Analysis of flow characteristics in poststenotic regions of the human coronary artery during bypass graft surgery

Investigation into the two-way interaction of coronary flow and heart function in coronary artery disease predicted by a computational model of autoregulation of coronary flow

This study presents a closed-loop computational model to investigate the interplay between heart function, coronary flow, and systemic circulation during exercise, with a specific focus on the impact of coronary artery stenosis. The model incorporates a lumped representation of the heart, main arteries, and coronary arteries, establishing a closed circulatory system. The simulation investigates the autoregulation of coronary flow in response to myocardial oxygen demands during physical exercise by incorporating sympathetic and parasympathetic functions. This study establishes a closed supply-demand loop and investigates the effect of coronary flow deficiency on heart function and systemic circulation in coronary artery diseases during exercise. In coronary artery diseases with low stenosis, heart function and systemic flow resemble those of a healthy person. However, as stenosis intensifies with physical exercise, an additional regulatory mechanism (reg2) is activated. This mechanism adjusts coronary flow by reducing myocardial contractility (E) and increasing heart rate (HR) while maintaining cardiac output (CO). The study results indicate that, at the highest exercise intensity for a healthy individual (HR = 150), the value of E increases from 6 to 8.65mmHg/ml. Meanwhile, for a patient with 85 % coronary artery stenosis in the same exercise intensity, the HR increases to 200, and the value of E decreases to 3.45mmHg/ml. The results also demonstrate that the initiation of the (reg2) mechanism at rest occurs at 83 % stenosis, while at the highest exercise intensity, this mechanism commences at 67 % stenosis.

Coronary microvascular resistance comparison of coronary arteries with and without considering vascular diameter: A retrospective, single-center study

Background and Aims

Measurement of coronary microvascular resistance (MR) is essential for diagnosing nonocclusive coronary artery ischemia, but whether coronary branches of different diameters can be similarly assessed using hyperemic microvascular resistance index (hMVRI) calculated from average peak velocity (APV) remains unclear. We investigated the relationship between coronary arteries of different diameters and hMVRI.

Methods

Thirty patients with suspected angina pectoris and nonobstructive coronary stenosis with fractional flow reserve >0.8 underwent evaluation of all coronary arteries using a Doppler velocity and pressure-equipped guidewire. Quantitative coronary angiography (QCA) was used to analyze vessel diameter (DQCA). Coronary blood flow (CBFQCA) was calculated as πDQCA 2/4 (0.5 × APV) and hMVRIQCA as distal coronary pressure divided by CBFQCA during maximal hyperemia.

Results

The hMVRI was significantly higher for the right coronary artery than for the left anterior descending artery, but no significant differences between arteries were seen for CBFQCA and hMVRIQCA. Although the correlation between CBFQCA and APV was weak, CBFQCA divided into three groups according to DQCA showed very strong correlations with APV. Slopes of the straight line between APV and CBFQCA for small-, middle-, and large-diameter groups were 0.48, 0.30, and 0.21, respectively, with slope decreasing as diameter increased.

Conclusions

Comparative evaluation of MR in coronary branches with varying vessel diameters requires vessel diameter to be accounted for.

Mid-term Patency of the Great Saphenous Bypass to Aorta vs. Non-aortic Arteries in Stanford Type A Aortic Dissection Surgery With Concomitant CABG

Background: Stanford type A aortic dissection (STAAD) is often associated with coronary artery problems requiring coronary artery bypass grafting (CABG). However, the prognosis of different proximal graft locations remains unclear.

Methods: From May 2015 to April 2020, 62 patients with acute STAAD who underwent aortic surgery concomitant with CABG were enrolled in our study. Aortic bypass was defined as connecting the proximal end of the vein bridge to the artificial aorta (SVG-AO); non-aortic bypass was defined as connecting the proximal end of the vein bridge to a non-aorta vessel, including left subclavian artery, left common carotid artery, and right brachiocephalic artery (non-SVG-AO). We compared early- and mid-term results between patients in the above two groups. Early results included death and bleeding, and mid-term results graft patency, aortic-related events, and bleeding. Grafts were evaluated by post-operative coronary computed tomography angiography. According to the Fitzgibbon classification, grade A (graft stenosis <50%) is considered a patent graft. Univariate and multivariate analyses were performed to assess differences between aortic and non-aortic bypass in STAAD.

