USMLE Step 1 Question 329

Question:

A man has a tragic horseback riding accident in which he is paralyzed below T4. Which of the following lung function measures will be decreased?

Correct Answer: D.  Expiratory Reserve Volume (ERV)

TV, IC, and IRV are all volumes that are generated by voluntary contraction of the diaphragm. Since the diaphragm is innervated by C3-C4-C5, inspiratory volumes will be unchanged with injury below C5. Remember TV+IRV=IC. Expiration is normally a passive process, but forceful expiration can be generated by contraction of abdominal muscles. Since the abdominal muscles (rectus abdominiis, external and internal obliques) are innervated by T7-T12, injury at T4 will paralyze voluntary abdominal contraction, and eliminate the ability to produce ERV (Answer D). RV is the volume that remains in the lungs after forceful expiration. This volume is dependent on the dead space. In this case the funcitonal dead space would actually increase due to lack of ERV.