Cardiac autonomic responses to progressive head-up tilt in individuals with paraplegia

Persistent Orthostatic Hypotension Following Surgery for Adult Spinal Deformity: Prevalence, Risk Factors, and Cardiovascular Evaluation

STUDY DESIGN

Retrospective case-control study.

PURPOSE

To understand the prevalence of persistent orthostatic hypotension (POH), as well as its risk factors and cardiovascular pathology, in patients receiving surgery for the adult spinal deformity (ASD).

OVERVIEW OF LITERATURE

Although reports of the incidence of and risk factors for POH in different spinal disorders have recently been published, we are not aware of the comprehensive evaluation of POH following surgery for ASD.

METHODS

We examined medical records from a single central database for 65 patients receiving surgical treatment for ASD. Statistical comparisons were made between patients who experienced postoperative POH and those who did not, by examining patient and operative characteristics, such as age, sex, comorbidities, functional status, preoperative neurological function, vertebral fractures, three-column osteotomy, total operative time, estimated blood loss, length of stay, and radiographic parameters. The determinants of POH were assessed using multiple logistic regression.

RESULTS

We showed that postoperative POH was a complication of ASD surgery, with a 9% incidence rate. Patients with POH were statistically considerably more likely to require supported walkers due to partial paralysis and to have comorbidities including diabetes and neurodegenerative disease (ND). Furthermore, ND was an independent risk factor for postoperative POH (odds ratio, 4.073; 95% confidence interval, 1.094-8.362; p =0.020). Moreover, perioperative evaluation of the inferior vena cava showed that patients with postoperative POH had preoperative congestive heart failure and had hypovolemia lower postoperative diameter of the inferior vena cava than patients without POH.

CONCLUSIONS

Postoperative POH is a potential complication of ASD surgery. The most pertinent risk factor is having an ND. According to our study, patients who have surgery for ASD may experience alterations in their hemodynamics.

Intractable postural hypotension resulting from cervical pseudomeningocele after a posterior cervical spinal surgery: a case report

INTRODUCTION

Postural hypotension (PH) is common in patients with spinal cord injury (SCI), especially those with tetraplegia. To effectively treat PH, identifying and eliminating treatable predisposing factors of PH are prerequisites before applying any interventions.

CASE PRESENTATION

We report a patient with post-acute cervical SCI who suffered from intractable PH resulting from pseudomeningocele causing unfavorable rehabilitation outcomes. A previously healthy 34-year-old man with complete C6 SCI resulting from C6-C7 fracture dislocation developed PH in the first week of the rehabilitation program. No specific predisposing factors including anemia, hyponatremia, and dehydration were identified. Non-pharmacological interventions and pharmacological treatment were administered to the patient without satisfactory result, causing a delay in rehabilitation progression. In the fourth week of rehabilitation program, a mass at the surgical site was detected. A cervical MRI revealed a large fluid collection at the posterior aspect of cervical spines with a size of 7.9 × 6.8 × 5.0 cm. A diagnosis of pseudomeningocele was made and surgical site debridement with closing dura by grafting was immediately conducted. One day after surgery, PH disappeared, and the patient could progress in his rehabilitation program and achieve his short-term goal within three weeks.

CONCLUSION

Pseudomeningocele could be one of the precipitating factors of PH in patients with tetraplegia. Healthcare providers should consider investigating pseudomeningocele in patients who have intractable and unexplainable PH.

Experimental high thoracic spinal cord injury impairs the cardiac and cerebrovascular response to orthostatic challenge in rats

Spinal cord injury (SCI) impairs the cardiovascular responses to postural challenge, leading to the development of orthostatic hypotension (OH). Here, we apply lower body negative pressure (LBNP) to rodents with high-level SCI to demonstrate the usefulness of LBNP as a model for experimental OH studies, and to explore the effect of simulated OH on cardiovascular and cerebrovascular function following SCI. Male Wistar rats (n = 34) were subjected to a sham or T3-SCI surgery and survived into the chronic period postinjury (i.e., 8 wk). Cardiac function was tracked via ultrasound pre- to post-SCI to demonstrate the clinical utility of our model. At study termination, we conducted left-ventricular (LV) catheterization and insonated the middle cerebral artery to investigate the hemodynamic, cardiac, and cerebrovascular response to a mild dose of LBNP that is sufficient to mimic clinically defined OH in rats with T3-SCI but not sham animals. In response to mimicked OH, there was a greater decline in stroke volume, cardiac output, maximal LV pressure, and blood pressure in SCI compared with sham (P < 0.034), whereas heart rate was increased in sham but decreased in SCI (P < 0.029). SCI animals also had an exaggerated reduction in peak, minimum and mean middle cerebral artery flow, for a given change in blood pressure, in response to LBNP (P < 0.033), implying impaired dynamic cerebral autoregulation. Using a preclinical SCI model of OH, we demonstrate that complete high thoracic SCI impairs the cardiac response to OH and disrupts dynamic cerebral autoregulation.NEW & NOTEWORTHY This is the first use of LBNP to interrogate the cardiac and cerebrovascular responses to simulated OH in a preclinical study of SCI. Here, we demonstrate the utility of our simulated OH model and use it to demonstrate that SCI impairs the cardiac response to simulated OH and disrupts dynamic cerebrovascular autoregulation.

Orthostatic Hypotension Following Cervical Spine Surgery: Prevalence and Risk Factors

STUDY DESIGN

Retrospective case series.

OBJECTIVES

This study aims to determine the prevalence and risk factors for orthostatic hypotension (OH) in patients undergoing cervical spine surgery.

