Ventilatory and P0.1 response to hypercapnia in quadriplegia

The Clinical Relevance of Autonomic Dysfunction, Cerebral Hemodynamics, and Sleep Interactions in Individuals Living With SCI

A myriad of physiological impairments is seen in individuals after a spinal cord injury (SCI). These include altered autonomic function, cerebral hemodynamics, and sleep. These physiological systems are interconnected and likely insidiously interact leading to secondary complications. These impairments negatively influence quality of life. A comprehensive review of these systems, and their interplay, may improve clinical treatment and the rehabilitation plan of individuals living with SCI. Thus, these physiological measures should receive more clinical consideration. This special communication introduces the under investigated autonomic dysfunction, cerebral hemodynamics, and sleep disorders in people with SCI to stakeholders involved in SCI rehabilitation. We also discuss the linkage between autonomic dysfunction, cerebral hemodynamics, and sleep disorders and some secondary outcomes are discussed. Recent evidence is synthesized to make clinical recommendations on the assessment and potential management of important autonomic, cerebral hemodynamics, and sleep-related dysfunction in people with SCI. Finally, a few recommendations for clinicians and researchers are provided.

Tetraplegia is associated with increased hypoxic ventilatory response during nonrapid eye movement sleep

People with cervical spinal cord injury (SCI) are likely to experience chronic intermittent hypoxia while sleeping. The physiological effects of intermittent hypoxia on the respiratory system during spontaneous sleep in individuals with chronic cervical SCI are unknown. We hypothesized that individuals with cervical SCI would demonstrate higher short- and long-term ventilatory responses to acute intermittent hypoxia (AIH) exposure than individuals with thoracic SCI during sleep. Twenty participants (10 with cervical SCI [9 male] and 10 with thoracic SCI [6 male]) underwent an AIH and sham protocol during sleep. During the AIH protocol, each participant experienced 15 episodes of isocapnic hypoxia using mixed gases of 100% nitrogen (N2 ) and 40% carbon dioxide (CO2 ) to achieve an oxygen saturation of less than 90%. This was followed by two breaths of 100% oxygen (O2 ). Measurements were collected before, during, and 40 min after the AIH protocol to obtain ventilatory data. During the sham protocol, participants breathed room air for the same amount of time that elapsed during the AIH protocol and at approximately the same time of night. Hypoxic ventilatory response (HVR) during the AIH protocol was significantly higher in participants with cervical SCI than those with thoracic SCI. There was no significant difference in minute ventilation (V.E. ), tidal volume (V.T. ), or respiratory frequency (f) during the recovery period after AIH in cervical SCI compared to thoracic SCI groups. Individuals with cervical SCI demonstrated a significant short-term increase in HVR compared to thoracic SCI. However, there was no evidence of ventilatory long-term facilitation following AIH in either group.

Is Sleep Disordered Breathing Confounding Rehabilitation Outcomes in Spinal Cord Injury Research?

The purpose of this article is to highlight the importance of considering sleep-disordered breathing (SDB) as a potential confounder to rehabilitation research interventions in spinal cord injury (SCI). SDB is highly prevalent in SCI, with increased prevalence in individuals with higher and more severe lesions, and the criterion standard treatment with continuous positive airway pressure remains problematic. Despite its high prevalence, SDB is often untested and untreated in individuals with SCI. In individuals without SCI, SDB is known to negatively affect physical function and many of the physiological systems that negatively affect physical rehabilitation in SCI. Thus, owing to the high prevalence, under testing, low treatment adherence, and known negative effect on the physical function, it is contended that underdiagnosed SDB in SCI may be confounding physical rehabilitation research studies in individuals with SCI. Studies investigating the effect of treating SDB and its effect on physical rehabilitation in SCI were unable to be located. Thus, studies investigating the likely integrated relationship among physical rehabilitation, SDB, and proper treatment of SDB in SCI are needed. Owing to rapid growth in both sleep medicine and physical rehabilitation intervention research in SCI, the authors contend it is the appropriate time to begin the conversations and collaborations between these fields. We discuss a general overview of SDB and physical training modalities, as well as how SDB could be affecting these studies.

