Outline the respiratory and cardiovascular consequences of an acute complete spinal cord transection at C6.

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College Answer

The main respiratory consequences of an acute C6 transection include the effects on the 
inspiratory muscles, the expiratory muscles, lung volumes, effects of changes in posture and 
effects on gas exchange. Sparing of the phrenic nerve, the main muscle of inspiration (C3 -
5), but paralysis of the external intercostal muscles innervated by thoracic nerve roots 
results in paradoxical inward movement of the chest wall on inspiration. Paralysis of all the 
expiratory muscles including the internal intercostal muscles innervated by thoracic nerve 
roots and the abdominal wall muscles, which are innervated by lower thoric and lumbar 
nerves. Many candidates did not mention these muscles or their innervation in their 
answers. While expiration is normally passive these muscles are required for manoeuvres
involving forced exhalation like coughing. Forced expiratory lung volumes (FEV1 and FVC) 
are reduced. Work of breathing is increased. Static lung volumes reveal a restrictive lung 
defect with most lung volumes decreased but in particular expiratory reserve volume (ERV) 
is significantly reduced. The reduction in FRC leads to airway closure, atelectasis and 
pathologic low V/Q and shunt and hence hypoxemia. These mechanisms can result in 
significant hypoxemia but were not described by many candidates. The second part of the 
question concerning the cardiovascular consequences of C6 transection was better 
answered. Areas that required mention in this section included the early massive 
sympathetic outflow and hypertension via the release of catecholamines from the adrenal 
medulla. Neurogenic shock is also seen due to interruption of the sympathetic outflow and 
impaired reflex vasoconstriction secondary to hypotension of any cause. Finally the loss of 
sympathetic innervation of the heart (T1-T4) results in unopposed parasympathetic cardiac 
stimulation and bradycardia and bradyarrthymias.