Alterations in ventilatory mechanics

The variations in respiratory mechanics that occur during the postoperative period have made it possible to identify the circumstances favoring atelectasis.

Atelectasis is essentially a restrictive syndrome with reduction of pulmonary volumes. Vital capacity is diminished by 40 to 60 per cent compared with the preoperative value after supraumbilical laparotomy, and by 20 to 40 per cent after subumbilical laparotomy. Tidal volume is diminished by 25 per cent and respiratory rate is increased by 20 per cent. This is accompanied by a 30 per cent reduction in functional residual capacity (FRC). These modifications are due to a marked diaphragmatic dyskinesia. This term includes all the diaphragmatic dysfunctions secondary to laparotomy or thoracic surgery.

Several possible causes of the mechanisms of diaphragmatic dyskinesia have been suggested. Decrease in diaphragmatic contractility after surgery seems to be unlikely, given the persistence of electromyographic contractility and the normal response secondary to bilateral phrenic contraction. A decrease in abdominal compliance after laparotomy is a possible cause, but is certainly not sufficient alone. The most probable mechanism is an inhibitory reflex of phrenic motor output, the origin of which is uncertain. Pain does not seem to be involved because a very satisfactory analgesia achieved using fentanyl administered by the peridural route does not improve diaphragmatic function (Simo.DDeau...et...a./ 1983).

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