Chapter 74: Pulmonary Hypertension
What physiologic characteristic of the pulmonary circulation makes it highly adaptable in the face of increased blood flow?
A. Large amount of smooth muscle cells in the vessels
B. Large vascular flow reserve
C. Small cross-sectional surface area
D. High pulmonary vascular resistance to flow
The answer is B. (Hurst’s The Heart, 14th Edition, Chap. 74) Because of its large capacity, its great distensibility, its low resistance to blood flow, and the modest amounts of smooth muscle in the small arteries and arterioles (option A), the pulmonary circulation is not predisposed to become hypertensive. In normal individuals lying supine, the mean driving pressure (ie, the difference between the mean blood pressure in the pulmonary artery and in the left atrium, the transpulmonary gradient) is usually < 10 mm Hg. Because blood flow (cardiac output) is the same in both circulations in the absence of any systemic to pulmonary communications, the pulmonary vascular resistance (PVR) is approximately one-eighth of systemic vascular resistance (option D). The large cross-sectional surface area of the pulmonary circulation (option C), coupled with the distensibility of its thin-walled vessels and the large recruitable vascular reserve, account for these unique characteristics. During exercise, as pulmonary blood flow increases, new regions of the pulmonary vascular bed are open, and existing vasculature dilates (option B); accordingly, the pulmonary circulation is capable of accommodating a fourfold or greater increase in resting blood flow with virtually no change in pulmonary artery pressure, with a concomitant decrease in PVR.
A mother brings her 9-week-old infant to clinic for a regular checkup. She notes that he has not been feeding well and gets tired easily. On physical examination, a continuous murmur is auscultated consistent with a patent ductus arteriosus (PDA). What pathophysiologic mechanism is responsible for this disease manifestation?
A. Pulmonary pressure drops significantly lower than systemic blood pressure after birth
B. Pulmonary pressure fails to rise above systemic blood pressure after birth
C. Pulmonary pressure is nearly equal to systemic blood pressure at birth
D. Pulmonary pressure fails to drop below systemic blood pressure after birth
The answer is D. (Hurst’s The Heart, 14th Edition, Chap. 74) Immediately before birth, pulmonary and systemic arterial blood pressures are near equal (option C) and approximately 70/40 mm Hg, with a mean of 50 mm Hg. Immediately after birth, with closure of the ductus ...