Stenotic Orifice Research Articles

Experimental Study of Pressure - Velocity Relationship Across Stenotic Orifices by Continuous Wave Doppler Ultrasound

Experimental Study of Pressure - Velocity Relationship Across Stenotic Orifices by Continuous Wave Doppler Ultrasound

Clinical applications of a new type of real-time two-dimensional doppler flow imaging system

The clinical significance of a newly developed real-time 2-dimensional (2-D) Doppler flow imaging technique was assessed. In the instrumentation of the echocardiograph, the pulsed Doppler mechanism was incorporated in a wide-angle, phased-array system. The Doppler flow signals obtained from the cardiac chamber were processed on the basis of the autocorrelation principle. The direction, velocity and variance of the intracardiac blood flow were calculated in real time and displayed in the color-coded mode on the television screen, and were superimposed on the 2-D echocardiographic image of the heart. The technique was used in 20 healthy subjects and 100 cardiac patients. The new technique clearly visualized the whole aspect of intracardiac blood flow by the cine mode in real time; thus, the technique may be called Doppler cineangiocardiography. The mitral inflow and the aortic ejection flow were clearly demonstrated. A regurgitant jet from the valve orifices was dynamically visualized as seen in the cineangiogram. The spatial orientation and extent of the regurgitant jet were easily assessed. The jet stream through the stenotic mitral orifice was well imaged in the left ventricular cavity, showing a variety of stream directions. Intracardiac shunts in ventricular septal defect and atrial septal defect were clearly visualized. The defect could be localized on the interventricular septum on the basis of the site where the shunt flow spurted, although the echocardiographic interruption was not demonstrated in the 2-D echocardiographic image of the cardiac structure. Although some technical problems remain, our new technique greatly improves the diagnostic efficacy of ultrasound.

Reliability and reproducibility of two dimensional echocardiographic measurement of the stenotic mitral valve orifice area

A wide angle phased array sector scanner was used to find the optimal method, the reliability and the reproducibility of measuring the mitral valve area with two dimensional echocardiography in patients with rheumatic mitral stenosis. Initial experience with 18 patients revealed that tracing the early diastolic actual black-white interface of the perceived orifice was the most reliable method for drawing the mitral valve orifice area. Good interobserver correlation was obtained when two observers used either method to calculate the mitral valve area ( r = 0.93). Similarly good intrastudy reliability was obtained when any one observer applied one measurement method to different diastolic cycles within the same study ( r = 0.89). The phased array two dimensional echocardiogram properly differentiated patients with critical mitral stenosis from those with non-critical mitral stenosis, but the correlation between the echocardiographically and the hemodynamically derived mitral valve areas was less good than previously reported ( r = 0.83). Imaging a test object with varied known orifice sizes and excised stenotic mitral valves of known orifice size with a phased array and mechanical sector scanner failed to reveal superiority of either instrument. Further testing with a phased array instrument revealed that the perceived orifice was critically dependent on receiver gains settings for any transmitted power level. Receiver gain settings too low led to image dropout, indicating a falsely large orifice. Receiver gain settings too high led to image saturation, indicating a falsely narrowed orifice. Six additional patients with predominant mitral stenosis later underwent imaging with strict attention paid to individual receiver gain settings. Combining the data from these 6 patients with those from the initial 18 patients gave a better correlation between the echocardiographic and hemodynamic calculated mitral valve areas ( r = 0.92). Accurate noninvasive measurement of the mitral valve area with two dimensional echocardiography in patients with mitral stenosis appears to depend on use of the proper echocardiographic technique to localize the true commissural edge of the valve in early diastole, the correct instrument settings and the appropriate method for drawing the perceived orifice. The noninvasive measurement of the mitral valve orifice with two dimensional echocardiography in mitral stenosis provides clinically useful data that are reliable and reproducible if these factors are taken into account.

