Cardiovascular control in spinal man

Intravesical injection of botulinum toxin type a may be an effective treatment option for autonomic dysreflexia in patients with high-level spinal cord injury

OBJECTIVE

To evaluate the efficacy of intravesical injection of botulinum toxin type A (BTX-A) for neurogenic detrusor overactivity (DO) in reducing the frequency and severity of autonomic dysreflexia (AD).

DESIGN

A cross-sectional nonrandomized trial with before (baseline) and after (follow-up) assessments.

SETTING

A single spinal cord injury (SCI) rehabilitation center in China.

PARTICIPANTS

Twenty-five patients with SCI at or above T6 and a history of AD who underwent urodynamic studies (UDS) before and 3 months after BTX-A injection.

INTERVENTIONS

Received bladder injection treatment wtih 200 U BTX-A.

OUTCOME MEASURES

The maximum detrusor pressure(Pdetmax) and voume at first DO(VFIDC), baseline and overall maximum systolic blood pressure (SBP) during UDS, and scores of Incontinence Specific Quality of Life Instrument (IQoL) were recorded before and 3 months after the injection. The change in SBP (ΔSBP) from baseline to maximum SBP during UDS was calculated to assess the severity. The frequency of AD was recorded using ambulatory blood pressure monitoring during a 24 h period before and 3 months after the injection.

RESULTS

BTX-A injection decreased the Pdetmax and increased the VFIDC and mean urine volume per catheterization increased. The maximum SBP and the ΔSBP during UDS decreased significantly decreased after the injection (151.44 ± 13.92 vs 133.32 ± 9.20 mmHg and 49.44 ± 12.81 vs 33.08 ± 9.11 mmHg respectively, P < 0.05). The frequency of bladder-related ADs (i.e. performed a clean intermittent catheterization or leakage) during a 24-h period significantly decreased from 11.04 ± 1.81-7.88 ± 2.15 (P < 0.001).

CONCLUSIONS

BTX-A decreases the severity of SBP increase and the number of AD episodes 3 months after intravesical injection.

Spinal Cord Stimulation Prevents Autonomic Dysreflexia in Individuals with Spinal Cord Injury: A Case Series

Spinal cord injury (SCI) results in severe cardiovascular dysfunction due to the disruption of supraspinal control. Autonomic dysreflexia (AD), an uncontrolled rise in blood pressure in response to peripheral stimuli including common bowel routine, digital anorectal stimulation (DARS), reduces the quality of life, and increases morbidity and mortality. Recently, spinal cord stimulation (SCS) has emerged as a potential intervention to mitigate unstable blood pressure following SCI. The objective of this case series was to test the real-time effect of epidural SCS (eSCS) at the lumbosacral spinal cord, the most common implant location, on mitigating AD in individuals with SCI. We recruited three individuals with cervical and upper thoracic motor-complete SCI who have an implanted epidural stimulator. We demonstrated that eSCS can reduce the elevation in blood pressure and prevent DARS-induced AD. The blood pressure variability analysis indicated that eSCS potentially reduced vascular sympathetic nervous system activity during DARS, compared to without eSCS. This case series provides evidence to support the use of eSCS to prevent AD episodes during routine bowel procedures, improving the quality of life for individuals with SCI and potentially reducing cardiovascular risks.

Severe Headache and Hypertension due to Cerebral Venous Sinus Thrombosis in a Patient With Cervical Spinal Cord Injury: A Case Report

Autonomic dysreflexia is suspected when patients with spinal cord injury report headaches and hypertension. A 52-year-old man was diagnosed with C5-C6-C7 cervical spinal cord injury, traumatic subdural hemorrhage, intracerebral hemorrhage, and skull fracture. The patient underwent surgery at another hospital. The patient was hospitalized for comprehensive rehabilitation after 7 months. The assessment revealed an American Spinal Cord Injury Association Impairment Scale grade A at the C7 level due to complete spinal cord damage. Evaluation of muscle weakness that occurred after experiencing severe headache and hypertension revealed an intracerebral hemorrhage caused by cerebral venous sinus thrombosis. The patient showed improvement in muscle strength over time and was monitored for warfarin administration. Furthermore, cerebral venous sinus thrombosis should be considered as a differential diagnosis when patients with spinal cord injury who have experienced polytrauma complain of headache and hypertension, as they share clinical symptoms with autonomic dysreflexia. Additional evaluations, such as imaging examinations, should be conducted, as necessary.

A porcine model for studying the cardiovascular consequences of high-thoracic spinal cord injury

KEY POINTS

We have developed a novel porcine model of high-thoracic midline contusion spinal cord injury (SCI) at the T2 spinal level. We describe this model and the ensuing cardiovascular and neurohormonal responses, and demonstrate the model is efficacious for studying clinically relevant cardiovascular dysfunction post-SCI. We demonstrate that the high-thoracic SCI model, but not a low-thoracic SCI model, induces persistent hypotension along with a gradual reduction in plasma noradrenaline and increases in plasma aldosterone and angiotensin II. We additionally conducted a proof-of-concept long-term (12 weeks) survival study in animals with T2 contusion SCI demonstrating the potential utility of this model for not only acute experimentation but also long-term drug studies prior to translation to the clinic.

ABSTRACT

Cardiovascular disease is a leading cause of morbidity and mortality in the spinal cord injury (SCI) population, especially in those with high-thoracic or cervical SCI. With this in mind, we aimed to develop a large animal (porcine) model of high-thoracic (T2 level) contusion SCI and compare the haemodynamic and neurohormonal responses of this injury against a low-thoracic (T10 level) model. Ten Yorkshire pigs were randomly subjected to 20 cm weight drop contusion SCI at either the T2 or the T10 spinal level. Systolic blood pressure (SBP), mean arterial pressure (MAP) and heart rate (HR) were continuously monitored until 4 h post-SCI. Plasma noradrenaline (NA), aldosterone and angiotensin II (ANGII) were measured pre-SCI and at 30, 60, 120 and 240 min post-SCI. Additionally, two Yucatan pigs were subjected to T2-SCI and survived up to 12 weeks post-injury to demonstrate the efficacy of this model for long-term survival studies. Immediately after T2-SCI, SBP, MAP and HR increased (P < 0.0001). Between decompression (5 min post-SCI) and 30 min post-decompression in T2-SCI, SBP and MAP were lower than pre-SCI (P < 0.038). At 3 and 4 h after T2-SCI, SBP remained lower than pre-SCI (P = 0.048). After T10-SCI, haemodynamic indices remained largely unaffected. Plasma NA was lower in T2- vs. T10-SCI post-SCI, whilst aldosterone and ANGII were higher. Both chronically injured pigs demonstrated a vast reduction in SBP at 12 weeks post-SCI. Our model of T2-SCI causes a rapid and sustained alteration in neurohormonal control and cardiovascular function, which does not occur in the T10 model.

Prevalence of Autonomic Dysreflexia in Patients with Spinal Cord Injury above T6

Objective

To investigate the prevalence of autonomic dysreflexia (AD) using ambulatory blood pressure monitoring (ABPM) and the autonomic dysfunction following spinal cord injury (ADFSCI) questionnaire in patients with spinal cord injury (SCI) above T6.

Methods

Twenty-eight patients diagnosed with SCI above T6 were enrolled. ABPM and ADFSCI were utilized to assess AD. Using ABPM, systolic blood pressure (SBP), diastolic blood pressure, and heart rate (HR) were measured at 30-minute intervals. AD was defined as SBP 20 mmHg higher than basal SBP, and the number of AD events was counted. The ADFSCI questionnaire evaluates the severity and frequency of the AD symptoms.

