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Kyphoscoliosis has long been recognized as a cause of cardiorespiratory failure. Only in recent years, however, has the combination of clinical picture, physiologic measurements, and anatomic observations at autopsy clarified the natural history of the cardiorespiratory disorder.

Unless there is independent lung disease, such as bronchitis or emphysema, only patients with severe spinal deformities are candidates for cardiorespiratory failure. Subjects with mild deformities are consistently asymptomatic. In contrast, those with severe degrees of deformity, particularly if considerable dwarfing has occurred, are often restricted in their activities by dyspnea on exertion. They are most prone to cardiorespiratory failure if an upper respiratory infection should supervene. From the point of view of disability and the likelihood of cardiorespiratory failure, the nature of the deformity (i.e., kyphosis, scoliosis, or both) is unimportant when compared with the severity of the deformity and dwarfing.

One approach to classifying individuals with kyphoscoliosis is on the basis of lung volumes. The more normal the total lung capacity, vital capacity, and tidal volume, the more the subject tends to remain asymptomatic. In those with severe reduction in lung volumes, the stage is set for cor pulmonale.
Estimates of the work of breathing, using pressure- volume loops, show an inordinate work load (and energy expenditure) attributable to the severe limitation of distensibility of the chest wall, which produces markedly reduced compliance. As a consequence of the high cost of breathing, the individual adopts a pattern of rapid, shallow breathing. Although this pattern is economical in terms of the work and energy required, it sacrifices alveolar ventilation for the sake of deadspace ventilation. The resultant alveolar hypoventilation brings about arterial hypoxemia, hypercapnia, and respiratory acidosis by hyperventilating the conducting airways and hypoventilating the alveoli. Thus, whereas individuals with asymptomatic kyphoscoliosis consistently manifest normal arterial blood gases, those with severe kyphoscoliosis often have cyanosis and show not only arterial hypoxemia but also hypercapnia. Between these two extremes are patients who remain breathless on exertion and whose arterial blood gases hover at the brink of important hypoxemia and hypercapnia. They are easily toppled into a state of cardiorespiratory failure by a bout of bronchitis or pneumonia.
In asymptomatic persons, the pulmonary arterial pressure is normal at rest and increases to clinically insignificant levels during exercise. In contrast, the pulmonary arterial pressure in those with severe kyphoscoliosis not only may be high at rest but also increases precipitously during modest exercise. The basis for this pulmonary hypertension is generally twofold: (1) a restricted pulmonary vascular bed caused by the compressing and distorting effects of the deformity on the lungs and on the pulmonary vasculature and (2) the pulmonary pressor effects of hypoxia. These two effects are most marked during exercise because of the increase in pulmonary blood flow into the restricted vascular bed and the pulmonary vasoconstriction elicited by the exercise-induced hypoxemia. The patients show enlargement of the right ventricle at autopsy. During an upper respiratory infection, the pulmonary pressor effects of the arterial hypoxemia may be sufficiently severe to increase pulmonary arterial pressure to very high levels to precipitate right ventricular failure.
In patients in whom chronic alveolar hypoventilation has caused sustained pulmonary hypertension, hypercapnia consistently accompanies arterial hypoxemia.
Hypercapnia contributes to pulmonary hypertension by way of the respiratory acidosis that it causes because acidosis acts synergistically with hypoxia in causing pulmonary vasoconstriction. However, hypercapnia exerts its predominant effects on the central nervous system rather than on the heart or circulation. In individuals with kyphoscoliosis who have chronic hypercapnia, there is generally no clinical manifestation of the hypercapnia per se. Ventilatory response to inhaled carbon dioxide is depressed compared with that of asymptomatic or individuals with kyphoscoliosis who do not have hypercapnia, reflecting impaired responsiveness to the major chemical stimulus to breathing. As a corollary, greater reliance is placed on the hypoxic drive via the peripheral chemoreceptors. But if a person with kyphoscoliosis develops acute hypercapnia during an upper respiratory infection or exaggerates the preexisting degree of hypercapnia, he or she may manifest personality changes, become unresponsive to conventional stimuli, and lapse into a coma. Accompanying these clinical disorders are cerebral vasodilation, cerebral edema, and an increase in cerebrospinal fluid pressure. The increase in intracranial pressure may be so large as to cause choking of the optic discs, simulating a brain tumor.
All of the disturbances in uncomplicated kyphoscoliosis are greatly exaggerated by intrinsic lung disease. Therefore, smoking and its attendant bronchitis increase the risk of respiratory insufficiency in individuals with kyphoscoliosis. Pneumonia may be disastrous.
From these observations, it is possible to reconstruct the pathogenesis of alveolar hypoventilation and cor pulmonale in individuals with kyphoscoliosis. The sequence begins with severe thoracic deformity, reducing the compliance of the thoracic cage and lung expansion. The work and energy cost of breathing are thus greatly increased. To minimize this work, the patient unconsciously adopts a pattern of rapid, shallow breathing, which results in chronic alveolar hypoventilation. Not only do the small, encased lungs contribute to the increased work of breathing, but they also limit the capacity and distensibility of the pulmonary vascular bed. Pulmonary arterial hypertension is caused by a disproportion between the level of pulmonary blood flow—which is normal for the subject’s metabolism— and the restricted vascular bed. After arterial hypoxemia is corrected, polycythemia, hypervolemia, and an increase in cardiac output help to sustain the pulmonary hypertension. The end result of the chronic pulmonary hypertension is enlargement of the right ventricle (cor pulmonale). In this situation, any additional mechanism for pulmonary hypertension, particularly an upper respiratory infection, may precipitate heart failure.

Hypercapnia goes hand in hand with arterial hypoxemia. This is generally well tolerated unless alveolar hypoventilation is acutely intensified, so that carbon dioxide elimination is further impaired. The acute increase in arterial Pco2 may evoke serious derangements in the central nervous system as well as contribute to the pulmonary hypertension and right ventricular failure.
Treatment of cardiorespiratory failure is directed toward reversing the pathogenetic sequence. In this emergency, generally precipitated by an upper respiratory infection, assisted ventilation may be required in conjunction with slightly enriched oxygen mixtures (25%-40%) to achieve tolerable levels of blood gases. The ventilatory insensitivity of the chronically hypercapnic patient to an increase in arterial Pco2, as well as his or her reliance on hypoxic stimulation of the peripheral chemoreceptors for an important part of the ventilatory drive, imposes a need for caution against using excessively high oxygen mixtures. Respiratory depressants are also hazardous because they may cause breathing to stop completely. Antibiotics and supportive measures usually suffice to tide the patient over the crisis brought on by acute respiratory infection. The goal of treatment is to restore the patient to the clinical state that existed before the acute episode. An individual with kyphoscoliosis who was dyspneic on exertion before an acute episode of cardiorespiratory failure can be expected to return to that condition after the crisis has passed. For many patients who have severe kyphoscoliosis, modest arterial hypoxemia and slight hypercapnia may remain. However, it is remarkable how successful adequate therapy can be in restoring the patient to the precrisis state of health.