pass 5-15 Hz + derivative-based
• 'hamilton2002': Band-pass 8-16 Hz
• 'elgendi2010': Band-pass 8-20 Hz
• 'engzeemod2012': FIR band-pass 0.5-40 Hz

Key parameters:

• powerline: Remove 50 or 60 Hz powerline noise (default: 50)

ecg_peaks()

Detect R-peaks in ECG signals with optional artifact correction.

peaks_dict, info = nk.ecg_peaks(cleaned_ecg, sampling_rate=1000, method='neurokit', correct_artifacts=False)

Available methods (13+ algorithms):

• 'neurokit': Hybrid approach optimized for reliability
• 'pantompkins1985': Classic Pan-Tompkins algorithm
• 'hamilton2002': Hamilton's adaptive threshold
• 'christov2004': Christov's adaptive method
• 'gamboa2008': Gamboa's approach
• 'elgendi2010': Elgendi's two moving averages
• 'engzeemod2012': Modified Engelse-Zeelenberg
• 'kalidas2017': XQRS-based
• 'martinez2004', 'rodrigues2021', 'koka2022', 'promac': Advanced methods

Artifact correction: Set correct_artifacts=True to apply Lipponen & Tarvainen (2019) correction:

• Detects ectopic beats, long/short intervals, missed beats
• Uses threshold-based detection with configurable parameters

Returns:

• Dictionary with 'ECG_R_Peaks' key containing peak indices

ecg_delineate()

Identify P, Q, S, T waves and their onsets/offsets.

waves, waves_peak = nk.ecg_delineate(cleaned_ecg, rpeaks, sampling_rate=1000, method='dwt')

Methods:

• 'dwt' (default): Discrete wavelet transform-based detection
• 'peak': Simple peak detection around R-peaks
• 'cwt': Continuous wavelet transform (Martinez et al., 2004)

Detected components:

• P waves: ECG_P_Peaks, ECG_P_Onsets, ECG_P_Offsets
• Q waves: ECG_Q_Peaks
• S waves: ECG_S_Peaks
• T waves: ECG_T_Peaks, ECG_T_Onsets, ECG_T_Offsets
• QRS complex: onsets and offsets

Returns:

• waves: Dictionary with all wave indices
• waves_peak: Dictionary with peak amplitudes

ecg_quality()

Assess ECG signal integrity and quality.

quality = nk.ecg_quality(ecg_signal, rpeaks=None, sampling_rate=1000, method='averageQRS')

Methods:

• 'averageQRS' (default): Template matching correlation (Zhao & Zhang, 2018)
  • Returns quality scores 0-1 for each heartbeat
  • Threshold: >0.6 = good quality
• 'zhao2018': Multi-index approach using kurtosis, power spectrum distribution

Use cases:

• Identify low-quality signal segments
• Filter out noisy heartbeats before analysis
• Validate R-peak detection accuracy

Analysis Functions

ecg_analyze()

High-level analysis that automatically selects event-related or interval-related mode.

analysis = nk.ecg_analyze(signals, sampling_rate=1000, method='auto')

Mode selection:

• Duration < 10 seconds → event-related analysis
• Duration ≥ 10 seconds → interval-related analysis

Returns: DataFrame with cardiac metrics appropriate for the analysis mode.

ecg_eventrelated()

Analyze stimulus-locked ECG epochs for event-related responses.

results = nk.ecg_eventrelated(epochs)

Computed metrics:

• ECG_Rate_Baseline: Mean heart rate before stimulus
• ECG_Rate_Min/Max: Minimum/maximum heart rate during epoch
• ECG_Phase_Atrial/Ventricular: Cardiac phase at stimulus onset
• Rate dynamics across epoch time windows

Use case: Experimental paradigms with discrete trials (e.g., stimulus presentations, task events).

ecg_intervalrelated()

Analyze continuous ECG recordings for resting state or extended periods.

results = nk.ecg_intervalrelated(signals, sampling_rate=1000)

Computed metrics:

• ECG_Rate_Mean: Average heart rate over interval
• Comprehensive HRV metrics (delegates to hrv() function)
  • Time domain: SDNN, RMSSD, pNN50, etc.
  • Frequency domain: LF, HF, LF/HF ratio
  • Nonlinear: Poincaré, entropy, fractal measures

Minimum duration:

• Basic rate: Any duration
• HRV frequency metrics: ≥60 seconds recommended, 1-5 minutes optimal

Utility Functions

ecg_rate()

Compute instantaneous heart rate from R-peak intervals.

heart_rate = nk.ecg_rate(peaks, sampling_rate=1000, desired_length=None)

Method:

• Calculates inter-beat intervals (IBIs) between consecutive R-peaks
• Converts to beats per minute (BPM): 60 / IBI
• Interpolates to match signal length if desired_length specified

Returns:

• Array of instantaneous heart rate values

ecg_phase()

Determine atrial and ventricular systole/diastole phases.

cardiac_phase = nk.ecg_phase(ecg_cleaned, rpeaks, delineate_info)

Phases computed:

