OLIVE, J. L., G. A. DUDLEY, and K. K. MCCULLY. Vascular Remodeling after Spinal Cord Injury. Med. Sci. Sports Exerc., Vol. 35, No. 6, pp. 901–907, 2003.
Purpose: Our purpose was to determine whether spinal cord injured (SCI) subjects have decreased femoral artery diameter and maximal hyperemic blood flow when expressed per unit of muscle volume compared with able-bodied (AB) individuals. A secondary purpose was to determine whether blood flow recovery rates were similar between groups.
Methods: Blood flow was measured in the femoral artery using Doppler ultrasound after distal thigh cuff occlusion of 4 and 10 min. Muscle mass of the lower leg was determined by magnetic resonance imaging (MRI).
Results: SCI individuals had smaller muscle cross-sectional areas (37%, P = 0.001) and volumes (38%, P = 0.001) than AB individuals. Furthermore, femoral artery diameter (0.76 ± 0.14 vs 0.48 ± 0.06 cm, AB vs SCI, P < 0.001) and femoral artery maximal blood flow (2050 ± 520 vs 1220 ± 240 mL·min−1, AB vs SCI, P < 0.001) were lower in SCI than AB individuals. Femoral artery diameter and maximal blood flow per unit muscle volume did not differ between SCI and AB individuals (P = 0.418 and P = 0.891, respectively). Blood flow recovery after ischemia was prolonged in SCI compared with AB individuals for both cuff durations (P = 0.048).
Conclusions: In summary, femoral artery diameter and maximal hyperemic blood flow response per unit muscle volume are not different between SCI and AB individuals. Vascular atrophy after SCI appears to be closely linked to muscle atrophy. Furthermore, the SCI compared with AB individuals had a prolonged time to recovery, which may suggest decreased vessel reactivity.
Spinal cord injury (SCI) results in muscle and vascular changes below the level of injury (7,8,14,16). These peripheral circulatory and skeletal muscle adaptations may contribute to the increased risk of cardiovascular disease in SCI patients (12). Severe muscle atrophy occurs after an SCI. Muscle cross-sectional area (CSA) is one half to two thirds of that of able-bodied individuals (3,6,14,24). Decreases in CSA occur as early as 6 months after injury but stabilize 12–17 months after injury (2,24).
Several other studies have reported that vascular atrophy occurs as well in this population. Thigh blood flow in paraplegics is reduced by 35% compared with able-bodied controls as early as 1 month after injury when measured by plethysmography (29). Femoral artery diameter size is 50% of able-bodied subjects in both paraplegics (1,7) and tetraplegics (16), indicating that vascular atrophy occurs to a similar extent regardless of the level of injury. Resting blood flow in paraplegics is reduced by 28% (8) and venous capacity reduced by 50% of able-bodied subjects (7). Furthermore, a 50% reduction in maximal blood flow was reported in sedentary tetraplegics after 5 min of cuff occlusion (16). A shortcoming of these studies is that vascular changes were not studied in relation to changes that occur in the muscle. A previous study in healthy individuals reported that vessel diameter size did not limit peak blood flow or oxygen uptake (21). It is not known, however, whether the large decreases in the vessel size of SCI individuals decreases muscle blood flow and impairs muscle function during exercise.
Vascular reactivity can be thought of as the ability of the vessel to regulate vascular tone (19). Vascular reactivity increases after exercise training (19) while it is impaired in deconditioned (10,26), immobilized patients (13), and in incomplete SCI individuals (17). Furthermore, vascular reactivity is impaired in individuals with diabetes (27,28), heart disease (4), and obesity (31). Alterations in vascular reactivity have been related to insulin insensitivity, hypertension, and increased risk of cardiovascular disease (31). SCI individuals who are inherently inactive and have high levels of obesity (12) vascular reactivity may be particularly affected in this population.
The purpose of this study was to compare femoral artery diameter size and reactive hyperemic response by Doppler ultrasound in complete, chronic SCI individuals, and in able-bodied individuals. Femoral artery diameter and blood flow was expressed per unit of muscle volume (as measured by MRI) to determine whether alterations in the vascular system are related to alterations in muscle volume. Halftime to recovery of blood flow was also investigated to determine whether vascular reactivity was altered in SCI individuals. We hypothesized that SCI subjects would have a smaller arterial diameter size and reduced blood flow response after cuff occlusion. However, this smaller difference would not persist when diameter and blood flow are expressed per unit of muscle volume. A second hypothesis was that SCI individuals will have a delayed recovery to reactive hyperemia compared with the able-bodied individuals.