Results

According to the ABPM, AD occurred in 26 patients and AD events occurred 5.8 ± 4.7 times. Average daytime and nighttime SBP were 119.9 ± 18.8 mmHg and 123.8 ± 21.2 mmHg, respectively, and the nighttime mean SBP appeared to be 4 mmHg higher than daytime mean SBP. These findings suggest the loss of nocturnal BP dipping in SCI patients. ADFSCI results revealed that 16 of the patients evaluated were symptomatic while 12 were asymptomatic.

Conclusion

AD following SCI above T6 was highly prevalent and several patients seemed asymptomatic. These results suggest the necessity of proper diagnostic and therapeutic interventions for managing AD.

Baroreflex autonomic control in human spinal cord injury: Physiology, measurement, and potential alterations

The arterial baroreflex is a primary regulator of autonomic outflow to effectively regulate acute changes in blood pressure. After a spinal cord injury (SCI), regulation of autonomic function is disrupted, although the damage of the autonomic pathways may not necessarily be related to the severity of injury (i.e. level and completeness). Nonetheless, it can be assumed that there would be greater loss of sympathetic innervation with higher level of injury and that cardiac parasympathetic control would remain intact regardless of injury level. In those with SCI, impaired baroreflex regulation has implications not only for adequate pressure regulation, but also for long term cardiovascular health. In this review, we discuss the expected impact ofan SCI on baroreflex control and the studies that have investigated baroreflex sensitivity in this population. The data generally indicates that baroreflex sensitivity is lesser in those with chronic injuries. However, these findings are counter to the expected effect of an SCI and hence may indicate that the effect of an SCI on baroreflex control might be secondary to long term deconditioning and/or vascular stiffening of baroreceptive arteries. Furthermore, the alterations in the ability to regulate pressure do not impact the relationship between spontaneous heart rate and blood pressure variabilities. In addition, those with SCI are not adequately able to control blood pressure changes in response to orthostasis, resulting in frank hypotension in a significant proportion of those with high level injuries.

Autonomic dysreflexia after spinal cord injury: Systemic pathophysiology and methods of management

Traumatic spinal cord injury (SCI) has widespread physiological effects beyond the disruption of sensory and motor function, notably the loss of normal autonomic and cardiovascular control. Injury at or above the sixth thoracic spinal cord segment segregates critical spinal sympathetic neurons from supraspinal modulation which can result in a syndrome known as autonomic dysreflexia (AD). AD is defined as episodic hypertension and concomitant baroreflex-mediated bradycardia initiated by unmodulated sympathetic reflexes in the decentralized cord. This condition is often triggered by noxious yet unperceived visceral or somatic stimuli below the injury level and if severe enough can require immediate medical attention. Herein, we review the pathophysiological mechanisms germane to the development of AD, including maladaptive plasticity of neural circuits mediating abnormal sympathetic reflexes and hypersensitization of peripheral vasculature that collectively contribute to abnormal hemodynamics after SCI. Further, we discuss the systemic effects of recurrent AD and pharmacological treatments used to manage such episodes. Contemporary research avenues are then presented to better understand the relative contributions of underlying mechanisms and to elucidate the effects of recurring AD on cardiovascular and immune functions for developing more targeted and effective treatments to attenuate the development of this insidious syndrome following high-level SCI.

Changes in sympathetic neurovascular function following spinal cord injury

The effects of spinal cord injury (SCI) on sympathetic neurovascular transmission have generally been ignored. This review describes changes in sympathetic nerve-mediated activation of arterial vessels to which ongoing sympathetic activity has been reduced or silenced following spinal cord transection in rats. In all vessels studied in rats, SCI markedly enhanced their contractile responses to nerve activity. However, the mechanisms that augment neurovascular transmission differ between the rat tail artery and mesenteric artery. In tail artery, the enhancement of neurovascular transmission cannot be attributed to changes in sensitivity of the vascular muscle to α₁- or α₂-adrenoceptor agonists. Instead the contribution of L-type Ca²⁺ channels to activation of the smooth muscle by nerve-released noradrenaline is greatly increased following SCI. By contrast, mesenteric arteries from SCI rats had increased sensitivity to phenylephrine but not to methoxamine. While both phenylephrine and methoxamine are α₁-adrenoceptor agonists, only phenylephrine is a substrate for the neuronal noradrenaline transporter. Therefore the selective increase in sensitivity to phenylephrine suggests that the activity of the neuronal noradrenaline transporter is reduced. While present evidence suggests that sympathetic vasoconstrictor neurons do not contribute to the normal regulation of peripheral resistance below a complete SCI in humans, the available evidence does indicate that these experimental findings in animals are likely to apply after SCI in humans and contribute to autonomic dysreflexia.

Development of an Algorithm to Perform a Comprehensive Study of Autonomic Dysreflexia in Animals with High Spinal Cord Injury Using a Telemetry Device

Spinal cord injury (SCI) is a debilitating neurological condition characterized by somatic and autonomic dysfunctions. In particular, SCI above the mid-thoracic level can lead to a potentially life-threatening hypertensive condition called autonomic dysreflexia (AD) that is often triggered by noxious or non-noxious somatic or visceral stimuli below the level of injury. One of the most common triggers of AD is the distension of pelvic viscera, such as during bladder and bowel distension or evacuation. This protocol presents a novel pattern recognition algorithm developed for a JAVA platform software to study the fluctuations of cardiovascular parameters as well as the number, severity and duration of spontaneously occurring AD events. The software is able to apply a pattern recognition algorithm on hemodynamic data such as systolic blood pressure (SBP) and heart rate (HR) extracted from telemetry recordings of conscious and unrestrained animals before and after thoracic (T3) complete transection. With this software, hemodynamic parameters and episodes of AD are able to be detected and analyzed with minimal experimenter bias.

Brain-heart interactions: physiology and clinical implications

The brain controls the heart directly through the sympathetic and parasympathetic branches of the autonomic nervous system, which consists of multi-synaptic pathways from myocardial cells back to peripheral ganglionic neurons and further to central preganglionic and premotor neurons. Cardiac function can be profoundly altered by the reflex activation of cardiac autonomic nerves in response to inputs from baro-, chemo-, nasopharyngeal and other receptors as well as by central autonomic commands, including those associated with stress, physical activity, arousal and sleep. In the clinical setting, slowly progressive autonomic failure frequently results from neurodegenerative disorders, whereas autonomic hyperactivity may result from vascular, inflammatory or traumatic lesions of the autonomic nervous system, adverse effects of drugs and chronic neurological disorders. Both acute and chronic manifestations of an imbalanced brain-heart interaction have a negative impact on health. Simple, widely available and reliable cardiovascular markers of the sympathetic tone and of the sympathetic-parasympathetic balance are lacking. A deeper understanding of the connections between autonomic cardiac control and brain dynamics through advanced signal and neuroimage processing may lead to invaluable tools for the early detection and treatment of pathological changes in the brain-heart interaction.

Functional electrical stimulation: cardiorespiratory adaptations and applications for training in paraplegia

Regular exercise can be broadly beneficial to health and quality of life in humans with spinal cord injury (SCI). However, exercises must meet certain criteria, such as the intensity and muscle mass involved, to induce significant benefits. SCI patients can have difficulty achieving these exercise requirements since the paralysed muscles cannot contribute to overall oxygen consumption. One solution is functional electrical stimulation (FES) and, more importantly, hybrid training that combines volitional arm and electrically controlled contractions of the lower limb muscles. However, it might be rather complicated for therapists to use FES because of the wide variety of protocols that can be employed, such as stimulation parameters or movements induced. Moreover, although the short-term physiological and psychological responses during different types of FES exercises have been extensively reported, there are fewer data regarding the long-term effects of FES. Therefore, the purpose of this brief review is to provide a critical appraisal and synthesis of the literature on the use of FES for exercise in paraplegic individuals. After a short introduction underlying the importance of exercise for SCI patients, the main applications and effects of FES are reviewed and discussed. Major findings reveal an increased physiological demand during FES hybrid exercises as compared with arms only exercises. In addition, when repeated within a training period, FES exercises showed beneficial effects on muscle characteristics, force output, exercise capacity, bone mineral density and cardiovascular parameters. In conclusion, there appears to be promising evidence of beneficial effects of FES training, and particularly FES hybrid training, for paraplegic individuals.

