Relation between QT interval variability and muscle sympathetic nerve activity in normal subjects

Abstract

Beat-to-beat variability of the QT interval (QTV) is sought to provide an indirect non-invasive measure of sympathetic nerve activity, but a formal quantification of this relationship has not been provided. In this study we used power contribution analysis to study the relationship between QTV and muscle sympathetic nerve activity (MSNA). Here, ECG and MSNA were recorded in 10 healthy subjects in the supine position, followed by a 40° degree head-up tilt. Power spectrum analysis was performed using a linear autoregressive model with two external inputs; heart period variability (RRV) and MSNA. Total and low frequency power of QTV was decomposed into contributions by RRV, MSNA, and sources independent of RRV and MSNA. Results show that the percentage of MSNA power contribution to QT is very small, and does not change with tilt. RRV power contribution to QT power is notable and decreases with tilt, while the greatest percentage of QTV is independent of RRV and MSNA in both the supine position and after 40° head-up tilt. In conclusion, beat-to-beat QTV in normal subjects does not appear to be significantly affected by the rhythmic modulations observed in MSNA following low-medium degrees of orthostatic stimulation. Therefore MSNA oscillations may not represent a useful surrogate for cardiac sympathetic nerve at moderate levels of activity or, alternatively, sympathetic influences on QTV are complex and not quantifiable with linear shift-invariant autoregressive models.