A 44-year-old woman comes to the office for follow-up. The patient sustained a complete C7 spinal cord injury a year ago after falling off a cliff while hiking. She is able to move her shoulders and upper arms but has paraplegia and uses a wheelchair. The patient has a suprapubic catheter due to neurogenic bladder. She has no other medical conditions. She is a lifelong nonsmoker. Which of the following changes in respiratory parameters are most likely present in this patient?

Question

Quizplus · Accepted Answer

11ec160b_3cd4_4b8c_9249_8f666256aea1_MD0001_00 Patients with spinal cord injuries often have significant alterations in respiratory dynamics. Because different muscles (with different sources of innervation) control inspiration and expiration, respiratory function is differentially impaired based on the level of spinal cord injury. Inspiration is driven by an increase in intrathoracic volume. This is mediated predominantly by contraction of the diaphragm, which is innervated by the phrenic nerve that originates from the C3-C5 nerve rootlets. Accessory muscles of inspiration elevate the sternum and ribs to further expand the thoracic cavity. These include the external intercostals (innervated by thoracic nerve roots), the sternocleidomastoid (innervated by C1-C4), and the scalenes (innervated by C4-C8). Passive expiration is driven by the net intrinsic elastic recoil of the lungs, chest wall, ribs, and diaphragm and therefore requires no innervation. Active expiration, as measured in this patient's pulmonary function testing, recruits the musculature to pull the ribs down and compress the abdominal contents to push the diaphragm up and decrease intrathoracic volume. This is driven by the internal intercostals (innervated by thoracic nerve roots) and abdominal muscles (innervated by thoracic and lumbar nerve roots). Because the spinal cord injury was at the level of C7 in this patient, the diaphragm and many of the accessory inspiratory muscles are likely functional, resulting in minimal impairment of maximal inspiratory pressure. However, the muscles involved in active expiration (ie, internal intercostals and abdominal muscles) are likely significantly impaired, leading to a significant decrease in maximal expiratory pressure. (Choices A and B) Although the external intercostals are innervated by thoracic nerve roots and may be impaired in this patient, the other accessory muscles of inspiration, including the sternocleidomastoid and scalenes, are typically able to compensate for their loss and adequately elevate the sternum/ribs. This leads to minimal or no change in the maximal inspiratory pressure. Educational objective: The diaphragm is the principal muscle of inspiration and is innervated by the phrenic nerve, which originates from the C3-5 nerve rootlets. Although expiration is largely achieved through passive recoil, active expiration is aided by the internal intercostals (innervated by thoracic nerve rootlets) and abdominal muscles (innervated by thoracic and lumbar nerve roots).

A 44-Year-Old Woman Comes to the Office for Follow-Up