Effect of heart disease on the pulmonary circulation
The pulmonary circulation differs from other vascular beds in at least three notable ways. Since it is a relatively low-pressure system the effects of gravity are prominent; it must transmit the entire cardiac output each minute and, most importantly, the patency of the pulmonary capillaries is influenced by the pressure within the surrounding alveolar space.
The classic model divides the lung into three zones.1 In the first zone, the apical zone of an upright subject, the alveolar pressure exceeds both pulmonary arterial and left atrial pressures so that there is no pulmonary blood flow. In the middle zone, pulmonary arterial pressure exceeds alveolar pressure, but flow is not continuous because the alveolar pressure exceeds the left atrial pressure during some part of the respiratory cycle. Pulmonary blood flow here depends on the difference between the left atrial and intraalveolar pressures. In the third zone, which is gravitationally dependent, both pulmonary arterial and left atrial pressures exceed alveolar pressure throughout the respiratory cycle and pulmonary blood flow continues without interruption, governed by the difference between the two intravascular pressures and uninfluenced during spontaneous ventilation by the intraalveolar pressure.
The model is a simple one and makes no allowance for pulmonary vasomotor tone, but it does provide a suitable framework on which to consider at least some of the effects of cardiac disease on the pulmonary circulation. These can be discussed under four headings: (1) transitory or sustained pulmonary venous hypertension; (2) increase in pulmonary blood flow; (3) decrease in pulmonary blood . . .