Disordered cardiovascular control after spinal cord injury

Damage to the spinal cord disrupts autonomic pathways, perturbing cardiovascular homeostasis. Cardiovascular dysfunction increases with higher levels of injury and greater severity. Disordered blood pressure control after spinal cord injury (SCI) has significant ramifications as cord-injured people have an increased risk of developing heart disease and stroke; cardiovascular dysfunction is currently a leading cause of death among those with SCI. Despite the clinical significance of abnormal cardiovascular control following SCI, this problem has been generally neglected by both the clinical and research community. Both autonomic dysreflexia and orthostatic hypotension are known to prevent and delay rehabilitation, and significantly impair the overall quality of life after SCI. Starting with neurogenic shock immediately after a higher SCI, ensuing cardiovascular dysfunctions include orthostatic hypotension, autonomic dysreflexia and cardiac arrhythmias. Disordered temperature regulation accompanies these autonomic dysfunctions. This chapter reviews the human and animal studies that have furthered our understanding of the pathophysiology and mechanisms of orthostatic hypotension, autonomic dysreflexia and cardiac arrhythmias. The cardiovascular dysfunction that occurs during sexual function and exercise is elaborated. New awareness of cardiovascular dysfunction after SCI has led to progress toward inclusion of this important autonomic problem in the overall assessment of the neurological condition of cord-injured people.

Sexual function and autonomic dysreflexia in men with spinal cord injuries: how should we treat?

STUDY DESIGN

Systematic review.

OBJECTIVES

Review the literature on the acute or prophylactic treatment of autonomic dysreflexia in the context of sexual activities.

SETTING

International.

METHODS

Medline search using AD and spinal cord injury and all years of publication.

RESULTS

Thirty-seven papers on the specific treatment of AD showed that nifedipine, prazosin, captopril and clonidine are candidates in the context of sexual activities. Prazosin, however, has an initial hypotensive effect requiring to begin treatment 12 h before intercourse, which makes it less ideal for spontaneous sexual activities. Captopril has an initial hypotensive effect and was only studied in acute AD. Its usefulness in prophylaxis remains to be demonstrated. Clonidine has successfully been used clinically for decades, but never studied in randomized control trials. Nifedipine remains the most widely studied and significant treatment of AD whether in acute or prophylactic conditions. Recent concerns suggest increased cardiovascular risks with sublingual nifedipine in non-SCI populations, but negative long-term effects have not been reported in the SCI population.

CONCLUSION

Sexual function is a priority for men with SCI. As sexual activities, in particular ejaculation, can be a source of AD, adequate treatments and prophylaxis must be considered in the context of sexual activities. Experts must meet and conclude on the thresholds, parameters and treatments that should be considered in the long-term management of AD in the context of sexual function in men with SCI.

Autonomic dysreflexia: current evidence related to unstable arterial blood pressure control among athletes with spinal cord injury

OBJECTIVE

To present the complex issues of the impact of spinal cord injury (SCI) in sport, with a specific focus on autonomic dysreflexia (AD) and the potential debilitating effects of unstable blood pressure control among athletes.

DATA SOURCES AND METHODS

A literature review based on a key word electronic literature search of articles, practice guidelines, and review articles pertaining to AD was conducted using MEDLINE, SportDiscus, and EMBASE.

RESULTS

Spinal cord injury results not only in devastating paralysis; it also commonly is associated with a range of autonomic dysregulation that can interfere with cardiovascular, bladder, bowel, temperature, and sexual function. Individuals with a cervical or high-thoracic SCI face lifelong abnormalities in systemic arterial pressure control. In general, their resting arterial pressure is lower than that in able-bodied individuals and is commonly associated with persistent orthostatic intolerance. In addition, they experience transient episodes of life-threatening hypertension, known as "AD," which often is associated with disturbances in heart rate and rhythm. Autonomic dysreflexia occurs in up to 90% of individuals with a cervical or high-thoracic SCI and requires prompt intervention. It also is known that, during athletic activities, self-induced AD is used by some individuals to improve their performance, a technique known as "boosting." For health safety reasons, boosting is officially banned by the International Paralympic Committee.

CONCLUSIONS

Devastating paralysis, a variety of autonomic dysfunctions, and abnormal cardiovascular control after SCI present significant challenges in terms of individuals remaining active in competitive sports. Medical practitioners who are involved in the care of wheelchair athletes should be aware of the unique cardiovascular dysfunction that results from SCI and may occur at any time, even with seemingly innocuous triggers. Prompt recognition and appropriate management of these conditions, including episodes of AD, could be life saving.

Bone and muscle loss after spinal cord injury: organ interactions

Spinal cord injury (SCI) results in paralysis and marked loss of skeletal muscle and bone below the level of injury. Modest muscle activity prevents atrophy, whereas much larger--and as yet poorly defined--bone loading seems necessary to prevent bone loss. Once established, bone loss may be irreversible. SCI is associated with reductions in growth hormone, IGF-1, and testosterone, deficiencies likely to exacerbate further loss of muscle and bone. Reduced muscle mass and inactivity are assumed to be contributors to the high prevalence of insulin resistance and diabetes in this population. Alterations in muscle gene expression after SCI share common features with other muscle loss states, but even so, show distinct profiles, possibly reflecting influences of neuromuscular activity due to spasticity. Changes in bone cells and markers after SCI have similarities with other conditions of unloading, although after SCI these changes are much more dramatic, perhaps reflecting the much greater magnitude of unloading. Adiposity and marrow fat are increased after SCI with intriguing, though poorly understood, implications for the function of skeletal muscle and bone cells.

Flexor reflex decreases during sympathetic stimulation in chronic human spinal cord injury

A better understanding of autonomic influence on motor reflex pathways in spinal cord injury is important to the clinical management of autonomic dysreflexia and spasticity in spinal cord injured patients. The purpose of this study was to examine the modulation of flexor reflex windup during episodes of induced sympathetic activity in chronic human spinal cord injury (SCI). We simultaneously measured peripheral vascular conductance and the windup of the flexor reflex in response to conditioning stimuli of electrocutaneous stimulation to the opposite leg and bladder percussion. Flexor reflexes were quantified using torque measurements of the response to a noxious electrical stimulus applied to the skin of the medial arch of the foot. Both bladder percussion and skin conditioning stimuli produced a reduction (43-67%) in the ankle and hip flexor torques (p<0.05) of the flexor reflex. This reduction was accompanied by a simultaneous reduction in vascular conductance, measured using venous plethysmography, with a time course that matched the flexor reflex depression. While there was an overall attenuation of the flexor reflex, windup of the flexor reflex to repeated stimuli was maintained during periods of increased sympathetic activity. This paradoxical depression of flexor reflexes and minimal effect on windup is consistent with inhibition of afferent feedback within the superficial dorsal horn. The results of this study bring attention to the possible interaction of motor and sympathetic reflexes in SCI above and below the T5 spinal level, and have implications for clinicians in spasticity management and for researchers investigating motor reflexes post SCI.

A systematic review of the management of autonomic dysreflexia after spinal cord injury

OBJECTIVE

To review systematically the clinical evidence on strategies to prevent and manage autonomic dysreflexia (AD).

DATA SOURCES

A key word search of several databases (Medline, CINAHL, EMBASE, and PsycINFO), in addition to manual searches of retrieved articles, was undertaken to identify all English-language literature evaluating the efficacy of interventions for AD.

