The effects of elevated body temperature on the complexity of the diaphragm EMG signals during maturation

In this paper, we examine the effect of elevated body temperature on the complexity of the diaphragm electromyography (EMGdia), the output of the respiratory neural network--using the approximate entropy method. The diaphragm EMG, EEG, EOG as well as other physiological signals (tracheal pressure, blood pressure and respiratory volume) in chronically instrumented rats were recorded at two postnatal ages: 25-35 days age (juvenile, n = 5) and 36-44 days age (early adult, n = 6) groups during control (36-37 degrees C), mild elevated body temperature (38 degrees C) and severe elevated body temperature (39-40 degrees C). Three to five trials of the recordings were performed at normal body temperature before raising the animal's core temperature by 1-4 degrees C with an electric heating pad. At the elevated temperature, another 3-5 trials were performed. Finally, the animal was cooled to the original temperature, and trials were again repeated. Complexity values of the diaphragm EMG signal were estimated and evaluated using the approximate entropy method (ApEn) over the ten consecutive breaths. Our results suggested that the mean approximate entropy values for the juvenile age group were 1.01 +/- 0.01 (standard error) during control, 0.91 +/- 0.02 during mild elevated body temperature and 0.81 +/- 0.02 during severe elevated body temperature. For the early adult age group, these values were 0.94 +/- 0.01 during control, 0.93 +/- 0.01 during mild elevated body temperature and 0.92 +/- 0.01 during severe elevated body temperature. Our results show that the complexity values and the durations of the diaphragm EMG (EMGdia) were significantly decreased when the elevated body temperature was shifted from control or mild to severe body temperature (p < 0.05) for the juvenile age group. However, for the early adult age group, an increase in body temperature slightly reduced the complexity measures and the duration of the EMGdia. But, these changes were not statistically significant. These results furthermore suggest that during maturation, the output of the central pattern generator becomes less complex probably because the elevated body temperature reduces the neural activity and alters the behavior of the central respiratory controller, making it more susceptible to sudden infant death syndrome (SIDS).