Injury to the cervical and upper thoracic spinal cord disrupts function of inspiratory and expiratory muscles, as reflected by reduction in spirometric and lung volume parameters and static mouth pressures. In association, subjects with
tetraplegia have decreased chest wall and lung compliance, increased abdominal wall compliance, and rib cage stiffness with paradoxical chest wall movements, all of which contribute to an increase in the work of breathing. Expiratory muscle function is more compromised than inspiratory muscle function among subjects with
tetraplegia and high
paraplegia, which can result in ineffective
cough and propensity to mucus retention and
atelectasis. Subjects with
tetraplegia also demonstrate heightened vagal activity with reduction in baseline airway caliber, findings attributed to loss of sympathetic innervation to the lungs. Significant increase in airway caliber following inhalation of
ipratropium bromide, an
anticholinergic agent, suggests that reduction in airway caliber is not due to acquired airway
fibrosis stemming from repeated
infections or to abnormal hysteresis secondary to chronic inability of subjects to inhale to predicted total lung capacity. Reduced baseline airway caliber possibly explains why subjects with
tetraplegia exhibit
airway hyperresponsiveness to
methacholine and ultrasonically nebulized distilled water. While it has been well demonstrated that bilateral phrenic nerve pacing or stimulation through intramuscular diaphragmatic
electrodes improves inspiratory muscle function, it remains unclear if inspiratory muscle training improves pulmonary function. Recent findings suggest that expiratory muscle training, electrical stimulation of expiratory muscles and administration of a long-acting beta(2)-agonist (
salmeterol) improve physiological parameters and
cough. It is unknown if baseline bronchoconstriction in
tetraplegia contributes to respiratory symptoms, of if the chronic administration of a
bronchodilator reduces the work of breathing and/or improves respiratory symptoms. Less is known regarding the benefits of treatment of
obstructive sleep apnea, despite evidence indicating that the prevalence of this condition in persons with
tetraplegia is far greater than that encountered in able-bodied individuals.