Diaphragmatic disease

Although diaphragmatic involvement is a significant component of many of the NM diseases that affect the muscles of respiration, additional etiologic and clinical considerations justify a separate discussion of diaphragmatic disease. First, we consider diaphragmatic fatigue, a potential consequence of disorders affecting other parts of the respiratory system that significantly increase the workload placed on the diaphragm. We then discuss diaphragmatic paralysis, with separate considerations of unilateral and bilateral paralysis, because the causes and clinical manifestations are often quite different.

Diaphragmatic fatigue

Excluding cardiac muscle, the diaphragm is the single muscle used most consistently and repetitively throughout the course of a person’s lifetime. It is well suited for sustained activity and aerobic metabolism, and under normal circumstances the diaphragm does not become fatigued.

However, if the diaphragm is required to perform an excessive amount of work or if its energy supplies are limited, fatigue may develop and may contribute to respiratory dysfunction in certain clinical settings. For example, if a healthy individual repetitively uses the diaphragm to generate 40% or more of its maximal force, fatigue develops and prevents this degree of effort from being sustained indefinitely. For patients with diseases that increase the work of breathing, particularly obstructive lung disease and disorders of the chest wall (described in the section on disorders affecting the chest wall), the diaphragm works at a level much closer to the point of fatigue. When a superimposed acute illness further increases the work of breathing or when an intercurrent problem (e.g., depressed cardiac output, anemia, or hypoxemia) decreases the energy supply available to the diaphragm, diaphragmatic fatigue may contribute to the development of hypoventilation and respiratory failure.

Inefficient diaphragmatic contraction is another factor that may contribute to diaphragmatic fatigue, especially in patients with obstructive lung disease. When the diaphragm is flattened and its fibers are shortened as a result of hyperinflated lungs, the force or negative pleural pressure developed during contraction is less for any given level of diaphragmatic excitation (see Chapter 17). Therefore, a higher degree of stimulation is necessary to generate comparable pressure by the diaphragm, and increased energy consumption results.

Factors contributing to diaphragmatic fatigue:

1.Increased work of breathing

2.Decreased energy supply to the diaphragm

3.Inefficient diaphragmatic contraction

Diaphragmatic fatigue is often difficult to detect because the force generated by the diaphragm cannot be measured conveniently. Ideally, diaphragmatic fatigue is documented by measuring the pressure across the diaphragm (i.e., the difference between abdominal and pleural pressure, called the transdiaphragmatic pressure) during diaphragmatic stimulation or contraction. As an alternative to measurement of transdiaphragmatic pressure, the strength of the inspiratory muscles in general can be assessed by measuring the pressure that a patient can generate with a maximal inspiratory effort against a closed mouthpiece (i.e., MIP). A useful finding on physical examination is the pattern of motion of the abdomen during breathing when the patient is supine. If diaphragmatic contraction is especially weak or absent, pleural pressure falls during inspiration mainly due to contraction of other inspiratory muscles.

The negative pleural pressure is transmitted across the relatively flaccid diaphragm to the abdomen,

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which then moves paradoxically inward during inspiration. Once a patient has reached the point of respiratory failure due to diaphragmatic fatigue, the most important intervention is mechanical ventilatory support, either noninvasive or invasive, while the diaphragm recovers and the acute cause of respiratory failure is treated.

Diaphragmatic weakness can be demonstrated in the supine position by inward motion of the abdomen during inspiration.

Unilateral diaphragmatic paralysis

Paralysis of the diaphragm on one side of the thorax (also called a hemidiaphragm) typically results from disease affecting the ipsilateral phrenic nerve. A particularly common cause of unilateral diaphragmatic paralysis is invasion of the phrenic nerve by malignancy. The underlying tumor is frequently lung cancer that has invaded or metastasized to the mediastinum, and either the primary tumor itself or mediastinal lymph node metastases of tumor invade the phrenic nerve somewhere along its course through the mediastinum. With treatment, some diaphragmatic function may return, but frequently diaphragmatic paralysis resulting from malignancy is irreversible.

