Cardiovascular Health and Fitness in Persons with Spinal Cord Injury




There are many issues after spinal cord injury that have an impact on cardiovascular health and fitness. This article discusses many of the secondary conditions and changes that occur and how they are affected by maintenance of an active lifestyle. It also discusses many of the benefits and difficulties individuals face in maintaining a regular exercise program after spinal cord injury.


As both the number of people and life expectancy increase for people with spinal cord injuries (SCI), many health concerns related to aging start to play a significant role in their overall health. Estimates for the incidence of new SCI remain approximately 11,000 per year, and the prevalence is approximately 230,000 and growing . Although life expectancies continue to rise with improved emergent and long-term management techniques, life expectancies remain below that of the general population . The effects of aging with spinal cord injury also have become more prevalent with greater lifespans. Among SCI survivors in the United States, it is believed that 40% are older than 45 years, and about 1 in 4 has lived 20 years or longer with their SCI .


Coronary heart disease (CHD) remains the leading cause of mortality among all Americans accounting for nearly 500,000 deaths per year . All individuals face numerous risk factors for CHD ( Table 1 ) . Individuals with SCI have been shown to be at increased risk of premature CHD development . Epidemiologic studies have found heart disease to be the cause or contributing factor in 22.4% of all deaths among those with SCI . In patients with chronic SCI, cardiovascular disease was the most frequent cause of death accounting for 46% of all deaths in people greater than 30 years after injury . Groah and colleagues has also shown that individuals with advancing age and higher levels and severity of SCI were at greater risk for CHD. Asymptomatic heart disease also has been detected among people with both tetraplegia and paraplegia, making early identification of risk factors, diagnosis, and primary prevention of CHD even more crucial .



Table 1

Major risk factors for CHD



















Modifiable risk factors Nonmodifiable risk factors
Cigarette smoking Age (men ≥45; women ≥55)
Hypertension Family history
Low HDL-C a <40
Diabetes

Data from National Cholesterol Education Program (NCEP) Expert Panel on Detection. Third Report of the National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III) final report. Circulation 2002;106(25):3143–421.

a HDL cholesterol ≥60 counts as a negative risk factor equivalent.



Several of the key cardiovascular risk factors have been shown to be more prevalent in individuals with SCI . Changes in body composition and lower levels of physical activity in persons with SCI are significant contributors to this increased risk for cardiovascular disease. These include changes in lipid metabolism, glucose intolerance/diabetes, obesity, and lack of physical fitness.


Lipid metabolism


High-density lipoproteins (HDL) account for approximately 20% to 30% of total levels of serum cholesterol. The precise mechanisms involved in the protective effects of HDL have not been elucidated fully. Thoughts on its benefit include its action as an antioxidant, antiinflammatory agent, and an inhibitor of atherogenesis . Studies have found a correlation with low levels of HDL and increased risk for the development of CHD. Conversely, elevated levels of serum HDL have shown protective effects. HDL levels <40 mg/dL are an independent risk factor for the development of CHD. Approximately 10% of the general population has HDL levels <35 mg/dL versus up to 40% in the SCI population . Those with tetraplegia have been shown to have lower levels of HDL versus paraplegia, suggesting that decreased physical activity is a major contributor to lower HDL levels . Increased levels of cardiopulmonary fitness have been shown to elevate levels of HDL in both the SCI and able-bodied populations .