Congenital Aortic Stenosis

The magnitude of the peak systolic pressure gradient (PSPG) over the aortic valve was often determining factor when deciding for or against operation of the congenital aortic stenosis. The impression that PSPG alone was an inadequate parameter for assessment of the degree of stenosis was supported by the present retrospective analysis of 32 patients operated upon for this anomaly. A gradient of 75 mmHg or more indicates a tight stenosis but a lower value does not exclude a pronounced constriction of the orifice, any more than does absence of symptoms or signs of left ventricular hypertrophy (LVH) on the ECG. It is probably not only the size of the stenotic orifice that determines the prognosis in aortic stenosis but also other factors, e.g. function of the left ventricular myocardium. A hypothesis was put forward by one of us (M. V.) that the location of the valvular ostium, central or peripheral, can be of importance and that turbulence-induced vibrations of the left ventricular wall may depress the myocardium. Differences in the pathoanatomy of the stenotic aortic valves did not explain the poor correlation between e.g. symptoms or LVH and PSPG or the size of the remaining orifice in the present study.

Hydraulic and hemodynamic studies on the blood flow through the cardiovascular system.

To investigate the hemodynamic aspects across a stenosis in the cardiovascular system, in vitro, in vivo, and clinical studies were performed. The results are summarized as follows: 1) Remarkable changes in the pressure profile were recognized downstream. The relationship between pressure gradient and flow across the stenotic orifice revealed an inverse linear relationship in in vitro studies, but showed a slight convex curve in in vivo studies. 2) The critical stenosis in vitro and in vivo studies 10% and 25%, respectively. 3) In the group of mitral valve area larger than 0.5 cm2, flow coefficient C was smaller than 1.0 and blood flow could be "vena contracta". 4) Gorlin's formula is considered to be appropriate in the cases with valve area larger than 0.5 cm2. But Gorlin's formula may over-estimate in the group with valve area smaller than 0.5 cm2.

Mitral valve area in combined mitral stenosis and regurgitation.

Eight patients with mixed mitral stenosis and regurgitation underwent hemodynamic and angiographic study prior to mitral valve replacement. The stenotic orifice of the mitral valve was calculated employing the total left ventricular stroke volume by cineangiography as the numerator of the Gorlin Formula. Excellent agreement with the measured orifice of the mitral valve was obtained using a value of 37.9 (0.85 X 44.5) for the constant in the Gorlin formula as recommended by Cohen and Gorlin. Recalculation of this constant independently by our data yielded a value that was almost identical. Regurgitant flows and orifice sizes were calculated for each patient using the same constant as for calculation of the stenotic orifices.

Arterial stenosis, pressure, and flow

The interaction between the effects of an arterial stenosis and postocclusion hyperemia was studied in a model of the capillaron system. A capillaron is a module consisting of soft-walled permeable vessels (capillaries) in a compliant capsule. Stenosis reduced flow in accord with arterial cross-sectional area. Addition of a capillaron reduced flow to a basal level. In this basal state, autoregulation and postocclusion hyperemia could be studied. Basal flow and autoregulation give no indication of the severity of the stenosis. The magnitude of the postocclusion hyperemia varied with the arterial pressure and, inversely, with the stenotic orifice area. In the absence of arterial pressure changes, the magnitude of the postocclusion hyperemia indicates the severity of the stenosis.

Studies on the orifice flow through stenotic vessels and mitral valve orifice

To investigate the discharge jet stream through the stenotic orifice, hydrodynamic and hemodynamic studies were performed in the artificial system, the thoracic aortic system of dogs, and the stenotic mitral valve orifice. In the experimental models with rigid plastic pipes and latex tubes an inverse linear relationship was observed between the pressure gradient and the flow across the orifice in both steady and pulsatile flow. The orifice flow revealed a slight decrease in the 30 percent throat area ratio(m), while in the thoracic aortic system it showed a marked decrease in the same throat area ratio and the critical closing throat area ratio (Q̇=0) was observed. The mitral valve flow in mitral stenosis was classified into two groups. In group A (larger group of the mitral valve orifice) the discharge blood stream through the mitral valve orifice is a vena contracta, while in the group B (smaller group) it is not. It is considered that this result explains the preservation of mitral valve flow during a chronic clinical course.

