Which tests may be performed in the evaluation of extrinsic lung disorder causes of restrictive lung disease?

Updated: Sep 16, 2020
  • Author: Jonathan Robert Caronia, DO; Chief Editor: John J Oppenheimer, MD  more...
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Answer

In nonmuscular diseases of the chest wall, severe kyphoscoliosis produces a restrictive pattern. The TLC is markedly reduced, with relative preservation of the RV. The vital capacity is reduced, and the RV-to-TLC ratio is elevated. Chest wall components are reduced, and inspiratory muscle weakness may also contribute to the restrictive process. Maximal inspiratory and expiratory pressures are modestly decreased in patients with mild disease but are severely reduced in patients with advanced disease.

Hypoxemia is due to a ventilation-perfusion mismatch caused by the underlying atelectasis and shunt.

In neuromuscular diseases, the maximal inspiratory and expiratory mouth pressures vary from normal to severely reduced. When maximal inspiratory pressure falls below 30 cm of water, ventilatory failure commonly ensues.

Patients with chronic muscular diseases have a decreased vital capacity and FRC, but the RV is preserved. TLC is also moderately reduced.   Breathing during sleep is often abnormal in these patients, resulting in nocturnal desaturation during rapid eye movement sleep, secondary to hypoventilation.

The diffusing capacity of lung for carbon monoxide (DLCO) is reduced in all patients with intrinsic lung disorders; however, the severity of this reduction does not correlate well with the stage of the disease. The DLCO is the most sensitive parameter, and findings may be abnormal even when the lung volumes are preserved. A normal DLCO value excludes intrinsic lung disease and indicates a chest wall, pleural, or neuromuscular cause of restrictive lung disease.

Arterial blood gas values at rest may reveal hypoxemia. Arterial oxygen desaturation occurs with exercise, along with an excessive increase in the respiratory rate and a high ratio of dead-space gas volume to tidal gas volume.

Cardiopulmonary exercise testing with measurements of gas exchange and oxygenation is more sensitive, and findings correlate better with lung biopsy but do not help predict the prognosis. A 6-minute walk test with oximetry provides a measure of oxygen requirement and a quantifiable measure of disease progression.


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