Mid-cervical spinal cord contusion causes robust deficits in respiratory parameters and pattern variability

Mid-cervical spinal cord contusion disrupts both the pathways and motoneurons vital to the activity of inspiratory muscles. The present study was designed to determine if a rat contusion model could result in a measurable deficit to both ventilatory and respiratory motor function under “normal” breathing conditions at acute to chronic stages post trauma. Through whole body plethysmography and electromyography we assessed respiratory output from three days to twelve weeks after a cervical level 3 (C3) contusion. Contused animals showed significant deficits in both tidal and minute volumes which were sustained from acute to chronic time points. We also examined the degree to which the contusion injury impacted ventilatory pattern variability through assessment of Mutual Information and Sample Entropy. Mid-cervical contusion significantly and robustly decreased the variability of ventilatory patterns. The enduring deficit to the respiratory motor system caused by contusion was further confirmed through electromyography recordings in multiple respiratory muscles. When isolated via a lesion, these contused pathways were insufficient to maintain respiratory activity at all time points post injury. Collectively these data illustrate that, counter to the prevailing literature, a profound and lasting ventilatory and respiratory motor deficit may be modelled and measured through multiple physiological assessments at all time points after cervical contusion injury.

Spinal cord injury is associated with enhanced peripheral chemoreflex sensitivity

Sleep‐disordered breathing (SDB) is prevalent in individuals with chronic spinal cord injury (SCI), but the exact mechanism is unknown. The aim of this study was to investigate whether peripheral chemoreceptors activity is enhanced in individuals with chronic SCI compared to abled‐bodied control subjects using CO₂ and O₂ chemical tests. In protocol (1) 30 subjects (8 cervical [cSCI], 7 thoracic [tSCI] and 15 able‐bodied [AB]) were studied to determine the ventilatory response to hyperoxia during wakefulness in the supine position. In protocol (2) 24 subjects (6 cSCI, 6 tSCI, and 12 AB subjects) were studied to determine the ventilatory response to a single breath of CO₂ (SBCO₂). The chemoreflex response to SBCO₂ was calculated as ∆V_E/∆CO₂ (L/min/mmHg). The ventilatory response to hyperoxia was defined as the % change in V_T following acute hyperoxia compared to preceding baseline. During hyperoxia SCI subjects had a significant decrease in V_T and V_E (63.4 ± 21.7% and 63.1 ± 23.0% baseline, respectively, P < 0.05) compared to AB (V_T: 87.1 ± 14.3% and V_E: 91.38 ± 15.1% baseline, respectively, P < 0.05). There was no significant difference between cSCI and tSCI in the V_T or V_E during hyperoxia (P = NS). There was no significant correlation between AHI and V_E% baseline (r = −0.28) in SCI and AB (n = 30). SCI participants had a greater ventilatory response to an SBCO₂ than AB (0.78 ± 0.42 L/min/mmHg vs. 0.26 ± 0.10 L/min/mmHg, respectively, P < 0.05). Peripheral ventilatory chemoresponsiveness is elevated in individuals with chronic SCI compared to able‐bodied individuals.

Intermittent hypoxia initiated plasticity in humans: A multipronged therapeutic approach to treat sleep apnea and overlapping co-morbidities

Over the past three decades exposure to intermittent hypoxia (IH) has generally been considered a stimulus associated with a number of detrimental outcomes. However, there is sufficient evidence to link IH to many beneficial outcomes but they have largely been ignored, particularly in the field of sleep medicine in the United States. Recent reviews have postulated that this apparent contradiction is related to the severity and duration of exposure to IH; mild forms of IH initiate beneficial outcomes while severe forms of IH are coupled to detrimental consequences. In the present review we explore the role that IH has in initiating respiratory plasticity and the potential this form of plasticity has to mitigate obstructive sleep apnea (OSA) in humans. In taking this approach, we address the possibility that IH could serve as an adjunct therapy coupled with continuous positive airway pressure (CPAP) to treat OSA. Our working hypothesis is that exposure to mild IH leads to respiratory plasticity that manifests in increased stability of the upper airway, which could ultimately reduce the CPAP required to treat OSA. In turn, this reduction could increase CPAP compliance and extend the length of treatment each night, which might improve the magnitude of outcome measures. Improved treatment compliance coupled with the direct effect that IH has on numerous overlapping conditions (i.e. asthma, chronic obstructive pulmonary disease, spinal cord injury) may well lead to substantial improvements that exceed outcomes following treatment with CPAP alone. Overall, this review will consider evidence from the published literature which suggests that IH could serve as an effective multipronged therapeutic approach to treat sleep apnea and its overlapping co-morbidities.

