Transient cardiorespiratory events during NREM sleep: A feline model for human microarousals

Microarousals (MAs) are brief transient events that occur during normal sle ep in humans and with increased frequency in disordered sleep, especially i n association with sleep apnoea. In a feline model, we discovered transient cardiorespiratory events during nonrapid eye movement (NREM) sleep that ex hibited consistent features with similarities to human MAs. It was observed that MAs have two distinct phases. Phase I (MAI) is characterized by an ab rupt increase in electromyogram (EMG) amplitude (> 50%), increased electroo culogram (EOG) activity and accelerated frequency of hippocampal electroenc ephalographic (EEG) activity. MAI lasts 4.1 +/- 0.3 s. Phase II (MAII), las ting 9.8 +/- 0.8 s, is characterized by high frequency EEG activity, but EM G, EOG and hippocampal activity remain at baseline levels. Mean inspiratory rate begins to increase 15 s before the onset of the MA, followed 10 s lat er by the increase in mean heart rate. Mean respiratory rate decreases shar ply through MAII, and returns to baseline levels 15 s after the MA. During MAII mean heart rate decreases quickly; there is increased respiratory irre gularity, followed by a prolonged ventilatory overshoot. The abrupt shift i n heart rate is coincident with the change in breath timing seen during MAI I. Heart rate returns to baseline levels 10 s following the MA. Integrating our findings with those described previously in humans, we propose that MA s may serve as a homeostatic mechanism which is designed to restore cardior espiratory function allowing the continuity of sleep.