Calibrated variability of muscle sympathetic nerve activity during graded head-up tilt in humans and its link with noradrenaline data and cardiovascular rhythms

Muscle sympathetic nerve activity (MSNA) variability is traditionally computed through a low-pass filtering procedure requiring normalization. We proposed a new beat-to-beat MSNA variability preserving dimensionality typical of an integrated neural discharge (i.e. bursts per unit of time). The calibrated MSNA (cMSNA) variability is contrasted with the traditional uncalibrated MSNA (ucMSNA) version. The powers of cMSNA and ucMSNA variabilities in low frequency (LF, from 0.04 to 0.15 Hz) band were computed with those of heart period (HP), systolic and diastolic arterial pressure (SAP and DAP) in 7 healthy subjects (age: 20-28 years, median = 22 years, 5 females) during graded head-up tilt. Subjects were sequentially tilted at 0°, 20°, 30°, 40°, 60° table inclinations. The LF powers of ucMSNA and HP variabilities were expressed in normalized units (LFnu), while all remaining spectral markers were expressed in absolute units. We found that: i) the LF power of cMSNA variability was positively correlated with tilt angle, while the LFnu power of ucMSNA series was uncorrelated; ii) the LF power of cMSNA variability was correlated with LF powers of SAP and DAP, LFnu power of HP and noradrenaline concentration, while the relation of LFnu power of ucMSNA variability on LF powers of SAP and DAP was weaker and on LFnu power of HP was absent; iii) the stronger relation of cMSNA variability on SAP and DAP spectral markers compared to the ucMSNA series was confirmed individually. The cMSNA variability appears to be more suitable in describing sympathetic control in humans than traditional ucMSNA variability.