Paralysis of the left hemidiaphragm may be seen following cardiac surgery, attributable to either a stretch injury or a cooling injury to the phrenic nerve. The latter type of injury relates to instillation of a cold potassium-rich solution into the pericardium during the procedure to stop cardiac contraction (cold cardioplegia) and allow surgery on a nonbeating heart while circulation is maintained by cardiopulmonary bypass. However, the cardioplegia solution also can cause injury to the left phrenic nerve, leading to diaphragmatic paralysis of variable severity and duration. With changes in surgical and cardiac anesthesia techniques, phrenic nerve injury has become less common following cardiac surgery. When it does occur, function usually recovers within 1 year.

In some patients with unilateral diaphragmatic paralysis, no underlying reason for the paralysis can be identified, and the problem is considered idiopathic. A viral infection affecting the phrenic nerve may be responsible in such cases. Many but not all of these patients recover some function over time.

The possibility of unilateral diaphragmatic paralysis is usually first suggested by a characteristic appearance on the chest radiograph (Fig. 19.2). The affected hemidiaphragm is elevated above its usual position in the absence of any associated lobar atelectasis or other reason for volume loss on the affected side. Standard chest radiographs taken during a full inspiration (to TLC) reveal that the normal hemidiaphragm descends fully during inspiration, whereas the paralyzed hemidiaphragm cannot. Patients may or may not be symptomatic with dyspnea as a result of the paralyzed hemidiaphragm, often depending on the presence or absence of additional underlying lung disease.

FIGURE 19.2 Chest radiograph shows elevation of right hemidiaphragm resulting

from unilateral (right) phrenic nerve paralysis.

Because an elevated diaphragm may result from causes other than diaphragmatic paralysis (e.g., processes below the diaphragm, such as a subphrenic abscess), it is important to confirm objectively that diaphragmatic paralysis is the cause of diaphragmatic elevation. This can be achieved relatively easily by real-time observation of diaphragmatic movement during a “sniff test.” With this technique, a radiologist observes diaphragmatic motion under fluoroscopy while the patient sniffs. During the act of sniffing, which is a rapid inspiratory activity, the normal diaphragm contracts and therefore descends, but the paralyzed diaphragm moves passively (and paradoxically) upward due to rapid development of negative intrathoracic pressure during the sniff. Ultrasonographic assessment of diaphragmatic motion may also be useful but requires considerable operator skill.

Many patients do not have symptoms related to unilateral diaphragmatic paralysis. For appropriate patients who are dyspneic in this setting, a treatment option is diaphragmatic plication; in this surgical procedure, the hemidiaphragm is fixed in a flattened position. Although the hemidiaphragm still does not move, the lung is maintained at a higher volume, and the hemidiaphragm can no longer move paradoxically upward during inspiration, thus improving ventilatory efficiency.

Bilateral diaphragmatic paralysis

Paralysis of both hemidiaphragms has much more serious clinical implications than unilateral paralysis because the patient must depend on the accessory muscles of inspiration to maintain minute ventilation. The causes of bilateral diaphragmatic paralysis are the NM diseases listed in Table 19.1, with bilateral diaphragmatic paralysis being the most severe consequence of respiratory involvement by these disorders.

A characteristic clinical manifestation of bilateral diaphragmatic paralysis is dyspnea that is significantly exacerbated when the patient assumes the recumbent position (i.e., severe orthopnea). In the supine patient, the abdominal contents push on the flaccid diaphragm when the beneficial effects of

gravity on abdominal contents and on lowering the position of the diaphragm are lost. On physical

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examination, patients typically demonstrate paradoxical inward motion of the abdomen during inspiration while they are supine, as described in the discussion on diaphragmatic fatigue. Pulmonary function testing will show the vital capacity measured in the supine position is significantly lower than that measured in the upright position.

Disorders affecting the chest wall

With certain diseases of the chest wall, difficulty in expanding the chest may impede normal inspiration (see Table 19.1). This section focuses on two specific disorders that pose the greatest clinical problems: kyphoscoliosis and obesity.

Kyphoscoliosis

Kyphoscoliosis is an abnormal curvature of the spine in both the anterior (kyphosis) and lateral (scoliosis) directions (Fig. 19.3). This deformity causes the rib cage to become stiffer and more difficult to expand (i.e., chest wall compliance is decreased). Respiratory difficulties are common in patients with significant kyphoscoliosis. In severe cases, chronic respiratory failure ensues. Although some cases of kyphoscoliosis are secondary to NM disease such as poliomyselitis, the majority of severe cases associated with respiratory impairment are idiopathic.

