Problems of the Chest Bellows

WHAT IS DISEASE OF THE CHEST BELLOWS?

The chest bellows is one of the three major divisions of the respiratory system (the other two are the central nervous system components that control breathing and the lungs themselves). The system and its components were introduced in Section A.

The chest bellows includes all those structures that make up the thoracic cage (muscles, bones, nerves, and connective tissue), plus the diaphragm muscles and the pleural membranes. The variety of different structures accounts for the diversity of diseases that can affect the chest bellows; the more common ones are listed in Table 1.

For example, polio may affect the spinal cord and damage the nerves that innervate the chest wall muscles, leading to respiratory problems. Massive obesity can likewise lead to respiratory difficulty as can severe deformity of the thoracic spine. Although three different conditions, polio, obesity and bone deformity can each interfere with normal movement of the chest bellows and impair breathing. In the most severe cases respiratory failure can result. Space does not permit thorough discussion of all conditions that may impair the chest bellows, but a few of the more common ones will be discussed in this chapter.

WHAT IS GUILLAIN BARRE SYNDROME?

This is a syndrome named after two French neurologists, Georges Guillain and Jean Alexander Barre, that is also known as "ascending paralysis." In GBS, paralysis usually starts in the legs and then ascends up the body to involve the arms and sometimes the respiratory muscles. Generally a rare condition, GBS is thought to be of viral origin and may strike anyone at any time. Several people who received the swine flu vaccine in the mid 1970s developed GBS and paralysis of their respiratory muscles.

Respiratory failure may result from GBS if it involves nerves of the respiratory muscles. Patients with respiratory muscle paralysis may require artificial ventilation until their condition improves. Barring complications, such as pneumonia, recovery usually occurs, although it may take weeks or months.

WHAT IS MYASTHENIA GRAVIS?

A disease that causes muscle weakness, myasthenia gravis is due to abnormal chemical transmission at the "neuromuscular junction," the space between the ends of the nerves and the beginning of the muscle cells. The disease appears due to a decrease in the number of chemical receptors in the muscles. Normally, these receptors are stimulated by a chemical, acetylcholine (ACh), that is released from the nerve endings whenever a muscle is moved. With a decreased number of muscle receptors the released ACh cannot stimulate the muscles normally.

Myasthenia gravis can present in a variety of ways, one of the most common being double vision (from eye muscle weakness) and drooping of the eyelids. Myasthenia can affect muscles anywhere in the body, including the respiratory muscles. Some patients can develop respiratory failure.

Once polio or Guillain Barré Syndrome have set in there is no specific treatment. By contrast, myasthenia gravis can be treated by drugs, thymectomy (removal of thymus gland) and/or a technique known as plasmapheresis ("washing" the blood to remove antibodies thought to affect the muscle receptors).

If respiratory failure occurs, ventilatory support will be required until reversal by treatment. Although patients with myasthenia may suffer complications such as aspiration pneumonia, atelectasis, or pulmonary embolism, the disease itself does not involve the lungs or airways.

WHAT IS KYPHOSCOLIOSIS?

Kyphoscoliosis is a condition of the bones manifested by abnormal curvature of the spine. "Kypho" refers to hunched over and "scoliosis" to curvature of the spine sideways. The kyphoscoliosis patient has a spine that causes him or her to be slightly hunched over and bent to one side. There are many degrees of kyphoscoliosis. The most severe spinal curvatures can lead to lung disease, respiratory failure, and death. Initially, patients with severe kyphoscoliosis may only have decreased lung volumes (restrictive lung disease) and decreased exercise tolerance. Compression of the bottom parts of the lungs due to the bent spine (and compressed thoracic cavity) may lead to recurrent lung infections, ultimately leading to a form of chronic bronchitis. Over many years this can eventually cause respiratory failure. Fortunately, most patients with kyphoscoliosis do not end up with chronic lung disease and respiratory failure.

HOW DOES SEVERE OBESITY LEAD TO TROUBLE BREATHING?

Obesity is a common and widespread condition. Distinction should be made between moderate obesity and so­called morbid or massive obesity, the latter being defined as in excess of 50 pounds overweight. When patients with morbid obesity are studied in the pulmonary laboratory, their routine breathing tests and blood gas analysis are usually normal or near normal.
*However, some massively obese patients do manifest respiratory impairment, for which there are several possible explanations.

Perhaps the simplest explanation is that excess weight of the chest wall caused by the fat makes it difficult to take a deep breath. Also the massive weight

of the abdominal fat makes it difficult to move the diaphragms. However, weight alone cannot be the only explanation for respiratory impairment, since patients equally overweight often have very different breathing test results.

Also contributing to the abnormal test results of some overweight patients is a decreased drive to breathe. This affects obese patients variably. On their own, some obese patients under breathe, but when coached they can increase their breathing. A decreased drive to breath plus excessive weight make up the Pickwickian Syndrome.

Finally, any underlying lung disease (not a disease of the chest bellows), such as bronchitis or emphysema, can certainly be aggravated by excess weight.

In summary, obesity per se may not lead to respiratory impairment at rest. When coupled with problems of decreased central drive to breathe or underlying lung disease, obesity may be a significant and aggravating factor.

[*This is at rest. As a group, morbidly obese patients who are otherwise healthy will always have decreased exercise ability compared to a group of thin, healthy people.]

