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Pathophysiology of the Respiratory System in SCI

What is the pathophysiology of the respiratory system in SCI? In describing this, identify the various lung volume and capacity measures and how SCI affects these.
1. Although the primary consequence of SCI is dennervation of the respiratory pump, secondary consequences occur within the lungs because of the inability to effectively distend and inflate the lung to its full capacity. As a consequence, the compliance of the lungs diminishes with increasing time after SCI.
2. Lung capacity measures are Inspiratory Capacity (IC), Functional Residual capacity (FRC), Vital Capacity (VC), and Total Lung Capacity (TLC).
3. Lung volume measures are Tidal Volume (VT but also known as TV), Inspiratory Reserve Volume (IRV), Expiratory Reserve Volume (ERV), and Residual Volume (RV).
4. Due to reduced inspiratory muscle force, the Forced Expiratory Volume in one second (volume of air that can be exhaled in the first second = FEV1) and Forced Vital Capacity (maximum volume of air that can be exhaled = FVC), are diminished in people after SCI with higher lesions, and especially in people with tetraplegia demonstrating moderate correlation with injury level. Longer duration of injury and smoking are associated with greater loss while incomplete lesions (compared to complete lesions) have less loss of forced expiratory measures of FEV1 and FVC.
5. The resting breathing pattern in patients with tetraplegia is characterized by a small tidal volume, and increased respiratory rate compared to that of age-matched healthy controls.
6. Partial or fully denervated expiratory muscles in those with SCI will diminish exercise ventilation and ventilatory reserve.
7. People with SCI at most levels affects innervation of the abdominal muscles which severely compromises the ability to generate cough and clear respiratory secretions.

Figure 4. Normal Anatomy Involved in Respiration

Figure 5. Innervation of the respiratory system. The main respiratory muscles are the diaphragm, intercostals and abdominals. The diaphragm is the major inspiratory muscle and is innervated by phrenic motor neurons that lie in the cervical spinal cord (C3–C5). Innervation of respiratory intercostal and abdominal muscles exits the thoracolumbar spinal cord, from T1–T11 and T7–L2, respectively. Activity of these muscles (as well as that of accessory muscles) is modulated by autonomic premotor neurons. The airways receive both parasympathetic and sympathetic inputs and extensive afferent innervation.