THE OPEN PNEUMOTHORAX
The lungs are normally kept expanded by a negative pleural pressure—the net result of the tendency of the lung to collapse and the chest wall to expand. When one side of the chest is opened, the negative pleural pressure is lost, and the elastic recoil of the lung on that side tends to collapse it. Spontaneous ventilation with an open pneumothorax in the lateral position results in paradoxical respirations and mediastinal shift. These two phenomena can cause progressive hypox-emia and hypercapnia, but, fortunately, their effects are overcome by the use of positive-pressure ventila-tion during general anesthesia and thoracotomy.
During spontaneous ventilation in the lateral posi-tion, inspiration causes pleural pressure to become
more negative on the dependent side, but not on the side of the open pneumothorax. This results in a downward shift of the mediastinum during inspiration and an upward shift during expiration (Figure25–3). The major effect of the mediastinal shift is to decrease the contribution of the dependent lung to the tidal volume.
Spontaneous ventilation in a patient with an open pneumothorax also results in to-and-fro gas flow between the dependent and nondependent lung(paradoxical respiration [pendeluft]). During inspi-ration, the pneumothorax increases, and gas flows from the upper lung across the carina to the depen-dent lung. During expiration, the gas flow reverses and moves from the dependent to the upper lung (Figure 25–4).
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