The sympathovagal balance during control and dobutamine stress MRI

Magnetic resonance imaging (MRI) of the heart precludes dynamic exercise. Therefore, stress MRI is accomplished by pharmacological stimulation, e.g., by the synthetic adrenergic agent dobutamine. With dobutamine stress, an autonomic defense (a shift of the sympathovagal balance towards vagal predominance) is to be expected. To assess this, the authors measured heart rate (HR) and the percentage 0.07-0.14 Hz low-frequency heart rate variability (LF) in 9 male patients (mean±SD age 60.6±8.6 years), 3 with one-, 3 with two-, and 3 with three-vessel disease, during control and dobutamine stress MRI. As the PQ-intervals were sometimes varying, we derived LF from PP-intervals (LF-P) as well as from QQ-intervals (LF-Q). With dobutamine, HR increased significantly (paired t-test, P<0.05) from (mean±SD) 67.0±6.5 to 102.9±13.9 bpm. LF-P decreased non-significantly from 38.1±15.7 to 30.0±14.0%; LF-Q decreased non-significantly from 41.8±16.2 to 37.9±21.1%. With submaximal exercise, HR and LF are known to increase; HR and LF reflect then both a modification of the sympathovagal balance towards sympathetic predominance. With dobutamine stress MRI, the increase in HR reflects a modification of the “total” (exogenous plus endogenous) sympathovagal balance towards sympathetic predominance. The opposite change of LF under dobutamine stress MRI suggests a modification of the endogenous sympathovagal balance towards vagal predominance (autonomic defense)