Effect of blood gases and acid-base disturbances on the pulmonary circulation.

A DISCUSSION OF THE HEMODYNAMICS of any circulation, and that of the pulmonary circulation in particular, must be concerned with three factors: (1) the flow through the circulation, which in the case of the lung is also the cardiac output; (2) the resistance to flow through the vas-cular bed, which is primarily a function of the number and diameter of resistance vessels , and to a lesser degree a function of blood viscosity; and (3) the pressure per-fusing the vascular bed, which in the case of the pulmonary circulation is the pressure drop between pulmonary artery and pulmonary vein. Actually, these three quantities are i n t e rr el a ted, so that if two are known, the third is determined. For example , at any given cardiac Output and any given diameter of pulmonary vessels (i.e., at any given resistance), the difference between pulmonary artery pressure and pulmonary vein pressure (approximately the same as left atrial pressure) is determined. For the sake of simplicity, it is therefore possible to discuss determinants of pulmonary hemodynamics in terms of the two basic factors, namely, (1) pulmonary blood flow or cardiac output and (2) resistance or vessel diameter. From these, the behavior of pulmonary artery pressure can also be predicted. Factors affecting pu 1m on a r y hemody-namics can be classified on this basis as follows: Original work reported done under a grant-in-aid from the United States Public Health Service (HE 08075). I. Factors affecting flow or cardiac output: A. Relation between work load of heart (pressure x flow) and state of myocardium (degree of competence or failure). 1. At any given work load, myocardial stimulation increases cardiac output. 2. For any given state of myocardium, increased work load (right or left heart) causes decreased cardiac output. B. Blood gases, as they affect state of myocardium. 1. Hypoxia: (a) Depresses myocardium directly; (b) Stimulates heart via sym-patho-adrenal system. 2. Hypercarbia: (a) Depresses myocar-dium directly; (b) Stimulates heart via sympatho-adrenal system; (c) Decreases effect of catecholamine hormones on heart. C. Blood volume; increase results in increased venous return and increased cardiac output.1 II. Factors affecting resistance or vessel diameter in lung: A. Factors passively affecting resistance.