WHAT DISEASES AFFECT THE DIAPHRAGMS?

The diaphragms are the major muscles of breathing. They sit at the bottom of the thoracic cavity and divide it from the abdominal cavity. The diaphragms are powered by nerves that arise from high up in the spinal cord in the neck region.

Injury to these nerves (called the phrenic nerves) or to the area of the spinal cord from which they arise can paralyze the diaphragms and lead to respiratory failure. Uncommonly a viral infection can affect these nerves and cause diaphragm paralysis.

Often only one diaphragm may be paralyzed, either from prior surgery (which involved the phrenic nerve), trauma, or tumor. Most patients with unilateral paralysis have no major problem with breathing. Bilateral or complete diaphragm paralysis definitely interferes with normal breathing. If only the diaphragms are paralyzed, patients can still breath using their abdominal muscles. However, this requires sitting up; recumbency makes the abdominal muscles ineffective and results in severe shortness of breath. If bilateral diaphragm paralysis is due to spinal cord damage, then all the muscles of breathing (including abdominal muscles) will be paralyzed. This is incompatible with life and artificial means must be used to support ventilation.

An artificial ventilator is one method of supporting patients with complete diaphragm paralysis who cannot use other muscles of breathing). A sophisticated technique called "diaphragm pacing" has also been tried on some patients. This involves attaching a pacing wire to one or both of the phrenic nerves and stimulating them via a battery that is located outside the body. To prevent nerve fatigue, pacing is only done for part of the day. When the pacer is not used, the patient has to go back on the ventilator (if that was required before pacing).

Complete diaphragm paralysis is rare and unilateral diaphragm paralysis does not cause major difficulty. A more common diaphragm problem is compression and immobilization from above (in the thoracic cavity) or below (in the abdomen). The diaphragms may be partly immobilized by many processes, including pleural disease and effusions from above, and obesity, pregnancy, ascites (fluid in the abdomen), and abdominal infection from below. A common problem in patients who have undergone major abdominal surgery is fluid or infection underneath the diaphragms; this can limit diaphragm activity and cause respiratory problems. When coupled with the effects of anesthesia and possible underlying lung disease the abdominal problem may precipitate respiratory failure.

WHAT ARE PLEURAL EFFUSIONS AND PLEURAL DISEASE?

The outside covering of the lungs and inside covering of the chest wall are very thin, transparent membranes called pleural membranes. They provide a smooth surface so the lungs can slide up and down within the chest cavity during breathing. When the membranes become inflamed, thickened, or involved with a disease process (such as cancer) we speak of the patient as having pleural disease. Pleural fluid, also known as pleural effusion, may be one manifestation of pleural disease.

Pleural effusion is an abnormal collection of fluid within the pleural space – actually the "potential" space between the membrane lining the lung and the membrane lining the inside of the thoracic cavity. Normally, these two membranes touch each other as the lungs slide up and down during breathing. Pleural fluid may accumulate between these two membranes, spreading them apart and filling the potential space with fluid; this creates an abnormal shadow on the chest X­ray, as shown in Figure 1. Pleural disease can also occur without any apparent fluid; in such cases the chest X­ray may only show scarring and thickening of the pleural membranes .

Pleural effusions and pleural disease may occur in a wide variety of disorders. The specific cause of a pleural effusion can often be diagnosed by examination of the fluid, obtained via a thin needle inserted into the chest cavity. This procedure is called a thoracentesis. Fluid removed this way is analyzed for several compounds and chemicals, such as protein and glucose.

On occasion, a piece of the pleural membrane itself will have to be removed this is called a pleural biopsy, and is performed with a larger needle than is used for thoracentesis. Pleural biopsy is generally useful only when the physician suspects either tuberculosis or cancer of the pleura. Cancer and TB in the pleural membranes cause specific changes that can be viewed under the microscope. Pleural biopsy is not helpful for other diagnoses because the pleura only shows non­specific changes under the microscope.

FIGURE 1. X­ray appearance of normal lungs. Figure on right shows enlargement of area in box. Between the pleura membranes lining the lung and the inside chest wall is a ''potential'' pleural space. It is called this because, under normal conditions, the two membranes come together. When pleural fluid forms, as shown in Figure 2, the space enlarges and the two membranes move apart.

FIGURE 2. X­ray appearance of patient with small pleural effusion; figure on right shows enlargement of area in box.

HOW DOES PLEURAL DISEASE INTERFERE WITH BREATHING?

Regardless of the underlying disease, pleural effusions and pleural disease may interfere with breathing. This generally occurs in one of three ways:

l. Inflammation of pleural membranes may cause pain. Pleural or pleuritic type pain is worse on inspiration and keeps the patient from taking a deep breath. When due to a viral infection this is commonly called pleurodynia. Pleurisy is a more general term for pain due to pleural inflammation of any cause.

2. Pleural disease and pleural effusion may interfere with breathing even without causing pain. If the membranes are very thickened or scarred, the patient may not be able to take a deep breath. Normally these surfaces are smooth and slide easily over each other; when inflamed they may move not at all or only with difficulty.

3. Large effusions may simply occupy so much of the thoracic cavity that the lung behind the fluid is compressed and the patient does not have a normal amount of room in which to expand the lungs.

Any of these mechanisms may led to a breathing problem or a feeling of shortness of breath.
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