I. The Nature of Pulmonary Hypertension

Pulmonary hypertension may be defined as an increase in the pressure within the pulmonary vascular bed. It is important to distinguish between pulmonary venous and pulmonary arterial hypertension. The term “pulmonary hypertension” is an over-simplified one, and gives only a limited indication of the state of the pulmonary vascular system. Pulmonary arterial hypertension is said to occur when the pulmonary arterial pressure exceeds the upper limit of normal (30/15 mm Hg measured from the sternal angle), but the average normal pressure is considerably lower, being 16/7 mm Hg (Wood, 1956). Pulmonary venous hypertension exists when the left atrial pressure exceeds the maximum normal of 8 to 10 mm Hg mean pressure above the sternal angle. The pulmonary arterial pressure is maintained by two principle factors—the output of the right ventricle and the resistance offered by the pulmonary vascular system. The latter is influenced by the left atrial resistance and by the arteriolar resistance. The arteriolar resistance is affected by the oxygen tension of the alveolar air, by reversible and organic occlusive arterial disease and by the pulmonary venous pressure. The normal pulmonary vascular bed is highly distensible, the vessels being capable of accommodating a high flow with little increase in resistance (Hickam and Cargill, 1948; Cournand, 1950, 1957), but in the face of extremely high blood flows of over 15 1./minute, the pressures may rise.