Inspiratory Muscles
realised as early as John Snow (1858) that deepening anaesthesia was associated with decreased thoracic excursion and that abdominal excursion was well maintained this is due to progressive failure of the intercostal muscles with preservation of diaphragm
in contrast, there is an increase in the thoracic component during IPPV in the anaesthetised paralysed patient
Bryan & Froese (1977) demonstrated that most of the ventilatory response to hypercapnia was due to the rib cage, rather than the abdominal component of total respiratory excursion
this is the basis of the statement that the reduction in the CO2 response is due to inhibition of intercostal muscle activity this loss of intercostal activity may be detrimental in patients with compromised abdominal excursion, or with hyperinflated lungs and flattened diaphragms the other major change is the loss of the tonic activity of the diaphragm, with the resultant decrease in the FRC
Expiratory and Other Muscles
GA results in phasic activity of the expiratory group which are normally silent during the respiratory cycle
this appears to serve no useful purpose and is unrelated to the decrease in FRC this increases abdominal muscle tone in the absence of paralysis
the genioglossus normally rhythmically contracts with respiration loss of tone to this may result in upper airway