Oxygen cost of resistive-loaded breathing in quadriplegia

The Potential Role of Inflammation in Modulating Endogenous Hippocampal Neurogenesis After Spinal Cord Injury

Currently there are approximately 291,000 people suffering from a spinal cord injury (SCI) in the United States. SCI is associated with traumatic changes in mobility and neuralgia, as well as many other long-term chronic health complications, including metabolic disorders, diabetes mellitus, non-alcoholic steatohepatitis, osteoporosis, and elevated inflammatory markers. Due to medical advances, patients with SCI survive much longer than previously. This increase in life expectancy exposes them to novel neurological complications such as memory loss, cognitive decline, depression, and Alzheimer's disease. In fact, these usually age-associated disorders are more prevalent in people living with SCI. A common factor of these disorders is the reduction in hippocampal neurogenesis. Inflammation, which is elevated after SCI, plays a major role in modulating hippocampal neurogenesis. While there is no clear consensus on the mechanism of the decline in hippocampal neurogenesis and cognition after SCI, we will examine in this review how SCI-induced inflammation could modulate hippocampal neurogenesis and provoke age-associated neurological disorders. Thereafter, we will discuss possible therapeutic options which may mitigate the influence of SCI associated complications on hippocampal neurogenesis.

Effect of overground locomotor training on ventilatory kinetics and rate of perceived exertion in persons with cervical motor-incomplete spinal cord injury

Study design

Pre-post, pilot study.

Objectives

To characterize ventilatory (VE) responses to exercise following warm-up walking in individuals with chronic incomplete spinal cord injury (iSCI) during constant work rate (CWR) exercise. Secondarily, to investigate VE and tidal volume (VT) variability, and ratings of perceived exertion (RPE) before and after overground locomotor training (OLT).

Setting

Research laboratory.

Methods

A 6-min CWR walking bout at preferred pace was used as a warm-up followed by 6 min of rest and a second 6-min CWR bout at above preferred walking pace. The second CWR bout was analyzed. Breath-by-breath ventilatory data were examined using a curvilinear least squares fitting procedure with a mono-exponential model. VE and VT variability was calculated as the difference between the observed and predicted values and RPE was taken every 2 min.

Results

Participants (n = 3, C4-C5) achieved a hyperpneic response to exercise in VE and VT. OLT resulted in faster ventilatory kinetics and reductions of 24 and 29% for VE and VT variability, respectively. A 30% reduction in RPE was concurrent with the reductions in ventilatory variability.

Conclusions

OLT may improve ventilatory control during CWR in patients with cervical motor-iSCI. These data suggest that in some participants with iSCI, ventilation may influence RPE during walking. Future research should investigate mechanisms of ventilatory variability and its implications in walking performance in patients with iSCI.

Evaluation of a clinical implementation of a respiratory muscle training group during spinal cord injury rehabilitation

STUDY DESIGN

Retrospective cohort study.

OBJECTIVE

To evaluate the clinical implementation of a respiratory muscle training group during rehabilitation of individuals with spinal cord injury.

SETTING

Spinal cord injury rehabilitation center.

METHODS

Individuals with complete or incomplete lesions during inpatient rehabilitation, level C4-T12.Ten or more training sessions of either an inspiratory or a combined in- and expiratory muscle training were performed in a group setting with respiratory function measurements before and after the training period.

RESULTS

Analysis of 79 persons. Inspiratory muscle training was performed for 7 weeks with a median of 3.1 training sessions per week. Median training intensity was at 33% of baseline PImax and 58 repetitions were performed per training session. Respiratory mucle strength parameters improved by 18-68% of baseline values and lung function parameters by 11-31% after inspiratory muscle training.The combined respiratory muscle training was performed for 13 weeks with a median of 2.8 sessions per week and 88 repetitions per training session. Median inspiratory training resistance was at 39% of baseline PImax and median expiratory training resistance was at 27% of baseline PEmax. Respiratory muscle strength parameters improved by 14-51% of baseline values and lung function parameters improved by 15-34% after the combined in- and expiratory muscle training.

CONCLUSION

Respiratory resistance training improved respiratory function of individuals with acute spinal cord injury. Even if the combined respiratory muscle training was performed with more repetitions per training and nearly twice as long, relative improvements of respiratory function parameters were comparable with isolated inspiratory muscle training.