STUDY SELECTION

Studies selected for review included randomized controlled trials (RCTs), prospective cohort studies, and cross-sectional studies. Treatments reviewed included pharmacologic and nonpharmacologic interventions for the management of AD in subjects with spinal cord injury. Studies that failed to assess AD outcomes (eg, blood pressure) or symptoms (eg, headaches, sweating) were excluded.

DATA EXTRACTION

Studies were critically reviewed and assessed for their methodologic quality by 2 independent reviewers.

DATA SYNTHESIS

Thirty-one studies were assessed, including 6 RCTs. Preventative strategies to reduce the episodes of AD caused by common triggers (eg, urogenital system, surgery) primarily were supported by level 4 (pre-post studies) and level 5 (observational studies) evidence. The initial acute nonpharmacologic management of an episode of AD (ie, positioning the patient upright, loosening tight clothing, eliminating any precipitating stimulus) is supported by clinical consensus and physiologic data (level 5 evidence). The use of antihypertensive drugs in the presence of sustained elevated blood pressure is supported by level 1 (prazosin) and level 2 evidence (nifedipine and prostaglandin E(2)).

CONCLUSIONS

A variety of options are available to prevent AD (eg, surgical, pharmacologic) and manage the acute episode (elimination of triggers, pharmacologic); however, these options are predominantly supported by evidence from noncontrolled trials, and more rigorous trials are required.

Effect of hypotensive challenge on systemic hemodynamics and cerebral blood flow in persons with tetraplegia

INTRODUCTION

Individuals with tetraplegia have impaired central control of sympathetic vascular modulation and blood pressure (BP); how this impairment affects cerebral blood flow (CBF) is unclear.

OBJECTIVES

To determine if persons with tetraplegia maintain CBF similarly to able-bodied controls after a hypotensive challenge.

METHODS

Seven individuals with chronic tetraplegia and seven age-matched, non-SCI control subjects underwent a hypotensive challenge consisting of angiotensin-converting enzyme (ACE) inhibition (1.25 mg enalaprilat) and 45 degrees head-up tilt (HUT). Heart rate (HR), low frequency systolic BP variability (LFsbp), brachial mean arterial pressure (MAP) and middle cerebral artery CBF were measured before and after the challenge. Group differences for the baseline (BL) to post-challenge response were determined by repeated measures ANOVA.

RESULTS

HR did not differ between the groups in response to the hypotensive challenge. LFsbp response was significantly reduced in the tetra compared to the control group (-38 +/- 51 vs. 72 +/- 93%, respectively). MAP did not differ between the groups at BL but was significantly lower in the tetra compared to the control group post-challenge (55 +/- 13 vs. 71 +/- 9 mmHg, respectively); the percent change in MAP was significantly greater in the tetra than in the control group (-29 +/- 14.1 vs. -13 +/- 9%, respectively). However, CBF did not differ between the groups at baseline or post-challenge; the percent change in CBF post-challenge was not different between the tetra and control groups (-29 +/- 13.2 vs. -23 +/- 10.3%, respectively).

INTERPRETATION

Despite impaired sympathetic vasomotor and BP control, CBF in persons with tetraplegia was comparable to that of control subjects during a hypotensive challenge.

Mutual control of breathing and blood pressure: case report

STUDY DESIGN

Trial outcome.

OBJECTIVES

To demonstrate the relationship between circulation and breathing with orthostasis in a tetraplegic man.

SETTING

A long-term care unit at a Veterans Administration Hospital, USA.

METHOD

A tetraplegic patient with a history of orthostatic hypotension was monitored for blood flow at the level of the supraorbital artery, for breathing by nasal air flow, and for pulse oxymetry on a tilt table at 0, 45 and 0 degrees of head elevation.

RESULTS

Tilting up caused a coincidental reduction in blood flow and enhanced breathing. These effects were coincidentally reversed by tilting down. Oxyhemoglobin concentration and pulse rates increased with orthostasis and returned to baseline on tilting down.

CONCLUSION

A reciprocal relationship between breathing and circulation has been demonstrated in a tetraplegic patient challenged by orthostasis.

Recognition and effective management of autonomic dysreflexia in spinal cord injuries

Autonomic dysreflexia is a potentially life-threatening hypertensive medical emergency that occurs most often in spinal cord-injured individuals with spinal lesions at or above the mid-thoracic spinal cord level. It is a condition that remains poorly recognised outside of spinal cord injury centres, which may result in adverse outcomes including mortality from potentially delayed diagnosis and treatment. Acute autonomic dysreflexia is characterised by severe paroxysmal hypertension associated with throbbing headaches, profuse sweating, nasal stuffiness, flushing of the skin above the level of the lesion, bradycardia, apprehension and anxiety, which is sometimes accompanied by cognitive impairment. The key to effective management is prevention of the condition, by recognition and avoidance of factors that initiate the condition. When it occurs, immediate recognition and reversal of trigger factors along with prompt administration of pharmacological treatment is of paramount importance in order to prevent complications, which include intracranial and retinal haemorrhage, convulsions, cardiac irregularities and death. Promising data from recent animal studies may hold the key to future treatment options.

Cardiovascular disease in spinal cord injury: an overview of prevalence, risk, evaluation, and management

Cardiovascular disease is a growing concern for the spinal cord-injured (SCI) population. For long-term SCI, morbidity and mortality from cardiovascular causes now exceeds that caused by renal and pulmonary conditions, the primary causes of mortality in previous decades. Although risk estimates commonly used for ambulatory individuals have not been established from follow-up studies in SCI, nearly all risk factors tend to be more prevalent in SCI subjects compared with ambulatory subjects. These risks include a greater prevalence of obesity, lipid disorders, metabolic syndrome, and diabetes. Daily energy expenditure is significantly lower in SCI individuals, not only because of a lack of motor function, but also because of a lack of accessibility and fewer opportunities to engage in physical activity. Autonomic dysfunction caused by SCI is also associated with several conditions that contribute to heightened cardiovascular risk, including abnormalities in blood pressure, heart rate variability, arrhythmias, and a blunted cardiovascular response to exercise that can limit the capacity to perform physical activity. Thus, screening, recognition, and treatment of cardiovascular disease should be an essential component of managing individuals with SCI, and judicious treatment of risk factors can play an important role in minimizing the incidence of cardiovascular disease in these individuals. This article reviews the cardiovascular consequences of chronic SCI, including the prevalence of cardiovascular disease and risk factors unique to these individuals, and provides a synopsis of management of cardiovascular disease in this population.

Assessment of the Autonomic Control of Heart Rate Variability in Healthy and Spinal-Cord Injured Subjects: Contribution of Different Complexity-Based Estimators

We investigated how complexity-based estimators of heart rate variability can detect changes in cardiovascular autonomic drive with respect to traditional measures of variability. This was done by analyzing healthy subjects and paraplegic patients with different autonomic impairment due to low (vascular impairment only) or high (cardiac and vascular impairment) spinal cord injury, during progressive autonomic activations. While traditional techniques only quantified the effects of the autonomic activation, not distinguishing the effects of the lesion level, some recently proposed complexity estimators could also reveal the pathologic alterations in the autonomic control of heart rate. These estimators included the detrended fluctuation analysis coefficient (sensitive to both low and high autonomic lesions), sample entropy (sensitive to low-level lesions) and the largest Lyapunov exponent (sensitive to high-level lesions). Thus complexity-based methods provide information on the autonomic function from the heart rate dynamics that cannot be obtained by traditional techniques. This finding supports the combined use of both complexity-based and traditional methods to investigate the autonomic cardiovascular control from a more comprehensive perspective.