Results: SVG-AO and non-SVG-AO were performed in 15 and 47 patients, respectively. There was no significant difference in death (log-rank test, p = 0.426) or bleeding (p = 0.766) between the two groups in the short term. One year of follow-up was completed in 37 patients (eight in the SVG-AO group and 29 in the non-SVG-AO group), among which 14/15 (93.3%) grafts were patent in the SVG-AO group and 32/33 (97.0%) grafts in the non-SVG-AO at 1 week, without a significant difference (p = 0.532). At 3 months, 12/13 (92.3%) grafts were patent in the SVG-AO group and 16/32 (50.0%) grafts in the non-SVG-AO, with a significant difference (p = 0.015), and 12/13 (92.3%) grafts in the SVG-AO group and 15/32 (46.9%) grafts in the non-SVG-AO group were patents, with a significant difference. Multivariate analysis showed proximal aortic bypass and dual anticoagulation to be protective factors for the 1-year patency of grafts.

Conclusion: In patients requiring aortic dissection surgery with concomitant CABG, no differencess' between SVG-AO and SVG-non-AO in early outcomes were detected, but SVG-AO may have higher mid-term patency.

Coronary Artery Assessment by Epicardial Pulsed Doppler Ultrasound

An intraoperative method was developed for direct investigation of the coronary arteries using an epicardial high-frequency two-dimensional Doppler ultrasound transducer. This fingertip 7.5-MHz transducer was used in 30 patients undergoing coronary artery bypass surgery to measure blood flow in the coronary arteries and bypass grafts. In all the patients, clear images of the coronary arteries and the bypass grafts were obtained in the beating heart, and coronary blood flow could be measured to evaluate the degree of stenosis. There were no significant differences between the measurements obtained with the Doppler transducer and a transit-time ultrasonic flowmeter of blood flow in the left internal thoracic artery graft to the left anterior descending coronary artery, indicating that the Doppler method produces accurate and reproducible measurements. This method can provide useful intraoperative information about the coronary artery system in emergency operations such as acute aortic dissection.

INTIMAL HYPERPLASIA AND WALL SHEAR IN ARTERIAL BYPASS Y-GRAFTING AND CONSEQUENCE GRAFTING: A NUMERICAL STUDY

The progression of intimal hyperplasia is considered to be the main cause of bypass failure and is directly related to the individual blood rheology, local arterial geometry and placement of the junctions, graft diameter and graft surface characteristics as well as the degree of compliance. In this paper we use commercial computational fluid dynamics (CFD) ANSYS to examine under the correct physiological flow conditions the hemodynamic forces of composite bypass with internal mammary artery in Y-grafting and consequence grafting which is known to achieve high patency rate and highly recommended by clinicians. Particular emphasis is given here on the parameters that could initiate the development of intimal hyperplasia within these bypass configurations. The hemodynamic flow patterns between the consequence grafting and the composite Y-grafting are observed here to be different. Moreover, on both end-to-side and side-to-side configurations, the circulating flows are detected in the vicinity of the junction area, while the Dean flow vortexes are only observed on the end-to-side configuration. Likewise, the hemodynamic flow on the end-to-side configuration on the LCX of both 45° and 90° Y-grafting is found to be smoother than that of the junction on the LCA, regardless of the changing of anastomosis angles. The high WSS gradients are observed at the vicinity of the toe and on the bed of the junction, while the low WSS are presented at the distal of the stenosis and at the stagnation point. The clinical relevance of the results are presented and discussed with particular focus on the factors and the flow patterns that trigger the development of intimal hyperplasia.

