SECTION 111 THE CARDIONASCULAR SYSTEM Aortie Stenosis A 64-year-old man,who experienced chest pain on exertion,was examined hy a cardiologist.When the physician listened to the patient's chest,he heard a loud systolic rurmur that was characteristic of aortic stenosis (narrowing of the aortic valve orifice).The physician advised the patient to have a cardiac catheterization, including coronary angiography. To assess the extent of the aortic stenosis,the cardiologist first used a catheter that had an end opening.As the catheter was advanced through the aorta to a location near the aortic valve,the maximun aortic pressure during each ventricular systole was 125 m Hg.The catheter was then removed and a different catheter inserted.which had an opening on the side of the catheter and near the tip.Whem the cardiologist advanced this side-hole catheter to the same position that had been reached by the preceding catheter,the maximum aortic pressure during ventricular systole was only 100 mm Hg. 1.Assuming that the patient's cardiovascular status was identical at the times that the pressure measurements were made with the end-hole and side-hole catheters. what do you estinate was the maximun velocity of blood flo through the aortic orifice? To assess the coronary circulation in this patient,the cardiologist injected a radiopaque dye into each of the major coronary arteries.On injection of dye into the left circumflex coronary artery.fluoroscopy disclosed an arteriosclerotic lesion about 2 cn distal to the origin of that vessel.The lesion was about I cm in length,and the radius of the vessel lumen appeared to he reduced to about one half of normal.The mean arterial pressure (over the entire cardiac cycle)in the norml vessel proxinal to the lesion was 90 mm Hg.When the catheter was advanced beyond the atherosclerotic lesion,the mean pressure was not appreciably different
SECTION III THE CARDIOVASCULAR SYSTEM Aortic Stenosis A 64-year-old man, who experienced chest pain on exertion, was examined by a cardiologist. When the physician listened to the patient's chest, he heard a loud systolic murmur that was characteristic of aortic stenosis (narrowing of the aortic valve orifice). The physician advised the patient to have a cardiac catheterization, including coronary angiography. To assess the extent of the aortic stenosis, the cardiologist first used a catheter that had an end opening. As the catheter was advanced through the aorta to a location near the aortic valve, the maximum aortic pressure during each ventricular systole was 125 mm Hg. The catheter was then removed and a different catheter inserted, which had an opening on the side of the catheter and near the tip. When the cardiologist advanced this side-hole catheter to the same position that had been reached by the preceding catheter, the maximum aortic pressure during ventricular systole was only 100 mm Hg. 1. Assuming that the patient's cardiovascular status was identical at the times that the pressure measurements were made with the end-hole and side-hole catheters, what do you estimate was the maximum velocity of blood flow through the aortic orifice? To assess the coronary circulation in this patient, the cardiologist injected a radiopaque dye into each of the major coronary arteries. On injection of dye into the left circumflex coronary artery, fluoroscopy disclosed an arteriosclerotic lesion about 2 cm distal to the origin of that vessel. The lesion was about 1 cm in length, and the radius of the vessel lumen appeared to be reduced to about one half of normal. The mean arterial pressure (over the entire cardiac cycle) in the normal vessel proximal to the lesion was 90 mm Hg. When the catheter was advanced beyond the atherosclerotic lesion, the mean pressure was not appreciably different
from that measured proximally:it was only about 1 m Hg less than the proximal pressure. 2.Explain why the arterioselerotic lesion,which reduced the radius of the vessel lumem by about one half,did not cause a substantial pressure drop in the artery distal to the lesion.For computational purposes,assune that (1)the resistance of the normal segments of the coronary artery is such that the pressure drop per centimeter length of vessel equals 0.1 m Hg at the average,normal flow rate,and(2)Poiseuille's law applies to the relations hetween pressure and flow in both the normal and narrow segments of the artery. 3.What would be the pressure drop across the narrow segment if the radius of that segment were reduced to one fourth,rather than to oee half.of normal? The same day that the cardiologist catheterized this adult patient with aortic stenosis,a baby was also catheterized.The following hemodynanic data was obtained on the two patients: 4.What were the total peripheral resistances of the adult and the bahy? 5.Explain the physical basis for the greater total peripheral resistance in the baby than in the adult. ANSVER 1.A catheter with an end-bole that faces upstream records the total pressure in a flowing strean:that is,it records the sun of a dynanic pressure (Pdy)corpoment and a statie pressure (Pst)component.Hoever.a side-hole catheter will record only Pst.Hence,the difference (25 m Hg)in the pressures recorded by the two types of catheters will equal the dynanic pressure component,Pdy.This pressure compoment reflects the influence of the kinetic energy of the noving fluid.Pdy equals pv2/2 (or 33,250 dynes/cn2),shere p is the density of the blood (about 1.06),and v is the velocity of the flowing blood.Hence the peak velocity of the blood flowing through the aortic valve orifice must have been about 258 cw/sec
