Ventilation during sleep onset
Creators: name: Colrain, Ian M
Publisher: name: University Of Tasmania
Year: 2023

description: Many aspects of human respiration are known to change from wakefulness to NREM sleep, including the level of ventilation, and the responsiveness to chemical respiratory stimuli. The differences in ventilation between wakefulness and sleep are due to a number of factors, one of which has been hypothesised to be the cessation of a wakefulness drive to respiration. Such an hypothesis would link the stimulation of respiration to the activity within the ascending reticular activating system, activity which varies greatly during the period of sleep onset, and which is responsible for the changes in activity seen at the cerebral cortex. The hypothesis allows the prediction that cessation of the wakefulness drive would occur rapidly and with a consistent relationship to EEG changes during the sleep onset period. The present literature lacks an accurate description of the relationship between changes in ventilation and alterations in cortical arousal as measured by the EEG. This thesis reports three experiments designed to investigate ventilation during sleep onset and to provide such a description, in both males and females. An assessment is also made of the extent to which respiratory data collected over the sleep onset period is influenced by the measurement devices used. In the first study, sleep onsets were studied in five young male adults in a series of single subject designs in which sleep onsets were replications. The results indicated that during sleep onset, the commencement of continuous theta activity in the EEG was associated with a substantial, rapid and highly predictable reduction in ventilation. The change in ventilation was typically due to a reduction in tidal volume, and was in part secondary to a reduction in metabolic rate. It was speculated that the non-metabolic component may reflect the loss of a wakefulness drive to respiration, though other interpretations were not eliminated. Sex differences exist in the incidence of sleep related respiratory dysfunction. In addition, there is uncertainty as to whether females show sleep related decreases in the responsiveness to chemical stimuli. This uncertainty has often been explained in terms of different subjects being in different cycles of the menstrual cycle when investigated. These data led to doubt as to whether females possess a wakefulness drive to respiration and, if so, whether it is affected by progesterone levels. The second experiment sought to investigate ventilation during sleep onset in normal young females, during both phases of the menstrual cycle. Sleep onsets were studied in five subjects in a series of single subject designs. The results obtained supported those of the male subjects. That is, decreases in ventilation were associated with the onset of theta activity in the EEG. Further, no menstrual phase differences existed in the magnitude of the ventilation decrease or in its timing relative to EEG variations. Thus, females demonstrated the same pattern of ventilation change as males, indicating that they also possess a wakefulness drive. In the first two experiments ventilation was monitored with a mask and breathing valve attached to a pneumotachograph and flow meter. There is evidence that during wakefulness, the increased dead space and respiratory load associated with such equipment substantially elevates the level of ventilation. The existence of such an effect while awake, but not during sleep, would also explain the above results. The third experiment was designed to determine whether the decrease in ventilation, associated with the onset of theta activity, could also be obtained without the mask and associated apparatus. Two young male subjects were investigated in three conditions. First, wearing a mask as per the previous experiments as well as abdominal and thoracic stain gauges. Second, wearing a nasal catheter with strain gauges, and third, wearing the strain gauges only. Once again the subjects were treated as single case designs with sleep onsets within each condition as replications. Data from the first condition was used to construct regression equations to enable the estimation of tidal volume from the strain gauges. Each subject required different regression equations for wakefulnes and sleep to accurately estimate ventilation from the strain gauge data. Different regression equations - were also required for each unique set of strain gauges used. The data indicated that the practice in earlier studies, of estimating ventilation during sleep, using regression equations based on short calibration sessions held during wakefulness, has probably involved the introduction of error. Both subjects displayed decreases in ventilation in all three conditions, associated with the onset of theta activity in the EEG. The elevation of the ventilation level associated with the mask did not vary significantly between wakefulness and sleep.. Thus the data from the first two experiments was not produced by the use of specific respiratory apparatus, and is thus consistent with the wakefulness drive hypothesis.