The application of Poincaré-plot analysis to heart-rate variability (HRV) is a common method for the assessment of autonomic balance. However, results obtained from the indexes provided by this analysis tend to be difficult to interpret. In this study the authors aimed to prove the usefulness of 2 new indexes: the stress score (SS) and the sympathetic:parasympathetic ratio (S:PS ratio).
25 professional Spanish soccer players from same team underwent 330 resting measurements of HRV. All subjects experienced 10 min of HRV monitoring through an R-R-interval recorder. The following parameters were calculated: (1) Poincaré-plot indexes: SD1 (transverse axis), which is proportional to parasympathetic activity; SD2 (longitudinal axis), which is inversely proportional to sympathetic activity; and the SD1:SD2 ratio; (2) time-domain parameters: standard deviation of R-R intervals (SDNN), root-mean-square differences of successive heartbeat intervals (rMSSD), and percentage of successive R-R-interval pairs differing in more than 50 ms in the entire recording divided by the total number of R-R intervals (pNN50); and (3) the proposed 2 new indexes: the SS and the S:PS ratio.
The study found a high negative correlation between the SS and SDNN (R 2 = .94). The S:PS ratio correlated inversely to rMSSD (R 2 = .95), SDNN (R 2 = .94), and pNN50 (R 2 = .74). The S:PS ratio showed a strong correlation with SD1 (R 2 = .95) and SS (r = .87, R 2 = .88).
The application of the SS as sympathetic-activity index and the S:PS ratio as a representation of autonomic balance (SS:SD1) provides a better understanding of the Poincaré-plot method in HRV.