Respiratory mechanics after abdominal surgery measured with continuous analysis of pressure, flow and volume signals

We studied 10 patients during the first night after upper abdominal surgery to assess the effect of airway obstruction on chest wall mechanics, by recording nasal gas flow and carbon dioxide concentration, rib cage and abdominal dimensions, abdominal muscle activity, and oesophageal and gastric pressures. The mean duration of study of each subject was 5.8 h, and 5.2 h were analysed. The median proportion of time spent breathing with normal mechanics was 29% (interquartile values 0-57%). Abnormal abdominal mechanical events were common and associated with airway obstruction (P < 0.001). Two common patterns of abnormal pressure and movement were found. In the first, abdominal pressure decreased at the onset of inspiration and there was a phase lag in abdominal movement. The incidence was 33 (14-50)%. In the second pattern, abdominal pressure decreased and in addition the abdominal wall moved inwards at the onset of inspiration. This occurred for 34 (0-52)% of the time. Both patterns were associated with evidence of increased activation of the abdominal muscles during expiration, changing the relationship of abdominal and pleural pressure changes and chest wall movements. Such changes have been interpreted previously as evidence of diaphragm dysfunction.