Thrombotic phenomena with nonanticoagulated, composite-strut aortic prostheses

Twenty-nine patients received no anticoagulant therapy after aortic valve replacement with a Starr-Edwards Model 2400 prosthesis. Hemodynamic studies were performed at 3 to 26 months (average 12 months) in 18 of 29 patients. Mean aortic valve gradients ranged from 14 to 62 mm. Hg and averaged 34 mm. Hg. Calculated aortic valve area varied from 0.20 to 1.75 sq. cm. and averaged 0.98 sq. cm. Thirteen of 18 patients had critically stenotic valve orifices. At reoperation or autopsy, examination of the prostheses consistently revealed pannus and thrombus which narrowed the inflow orifice and usually extended to the struts. Of the remaining 11 patients, 3 have died (2 suddenly and one after a cerebrovascular accident), 2 have had embolic episodes, and 6 have refused a repeat study but are being given anticoagulant therapy. Clinical examination, serum lactic dehydrogenase (LDH) levels, phonocardiography, echocardiography, and fluoroscopy of the prosthesis were often unrevealing. Cardiac catheterization was the only reliable method for critically evaluating prosthetic function. In conclusion, close follow-up, preferably with cardiac catheterization, is recommended in any patient who received a Starr-Edwards Model 2400 aortic valve prosthesis without anticoagulation. Long-term anticoagulation with sodium warfarin is indicated in all patients with a Model 2400 aortic valve prosthesis unless there is a contraindication to such therapy.

Circulatory dynamics across the stenotic mitral valve orifice.

Considering the configulation of the stenotic mitral valve orifice as a sort of pipe orifice in the hydraulic system, the above-mentioned formula has been derived from the usual form of continuity equation and energy equation. Vena contracta has not been cuased by the configulation of mitral orifice in about fifty-one percent of mitral stenosis. This result is different from Gorlin's one. The reasons why vena contracta was not brought about in the small mitral valve orifice as follow; the greater part of the mitral valve orifices were situated downward from the annulus and considered as so-called nozzled shape; there are little precision in the measurement of blood flow and pressure; mitral valve orifices themselves have various configulations. And so flow coefficient C is not constant such as Gorlin's formula, but a function of Reynolds number and throat area ratio. This coefficient is attributable to the configulation of the mitral orifice, the velocity of mitral valve flow and the viscosity of blood flow.

Natural hemodynamic history of congenital aortic stenosis in childhood

The natural hemodynamic history of 37 patients with congenital aortic stenosis (18 valvular, 10 valvular with slight or moderate aortic insufficiency, 6 discrete subvalvular and 3 supravalvular) was documented by 2 catheterizations using the same methods of study. In the majority of patients with valvular, discrete subvalvular and supravalvular aortic stenosis the anomaly increased in severity, as evidenced by (1) a statistically significant increase in left ventricular peak systolic pressure, left ventricular to aortic peak systolic pressure gradient and arteriovenous oxygen difference; and (2) by a significant decrease in cardiac index and stenotic orifice area per square meter body surface area. The changes were more severe in patients with supra- and subvalvular stenosis. There were no statistically significant changes in aortic pressure, heart rate, systolic ejection period or wedge pressure. Left ventricular end-diastolic pressure increased significantly only in patients with discrete subvalvular aortic stenosis. Although the stenotic area index was lower in all patients without aortic insufficiency, absolute stenotic area decreased in only a small percentage. In patients with both valvular aortic stenosis and insufficiency, there was a significant unfavorable change in cardiac index, arteriovenous oxygen difference and left ventricular end-diastolic pressure but no significant increase in left ventricular peak systolic pressure or left ventricular to aortic peak systolic pressure gradient. Ninety percent of the patients remained asymptomatic during the study. Aortic stenosis is likely to increase in severity, usually by an increase in flow across a “fixed” stenotic area as a result of body growth and sometimes by a concomitant actual decrease in stenotic area.

