Respiratory physiology & neurobiology
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Respir Physiol Neurobiol · Nov 2009
ReviewPhrenic nerve stimulation in patients with spinal cord injury.
Phrenic nerve pacing (PNP) is a clinically useful technique to restore inspiratory muscle function in patients with respiratory failure secondary to cervical spinal cord injury. In this review, patient evaluation, equipment, methods of implementation, clinical outcomes, and the complications and side effects of PNP are discussed. Despite considerable technical development, and clinical success, however, current PNP systems have significant limitations. ⋯ Inadequate inspired volume generation may arise secondary to incomplete diaphragm activation, reversed recruitment order of motor units, fiber type conversion resulting in reduced force generating capacity and lack of coincident intercostal muscle activation. A novel method of pacing is under development which involves stimulating spinal cord tracts which synapse with the inspiratory motoneuron pools. This technique results in combined activation of the intercostal muscles and diaphragm in concert and holds promise to provide a more physiologic and effective method of PNP.
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Respir Physiol Neurobiol · Nov 2009
ReviewAutonomic 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. ⋯ 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.
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Phox2b-expressing cells in the parafacial region of the ventral medulla are proposed to play a role in central chemoreception and postnatal survival. Recent findings in the adult rat and neonatal mouse suggest that the Phox2b-immunoreactive (ir) cell cluster in the rostral ventrolateral medulla is composed of glutamatergic neurons and expresses neurokinin 1 receptor (NK1R), indicating that the cluster may be identical to the retrotrapezoid nucleus. ⋯ Our findings suggest that Phox2b-expressing pFRG/Pre-I neurons play a role in respiratory rhythm generation as well as central chemoreception and thus are essential for postnatal survival. In this brief review, we focused on these recent findings and discuss the functional role of pFRG/Pre-I neurons.
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Respir Physiol Neurobiol · May 2009
Editorial ReviewMechanisms of activity-related dyspnea in pulmonary diseases.
Progressive activity-related dyspnea dominates the clinical presentation of patients afflicted by chronic obstructive and restrictive lung diseases. This symptom invariably leads to activity limitation, global skeletal muscle deconditioning and an impoverished quality of life. The effective management of exertional dyspnea remains an elusive goal but our understanding of the nature and mechanisms of this distressing symptom continues to grow. ⋯ Reductionist experimental approaches that attempt to partition, or isolate, the contribution of central and multiple peripheral sensory afferent systems to activity-induced dyspnea have met with limited success. Integrative approaches which explore the possible neurophysiological mechanisms involved in the two dominant qualitative descriptors of activity-related dyspnea in both diseases may prove to be more fruitful. In this review, we present a hypothetical model for exertional dyspnea that is based on current neurophysiological constructs that have been rigorously developed to explain the origins of perceptions of "effort," "air hunger" and the accompanying affective "distress" response.
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Dyspnea, or the uncomfortable awareness of respiratory distress, is a common symptom experienced by most people at some point during their lifetime. It is commonly encountered in individuals with pulmonary disease, such as chronic obstructive pulmonary disease (COPD), but can also be seen in healthy individuals after strenuous exercise, at altitude or in response to psychological stress. Dyspnea is a multifactorial sensation involving the brainstem, cortex, and limbic system, as well as mechanoreceptors, irritant receptors and chemoreceptors. ⋯ They stimulate the respiratory control system in response to hypoxia and/or hypercapnia, and the resultant increase respiratory motor output can be consciously perceived as unpleasant. They also can induce the sensation of dyspnea through an as yet undetermined mechanism-potentially via direct ascending connections to the limbic system and cortex. The goal of this article is to briefly review how changes in blood gases reach conscious awareness and how chemoreceptors are involved in dyspnea.