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Figure 2. A, The slope of the regression (sensitivity parameter) is thought to be a stable characteristic of the individual, whereas the intercept varies with changes in cardiac vagal tone (reproduced with permission from Grossman and Kollai, 1993); x-axis: respiration rate (in Hz), y-axis: RSA/VT. The two lines represent the same individual at different levels of cardiac vagal tone. B, Example for two individuals with different sensitivities of the respiratory modulation of RSA/VT. In this illustrative case, both show decreases of cardiac vagal tone from baseline to the experimental phase; solid lines: RSA/VT regression line during baseline paced breathing calibration with three respiration rates (TTOT = 6.5, 5, and 3.5 s, equaling 9.2, 12, and 17.1 breaths/min); dashed lines: hypothetical level of RSA/VT for the three respiration rates during a subsequent experiment that led to a decrease in vagal tone. RSA/VT for the experiment is calculated for each breath as observed RSA/VT minus RSA/VT baseline at the observed TTOT of the respective breath.