Numerical analysis of coronary artery bypass grafts: an over view

Arterial bypass grafts tend to fail after some years due to the development of intimal thickening (restenosis). Non-uniform hemodynamics following a bypass operation contributes to restenosis and bypass failure can occur due to the focal development of anastomotic intimal hyperplasia. Additionally, surgical injury aggravated by compliance mismatch between the graft and artery has been suggested as an initiating factor for progress of wall thickening along the suture line Vascular grafts that are small in diameter tend to occlude rapidly. Computational fluid dynamics (CFD) methods have been effectively used to simulate the physical and geometrical parameters characterizing the hemodynamics of various arteries and bypass configurations. The effects of such changes on the pressure and flow characteristics as well as the wall shear stress during a cardiac cycle can be simulated. Recently, utilization of fluid and structure interactions have been used to determine fluid flow parameters and structure forces including stress and strains relationships under steady and transient conditions. In parallel to this, experimental diagnostics techniques such as Laser Doppler Anemometry, Particle Image Velocimetry, Doppler Guide wire and Magnetic Resonance Imaging have been used to provide essential information and to validate the numerical results. Moreover, clinical imaging techniques such as magnetic resonance or computed tomography have assisted considerably in gaining a detailed patient-specific picture of the blood flow and structure dynamics. This paper gives a review of recent numerical investigations of various configurations of coronary artery bypass grafts (CABG). In addition, the paper ends with a summary of the findings and the future directions.

Microcalcifications increase coronary vulnerable plaque rupture potential: a patient-based micro-CT fluid-structure interaction study

Asymptomatic vulnerable plaques (VP) in coronary arteries accounts for significant level of morbidity. Their main risk is associated with their rupture which may prompt fatal heart attacks and strokes. The role of microcalcifications (micro-Ca), embedded in the VP fibrous cap, in the plaque rupture mechanics has been recently established. However, their diminutive size offers a major challenge for studying the VP rupture biomechanics on a patient specific basis. In this study, a highly detailed model was reconstructed from a post-mortem coronary specimen of a patient with observed VP, using high resolution micro-CT which captured the microcalcifications embedded in the fibrous cap. Fluid-structure interaction (FSI) simulations were conducted in the reconstructed model to examine the combined effects of micro-Ca, flow phase lag and plaque material properties on plaque burden and vulnerability. This dynamic fibrous cap stress mapping elucidates the contribution of micro-Ca and flow phase lag VP vulnerability independently. Micro-Ca embedded in the fibrous cap produced increased stresses predicted by previously published analytical model, and corroborated our previous studies. The 'micro-CT to FSI' methodology may offer better diagnostic tools for clinicians, while reducing morbidity and mortality rates for patients with vulnerable plaques and ameliorating the ensuing healthcare costs.

Detection of severe left anterior descending coronary artery stenosis by transthoracic evaluation of resting coronary flow velocity dynamics

In the presence of severe stenosis, coronary artery flow may be reduced at rest. Recent advances in echocardiography have made non-invasive sampling of velocities in the left anterior descending coronary artery (LAD) possible. The aim of our study was to evaluate feasibility and capability of transthoracic Doppler to detect severe stenosis of the LAD. The study population consisted of 42 subjects with suspected coronary artery disease scheduled for coronary angiography. All had complete transthoracic echocardiography and Doppler sampling of LAD velocities. Quantitative coronary angiography was performed within 24 hours of the echocardiogram. Correlations between LAD velocity profile, measurements and calculations, and the angiographic results were performed. Six subjects had LAD occlusion, 10 had severe (>80% diameter) LAD stenosis, and 26 had normal or non-occlusive LAD disease. In all six subjects with LAD occlusion, distal LAD velocities were not detectable, while in the other 36 subjects, LAD velocities were recorded indicating the vessels were patent. In the 10 subjects with severe LAD stenosis, the diastolic/systolic velocity ratio was <1.5, while in those with non-significant LAD disease, the diastolic/systolic velocity ratio was >1.5 (P<0.005). Diastolic LAD flow was 21.8±13 mL/min in the presence of severe stenosis as compared to 48.5±20 mL/min in subjects without severe stenosis (P<0.0013). LAD velocities had high sensitivity and specificity for the prediction of severe angiographic stenosis. Thus transthoracic Doppler measurement of LAD velocities is feasible and can predict the presence of severe LAD stenosis or occlusion.

Womersley number-based estimation of flow rate with Doppler ultrasound: sensitivity analysis and first clinical application

In this paper we continue in investigating the approach we have proposed in a paper recently published, for a reliable estimate of (peak systolic) blood flow rate from velocity Doppler measurements. Basic features of this approach together with some in silico test cases were discussed in that work. Here, we provide more insights of this approach by performing a sensitivity analysis of the formulas relating blood flow rate to velocity. In particular we analyze how our estimates are affected by perturbation or errors in measurements in comparison with a standard method for catheter based estimates based on the assumption of a parabolic velocity profile. A first glance to in vivo clinical applications is given as well.