METHODS

Data was collected from records of 190 consecutive patients who underwent cervical spine procedures at our center over 24 months. Statistical comparison was made between patients who developed postoperative OH and those who did not by analyzing characteristics such as age, gender, premorbid medical comorbidities, functional status, mechanism of spinal cord injury, preoperative neurological function, surgical approach, estimated blood loss, and length of stay.

RESULTS

Twenty-two of 190 patients (11.6%) developed OH postoperatively. No significant differences in age, gender, medical comorbidities, or premorbid functional status were observed. Based on univariate comparisons, traumatic mechanism of injury (P = .002), poor ASIA (American Spinal Injury Association) grades (A, B, or C) (P < .001), and posterior surgical approach (P = .045) were found to significantly influence occurrence of OH. Among the significant variables, after adjusting for mechanism of injury and surgical approach, only ASIA grade was found to be an independent predictor. Having an ASIA grade of A, B, or C increased the likelihood of developing OH by approximately 5.978 times (P = .003).

CONCLUSION

Our study highlights that OH is not an uncommon manifestation following cervical spine surgery. Patients with poorer ASIA grades A, B, or C were more likely to have OH when compared with those with ASIA grades D or E (43.5% vs 7.2%). Hence, we suggest that postural blood pressure should be routinely monitored in this group of patients so that early intervention can be initiated.

Heart rate variability in spinal cord injury: Asymptomatic orthostatic hypotension is a confounding variable

STUDY DESIGN

observational study.

OBJECTIVES

Heart rate variability (HRV) studies have shown that the low-frequency (LF) domain is decreased in postural challenge of spinal cord injury (SCI). However, there are inconsistencies in the literature regarding HRV with postural challenge. We believed that asymptomatic orthostatic hypotension (OH) may contribute to the discrepancies in the literature. We also evaluated the time-domain parameters of HRV to evaluate for changes in SCI postural challenge. We looked at the association of Frequency and Time-domain HRV parameters to blood pressure (BP) and postural challenge in SCI based on groups with and without OH.

SETTING

SCI clinic.

METHODS

Patients from SCI clinic were enrolled and 5-minute electrocardiogram (ECG) recordings were performed before and after a postural challenge from supine to sitting position with blood pressure (BP) recordings at each time point. Between and within group analyses were performed based on tetraplegia, paraplegia, and the presence of asymptomatic OH.

RESULTS

There were 6 controls and 18 SCI subjects, 4 with OH based on change in diastolic BP. After postural challenge, mean heart rate was lower in tetraplegia compared to controls in all groupings. When OH was removed from the groups, total power was consistently reduced in tetraplegia compared to paraplegia across all positions. There were both time- and frequency-domain differences observed in SCI with OH compared to SCI without.

CONCLUSIONS

Autonomic variability is increased in SCI with OH and can lead to misinterpretation of findings if asymptomatic OH is included in samples. Our data suggest that there are frequency- and time-domain differences in HRV variables in those with OH compared to those without. Future HRV studies should account for asymptomatic OH and include analyses of the time-domain variables in postural challenge of SCI.

Characteristics of cardiovascular responses to an orthostatic challenge in trained spinal cord-injured individuals

Background

We investigated cardiovascular responses to an orthostatic challenge in trained spinal cord-injured (SCI) individuals compared to able-bodied (AB) individuals.

Methods

A total of 23 subjects participated, divided into three groups: seven were trained as spinal cord-injured (Tr-SCI) individuals, seven were able-bodied individuals trained as runners (Tr-AB), and nine were untrained able-bodied individuals (UnTr-AB). We measured the cardiovascular autonomic responses in all three groups during each 5-min head-up tilt (HUT) of 0°, 40°, and 80°. Stroke volume (SV), heart rate (HR), and cardiac output (Qc) as cardiovascular responses were measured by impedance cardiography. Changes in deoxyhemoglobin (∆[HHb]) and total hemoglobin (∆[Hb_tot]) concentrations of the right medial gastrocnemius muscle were measured using near-infrared spectroscopy (NIRS).

Results

As the HUT increased from 0° to 80°, Tr-SCI group showed less change in SV at all HUT levels even if HR increased significantly. Mean arterial pressure (MAP) also did not significantly increase as tilting increased from 0° to 80°. Regarding peripheral vascular responses, the alterations of ∆[Hb_tot] from 0° to 80° were less in Tr-SCI group compared to AB individuals.

Conclusion

There is a specific mechanism whereby blood pressure is maintained during a HUT in Tr-SCI group with the elicitation of peripheral vasoconstriction and the atrophy of the vascular vessels in paraplegic lower limbs, which would be associated with less change in SV in response to an orthostatic challenge.

Effects of Respiratory Training on Heart Rate Variability and Baroreflex Sensitivity in Individuals With Chronic Spinal Cord Injury

OBJECTIVE

To evaluate the effects of pressure threshold respiratory training (RT) on heart rate variability and baroreflex sensitivity in persons with chronic spinal cord injury (SCI).

DESIGN

Before-after intervention case-controlled clinical study.

SETTING

SCI research center and outpatient rehabilitation unit.

PARTICIPANTS

Participants (N=44) consisted of persons with chronic SCI ranging from C2 to T11 who participated in RT (n=24), and untrained control subjects with chronic SCI ranging from C2 to T9 (n=20).

INTERVENTIONS

A total of 21±2 RT sessions performed 5 days a week during a 4-week period using a combination of pressure threshold inspiratory and expiratory devices.

