Afferent vagal modulation - Clinical studies of visceral sensory input

The frequency composition of a continuous time series of R-R intervals may be viewed as the phasic output of a central processing system intimately de pendent on sensory input from a variety of afferent sources. While differen t measures of heart rate variability permit a glimpse into the autonomic ef ferent limb of this complex system, direct access of afferent fibers in hum ans has remained elusive. Using a specially designed esophageal catheter/ma nometer probe, we have been able to gain access to vagal afferent fibers in the distal esophagus. Our studies on the effect of vagal afferent electros timulation on both cerebral evoked potentials (EvP) and the power spectrum of heart rate variability have yielded the following observations: 1. Stimulation of esophageal vagal afferents dramatically and reproducibly increases the high frequency (HF) vagal power and reduces the low frequency (LF) power of the heart rate autospectrum. 2. This effect is constant across stimulation frequencies from 0.1 to 1.0 H z and across stimulation intensities from 2.5 to 20 mA. 3. Regardless of the stimulation parameters, there are only minimal changes in heart rate (2-6 bpm) and no change in respiratory frequency. 4. There is a linear correlation between electrical stimulation intensity a nd the amplitude of cerebral evoked potentials, whereas there is a non-line ar relationship with all short-term power spectral indices. 5. While cerebral evoked potentials are only elicited at stimulation intens ities above perception threshold, there is already a significant shift to i ncreased vagal efferent modulation well below perception threshold. Conclusion: These studies support the concept that power spectral indices o f heart rate variability represent phasic output responses to tonic afferen t viscerosensory signals in humans. These studies also demonstrate the feas ibility of accessing vagal afferents in humans. (C) 2001 Elsevier Science B .V. All rights reserved.