Stenosis of the Pulmonary Arteries: Evolution and Hemodynamic Effects

The evolution of stenosis of the pulmonary arteries may be enhanced by congenital heart disease leading to increased or decreased pulmonary blood flow. A third possibility is that an intrapulmonary right-to-left shunt through a vascular malformation hinders normal maturation of the pulmonary vascular bed. This evolution is illustrated by an example of severe stenosis of the pulmonary arteries occurring with pulmonary arteriovenous fistula. In most patients with isolated bilateral stenosis of the pulmonary arteries there is no significant increase in right ventricular pressure with age. A discrepancy between the increase in cardiac output and growth of the stenotic orifice in children may however aggravate it. Although the hemodynamic effect of bilateral stenosis of the pulmonary arteries is greater on the pressures than on the morphology of the pulmonary circulation, angiocardiographic diagnosis is usually possible. Pressure measurements show a gradient across and an abnormal pulse contour proximal to the stenosis.

The angiographic features of bicuspid and unicommissural aortic stenosis

Stenosis of a congenitally bicuspid aortic valve results in a characteristic angiographic appearance: In systole the valve leaflets are domed, and there is a central jet that corresponds to the orifice of the stenotic valve. In diastole the sinuses of Valsalva reflect the morphologic features of the 2 aortic cusps. In contrast, the anatomic derangement of unicommissural aortic stenosis is reflected in an angiogram which can be differentiated from that of bicuspid aortic stenosis. In this case, the stenotic orifice visualized by the jet in systole can be seen in contact with the posterior wall of the aorta; leaflet tissue and valve motion are seen only anteriorly. The diastolic appearance differs from that of the bicuspid valve by the presence of a deep sinus of Valsalva anteriorly and a very shallow posterior sinus, also resulting from the lack of leaflet tissue posteriorly. This differentiation is important since extensive valvulotomy on the unicommissural valve will result in significant aortic regurgitation postoperatively.

An Explanation of Asymmetric Upper Extremity Blood Pressures in Supravalvular Aortic Stenosis

The Coanda effect, the tendency of a jet stream to adhere to a wall, was investigated as an explanation of the unequal pressures in the upper extremities in patients with supravalvular aortic stenosis (SVAS). Of 56 patients with SVAS reviewed, 48 had unequal blood pressures in the upper extremities. The average difference was 18 mm Hg systolic. Although 11 of the 20 patients in the control group (valvular aortic stenosis) had some blood pressure asymmetry, the average difference was 3.5 mm Hg systolic. In valvular aortic stenosis, the velocity of the jet is quickly dissipated beyond the stenotic orifice, preventing any sustained high-velocity stream. However, the smooth, annular narrowing of SVAS creates a "step" between the orifice and the ascending aortic wall which enhances the natural affinity of a jet for a boundary wall and conserves the kinetic energy of the jet stream. In most patients with SVAS, the high-velocity stream is along the right aortic wall, causing disproportionately high pressure in the right arm.

Mechanism of elevated innominate artery pressures in supravalvular aortic stenosis.

Patients with supravalvular aortic stenosis frequently have higher phasic and mean blood pressures in the innominate artery than in the aorta distal to the stenosis, a finding never fully explained. Theoretically, this phenomenon could result from the impact of a jet upon blood within the innominate artery. Slowing of the jet would convert kinetic into potential energy, thus increasing pressure within the innominate artery. The aorta distal to the innominate artery would receive blood whose kinetic energy had been dissipated and would consequently remain at the same pressure as the aorta proximal to the origin of the innominate artery. This hypothesis was tested in a model simulating supravalvular aortic stenosis. Cinematography showed a jet directed into the innominate artery. Only after this jet was dispersed, did fluid turn backward to flow into the distal portion of the aorta. Observed pressure rises in the innominate artery corresponded closely with values predicted from the estimated kinetic energy of the jet. Dispersion of the jet by replacement of the stenotic orifice with an equally obstructive screen or proximal displacement of the stenosis (simulating valvular aortic stenosis) abolished the pressure difference. The results obtained were thus entirely consistent with the mechanism proposed to explain the elevated innominate artery pressures seen in supravalvular aortic stenosis.

