Kyphoscoliosis
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.
