III. Interstitial Pulmonary Oedema and its Radiological Diagnosis. A Sign of Pulmonary Venous and Capillary Hypertension

Oedema of the ankles is a traditional physical sign of the increased systemic venous pressure, resulting from embarrassment of the right side of the heart. The high venous pressure, most marked in the dependent parts of the body, causes an increase in the capillary pressure tending to overcome the restraining plasma osmotic pressure, and is an important factor in the excessive transudation of tissue fluid. Clinical oedema of the ankles results when the capacity of the draining lymphatics to drain the excessive capillary transudate becomes exceeded. In an exactly comparable way, pulmonary oedema may be used as a physical sign of a high pulmonary capillary and venous pressure due to embarrassment of the left side of the heart. This is, of course, standard clinical practice, but it is considered that many cases of pulmonary oedema are still not being correctly diagnosed, either by clinical or radiological techniques. Pulmonary oedema fluid may lie either in the alveolar air spaces, or in the connective tissue framework which supports and permeates the human lung. The former distribution, which we can call alveolar pulmonary oedema, presents no real problem of diagnosis. Its diagnosis is essentially clinical and usually patently evident. The second variety of pulmonary oedema, in which the oedema fluid lies only in the interstitial connective tissue of the lung (interstitial pulmonary oedema), does not lend itself, however, to clinical diagnosis. Paul Wood (1956) emphasised that “a raised pulmonary venous pressure per se gives rise to no auscultatory signs whatever”.