Hemodynamics during rest and exercise in patients with mixed aortic stenosis and insufficiency

below 2,200 set+ are seen in myocardial dysfunction, someafter catheterization, appear to be useful in the total assesstimes before t,he ejection fraction begins to decrease. ment of t.he condit,ion of the individual pat.ient studied in a These measurements, which can be obtained in minutes clinical laborat,ory. Computer Analyses and Telephone Transmission of Pacemaker Artifact Information DORIS J. W. ESCHER, MD, FACC/SEYMOUR FURMAN, MD, FACC/BRYAN PARKER NORMON SOLOMON, MD and SRIRAMALU NAIDU, MD Bronx, New York Programmed follow-up st’udies of patients after implantation of cardiac pacemakers is the major recourse against unant.icipated catastrophic failures. In a series, dating from 1961, of approximately 470 such pat,ient,s, 375 have been followed up since 1967 by electronic analysis 3 to 4 t*imes a year of pacemaker artifact rate, contour, amplitude and pulse duration, with mandat’ory change of the pulse generator at 24 to 30 months (depending upon statistical experience with specific systems). Of premature failures, 95% have been of the pulse generator, 10% under 1 or after 3 years, 80% in the second yea.r. Overall sudden failures of 16% have been reduced to 12% in the last year. Hemodynamics During Rest and Exercise in Patients with Mixed Aortic Stenosis and Insufficiency PHILIP 0. ElTINGER, MD/MARTIN J. FRANK, MD, FACC and GILBERT E. LEVINSON, MD, FACC Jersey City, New Jersey Recent work by others has shown that many patients with mixed aortic stenosis and regurgitation exhibit a decrease or no change in the aortic systolic gradient during exercise despite increased systemic flow. It has been suggested that the Gorlin formula may, therefore, sometimes be inapplicable because the stenotic aortic orifice is, or behaves as if it were, inconstant in size. However, studies reported from our laboratory have shown that regurgitant fraction falls during exercise in pure aortic regurgitation while total flow is remarkably constant. The present study investigated this problem in 9 patients with mixed aortic lesions. Regurgitation was measured during rest and exercise by simultaneous upstream and downstream sampling with the use of continuous indicator infusions. Hea,rt rate increased and peripheral resistance decreased in every patient, but the aortic valve gradient, (mean 42 + 10 Computer programming with trend analyses for each patient of decay rate or unusual changes and comparative analyses of system groups allows better prediction of individual and group performance. Further safety would be insured by more frequent, examination. Where t,his becomes a logist,i,c problem, use of newly developed instrumentation, a.llowing transmission of artifact rate and possibly artifact amplitude and duration by signals over telephone lines from home to follow-up clinic, supplements or reduces direct visits. Tape-recorded transmissions can be transferred to visual displays or fed into the computer to become part of the ongoing record. mm Hg) did not change significantly. Increased forward flow (mean 4.52 + 0.34 at rest and 6.66 + 0.44 liter/min with exercise, P < 0.01) was balanced by decreased regurgitation (mean 4.54 % 1.17 and 2.81 r+ 1.12 liter/min, P = 0.05), and total flow did not change significantly. The systolic ejection period declined significantly, but systolic and diastolic seconds per minut.e changed very little. The mean aortic valve area was 1.8 +0.3 cmz bot#h at rest and during exercise. This study confirms the necessity for measurement’s of tot.al valve flow in eva.luation of mixed valve lesions. It also demonst’rates that (1) exercise reduces the regurgitant fraction in mixed aortic lesions as it does in pure aortic insufficiency; (2) this effect is not due to a shortened diastolic time per minute but must reflect an altered relation of peripheral resistance to ventricular compliance; and (3) the stenotic aortic valve, contrary t,o recent suggestion, behaves as a fixed orifice. Maximal Contractile Element Velocity (V ,,,& as an Index of Contractile State in Man HERMAN L. FALSETTI, MD/ROBERT E. MATES, PhD/DAVID G. GREENE, MD, FACC and IVAN L. BUNNELL, MD

