Attached image shows plot of Fast fourier transform of ECG that had applied a lowpass filter to (15 Hz, zero-phase shift FIR with 24 dB/octave roll-off). The frequency bands increase in increments of 0.03125Hz from 0 to 15 Hz. HRV (< 0.4 Hz) and HR (~1.5 Hz) peaks are clear, but I'm unsure what the harmonics are specifically or their significance. I've found suggestions they are related to subtle variations in the PQRST waveform but find very little literature detailing their significance or meaning, besides a preprint Kotriwar, Y., Kachhara, S., Harikrishnan, K. P. & Ambika, G. Higher order spectral analysis of ECG signals. arXiv (2018) doi:10.48550/arxiv.1809.08451.

Any advice or further references related to these harmonics and their meaning would be greatly appreciated. Thanks