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NBME 19 Answers

nbme19/Block 3/Question#41 (10.1 difficulty score)
41 yo woman, 34 weeks gestation; ...
Decreased functional residual capacityπŸ”,πŸ“Ί

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submitted by cassdawg(1172),

This ratio is used to determine appropriate production of lung surfactant, so this baby would not have appropriate production of surfactant if birthed at the time of the ratio calculation. It would go into neonatal respiratory distress syndrome.

TL;DR: surfactant decreases lung recoil, so the lack of surfactant in this baby will cause an increased recoil which will decrease the functional residual capacity (FA 2020 p661-662)

Surfactant is necessary to decrease surface tension of alveoli and increase compliance of the lungs (remember when the lungs have greater compliance, this makes the lungs easier to fill). Surfactant also helps to prevent alveolar collapse as the lack of surfactant allows there to be varying surface tensions between large and small alveoli (Law of Laplace), so lack of surfactant would lead to widespread atelectasis (alveolar collapse). Because surfactant serves to decrease these forces which normally favor collapse of the lung, it also serves to decrease the lung recoil.

Lack of surfactant in a baby = increased alveolar surface tension, lower compliance, more alveolar collapse, more recoil (less residual volumes)

Total lung capacity is unchanged because with enough force you can still expand the lungs to full capacity.

geekymle  hey ya! thanks for the explanation, but i'm still not able to understand why decreased functional residual capacity. +1  
cassdawg  Not sure if this will help but another way of thinking of this is that it is similar to pulmonary fibrosis with decreased compliance leading to lower functional residual capacity! The only difference is here you retain total lung capacity because it is a "reversible" compliance issue (give them surfactant) and the lungs themselves are not the issue. Also, like you said in your comment, lungs won't open up so there is a lower starting point for breathing, and thus a lower functional residual capacity (volume left in the lungs after normal expiration). +2  

submitted by geekymle(9),

Surfactant deficiency β†’ no reduction of alveolar surface tension β†’ reduced pulmonary unfolding β†’ lungs wont open up β†’ decreased lung compliance and functional residual capacity β†’ hypoxemia and hypercapnia

this should help

submitted by step12395(0),

could someone please explain why increased static lung compliance is wrong

an_improved_me  Increased compliance indicates an increased ability to maintain a certain volume at lower pressures. In other words, at equilibrium (static), the lung can hold volume at lower pressures. This is the oppisite of what would happen if physiologic levels of surfactant cannot be produced, since elastic recoil goes up (compliance goes down). In this case, certain volumes of fluid would correspond to higher pressures. +