Low-density lipoproteins (LDL) make up approximately 60% to 70% of serum cholesterol levels. It is believed to be the primary atherogenic cholesterol compound. Deposits of cholesterol, primarily LDL, originate in the coronary arteries and, over time, mature into occlusive plaques in the coronary arteries. Approximately 25% of the general population is considered to have elevated levels of LDL cholesterol. Individuals with SCI have been shown to have LDL levels similar to that of the general population . Because of the positive association with LDL and the increased risk of CHD, LDL is the primary target for lipid-lowering therapy, with the goal of treatment to lower serum LDL levels. Strategies such as low-fat, low-cholesterol dietary changes and increased physical activity often are first-line therapeutic strategies. Drugs such as the 5-hydroxy-3-methylglutaryl-coenzyme+A+reductase (HMG Co-A) reductase inhibitors may also be necessary when LDL levels are resistant to the lifestyle changes. These agents act in the liver to decrease the biosynthesis of LDL levels. Other agents such as bile acid sequestrants or resins can also be useful adjunct therapy in the management of hypercholesterolemia. Drug therapy is also predicated on the risk factor analysis of an individual. Those people with elevated LDL levels and two or more risk factors are treated much more aggressively with lipid-lowering medications and have lower target LDL values ( Tables 2 and 3 ) . Because individuals with SCI have an increased likelihood of having an increased number of risk factors (elevated HDL, obesity, altered glucose metabolism), aggressive monitoring of LDL levels are crucial.



Table 2

Target for LDL-cholesterol (mg/dL)


















<100 Optimal
100–129 Near optimal
130–159 Borderline
160–189 High
≥190 Very high

Data from National Cholesterol Education Program (NCEP) Expert Panel on Detection. Third Report of the National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III) final report. Circulation 2002;106(25):3143–421.


Table 3

Risk category












CHD present or CHD risk >20% <100
2+ risk factors (10-year risk ≤20%) <130
0–1 risk factors <160

Data from National Cholesterol Education Program (NCEP) Expert Panel on Detection. Third Report of the National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III) final report. Circulation 2002;106(25):3143–421.


Serum triglycerides are also thought to contribute to CHD. Although they are not identified as an independent risk factor for CHD, elevated levels of triglycerides are very commonly associated with some nonlipid risk factors for CHD including those with diabetes, obesity, high carbohydrate diets, sedentary lifestyles, hypertension, excess alcohol intake, and cigarette smoking . Both individuals with SCI and able-bodied controls show an inverse correlation with HDL cholesterol (HDL-C) levels and triglyceride levels .




Glucose intolerance/diabetes


Abnormalities in glucose metabolism are also more prevalent in individuals with SCI . After an SCI, atrophy of skeletal muscle decreases the muscle mass. It has been shown that by 24 weeks after injury the cross-sectional muscle mass of the leg is 45% to 80% of age-matched controls . Subjects with tetraplegia have been found to have a reduction in glucose transport that is proportional to the loss of the muscle mass . The loss of muscle mass is critical because insulin acts peripherally on the individual’s muscle mass for metabolism of glucose. Glucose tolerance ensues as higher plasma levels of insulin are required to maintain normal blood glucose levels . Elevated serum insulin levels and other abnormalities in glucose metabolism, even if not meeting criteria for absolute diabetes, are considered to be atherogenic conditions, increasing the risk for CHD.


Bauman and colleagues have shown the increased rates of abnormal glucose tolerance and presence of frank diabetes in individuals with SCI compared with age-matched controls. After a 75-g oral glucose tolerance test, 22% of individuals with SCI had met criteria for diabetes versus only 6% of the able-bodied subjects tested. Only 38% of individuals with tetraplegia and 50% of those with paraplegia had a normal response to this glucose tolerance test versus 82% of age-matched controls. Bauman also found that those with complete tetraplegia were found to have the highest incidence of impaired glucose metabolism compared with other neurologic classification groups of spinal cord injury, stressing the importance of muscle mass and immobility.




Glucose intolerance/diabetes


Abnormalities in glucose metabolism are also more prevalent in individuals with SCI . After an SCI, atrophy of skeletal muscle decreases the muscle mass. It has been shown that by 24 weeks after injury the cross-sectional muscle mass of the leg is 45% to 80% of age-matched controls . Subjects with tetraplegia have been found to have a reduction in glucose transport that is proportional to the loss of the muscle mass . The loss of muscle mass is critical because insulin acts peripherally on the individual’s muscle mass for metabolism of glucose. Glucose tolerance ensues as higher plasma levels of insulin are required to maintain normal blood glucose levels . Elevated serum insulin levels and other abnormalities in glucose metabolism, even if not meeting criteria for absolute diabetes, are considered to be atherogenic conditions, increasing the risk for CHD.


