Quantification of respiratory sinus arrhythmia using Hilbert-Huang transform

To investigate whether the first intrinsic mode function, obtained from Hilbert-Huang transform (HHT), of heart rate variability is respiratory related. Electrocardiogram and chest circumference signals were recorded from 10 healthy subjects at supine rest. The HHT was applied to both R-R interval and chest circumference signals to figure out their first intrinsic mode functions (C1RR and C1RESP, respectively) from which the instantaneous amplitude, phase, and frequency were calculated. Although the instantaneous amplitudes and frequencies of C1RR and C1 RESP were variable, linear regression analysis indicated a phase lock between C1RR and C1RESP. Intake of 500 ml water significantly elevated the amplitude ratio of C1RR to C1 RESP; however, the phase difference of C1RR to C1 RESP was still unchanged. The data indicate that the first intrinsic mode function of heart rate variability is respiratory related and may be equivalent to respiratory sinus arrhythmia. As compared to fast Fourier transform, HHT of respiratory sinus arrhythmia provides a comparative spatial measurement with a much higher temporal resolution.