Reprint of "Drawing breath without the command of effectors: the control of respiration following spinal cord injury"

The maintenance of blood gas and pH homeostasis is essential to life. As such breathing, and the mechanisms which control ventilation, must be tightly regulated yet highly plastic and dynamic. However, injury to the spinal cord prevents the medullary areas which control respiration from connecting to respiratory effectors and feedback mechanisms below the level of the lesion. This trauma typically leads to severe and permanent functional deficits in the respiratory motor system. However, endogenous mechanisms of plasticity occur following spinal cord injury to facilitate respiration and help recover pulmonary ventilation. These mechanisms include the activation of spared or latent pathways, endogenous sprouting or synaptogenesis, and the possible formation of new respiratory control centres. Acting in combination, these processes provide a means to facilitate respiratory support following spinal cord trauma. However, they are by no means sufficient to return pulmonary function to pre-injury levels. A major challenge in the study of spinal cord injury is to understand and enhance the systems of endogenous plasticity which arise to facilitate respiration to mediate effective treatments for pulmonary dysfunction.

A center's experience: pulmonary function in spinal cord injury

Traumatic spinal cord injury (SCI) is associated with significant psychological and physical challenges. A multidisciplinary approach to management is essential to ensure recovery during the acute phase, and comprehensive rehabilitative strategies are necessary to foster independence and quality of life throughout the chronic phase of injury. Complications that beset these individuals are often a unique consequence of SCI, and knowledge of the effects of SCI upon organ systems is essential for appropriate management. According to the National SCI Statistical Center (NSCISC), as of 2010 there were an estimated 265,000 persons living with SCI in the United States, with approximately 12,000 incidence cases annually. Although life expectancy for newly injured individuals with SCI is markedly reduced, persons with chronic SCI are expected to live about as long as individuals without SCI; however, longevity varies inversely with level of injury. Since 2005, 56 % of persons with SCI are tetraplegic, and due to paralysis of respiratory muscles, these individuals may be especially prone to pulmonary complications, which remain a major cause of mortality among persons with chronic SCI. We at the VA Rehabilitation Research and Development Center of Excellence for the Medical Consequences of SCI at the James J. Peters VA Medical Center have devoted more than 25 years to the study of secondary medical conditions that complicate SCI. Herein, we review pulmonary research at the Center, both our past and future endeavors, which form an integral part of our multidisciplinary approach toward achieving a greater understanding of and improving care for veterans with SCI.

Noninvasive ventilation reduces energy expenditure in amyotrophic lateral sclerosis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) leads to chronic respiratory failure. Diaphragmatic dysfunction, a major driver of dyspnea and mortality, is associated with a shift of the burden of ventilation to extradiaphragmatic inspiratory muscles, including neck muscles. Besides, energy expenditure is often abnormally high in ALS, and this is associated with a negative prognostic value. We hypothesized that noninvasive ventilation (NIV) would relieve inspiratory neck muscles and reduce resting energy expenditure (REE).

METHODS

Using indirect calorimetry, we measured REE during spontaneous breathing (REESB) and NIV (REENIV) in 16 ALS patients with diaphragmatic dysfunction, during the first 3 months of NIV. Measured values were compared with predicted REE (REEpred)(Harris-Benedict equation).

RESULTS

NIV abolished inspiratory neck muscle activity. Even though our patients were not hypermetabolic, on the contrary, with a REESB that was lower than REEpred (average 11%), NIV did reduce energy expenditure. Indeed, median REENIV, in this population with a mean body mass index of 21.4 kg.m-2, was 1149 kcal/24 h [interquartile 970-1309], lower than REESB (1197 kcal/24 h, 1054-1402; mean difference 7%; p = 0.03, Wilcoxon). REESB and REENIV were correlated with forced vital capacity and maximal inspiratory pressure.

CONCLUSIONS

NIV can reduce energy expenditure in ALS patients probably by alleviating the ventilatory burden imposed on inspiratory neck muscles to compensate diaphragm weakness. It remains to be elucidated whether or not, in which population, and to what extent, NIV can be beneficial in ALS through the corresponding reduction in energy expenditure.

Effect of resistive load on the inspiratory work and power of breathing during exertion

The resistive work of breathing against an external load during inspiration (WR_I) was measured at the mouth, during sub-maximal exercise in healthy participants. This measure (which excludes the elastic work component) allows the relationship between resistive work and power, ventilation and exercise modality to be explored. A total of 45 adult participants with healthy lung function took part in a series of exercise protocols, in which the relationship between WR_I, power of breathing, PR_I and minute ventilation, were assessed during rest, while treadmill walking or ergometer cycling, over a range of exercise intensities (up to 150 Watts) and ventilation rates (up to 48 L min⁻¹) with applied constant resistive loads of 0.75 and 1.5 kPa.L.sec⁻¹. Resting WR_I was 0.12 JL⁻¹ and PR_I was 0.9 W. At each resistive load, independent of the breathing pattern or exercise mode, the WR_I increased in a linear fashion at 20 mJ per litre of , while PR_I increased exponentially. With increasing resistive load the work and power at any given increased exponentially. Calculation of the power to work ratio during loaded breathing suggests that loads above 1.5 kPa.L.sec⁻¹ make the work of resistive breathing become inhibitive at even a moderate (>30 L sec⁻¹). The relationship between work done and power generated while breathing against resistive loads is independent of the exercise mode (cycling or walking) and that ventilation is limited by the work required to breathe, rather than an inability to maintain or generate power.

