An Efficient Linear Phase High-Pass Filter for ECG.

Simple infinite impulse response (IIR) high-pass filters for ECG cause ST-segment distortion due to the overshoot in the impulse response unless the cutoff frequency is very low. Linear-phase filters solve the ST distortion problem but these filters are typically computationally expensive. Four 0.5 Hz linear-phase high-pass filters were compared. The reference high-pass filter was a single pole IIR filter run in the forward and backward direction (IIR). The other filters were a boxcar, a weighted average 9 boxcar filter to approximate a Blackman window (Multi-Boxcar), and a moving window median (Median). The filters were compared by average power spectrum on a 12-lead ECG database of prehospital ECGs containing significant baseline wander (n=5,368). The average power spectrum for the boxcar high-pass filters shows higher levels of low frequency attenuation. The multi-boxcar filter shows a lower passband ripple than the boxcar. The maximum (mean) absolute deviation from the raw signal power spectrum in the 1.0 to 10 Hz range was 0.86 (0.09), 1.6 (0.34), 0.27 (0.05) and 0.19 (0.04) dB for the IIR, boxcar, multi-Boxcar and median filters, respectively. The multi-boxcar filter improves the passband ripple compared to a simple boxcar high-pass while retaining an efficient design. The trade-off is a delay on the order of one second.