Orthostatic hypotension and paroxysmal hypertension in humans with high spinal cord injury

The spinal cord is essential for normal autonomic nervous system regulation of the cardiovascular system as the preganglionic neurons controlling the heart and blood vessels originate in the thoracolumbar spinal segments. The site and extent of a spinal cord injury determine the degree of autonomic involvement in cardiovascular dysfunction after the injury. After complete cervical cord lesions the entire sympathetic outflow is separated from cerebral control; this may cause orthostatic hypotension. Commonly after traumatic injuries to the spinal cord, one or more segments are totally destroyed. However, the distal portion of the spinal cord often retains function and activation of spinal cord reflexes working independently of the brain can result in paroxysmal hypertension. This chapter will focus on orthostatic hypotension and paroxysmal hypertension in cord-injured people with lesions affecting the cervical and upper thoracic spinal cord. Conditions promoting these abnormalities in blood pressure will be elaborated. Possible mechanisms for the hypo- and hypertension will be discussed, as will strategies for managing these problems.

Cardiorespiratory responses during passive walking-like exercise in quadriplegics

STUDY DESIGN

Cross-sectional and comparative investigation using quadriplegics (QP) and nondisabled subjects (ND).

OBJECTIVE

To evaluate cardiorespiratory responses during passive walking-like exercise (PWE) in QP.

SETTING

National Rehabilitation Center for Persons with Disabilities in Japan.

METHOD

The subjects were seven male QP with complete lesion (age: 27.0 +/- 5.4, injured level: C6-C7) and six male ND (age: 26.3 +/- 4.5). Cardiorespiratory responses were measured until voluntary fatigue during PWE, the rhythmical activity of paralyzed lower limbs synchronized with arm movements.

RESULTS

There were no significant differences in oxygen consumption (VO(2)), pulmonary ventilation (VE), heart rate (HR) and oxygen pulse (O(2) pulse) between QP and ND during PWE. ND showed increased ventilatory equivalent for oxygen (VE/VO(2) ratio) during exercise, while QP showed a significantly greater respiratory rate (RR) during exercise than ND (P < 0.05).

CONCLUSION

PWE elicited an increase in VO(2) with workload increment in QP similar to ND. However, higher RR suggested the intrinsic dysfunction of RR control during submaximal exercise in QP. From these results, it was thought that respiratory response would be the restriction factor of efficient oxygen transportation during PWE in QP.

The effects of lower-extremity functional electric stimulation on the orthostatic responses of people with tetraplegia

OBJECTIVE

To determine whether application of functional electric stimulation (FES) to lower-limb muscles during postural tilting improves orthostatic tolerance in people with tetraplegia.

DESIGN

A crossover design.

SETTING

A rehabilitation hospital.

PARTICIPANTS

Sixteen acute and chronic subjects with tetraplegia (15 men, 1 woman) with complete motor function loss at the C3-7 levels were recruited. Time since injury ranged from 2 to 324 months (mean, 118.9+/-104.2 mo).

INTERVENTION

Subjects were tested on a progressive head-up tilting maneuver with and without the application of FES at 0 degrees , 15 degrees , 30 degrees , 45 degrees , 60 degrees , 75 degrees , and 90 degrees continuously for up to 1 hour. FES was administered to 4 muscle groups including the quadriceps, hamstrings, tibialis anterior, and gastrocnemius muscles bilaterally at an intensity that provided a strong, visible, and palpable contraction. This was to produce a muscle pumping mechanism during the tilting maneuver.

MAIN OUTCOME MEASURES

Systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate, perceived presyncope score, and the overall duration of orthostatic tolerance, that is, the time that subjects could tolerate the tilting maneuver without developing severe hypotension or other intolerance symptoms.

RESULTS

When the tilt angle was increased, the subjects' SBP and DBP tended to decrease, whereas the heart rate tended to increase in both testing conditions. Adding FES to tilting significantly attenuated the drop in SBP by 3.7+/-1.1 mmHg (P = .005), the drop in DBP by 2.3+/-0.9 mmHg (P = .018), and the increase in heart rate by 1.0+/-0.5 beats/min (P = .039) for every 15 degrees increment in the angle of the tilt. FES increased the overall mean standing time by 14.3+/-3.9 min (P = .003).

CONCLUSIONS

An FES-induced leg muscle contraction is an effective adjunct treatment to delay orthostatic hypotension caused by tilting; it allows people with tetraplegia to stand up more frequently and for longer durations.

Recognition and management of autonomic dysreflexia in pediatric spinal cord injury

BACKGROUND

Autonomic dysreflexia (AD) is a well-documented complication of spinal cord injury (SCI) at or above the T6 level. However, research into AD has focused primarily on the adult. Because research that involves children with SCI is scarce, current guidelines may not be appropriate for children. Therefore, many episodes of AD may be unrecognized or inappropriately treated. To address this issue, Shriners Hospitals for Children undertook the development of a protocol specific to children and adolescents.

METHOD

A task force was developed to look at current literature on AD and blood pressure in children. Utilizing this literature and consensus among the task force members, the tools necessary to treat children with SCI at risk for AD were developed.

RESULTS

The task force developed several products intended to assist in the recognition and management of AD. These include an event flow sheet for recording incidents of AD, a letter for the child's school or primary care physician that provides a brief summary of AD and the child's baseline blood pressure, and a policy/protocol with 2 age-specific algorithms to standardize interventions across the 3 Shriners Hospitals in the United States with SCI programs (California, Illinois, and Pennsylvania).

CONCLUSION

The Shriners Hospitals for Children Task Force on Autonomic Dysreflexia in Children with Spinal Cord Injury has developed several tools specific to children. However, many questions remain to be answered concerning blood pressure norms and the clinical presentation of AD in children.

Differences in overshoot of blood pressure after head-up tilt in two groups with chronic autonomic failure: pure autonomic failure and multiple system atrophy

On head down tilt to the supine horizontal position (tilt reversal) after head up tilt (HUT), patients with orthostatic hypotension may show an increase in blood pressure (BP) relative to baseline readings. We assessed this BP overshoot in 8 patients with pure autonomic failure (PAF, 64+/-13 years) and 8 patients with multiple system atrophy (MSA, 66+/-10 years). BP was intermittently measured during pre-tilt supine, HUT (60 degrees , 10 min), and post-tilt supine periods. In addition, beat-to-beat BP was measured continuously using the Portapres model 2 device to calculate stroke volume (SV), cardiac output (CO) and total peripheral resistance (TPR). There was systolic BP overshoot of > or = 15 mmHg after tilt reversal in 5 out of 8 PAF, but in only one of 8 MSA. A mean increase of systolic BP in PAF was significantly higher than that in MSA (p<0.01). TPR increased over baseline level after tilt reversal, although there was no significant difference. SV and CO levels during the post-tilt supine period were similar to baseline levels. In conclusion, BP overshoot was prominent in the PAF group but not in the MSA group. The phenomenon of BP overshoot while supine, especially in PAF, may have implications for long term cardiac and vascular damage in such patients.

Cardiovascular control during voluntary static exercise in humans with tetraplegia

The purpose of the present study was 1) to investigate whether an increase in heart rate (HR) at the onset of voluntary static arm exercise in tetraplegic subjects was similar to that of normal subjects and 2) to identify how the cardiovascular adaptation during static exercise was disturbed by sympathetic decentralization. Mean arterial blood pressure (MAP) and HR were noninvasively recorded during static arm exercise at 35% of maximal voluntary contraction in six tetraplegic subjects who had complete cervical spinal cord injury (C(6)-C(7)). Stroke volume (SV), cardiac output (CO), and total peripheral resistance (TPR) were estimated by using a Modelflow method simulating aortic input impedance from arterial blood pressure waveform. In tetraplegic subjects, the increase in HR at the onset of static exercise was blunted compared with age-matched control subjects, whereas the peak increase in HR at the end of exercise was similar between the two groups. CO increased during exercise with no or slight decrease in SV. MAP increased approximately one-third above the control pressor response but TPR did not rise at all throughout static exercise, indicating that the slight pressor response is determined by the increase in CO. We conclude that the cardiovascular adaptation during voluntary static arm exercise in tetraplegic subjects is mainly accomplished by increasing cardiac pump output according to the tachycardia, which is controlled by cardiac vagal outflow, and that sympathetic decentralization causes both absent peripheral vasoconstriction and a decreased capacity to increase HR, especially at the onset of exercise.