Bauman and colleagues have shown the increased rates of abnormal glucose tolerance and presence of frank diabetes in individuals with SCI compared with age-matched controls. After a 75-g oral glucose tolerance test, 22% of individuals with SCI had met criteria for diabetes versus only 6% of the able-bodied subjects tested. Only 38% of individuals with tetraplegia and 50% of those with paraplegia had a normal response to this glucose tolerance test versus 82% of age-matched controls. Bauman also found that those with complete tetraplegia were found to have the highest incidence of impaired glucose metabolism compared with other neurologic classification groups of spinal cord injury, stressing the importance of muscle mass and immobility.




Obesity


Obesity is also a major risk factor contributing to increased risk of CHD in the population with SCI. Obesity after SCI can come about from a number of factors. Buchholz estimates that the resting metabolic rate (RMR) is overestimated by 5% to 32% in individuals with SCI. This is explained primarily by a reduction in the fat-free lean body mass. In normal individuals, the RMR typically will account for approximately 65% of the total energy expenditure. After SCI, there is a decrease in the fat-free body mass that occurs primarily from atrophy of skeletal muscle. Studies have shown that the RMR has been found to be 14% to 27% lower in individuals with chronic SCI versus age-matched control subjects . Absence of this metabolically active muscle can account for a significant portion of the decreased resting metabolic rate. It is also thought in high-level injuries that the decreased sympathetic nervous system activity can contribute to a decreased RMR. The issue of obesity in people with SCI is extensively reviewed by Dr. Gater in this edition of PM&R clinics .




Physical fitness


Physical activity accounts for 25% to 30% of the total daily energy expenditure . Not surprisingly, numerous studies suggest that individuals with SCI have significantly lower levels of regular physical activity than the able-bodied population . It is also noted that individuals with tetraplegia had lower levels of fitness versus those with paraplegia . The thermal effect of food accounts for approximately 10% of the total daily energy expenditure and is not felt to be significantly different in the able-bodied population versus the population with SCI. This decrease in RMR and decreased physical activity in individuals with SCI can lead to a positive energy balance. Individuals with SCI have been found to have lower metabolic demands and decreased energy expenditures. To maintain weight at baseline levels, caloric intake must be reduced accordingly or weight gain will occur .


Individuals with SCI, by the nature of their impairments, are susceptible to the constellation of metabolic abnormalities discussed above (decreased HDL-C levels, glucose intolerance/insulin resistance, type II diabetes mellitus, obesity, and sedentary lifestyle). This cluster of clinical and subclinical metabolic risk factors, the metabolic syndrome, has been associated with increased risk for CHD . Regular physical activity can increase exercise capacity, endurance, and muscle strength and has clearly been associated with decreased risk of cardiovascular disease. High levels of exercise have shown positive effects on lipoprotein profiles . Meta-analysis of 52 exercise trials of exercise training programs of >12 weeks’ duration yielded significant increases of HDL-C and decreases in both LDL cholesterol (LDL-C) and triglycerides . The HEalth, RIsk factors, exercise Training, And GEnetics (HERITAGE) study, a large and carefully controlled exercise study, also found positive effects on HDL-C, triglycerides, and LDL-C levels .


Physical activity has also shown positive effects on glucose metabolism . A diabetes prevention program showed that with an 8–metabolic equivalent (MET)-hour/week increase of activity (roughly equivalent to 6 miles of walking per week) showed a 58% decrease in the onset of type II diabetes and an average of a 4-kg weight loss compared with usual management . A review of 337 patients with type II diabetes showed average reductions in hemoglobin A1c levels of 0.5% to 1.0% .


