Autonomic nervous system-controlled cardiac pacing: A comparison between intracardiac impedance signal and muscle sympathetic nerve activity

A recently introduced rate responsive cardiac pacing system is based on inf ormation derived from the intracardiac impedance signal containing informat ion on the inotropic state of the ventricle. This study compared the inotro pic state index (ISI) with muscle sympathetic activity (MSA), both being mo dulated by the autonomic nervous system. Nine patients (66 +/- 3 years, mea n +/- SEM) with Inos(2)DR pacemakers were included. Each patient was studie d at rest and during cold presser test (CPT). Microneurography of the peron eal nerve was performed to measure MSA continuously, which was digitally st ored along with continuous surface ECG and blood pressure. The intracardiac impedance signal was transmitted by the pacemaker and stored simultaneousl y. Linear correlation between ISI and MSA wets calculated for the period of the CPT. During CPT, mean systolic blood pressure increased from 122 +/- 4 to 249 +/- 6 mmHg (P < 0.0001), diastolic blood pressure increased from 74 +/- 8 to 86 +/- 4 mmHg (P = 0,02), and intrinsic heart rate increcrsed rom 69 +/- 7 to 75 +/- 7 beats/mill (P = 0.019). ISI increased by 21 +/- 7% (P = 0.018), MSA by 26 +/- 6% (P = 0.004). ISI and MSA were positively correl ated during the CPT in eight of nine patients (R-2 = 0.86-0.99, P < 0.0001) . Negative correlation was found in one patient (R-2 = 0.94). This study de monstrates parallel increases of the ISI and MSA during CPT. ISI and MSA sh owed a close linear relationship during provoked changes of sympathetic act ivity. These results provide further evidence that the sympathetic nervous system is responsible for the observed ISI changes.