This week in Pulmonary Report, Dr. Ludwig presented the case of a 68 year old man with chronic exertional dyspnea and abnormal PFTs. Her evaluation (which is still ongoing!) tackled a a high-yield and challenging topic – what happens when your patient has restrictive PFTs but doesn’t have parenchymal disease to explain it?
The broad categories of restrictive lung disease can be remembered with the mnemonic PAINT:
In our patient’s case, an HRCT was performed and did not reveal clear parenchymal/pleural causes of restriction, leading us down the “extra-parenchymal” pathway:
Slide: extra-parenchymal causes of restrictive lung dz
Several maneuvers are available to aid in the differential diagnosis of a restrictive PFT
Supine and upright VC may suggest neuromuscular causes of restrictive lung disease.
- Normal lung function – decrease of 3-8% from upright to supine
- Significant diaphragm dysfunction – >15%
Chest wall and diaphragm mechanics
MVV (or maximal voluntary ventilation) wherein patients are asked to take rapid deep breaths for 12 seconds, is demonstrated in the video below:
MVVpred = FEV1 x ~35-40 (lower values suggestive of neuromuscular weakness)
Maximum inspiratory/expiratory pressure (MIP and MEP) are also decreased in neuromuscular disease:
Pre (upright) and post (supine) flow-volume loop
Pre (upright) and post (supine) vital capacity
MIP and MEP
Diaphragm function may also be evaluated using other modalities:
Diaphragm ultrasound for excursion and fractional thickening
Obtaining a diaphragmatic ultrasound
Thickening fraction: [(thickness at end-inspiration—thickness at end-expiration)/thickness at end-expiration]. <20% is indicative of diaphragmatic weakness
Normal diaphragm excursion: 6cm (female) 7cm (male) during deep breathing
Electromyography of diaphragm to assess for innervation
- Using esophageal or surface electrodes below lower frontal/dorsal ribs
- Negative conduction does not distinguish between neuropathic/myopathic causes and may be followed up with nerve stimulation test
Fluoroscopic sniff test can be used to detect unilateral pathology
- Poor sensitivity in bilateral paralysis; accessory muscle use may cause upward displacement of ribs creating appearance of downward displacement of diaphragm
- Sensitivity in unilateral paralysis is ~90%
Finally, some management considerations for respiratory symptoms in the patient with neuromuscular disease:
Consider blood gas testing for hypercapnia when FVC <40% pred
Thanks Amy!
