Time-domain and spectral analysis of heart rate variability in rats challenged with hypoxia

Different approaches based on spectral and time-domain analysis of heart rate variability (HRV) were used to evaluate cardiac rhythm regulatory response to hypoxia. Nineteen male Wistar rats with previously implanted subcutaneous ECG electrodes, were maintained unrestrained in originally designed experimental setup, during normoxia and hypobaric hypoxia (-400 mmHg). RR intervals were extracted from continuous ECG recording. Time- and frequency-domain HRV estimates were computed from time-series of 1024 consecutive RRi. Power spectrum was obtained using both, fast Fourier transformation and autoregressive (AR) modeling. Compared to normoxia, hypoxia resulted in a significant increase of mean RRi, SDNN, rMSSD. AR and FFT analyses provided concordant power spectra across all tested bands. TP, including VLF, HF and predominantly LF, was increased in hypoxia, whereas LF/HF index of the sympatho-vagal balance remained unchanged. Changes in spectral power (TP, VLF, LF, HF) evoked by hypoxia correlated positively with simultaneous changes in SDNN and rMSSD. Interestingly, correlation coefficients derived from AR and FFT did not vary.