Complex sentence: Biopsy of the lung is usually not indicated unless the pulmonary hypertension is thought to be due to an underlying interstitial lung disease; further, lung biopsies are fraught with risks of bleeding due to the high intrapulmonary blood pressure.
Split into simpler sentences: Biopsy of the lung is usually not indicated unless the pulmonary hypertension is thought to be due to an underlying interstitial lung disease. But lung biopsies are fraught with risks of bleeding due to the high intrapulmonary blood pressure.