Interactions between cardiac, respiratory and EEG‐δ oscillations in rats during anaesthesia

Abstract

We hypothesized that, associated with the state of anaesthesia, characteristic changes exist in both cardio-respiratory and cerebral oscillator parameters and couplings, perhaps varying with depth of anaesthesia. Electrocardiograms (ECGs), respiration and electroencephalograms (EEGs) were recorded from two groups of 10 rats during the entire course of anaesthesia following the administration of a single bolus of ketamine-xylazine (KX group) or pentobarbital (PB group). The phase dynamics approach was then used to extract the instantaneous frequencies of heart beat, respiration and slow δ-waves (within 0.5-3.5 Hz). The amplitudes of δ-and θ-waves were analysed by use of a time-frequency representation of the EEG signal within 0.5-7.5 Hz obtained by wavelet transformation, using the Morlet mother wavelet. For the KX group, where slow δ-waves constituted the dominant spectral component, the Hilbert transform was applied to obtain the instantaneous δ-frequency. The θ-activity was spread over too wide a spectral range for its phase to be meaningfully defined. For both agents, we observed two distinct phases of anaesthesia, with a marked increase in θ-wave activity occurring on passage from a deeper phase of anaesthesia to a shallower one. In other respects, the effects of the two anaesthetics were very different. For KX anaesthesia, the two phases were separated by a marked change in all three instantaneous frequencies: stable, deep, anaesthesia with small frequency variability was followed by a sharp transition to shallow anaesthesia with large frequency variability, lasting until the animal awoke. The transition occurred 16-76 min after injection of the anaesthetic, with simultaneous reduction in the δ-wave amplitude. For PB anaesthesia, the two epochs were separated by the return of a positive response to the pinch test at 53-94 min, following which it took a further period of 45-70 min for the animal to awaken. δ-Waves were not apparent at any stage of PB anaesthesia. We applied non-linear dynamics and information theory to seek evidence of causal relationships between the cardiac, respiratory and slow δ-oscillations. We demonstrate that, for both groups, respiration drives the cardiac oscillator during deep anaesthesia. During shallow KX anaesthesia the direction either reverses, or the cardio-respiratory interaction becomes insignificant; in the deep phase, there is a unidirectional deterministic interaction of respiration with slow δ-oscillations. For PB anaesthesia, the cardio-respiratory interaction weakens during the second phase but, otherwise, there is no observable change in the interactions. We conclude that non-linear dynamics and information theory can be used to identify different stages of anaesthesia and the effects of different anaesthetics.

Effects of the anesthetic agent propofol on neural populations

“…In addition to these studies of microscopic actions, much research has been devoted to macroscopic effects of AA, such as the cardiovascular response of subjects to AAs (Mustola et al 2003;Musizza et al 2007) and the power spectrum of the subjects' resting electroencephalogram (EEG) as a function of the blood concentration of AAs (Forrest et al 1994;Dutta et al 1997;Kuizenga et al 2001;Fell et al 2005;Han et al 2005). The resting EEG power spectrum especially reflects the anesthetic action in a characteristic way and permits the classification of the depth of anesthesia by so-called monitors, see e.g.…”

Event-Related Desynchronization in Diffusively Coupled Oscillator Models

“…Dramatic decrease in delta band (1)(2)(3)(4) intensity and an increase in theta (4)(5)(6)(7)(8) intensity is reported to be a signature of transition from deep to light anesthesia [3] (see Fig. 1).…”

“…Task-specific experimental observations have been made in this direction [1] and task/brain wave -specific detailed models have been proposed for ERD/ERS [5]; however a more general phe-FIG. 1: (Color online) Experimental observation of dramatic diminution of EEG-delta intensity (left) and emergence of EEG-theta intensity (right) during transition from deep to light anesthesia, the transition time is around 40 mins [3]. Note the substantial difference in the intensities of the delta and theta oscillations.…”

“…Note the substantial difference in the intensities of the delta and theta oscillations. Reproduced from Musizza et al [3], with permission.…”

“…Therefore, the state with which we began was a state of complete synchronization in a specific frequency band that corresponds to a particular functional/behavioral task. In the case of the motivating experiment [3], this is the state of deep anaesthesia corresponding to synchronization in the δ frequency band. However, as the strength of the external stimulus (say, the need to code information) increases and when the need to accomplish this task arises (sufficient strength of B), the intensity of the previously synchronized frequency band is reduced (which is termed as ERD) by desynchronization of few oscillators from the original frequency.…”