Chapter 5. Surfactant Biology and Lung Compliance
A 2100-g (4 lb, 10 oz) male neonate is delivered by cesarean section at 31 weeks gestation after his mother sustained trauma to the abdomen that caused near-total loss of amniotic fluid. Physical exam of the neonate shows cyanosis. Interventional intubation and mechanical ventilation are started, but the neonate dies 15 minutes later. Which of the following findings are most likely on autopsy?
a. Ventilator-induced atelectasis and edema
b. Alveolar septa lacking capillaries
c. Cuboidal epithelial cells with few lamellar inclusions
d. Absence of secondary bronchi
e. Interstitial spaces containing no fibroblasts
The most correct answer is c.
Lungs at this presumed gestational age contain few if any differentiated type 2 cells, while secondary bronchi (answer d) and septal development (answer b) will be evident (see Chap. 2). Septation requires fibroblasts that drive the remodeling process (answer e). Ventilator-associated injury would not likely develop after only 15 minutes of intervention (answer a).
Lungs are excised from a healthy adult donor, and the trachea is fitted with a cuffed endotracheal tube. By fluid displacement, their initial volume is 0.9 L at an intratracheal pressure (PIT) of 0 cm H2O (= ambient PB). When inflated with room air, the lungs expand maximally to 5.4 L as PIT is increased to 23 cm H2O. When the lungs are inflated with sterile 0.9% NaCl, they achieve the same 5.4 L at a PIT of 8 cm H2O. What single factor best explains the greater PIT needed to achieve 5.4 L with air?
a. Peripheral lobe atelectasis
b. Surface tension recoil forces
c. Saline inactivation of surfactant
d. Tissue elastic recoil forces
e. Surfactant-increased hysteresis
The most correct answer is b.
Overcoming surface tension recoil is the single greatest determinant of the work required to inflate lungs with air. Atelectasis of freshly harvested healthy lungs (answer a) is common and might raise critical opening pressure but should not affect the PIT needed to reach total lung capacity. Filling the lungs with saline may inactivate surfactant on their alveolar surfaces (answer c). However inflating such lungs is easier, needing to overcome only tissue elastic recoil (answer d). Surfactant reduces hysteresis, rather than increasing it (answer e...