Noninvasive diagnosis of ischemic and nonischemic cardiomyopathy using coronary flow velocity measurements of the left anterior descending coronary artery by transthoracic Doppler echocardiography

OBJECTIVES

The purpose of this study was to assess the feasibility and usefulness of coronary flow velocity measurements of the left anterior descending coronary artery (LAD) by transthoracic Doppler echocardiography (TTDE) to differentiate ischemic cardiomyopathy (ICM) from non-ICM in patients.

BACKGROUND

ICM and non-ICM have similar 2-dimensional echocardiographic features, left ventricular dilatation, and diffuse wall-motion abnormalities. TTDE may be useful to differentiate ICM from non-ICM by detecting significant LAD stenosis based on LAD flow signal analysis.

METHODS

TTDE was performed in 52 consecutive patients with left ventricular dilatation and diffuse wall-motion abnormalities of unknown origin. Peak and averaged systolic and diastolic flow velocities of the distal LAD flow could be recorded and measured from 44 patients (85%). Peak and mean diastolic/systolic velocity ratio (DSVR) were calculated.

RESULTS

By coronary angiogram, 13 patients were given the diagnosis of ICM and 31 of non-ICM. Left ventricular end-diastolic and end-systolic volumes and ejection fraction were similar between ICM and non-ICM. On the other hand, peak DSVR (1.47 +/- 0.38 vs 2.34 +/- 0.67, P < .0001) and mean DSVR (1.40 +/- 0.42 vs 2.24 +/- 0.61, P < .0001) were significantly lower in patients with ICM than non-ICM. Either peak DSVR less than 1.8 or mean DSVR less than 1.8 had a sensitivity of 77% and a specificity of 77% for detecting the presence of severe LAD stenosis and, therefore, the diagnosis of ICM.

CONCLUSION

TTDE is a useful noninvasive method to differentiate ICM from non-ICM.

Characterizing momentum change and viscous loss of a hemodynamic endpoint in assessment of coronary lesions

Myocardial fractional flow reserve (FFR(myo)) and coronary flow reserve (CFR), measured with guidewire, and quantitative angiography (QA) are widely used in combination to distinguish ischemic from non-ischemic coronary stenoses. Recent studies have shown that simultaneous measurements of FFR(myo) and CFR are recommended to dissociate conduit epicardial coronary stenoses from distal resistance microvascular disease. In this study, a more comprehensive diagnostic parameter, named as lesion flow coefficient, c, is proposed. The coefficient, c, which accounts for mean pressure drop, Delta p, mean coronary flow, Q, and percentage area stenosis, can be used to assess the hemodynamic severity of a coronary artery stenoses. Importantly, the contribution of viscous loss and loss due to momentum change for several lesion sizes can be distinguished using c. FFR(myo), CFR and c were calculated for pre-angioplasty, intermediate and post-angioplasty epicardial lesions, without microvascular disease. While hyperemic c decreased from 0.65 for pre-angioplasty to 0.48 for post-angioplasty lesion with guidewire of size 0.35 mm, FFR(myo) increased from 0.52 to 0.87, and CFR increased from 1.72 to 3.45, respectively. Thus, reduced loss produced by momentum change due to lower percentage area stenosis decreased c. For post-angioplasty lesion, c decreased from 0.55 to 0.48 with the insertion of guidewire. Hence, increased viscous loss due to the presence of guidewire decreased c compared with a lesion without guidewire. Further, c showed a linear relationship with FFR(myo), CFR and percentage area stenosis for pre-angioplasty, intermediate and post-angioplasty lesion. These baseline values of c were developed from fluid dynamics fundamentals for focal lesions, and provided a single hemodynamic endpoint to evaluate coronary stenosis severity.