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.

Respiratory motor outputs following unilateral midcervical spinal cord injury in the adult rat

The present study was designed to investigate the impact of midcervical spinal cord injury on respiratory outputs and compare respiratory recovery following high- vs. midcervical spinal injury. A unilateral hemisection (Hx) in the spinal cord at C2 or C4 was performed in adult rats. Respiratory behaviors of unanesthetized animals were measured at normoxic baseline and hypercapnia by whole body plethysmography at 1 day and 1, 2, 4, and 8 wk after spinal injury. C2Hx and C4Hx induced a similar rapid shallow breathing pattern at 1 day postinjury. The respiratory frequency of C4Hx animals gradually returned to normal, but the tidal volume from 1 to 8 wk postinjury remained lower than that of the control animals. Linear regression analyses indicated that the tidal volume recovery was greater in the C4Hx animals than in the C2Hx animals at the baseline, but not at hypercapnia. The bilateral phrenic nerve activity was recorded in anesthetized animals under different respiratory drives at 8–9 wk postinjury. The phrenic burst amplitude ipsilateral to the lesion reduced following both high- and midcervical Hx; however, the ability to increase activity was lower in the C4Hx animals than in the C2Hx animals. When the data were normalized by the maximal inspiratory effort during asphyxia, the phrenic burst amplitude enhanced in the C4Hx animals, but reduced in the C2Hx animals compared with the control animals. These results suggest that respiratory deficits are evident following midcervical Hx, and that respiratory recovery and neuroplasticity of phrenic outputs are different following high- vs. midcervical spinal injury.

Early phrenic motor neuron loss and transient respiratory abnormalities after unilateral cervical spinal cord contusion

Contusion-type cervical spinal cord injury (SCI) is one of the most common forms of SCI observed in patients. In particular, injuries targeting the C3-C5 region affect the pool of phrenic motor neurons (PhMNs) that innervates the diaphragm, resulting in significant and often chronic respiratory dysfunction. Using a previously described rat model of unilateral midcervical C4 contusion with the Infinite Horizon Impactor, we have characterized the early time course of PhMN degeneration and consequent respiratory deficits following injury, as this knowledge is important for designing relevant treatment strategies targeting protection and plasticity of PhMN circuitry. PhMN loss (48% of the ipsilateral pool) occurred almost entirely during the first 24 h post-injury, resulting in persistent phrenic nerve axonal degeneration and denervation at the diaphragm neuromuscular junction (NMJ). Reduced diaphragm compound muscle action potential amplitudes following phrenic nerve stimulation were observed as early as the first day post-injury (30% of pre-injury maximum amplitude), with slow functional improvement over time that was associated with partial reinnervation at the diaphragm NMJ. Consistent with ipsilateral diaphragmatic compromise, the injury resulted in rapid, yet only transient, changes in overall ventilatory parameters measured via whole-body plethysmography, including increased respiratory rate, decreased tidal volume, and decreased peak inspiratory flow. Despite significant ipsilateral PhMN loss, the respiratory system has the capacity to quickly compensate for partially impaired hemidiaphragm function, suggesting that C4 hemicontusion in rats is a model of SCI that manifests subacute respiratory abnormalities. Collectively, these findings demonstrate significant and persistent diaphragm compromise in a clinically relevant model of midcervical contusion SCI; however, the therapeutic window for PhMN protection is restricted to early time points post-injury. On the contrary, preventing loss of innervation by PhMNs and/or inducing plasticity in spared PhMN axons at the diaphragm NMJ are relevant long-term targets.

