Sloan et al. (A-31) note that cardiac sympathetic activation and parasympathetic withdrawal result in heart rate increases during psychological stress. As indicators of cardiac adrenergic activity, plasma epinephrine (E) and norepinephrine (NE) generally increase in response to psychological challenge.

Power spectral analysis of heart period variability also provides estimates of cardiac autonomic nervous system activity. The authors conducted a study to determine the relationship between neurohumoral and two different spectral estimates of cardiac sympathetic nervous system activity during a quiet resting baseline and in response to a psychologically challenging arithmetic task.
Subjects were healthy, medication-free male and female volunteers with a mean age of 37.8 years.
None had a history of cardiac, respiratory, or vascular disease. Among the data collected were the following measurements on E, NE, low-frequency (LF) and very-low-frequency (VLF) power spectral indices, and low-frequency/high frequency ratios (LH/HF). Measurements are given for three periods: baseline (B), a mental arithmetic task (MA), and change from baseline to task (DELTA).
Patient No. E NE LF/HF LF Period VLF 5 3.55535 6.28040 0.66706 7.71886 B 7.74600 5 0.05557 0.13960 0.48115 0.99826 DELTA 2.23823 5 3.61092 6.41999 0.18591 6.72059 MA 5.50777 6 3.55535 6.24611 2.48308 7.33729 B 6.64353 6 0.10821 0.05374 2.03738 0.77109 DELTA 1.27196 6 3.66356 6.19236 0.44569 6.56620 MA 5.37157 7 3.29584 4.91998 0.15473 7.86663 B 7.99450 7 0.59598 0.53106 0.14086 0.81345 DELTA 2.86401 7 3.89182 5.45104 0.01387 7.05319 MA 5.13049 8 4.00733 5.97635 1.58951 8.18005 B 5.97126 8 0.29673 0.11947 0.11771 1.16584 DELTA 0.39078 8 4.30407 6.09582 1.47180 7.01421 MA 5.58048 12 3.87120 5.35659 0.47942 6.56488 B 5.94960 12   0.19379 0.03415 DELTA 0.50134 12   0.67321 6.59903 MA 6.45094 13 3.97029 5.85507 0.13687 6.27444 B 5.58500 13 0.20909 0.10851 1.05965 0.49619 DELTA 1.68911 13 3.76120 5.96358 1.19652 5.77825 MA 3.89589 14 3.63759 5.62040 0.88389 6.08877 B 6.12490 14 0.31366 0.07333 1.06100 1.37098 DELTA 1.07633 14 3.95124 5.69373 1.94489 7.45975 MA 5.04857 18 4.44265 5.88053 0.99200 7.52268 B 7.19376 18 0.35314 0.62824 0.10297 0.57142 DELTA 2.06150 18 4.79579 6.50877 0.88903 6.95126 MA 5.13226 19  5.03044 0.62446 6.90677 B 7.39854 19  0.69966 0.09578 0.94413 DELTA 0.88309 19 2.94444 5.73010 0.72024 7.85090 MA 6.51545 20 3.91202 5.86363 1.11825 8.26341 B 6.89497 20 0.02020 0.21401 0.60117 1.13100 DELTA 1.12073 20 3.89182 6.07764 0.51708 7.13241 MA 5.77424 21 3.55535 6.21860 0.78632 8.74397 B 8.26111 21 0.31585 0.52487 1.92114 2.38726 DELTA 2.08151 21 3.87120 5.69373 1.13483 6.35671 MA 6.17960 22 4.18965 5.76832 0.02785 8.66907 B 7.51529 22 0.16705 0.05459 0.93349 0.89157 DELTA 1.00414 22 4.35671 5.71373 0.90563 7.77751 MA 6.51115