MAIN OUTCOME MEASURES

Forced vital capacity (FVC), forced expiratory volume in 1 second (FEV₁), and beat-to-beat arterial blood pressure and heart rate changes during the 5-second-long maximum expiratory pressure maneuver (5s MEP) and the sit-up orthostatic stress test, acquired before and after the RT program.

RESULTS

In contrast to the untrained controls, individuals in the RT group experienced significantly increased FVC and FEV₁ (both P<.01) in association with improved quality of sleep, cough, and speech. Sympathetically (phase II) and parasympathetically (phase IV) mediated baroreflex sensitivity both significantly (P<.05) increased during the 5s MEP. During the orthostatic stress test, improved autonomic control over heart rate was associated with significantly increased sympathetic and parasympathetic modulation (low- and high-frequency change: P<.01 and P<.05, respectively).

CONCLUSIONS

Inspiratory-expiratory pressure threshold RT is a promising technique to positively affect both respiratory and cardiovascular dysregulation observed in persons with chronic SCI.

Acute Effects of Caffeine on Heart Rate Variability, Blood Pressure and Tidal Volume in Paraplegic and Tetraplegic Compared to Able-Bodied Individuals: A Randomized, Blinded Trial

Caffeine increases sympathetic nerve activity in healthy individuals. Such modulation of nervous system activity can be tracked by assessing the heart rate variability. This study aimed to investigate the influence of caffeine on time- and frequency-domain heart rate variability parameters, blood pressure and tidal volume in paraplegic and tetraplegic compared to able-bodied participants. Heart rate variability was measured in supine and sitting position pre and post ingestion of either placebo or 6 mg caffeine in 12 able-bodied, 9 paraplegic and 7 tetraplegic participants in a placebo-controlled, randomized and double-blind study design. Metronomic breathing was applied (0.25 Hz) and tidal volume was recorded during heart rate variability assessment. Blood pressure, plasma caffeine and epinephrine concentrations were analyzed pre and post ingestion. Most parameters of heart rate variability did not significantly change post caffeine ingestion compared to placebo. Tidal volume significantly increased post caffeine ingestion in able-bodied (p = 0.021) and paraplegic (p = 0.036) but not in tetraplegic participants (p = 0.34). Systolic and diastolic blood pressure increased significantly post caffeine in able-bodied (systolic: p = 0.003; diastolic: p = 0.021) and tetraplegic (systolic: p = 0.043; diastolic: p = 0.042) but not in paraplegic participants (systolic: p = 0.09; diastolic: p = 0.33). Plasma caffeine concentrations were significantly increased post caffeine ingestion in all three groups of participants (p<0.05). Plasma epinephrine concentrations increased significantly in able-bodied (p = 0.002) and paraplegic (p = 0.032) but not in tetraplegic participants (p = 0.63). The influence of caffeine on the autonomic nervous system seems to depend on the level of lesion and the extent of the impairment. Therefore, tetraplegic participants may be less influenced by caffeine ingestion.

Baroreceptor reflex during forced expiratory maneuvers in individuals with chronic spinal cord injury

Pulmonary and cardiovascular dysfunctions are leading causes of morbidity and mortality in patients with chronic Spinal Cord Injury (SCI). Impaired respiratory motor function and decreased Baroreflex Sensitivity (BS) are predictors for the development of cardiopulmonary disease. This observational case-controlled clinical study was undertaken to investigate if respiratory motor control deficits in individuals with SCI affect their ability to perform the Valsalva maneuver, and to determine if a sustained Maximum Expiratory Pressure (MEP) effort can serve as an acceptable maneuver for determination of the BS in the event that the Valsalva maneuver cannot be performed. The BS outcomes (ms/mmHg) were obtained using continuous beat-to-beat arterial blood pressure (BP) and heart rate (HR) recordings during Valsalva or MEP maneuvers in thirty nine individuals with chronic C3-T12 SCI. Twenty one participants (54%) reported signs of intolerance during the Valsalva maneuver and only 15 individuals (39%) were able to complete this task. Cervical level of injury was a significant risk factor (p=0.001) for failing to complete the Valsalva maneuver, and motor-complete injury was a significant risk factor for symptoms of intolerance (p=0.04). Twenty eight participants (72%) were able to perform the MEP maneuver; the other 11 participants failed to exceed the standard airway pressure threshold of 27cm H2O. Neither level nor completeness of injury were significant risk factors for failure of MEP maneuver. When the required airway pressure was sustained, there were no significant differences between BS outcomes obtained during Valsalva and MEP maneuvers. The results of this study indicate that individuals with high-level and motor-complete SCI are at increased risk of not completing the Valsalva maneuver and that baroreflex-mediated responses can be evaluated by using sustained MEP maneuver when the Valsalva maneuver cannot be performed.

Cardiovascular dysfunction following spinal cord injury

Both sensorimotor and autonomic dysfunctions often occur after spinal cord injury (SCI). Particularly, a high thoracic or cervical SCI interrupts supraspinal vasomotor pathways and results in disordered hemodynamics due to deregulated sympathetic outflow. As a result of the reduced sympathetic activity, patients with SCI may experience hypotension, cardiac dysrhythmias, and hypothermia post-injury. In the chronic phase, changes within the CNS and blood vessels lead to orthostatic hypotension and life-threatening autonomic dysreflexia (AD). AD is characterized by an episodic, massive sympathetic discharge that causes severe hypertension associated with bradycardia. The syndrome is often triggered by unpleasant visceral or sensory stimuli below the injury level. Currently the only treatments are palliative - once a stimulus elicits AD, pharmacological vasodilators are administered to help reduce the spike in arterial blood pressure. However, a more effective means would be to mitigate AD development by attenuating contributing mechanisms, such as the reorganization of intraspinal circuits below the level of injury. A better understanding of the neuropathophysiology underlying cardiovascular dysfunction after SCI is essential to better develop novel therapeutic approaches to restore hemodynamic performance.

