Which of the following are true about absorption atelectasis?

1. It can occur only when breathing supplemental O2.
2. Its risk is increased in patients breathing at low tidal volumes (VT values).
3. Its risk is decreased through the natural "sigh" mechanism.
4. It results in an increase in the physiologic shunt fraction.
a. 1 and 3 only
b. 1, 2, and 3 only
c. 3 and 4 only
d. 2, 3, and 4 only

ANS: D
The risk of absorption atelectasis is greatest in patients breathing at low tidal volumes as a result of sedation, surgical pain, or CNS dysfunction. In these cases, poorly ventilated alveoli may be-come unstable when they lose O2 faster than it can be replaced. The result is a more gradual shrinking of the alveoli that may lead to complete collapse, even when the patient is not breathing supplemental O2. For an alert patient this is not a great risk, because the natural sigh mechanism periodically hyperinflates the lung.