Pulmonary function and spinal cord injury

Injury to the cervical and upper thoracic spinal cord disrupts function of inspiratory and expiratory muscles, as reflected by reduction in spirometric and lung volume parameters and static mouth pressures. In association, subjects with tetraplegia have decreased chest wall and lung compliance, increased abdominal wall compliance, and rib cage stiffness with paradoxical chest wall movements, all of which contribute to an increase in the work of breathing. Expiratory muscle function is more compromised than inspiratory muscle function among subjects with tetraplegia and high paraplegia, which can result in ineffective cough and propensity to mucus retention and atelectasis. Subjects with tetraplegia also demonstrate heightened vagal activity with reduction in baseline airway caliber, findings attributed to loss of sympathetic innervation to the lungs. Significant increase in airway caliber following inhalation of ipratropium bromide, an anticholinergic agent, suggests that reduction in airway caliber is not due to acquired airway fibrosis stemming from repeated infections or to abnormal hysteresis secondary to chronic inability of subjects to inhale to predicted total lung capacity. Reduced baseline airway caliber possibly explains why subjects with tetraplegia exhibit airway hyperresponsiveness to methacholine and ultrasonically nebulized distilled water. While it has been well demonstrated that bilateral phrenic nerve pacing or stimulation through intramuscular diaphragmatic electrodes improves inspiratory muscle function, it remains unclear if inspiratory muscle training improves pulmonary function. Recent findings suggest that expiratory muscle training, electrical stimulation of expiratory muscles and administration of a long-acting beta(2)-agonist (salmeterol) improve physiological parameters and cough. It is unknown if baseline bronchoconstriction in tetraplegia contributes to respiratory symptoms, of if the chronic administration of a bronchodilator reduces the work of breathing and/or improves respiratory symptoms. Less is known regarding the benefits of treatment of obstructive sleep apnea, despite evidence indicating that the prevalence of this condition in persons with tetraplegia is far greater than that encountered in able-bodied individuals.

Effect of spinal cord injury on the respiratory system: basic research and current clinical treatment options

Spinal cord injury (SCI) often leads to an impairment of the respiratory system. The more rostral the level of injury, the more likely the injury will affect ventilation. In fact, respiratory insufficiency is the number one cause of mortality and morbidity after SCI. This review highlights the progress that has been made in basic and clinical research, while noting the gaps in our knowledge. Basic research has focused on a hemisection injury model to examine methods aimed at improving respiratory function after SCI, but contusion injury models have also been used. Increasing synaptic plasticity, strengthening spared axonal pathways, and the disinhibition of phrenic motor neurons all result in the activation of a latent respiratory motor pathway that restores function to a previously paralyzed hemidiaphragm in animal models. Human clinical studies have revealed that respiratory function is negatively impacted by SCI. Respiratory muscle training regimens may improve inspiratory function after SCI, but more thorough and carefully designed studies are needed to adequately address this issue. Phrenic nerve and diaphragm pacing are options available to wean patients from standard mechanical ventilation. The techniques aimed at improving respiratory function in humans with SCI have both pros and cons, but having more options available to the clinician allows for more individualized treatment, resulting in better patient care. Despite significant progress in both basic and clinical research, there is still a significant gap in our understanding of the effect of SCI on the respiratory system.

Waist and neck enlargement after quadriplegia

Changes in waist and neck size in quadriplegic patients after paralysis, noted clinically, were assessed systematically. Twenty quadriplegic men, aged 60 +/- 13 years (mean +/- 1 SD) and 20 neurologically intact men, aged 63 +/- 17 years, selected by absence of weight gain, were questioned about changes in their waist and shirt collar sizes since the onset of paralysis (20 +/- 13 years) or during the previous 20 years for control subjects. Waist size expanded 7.0 +/- 0.3 inches for quadriplegic and 1.7 +/- 1.7 inches for control subjects (p < 0.001). Changes in neck size of 0.7 +/- 1.1 inches for quadriplegic and 0 +/- 0.7 inches for control subjects were found (p < 0.02). We conclude that quadriplegia is often followed by increased waist and neck size. These changes may relate to the impaired breathing mechanisms in quadriplegia.