• ECG_Phase_Atrial: Atrial systole (1) vs. diastole (0)
• ECG_Phase_Ventricular: Ventricular systole (1) vs. diastole (0)
• ECG_Phase_Completion_Atrial/Ventricular: Percentage of phase completion (0-1)

Use case:

• Cardiac-locked stimulus presentation
• Psychophysiology experiments timing events to cardiac cycle

ecg_segment()

Extract individual heartbeats for morphological analysis.

heartbeats = nk.ecg_segment(ecg_cleaned, rpeaks, sampling_rate=1000)

Returns:

• Dictionary of epochs, each containing one heartbeat
• Centered on R-peak with configurable pre/post windows
• Useful for beat-to-beat morphology comparison

ecg_invert()

Detect and correct inverted ECG signals automatically.

corrected_ecg, is_inverted = nk.ecg_invert(ecg_signal, sampling_rate=1000)

Method:

• Analyzes QRS complex polarity
• Flips signal if predominantly negative
• Returns corrected signal and inversion status

ecg_rsp()

Extract ECG-derived respiration (EDR) as respiratory proxy signal.

edr_signal = nk.ecg_rsp(ecg_cleaned, sampling_rate=1000, method='vangent2019')

Methods:

• 'vangent2019': Bandpass filtering 0.1-0.4 Hz
• 'charlton2016': Bandpass 0.15-0.4 Hz
• 'soni2019': Bandpass 0.08-0.5 Hz

Use case:

• Estimate respiration when direct respiratory signal unavailable
• Multi-modal physiological analysis

Simulation and Visualization

ecg_simulate()

Generate synthetic ECG signals for testing and validation.

synthetic_ecg = nk.ecg_simulate(duration=10, sampling_rate=1000, heart_rate=70, method='ecgsyn', noise=0.01)

Methods:

• 'ecgsyn': Realistic dynamical model (McSharry et al., 2003)
  • Simulates P-QRS-T complex morphology
  • Physiologically plausible waveforms
• 'simple': Faster wavelet-based approximation
  • Gaussian-like QRS complexes
  • Less realistic but computationally efficient

Key parameters:

• heart_rate: Average BPM (default: 70)
• heart_rate_std: Heart rate variability magnitude (default: 1)
• noise: Gaussian noise level (default: 0.01)
• random_state: Seed for reproducibility

ecg_plot()

Visualize processed ECG with detected R-peaks and signal quality.

nk.ecg_plot(signals, info)

Displays:

• Raw and cleaned ECG signals
• Detected R-peaks overlaid
• Heart rate trace
• Signal quality indicators

ECG-Specific Considerations

Sampling Rate Recommendations

• Minimum: 250 Hz for basic R-peak detection
• Recommended: 500-1000 Hz for waveform delineation
• High-resolution: 2000+ Hz for detailed morphology analysis

Signal Duration Requirements

• R-peak detection: Any duration (≥2 beats minimum)
• Basic heart rate: ≥10 seconds
• HRV time domain: ≥60 seconds
• HRV frequency domain: 1-5 minutes (optimal)
• Ultra-low frequency HRV: ≥24 hours

Common Issues and Solutions

Poor R-peak detection:

• Try different methods: method='pantompkins1985' often robust
• Ensure adequate sampling rate (≥250 Hz)
• Check for inverted ECG: use ecg_invert()
• Apply artifact correction: correct_artifacts=True

Noisy signal:

• Use appropriate cleaning method for noise type
• Adjust powerline frequency if outside US/Europe
• Consider signal quality assessment before analysis

Missing waveform components:

• Increase sampling rate (≥500 Hz for delineation)
• Try different delineation methods ('dwt', 'peak', 'cwt')
• Verify signal quality with ecg_quality()

Integration with Other Signals

ECG + RSP: Respiratory Sinus Arrhythmia

# Process both signals
ecg_signals, ecg_info = nk.ecg_process(ecg, sampling_rate=1000)
rsp_signals, rsp_info = nk.rsp_process(rsp, sampling_rate=1000)

# Compute RSA
rsa = nk.hrv_rsa(ecg_info['ECG_R_Peaks'], rsp_signals['RSP_Clean'], sampling_rate=1000)

Multi-modal Integration

# Process multiple signals at once
bio_signals, bio_info = nk.bio_process(
    ecg=ecg_signal,
    rsp=rsp_signal,
    eda=eda_signal,
    sampling_rate=1000
)

References

• Pan, J., & Tompkins, W. J. (1985). A real-time QRS detection algorithm. IEEE transactions on biomedical engineering, 32(3), 230-236.
• Hamilton, P. (2002). Open source ECG analysis. Computers in cardiology, 101-104.
• Martinez, J. P., Almeida, R., Olmos, S., Rocha, A. P., & Laguna, P. (2004). A wavelet-based ECG delineator: evaluation on standard databases. IEEE Transactions on biomedical engineering, 51(4), 570-581.
• Lipponen, J. A., & Tarvainen, M. P. (2019). A robust algorithm for heart rate variability time series artefact correction using novel beat classification. Journal of medical engineering & technology, 43(3), 173-181.