Seers, Christopher Paul; (1997) Respiratory modulation of the vagal regulation of the heart rate and its beat to beat variability. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis examines concepts for respiratory rhythm generation in relation to how the central inspiratory activity interacts with the peripheral inputs responsible for the reflex control of the heart rate by the vagus nerve. The aim of the study was to examine the beat to beat parasympathetic control of the heart in the anaesthetized cat in eupnoea, in ventilation-induced hyperoxic-hypocapnic apnoea and during transitions between the two states. Morphine was used to slow respiration and elevate the threshold for rhythm generation, and [beta]-blockade to eliminate the cardiac sympathetic drive. Hypocapnic apnoea caused a largely parasympathetically mediated tachycardia, subsequent CO2 titrations showed a CO2 dependent bradycardia and a titratable increase in the beat to beat heart rate variability (HRV). When expiration was prolonged the bradycardia phase of the respiratory sinus arrhythmia (RSA) was generally larger, and closely followed the firing pattern of the expiratory EMG. Late in the expiratory phase there was a greater HRV which is analogous to the CO2 dependent HRV seen during hypocapnic apnoea. Morphine significantly augmented RSA and HRV by an increase in vagal tone (VT), which was not completely eliminated during inspiration. In the young cat changing from normoxic to hyperoxic breathing caused a marked bradycardia and increase in RSA. During apnoea hyperoxia significantly increased VT and HRV, which was then phasically inhibited at the onset of rhythm to give a substantial RSA. The results show that both morphine and the central CO2 drive can evoke a tonic VT which is not completely inhibited by the inspiratory related inhibition of the cardiovagal motoneurones (CVMs). In the young animal the peripheral chemoreceptors powerfully inhibit VT via the respiratory rhythm generating mechanisms, which is not seen in the adult. During apnoea the CVMs receive a central CO2 dependent tonic cardiac drive, which is rendered phasic with the onset of rhythm, and is analogous to the tonic expiratory drive to the expiratory bulbospinal neurones.

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