Exercise alone is an inefficient means to achieve weight loss, and programs incorporating both an increase in physical activity and lifestyle modification produce better results . Exercise is certainly an important component in the battle with obesity. Exercise assists in fat metabolism and gains in lean muscle mass that can very well improve the general body composition . In contrast, weight loss programs with dieting can contribute to a 30% loss in lean muscle mass. A loss in weight of 5% to 10% can produce improvements in other cardiovascular risk factors . In general, the benefits of increased physical activity have widely been defined as an important component to a wellness program.


The American College of Sports Medicine (ACSM) and Centers for Disease Control (CDC) recommend that healthy adults participate in 30 minutes or more of moderate-intensity physical activity (ie, brisk walking) on most, and preferably all, days of the week . Moderate intensity exercises are classified as 40% to 60% of one’s peak VO 2 max or 4 to 6 metabolic equivalents (METs). One MET (3.5 mL O 2 /kg/min) is equivalent to one’s energy expenditure at total rest. A brisk walk of about 3 miles per hour is roughly equivalent to 4 METs . Evidence shows that exercise can also decrease the risks of other chronic diseases such as type II diabetes mellitus and obesity .


Sedentary lifestyles and decreased amounts of physical activity, however, are common among individuals with and without SCI. One survey estimates that more than 60% of Americans reported infrequent activity and one of four people stated they were totally sedentary . Individuals with SCI are perhaps even more susceptible to the perils of a sedentary lifestyle as a direct result of there injury and limited options for exercise . Studies suggest that those with SCI have significantly lower levels of physical activity than those in the able-bodied population . In fact, individuals living with SCI have been placed in the low end of the physical fitness spectrum. As expected, individuals with tetraplegia have lower levels of daily energy expenditure and aerobic power than those with paraplegia . Those with paraplegia, despite their increased upper extremity mobility and options for exercise, have been found to be only marginally more fit than those with tetraplegia . Noreau and colleagues has shown that approximately 25% of young people with paraplegia were able to achieve peak oxygen consumption levels that were only marginally sufficient to maintain independent living.


In an effort to combat many of the secondary conditions that occur after spinal cord injury, regular exercise and active lifestyles should be encouraged for all people with spinal cord injuries. The options for activity and types of exercise available vary depending on local resources, interests, and accessibility. In addition to the physical and metabolic benefits of exercise, persons with spinal cord injury who exercised reported less stress, less depression, and an improved quality of life . With regard to exercising after spinal cord injury, there are several physiologic changes that occur that have significant effect on exercise response and tolerance. These include the impairment or loss of sensorimotor function below the neurologic level of injury and impairments of the autonomic nervous system.


After spinal cord injury, there is loss of voluntary motor function in the large muscle groups in the lower extremities. This loss generally limits exercise routines to the smaller muscle groups of the upper extremities and trunk. The overall availability of muscle mass is dependent on level and completeness of injury and varies between individuals. An inverse relationship between level of injury and the peak oxygen consumption and cardiac output has been observed during exercise in those with spinal cord injury . Individuals with paraplegia have peak power output and cardiac output levels approximately half of those on able-bodied persons performing lower extremity exercise . The levels in tetraplegia are even further reduced from those seen with paraplegia . As the level of injury increases, less muscle mass is available during exercise. During peak exercise, individuals with higher-level lesions may have more difficulty reaching levels of O 2 consumption that stress the cardiovascular system appropriately before they reach peripheral fatigue. Persons with tetraplegia may also be less efficient during exercise because of the loss of trunk control and the need to stabilize themselves with the same muscle groups they are attempting to exercise .


In the able-bodied population, during exercise, there is a physiologic response that accounts for appropriate flow and distribution of blood throughout the body. This is accomplished with the venous pumping action of the lower extremities as well as through sympathetic-mediated input to maintain systemic blood pressure and flow. This ultimately allows for an increase in the cardiac end-diastolic volume, which increases the stroke volume of the heart and ultimately the cardiac output. This effect allows for greater delivery of oxygenated blood to the active muscles and compensation for their increased metabolic rate. In a person with spinal cord injury, this response can be significantly impaired. After spinal cord injury, in addition to the loss of lower extremity muscle activation during exercise, there are significant alterations in the flow and distribution of blood throughout the cardiovascular system. This results from loss of the venous pumping action in the muscles of the lower extremities as well as alterations in the autonomic (particularly sympathetic) nervous system that affects the cardiovascular system’s response to exercise. Persons with higher-level thoracic and cervical injuries, in addition to impaired function and blood flow in the lower extremities, also may experience a partial or complete separation of the autonomic nervous system from central control. This autonomic dysfunction further affects the cardiac response to exercise and ultimately may lead to further impairment in peak cardiac output achieved during exercise.