Analysis of blood flow in an out-of-plane CABG model

Coronary artery bypass graft (CABG) is a routine surgical treatment for ischemic and infarcted myocardium. A large number of CABG fail postoperatively because of intimal hyperplasia within months or years. The cause of this failure is thought to be partly related to the flow patterns and shear stresses acting on the endothelial cells. An accurate representation of the flow field and associated wall shear stress (WSS) requires a detailed three-dimensional (3D) model of the CABG. The purpose of this study is to present a detailed analysis of blood flow in a 3D aorto/left CABG, bypassing the occluded left anterior descending coronary (LAD) artery. The analysis takes into account the influence of the out-of-plane geometry of the graft. The finite volume technique was employed to model the 3D blood flow pattern to determine the velocity and WSS distributions. This study presents the flow field distributions of the velocity and WSS at four instances of the cardiac cycle, two in systole and two in diastole. Our results reveal that the CABG geometry has a significant effect on the velocity distribution. The axial velocity profiles at different instances of the cardiac cycle exhibit strong skewing; significant secondary flow and vortex structures are seen in the in-plane velocity patterns. The maximum WSS on the bed of the occluded LAD artery opposite to the graft junction is 14 Pa in middiastole, whereas there is a significantly lower and more uniform distribution of WSS on the bed of the anastomosis. The present results indicate that nonplanarity of the blood vessel along with the inflow conditions has a substantial effect on the fluid mechanics of CABG that contribute to the patency of graft.

Physiologic Assessment of Coronary Artery Stenosis without Stress Tests: Noninvasive Analysis of Phasic Flow Characteristics by Transthoracic Doppler Echocardiography

We evaluated the significance of the diastolic-to-systolic blood flow velocity ratio (DSVR) determined by transthoracic Doppler echocardiography, for a physiologic assessment of the severity of coronary stenosis without stress tests, as compared with thallium 201 single photon emission computed tomography. In 95 patients undergoing thallium 201 single photon emission computed tomography for coronary artery disease, the flow velocity in the distal left anterior descending coronary artery was obtained with transthoracic Doppler echocardiography. The mean and peak DSVR values were calculated using mean and peak coronary flow velocity. DSVR was successfully measured for 82 patients (86.3%), including 33 patients with reversible perfusion defects in the left anterior descending coronary artery territories. For predicting reversible perfusion defects in thallium 201 single photon emission computed tomography, the best cut-off points were 1.5 for mean DSVR (sensitivity 81.8%, specificity 85.7%) and 1.6 for peak DSVR (sensitivity 75.7%, specificity 83.6%). Noninvasive measurement of DSVR with transthoracic Doppler echocardiography provides physiologic estimation of the left anterior descending coronary artery stenosis severity at high success rate, without stress tests.

Delineating the guide-wire flow obstruction effect in assessment of fractional flow reserve and coronary flow reserve measurements

Hemodynamic analysis was conducted to determine uncertainty in clinical measurements of coronary flow reserve (CFR) and fractional flow reserve (FFR) over pathophysiological conditions in a patient group with coronary artery disease during angioplasty. The vasodilation-distal perfusion pressure (CFR-p(rh)) curve was obtained for 0.35- and 0.46-mm guide wires. Our hypothesis is that a guide wire spanning the lesions elevates the pressure gradient and reduces the flow during hyperemic measurements. Maximal CFR-p(rh) was uniquely determined by the intersection of measured CFR and calculated p(rh) of native and residual epicardial lesions in patients without microvascular disease, during angioplasty. Extrapolation of the linear curve gave a zero-coronary flow mean pressure (p(zf)) of approximately 20 mmHg and a corresponding p(rh) of 55 mmHg in the native lesions, which coincided with the level that causes ischemia in human hearts. On this linear curve, values of CFR and FFRmyo (pathophysiological condition) and CFRg and FFRmyog (in the presence of the guide wire) were obtained in native and residual lesions. A strong linear correlation was found between CFR and CFRg [CFR = CFRg x 0.689 + 1.271 (R2= 0.99) for 0.46 mm and CFR = CFRg x 0.757 + 1.004 (R2= 0.99) for 0.35 mm] and between FFRmyo and FFRmyog [FFRmyo = FFRmyog x 0.737 + 0.263 (R2= 0.99) for 0.46 mm and FFRmyo = FFRmyog x 0.790 + 0.210 (R2= 0.99) for 0.35 mm]. This study establishes a strong correlation between CFR and CFRg and between FFRmyo and FFRmyog, which could be used to obtain the true state of occlusion in the coronary artery during angioplasty.