FIGURE 19.3 Chest radiograph of patient with severe kyphoscoliosis. Note

marked spinal curvature and chest wall distortion.

Several pathophysiologic features contribute to respiratory dysfunction in patients with kyphoscoliosis. A crucial underlying problem is the increased work of breathing resulting from the poorly compliant chest wall. To maintain a normal minute ventilation, the work expenditure of the respiratory muscles is greatly increased. In addition, patients decrease their tidal volume and increase respiratory frequency because of difficulty expanding the abnormally stiff chest wall. Consequently, the proportion of dead space ventilation rises, and alveolar ventilation falls unless total ventilation undergoes a compensatory increase. Hence, the increased work of breathing acts together with the altered pattern of breathing to decrease alveolar ventilation and increase PCO2. Chest wall compliance further decreases with age, and respiratory complications of uncorrected kyphoscoliosis become increasingly prevalent as the patient grows older.

Marked distortion of the chest wall causes underventilation of some regions of the lung, microatelectasis, ventilation-perfusion mismatch, and hypoxemia. Thus, two frequent causes of hypoxemia in kyphoscoliosis are hypoventilation and ventilation-perfusion mismatch.

A common complication of severe kyphoscoliosis is pulmonary hypertension and cor pulmonale. Hypoxemia is the most important trigger for the development of pulmonary hypertension. However, increased resistance of the pulmonary vessels also results from compression and possibly from impaired development in regions where the chest wall is especially distorted. Long-standing hypoxemia and pulmonary hypertension eventually result in remodeling of the pulmonary vasculature, and the pulmonary hypertension becomes irreversible, even with correction of hypoxemia.

Features of severe kyphoscoliosis:

1.Increased work of breathing

2.Altered pattern of breathing (↑ rate, ↓ tidal volume)

3.Exertional dyspnea

4.Ventilation-perfusion mismatch

5.↑ PCO2, often with ↓ PO2

6.Pulmonary hypertension, cor pulmonale

7.Restrictive pattern on pulmonary function tests

Exertional dyspnea is probably the most common symptom experienced by patients with severe kyphoscoliosis and respiratory impairment. Unlike patients with NM disease, those with a chest wall deformity such as kyphoscoliosis have normal skeletal muscle strength and therefore are capable of normal levels of exertion. Unlike patients with NM disease, patients with kyphoscoliosis are not subject to the same difficulty in generating an effective cough. Expiratory muscle function is preserved, an effective cough is maintained, and problems with secretions and recurrent respiratory tract infections are not prominent clinical features.

Pulmonary function tests in patients with kyphoscoliosis are notable for a restrictive pattern of impairment with a decrease in TLC. Vital capacity is significantly decreased, whereas RV tends to be relatively preserved. FRC, determined by the outward recoil of the chest wall balanced by the inward recoil of the lung, is decreased because the poorly compliant chest wall has a diminished propensity to recoil outward (see Fig. 19.1).

Severe cases of kyphoscoliosis are generally characterized by hypercapnia and hypoxemia. The latter usually is due to both hypoventilation and ventilation-perfusion mismatch. Chronic respiratory insufficiency and cor pulmonale are the end results of severe kyphoscoliosis, and the level of respiratory

difficulty appears to correlate with the severity of chest wall deformity.

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Surgical therapy aimed at improving or correcting the spinal deformity may be useful in children or adolescents but rarely is effective in adults. Supportive therapy that may be beneficial includes a variety of measures that provide ventilatory assistance to the patient. Treatments with an intermittent positivepressure breathing machine augment tidal volume by delivering positive pressure to the patient during inspiration. The increase in tidal volume improves microatelectasis and lung compliance, affording the patient several hours with decreased work of breathing after each treatment. At night, ventilatory assistance with inspiratory positive pressure delivered via a mask or through a tracheostomy tube allows the respiratory muscles to rest. Nocturnal ventilatory support may provide sufficient rest for the inspiratory muscles to diminish daytime respiratory muscle fatigue. This type of ventilatory support is discussed further in Chapter 30.