Tetraplegia is a risk factor for central sleep apnea

Sleep-disordered breathing (SDB) is highly prevalent in patients with spinal cord injury (SCI); the exact mechanism(s) or the predictors of disease are unknown. We hypothesized that patients with cervical SCI (C-SCI) are more susceptible to central apnea than patients with thoracic SCI (T-SCI) or able-bodied controls. Sixteen patients with chronic SCI, level T6 or above (8 C-SCI, 8 T-SCI; age 42.5 ± 15.5 years; body mass index 25.9 ± 4.9 kg/m(2)) and 16 matched controls were studied. The hypocapnic apneic threshold and CO2 reserve were determined using noninvasive ventilation. For participants with spontaneous central apnea, CO2 was administered until central apnea was abolished, and CO2 reserve was measured as the difference in end-tidal CO2 (PetCO2) before and after. Steady-state plant gain (PG) was calculated from PetCO2 and VE ratio during stable sleep. Controller gain (CG) was defined as the ratio of change in VE between control and hypopnea or apnea to the ΔPetCO2. Central SDB was more common in C-SCI than T-SCI (63% vs. 13%, respectively; P < 0.05). Mean CO2 reserve for all participants was narrower in C-SCI than in T-SCI or control group (-0.4 ± 2.9 vs.-2.9 ± 3.3 vs. -3.0 ± 1.2 l·min(-1)·mmHg(-1), respectively; P < 0.05). PG was higher in C-SCI than in T-SCI or control groups (10.5 ± 2.4 vs. 5.9 ± 2.4 vs. 6.3 ± 1.6 mmHg·l(-1)·min(-1), respectively; P < 0.05) and CG was not significantly different. The CO2 reserve was an independent predictor of apnea-hypopnea index. In conclusion, C-SCI had higher rates of central SDB, indicating that tetraplegia is a risk factor for central sleep apnea. Sleep-related hypoventilation may play a significant role in the mechanism of SDB in higher SCI levels.

Respiratory CO₂ response in acute cervical spinal cord injury (CO₂ response in spinal cord injury)

STUDY DESIGN

Retrospective study.

OBJECTIVES

The objective of this study was to compare the CO₂ response of acute tetraplegic cervical spinal cord injury (SCI) patients undergoing mechanical ventilation with a control group of critically ill patients ready for weaning of mechanical ventilation and successfully extubated.

SETTING

This study was conducted at the intensive care unit of a University Hospital in Mallorca, Spain.

METHODS

CO₂ response was studied in 12 acute tetraplegic cervical SCI patients at the C4-C7 level and 22 control patients. The control group patients were consecutively selected from a database of patients with mechanical ventilation and who were successfully extubated after a CO₂ response test. To increase the CO₂ , we used the method of re-inhalation of expired air, and we evaluated the hypercapnic ventilatory response, the change in minute ventilation induced by the increase of partial pressure of arterial carbon dioxide (PaCO₂ ), which measures the whole respiratory system (metabolic control, neuromuscular or ventilatory apparatus), and the hypercapnic drive response, the change in the airway occlusion pressure at 100 ms induced by the increase in PaCO2, which measures the chemosensitivity of the respiratory center.

RESULTS

Cervical SCI patients were younger than the control group patients (26±7 and 62±12 years, respectively; P<0.001). Mean values of the hypercapnic ventilatory response in cervical SCI and control groups were 0.52±0.31 and 0.64±0.33 l min(-1) per mm Hg (P=0.40), respectively, and the hypercapnic drive response was 0.24±0.16 and 0.48±0.23 cm H₂O per mm Hg (P=0.001), respectively.

CONCLUSION

Acute tetraplegic cervical SCI patients had reduced hypercapnic drive response that may contribute to the difficult weaning, without reduction in hypercapnic ventilatory response.

Respiration following spinal cord injury: evidence for human neuroplasticity

Respiratory dysfunction is one of the most devastating consequences of cervical spinal cord injury (SCI) with impaired breathing being a leading cause of morbidity and mortality in this population. However, there is mounting experimental and clinical evidence for moderate spontaneous respiratory recovery, or "plasticity", after some spinal cord injuries. Pre-clinical models of respiratory dysfunction following SCI have demonstrated plasticity at neural and behavioral levels that result in progressive recovery of function. Temporal changes in respiration after human SCI have revealed some functional improvements suggesting plasticity paralleling that seen in experimental models-a concept that has been previously under-appreciated. While the extent of spontaneous recovery remains limited, it is possible that enhancing or facilitating neuroplastic mechanisms may have significant therapeutic potential. The next generation of treatment strategies for SCI and related respiratory dysfunction should aim to optimize these recovery processes of the injured spinal cord for lasting functional restoration.

