Modes of ventilation and changes in the mechanical properties of the thoracic wall

Mechanical ventilation

It has been shown that the thoracopulmonary structure is less compliant under general anesthesia. The decrease in total compliance is due to the change in the elastic properties of the lung which would explain the occurrence of atelectasis. CT scanning has revealed that parenchymatous zones with increased density appear during the first few minutes of anesthesia. These dependent zones in the lung, whose extension is not influenced by the fraction of inspired oxygen, disappear during positive end-expiratory pressure (PEEP) of 10 cmH2O (Brismar.eL§L 1985). These zones correspond more to atelectasis caused by compression than to that caused by adsorption. There is a significant correlation between these retention zones and the development of an intrapulmonary shunt ( Brismar etal 1985).

Spontaneous ventilation

Patients in the postoperative period ventilate at a greater respiratory rate and a smaller tidal volume. This type of ventilation causes alveolar collapse with reduction of FRC. When pulmonary volume falls below a certain value, the small bronchi collapse. Alveoli are then ventilated less well or not at all. The pulmonary volume at which the airways close is known as the closing volume. Its value is normally less that of the FRC in a standing subject aged less than 60 years. Atelectasis occurs when the FRC is less than the closing volume.

The FRC can be reduced by many other factors, including maintaining the patient in a supine or lateral position, obesity or abdominal distention, pain that limits thoracic expansion, postanesthesia alveolar hypoventilation, etc.

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