Impact of ostial stenosis on the Doppler flow profiles in internal thoracic artery graft

As ostial stenoses of internal thoracic artery (ITA) grafts rarely occur after coronary artery bypass grafting, little is known about their Doppler flow profile. This report describes changes in the Doppler flow of ITA grafts with ostial stenosis after surgical repair of the stenosis. A 54-year-old male underwent coronary artery bypass grafting (CABG) in which the left ITA was anastomosed to the left anterior descending coronary artery. The follow-up coronary angiography revealed an ostial 90% stenosis of the ITA. The patient underwent elective surgery during which the radial artery was interposed between the left subclavian artery and the ITA. Intraoperative ultrasonography was performed immediately before cut down of the ITA graft and again immediately after completion of all anastomoses. Both diastolic and systolic velocities and the velocity time integral increased more than 2-fold after the repair. Neither the diastolic-to-systolic peak velocity ratio nor the diastolic velocity time integral fraction showed remarkable change. These profiles were different from those reported previously for distal stenosis.

Blood Flow in Coronary Artery Bypass Vein Grafts: Volume versus Velocity at Cardiovascular MR Imaging

Forty-nine patients with previous bypass surgery underwent coronary angiography and cardiovascular magnetic resonance (MR) imaging of single-vein bypass grafts. Volume flow and velocity analyses were performed and compared on MR velocity maps. Bland-Altman analysis showed close agreement between the two types of analysis. Comparison of areas under the receiver operating characteristic curve revealed no significant differences between the analyses for detection of stenoses of 70% or greater. Diagnostic accuracy for volume flow and velocity parameters was 92% and 93%, respectively. Velocity analysis appears to be the preferred method, because it is less time-consuming and has a similar diagnostic accuracy to volume flow analysis.

Flow resistance over technical anastomoses in relation to the angle of distal end-to-side connections

OBJECTIVE

The fluid dynamics within the anastomotic region may contribute to graft failure. The aim was to investigate how flow resistance is affected by anastomotic angle and viscosity.

DESIGN

Technical end-to-side anastomoses of 3-mm inlet/outlet diameter were precision-drilled in plastic blocks at 90 degrees, 60 degrees, 45 degrees, and 30 degrees angles, with 0 degrees as control to calculate relative resistance. Five pressure loads of 200-1000 mmH (2) O and two fluid viscosities were tested using pulsatile perfusion.

RESULTS

Absolute resistances showed a significant linear correlation with pressure, regardless of anastomotic angle and with similar slope values. Increased viscosity produced an upward shift in resistance but otherwise had no effect. However, the relative resistances showed transition zones at increased pressures, which were sensitive to viscosity. The 30 degrees angle produced less resistance than steeper angles.

CONCLUSION

Previous studies gave contradictory results about the importance of anastomotic angle. Our findings suggest that in iso-diametrical technical anastomoses a low angle is favorable. The complexity of angularity in distal aorto-coronary anastomoses was illustrated by threshold phenomena at different pressures, possibly due to induced turbulence and anastomotic resonance.

Assessment of coronary flow reserve by adenosine transthoracic echocardiography: validation with intracoronary Doppler

OBJECTIVE

Data on the accuracy of transthoracic echocardiographic (TTE) analysis of coronary flow reserve are scarce. We compared coronary flow reserve measurements assessed using TTE with those achieved using the gold standard of intracoronary Doppler.

METHODS

Twenty-one patients admitted for elective coronary angioplasty to the circumflex or left anterior descending (LAD) coronary artery underwent TTE immediately before angioplasty, both at rest and during intravenous administration of adenosine 140 microg/kg/min. Transthoracic images of distal LAD coronary diameter and coronary flow were obtained in 14 patients (66%). These patients then underwent intracoronary Doppler analysis of coronary flow reserve in the distal LAD coronary artery. In 1 patient with a proximal LAD artery lesion, the narrowing could not be crossed with the Doppler guidewire. Paired data on coronary flow reserve were therefore available in 13 patients.