Physiological and metabolic responses of wheelchair athletes in different racing classes to prolonged exercise

The aims of this study were to examine and compare selected physiological and metabolic responses of wheelchair athletes in two paraplegic racing classes [T3: n = 8 (lesion levels T1-T7; paraplegics); T4: n = 9 (lesion levels below T7; paraplegics)] to prolonged exercise. In addition, we describe the responses of three tetraplegic athletes [T2: n = 3 (lesion levels C6/C7: tetraplegics)]. Twenty athletes completed 90 min of exercise at 75% VO2peak on a motorized treadmill adapted for wheelchairs. The mean (+/- s) heart rates of the T3 and T4 racing classes were 165 +/- 2 and 172 +/- 6 beats.min-1, respectively. For the T4 racing class, heart rate gradually increased during the test (P < 0.05), whereas for the T3 racing class, heart rate reached a plateau after an initial increase. The mean heart rate of the tetraplegics was 114 +/- 3 beats.min-1. The T3 and T4 classes exhibited similar respiratory exchange ratios, plasma lactate and glucose concentrations throughout the test. For both the T3 and T4 racing class, free fatty acid, glycerol, ammonia, urea and potassium concentrations had increased from resting values by the end of the test (P < 0.05). In conclusion, the results of this study suggest that endurance-trained wheelchair athletes are able to maintain velocities equivalent to the same relative exercise intensity (75% VO2peak) for prolonged periods irrespective of lesion level.

Is there a relationship between chronic pain and autonomic dysreflexia in persons with cervical spinal cord injury?

The purpose of this study was to define the relationships among clinical characteristics of chronic pain, injury characteristics, affective factors and autonomic dysreflexia (AD) in people with chronic cervical spinal cord injury (SCI). A survey containing questions concerning the presence of AD, demographic factors, clinical characteristics of pain and affective symptoms, was mailed to people with traumatic SCI and chronic pain included in the Miami Project to Cure Paralysis database. Of all subjects (n = 330) reporting chronic pain in a previous study, 181 had cervical injuries. Of this subset, 117 (64.6%) participated in the study. Thirty-five people (29.9%) reported AD in our study. Although intense pain has previously been reported to trigger AD, the average pain intensity was not predictive of AD. Pair-wise comparisons indicated that people reporting AD marked significantly more painful areas in the drawing and used more descriptive pain adjectives. Anxiety and sadness were significantly more frequent among people with AD. Furthermore, aggravation of pain due to muscle spasms, infections, full bladder, and constipation was significantly more common among people with AD. The logistic regression analysis indicated that a combination of having widespread pain, experiencing aggravation of pain due to infections, having a complete injury, and experiencing anxiety significantly predicted AD. Our results suggest that relationships between chronic pain and AD exist. Since multiple pain types are common after SCI and may be associated with AD, it is important to determine which pain types that may be particularly important for the condition of AD.

Increased blood pressure can reduce fatigue of thenar muscles paralyzed after spinal cord injury

The aim of this study was to evaluate whether increases in blood pressure, and presumably muscle perfusion pressure, improve the endurance of thenar muscles paralyzed chronically by cervical spinal cord injury (SCI). Resting mean arterial pressure (MAP) was low in all eight subjects (64 +/- 2 mmHg). Muscle fatigue (force decline) was produced on 2 days by intermittent supramaximal electrical stimulation of the median nerve at 20 Hz for 2 min. During one of the fatigue tests, a concurrent sustained voluntary contraction of the contralateral elbow flexors was used to increase resting MAP (by 22%, on average). Although this change in blood pressure resulted in no significant change in mean fatigue for the group, changes in MAP with exercise (median nerve stimulation with and without voluntary contraction) correlated with changes in thenar muscle fatigue in seven subjects. For every 10% increase in MAP, fatigue was reduced by approximately 3%. The data suggest that low blood pressure after chronic cervical SCI and poor blood pressure control during exercise exacerbate the fatigability of paralyzed muscles.

Increased vascular resistance in paralyzed legs after spinal cord injury is reversible by training

The purpose of the present study was to determine the effect of a spinal cord injury (SCI) on resting vascular resistance in paralyzed legs in humans. To accomplish this goal, we measured blood pressure and resting flow above and below the lesion (by using venous occlusion plethysmography) in 11 patients with SCI and in 10 healthy controls (C). Relative vascular resistance was calculated as mean arterial pressure in millimeters of mercury divided by the arterial blood flow in milliliters per minute per 100 milliliters of tissue. Arterial blood flow in the sympathetically deprived and paralyzed legs of SCI was significantly lower than leg blood flow in C. Because mean arterial pressure showed no differences between both groups, leg vascular resistance in SCI was significantly higher than in C. Within the SCI group, arterial blood flow was significantly higher and vascular resistance significantly lower in the arms than in the legs. To distinguish between the effect of loss of central neural control vs. deconditioning, a group of nine SCI patients was trained for 6 wk and showed a 30% increase in leg blood flow with unchanged blood pressure levels, indicating a marked reduction in vascular resistance. In conclusion, vascular resistance is increased in the paralyzed legs of individuals with SCI and is reversible by training.

TENS attenuates response to colon distension in paraplegic and quadriplegic rats

Individuals with spinal cord injuries above thoracic level 6 experience episodic bouts of life-threatening hypertension as part of a condition termed autonomic dysreflexia (AD). The hypertension can be caused by stimulation of the skin, distension of the urinary bladder or colon, and/or muscle spasms. Transcutaneous electrical nerve stimulation (TENS) may reduce the severity of AD because TENS has been used to inhibit second-order neurons in the dorsal horn. Therefore, we tested the hypothesis that TENS attenuates the hemodynamic responses to colon distension. Eleven Wistar rats underwent spinal cord transection between thoracic vertebrae 4 and 5 (paraplegic, n = 6) or between cervical vertebra 7 and thoracic vertebra 1 (quadriplegic, n = 5). After recovery, all rats were instrumented with a radiotelemetry device for recording arterial pressure. Subsequently, the hemodynamic responses to graded colon distension were determined before and during TENS. During TENS the hemodynamic responses to colon distension were significantly attenuated. Thus TENS may be a preventive approach to reduce the severity of AD in paraplegic and quadriplegic individuals.

Changes in cerebral oxygenation and blood flow during LBNP in spinal cord-injured individuals

Spinal cord-injured (SCI) individuals, having a sympathetic nervous system lesion, experience hypotension during sitting and standing. Surprisingly, they experience few syncopal events. This suggests adaptations in cerebrovascular regulation. Therefore, changes in systemic circulation, cerebral blood flow, and oxygenation in eight SCI individuals were compared with eight able-bodied (AB) individuals. Systemic circulation was manipulated by lower body negative pressure at several levels down to -60 mmHg. At each level, we measured steady-state blood pressure, changes in cerebral blood velocity with transcranial Doppler, and cerebral oxygenation using near-infrared spectroscopy. We found that mean arterial pressure decreased significantly in SCI but not in AB individuals, in accordance with the sympathetic impairment in the SCI group. Cerebral blood flow velocity decreased during orthostatic stress in both groups, but this decrease was significantly greater in SCI individuals. Cerebral oxygenation decreased in both groups, with a tendency to a greater decrease in SCI individuals. Thus present data do not support an advantageous mechanism during orthostatic stress in the cerebrovascular regulation of SCI individuals.

