Paradoxical respiratory sinus arrhythmia in the anesthetized rat

This study examines respiratory sinus arrhythmia (RSA) in the isoflurane-anesthetized rat. In fifteen female Sprague-Dawley rats, we recorded continuous ECG and respiratory airflow before and after bilateral vagotomy. RSA was assessed using power spectral analysis and by plotting the normalised changes in heart period as a function of the time during the respiratory cycle. Contrary to descriptions of RSA in conscious rats, we observed in all rats in the current study a 'reversed' pattern of RSA in which heart rate decelerated during inspiration. Elimination of vagal efferent fibres to the heart by vagotomy did not abolish the presence of reversed RSA suggesting that the pattern of heart period variation is not neural, and may be largely mechanical. Vagotomy altered breathing by increasing respiratory period, tidal volume, and the time to peak inspiratory flow. These changes did not alter the magnitude of RSA but reduced the latency period between inspiratory onset and the onset of respiratory related prolongation of heart period. Periods of positive pressure ventilation were associated with reversal of the inspiratory cardiac-deceleration pattern of RSA to resemble the more widely described pattern of inspiratory cardiac-acceleration. We conclude that RSA is not a suitable measure of vagal tone during anesthesia in the rat and reiterate the caution that needs to be taken when working with anesthetized experimental models of cardiac control.