Autonomic consequences of spinal cord injury

Spinal cord injury (SCI) results not only in motor and sensory deficits but also in autonomic dysfunctions. The disruption of connections between higher brain centers and the spinal cord, or the impaired autonomic nervous system itself, manifests a broad range of autonomic abnormalities. This includes compromised cardiovascular, respiratory, urinary, gastrointestinal, thermoregulatory, and sexual activities. These disabilities evoke potentially life-threatening symptoms that severely interfere with the daily living of those with SCI. In particular, high thoracic or cervical SCI often causes disordered hemodynamics due to deregulated sympathetic outflow. Episodic hypertension associated with autonomic dysreflexia develops as a result of massive sympathetic discharge often triggered by unpleasant visceral or sensory stimuli below the injury level. In the pelvic floor, bladder and urethral dysfunctions are classified according to upper motor neuron versus lower motor neuron injuries; this is dependent on the level of lesion. Most impairments of the lower urinary tract manifest in two interrelated complications: bladder storage and emptying. Inadequate or excessive detrusor and sphincter functions as well as detrusor-sphincter dyssynergia are examples of micturition abnormalities stemming from SCI. Gastrointestinal motility disorders in spinal cord injured-individuals are comprised of gastric dilation, delayed gastric emptying, and diminished propulsive transit along the entire gastrointestinal tract. As a critical consequence of SCI, neurogenic bowel dysfunction exhibits constipation and/or incontinence. Thus, it is essential to recognize neural mechanisms and pathophysiology underlying various complications of autonomic dysfunctions after SCI. This overview provides both vital information for better understanding these disorders and guides to pursue novel therapeutic approaches to alleviate secondary complications.

Kalpakjian C, W. Choi S, Jette AM, Forchheimer M, Cella D. Overview of the Spinal Cord Injury--Quality of Life (SCI-QOL) measurement system

CONTEXT/OBJECTIVE

The Spinal Cord Injury--Quality of Life (SCI-QOL) measurement system was developed to address the shortage of relevant and psychometrically sound patient reported outcome (PRO) measures available for clinical care and research in spinal cord injury (SCI) rehabilitation. Using a computer adaptive testing (CAT) approach, the SCI-QOL builds on the Patient Reported Outcomes Measurement Information System (PROMIS) and the Quality of Life in Neurological Disorders (Neuro-QOL) initiative. This initial manuscript introduces the background and development of the SCI-QOL measurement system. Greater detail is presented in the additional manuscripts of this special issue.

DESIGN

Classical and contemporary test development methodologies were employed. Qualitative input was obtained from individuals with SCI and clinicians through interviews, focus groups, and cognitive debriefing. Item pools were field tested in a multi-site sample (n=877) and calibrated using item response theory methods. Initial reliability and validity testing was performed in a new sample of individuals with traumatic SCI (n=245).

SETTING

Five Model SCI System centers and one Department of Veterans Affairs Medical Center across the United States.

PARTICIPANTS

Adults with traumatic SCI.

INTERVENTIONS

n/a

OUTCOME MEASURES

n/a

RESULTS

The SCI-QOL consists of 19 item banks, including the SCI-Functional Index banks, and 3 fixed-length scales measuring physical, emotional, and social aspects of health-related QOL (HRQOL).

CONCLUSION

The SCI-QOL measurement system consists of psychometrically sound measures for individuals with SCI. The manuscripts in this special issue provide evidence of the reliability and initial validity of this measurement system. The SCI-QOL also links to other measures designed for a general medical population.

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.

Effects of compression stockings on sympathetic activity and heart rate variability in individuals with spinal cord injury

OBJECTIVE

To investigate whether wearing graduated compression stockings (GCS) could affect the sympatho-adrenergic and heart rate variability (HRV) responses at rest and after a strenuous wheelchair exercise in individuals with spinal cord injury (SCI).

DESIGN

Crossover trial.

SETTING

Department of Physical Medicine and Rehabilitation, Saint Etienne, France.

PARTICIPANTS

Nine men with SCI (five with low paraplegia: LP, four with high paraplegia: HP).

INTERVENTIONS

Two maximal wheelchair exercise tests: with and without GCS (21 mmHg).

MAIN OUTCOME MEASURES

HRV measurements: high frequency (HF), low frequency (LF), and LF/HF ratio. Norepinephrine (NOR) and epinephrine (EPI), at rest and post-exercise. Secondary measures were: blood pressure, heart rate, maximal power output, oxygen uptake, stroke volume, cardiac output, at rest, during and after exercise.

RESULTS

When wearing GCS: LFnu(wavelet-post) significantly increased and HFnu(wavelet-post) significantly decreased (P < 0.05) in SCI subjects, leading to an enhance ratio of LF(wavelet)/HF(wavelet) and a significantly increased in NOR(rest) (P < 0.05).

CONCLUSIONS

GCS induces an enhanced sympathetic activity in individuals with paraplegia, regardless of the level of the injury. Enhanced post-exercise sympathetic activity with GCS may help prevent orthostatic hypotension or post-exercise hypotension.

