19: Disorders of the respiratory pump
OUTLINE
Neuromuscular Disease Affecting the Muscles of Respiration, 232
Specific Diseases, 232
Pathophysiology and Clinical Consequences, 233
Diaphragmatic Disease, 235
Diaphragmatic Fatigue, 235
Unilateral Diaphragmatic Paralysis, 235
Bilateral Diaphragmatic Paralysis, 236
Disorders Affecting the Chest Wall, 237
Kyphoscoliosis, 237
Obesity, 238
The chest wall, diaphragm, and related neuromuscular (NM) apparatus moving the chest wall act in concert to translate signals from the ventilatory controller into expansion of the thorax. Together these structures constitute the respiratory pump, an important system that may fail as a result of diseases affecting any of its parts. Because disorders of the respiratory pump include a variety of problems, this discussion is limited to those disorders that are most common and most important clinically: (1) NM disease affecting the muscles of respiration (Guillain-Barré syndrome, myasthenia gravis, poliomyelitis, and amyotrophic lateral sclerosis), (2) diaphragmatic fatigue, (3) diaphragmatic paralysis, and (4) disorders affecting the chest wall (kyphoscoliosis, obesity).
Neuromuscular disease affecting the muscles of respiration
Several NM diseases have the potential for affecting the muscles of respiration. In some cases, the underlying process is acute and generally reversible (e.g., Guillain-Barré syndrome), and the muscles of respiration are transiently affected. In other cases the NM damage is permanent, and any consequences that impair function of the muscles of respiration are chronic and irreversible. This chapter provides brief definitions of some specific neurologic disorders with respiratory sequelae, followed by a discussion of the pathophysiology and clinical consequences of these diseases as they relate to the respiratory system.
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Specific diseases
The major NM diseases that can affect the muscles of respiration are listed in Table 19.1; several are discussed here.
TABLE 19.1
Disorders of the Respiratory Pump
Neuromuscular Diseases |
Chest Wall Diseases |
Guillain-Barré syndrome |
Kyphoscoliosis |
|
|
Myasthenia gravis |
Obesity |
|
|
Poliomyelitis |
Ankylosing spondylitis |
|
|
Postpolio syndrome |
|
|
|
Amyotrophic lateral sclerosis |
|
|
|
Cervical or thoracic spinal cord injury |
|
|
|
Polymyositis |
|
|
|
Muscular dystrophies |
|
|
|
Guillain-Barré syndrome is a heterogeneous disorder with different variants all involving immune injury to peripheral nerves. Acute inflammatory demyelinating polyradiculoneuropathy (AIDP), the most common variant accounting for 85% to 90% of cases, is a disorder characterized by demyelination of peripheral nerves. It is thought to be triggered by exposure to an antigen (typically an infectious agent such as Campylobacter jejuni) resulting in production of an antibody that cross-reacts with similar antigenic determinants (epitopes) on neural tissue or Schwann cells. Patients frequently report a history of a recent viral or bacterial illness followed by development of ascending paralysis and variable sensory symptoms. C. jejuni is the most frequently identified infection, but human immunodeficiency virus (HIV), cytomegalovirus, Epstein-Barr virus, Zika virus, and other infections also have been implicated. Typically, weakness or paralysis starts symmetrically in the lower extremities and progresses or ascends proximally to the upper extremities and trunk. In up to one-third of cases, the disease is more severe, with respiratory muscle weakness or paralysis accompanying the more usual limb and trunk symptoms. Respiratory muscle involvement may progress to respiratory failure, which is usually reversible over the course of weeks to months. In general, the natural history of the disease leads to recovery, although mortality is 3% to 8% and up to 10% of survivors have permanent sequelae.
In myasthenia gravis, patients experience weakness and fatigue of skeletal muscles, most frequently those innervated by cranial nerves, but peripheral (limb) and respiratory muscles also may be affected. The primary abnormality is found at the NM junction, where transmission of impulses from nerve to muscle is impaired by a decreased number of receptors on the muscle for the neurotransmitter acetylcholine and by the presence of antibodies against these receptors. Although myasthenia gravis is a chronic illness, the manifestations often can be controlled by appropriate therapy, and individual episodes of respiratory failure are potentially reversible.
Poliomyelitis is a viral disease in which the poliovirus attacks motor nerve cells of the spinal cord and brainstem. Both the diaphragm and intercostal muscles can be affected, with resulting weakness or paralysis and respiratory failure. Surviving patients usually recover respiratory muscle function, although
some patients have chronic respiratory insufficiency from prior disease. Mass vaccination of the population in developed countries makes new cases rare. In postpolio syndrome, patients develop new or progressive symptoms of weakness that occur decades after the initial episode of poliomyelitis. Involvement typically occurs in muscles originally affected by the disease, so respiratory muscle involvement is more likely in patients who had respiratory failure with their initial disease.