Obesity

Obesity has many consequences for health, and respiratory symptoms are one aspect. Obesity can produce a wide spectrum in severity of respiratory impairment, ranging from no symptoms to marked limitation in function. Surprisingly, the degree of obesity does not correlate very well with the presence or severity of respiratory dysfunction. Some patients who are massively obese have little difficulty in comparison with much less obese patients who may be severely limited. This is partially explained by the distribution of body fat: central (abdominal) distribution of fat is more associated with decreased lung function as measured by pulmonary function testing. A full explanation of the discrepancies in symptoms among different patients is based on several factors, including smoking history, underlying lung disease, effects of obesity on the cardiovascular system, and underlying physical deconditioning.

The problem of respiratory impairment in obesity was popularly known for years as the Pickwickian syndrome or obesity-hypoventilation syndrome (OHS). The term Pickwickian was applied because of the description of Joe, the obese character in Charles Dickens’ Pickwick Papers, who had many of the characteristics described in this syndrome. Specifically, Joe had features of massive obesity, hypersomnolence, and peripheral edema, the latter presumably related to cor pulmonale and right ventricular failure. With the accumulation of knowledge about the pathogenesis of respiratory impairment in obesity, the term Pickwickian syndrome has become less meaningful.

Obesity appears to exert two main mechanical effects on the respiratory system. As a result of excess soft tissue, more work is necessary for expansion of the thorax. In addition, the massive accumulation of soft tissue in the abdominal wall exerts pressure on abdominal contents, forcing the diaphragm up to a higher resting position. The high resting position of the diaphragm in obesity is associated with decreased expansion of the lung and closure of small airways and alveoli at the bases. Thus, the dependent regions are hypoventilated relative to their perfusion, and this ventilation-perfusion mismatch contributes to arterial hypoxemia.

Similar to kyphoscoliosis, obesity results in lower tidal volumes and increased wasted or dead space ventilation. To maintain adequate alveolar ventilation, overall minute ventilation must increase in the face of increased work of breathing. Most patients compensate appropriately by increasing their overall minute ventilation, and PCO2 remains normal. Other patients do not compensate fully, and hypercapnia is the consequence.

Exactly what distinguishes these two types of patients is not clearly understood. Most obese patients have increased central respiratory drive in response to the added mechanical load. However, patients with OHS do not develop this increased drive. Patients with OHS also show decreased sensitivity to hypoxemia and hypercapnia. This appears to be an acquired phenomenon and may develop in conjunction with sleep-disordered breathing. In addition, the obesity-associated hormone leptin may influence the development of OHS. After hypercapnia develops, it is much more difficult to assess the innate

FIGURE 19.4 Venn diagram shows hypothetical indication of the way obesity

interacts with obstructive apnea and abnormal respiratory drive. Overlap on left

indicates obese normocapnic patients with obstructive apnea. Overlap on right

indicates hypercapnic obese patients without obstructive apnea. Overlap at center

indicates obese hypercapnic patients with obstructive apnea.

The symptoms that may occur in obese patients can be associated with increased work of breathing (e.g., dyspnea) or sleep apnea syndrome (e.g., daytime somnolence and disordered sleep with profound snoring). Patients may have clinical manifestations related to the complications of pulmonary hypertension, cor pulmonale, and right ventricular failure. These complications are largely related to arterial hypoxemia both during the day and at night, particularly if patients have sleep apnea syndrome.

Pulmonary function tests typically demonstrate a restrictive pattern, with a decrease in TLC. The diaphragm is pushed up in massively obese patients, reducing FRC, which in these patients is much closer to RV; thus, spirometric examination shows the expiratory reserve volume is greatly reduced. This pattern of functional impairment is shown in Fig. 19.1.

In most obese patients, arterial blood gases show a decrease in PO2 and an increase in AaDO2 as a consequence of high diaphragms, airway and alveolar closure, and ventilation-perfusion mismatch. When patients have OHS and PCO2 is elevated, hypoventilation is another factor contributing to hypoxemia. If patients have sleep apnea syndrome, arterial blood gas values become even more deranged at night due to episodes of disordered breathing.

Weight loss is crucial in the treatment of obese patients with respiratory dysfunction. If weight loss is successfully achieved by either diet or bariatric surgery, respiratory problems may resolve in some patients. Unfortunately, attempts at significant and sustained weight loss are often unsuccessful, and some patients who successfully lose weight do not manifest respiratory benefits. Thus, in most patients, other