The isolated sympathetic spinal cord: Cardiovascular and sudomotor assessment in spinal cord injury patients: A literature survey

OBJECTIVES

To present a comprehensive approach to the assessment of the severity of the autonomic lesion in spinal cord injury (SCI) patients, with regard to the level of lesion. To discuss how to assess an isolated sympathetic spinal cord that has lost supraspinal control (sympathetically complete lesion).

METHOD

PubMed was searched for articles related to cardiovascular (mainly cold pressor test, respiratory and postural challenges) and sudomotor (sympathetic skin responses) tests that have been used. The results of these evaluations are analysed with regard to the site of stimulation (above or below the lesion) according to three types of SCI that offer typical autonomic reactions (tetraplegics, paraplegics at T6 and at T10).

RESULTS

Non-invasive cardiovascular and sudomotor testing allows the assessment of the isolated sympathetic spinal cord in SCI patients. Typical responses are found in relation with the level of the sympathetic lesion. Its definition would allow comparison with the somatic motor and sensory level of lesion of SCI patients and provide additional aid to the classification of those patients.

CONCLUSION

For research purposes on the integrity of the spinal sympathetic pathways, a battery of test approach is probably needed, using a combination of stimuli above and below the lesion, evaluating both cardiovascular and sudomotor pathways.

Challenges, concerns and common problems: physiological consequences of spinal cord injury and microgravity

INTRODUCTION

Similarities between the clinical presentation of individuals living with spinal cord injury (SCI) and astronauts are remarkable, and may be of great interest to clinicians and scientists alike.

OBJECTIVES

The primary purpose of this review is to outline the manner in which cardiovascular, musculoskeletal, renal, immune and sensory motor systems are affected by microgravity and SCI.

METHODS

A comprehensive review of the literature was conducted (using PubMed) to evaluate the hallmark symptoms seen after spaceflight and SCI. This literature was then examined critically to determine symptoms common to both populations.

RESULTS

Both SCI and prolonged microgravity exposure are associated with marked deteriorations in various physiological functions. Atrophy in muscle and bone, cardiovascular disturbances, and alterations in renal, immune and sensory motor systems are conditions commonly observed not only in individuals with SCI, but also in those who experience prolonged gravity unloading.

CONCLUSION

The preponderance of data indicates that similar physiological changes occur in both SCI and prolonged space flight. These findings have important implications for future research in SCI and prolonged space flight.

Limitations of oximetry to measure heart rate variability measures

Measuring heart rate variability (HRV) is widely used to assess autonomic nervous system function. It requires accurate measurement of the interval between successive heartbeats. This can be achieved from recording the electrocardiogram (ECG), which is non-invasive and widely available. However, methodological problems inherent in recording and analyzing ECG traces have motivated a search for alternative means of measuring the interval between successive heartbeats. Recording blood oxygenation pulsations (photoplethysmography-PPG) is also convenient, non-invasive and widely available, and has been suggested as an effective alternative to ECG to derive HRV. Moreover, it has been claimed that the pulse waveforms produced by oximetry may be more practicable than R-R intervals measured from the by ECG, especially for ambulatory recordings. We have therefore compared PPG with ECG recordings to measure HRV applying the same signal analysis techniques to PPG and ECG recordings made simultaneously. Comparison of 5 min recording epochs demonstrated a very high degree of correlation, in temporal, frequency domains and non-linear analysis, between HRV measures derived from the PPG and ECG. However, we found that the PPG signal is especially vulnerable to motion artifacts when compared to the ECG, preventing any HRV analysis at all in a significant minority of PPG recordings. Our results demonstrate that even though PPG provides accurate interpulse intervals to measure heart rate variability under ideal conditions, it is less reliable due to its vulnerability to motion artifacts. Therefore it is unlikely to prove a practical alternative to the ECG in ambulatory recordings or recordings made during other activities.

Autonomic function following cervical spinal cord injury

Spinal cord injury (SCI) is commonly associated with devastating paralysis. However, this condition also results in a variety of autonomic dysfunctions, primarily: cardiovascular, broncho-pulmonary, urinary, gastrointestinal, sexual, and thermoregulatory. SCI and the resultant unstable autonomic control are responsible for increased mortality from cardiovascular and respiratory disease among individuals with SCI. Injury level and severity directly correlate to the severity of autonomic dysfunctions following SCI. Following high cervical SCI, parasympathetic (vagal) control will remain intact, while the spinal sympathetic circuits will lose their tonic supraspinal autonomic control. On the other hand, in individuals with injury below the 5th thoracic segment, both the sympathetic and parasympathetic control of the heart and broncho-pulmonary tree are intact. As a result of injury level, individuals with quadriplegia versus those with paraplegia will have very different cardiovascular and respiratory responses. Furthermore, similar relationships can exist between the level of SCI and function of other organs that are under autonomic control (bladder, bowel, sweat glands, etc.). It is also important to appreciate that high cervical injuries result in significant respiratory dysfunctions due to the involvement of the diaphragm and a larger portion of the accessory respiratory muscles. Early recognition and timely management of autonomic dysfunctions in individuals with SCI are crucial for the long term health outcomes in this population.

A systematic review of the management of orthostatic hypotension after spinal cord injury

OBJECTIVE

To review systematically the evidence for the management of orthostatic hypotension (OH) in patients with spinal cord injuries (SCIs).

DATA SOURCES

A key word literature search was conducted of original and review articles as well as practice guidelines using Medline, CINAHL, EMBASE, and PsycInfo, and manual searches of retrieved articles from 1950 to July 2008, to identify literature evaluating the effectiveness of currently used treatments for OH.

STUDY SELECTION

Included randomized controlled trials (RCTs), prospective cohort studies, case-control studies, pre-post studies, and case reports that assessed pharmacologic and nonpharmacologic intervention for the management of OH in patients with SCI.