Dynamic autofluoroscopic pulmonary vascular examination in thromboembolism

below 2,200 set+ are seen in myocardial dysfunction, someafter catheterization, appear to be useful in the total assesstimes before t,he ejection fraction begins to decrease. ment of t.he condit,ion of the individual pat.ient studied in a These measurements, which can be obtained in minutes clinical laborat,ory. Computer Analyses and Telephone Transmission of Pacemaker Artifact Information DORIS J. W. ESCHER, MD, FACC/SEYMOUR FURMAN, MD, FACC/BRYAN PARKER NORMON SOLOMON, MD and SRIRAMALU NAIDU, MD Bronx, New York Programmed follow-up st’udies of patients after implantation of cardiac pacemakers is the major recourse against unant.icipated catastrophic failures. In a series, dating from 1961, of approximately 470 such pat,ient,s, 375 have been followed up since 1967 by electronic analysis 3 to 4 t*imes a year of pacemaker artifact rate, contour, amplitude and pulse duration, with mandat’ory change of the pulse generator at 24 to 30 months (depending upon statistical experience with specific systems). Of premature failures, 95% have been of the pulse generator, 10% under 1 or after 3 years, 80% in the second yea.r. Overall sudden failures of 16% have been reduced to 12% in the last year. Hemodynamics During Rest and Exercise in Patients with Mixed Aortic Stenosis and Insufficiency PHILIP 0. ElTINGER, MD/MARTIN J. FRANK, MD, FACC and GILBERT E. LEVINSON, MD, FACC Jersey City, New Jersey Recent work by others has shown that many patients with mixed aortic stenosis and regurgitation exhibit a decrease or no change in the aortic systolic gradient during exercise despite increased systemic flow. It has been suggested that the Gorlin formula may, therefore, sometimes be inapplicable because the stenotic aortic orifice is, or behaves as if it were, inconstant in size. However, studies reported from our laboratory have shown that regurgitant fraction falls during exercise in pure aortic regurgitation while total flow is remarkably constant. The present study investigated this problem in 9 patients with mixed aortic lesions. Regurgitation was measured during rest and exercise by simultaneous upstream and downstream sampling with the use of continuous indicator infusions. Hea,rt rate increased and peripheral resistance decreased in every patient, but the aortic valve gradient, (mean 42 + 10 Computer programming with trend analyses for each patient of decay rate or unusual changes and comparative analyses of system groups allows better prediction of individual and group performance. Further safety would be insured by more frequent, examination. Where t,his becomes a logist,i,c problem, use of newly developed instrumentation, a.llowing transmission of artifact rate and possibly artifact amplitude and duration by signals over telephone lines from home to follow-up clinic, supplements or reduces direct visits. Tape-recorded transmissions can be transferred to visual displays or fed into the computer to become part of the ongoing record. mm Hg) did not change significantly. Increased forward flow (mean 4.52 + 0.34 at rest and 6.66 + 0.44 liter/min with exercise, P < 0.01) was balanced by decreased regurgitation (mean 4.54 % 1.17 and 2.81 r+ 1.12 liter/min, P = 0.05), and total flow did not change significantly. The systolic ejection period declined significantly, but systolic and diastolic seconds per minut.e changed very little. The mean aortic valve area was 1.8 +0.3 cmz bot#h at rest and during exercise. This study confirms the necessity for measurement’s of tot.al valve flow in eva.luation of mixed valve lesions. It also demonst’rates that (1) exercise reduces the regurgitant fraction in mixed aortic lesions as it does in pure aortic insufficiency; (2) this effect is not due to a shortened diastolic time per minute but must reflect an altered relation of peripheral resistance to ventricular compliance; and (3) the stenotic aortic valve, contrary t,o recent suggestion, behaves as a fixed orifice. Maximal Contractile Element Velocity (V ,,,& as an Index of Contractile State in Man HERMAN L. FALSETTI, MD/ROBERT E. MATES, PhD/DAVID G. GREENE, MD, FACC and IVAN L. BUNNELL, MD Buffalo, New York The maximal, no-load velocity for the contractile element (V,,.) was estimated in 45 patients. The patients included 17 with normal left ventricular dynamics; 8 with volume overload, compensated; 11 with volume overload, decompensated; 3 with pressure overload; and 6 with cardiomyopathy. Contractile element velocity (V,) during isovolumic 94 The American Journal of CARDlOLOaY

