CARDIOVASCULAR CONSEQUENCES AFTER SPINAL CORD INJURY AND THE EFFECTS OF ELECTRICAL STIMULATION
A spinal cord injury (SCI) is an extremely complex condition and is associated with numerous adverse health complications. One concern in particular is an increased prevalence and risk of cardiovascular disease (CVD). Cardiovascular disease, which encompasses pathologies of the heart and vascular tree, is one of the leading causes of morbidity and premature mortality in this population. Current clinical practice guidelines for CVD risk estimation in SCI use traditional cardiovascular risk factors and risk algorithms validated in able-bodied populations. However, despite the increased prevalence of CVD in people with SCI, research has demonstrated no difference in traditional cardiovascular risk factors when compared to the able-bodied population. The aim of Chapter 3 was to explore whether traditional models of CVD risk prediction are accurate in estimating CVD events in people with SCI. Firstly, this study used retrospective data from 200 individuals with a SCI to prospectively estimate their 5-year risk of developing CVD using the FRS. The difference in clinical outcomes of individuals with a below median FRS (1.36%) were compared to individuals with an above median FRS (>1.36%) using Kaplan-Meier curves and log-rank test. The end point was defined as either a CVD event or CVD mortality. Across the 5-year period, 39 (19.5%) participants developed a CVD event, 10 of which were fatalities. The log-rank test demonstrated that individuals with >median FRS vs. <median FRS had a 3.2 fold greater risk for developing a CVD event. Secondly, this study used receiver operating characteristic curves (ROC) with corresponding area under the curve (AUC) to explore the prognostic accuracy of the FRS in being able to predict CVD events in people with SCI. The ROC-AUC suggests acceptable accuracy of the FRS in identifying individuals with increased risk for CVD events (ROC-AUC of 0.71, 95% CI 0.62-0.82). Finally, SCI-related factors were explored using univariate cox proportional hazard regression to try to determine individual predictors for CVD. These factors were separately added to the FRS in an attempt to improve the prognostic capability of the FRS when used for people with SCI. Adding ASIA impairment (0.74; 95% CI 0.66-0.82), motor impairment (0.74; 95% CI 0.66-0.83), level of injury (0.72; 95% CI 0.63-0.81) or active engagement in sport prior to injury (0.72; 95% CI 0.63-0.88) to the FRS did not improve the level of discrimination. The findings in Chapter 3, together with other research, suggest that traditional CVD risk factors cannot solely be accountable for the increased CVD morbidity and mortality in SCI. Instead, the effects of chronic physical inactivity and extreme vascular deconditioning resulting from lower limb paralysis have been proposed as an alternative mechanism explaining the greater CVD prevalence in the absence of abnormal CVD risk factors. This provides a rationale for needing appropriate interventions targeting the deconditioned vasculature. Reactivating the muscles using electrical stimulation has shown to reverse the rapid vascular and muscular deconditioning that occurs in people with SCI. However, the majority of these ES methods are not easily accessible for individuals with SCI or practical for regular homebased use. The primary aim of the study outlined in Chapter 4 was to examine the acute effects of low intensity ES using a wearable clothing garment on macrovascular (femoral) and microvascular (skin) perfusion and oxygen consumption in people with SCI. This particular method of ES has significant clinical potential because it can be applied for longer periods and in a home based environment. Eight participants with a motor complete SCI received 4x3-minutes of unilateral gluteal and hamstring ES. Deep femoral artery (DFA) blood flow, oxygen consumption, and gluteal and hamstring cutaneous perfusion were measured at baseline and during each block of ES. DFA blood flow increased by 18.1% with the application of ES. Skin perfusion did not change during an individual block of stimulation but did progressively increase over time with repeated stimulation and was 80% higher during the last 3-minute block compared with the first 3-minute block of ES. There was no change in DFA blood flow in the contralateral control leg or in oxygen consumption. Individuals with SCI are subject to additional secondary complications such as skin ulcers resulting from poor microvascular perfusion and prolonged sitting pressure. This, along with the findings from Chapter 4, provide a compelling rationale for studies to examine the long-term effects of low-level ES on the vasculature, pressure ulcer risk factors, and the feasibility of home based ES devices such as a wearable clothing garment. The aim of Chapter 5 was to explore the changes in femoral artery diameter, basal blood flow, cutaneous microvascular function, limb volume and sitting pressure in people with SCI after 12 weeks of daily low-level ES using a wearable clothing garment. Twelve participants were instructed to wear the ES garment every day or night. The ES protocol lasted for 6 hours and consisted of 8x30 minutes of active ES with 15 minutes rest in-between. Three participants did not complete the study therefore data analysis was performed on the nine remaining individuals. After 12 weeks of ES, resting diameter and mean basal blood flow of the common femoral artery increased by 8.3% (P<0.001) and 43% (P<0.001), respectively. In response to graded local heating a significant interaction of heating stage and time was observed for gluteal CVC (P = 0.02) and %CVCmax (P <0.001) but not for hamstring CVC (P = 0.86) or %CVCmax. There was a significant increase in average thigh circumference (P = 0.008) and calculated thigh volume (P = 0.01) after 12 weeks of ES. A significant decrease in peak sitting pressure (P = 0.003) and pressure gradient (P = 0.007) was observed. Questionnaires were used to examine the feasibility of using the ES garment on a daily basis. Majority of participants wore the device at night whilst sleeping and indicated it did not disrupt their sleep quality. Applying the garment takes 10 minutes or less, can be worn under suitable clothing and does not disrupt daily living activities. More importantly, all the individuals who completed the intervention indicated they would be happy to continue using the device. The findings from this thesis suggest that, firstly, traditional cardiovascular risk factors and the FRS are able to distinguish individuals who are at a higher vs. lower risk for CVD, but the absolute prediction values markedly underestimate the occurrence of CVD in people with SCI. Additionally, the increased risk of CVD in people with SCI appears independent of level and severity of injury. Secondly, this thesis indicates that acute low-intensity ES using a wearable clothing garment improves micro- and macrovascular perfusion. Finally, the research undertaken in this thesis provides evidence that home based, daily, low-intensity ES using a wearable clothing garment represents a practical and effective strategy to reduce the risk of pressure ulcers, improve vascular perfusion and help prevent vascular complications in people with SCI.
Networked Digital Library of Theses and Dissertations