DATA EXTRACTION

Two independent reviewers evaluated the quality of each study, using the Physiotherapy Evidence Database score for RCTs and the Downs and Black scale for all other studies. Study results were tabulated and levels of evidence assigned.

DATA SYNTHESIS

A total of 8 pharmacologic and 21 nonpharmacologic studies were identified that met the criteria. Of these 26 studies (some include both pharmacologic and nonpharmacologic interventions), only 1 pharmacologic RCT was identified (low-quality RCT producing level 2 evidence), in which midodrine was found to be effective in the management of OH after SCI. Functional electrical stimulation was one of the only nonpharmacologic interventions with some evidence (level 2) to support its utility.

CONCLUSIONS

Although a wide array of physical and pharmacologic measures are recommended for the management of OH in the general population, very few have been evaluated for use in SCI. Further research needs to quantify the efficacy of treatment for OH in subjects with SCI, especially of the many other pharmacologic interventions that have been shown to be effective in non-SCI conditions.

Cardiovascular complications after acute spinal cord injury: pathophysiology, diagnosis, and management

Cardiovascular complications in the acute stage following traumatic spinal cord injury (SCI) require prompt medical attention to avoid neurological compromise, morbidity, and death. In this review, the authors summarize the neural regulation of the cardiovascular system as well as the pathophysiology, diagnosis, and management of major cardiovascular complications that can occur following acute (up to 30 days) traumatic SCI. Hypotension (both supine and orthostatic), autonomic dysreflexia, and cardiac arrhythmias (including persistent bradycardia) are attributed to the loss of supraspinal control of the sympathetic nervous system that commonly occurs in patients with severe spinal cord lesions at T-6 or higher. Current evidence-based guidelines recommend: 1) monitoring of cardiac and hemodynamic parameters in the acute phase of SCI; 2) maintenance of a minimum mean arterial blood pressure of 85 mm Hg during the hyperacute phase (1 week after SCI); 3) timely detection and appropriate treatment of neurogenic shock and cardiac arrhythmias; and 4) immediate and adequate treatment of episodes of acute autonomic dysreflexia. In addition to these forms of cardiovascular dysfunction, individuals with acute SCIs are at high risk for deep venous thrombosis (DVT) and pulmonary embolism due to loss of mobility and, potentially, altered fibrinolytic activity, abnormal platelet function, and impaired circadian variations of hemostatic and fibrinolytic parameters. Current evidence supports a recommendation for thromboprophylaxis using mechanical methods and anticoagulants during the acute stage up to 3 months following SCI, depending on the severity and level of injury. Low-molecular-weight heparin is the first choice for anticoagulant prophylaxis in patients with acute SCI. Although there is insufficient evidence to recommend (or refute) the use of screening tests for DVT in asymptomatic adults with acute SCI, this strategy may detect asymptomatic DVT in at least 9.4% of individuals who undergo thromboprophylaxis using lowmolecular- weight heparin. Indications and treatment of DVT and acute pulmonary embolism are well established and are summarized in this review. Recognition of cardiovascular complications after acute SCI is essential to minimize adverse outcomes and to optimize recovery.

Clinical correlates of frequency analyses of cardiovascular control after spinal cord injury

Spinal cord injury (SCI) has profound effects on cardiovascular autonomic function due to injury to descending autonomic pathways, and cardiovascular diseases are the leading causes of morbidity and mortality after SCI. Evaluation of cardiovascular autonomic dysfunction after SCI and appraisal of simple noninvasive autonomic assessments that are clinically meaningful would be useful to SCI clinicians and researchers. We aimed to assess supine and upright cardiovascular autonomic function from frequency analyses of heart rate and blood pressure variability (HRV and BPV) after SCI. We studied 26 subjects with chronic cervical or thoracic SCI and 17 able-bodied controls. We continuously recorded R-R interval (RRI, by ECG) and beat-to-beat blood pressure (by Finometer) in supine and seated positions. Cardiovascular control was assessed from spectral analysis of RRI and blood pressure time series. Cardiac baroreflex control was assessed from cross-spectral analyses of low-frequency spectra. Supine and upright low-frequency HRV and BPV were reduced in cervical SCI subjects, as were total BPV and HRV. Supine high-frequency HRV was reduced in thoracic SCI subjects. Cardiac baroreflex delay was increased in cervical SCI subjects. Supine frequency domain indexes were correlated with sympathetic skin responses, orthostatic cardiovascular responses, and plasma catecholamine levels. SCI results in reduced sympathetic drive to the heart and vasculature and increased baroreflex delay in cervical SCI subjects and reduced cardiac vagal tone in thoracic SCI subjects. Frequency analyses of autonomic function are related to clinical measures of autonomic control after SCI and provide useful noninvasive clinical tools with which to assess autonomic completeness of injury following SCI.

Orthostatic hypotension in the first month following acute spinal cord injury

STUDY DESIGN

Retrospective data analysis.

OBJECTIVES

To determine prevalence of orthostatic hypotension (OH) in patients with spinal cord injury (SCI) during the acute rehabilitation period.

SETTING

Quaternary care spinal unit, Vancouver General Hospital, British Columbia, Canada.

METHODS

Eighty-nine patients with acute SCI stratified by neurological level (cervical, 55 (62%); upper thoracic, 12 (13%); lower thoracic, 22 (25%)), and graded by American Spinal Injury Association standards. Non-invasive measurement of systolic and diastolic blood pressure and heart rate were made at baseline and 3 min following an orthostatic challenge test administered during the first month after SCI.

