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

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Research
• Noninvasive Cardiac Monitoring

Background:

• Beat-to-beat QT interval variability (QTV) is proposed as a noninvasive index of sympathetic nerve activity.
• A precise quantification of the relationship between QTV and muscle sympathetic nerve activity (MSNA) is lacking.

Purpose of the Study:

• To quantitatively assess the relationship between QTV and MSNA using power contribution analysis.
• To determine if QTV can serve as a surrogate for cardiac sympathetic nerve activity.

Main Methods:

• ECG and MSNA were recorded in 10 healthy subjects under supine and head-up tilt (40°) conditions.
• Power spectrum analysis using a linear autoregressive model decomposed QTV into contributions from heart rate variability (RRV) and MSNA.

Main Results:

• MSNA contributed minimally to QTV and was unaffected by tilt.
• RRV significantly contributed to QTV, decreasing with tilt.
• The majority of QTV originated from sources independent of both RRV and MSNA.

Conclusions:

• Beat-to-beat QTV is not significantly influenced by MSNA modulations during mild orthostatic stress.
• MSNA oscillations may not be a reliable surrogate for cardiac sympathetic nerve activity at moderate activation levels.
• Complex sympathetic influences on QTV may not be captured by linear autoregressive models.