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Low DLCO (36%), shortness of breath, oxygen desaturation, all else pretty normal?

After complaining for years about exercise induced shortness of breath and after being prescribed inhalers which did no good, HMO performed a Pulmonary Function Test (PFT) with a gas diffusion component.  Previous tests did not perform the single breath carbon-monoxide test.  My oxygen saturation is normally in the low 90's.  It drops into the mid 80's when walking on flat surface at sea level at a reasonable walking pace.  Walking uphill, especially at altitude, taxes my breathing heavily and cause a cramp-like sensation in the upper abdomen.  After a short period of rest, oxygen saturation returns to my normal and the cramping sensation goes away.

PFT calculated DLCO at 41% and a second PFT calculated DLCO at 36% of predicted value.  Otherwise spirometry and lung volumes are normal.   COPD, interstitial lung disease, etc. have been ruled out through CT scans, Xrays and a triple lung wedge biopsy.  Right heart catherization was normal with a pulmonary artery pressure of 25/10.  Echocardiograms showed normal heart function, with 62% ejection fraction.  A small shunt estimated at 5.15% was seen on a nuclear medicine study.  However, even though bubble saline echocardiogram showed small shunt at rest, that shunt did not increase with exertion. CT angiogram showed no parenchymal disease nor evidence for a pulmonary embolus.  Vascular specialist said no evidence of vascular disease after reviewing CPET (cardiopulmonary exercise test).  CPET showed a reduced VO2 (doc said it was about 80% of normal for someone my age), but a normal anaerobic threshold, a reduced breathing reserve, and elevated Ve/VO2 and Ve/VCO2.  Vd/Vt was normal at rest with expected reduction with exercise, but dead space was lower than expected with exercise.  Blood tests have all been normal, including those looking for emphysema, sickle cell and the like.  Hemoglobin counts are normal.  

I have tried supplemental oxygen, but it appears to have little to no effect.

Long post I know, but case is odd.  Would appreciate if anybody has seen something like this or has some other insight to post back.

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