RESULTS

Patients with cervical or upper thoracic motor complete SCI more frequently experienced OH (P<0.01). OH persisted during the first month following SCI in 74% of cervical and only 20% of upper thoracic motor complete SCI patients.

CONCLUSION

Patients with cervical and upper thoracic motor complete SCI are more likely to experience persistent OH than those with lower level or motor incomplete SCI during the first month of rehabilitation.

Glutamine concentration and immune response of spinal cord-injured rats

BACKGROUND/OBJECTIVES

Glutamine plays a key role in immune response. Spinal cord injury (SCI) leads to severe loss of muscle mass and to a high incidence of infections. This study investigated the acute effect of SCI (2 and 5 days) on the plasma glutamine and skeletal muscle concentrations and immune responses in rats.

METHODS

A total of 29 adult male Wistar rats were divided as follows: control (C; n = 5), sham-operated (S2; n = 5) and spinal cord-transected (T2; n = 7). They were killed on day 2 after surgery/transection (acute phase). Another set was sham-operated (S5; n = 5), spinal cord-transected (T5; n = 7), and killed at day 5 after surgery/transection (secondary phase). Blood was collected; the white portion of the epitrochlearis and gastrocnemius muscles and the red portion of soleus muscles were dissected to measure the glutamine concentration. Gut-associated lymphocytes and peritoneal macrophages were obtained for immune parameters measurements.

RESULTS

Glutamine concentration in the plasma, gastrocnemius, and soleus muscles in rats with SCI were significantly reduced but not in the epitrochlearis muscle in the acute (2 days) and secondary (5 days) phases. Phagocytic response was reduced in the acute phase but increased in the secondary phase in rats with SCI. Superoxide production, on the other hand, was significantly increased at days 2 and 5 after SCI, and CD8+ lymphocytes subset decreased significantly on days 2 and 5.

CONCLUSIONS

Our results showed reduction in plasma glutamine and skeletal muscle concentrations after spinal cord transection. They also suggest that SCI and glutamine reduction contribute to an alteration in immune competence.

Blood pressure regulation in neurally intact human vs. acutely injured paraplegic and tetraplegic patients during passive tilt

We investigated autonomic control of cardiovascular function in able-bodied (AB), paraplegic (PARA), and tetraplegic (TETRA) subjects in response to head-up tilt following spinal cord injury. We evaluated spectral power of blood pressure (BP), baroreflex sensitivity (BRS), baroreflex effectiveness index (BEI), occurrence of systolic blood pressure (SBP) ramps, baroreflex sequences, and cross-correlation of SBP with heart rate (HR) in low (0.04-0.15 Hz)- and high (0.15-0.4 Hz)-frequency regions. During tilt, AB and PARA effectively regulated BP and HR, but TETRA did not. The numbers of SBP ramps and percentages of heartbeats involved in SBP ramps and baroreflex sequences increased in AB, were unchanged in PARA, and declined in TETRA. BRS was lowest in PARA and declined with tilt in all groups. BEI was greatest in AB and declined with tilt in all groups. Low-frequency power of BP and the peak of the SBP/HR cross-correlation magnitude were greatest in AB, increased during tilt in AB, remained unchanged in PARA, and declined in TETRA. The peak cross-correlation magnitude in HF decreased with tilt in all groups. Our data indicate that spinal cord injury results in decreased stimulation of arterial baroreceptors and less engagement of feedback control as demonstrated by lower 1) spectral power of BP, 2) number (and percentages) of SBP ramps and barosequences, 3) cross-correlation magnitude of SBP/HR, 4) BEI, and 5) changes in delay between SBP/HR. Diminished vasomotion and impaired baroreflex regulation may be major contributors to decreased orthostatic tolerance following injury.

Blunted heart rate response to vagal withdrawal in persons with tetraplegia

Cardiovascular autonomic mechanisms control heart rate (HR) and determination of heart rate variability (HRV) permits the quantitative assessment of relative shifts in autonomic cardiac control during head-up tilt (HUT). The study herein used HRV techniques to determine the vagal and sympathetic contribution to the change in HR during HUT in persons with tetraplegia (T; n = 7) paraplegia (P; n = 7) and a non-spinal cord injured (non-SCI; n = 8) control group. Heart rate (HR) was continuously monitored and cardiovascular autonomic responses were assessed for 5-minutes at supine and at 45 degrees HUT. Change associated with tilt from supine to 45 degrees HUT was calculated for HR (deltaHR), high & low frequency HRV (HF & LF) and the LF/HF ratio. HR and LF power were lower in the T compared to the P and non-SCI groups at 45 degrees HUT, whereas there were no group differences for HF at 45 degrees HUT. The LF/HF ratio was lower in the T compared to the non-SCI group at 45 degrees HUT. The relationship between delta HR and delta HF response differed between groups (significant group x delta HF interaction) such that the slope of this relationship was reduced in the T (-1.026: 95% CI: -2.623 to 0.571) compared with the non-SCI (-6.985: 95% CI: -11.25 to 2.72) and P (-5.218: 95% CI: -8.197 to -2.239) groups. There was no significant interaction effect for the relationships between deltaHR and deltaLF or deltaLF/HF. In summary, although the magnitude of vagal withdrawal was comparable among the groups, the increase in HR was attenuated in the group with tetraplegia, which may reflect reduced sympathetic cardiac modulation or altered SA node responsiveness to vagal withdrawal.

