Doctor of Medicine and Doctor of Philosophy (MDPhD)
Autonomic dysfunction following spinal cord injury: mechanisms, novel treatments, and clinical reality
Cervical spinal cord injury (C-SCI) is a devastating condition that leads to significant impairments in both the cardiovascular and respiratory response to aerobic exercise. As the heart and lungs share space within the thoracic cavity it follows that changes in one system will affect the other. Therefore, the purpose of this thesis was to examine heart-lung interactions in C-SCI and to utilize this knowledge to enhance exercise capacity in athletes with C-SCI.The aims of this thesis were to (1) compare the cardiorespiratory response to maximal and sub-maximal exercise following C-SCI to able-bodied individuals, with a particular focus on operating lung volumes (Study #1, Chapter 3); (2) to examine the effects of respiratory loading on lung volumes and left-ventricular function during head-up tilt (Study #2, Chapter 4); and (3) to assess the effects of a combined inspiratory and expiratory respiratory muscle training (i.e. RMT) intervention in elite athletes with C-SCI (Study #3, Chapter 5).Laboratory-based incremental arm ergometry testing demonstrated that C-SCI is associated with a limited exercise capacity compared to able-bodied individuals along with an altered respiratory pattern that is characterized by dynamic hyperinflation and reduced tidal volume. By manipulating inspiratory and expiratory esophageal pressure in individuals with C-SCI, it was demonstrated that expiratory loading elicited dynamic hyperinflation that was associated with impaired left-ventricular filling, likely due to direct ventricular interaction and/or mediastinal constraint. Finally, a six-week RMT intervention in elite athletes with C-SCI was found to significantly enhance respiratory muscle strength and measures of pulmonary function and prevent dynamic hyperinflation during exercise. These changes in pulmonary function were accompanied by enhanced exercise capacity during an incremental arm ergometry test and were partly ameliorated following six-weeks of wash-out (i.e., no RMT).This thesis demonstrates that dynamic hyperinflation in individuals with C-SCI, which likely occurs due to expiratory muscle weakness, limits left ventricular filling and is associated with an attenuated exercise capacity compared to able-bodied individuals. RMT improved respiratory muscle strength and prevented dynamic hyperinflation in individuals with C-SCI and enhanced exercise capacity.
Individuals with spinal cord injury (SCI) are at greatly increased risk of cardiovascular disease (CVD). This is likely due to physical inactivity and impaired sympathetic control of the heart and blood vessels, resulting in cardiovascular dysfunction. Cardiovascular dysfunction in individuals with SCI is associated with injury level, whereby individuals with higher lesions exhibit greater dysfunction. In people without SCI, cardiac dysfunction predicts CVD. The studies that have investigated cardiac indices in individuals with SCI tend to agree that cardiac atrophy and impaired systolic function occur following SCI. Physical activity is a key method to decrease CVD risk and improve cardiac function, yet few studies have examined the relationship between cardiac function and physical activity in individuals with SCI. Those that have investigated this relationship have used subjective measures of physical activity. The current guidelines for physical activity participation for individuals with SCI were based on a systematic review of the evidence on the benefits of physical activity, yet there was inadequate evidence to prescribe activity intensity and duration to improve cardiovascular health in this population. Individuals with SCI also experience numerous barriers and facilitators to physical activity participation that affect their ability to meet the guideline recommendations. The objectives of this thesis, therefore, were: 1) to objectively measure physical activity in individuals with SCI, using wrist-worn accelerometry during a six-day physical activity monitoring period, and to evaluate the utility of group based wrist accelerometry cut-points to estimate physical activity intensity by comparing MVPA determined by individual cut-points to MVPA determined by group-based cut-points; 2) to determine the relationship between objectively measured physical activity and cardiac structure and function in individuals with SCI across a range of injury levels, and 3) to explore the barriers and facilitators to physical activity participation experienced by individuals with SCI during a six-day physical activity monitoring period.