Positive and negative feedback mechanisms in the neural regulation of cardiovascular function in healthy and spinal cord-injured humans

BACKGROUND

We tested the hypothesis that in humans, hypertension/tachycardia and hypotension/bradycardia nonbaroreflex sequences that occur within spontaneous arterial pressure (AP) and R-R interval fluctuations are an expression of positive feedback mechanisms neurally regulating the cardiovascular system.

METHODS AND RESULTS

We studied 15 spinal cord-injured (SCI) subjects (8 tetraplegics and 7 paraplegics) and 8 healthy subjects. The occurrence of nonbaroreflex (NBseq) and baroreflex (Bseq) sequences, ie, hypertension-bradycardia and hypotension-tachycardia sequences, was assessed during rest and head-up tilt (HUT). The ratio between Bseq and NBseq (B/NB ratio) was also calculated. In resting conditions, the occurrence of NBseq was significantly lower (P:<0.05) in tetraplegics (7.9+/-1.5) than in paraplegics (16.2+/-3.2) and normal subjects (19.0+/-3.5), whereas the occurrence of Bseq was not significantly different between the 3 groups (38.6+/-11.9 versus 45.4+/-6.0 versus 47.0+/-11.9). In tetraplegics, the B/NB ratio showed a marked, significant decrease (from 8.4+/-4.2 to 1.9+/-0.8, P:<0.05) in response to HUT, whereas in normal subjects, it showed a significant increase (from 3.5+/-0.7 to 9.4+/-2.7, P:<0.05). In paraplegics, the B/NB ratio did not change significantly in response to HUT (from 4.5+/-1.6 to 4.8+/-1.1).

CONCLUSIONS

Our data suggest that nonbaroreflex sequences occur in humans and might represent the expression of an integrated, neurally mediated, feed-forward type of short-term cardiovascular regulation that is able to interact dynamically with feedback mechanisms of baroreflex origin.

Sympathetic nervous system activity and cardiovascular homeostatis during head-up tilt in patients with spinal cord injuries

The relationship between sympathetic nervous system activity and cardiovascular responses to head-up tilt in patients with spinal cord injuries and in able-bodied subjects was studied. Twenty-seven adults, nine in each of the three groups (tetraplegia, paraplegia, and able-bodied subjects) were tilted 70 degrees, head up, for 12 minutes after 20 minutes supine rest. Differences between steady-state measurements of mean arterial pressure, stroke volume, and sympathetic nervous system activity were estimated in both positions. Sympathetic nervous system activity was reflected by the low-frequency peak of the blood pressure variability spectrum. From supine rest to head-up tilt, low-frequency power increased in able-bodied subjects (median, 0.42 mm Hg2, p = 0.003), which was different (p = 0.015) from patients with tetraplegia and paraplegia (-0.15 and -0.10 mm Hg2, respectively). Stroke volume and mean arterial pressure decreased in patients with tetraplegia (-40% and -9 mm Hg, respectively; p = 0.008, both variables) more than in able-bodied subjects (-33%, 11 mm Hg, respectively) or patients with paraplegia (-24%, 8 mm Hg, respectively). Results indicated increased sympathetic nervous system activity during head-up tilt in able-bodied subjects, but not in patients with paraplegia or tetraplegia, whereas patients with tetraplegia, but not paraplegia, showed poorer cardiovascular homeostasis than able-bodied subjects. This suggests that patients with paraplegia maintained cardiovascular homeostasis during head-up tilt without increased sympathetic nervous system activity.

Static exercise-induced increase in blood pressure in individuals with cervical spinal cord injury

OBJECTIVE

To compare the pressor response to static exercise in subjects with cervical spinal cord injury (SCI) at the C6 to C8 level with that in able-bodied control subjects. In these SCI subjects, the descending supraspinal sympathetic neurons and afferent pathways from the contracting muscles to peripheral vessels via the medullary cardiovascular center are damaged.

DESIGN

Mean arterial blood pressure, heart rate, and plasma concentrations of norepinephrine, epinephrine, renin activity, vasopressin, aldosterone, and human atrial natriuretic peptide were measured during a 2-minute period of sustained contraction of elbow flexor group muscle in 7 SCI subjects and 7 age-matched able-bodied control subjects.

RESULTS

Static exercise resulted in a significant increase in mean blood pressure (p<.05) in both SCI subjects (pre-exercise. 74.7+/-2.2 mm Hg; static exercise, 81.9+/-4.1 mm Hg) and control subjects (pre-exercise, 101.0+/-4.2 mm Hg; static exercise, 117.0+/-4.9 mm Hg). In SCI subjects, there was no change in heart rate during exercise, whereas in control subjects heart rate increased during exercise (p<.05) (pre-exercise, 8.7+/-3.8 beats/min: static exercise, 76.0+/-3.1 beats/min). There were no significant changes in the hormone levels in the SCI subjects throughout the experiment.

CONCLUSION

The significant increase in mean blood pressure observed in the present study indicates the presence of peripheral control from muscle receptors and evoked pressor response during static exercise in SCI subjects.

Stomach distension increases efferent muscle sympathetic nerve activity and blood pressure in healthy humans

Although the enteric nervous system is usually described as a separate and independent entity, animal studies show that gastric distension causes a reflex increase in arterial pressure and a sympathetically mediated increase in heart rate and peripheral vascular resistance. To assess the influence of gastric distension on sympathetic outflow and blood pressure, we recorded muscle sympathetic nerve activity (MSNA) from the peroneal nerve by microneurography in eight healthy volunteers. The stomach was distended by means of a barostat, using a single staircase protocol by which pressure was increased by 2 mmHg every 3 min. Gastric sensory function was assessed at each distension step by using a visual analog scale (VAS) for sensations of fullness, nausea and pain. For comparison, we also performed a cold pressor test. The MSNA increased on barostat-induced gastric distension with an almost concomitant elevation of blood pressure. The increase in both was proportional to the intragastric pressure and both decreased towards initial values after the end of distension. Heart rate increased inconsistently and only at higher distension pressures that were associated with high VAS scores. The opposite was found for the cold pressor test. The results of this study confirm the existence of a functional relationship between gastrointestinal distension and cardiovascular function. Decrease in this gastrovascular response may play a role in postprandial hypotension in the elderly, since the MSNA responses to simulated microgravity decrease with age.

Effect of spinal cord transection on N-methyl-D-aspartate receptors in the cord

Spinal cord injury can lead to an exaggeration of transmission through spinal pathways, resulting in muscle spasticity, chronic pain, and abnormal control of blood pressure and bladder function. These conditions are mediated, in part, by N-methyl-D-aspartate (NMDA) receptors on spinal neurons, but the effects of cord injury on the expression or function of these receptors is unknown. Therefore, antibodies to the NMDA-R1 receptor subunit and binding of [3H]MK-801 were used to assess NMDA receptors in the spinal cord. Receptor density in rats with intact spinal cords was compared to that in rats 1 and 2 weeks after spinal cord transection (SCT) at the mid-thoracic level. At 1 and 2 weeks after SCT, [3H]MK-801 binding was reduced in most laminae in cord segments caudal to the injury, whereas no decrease in amount of R1 subunit immunoreactivity was observed. No significant changes in [3H]MK-801 binding and NMDA-R1 immunoreactivity could be seen rostral to the transection. Since [3H]MK-801 binding requires an open ion channel, the discrepancy between [3H]MK-801 binding and immunocytochemistry may indicate a loss of functional receptors without a consistent change in their total number. Therefore, the exaggerated reflexes that are well established in rats 2 weeks after cord injury must be mediated by a mechanism that withstands attenuation of NMDA receptor function.