The impact of spinal cord injury on breathing during sleep

The prevalence of sleep disordered breathing (SDB) following spinal cord injury (SCI) is considerably greater than in the general population. While the literature on this topic is still relatively small, and in some cases contradictory, a few general conclusions can be drawn. First, while both central and obstructive sleep apnea (OSA) has been reported after SCI, OSA appears to be more common. Second, SDB after SCI likely reflects a complex interplay between multiple factors including body mass, lung volume, autonomic function, sleep position, and respiratory neuroplasticity. It is not yet possible to pinpoint a "primary factor" which will predispose an individual with SCI to SDB, and the underlying mechanisms may change during progression from acute to chronic injury. Given the prevalence and potential health implications of SDB in the SCI population, we suggest that additional studies aimed at defining the underlying mechanisms are warranted.

Respiratory function following bilateral mid-cervical contusion injury in the adult rat

The consequences of spinal cord injury (SCI) are often viewed as the result of white matter damage. However, injuries occurring at any spinal level, especially in cervical and lumbar enlargement regions, also entail segmental neuronal loss. Yet, the contributions of gray matter injury and plasticity to functional outcomes are poorly understood. The present study addressed this issue by investigating changes in respiratory function following bilateral C(3)/C(4) contusion injuries at the level of the phrenic motoneuron (PhMN) pool which in the adult rat extends from C(3) to C(5/6) and provides innervation to the diaphragm. Despite extensive white and gray matter pathology associated with two magnitudes of injury severity, ventilation was relatively unaffected during both quiet breathing and respiratory challenge (hypercapnia). On the other hand, bilateral diaphragm EMG recordings revealed that the ability to increase diaphragm activity during respiratory challenge was substantially, and chronically, impaired. This deficit has not been seen following predominantly white matter lesions at higher cervical levels. Thus, the impact of gray matter damage relative to PhMNs and/or interneurons becomes evident during conditions associated with increased respiratory drive. Unaltered ventilatory behavior, despite significant deficits in diaphragm function, suggests compensatory neuroplasticity involving recruitment of other spinal respiratory networks which may entail remodeling of connections. Transynaptic tracing, using pseudorabies virus (PRV), revealed changes in PhMN-related interneuronal labeling rostral to the site of injury, thus offering insight into the potential anatomical reorganization and spinal plasticity following cervical contusion.

Breathing patterns after mid-cervical spinal contusion in rats

Respiratory failure is the leading cause of death after cervical spinal injury. We hypothesized that incomplete cervical spinal injuries would alter respiratory pattern and initiate plasticity in the neural control of breathing. Further, we hypothesized that the severity of cervical spinal contusion would correlate with changes in breathing pattern. Fourteen days after C4-C5 contusions, respiratory frequency and tidal volume were measured in unanesthetized Sprague Dawley rats in a whole body plethysmograph. Phrenic motor output was monitored in the same rats which were anesthetized, vagotomized, paralyzed and ventilated to eliminate and/or control sensory feedback that could alter breathing patterns. The extent of spinal injury was approximated histologically by measurements of the injury-induced cyst area in transverse sections; cysts ranged from 2 to 28% of spinal cross-sectional area, and had a unilateral bias. In unanesthetized rats, the severity of spinal injury correlated negatively with tidal volume (R(2)=0.85; p<0.001) and positively with breathing frequency (R(2)=0.65; p<0.05). Thus, the severity of C4-C5 spinal contusion dictates post-injury breathing pattern. In anesthetized rats, phrenic burst amplitude was decreased on the side of injury, and burst frequency correlated negatively with contusion size (R(2)=0.51; p<0.05). A strong correlation between unanesthetized breathing pattern and the pattern of phrenic bursts in anesthetized, vagotomized and ventilated rats suggests that changes in respiratory motor output after spinal injury reflect, at least in part, intrinsic neural mechanisms of CNS plasticity initiated by injury.

