The change in the single-breath carbon dioxide (CO2) curve from "A" to "B" shown in the figure may be a result of which of the following?

a. Hypervolemia
b. Decreased positive end-expiratory pres-sure (PEEP)
c. Increased mean airway pressure
d. Excessive bronchodilator administration

ANS: C
The change in the figure shows an increase in phase 1 and a decrease in phase 2 of the sin-gle-breath carbon dioxide (SBCO2) curve. An increase in phase 1 may be caused by an increase in anatomical dead space. This is possible as a result of increased airway obstruction or excessive positive end-expiratory pressure (PEEP). Therefore, answers "B" and "D" cannot be correct. A decrease in phase 2 means there is a decrease in venous return or an increase in intrathoracic pressure. This could be caused by hypovolemia or an increased intrathoracic pressure as seen with increased mean airway pressures. Therefore, answer "A" cannot be correct. Answer "C" is a measure of increased intrathoracic pressure and is the correct answer.