| Grade | Microscopic Features |
| Potenially Reversable | |
| I | Hypertrophy of the media of muscular pulmonary arteries. Extension of muscle into the wall of pulmonary arterioles. |
| II | Muscle hypertrophy plus proliferation of intimal cells in arterioles and small muscular arteries. |
| III | "Muscle hypertrophy plus subendothelial fibrosis. Eventually, concentric masses of fibrous tissue and reduplicated internal elastic lamina occlude the vascular lumen of arterioles and small muscular arteries. Large elastic arteries show atherosclerosis." |
| Usually Irreversible | |
| IV | "Muscle hypertrophy is less apparent; progressive dilatation of small arteries, especially those near vessels with intimal fibrous occlusion. Plexiform lesions occur." |
| V | Plexiform and angiomatoid lesions plus intra-alveolar hemosiderin-filled macrophages. |
| VI | Necrotizing arteritis with thrombosis. Fibrinoid necrosis of the arterial wall wih a transmural infiltrate of polymorphonuclear leukocytes and eosinophils. |
PULMONARY HYPERTENSION - HEATH-EDWARDS GRADING SYSTEM - Self-explanatory.
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