Outline the respiratory and cardiovascular consequences of an acute complete spinal cord transection at C6.
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.
A quick answer in point form would look like this:
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