RESPIRATORY FUNCTION DURING ANESTHESIA

Arterial oxygenation is impaired in most patients during anesthesia with either spontaneous or controlled ventilation.^{[116] [117] [118] [119] [120] [121]} In otherwise normal patients, it is generally accepted that the impairment in arterial oxygenation during anesthesia is more severe in the elderly,^{[122] [123]}

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In addition to the foregoing generalizations concerning respiratory function during anesthesia, the effect of a given anesthetic on respiratory function depends on the depth of general anesthesia, the patient's preoperative respiratory condition, and the presence of special intraoperative anesthetic and surgical conditions.

Effect of Anesthetic Depth on Respiratory Pattern

The respiratory pattern is altered by the induction and deepening of anesthesia. When the depth of anesthesia is inadequate (less than MAC), the respiratory pattern may vary from excessive hyperventilation and vocalization to breath-holding. As anesthetic depth approaches or equals MAC (light anesthesia), irregular respiration progresses to a more regular pattern that is associated with a larger than normal VT. However, during light but deepening anesthesia, the approach to a more regular respiratory pattern may be interrupted by a pause at the end of inspiration (a "hitch" in inspiration), followed by a relatively prolonged and active expiration in which the patient seems to exhale forcefully rather than passively. As anesthesia deepens to moderate levels, respiration becomes faster and more regular, but shallower. The respiratory pattern is a sine wave losing the inspiratory hitch and lengthened expiratory pause. There is little or no inspiratory or expiratory pause, and the inspiratory and expiratory periods are equivalent. Intercostal muscle activity is still present, and there is normal movement of the thoracic cage with lifting of the chest during inspiration. The respiratory rate is generally slower and the VT larger with nitrous oxide-narcotic anesthesia than with anesthesia involving halogenated drugs. During deep anesthesia with halogenated drugs, increasing respiratory depression is manifested by increasingly rapid and shallow breathing (panting). On the other hand, with deep nitrous oxide-narcotic anesthesia, respirations become slower but may remain deep. In the case of very deep anesthesia with all inhaled drugs, respirations often become jerky or gasping in character and irregular in pattern. This situation results from loss of the active intercostal muscle contribution to inspiration. As a result, a rocking boat movement occurs in which there is out-of-phase depression of the chest wall during inspiration, flaring of the lower chest margins, and billowing of the abdomen. The reason for this type of movement is that inspiration is dependent solely on diaphragmatic effort. Independent of anesthetic depth, similar chest movements may be simulated by upper and lower airway obstruction and by partial paralysis.