Fluid dynamics of coronary artery stenosis.

Abstract

SUMMARY A large-scale model of the coronary circulation, instrumented to permit detailed pressure and velocity measurements, has been used to study flow through isolated stenotic elements in large coronary arteries. Pulsatile aortic pressure and instantaneous peripheral resistance were simulated with servo valves. A variety of axisymmetric and asymmetric stenoses were studied and flow separation was found to occur for all but very mild stenoses. Pressure recovery downstream of the stenosis throat was limited and, in some cases, no recovery was observed. Pressure drop was primarily dependent upon the minimum area of the stenosis and relatively independent of stenosis geometry. Flow was quasi-steady at normal heart rates, and simple steady flow theory proved adequate to describe the pressure drop through the stenosis. The theory yielded results that agreed well with published data for dogs and appears promising for predicting effects of hemodynamic variables on a given stenotic lesion. Thus, principal findings of the study are that a relatively severe stenosis behaves essentially like an orifice and that a simple quasi-steady theory appears adequate to predict effects of a stenosis on coronary flow.IT HAS BEEN known for some time that a relatively severe constriction of a coronary artery is required to alter mean resting coronary flow significantly. Studies in open-chested dogs 1 "5 have delineated the following characteristics. An area occlusion of approximately 80-85% (corresponding to a diameter reduction of 60-80%) is required to reduce resting coronary flow. This value has been referred to as a "critical" occlusion. As this critical occlusion is approached, reactive hyperemia is almost entirely abolished indicating that full peripheral vasodilation has already occurred. For smaller degrees of occlusion, reactive hyperemia is reduced in comparison to the control state indicating partial vasodilation. Similar results were obtained in steady flow in vitro experiments conducted using postmortem coronary arteries with partial occlusion." In this study, in which perfusion pressure and downstream resistance could be independently controlled, it was found that at low flow rates stenotic resistance (ratio of pressure drop to flow) was essentially constant, suggesting fully developed laminar flow. At high flows resistance increased with flow, indicating the importance of turbulence, flow separation, or entrance effects. It was further observed that the resistance of the stenosis was primarily dependent on its minimum cross-sectional area rather than its length.The foregoing studies have been confined to a description of the overall behavior of coronary flow in the presence of a stenosis. Because of the small size of the arteries, it is very difficult to make any detailed pressure or flow measurements to describe in detail the flow mechanism responsible for stenotic resistance. In order to study the flow in detail, it is necessary to develop large scale laboratory models. Most studies of flow through an isolated area of...

The importance of renal vascular tone in determining the severity of renal artery stenosis in dogs.

“…This did not occur with angiotensin II which is rapidly metabolized by intrarenal enzymes (Oparil & Bailie, 1973). The resistance to blood flow exerted by a fixed-diameter arterial stenosis is not constant but varies with arterial pressure and distal vascular resistance (Anderson et al 1979a;Gould et al 1974;Mates et al 1978;May et al 1963;Schwartz et al 1979;Walinsky et al 1979;Young et al 1977). In all groups in the present study, the calculated stenosis resistance fell from its initial value over the hour of stenosis.…”

Quantification of coronary artery stenosis in vivo.

“…Intrastenotic flow for most coronary stenoses is therefore likely to be underdeveloped, with parabolically shaped velocity profiles being approached only with very long stenoses, under resting flow conditions. For such flow conditions, viscous losses across coronary stenoses would be greater than that predicted by the presently used equations (Seeley and Young, 1976;Iipscomb and Hooten, 1978), as would the inertial expansion losses as previously hypothesized (Mates et al, 1978;Lipscomb and Hooten, 1978). Entrance losses, previously assumed to be negligible in quantitative arteriography, are related to the issue of flow profile effects, since entrance geometry may profoundly affect flow profiles in the stenotic segment and therefore influence exit losses as well.…”

“…According to presently used fluid dynamic equations for flow in rigid tubes (Young andTsai, 1973a, 1973b;Brown et al, 1977;Mates et al, 1978;lipscomb and Hooten, 1978;Gould, 1978a;Gould et al, 1982), the energy or pressure losses across a coronary artery stenosis occur primarily because of two geometric characteristics: narrowness of the arterial lumen and its abrupt expansion at the distal end where it opens into the normal-sized artery. Along the narrowed lumen of the stenosis, viscous friction is generated by the layers of blood sliding against each other and against the endothelium of the artery.…”

“…Progressively severe coronary stenoses increasingly dampen the coronary flow waveform, thereby reducing its pulsatile nature. Thus, at least for purposes of analyzing the mean or instantaneous diastolic pressure drops across clinically significant coronary stenoses, blood flow can be considered as quasi-steady (Young et al, 1975;Mates et al, 1978;Gould, 1978a). The total pressure drop across a stenosis is then the sum of the viscous and expansion losses, expressed in the general form of a quadratic equation relating pressure gradient, AP, across a given stenosis to the volumetric flow, Q, through it as follows:…”

Accuracy and Reliability of Quantitative Measurement of Coronary Arterial Stenosis by Videodensitometry on Coronary Angiogram.

“…In videodensitometry, the concentration of the contrast medium at the objective site to be measured is converted to the intensity of videosignals, and the concentration of the contrast medium can be considered to reflect the cross-sectional area of the lumen of the coronary artery rather than the luminal diameter.6)-8) The luminal cross-sectional area is well known to be a major determinant of coronary flow, and is clinically more important than the luminal diameter.4), 9) In the present study, the intraobserver variability in videodensitometry was found to be small. The results obtained by 2 observers were almost iden- tical, indicating that interobserver variability is also minimum.…”