RESULTS

Patients were aged 61.7 +/- 8.3 years. Ten were men. Body mass index was 26.3 +/- 4.6 kg/m(2). Resting distal LAD artery blood flow was 18.4 +/- 9.0 mL/min assessed by TTE versus 17.6 +/- 8.1 mL/min by intracoronary Doppler. Hyperemic flow was 36.3 +/- 23.4 versus 33.1 +/- 19.2 mL/min, respectively. Coronary flow reserve was therefore 1.89 +/- 0.66 by TTE compared with 1.83 +/- 0.62 by intracoronary Doppler. Limits of agreement for coronary flow reserve were -0.28 to +0.44, well within boundaries of clinical acceptability.

CONCLUSION

Transthoracic echocardiography is capable of providing accurate data on coronary flow reserve in the distal LAD coronary artery. As a truly noninvasive modality, this technique offers advantages over traditional invasive procedures.

Comparison of the postoperative blood flow waveforms of the bypassing grafts in patients following minimally invasive direct coronary artery bypass

PURPOSE

To use Doppler ultrasound velocimetry to detect and compare the postoperative flow characteristics of the bypassing grafts in patients following minimally invasive direct coronary artery bypass surgery (MIDCAB).

MATERIALS AND METHODS

From January 1997 to June 1999, 34 patients underwent MIDCAB with the left internal thoracic artery (LITA) to the left anterior descending coronary artery (LAD) [n = 23], with the right gastroepiploic artery (RGEA) to the right posterior descending artery (RPD) [n = 3], or with the LITA with a saphenous vein graft extension to the LAD (n = 6), the diagonal coronary artery (n = 1), or the right acute coronary artery (n = 1). There were two patients with LITA to the LAD and RGEA to the RPD. Patients underwent MIDCAB due to coronary artery stenosis (100% occlusion, n = 10; 90 to 99% stenosis, n = 18; < 90% stenosis, n = 5) or unsuccessful percutaneous transcoronary angioplasty with dissection (n = 1). All patients underwent flow velocity measurement by Doppler ultrasound velocimetry in the immediate postoperative period, and at 6-month and 12-month intervals; graft flows were quantified based on Doppler velocimetric data.

RESULTS

The results showed that in a patient with a totally occluded LAD or RPD, typical biphasic velocity waveforms were consistently observed. However, a delayed diastolic wave was noted in RGEA grafts. In patients with less-occluded stenotic lesions or with strong back flows, the flow velocity patterns showed biphasic waveforms but systolic reversal was observed in the area closest to the anastomotic site.

CONCLUSION

The presence of an LAD or RPD stenosis proximal to the anastomotic site significantly affects the LITA or RGEA graft flow volume. The biphasic flow pattern proves that an LITA or RGEA graft transports the blood primarily to coronary arteries during the diastolic phase.

Flow dynamics in internal thoracic artery grafts 10 years after coronary artery bypass grafting

BACKGROUND

Flow dynamics in internal thoracic artery grafts 10 years after surgery are not known.

METHODS

Doppler examination was performed in native internal thoracic arteries as a control (n = 8) and in internal thoracic artery grafts to the left anterior descending coronary artery 6 months postoperatively (group A, n = 25), at 5 years (group B, n = 14), and at 10 years (group C, n = 11).

RESULTS

Each graft group showed a diastolic to systolic peak velocity ratio of less than 1.0 at the proximal end, and more than 1.0 at the distal end, but the control group showed a ratio of less than 1.0 throughout the length of the artery. The diastolic peak velocity of the graft groups was significantly faster than that of the control group at the distal end (versus group A, p < 0.01; versus group B, p < 0.005; and versus group C, p < 0.05). The systolic peak velocity of the graft groups was significantly lower than that of the control at the proximal end (versus group A, p < 0.0001; versus group B, p < 0.005; and versus group C, p < 0.0005). There were no significant differences of flow velocities among the graft groups and graft diameter among the four groups.

CONCLUSIONS

Although the internal thoracic artery is systolic predominant, when native artery is used as graft, it changes its hemodynamics to diastolic predominance, especially at the distal end. Even after 10 years, graft flow dynamics are unchanged. This hemodynamic character may be one of the factors related to the superior long-term patency.

