Thoracic surgery presents a unique set of physiologi-cal problems for the anesthesiologist. These include physiological derangements caused by placing the patient in the lateral decubitus position, opening the chest (open pneumothorax), and the need for one-lung ventilation.
THE LATERAL DECUBITUS POSITION
The lateral decubitus position provides optimal access for most operations on the lungs, pleura, esophagus, the great vessels, other mediastinal structures, and vertebrae. Unfortunately, this posi-tion may significantly alter the normal pulmonary ventilation/perfusion relationships. These derange-ments are further accentuated by induction of anesthesia, initiation of mechanical ventilation, neu-romuscular blockade, opening the chest, and surgical retraction. Although perfusion continues to favor the dependent (lower) lung, ventilation progressively favors the less perfused upper lung. The resulting mismatch increases the risk of hypoxemia.
When a supine patient assumes the lateral decu-bitus position, ventilation/perfusion matching is preserved during spontaneous ventilation. The dependent (lower) lung receives more perfusion than does the upper lung due to gravitational influ-ences on blood flow distribution in the pulmonary circulation. The dependent lung also receives more ventilation because: (1) contraction of the depen-dent hemidiaphragm is more efficient compared with the nondependent [upper] hemidiaphragm and (2) the dependent lung is on a more favorable part of the compliance curve ( Figure25–1).
The decrease in functional residual capacity (FRC) with induction of general anesthesia moves the upper lung to a more favorable part of the compliance
curve, but moves the lower lung to a less favorable-position (Figure25–2). As a result, the upper lung is ventilated more than the dependent lower lung; ven-tilation/perfusion mismatching occurs because the dependent lung continues to have greater perfusion.
Controlled positive-pressure ventilation favors the upper lung in the lateral position because it is more compliant than the lower lung. Neuromuscular blockade enhances this effect by allowing the abdominal contents to rise up further against the dependent hemidiaphragm and impede ventila-tion of the lower lung. Using a rigid “bean bag” to maintain the patient in the lateral decubitus position further restricts movement of the dependent hemi-thorax. Finally, opening the nondependent side of the chest further accentuates differences in compli-ance between the two sides because the upper lung is now less restricted in movement. All of these effectsworsen ventilation/perfusion mismatching and pre-dispose the patient to hypoxemia.
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