Amyotrophic lateral sclerosis is a degenerative disease of the nervous system that involves both upper and lower motor neurons. Commonly, muscles innervated by either cranial nerves or spinal nerves are affected. Clinically, progressive muscle weakness and wasting develop, eventually leading to profound weakness of respiratory muscles and death. Although the time course of the disease is variable among patients, the natural history is one of progressive and irreversible deterioration. As a result, patients and families must confront the difficult decision of whether to use mechanical ventilation either noninvasively or through a tracheostomy tube when the patient develops respiratory failure, knowing that no treatment will arrest the progressive neurologic deterioration.
Pathophysiology and clinical consequences
Weakness of respiratory muscles is the hallmark of respiratory involvement in the NM diseases. Depending on the specific disease, chest wall (intercostal) muscles, diaphragm, and expiratory muscles of the abdominal wall can be affected to variable extents.
Impairment of inspiratory muscle strength may render patients unable to maintain sufficient minute ventilation for adequate CO2 elimination. In addition, patients often alter their pattern of breathing, taking shallower and more frequent breaths. Although this pattern of breathing may require less muscular effort and be more comfortable, it is also less efficient because a greater proportion of each breath is wasted on ventilating the anatomic dead space (see Chapter 1). Therefore, even if total minute ventilation is maintained, alveolar ventilation (and thus CO2 elimination) is impaired by this altered pattern of breathing.
The respiratory difficulty that develops in patients with NM disease is exacerbated by weakness of expiratory muscles leading to ineffective cough. Recurrent respiratory tract infections, accumulation of secretions, and areas of collapse or atelectasis significantly contribute to the clinical problems seen in these patients. Respiratory failure usually occurs in the context of a respiratory infection and impaired secretion clearance rather than as a result of simple progression of weakness beyond a critical point in the absence of these factors.
Symptoms include dyspnea and anxiety. Patients may also have a feeling of suffocation. Often the presence of generalized muscle weakness severely limits patients’ activity and lessens the degree of dyspnea that would be present if they were capable of more exertion.
Features of NM disease:
1.Altered pattern of breathing (↑ rate, ↓ tidal volume)
2.Ineffective cough
3.Restrictive pattern on pulmonary function tests
4.Decreased maximal inspiratory/expiratory pressures
5.↑ PCO2, often with ↓ PO2
With severe NM disease, pulmonary function tests show a restrictive pattern of impairment. Although muscle weakness is the primary cause of restriction, compliance of the lung and chest wall may be
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secondarily affected, further contributing to the restrictive pattern. The decrease in pulmonary compliance presumably is due to microatelectasis (i.e., multiple areas of alveolar collapse) resulting from the shallow tidal volumes. At the same time, stiffening of various components of the chest wall (e.g., tendons, ligaments, and joints) over time is thought to be responsible for decreased distensibility of the chest wall. Functional residual capacity (FRC) is normal or decreased, depending on how much respiratory system compliance is altered. Total lung capacity (TLC) is decreased primarily due to inspiratory muscle weakness, but changes in respiratory system compliance may also contribute. Residual volume (RV) frequently is increased due to expiratory muscle weakness (Fig. 19.1). The degree of muscle weakness can be quantified by measuring the maximal inspiratory and expiratory pressures the patient is able to generate with maximal inspiratory and expiratory efforts against a closed mouthpiece. Both maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) may be significantly depressed.
FIGURE 19.1 Examples of lung volumes (total lung capacity [TLC] and its
subdivisions) in patients with chest wall and neuromuscular (NM) disease
compared with values in a normal subject. Nonshaded area represents vital capacity
and its subdivisions. ALS, amyotrophic lateral sclerosis; ERV, expiratory reserve
volume; IC, inspiratory capacity; RV, residual volume. Source: (Modified from
Bergofsky, E. H. (1979). Respiratory failure in disorders of the thoracic cage.
American Review of Respiratory Disease, 119, 643–669.)
In the setting of severe muscle weakness, arterial blood gas analysis is most notable for hypercapnia. Hypoxemia due to alveolar hypoventilation and atelectasis also occurs. When hypoventilation is the sole cause of hypoxemia, the alveolar-arterial oxygen difference (AaDO2) is normal. However, complications such as atelectasis, respiratory tract infections, and inadequately cleared secretions may add a component of ventilation-perfusion mismatch or shunt that further depresses PO2 and increases AaDO2.