Ventilatory response to hypercapnia in C(5-8) chronic tetraplegia: the effect of posture

OBJECTIVE

To study the effect of posture on the hypercapnic ventilatory responses (HCVR).

DESIGN

Nonrandomized controlled study.

SETTING

Rehabilitation hospital and a pulmonary institute.

PARTICIPANTS

Patients with neurologically stable C(5-8) tetraplegia (n=12) and healthy control subjects (n=7).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Supine and seated forced vital capacity (FVC) and HCVR, and supine and erect blood pressure.

RESULTS

FVC in the sitting position was reduced in patients with tetraplegia (52+/-13% predicted); supine FVC was 21% higher (P=.0005). In the sitting position, HCVR was lower in patients than in controls (0.8+/-0.4 vs 2.46+/-0.3 L/min/mmHg, P<.001). Supine HCVR was not significantly different between the groups. When HCVR was normalized to FVC, there was still a significant difference between patients and controls in the sitting position. Patients with tetraplegia were orthostatic (mean supine blood pressure 91+/-13 mmHg vs mean erect blood pressure 61+/-13 mmHg, respectively, P<.0001). The magnitude of the orthostatism correlated with that of the postural change in HCVR (r=.93, P<.0001).

CONCLUSIONS

Respiratory muscle weakness may contribute to the attenuated HCVR in tetraplegia. However, the observation that supine HCVR is still low even when normalized to FVC suggests a central posture-dependent effect on the HCVR, which may be linked to the postural effect on arterial blood pressure.

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.

Spinal cord injury in neonates alters respiratory motor output via supraspinal mechanisms

Upper cervical spinal cord injury (SCI) alters respiratory output and results in a blunted respiratory response to pH/CO2. Many SCI studies have concentrated on respiratory changes in neural function caudal to injury; however few have examined whether neural plasticity occurs rostral to SCI. Golder et al. (2001a) showed that supraspinal changes occur to alter respiratory output after SCI. Furthermore, Brown et al. (2004) showed that neural receptors change rostral to a thoracic SCI. We hypothesized that SCI in neonates will alter supraspinal output, show a blunted response to pH and alter receptor protein levels in the medulla. On postnatal day 0/1, a C2 SCI surgery was performed. Two days later, neonates were anesthetized and brainstem-spinal cords removed. Respiratory-related activity was recorded using the in vitro brainstem-spinal cord preparation and the superfusate pH was changed (pH 7.2, 7.4 and 7.8). The respiratory-like frequency was significantly reduced in SCI rats indicating supraspinal plasticity. Increasing the pH decreased respiratory-like frequency and peak amplitude in injured and sham controls. Increasing the pH increased burst duration and area in sham controls, whereas in injured rats, the burst duration and area decreased. Western blot analysis demonstrated significant changes in glutamate receptor subunits (NR1, NR2B and GluR2), adenosine receptors (A1, A2A), glutamic acid decarboxylase (65) and neurokinin-1 receptors in medullary tissue ipsilateral and contralateral to injury. These data show that supraspinal plasticity in the respiratory system occurs after SCI in neonate rats. The mechanisms remain unknown, but may involve alterations in receptor proteins involved in neurotransmission.

Respiratory abnormalities resulting from midcervical spinal cord injury and their reversal by serotonin 1A agonists in conscious rats

Respiratory dysfunction after cervical spinal cord injury (SCI) has not been examined experimentally using conscious animals, although clinical SCI most frequently occurs in midcervical segments. Here, we report a C5 hemicontusion SCI model in rats with abnormalities that emulate human post-SCI pathophysiology, including spontaneous recovery processes. Post-C5 SCI rats demonstrated deficits in minute ventilation (Ve) responses to a 7% CO2 challenge that correlated significantly with lesion severities (no injury or 12.5, 25, or 50 mm x 10 g weight drop; New York University impactor; p < 0.001) and ipsilateral motor neuron loss (p = 0.016). Importantly, C5 SCI resulted in at least 4 weeks of respiratory abnormalities that ultimately recovered afterward. Because serotonin is involved in respiration-related neuroplasticity, we investigated the impact of activating 5-HT1A receptors on post-C5 SCI respiratory dysfunction. Treatment with the 5-HT1A agonist 8-hydroxy-2-(di-n-propylmino)tetralin (8-OH DPAT) (250 microg/kg, i.p.) restored hypercapnic Ve at 2 and 4 weeks after injury (i.e., approximately 39.2% increase vs post-SCI baseline; p < or = 0.033). Improvements in hypercapnic Ve response after single administration of 8-OH DPAT were dose dependent and lasted for approximately 4 h(p < or = 0.038 and p < or = 0.024, respectively). Treatment with another 5-HT1A receptor agonist, buspirone (1.5 mg/kg, i.p.), replicated the results, whereas pretreatment with a 5-HT1A-specific antagonist, 4-iodo-N-[2-[4(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzamide (3 mg/kg, i.p.) given 20 min before 8-OH DPAT negated the effect of 8-OH DPAT. These results imply a potential clinical use of 5-HT1A agonists for post-SCI respiratory disorders.

