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  • image The trachea serves as a conduit for ventilation and for clearance of tracheal and bronchial secretions and has an average length of 10 to 13 cm. The trachea bifurcates at the carina into the right and left mainstem bronchi. The right mainstem bronchus lies in a more linear arrangement with the trachea, whereas the left mainstem bronchus lies in a more angular orientation with the trachea.

  • image The periodic exchange of alveolar gas with the fresh gas from the upper airway reoxygenates desaturated blood and eliminates carbon dioxide (CO2). This exchange is normally brought about by small cyclic pressure gradients established within the airways. During spontaneous ventilation, these gradients are secondary to variations in intrathoracic pressure; during mechanical ventilation, they are produced by intermittent positive pressure in the upper airway.

  • image The lung volume at the end of a normal exhalation is called functional residual capacity (FRC). At this volume, the inward elastic recoil of the lung approximates the outward elastic recoil of the chest (including resting diaphragmatic tone).

  • image Closing capacity is normally well below FRC, but it rises steadily with age. This increase is probably responsible for the normal age-related decline in arterial O2 tension.

  • image Whereas both forced expiratory volume in the first second of exhalation (FEV1) and forced vital capacity (FVC) are effort dependent, forced midexpiratory flow (FEF25–75%) is more effort independent and may be a more reliable measure of obstruction.

  • image Changes in lung mechanics due to general anesthesia occur shortly after induction. The supine position reduces the FRC by 0.8 to 1.0 L, and induction of general anesthesia further reduces the FRC by 0.4 to 0.5 L. FRC reduction is a consequence of alveolar collapse, and compression atelectasis is due to loss of inspiratory muscle tone, change in chest wall rigidity, and upward shift of the diaphragm.

  • image Local factors are more important than the autonomic system in influencing pulmonary vascular tone. Hypoxia is a powerful stimulus for pulmonary vasoconstriction (the opposite of its systemic effect).

  • image Because alveolar ventilation (V̇A) is normally about 4 L/min and pulmonary capillary perfusion (Q̇) is 5 L/min, the overall ratio is about 0.8.

  • image Shunting denotes the process whereby desaturated, mixed venous blood from the right heart returns to the left heart without being oxygenated in the lungs. The overall effect of shunting is to decrease (dilute) arterial O2 content; this type of shunt is referred to as right-to-left.

  • image General anesthesia commonly increases venous admixture to 5% to 10%, probably as a result of atelectasis and airway collapse in dependent areas of the lung.

  • image Note that large increases in PaCO2 (>75 mm Hg) readily produce hypoxia (PaO2 <60 mm Hg) at room air but not at high inspired O2 concentrations.

  • image The binding of O2 to hemoglobin seems to be the principal rate-limiting factor in the transfer of O2 from alveolar gas to blood.

  • image The greater ...

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