Fig 1.
Heart rate profile and indices.
A: Definition of heart rate (HR) profile and HR indices during the exercise stress test. The heart rate profile, xHR(t) (solid dark line) is a function of time obtained by filtering the instantaneous heart rate (dots) with a median filter 15 beats long. HRrest: Mean xHR(t) over 15 sec resting pre-test; HRex: Maximum xHR(t) during exercise; HRrec: minimum xHR(t) during recovery; ΔHRex = HRex − HRrest: HR increase during exercise; ΔHRrec = HRrec − HRex HR decrease during the 1 min recovery phase. HRm: Mean HR during the entire test; B: Distribution of the heart rate profile across all participants. Black solid line, dark and light shadowed areas represent median, 25th-75th percentiles and 5th-95th percentile intervals, respectively.
Table 1.
Study population.
Fig 2.
Intra-individual correlation of HR profile.
Distribution of the Pearson’s correlation coefficients between heart rate profiles during EST1 and EST2 for all individuals (n = 821). First, second and third quartiles correspond to cases A, B and C, respectively. The HR profiles corresponding to these values are shown in panels (A)-(C). Each panel is composed of two sub-panels, one on the left showing xHR(t) against time and using the same vertical axis for EST1 and EST2, and another one on the right showing xHR(t) against EST phase (0 and 1 correspond to the beginning and end of each EST) and with different vertical scales adjusted to span the HR range of xHR(t) during EST1 and EST2.
Fig 3.
Distribution of heart rate indices.
Distribution of heart rate indices during first and second exercise stress test (EST1 and EST2). Every HR index decreased during EST2 with respect to EST1. The decrease was small (<4%) but consistent across individuals. (***) p<5·10−4 (Paired, two-sided Wilcoxon signed rank test).
Fig 4.
Intra-individual correlation of HR indices.
Scatter-plots showing the correlation between HR indices at first (EST1) and second (EST2) assessment. The Spearman’s correlation coefficient, ρsp, and the coefficient of determination, R2, quantify the intra-individual correlation for each HR index and are reported in each panel. Dashed grey and red lines represent the identity line and the linear regression line, respectively.
Fig 5.
Intra-individual agreement for heart rate indices.
For each HR index, Bland-Altman plots were used to assess intra-individual agreement. For each individual, the difference in each index between EST2 and EST1 (x2-x1) is plotted against the mean value (x1+x2)/2. Each subplot reports the bias, i.e. mean(x2-x1), the standard deviation of the differences, i.e. σ = std(x2-x1), the number of individuals outside the limits of agreements (%), and the total number of individuals. The confidence interval (bias ± 2σ, red lines) is reported in dashed lines.
Fig 6.
Intra-individual reproducibility of extreme values.
Intra-individual reproducibility of abnormality, defined as the probability for an individual who was in a given quantile during EST1 to remain in the same quantile or to change of quintile during EST2. For example, intra-individual reproducibility of extreme values for ΔHRex (first quintile) and ΔHRrec (fifth quintile) was 58% and 57%, respectively.
Fig 7.
Influence of maximum workload differences on intra-individual correlation.
Intra-individual correlation of HR indices was assessed considering individuals who underwent ESTs with maximum work load that did not differ more than |ΔWL|. Bars represent the Spearman’s correlation coefficient. The number of individuals for each group is reported on top of each bar. Horizontal line is the mean Spearman’s correlation coefficient for resting HR.