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The Mechanical Properties of the Respiratory System during Anesthesia

1. Static compliance of the total respiratory system is decreased during anesthesia, 2. The fall in total static compliance is probably caused by a fall in lung compliance. 3. It seems unlikely that the fall in lung compliance is the primary event. The change in pleural pressures at FRC before and during anesthesia would answer this question, but the data are conflicting. 4. It is more likely that the position of the chest wall pressure-volume curve shifts to the right as a consequence of anesthesia, so that the position of equilibrium for the lung and chest wall at FRC is at a lower volume. The alteration in the chest wall is possibly because of loss of respiratory muscle tone. 5. FRC is reduced. Gas trapping occurs as a secondary event if FRC falls to below CC. 6. Lung compliance falls as a consequence of the fall in FRC, but only if FRC drops below CC, and closure occurs. No data of the correlation of change in lung compliance to the relationship of FRC to CC have been published. 7. A-aDo2 increases because of an increase in the amount of lung where ventilation is reduced compared to perfusion. Areas of zero ventilation (shunt) will also be produced. 8. The changes in lung compliance, FRC, and A-aDo2 appear to have similar characteristics. They are not influenced by depth or type of general anesthesia or the presence of neuromuscular blockade. They appear as soon as anesthesia is induced and are not progressive with time during anesthesia. They are less in the sitting position and in taller, thinner subjects. They appear to be only transiently affected by hyperinflations. 9. The fall in total respiratory system compliance may contribute to the apparent depression of the ventilatory response to carbon dioxide.