Impaired blood flow caused by the loss of venous pumping action and compensatory vasoconstriction can lead to blood pooling in the lower extremities, which results in decreased stroke volume both at rest and during exercise. Impairment in stroke volume has a direct effect on the resulting cardiac output. Several techniques have been evaluated to decrease the blood pooling in the lower extremities and improve blood return to the heart. These techniques include arm crank exercise in the supine or leg-elevated position, lower extremity compression garments, abdominal binders, antigravity suits (increase external pressure on the lower extremities), and lower extremity functional electrical stimulation (FES) .


Hopman and colleagues , conducted a comprehensive study comparing submaximal arm crank exercise response in tetraplegia and paraplegia using several different techniques to improve blood flow and redistribution. When compared with submaximal exercise in the sitting position, this study showed varied responses depending on the technique used. Supine positioning resulted in decreased heart rate and increased stroke volume significantly in both groups. The antigravity suit decreased heart rate in both groups and decreased oxygen uptake in those with paraplegia. Compressive stockings decreased heart rate and increased stroke volume in those with paraplegia only. FES improved stroke volume and increased oxygen uptake in the group with tetraplegia. This finding showed that the benefits of different techniques vary depending on mechanism and level of spinal cord injury .


In addition to the mechanical effects of lower-extremity paralysis and the affect it has on blood flow, the autonomic dysfunction following spinal cord injury also has a significant influence on exercise response. This dysfunction leads to alterations in heart rate, stroke volume, cardiac output, and blood pressure when compared with able-bodied controls . The sympathetic output from the spinal cord occurs from the T1 through the L2 level, and autonomic function and the body’s response may vary depending on the exact neurologic level and completeness of injury. In persons with tetraplegia, where injury occurs above sympathetic output of the spinal cord, there is a separation of the central control for autonomic nervous system from the periphery. Persons with tetraplegia typically have resting hypotensive blood pressures with systolic pressures in the 70- to 80-mmHg range when in the sitting position. The autonomic impairment affects the central control over input to the heart, adrenals, and peripheral vascular system . At maximal exercise effort, the peak heart rate seen with tetraplegia is generally limited to approximately 120 to 130 beats per minute. It has been suggested that this is most likely regulated through a decrease in parasympathetic input without sympathetic compensation . During exercise, sympathetic impairment also leads to loss of the sympathetic-mediated vasoconstriction used to regulate blood flow in the muscles and skin with the end result being the inability to maintain systemic blood pressure and thermoregulation; this may result in uncompensated vasodilation, with further hypotension and dizziness during more strenuous exercise. The long-term effects of low systemic blood pressure and impaired cardiac preload eventually can result in myocardium atrophy and impaired cardiac efficiency .


In studies of persons with paraplegia with injury levels T1 through T4, sympathetic input and exercise response have shown inconclusive results . Approaching levels of T6 and below, there is generally sustained central control of sympathetic regulation. Individuals with mid-level thoracic lesions and below are able to maintain normotensive blood pressures and cardiac output levels similar to able-bodied controls at rest. The sympathetic cardiac response allows for these individuals to compensate for decreased stroke volume secondary to the venous pooling in the lower extremities by elevating the heart rate, which is not seen in tetraplegia. Therefore, to maintain a similar cardiac output as able-bodied persons in the face of a lower stroke volume, resting heart rates are generally higher in those with lesions below T5 than in those in the uninjured population. Heart rates in paraplegia patients are also higher at similar workloads during exercise . The overall effect of heart rate on cardiac output is somewhat limited in paraplegia, and elevation of heart rate is unable to fully compensate adequately to increase the cardiac output to a level similar to able-bodied controls during more rigorous exercise using the upper extremities .


