Pressures in the Right Ventricle. The pressure pulse curves of the right ventricle and pulmonary artery are shown in the lower portion of Figure 1. These curves are contrasted with the much higher aortic pressure curve shown in the upper portion of the figure. The systolic pressure in the right ventricle of the normal human averages about 25 mm Hg, and the diastolic pressure averages about 0 to 1 mm Hg, values that are only one fifth those for the left ventricle.
Fig1. Pressure pulse contours in the right ventricle, pulmonary artery, and aorta.
Pressures in the Pulmonary Artery. During systole, the pressure in the pulmonary artery is essentially equal to the pressure in the right ventricle, as also shown in Figure 1. However, after the pulmonary valve closes at the end of systole, the ventricular pressure falls precipitously, whereas the pulmonary arterial pressure falls more slowly as blood flows through the capillaries of the lungs.
As shown in Figure 2, the systolic pulmonary arterial pressure normally averages about 25 mm Hg in the human being, the diastolic pulmonary arterial pressure is about 8 mm Hg, and the mean pulmonary arterial pressure is 15 mm Hg.
Fig2. Pressures in the different vessels of the lungs. The red curve denotes arterial pulsations. D, diastolic; M, mean; S, systolic.
Pulmonary Capillary Pressure. The mean pulmonary capillary pressure, as diagrammed in Figure 2, is about 7 mm Hg. The importance of this low capillary pressure is discussed in detail later in the chapter in relation to fluid exchange functions of the pulmonary capillaries.
Left Atrial and Pulmonary Venous Pressures. The mean pressure in the left atrium and the major pulmonary veins averages about 2 mm Hg in the recumbent human being, varying from as low as 1 mm Hg to as high as 5 mm Hg. It usually is not feasible to measure a human being’s left atrial pressure using a direct measuring device because it is difficult to pass a catheter through the heart chambers into the left atrium. However, the left atrial pressure can be estimated with moderate accuracy by measuring the so-called pulmonary wedge pressure. This measurement is achieved by inserting a catheter first through a peripheral vein to the right atrium, then through the right side of the heart and through the pulmonary artery into one of the small branches of the pulmonary artery, finally pushing the catheter until it wedges tightly in the small branch.
The pressure measured through the catheter, called the “wedge pressure,” is about 5 mm Hg. Because all blood f low has been stopped in the small wedged artery, and because the blood vessels extending beyond this artery make a direct connection with the pulmonary capillaries, this wedge pressure is usually only 2 to 3 mm Hg greater than the left atrial pressure. When the left atrial pressure rises to high values, the pulmonary wedge pressure also rises. Therefore, wedge pressure measurements can be used to study changes in pulmonary capillary pressure and left atrial pressure in patients with congestive heart failure.
