[Polycythemia in patients with chronic respiratory insufficiency].

Tissue hypoxia is the main stimulus of erythropoietin production. Erythropoietin stimulates erythropoiesis by acting on medullary stem cells. Unlike healthy subjects adjusting to high altitude, patients with chronic respiratory failure do not always develop polycythaemia. In those who do, the physiopathology of polycythaemia remains unknown. It seems to be due to insufficient action more than to insufficient production of erythropoietin. Most authors agree that polycythaemia is harmful when the haematocrit exceeds 60%. The most commonly used corrective method nowadays is phlebotomy. Blood withdrawal is effective on clinical symptoms and on pulmonary arterial hypertension, but not on blood gases and ventilatory parameters at rest or during exercise.