Thoracic gas volume and chest-abdomen dimensions during anesthesia and muscle paralysis

The aim was to study thoracic gas volume (TGV) in anesthetized and paralyzed subjects (n = 16) and to analyze any change in TGV in terms of thoraco-abdominal dimensions. TGV was measured by means of a body plethysmograph, using a super syringe for lung inflation during muscle paralysis. The thoraco-abdominal shape at the mamillary and umbilical levels was studied by means of magnetometry for the assessment of antero-posterior diameters (A-P), a strain gauge technique for measuring circumferences and respiratory inductive plethysmography for the recording of cross-sectional areas. TGV was reduced by an average of 28 per cent, from 2.38 1 awake to 1.72 1 during pentothal anesthesia and spontaneous breathing. No change in A-P diameters or cross-sectional areas were noticed. Muscle paralysis and mechanical ventilation caused no further change in TGV, nor were there any changes in A-P diameters, circumferences or cross-sectional areas of the chest or abdomen. The rib cage contribution to the tidal breath was significantly increased, from 41 per cent to 62 per cent to the tidal volume. The result of a lowered TGV during anesthesia concurrently with no measurable change in chest-abdomen dimensions remains a paradox not yet fully understood.