Comparison of indexes of respiratory sinus arrhythmia

Several indexes at time and spectral domains for assessment of the respiratory sinus arrhythmia have been proposed but none of them is yet considered as the reference. The goal of the present work was to describe how those indexes represent the low-pass characteristic of the sinus node in a broad range of breathing frequencies, and to evaluate the discriminative capacity of them over heart rate. Time and frequency domain parameters were calculated from the RR intervals and breathing series obtained at eight frequency rates of controlled breathing. We assumed an autoregressive with exogenous input variable model that related the changes in heart rate (output) to changes in respiration (input or exogenous variable). Spectral parameters were computed using this model. The statistical discrimination capacity of pairs of means at those frequencies was evaluated. For those indexes with higher capacity, the cut off frequency was computed. The results proved that the temporal index gain had the best discriminative capacity because it is less sensitive to the inter-individual variability. However, the indexes with higher capacity reflected appropriately the low-pass behavior of the sinus node.