Even in the presence of the changes in exercise response, benefits to exercise have been defined clearly in individuals with SCI. In studies that have examined the relationship of peak oxygen consumption (VO 2 max) obtained via arm ergometry and serum lipids, significant inverse relationships were found between the VO 2 max and total cholesterol to HDL-C ratios, triglycerides, and LDL-C to HDL-C ratio . High-intensity training also showed increased levels of HDL-C and decreased triglycerides, LDL-C, and total cholesterol to HDL-C ratios . Exercise testing of 22 men with paraplegia found that those with lower peak aerobic capacities (based on exercise testing) and lower physical activity levels had higher fasting glucose levels, decreased HDL-C levels, and larger abdominal girth . With regard to cardiovascular improvements, in a review of 13 exercise studies, Hoffmann noted VO 2 max improvements after several weeks of training. In a study evaluating benefits of a 12-week circuit training program for person with paraplegia, Jacobs and colleagues , observed increased peak oxygen consumption, time to fatigue, peak power output, and improved isoinertial and isokinetic strength. Many of the benefits resulting from arm exercise routines are felt to be peripheral in nature or resulting more from changes in the arm musculature that leads to improvements in strength and endurance rather than from cardiovascular adaptations.


In addition to arm exercise, FES leg cycle ergometry (FES-LCE) also has been evaluated for its potential to activate lower-extremity muscles and improve blood flow back to the heart where it could improve stroke volume and cardiac output . In a 12- to 16-week study involving 36 exercise sessions in which persons with both tetraplegia and paraplegia performed FES-LCE, Hooker and colleagues observed higher posttraining power output, oxygen uptake, pulmonary ventilation, heart rate and cardiac output, and lower total peripheral resistance. In another study involving FES-LCE over 12 weeks with 36 sessions, Faghri and colleagues observed increased power output, in both tetraplegia and paraplegia. He also noted increased resting heart rate and systolic blood pressure in the group with tetraplegia, suggesting better cardiovascular stability. In both groups, heart rate and blood pressure decreased while stroke volume and cardiac output increased after 12 weeks. Exercise programs with FES-LCE also have been reported to reverse the left ventricular myocardial atrophy seen with tetraplegia .


The need for exercise in the SCI population has been well recognized and does not vary dramatically from the general population. Generally, it is suggested that three to five exercise sessions should occur on a weekly basis. These sessions should be 20 to 60 minutes in duration with an intensity of 50% to 80% of the individual’s peak heart rate . The American College of Sports medicine has published recommended exercise programming for individuals with SCI . The recommendations for these exercise programs are summarized as follows: different modes of cardiopulmonary training, which includes arm crank ergometry, wheelchair propulsion, swimming, vigorous wheelchair sports, ambulation with crutches or braces, seated aerobic exercise, and the use of electrically stimulated leg cycle activities. Important considerations in the prevention of overuse injuries are important components. It is suggested to try to vary activities as much as possible to avoid overuse injuries to the upper extremities as well as to provide strengthening activities to all major muscle groups. Emphasis on precautions, to minimize the risk of secondary conditions that individuals with SCI are susceptible to are also emphasized. Using appropriate pressure relief cushions and proper positioning and balancing to avoid falls and risk of fractures are outlined. In addition, autonomic dysfunction and autonomic dysreflexia are addressed, and it is suggested that consultation with a physician should occur if there are any questions regarding potential complications or secondary conditions. Slow, progressive improvements should be the goal, and expectations should be based on the amount of the muscle mass being exercised (ie, the greater the muscle mass being exercised, the greater the improvements in fitness that would be expected).

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Apr 19, 2017 | Posted by in PHYSICAL MEDICINE & REHABILITATION | Comments Off on Cardiovascular Health and Fitness in Persons with Spinal Cord Injury

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