Effect of spinal cord injury on the heart and cardiovascular fitness

The use of various FES protocols to encourage increases in physical activity and to augment physical fitness and reduce heart disease risk is a relatively new, but growing field of investigation. The evidence so far supports its use in improving potential health benefits for patients with SCI. Such benefits may include more efficient and safer cardiac function; greater stimulus for metabolic, cardiovascular, and pulmonary training adaptations; and greater stimulus for skeletal muscle training adaptations. In addition, the availability of relatively inexpensive commercial FES units to elicit muscular contractions, the ease of use of gel-less, reusable electrodes, and the increasing popularity of home and commercial upper body exercise equipment mean that such benefits are likely to be more accessible to the SCI population through increased convenience and decreased cost. The US Department of Health and Human Services has identified those with SCI as a "special population" whose health problems are accentuated, and so need to be specifically addressed. FES presents "a clear opportunity.... For health promotion and disease prevention efforts to improve the health prospects and functional independence of people with disabilities." As a corollary to this, the Centers for Disease Control and Prevention have recommended the development of techniques to prevent or ameliorate secondary disabilities in persons with a SCI. Patients with SCI have an increased susceptibility to cardiac morbidity and mortality in the acute and early stages of their injury. Most of these patients make an excellent adaptation except when confronted with infection or hypoxia. SCI by itself does not promote atherosclerosis; however, in association with multiple secondary conditions related to SCI, along with advancing age, patients with SCI are predisposed to relatively greater risk of heart disease. The epidemiologic significance of this is reflected in demographic studies that indicate an increasing number of SCI patients becoming aged. Currently 71,000 (40%) of the total 179,000 patients with SCI living in the United States are older than 40 years, and 45,000 have injuries sustained more than 20 years earlier. In addition, new injuries in the older population are increasing (currently 11% of all injuries), and some of these new patients with SCI already have pre-existing cardiac disease. Studies have demonstrated that improved lifestyle, physical activity, lipid management, and dietary restrictions can affect major risk factors for coronary artery disease. Therefore an aggressive cardiac prevention program is appropriate for patients with SCI as part of their rehabilitation. At a given submaximal workload, arm exercise is performed at a greater physiologic cost than is leg exercise. At maximal effort, however, physiologic responses are generally greater in leg exercise than arm exercise. Arm exercise is less efficient and less effective than lower body exercise in developing and maintaining both central and peripheral aspects of cardiovascular fitness. The situation is further compounded in SCI because of poor venous return as a result of lower-limb blood pooling, as a result of lack of sympathetic tone, and a diminished or absent venous "muscle pump" in the legs. This latter mechanism perhaps contributes the greatest diminution in the potential for aerobic performance in the SCI population. Obtaining a cardiopulmonary training effect in individuals with SCI is quite possible. Current studies indicate decreases in submaximal HR, respiratory quotient, minute ventilation, and oxygen uptake, with increases in maximal power output, oxygen uptake, minute ventilation, and lactic acid. Individuals with SCI have been shown to benefit from lower limb functional electrical stimulation (FES)-induced exercise. Studies have consistently reported increases in lower limb strength and cycle endurance performance with these protocols, as well as improvements in metabolic and

Impaired cerebral cortex development and blood pressure regulation in FGF-2-deficient mice

Fibroblast growth factor-2 (FGF-2) has been implicated in various signaling processes which control embryonic growth and differentiation, adult physiology and pathology. To analyze the in vivo functions of this signaling molecule, the FGF-2 gene was inactivated by homologous recombination in mouse embryonic stem cells. FGF-2-deficient mice are viable, but display cerebral cortex defects at birth. Bromodeoxyuridine pulse labeling of embryos showed that proliferation of neuronal progenitors is normal, whereas a fraction of them fail to colonize their target layers in the cerebral cortex. A corresponding reduction in parvalbumin-positive neurons is observed in adult cortical layers. Neuronal defects are not limited to the cerebral cortex, as ectopic parvalbumin-positive neurons are present in the hippocampal commissure and neuronal deficiencies are observed in the cervical spinal cord. Physiological studies showed that FGF-2-deficient adult mice are hypotensive. They respond normally to angiotensin II-induced hypertension, whereas neural regulation of blood pressure by the baroreceptor reflex is impaired. The present genetic study establishes that FGF-2 participates in controlling fates, migration and differentiation of neuronal cells, whereas it is not essential for their proliferation. The observed autonomic dysfunction in FGF-2-deficient adult mice uncovers more general roles in neural development and function.

Sprouting of primary afferent fibers after spinal cord transection in the rat

After spinal cord injury, hyper-reflexia can lead to episodic hypertension, muscle spasticity and urinary bladder dyssynergia. This condition may be caused by primary afferent fiber sprouting providing new input to partially denervated spinal interneurons, autonomic neurons and motor neurons. However, conflicting reports concerning afferent neurite sprouting after cord injury do not provide adequate information to associate sprouting with hyper-reflexia. Therefore, we studied the effect of mid-thoracic spinal cord transection on central projections of sensory neurons, quantified by area measurements. The area of myelinated afferent arbors, immunolabeled by cholera toxin B, was greater in laminae I-V in lumbar, but not thoracic cord, by one week after cord transection. Changes in small sensory neurons and their unmyelinated fibers, immunolabeled for calcitonin gene-related peptide, were assessed in the cord and in dorsal root ganglia. The area of calcitonin gene-related peptide-immunoreactive fibers in laminae III-V increased in all cord segments at two weeks after cord transection, but not at one week. Numbers of sensory neurons immunoreactive for calcitonin gene-related peptide were unchanged, suggesting that the increased area of immunoreactivity reflected sprouting rather than peptide up-regulation. Immunoreactive fibers in the lateral horn increased only above the lesion and in lumbar segments at two weeks after cord transection. They were not continuous with dorsal horn fibers, suggesting that they were not primary afferent fibers. Using the fluorescent tracer DiI to label afferent fibers, an increase in area could be seen in Clarke's nucleus caudal to the injury two weeks after transection. In conclusion, site- and time-dependent sprouting of myelinated and unmyelinated primary afferent fibers, and possibly interneurons, occurred after spinal cord transection. Afferent fiber sprouting did not reach autonomic or motor neurons directly, but may cause hyper-reflexia by increasing inputs to interneurons.

Anaesthesia for chronic spinal cord lesions

Increasing numbers of patients with spinal cord injury present for surgery or obstetric care. Spinal cord injury causes unique pathophysiological changes. The most important peri-operative dangers are autonomic dysreflexia, bradycardia, hypotension, respiratory inadequacy and muscle spasms. Autonomic dysreflexia is suggested by headache, sweating, bradycardia and severe hypertension and may be precipitated by surgery, especially bladder distension. Patients with low, complete lesions, undergoing surgery below the level of injury, may safely do so without anaesthesia provided there is no history of autonomic dysreflexia or troublesome spasms. An anaesthetist should be present to monitor the patient in this situation. General anaesthesia of sufficient depth is effective at controlling spasms and autonomic dysreflexia but hypotension and respiratory dysfunction are risks. There is a growing consensus that spinal anaesthesia is safe, effective and technically simple to perform in this group of patients. We present a survey of 515 consecutive anaesthetics in cord-injured patients and a review of the current literature on anaesthesia for patients with chronic spinal cord lesions.

Changes in the morphology of sympathetic preganglionic neurons parallel the development of autonomic dysreflexia after spinal cord injury in rats

Following spinal cord transection (SCT), sensory input to the spinal cord causes increases in arterial pressure that are small in rats 1 week after SCT, but become large and well established by 2 weeks. Moreover, sympathetic preganglionic neurons (SPNs) undergo atrophy by 1 week after SCT, and regeneration of these neurons may be an important factor in the etiology of this autonomic dysreflexia. Therefore, we examined the morphology of SPNs 2 weeks after SCT using retrograde transport of the cholera toxin subunit B. The dendritic arbors of SPNs were re-established by 2 weeks after SCT. This regeneration parallels the time course of the development of autonomic dysreflexia after cord injury in the rat, and may play a role in initiating this disorder.