Frequency domain heart rate variability and plasma norepinephrine level inthe coronary sinus during handgrip exercise

Background: Heart rate (HR) variability has been recognized as an important noninvasive index of autonomic nervous activities. However, the relationsh ip between HR variability and cardiac circulating norepinephrine (NE), espe cially with respect to coronary ischemia, remains unclear. Hypothesis: This study was undertaken to determine whether HR variability i ndices can reflect cardiac NE levels during handgrip exercise. Methods: We simultaneously measured HR variability and cardiac NE overflow rate in 32 patients (30 men, 2 women) during a 6-min isometric handgrip exe rcise. Among the 32 subjects, 20 (19 men, 1 woman) had coronary artery dise ase (CAD) and 12 (control group; 11 men, 1 woman) did not. Results: Hemodynamics and cardiac NE overflow rates among subjects at rest were not significantly different between the two groups. In the normal subj ects, low-frequency (LF) spectra and LF/HF (high-frequency) ratios were not significantly changed during handgrip exercise, but HF spectra significant ly increased from 10.1 +/- 4.5 to 12.2 +/- 7.0 ms (p < 0.05). In the subjec ts with CAD, LF and LF/HF spectra were significantly (p < 0.05 and 0.01, re spectively) increased by handgrip exercise. High-frequency spectra were not significantly changed by handgrip exercise. In the normal subjects, a sign ificant negative relation (r = -0.76, p < 0.01) was obtained between HF cha nge and cardiac NE overflow rate, whereas this relationship was not signifi cant in the subjects with CAD. The correlation between changes of LF/HF and cardiac NE overflow rate was significant in the normal (r = 0.56, p < 0.05 ) but not in subjects with CAD. Conclusion: These results suggest that vagal modulation of HR variability i s more prominent in normal coronary artery subjects than in CAD subjects du ring handgrip exercise. Heart rate variability indices may thus serve as ad equate indicators of autonomic nerve activity in subjects with normal coron ary arteries but not in those with CAD, probably due to decreased adaptatio n to physical stress during handgrip exercise.