Left atrial transport in mitral stenosis.

Active left atrial transport was examined acutely in five patients with mitral stenosis and sinus rhythm. To observe the effect of active left atrial contraction on subsequent left ventricular output, the timing of atrial contraction with respect to ventricular contraction was varied. Atrioventricular (A-V) dissociation produced by right ventricular pacing was used to change continually the relationship of left atrial (LA) to left ventricular (LV) contraction. Measurements of each beat included mitral valve gradient, left ventricular ejection time (LVET), and left ventricular peak-systolic pressure (LVPSP). The diastolic gradient was used to calculate diastolic flow per beat after the area of the stenotic mitral orifice had been previously determined. Calculated diastolic flow increased significantly (24%) as did the parameters of left ventricular output, LVPSP and LVET ( P &lt; 0.005 for both), when atrial contraction was optimally timed. Cumulative effects were observed when a series of well-placed or poorly placed atrial contractions occurred. The method documents change in LV output which depends on appropriate timing of LA contraction that may not have been seen with a steady-state approach.

Dynamics of blood flow in stenotic vascular lesions

A normal artery can serve as a conduit for many times the resting flow without significant loss of energy or production of murmurs. In such vessels, the rate of flow is determined primarily by the peripheral vascular conductance (rate of flow/drop in pressure) rather than by the characteristics of the artery. In a system of vessels connected in series, the rate of flow is limited by the segment of lowest conductance. Thus, when the conductance of the peripheral vascular bed is low, a partially stenotic lesion has no effect on the rate of flow. When an arteriovenous fistula or an increase in metabolic activity markedly increases the delivery through an arterial narrowing, the stenotic segment becomes the limiting factor, and a systolic bruit indicates that the stream is no longer laminar. Thus, a murmur is the announcement that the caliber of a vessel or valve is inadequate to provide for laminar flow. With further increases in peripheral vascular conductance and rate of flow, the murmur increases in intensity, pitch, and duration. Cross flow is minimal in an anastomosis between two parallel arteries which deliver blood to separate vascular beds of equal conductance. If a stenosis is present in one of the arterial roots, blood will shunt from the normal vessel to the vascular bed of the narrowed vessel. If the stenosis is severe, the deviation of blood will be marked and the tissues normally supplied by the normal vessel will become ischemic (steal syndrome). The interaction (hydrodynamic drag) of the stream with the vessel wall determines vascular caliber. Laminar streams interact normally with the wall. The separation of the stream lines from the wall at the downstream lip of a stenotic orifice produces a locus at which stream-wall interaction is minimal, with the result that the narrowing becomes progressively marked. In the poststenotic segment, turbulence increases the interaction of stream and wall, and stimulates a localized enlargement (poststenotic dilatation).

IDIOPATHIC HYPERTROPHIC SUBAORTIC STENOSIS. I. A DESCRIPTION OF THE DISEASE BASED UPON AN ANALYSIS OF 64 PATIENTS.

Idiopathic hypertrophic subaortic stenosis (IHSS) is a disease characterized by marked hypertrophy of the left ventricle, involving in particular the interventricular septum and the left ventricula...