Hemodynamic responses to head-up tilt after spinal cord injury support a role for the mid-thoracic spinal cord in cardiovascular regulation

Influence of neurological lesion level on heart rate variability and fatigue in adults with spinal cord injury

STUDY DESIGN

Group cohort design.

OBJECTIVES

To determine the influence of spinal cord injury (SCI) and neurological level on heart rate variability (HRV) and associations with fatigue.

SETTING

SCI rehabilitation outpatient and community settings in New South Wales, Australia.

METHODS

Participants included 45 adults with SCI living in the community and 44 able-bodied controls. Socio-demographic, neurological injury, psychological, HRV and eye blink variables were assessed. Multivariate analysis of variance and post hoc protected t-tests were used to determine differences in HRV and fatigue as a function of the neurological level. Pearson's correlation analysis was used to determine the relationships between these factors.

RESULTS

Participants with SCI had significantly reduced sympathetic activity. Those with tetraplegia had lowered sympathetic activity compared with those with paraplegia and able-bodied controls. Neither were differences in parasympathetic activity found between groups nor were there any significant differences found for the time domain or non-linear domains. Higher levels of fatigue were found in the SCI sample, and participants with tetraplegia had higher fatigue levels compared with those with paraplegia. Fatigued participants were more likely to have altered autonomic function-that is, reduced sympathetic activity.

CONCLUSIONS

Higher levels of neurological impairment in people with SCI are more likely to result in disordered cardiovascular control involving reduced sympathetic activity, whereas elevated fatigue was found to be associated with increased sympathetic dysfunction. Findings highlight the need to address risks associated with this dysfunction, such as improved HRV and fatigue screening for people with SCI and improved education on cardiovascular risk factors.

Tilt testing with combined lower body negative pressure: a "gold standard" for measuring orthostatic tolerance

Orthostatic tolerance (OT) refers to the ability to maintain cardiovascular stability when upright, against the hydrostatic effects of gravity, and hence to maintain cerebral perfusion and prevent syncope (fainting). Various techniques are available to assess OT and the effects of gravitational stress upon the circulation, typically by reproducing a presyncopal event (near-fainting episode) in a controlled laboratory environment. The time and/or degree of stress required to provoke this response provides the measure of OT. Any technique used to determine OT should: enable distinction between patients with orthostatic intolerance (of various causes) and asymptomatic control subjects; be highly reproducible, enabling evaluation of therapeutic interventions; avoid invasive procedures, which are known to impair OT(1). In the late 1980s head-upright tilt testing was first utilized for diagnosing syncope(2). Since then it has been used to assess OT in patients with syncope of unknown cause, as well as in healthy subjects to study postural cardiovascular reflexes(2-6). Tilting protocols comprise three categories: passive tilt; passive tilt accompanied by pharmacological provocation; and passive tilt with combined lower body negative pressure (LBNP). However, the effects of tilt testing (and other orthostatic stress testing modalities) are often poorly reproducible, with low sensitivity and specificity to diagnose orthostatic intolerance(7). Typically, a passive tilt includes 20-60 min of orthostatic stress continued until the onset of presyncope in patients(2-6). However, the main drawback of this procedure is its inability to invoke presyncope in all individuals undergoing the test, and corresponding low sensitivity(8,9). Thus, different methods were explored to increase the orthostatic stress and improve sensitivity. Pharmacological provocation has been used to increase the orthostatic challenge, for example using isoprenaline(4,7,10,11) or sublingual nitrate(12,13). However, the main drawback of these approaches are increases in sensitivity at the cost of unacceptable decreases in specificity(10,14), with a high positive response rate immediately after administration(15). Furthermore, invasive procedures associated with some pharmacological provocations greatly increase the false positive rate(1). Another approach is to combine passive tilt testing with LBNP, providing a stronger orthostatic stress without invasive procedures or drug side-effects, using the technique pioneered by Professor Roger Hainsworth in the 1990s(16-18). This approach provokes presyncope in almost all subjects (allowing for symptom recognition in patients with syncope), while discriminating between patients with syncope and healthy controls, with a specificity of 92%, sensitivity of 85%, and repeatability of 1.1±0.6 min(16,17). This allows not only diagnosis and pathophysiological assessment(19-22), but also the evaluation of treatments for orthostatic intolerance due to its high repeatability(23-30). For these reasons, we argue this should be the "gold standard" for orthostatic stress testing, and accordingly this will be the method described in this paper.

Cardiovascular complications of spinal cord injury

BACKGROUND

The aim of this paper is to provide an overview of the autonomic innervation of the cardiovascular system and the cardiovascular sequelae of spinal cord injuries.

METHOD

A literature search was carried out in the PubMed database, with the search phrases "traumatic spinal cord injury"/"traumatic spinal cord injuries" together with "autonomic dysfunction", "autonomic dysreflexia" and "cardiovascular disease".

RESULTS

The most important cardiovascular complications in the acute phase are bradyarrhythmia, hypotension, enhanced vasovagal reflexes, supraventricular/ventricular ectopic beats, vasodilation and venous stasis. Important in the chronic phase are orthostatic hypotension and impaired regulation of blood pressure, blood volume and body temperature. Tetraplegia is frequently accompanied by autonomic dysreflexia, impaired transmission of cardial pain, loss of muscle mass in the left ventricle and pseudoinfarction. Patients with injuries above the sixth thoracic vertebra have a predisposition for autonomic dysreflexia. This is a condition characterised by sudden, uncontrolled sympathetic response accompanied by a rise in blood pressure. Autonomic dysreflexia usually leads to headaches and erythema on the upper chest. The condition may cause cerebral haemorrhage and is potentially life threatening. Patients with spinal cord injuries have an increased risk of atherosclerotic disease due to overweight, lipid disorders, metabolic syndrome and diabetes. They are predisposed to thrombotic emboli due to venous stasis and hypercoagulopathy, particularly immediately after the injury.

INTERPRETATION

Knowledge of cardiovascular sequelae after spinal cord injuries and assessment of these is important for correct diagnostics, planning of preventive measures and optimal treatment.

Insulin resistance in tetraplegia but not in mid-thoracic paraplegia: is the mid-thoracic spinal cord involved in glucose regulation?

STUDY DESIGN

Controlled experimental human study.

OBJECTIVES

To assess insulin resistance (IR) in tetraplegia and paraplegia, and the role of the spinal cord (SC) in glucose regulation.

SETTING

Laboratory of Spinal Research, Loewenstein Rehabilitation Hospital.

METHODS

Glucose and insulin levels and the heart rate variation spectral components LF (low frequency), HF (high frequency) and LF/HF were studied at supine rest, head-up tilt and after a standard meal in three groups: 13 healthy subjects, 7 patients with T(4)-T(6) paraplegia and 11 patients with C(4)-C(7) tetraplegia.

RESULTS

Glucose and insulin increased significantly after the meal in all groups (P<0.001). Glucose increased significantly more in the tetraplegia than in the other groups (P<0.01). Increases in insulin level tended to accompany increases in LF/HF after the meal in the tetraplegia and control groups but not in the paraplegia group.

CONCLUSION

Post-prandial IR appears in C(4)-C(7) but not in T(4)-T(6) SC injury. The results of the study, combined with previously published findings, are consistent with the hypotheses that IR is related to activation of the sympathetic nervous system, and that below T(4) the mid-thoracic SC is involved in the regulation of glucose and insulin levels.