It’s difficult to read that plot. It would help to see the original 12-lead EKG and a 1-minute Lead II rhythm strip.
At such a low frequency, I would imagine that the peak that follows the ‘notch’ is due to ‘respiratory sinus arrythmia’ ( Respiratory sinus arrhythmia: why does the heartbeat synchronize with respiratory rhythm? ), the Hering-Breuer reflex, or some similar physiologic phenomenon. The ‘notch’ you see is itself simply the frequencies at which less energy are present. This is not uncommon with physiologic signals, and unless it represents a change from a previous normal record, is likely no cause for concern.
Admittedly, I have not routinely read fft analyses of EKGs, preferring to read the original EKGs themselves, where all the clinical information resides.
I am troubled by the magnitude of the fft of any EKG that is lower than zero. The PQRSTU complex is generally about -2 mV +10 mV, (in Lead II, it varies considerably in other leads) and has an isoelectric reference at zero mV.
I’ll let that go for now, because I do not know what EKG you started with or how you did the fft. However I consider the entire fft analysis of that EKG to be troubling. Most of the information in an EKG should be between about 1 and 100 Hz, and this one clearly defies that generally-held rule. The spikes at about 300 and 420 Hz bother me. Nothing in the normal heart (and presumably in well-functioning instrumentation as well) would produce significant energy at such frequencies.
Without the original EKG and seeing the code you used to produce that plot, it is impossible to come to any firm conclusions on your analysis.