Effect of Spinal Cord Injury on the Respiratory System

There are >200,000 persons living with a spinal cord injury in the United States, with approximately 10,000 new cases of traumatic injury per year. Advances in the care of these patients have significantly reduced acute and long-term mortality rates, although life expectancy remains decreased. This article will review the alterations in respiratory mechanics resulting from a spinal cord injury and will examine the contribution of respiratory complications to morbidity and mortality associated with various types of spinal cord injury.

Ventilatory and respiratory muscle responses to hypercapnia in patients with paraplegia

To evaluate ventilatory and respiratory muscle responses to hypercapnia in patients with paraplegia with paralysis of abdominal muscles, we studied seven patients with complete transection of the midthoracic cord (Th6-Th7) and six normal subjects. Minute ventilation (V E) and mean inspiratory flow responses to hypercapnia were similar in normal subjects and patients with paraplegia, but in the latter, at any given level of end-tidal CO(2) partial pressure (PET(CO(2))), tidal volume (VT) was reduced and frequency was increased. In normal subjects during hypercapnia, end-expiratory transpulmonary pressure (PL) and abdominal volume at end expiration decreased markedly, whereas end-expiratory volume of the rib cage (Vrc,E) remained constant, suggesting progressive recruitment of abdominal muscles. In patients with paraplegia compared to normal subjects the decrease in end-expiratory PL was reduced, and it was associated with a decrease in Vrc,E, suggesting recruitment of rib cage expiratory muscles. For a PET(CO(2)) of 70 mm Hg the estimated expiratory muscle contribution to VT was 10.3 and 28.4% (p < 0.02) in patients with paraplegia and normal subjects, respectively. We conclude that the V E-CO(2) relationship is preserved in patients with paraplegia with the development of a rapid and shallow pattern of breathing. This suggests that expiratory muscle paralysis elicits adaptation of the ventilatory control system similar to that observed in patients with generalized respiratory muscle weakness.

Ventilatory and mouth occlusion pressure responses to hypercapnia in chronic tetraplegia

OBJECTIVE

To compare the ventilatory response to hypercapnia and the mouth occlusion pressure measured at 0.1 second following inspiration (P0.1) in hypercapnia between chronic tetraplegic and normal subjects.

DESIGN

A case-control study with an uneven sample size for the study of clinical disorders.

SETTING

Patients were recruited from the outpatient clinic of a rehabilitation department.

PARTICIPANTS

Seven normal men and 9 men with tetraplegia who had cervical cord injuries (C5-C8), with a mean injury duration of 9.7 yrs.

INTERVENTIONS

Pulmonary function tests were performed during resting, whereas minute ventilation (VE) and P0.1 were measured during CO2 rebreathing.

RESULTS

The maximal voluntary ventilation (MVV), vital capacity (VC), and maximal respiratory muscle strength in the tetraplegic subjects were significantly less than in the normal subjects. Both the ventilatory and P0.1 responses to hypercapnia were significantly reduced in tetraplegic as compared with normal subjects, but the reductions were eliminated by normalizing with maximal ventilatory performance (MVV or VC) and maximal inspiratory muscle strength (PImax), respectively.

CONCLUSIONS

Chronic tetraplegic persons have diminished ventilatory and P0.1 responses to hypercapnia. Respiratory muscle weakness may be a primary factor contributing to the diminished ventilatory response observed in these patients.

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.