Spontaneous arousals during quiet sleep in piglets: a visual and wavelet-based analysis

The objectives of the study were to characterize spontaneous arousals during NREM sleep in piglets and to compare two methods of identifying these events: a "visual" technique using spectral analysis and an automated technique using wavelets. Our goal was to understand the benefits and limits of these methods when applied to sleep in human infants.

Arousals were identified by evaluating rapid changes in EEG low frequency activity, blood pressure (BP), and heart rate (HR). A cortical arousal was defined as a rapid decrease in EEG low frequency activity. An autonomic arousal was defined by a transient increase in heart rate or a transient change in mean arterial BP (MAP).

Laboratory study in sleeping and awake piglets.

Five 1-2 week old piglets.

Chronically instrumented with a femoral arterial line, EEG, EOG, EMG electrodes, and a micro-dialysis probe with its tip located in the rostral ventral medulla. Artificial CSF (aCSF) was dialyzed into the RVM throughout the experiments Measurements: For the visual analysis, the average delta power (0.5-4 Hz) for each 5-second epoch was determined using spectral analysis. MAP and HR were analyzed in 1-second bins. Video images were analyzed for body movements and eye openings. Transient changes in blood pressure, HR, and delta power were then visually identified. For the wavelet analysis, a quantitative, automated technique with a defined "wakefulness threshold" was used to identify rapid decreases in EEG low frequency activity and the rate of change of MAP.

Using the visual method, 117 episodes associated with stereotypical hemodynamic, EEG, and behavioral changes (startle) were identified. Seventy five events occurred in isolation or were first in a series of "multiple" events, 41 "multiple" events were defined as events occurring <20 seconds following a previous event. Eighteen events were associated with the termination of apnea. In isolated events or those occurring first in a series, the onset of changes in HR and BP clearly preceded the decrease in EEG amplitude and delta power. Using wavelet analysis, 73 EEG arousals and 115 MAP transients were identified independently; 62% of the EEG events were associated with a transient change in MAP and HR, and in these cases the onset of the hemodynamic events preceded EEG arousals. EEG arousals and MAP transients, however, also occurred alone and not associated with a stereotypical pattern of a startle, changes in MAP and HR and the EEG.

Many of these spontaneous arousals represent integrated EEG, hemodynamic, and behavioral processes similar to arousal phenomena described in adult rats and human infants, but the pattern of spontaneous arousals appears to be more heterogeneous than has been described for arousals induced by exogenous stimuli. Both the visual and wavelet analysis identified these events, but the wavelet technique has the potential advantage of better time resolution and automation of the analysis.