Frequency domain analysis of heart rate variability in horses at rest and during exercise

The pattern of variation in heart rate on a beat-to-beat basis contains inf ormation concerning sympathetic (SNS) and parasympathetic (PNS) contributio ns to autonomic nervous system (ANS) modulation of heart rate (HR). In the present study, heart period (RR interval) time series data were collected a t rest and during 3 different treadmill exercise protocols from 6 Thoroughb red horses. Frequency and spectral power were determined in 3 frequency ban ds: very low (VLF) 0-less than or equal to 0.01, low (LO) >0.01-less than o r equal to 0.07 and high (HI) >0.07-less than or equal to 0.5 cycles/beat. Indicators of sympathetic (SNSI = LO/HI) and parasympathetic (PNSI = HI/TOT AL) activity were calculated. Power in all bands fell progressively with in creasing exercise intensity from rest to trot. At the gallop VLF and LO pow er continued to fall but III power rose. SNSI rose from rest to walk, then fell with increasing effort and was lowest at the gallop. PNSI fell from re st to walk, then rose and was highest at the gallop. Normalised HI power ex ceeded combined VLF and LO power at all gaits, with the ratio HI to LO powe r being lowest at the walk and highest at the gallop. ANS indicators showed considerable inter-horse variation, and varied less consistently than raw power with increasing physical effort. In the horses studied, the relations hip between power and HR changed at exercise intensities associated with he art rates above approximately 120-130 beats/min. At this level, humoral and other non-neural mechanisms may become more important than autonomic modul ation in influencing heart rate and heart rate variability (HRV). HRV at in tense effort may be influenced by respiratory-gait entrainment, energetics of locomotion and work of breathing. HRV analysis in the frequency domain w ould appear to be of potential value as a noninvasive means of assessing au tonomic modulation of heart rate at low exercise intensities, only. The tec hnique may be a sensitive method for assessing exercise response to experim ental manipulations and disease states.