from that measured proximally; it was only about 1 mm Hg less than the proximal pressure. 2. Explain why the arteriosclerotic lesion, which reduced the radius of the vessel lumen by about one half, did not cause a substantial pressure drop in the artery distal to the lesion. For computational purposes, assume that (1) the resistance of the normal segments of the coronary artery is such that the pressure drop per centimeter length of vessel equals 0.1 mm Hg at the average, normal flow rate, and (2) Poiseuille's law applies to the relations between pressure and flow in both the normal and narrow segments of the artery. 3. What would be the pressure drop across the narrow segment if the radius of that segment were reduced to one fourth, rather than to one half, of normal? The same day that the cardiologist catheterized this adult patient with aortic stenosis, a baby was also catheterized. The following hemodynamic data was obtained on the two patients: 4. What were the total peripheral resistances of the adult and the baby? 5. Explain the physical basis for the greater total peripheral resistance in the baby than in the adult. ANSWER 1. A catheter with an end-hole that faces upstream records the total pressure in a flowing stream; that is, it records the sum of a dynamic pressure (Pdy) component and a static pressure (Pst) component. However, a side-hole catheter will record only Pst. Hence, the difference (25 mm Hg) in the pressures recorded by the two types of catheters will equal the dynamic pressure component, Pdy. This pressure component reflects the influence of the kinetic energy of the moving fluid. Pdy equals ρv2/2 (or 33,250 dynes/cm2), where ρ is the density of the blood (about 1.06), and ν is the velocity of the flowing blood. Hence the peak velocity of the blood flowing through the aortic valve orifice must have been about 258 cm/sec
2.Generally,in vascular beds,including the coroeary vascular bed,the resistance per unit length is normally very small in the major distributing arteries, but it is substantial in the small 'resistance"vessels.Bence in a nornal left circunflex coronary artery.the pressure drop per cm length along the vessel is negligible.Let us assume that the normal drop in pressure per unit length is 0.1 aHg per cm of vessel length at the pormal rate of blood flow through the vessel. Because resistance (R)equals pressure drop (AP)divided by flow (Q).the pressure drop per unit length is proportional to the resistance per unit length of vessel. Fron Poiseuille's law (which vill yield a crude estimate of the relations anong AP. Q.and vessel dinensions),we would expect that a reduction in radius (r)to one half of normal would produce a sixteenfold increase in resistance,and hence in AP (hecause R is inversely proportional to r4).Thus the pressure drop across the narrowed arterial segment would be 16 x 0.1,or 1.6,mm Hg.Therefore,such a restriction in the arterial lumen would be expected to have only a negligible effect on blood flow to the myocardium supplied by that vessel. 3.If the radius of the atherosclerotic segnent were reduced to one-fourth. rather than to one-half,of normal,the consequence would he mach more pronounced. Again,if we assure that Poiseuille's law applies,a reduction in radius to one-fourth of normal would increase the resistance by a factor of 256 (i.e.,44). Thus if the flow were unchanged from the preceding exanple,the pressure drop across the narrowed segment would now be 256 x 0.01,or 25.6,mn Hg:that is,the downstream pressure would be 25.6 m Hg less than the upstrean pressure.This dininished pressure distal to the lesion would be the pressure available for perfusing the myocardial vascular bed supplied by the left circumflex coronary artery.Hence,such a vascular lesion would be associated with a redaced blood supply to the perfused myocardium Furthermore,slight additional narrowings of the affected vascular segment would evoke disproportiomately greater curtailments of blood flow. 4.The total peripheral resistance (TPR)of the adult was (95-5)/5 18 mm Hg/L/nin,and that of the baby was (95-5)/1 "90 m Hg/L/min
2. Generally, in vascular beds, including the coronary vascular bed, the resistance per unit length is normally very small in the major distributing arteries, but it is substantial in the small "resistance" vessels. Hence in a normal left circumflex coronary artery, the pressure drop per cm length along the vessel is negligible. Let us assume that the normal drop in pressure per unit length is 0.1 mm Hg per cm of vessel length at the normal rate of blood flow through the vessel. Because resistance (R) equals pressure drop (∆P) divided by flow (Q), the pressure drop per unit length is proportional to the resistance per unit length of vessel. From Poiseuille's law (which will yield a crude estimate of the relations among ∆P, Q, and vessel dimensions), we would expect that a reduction in radius (r) to one half of normal would produce a sixteenfold increase in resistance, and hence in ∆P (because R is inversely proportional to r4). Thus the pressure drop across the narrowed arterial segment would be 16 x 0.1, or 1.6, mm Hg. Therefore, such a restriction in the arterial lumen would be expected to have only a negligible effect on blood flow to the myocardium supplied by that vessel. 3. If the radius of the atherosclerotic segment were reduced to one-fourth, rather than to one-half, of normal, the consequence would be much more pronounced. Again, if we assume that Poiseuille's law applies, a reduction in radius to one-fourth of normal would increase the resistance by a factor of 256 (i.e., 44). Thus if the flow were unchanged from the preceding example, the pressure drop across the narrowed segment would now be 256 x 0.01, or 25.6, mm Hg; that is, the downstream pressure would be 25.6 mm Hg less than the upstream pressure. This diminished pressure distal to the lesion would be the pressure available for perfusing the myocardial vascular bed supplied by the left circumflex coronary artery. Hence, such a vascular lesion would be associated with a reduced blood supply to the perfused myocardium. Furthermore, slight additional narrowings of the affected vascular segment would evoke disproportionately greater curtailments of blood flow. 4. The total peripheral resistance (TPR) of the adult was (95-5)/5 = 18 mm Hg/L/min, and that of the baby was (95-5)/1 = 90 mm Hg/L/min
5.The TPR of the baby is so much greater tham that of the adult because the haby has a much smaller mumher of parallel resistance vessels (mainly arterioles) than does the adult.Therefore the same pressure gradient (90 m Hg)from the aorta to the right atriun is able to foree a blood flow of only 1 L/min through the systemic resistance vessels in the haby,whereas that same pressure gradient forces 5 L/min to flow through the much larger mumber of parallel resistance vessels in the adult
5. The TPR of the baby is so much greater than that of the adult because the baby has a much smaller number of parallel resistance vessels (mainly arterioles) than does the adult. Therefore the same pressure gradient (90 mm Hg) from the aorta to the right atrium is able to force a blood flow of only 1 L/min through the systemic resistance vessels in the baby, whereas that same pressure gradient forces 5 L/min to flow through the much larger number of parallel resistance vessels in the adult