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Ventilation during sleep onset

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Colrain, Ian M(Ian Michael) (1988) Ventilation during sleep onset. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
Keywords: Sleep, Sleep, Respiration
Copyright Holders: The Author
Copyright Information:

Copyright 1998 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1989. Bibliography: leaves 134-154

Date Deposited: 25 Nov 2014 00:44
Last Modified: 19 Jul 2016 02:26
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