step up pattern in restrictive lung disease is result of respiratory muscle weakness
- related: PFT and lung functions
- tags: #literature #pulmonology
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This patient has respiratory muscle weakness on the basis of her chest radiographic findings and PFT results. Her chest radiograph shows bilateral elevated hemidiaphragms, small lung volumes, and subsegmental atelectasis. The findings on her PFTs strongly suggest respiratory muscle weakness (choice B is correct).
Total lung capacity (TLC) is most accurate when measured with plethysmography, which uses Boyle's law to calculate lung volumes. During the TLC maneuver, the patient is instructed to inhale as deeply as possible and hold breath at the end of this maneuver. Those with respiratory muscle weakness will have a TLC that is reduced. The functional residual capacity (FRC) is the volume of air remaining in the lungs after a normal, passive exhalation. The lungs themselves have an elastic recoil that leads them to collapse, while the chest wall has an elastic recoil that leads it to expand (ie, spring outward). These two opposing forces offset one another at FRC. Therefore, the FRC at end expiration should be normal in those with respiratory muscle weakness. The residual volume (RV) is the volume of air that remains in the lungs after an FVC maneuver. With respiratory muscle weakness, the FVC is decreased lower than normal, leading to an abnormally increased RV. This pattern of low TLC, normal FRC, and elevated RV is sometimes referred to as a "step-up" pattern. Patients with respiratory muscle weakness will also have reductions in both FEV1 and FVC with a normal or sometimes elevated FEV1/FVC ratio, as well as an elevated RV/TLC ratio. Abnormalities in negative inspiratory pressure and positive expiratory pressure are also seen in respiratory muscle weakness. The measured DLCO is a measure of pulmonary capillary surface area and should not be affected by respiratory muscle weakness.
It is tempting to attribute this patient's normal DLCO to polycythemia (in which increased hemoglobin concentrations will increase carbon monoxide diffusion) that is offset by interstitial lung disease. Certainly, any interstitial lung disease can reduce DLCO; however, as discussed in the previous paragraph, this patient's imaging and PFTs are not consistent with interstitial lung disease. With interstitial lung disease, one would expect the FRC and RV to be reduced along with the TLC, which is not seen on these PFTs (choice A is incorrect).
The accumulation of rheum in the patient's eyes while she is sleeping reflects eyelid weakness, most likely from myasthenia gravis (a condition associated with respiratory muscle weakness). Here, the eyelid weakness reduces blinking strength, leading to rheum accumulation. Although respiratory muscle weakness may lead to hypercapnia, hypercapnia does not typically manifest as blurry vision (choice C is incorrect).
As mentioned already, the reduced TLC might suggest that the patient has interstitial lung disease; however, with interstitial lung disease, one would expect the FRC and RV to be reduced along with the TLC, which is not seen on these PFTs (choice D is incorrect). This patient had myasthenia gravis diagnosed with the presence of an anti-acetylcholine receptor antibody at assay.1