Autonomic Recovery from Peak Arm Exercise in Fit and Unfit Individuals with Paraplegia

INTRODUCTION

Altered autonomic cardiovascular control in persons with paraplegia may reflect peripheral sympathetic denervation caused by the injury or deconditioning due to skeletal muscle paralysis. Parameters of autonomic cardiovascular control may be improved in fit persons with paraplegia similar to effects reported in the noninjured population.

PURPOSE

To determine differences in resting and recovery HR and cardiac autonomic control in fit and unfit individuals with paraplegia.

METHODS

Eighteen healthy males with paraplegia below T6 were studied; nine participated in aerobic exercise conditioning (fit: >or=30 min.d, >or=3 d.wk, >or=6 months), and nine were sedentary (unfit). Analysis of heart rate variability (HRV) was used to determine spectral power (ln transformed) in the high- (lnHF) and low-frequency (lnLF) bandwidths, and the LF/HF ratio was calculated. Data were collected at baseline (BL) and at 2, 10, 30, 60, and 90 min of recovery from peak arm cycle ergometry.

RESULTS

The relative intensity achieved on the peak exercise test was comparable between the groups (i.e., 88% peak predicted HR). However, peak watts (P<0.001) and oxygen consumption (P<0.01) were higher in the fit compared with the unfit group (56 and 51%, respectively). Recovery lnHF was increased (P<0.05), and recovery lnLF (P<0.01) and LF/HF (P<0.05) were reduced in the fit compared with the unfit group. Mean recovery autonomic activity was not different from BL in the fit group. In the unfit group, mean recovery lnHF was reduced, and mean recovery lnLF and LF/HF remained elevated above BL.

CONCLUSION

These data suggest that fit individuals with paraplegia have improved cardiac autonomic control during the postexercise recovery period compared with their unfit counterparts.

Effect of harness application and postural changes on cardiovascular parameters of individuals with spinal cord injury

STUDY DESIGN

Prospective assessment of cardiovascular parameters in individuals with spinal cord injury (SCI) in response to harness application and postural changes including orthostatic stress.

OBJECTIVE

To evaluate arterial blood pressure and heart rate (HR) with and without harness application during sitting, supine, and standing positions in able-bodied and SCI individuals.

METHODS

Measurements were obtained in all SCI research participants (n=11) before a locomotor training intervention and compared to data with able-bodied individuals (n=9). During standing, all research participants wore a harness and were suspended by an overhead, pneumatic body weight support system.

RESULTS

Resting arterial blood pressure and HR in individuals with cervical SCI were significantly lower during sitting than in thoracic SCI and able-bodied individuals (P<0.05). Orthostatic stress significantly decreased arterial blood pressure only in individuals with cervical SCI (P<0.05). Harness application had no effect on cardiovascular parameters in able-bodied individuals, whereas diastolic blood pressure was significantly increased in those with SCI. Orthostatic changes in cervical SCI when sitting were ameliorated by harness application. However, while standing with harness, individuals with cervical SCI still developed orthostatic hypotension.

CONCLUSIONS

Level of injury to the spinal cord influences baseline cardiovascular parameters. Application of harness in individuals with SCI could alter baseline cardiovascular parameters and the response to orthostatic stress. This should be carefully considered when assessing effects of therapeutic interventions using body weight support in individuals with SCI.

The clinical problems in cardiovascular control following spinal cord injury: an overview

On a daily basis, individuals with cervical and upper thoracic spinal cord injury face the challenge of managing their unstable blood pressure, which frequently results in persistent hypotension and/or episodes of uncontrolled hypertension. This chapter will focus on the clinical issues related to abnormal cardiovascular control in individuals with spinal cord injury, which include neurogenic shock, autonomic dysreflexia and orthostatic hypotension. Blood pressure control depends upon tonic activation of sympathetic preganglionic neurons by descending input from the supraspinal structures (Calaresu and Yardley, 1988). Following spinal cord injury, these pathways are disrupted, and thus spinal circuits are solely responsible for the generation of sympathetic activity (Osborn et al., 1989; Maiorov et al., 1997). This results in a variety of cardiovascular abnormalities that have been well documented in human studies, as well as in animal models (Osborn et al., 1990; Mathias and Frankel, 1992a, b; Krassioukov and Weaver, 1995; Maiorov et al., 1997, 1998; Teasell et al., 2000). However, the recognition and management of these cardiovascular dysfunctions following spinal cord injury represent challenging clinical issues. Moreover, cardiovascular disorders in the acute and chronic stages of spinal cord injury are among the most common causes of death in individuals with spinal cord injury (DeVivo et al., 1999).

Orthostatic hypotension following spinal cord injury: understanding clinical pathophysiology

Motor and sensory deficits are well-known consequences of spinal cord injury (SCI). During the last decade, a significant number of experimental and clinical studies have focused on the investigation of autonomic dysfunction and cardiovascular control following SCI. Numerous clinical reports have suggested that unstable blood pressure control in individuals with SCI could be responsible for their increased cardiovascular mortality. The aim of this review is to outline the incidence and pathophysiological mechanisms underlying the orthostatic hypotension that commonly occurs following SCI. We describe the clinical abnormalities of blood pressure control following SCI, with particular emphasis upon orthostatic hypotension. Possible mechanisms underlying orthostatic hypotension in SCI, such as changes in sympathetic activity, altered baroreflex function, the lack of skeletal muscle pumping activity, cardiovascular deconditioning and altered salt and water balance will be discussed. Possible alterations in cerebral autoregulation following SCI, and the impact of these changes upon cerebral perfusion are also examined. Finally, the management of orthostatic hypotension will be considered.