byAle jan d roO eli s, R a ul 0 ice r 0, Her milo Del 0 a still 0, and E n r i que Arc e G.If an opaque substance is injected into the superior vena cava above the azygos arch, it diffuses in the same direction as the blood flow. One of the most important causes for the direction of circulation within the thorax is the endothoracic negative pressure.The contrast medium during angiocardiography never flows towards the tributary veins of the superior vena cava (i. e. the azygos vein), because the negative endothoracic pressure opposes it. By making the pressure positive, however, the circulation of the blood may be then arrested. An angiocardiographic study under these circumstances offers an interesting picture of the dynamic phenomena occurring within the thorax. An easy way to produce positive values of the endothoracic pressure is by means of the Valsalva experiment (4).Review ot literature. In 1761 MORGAGNI quoted VALSALVA'S words: "If the glottis be closed after a deep inspiration, and a strenuous and prolonged expiratory effort be then made, such pressure can be exerted upon the heart and intrathoracic vessels that the movement of the blood is temporarily arrested." This fundamental physiologic explanation given by VALSALVA, remains unaltered.As early as 1894, CRIEGERN found that the cardiac outline became smaller during Valsalva's experiment, an observation verified by DIET-LIN in 1906 by fluoroscopic examination; a diminution amounting to 25 % of the cardiac area was observed. This same observation was again made much later by several authors (DAWSON 11).