DALHOUSIE MEDICAL JOURNAL 9 The Pathophysiology Of Pulmonary Hypertension

For the student in the early years of medicine, no general topic points out his inadequate knowledge more than does pulmonary physiology and function. At best, any acquaintance with the topic is in knowing a few isolated but ill-understood terms, and this poor background works in a vicious circle to ensure that this state will continue. Part of the reason for his lack of knowledge, no doubt, lies in the fact that much of the available data is supported by diverse and hard to prove theories, and the budding stud-ent is soon as confused as are many of his more senior colleagues. When I first thought of the topic of pulmonary hypertension for this paper, I did so because I felt it would enable me to correlate some of the aspects of cardiac and of pulmonary function; however, I must admit that though I had heard the term used on many occasions, I neither understood its full meaning, nor was I aware, to any extent, of its clinical significance. I shall attempt to cover the topic by defining my terms and giving a classification which can be easily followed, then commenting on some of the general features of pulmonary hypertension, some evidence of its pathological sequelae, and some of the hemodynamics which accompany certain specific disease processes. The term pulmonary hypertension is commonly used to denote arterial hyper-tension, but the importance of pulmonary venous hypertension must not be forgotten, and some authors suggest qualifying pulmonary hypertension with the appropriate adjective, arterial or venous. By definition, pulmonary arterial hypertension exists when the pulmonary artery pressure exceeds the maximum normal limit, defined by Wood as 30/15 mm. of mercury, measured from the sternal angle, the maximum normal mean pressure being 22 mm. of mercury. The main factors maintaining pulmonary arterial pressure are right ventricular output, and resistance to flow offered by the pulmonary vascular bed. CLASSIFICATION 1. Hyperdynamic—Increased pulmonary flow —Low pulmonary resistance 2. Obstructive—Increased pulmonary resistance —Low pulmonary flo