Simple detection of severe coronary stenosis using transthoracic doppler echocardiography at rest

Coronary flow velocity can be measured by transthoracic Doppler echocardiography (TTDE). The purpose of this study was to detect severe coronary stenosis using the diastolic-to-systolic flow velocity ratio (DSVR) determined by TTDE at rest. We prospectively examined 190 consecutive patients with angina pectoris for whom coronary angiography was planned. Doppler spectral tracings of flow velocity in the distal left anterior descending artery were recorded by TTDE at rest. The mean and peak DSVR values were computed using mean and peak coronary flow velocities. DSVR measurement by TTDE at rest was performed within 24 hours before angiography, and in patients who underwent coronary intervention it was performed again within 48 hours after the intervention. The success rate for DSVR measurement by TTDE was 83.7%. There were significant differences in peak DSVR and mean DSVR between the patients with severe stenosis (percent diameter stenosis >85%) and those without severe stenosis (1.3 +/- 0.4 vs 1.9 +/- 0.50 and 1.2 +/- 0.4 vs 1.8 +/- 0.5, respectively; p <0.0001). In the 17 patients with successful intervention, DSVR was significantly increased after the procedure (mean 1.2 +/- 0.1 vs 2.0 +/- 0.2; peak 1.2 +/- 0.2 vs 2.0 +/- 0.3, respectively; p <0.0001). For percent diameter stenosis >85%, the best cut-off points were 1.6 for peak DSVR (sensitivity 79.0%, specificity 75.7%) and 1.5 for mean DSVR (sensitivity 77.0%, specificity 77.9%). Thus, DSVR measurement by TTDE is a simple, noninvasive method for detection of severe coronary stenosis at rest.

Intermediate coronary artery stenosis: evidence-based decisions in interventions to avoid the oculostenotic reflex

The prevalence of intermediate coronary artery stenosis (defined as a diameter stenosis of 40% to 70%) is quite large in patients undergoing PTCA. The coronary angiogram is considered the 'gold standard' for the definition of coronary anatomy, in spite of various limitations associated with its use. In recent years, sensor tipped guidewire based methods of physiologic assessment of stenosis severity, like myocardial fractional flow reserve, and poststenotic coronary flow reserve had established their role in the decision making in catheterization laboratory. The decision making should combine morphologic and physiologic assessment as better evidence based approach in guiding therapy to avoid the 'oculostenotic reflex'.

Estimated flow resistance increase in a spiral human coronary artery segment

Coronary flow estimates were made for a spiral coronary artery segment (identified from a post-mortem replica casting) by using a modified Dean number based on the approximate coil radius of curvature, as suggested earlier. The estimates were found to correlate experimental pressure drop data for helical coiled tubes. Over a physiological range of mean Reynolds numbers from 100 to 400 for blood flow through main coronary arteries, estimates of the flow resistance increase relative to a straight lumen segment ranged from about 20 to 80 percent, and were of similar magnitude to those found in a flow study in a sinuous coronary vessel segment with no spiral.

Noninvasive assessment of coronary flow velocity and coronary flow velocity reserve in the left anterior descending coronary artery by Doppler echocardiography: comparison with invasive technique

OBJECTIVES

The purpose of this study was to evaluate whether transthoracic Doppler echocardiography (TTDE) can reliably measure coronary flow velocity (CFV) and coronary flow velocity reserve (CFVR) in the left anterior descending coronary artery (LAD) in the clinical setting.

BACKGROUND

Coronary flow velocity measurement has provided useful clinical and physiologic information. Advancement in TTDE provides noninvasive measurement of CFV and CFVR in the distal LAD.

METHODS

In 23 patients, CFV in the distal LAD was measured by TTDE (5 or 3.5 MHz) under the guidance of color Doppler flow mapping at the time of Doppler guide wire (DGW) examination. Coronary flow velocity in the distal LAD were measured at baseline and hyperemic conditions (intravenous administration of adenosine 0.14 mg/kg/min) by both TTDE and DGW techniques. Coronary flow velocity reserve was defined as the ratio of peak hyperemic to basal averaged peak velocity in the distal LAD.

RESULTS

Clear envelopes of basal and hyperemic CFV in the distal LAD were obtained in 18 (78%) of 23 study patients by TTDE. There were excellent correlations between TTDE and DGW methods for the measurements of CFV (averaged peak velocity: r=0.97, y=0.94x + 0.40; averaged diastolic peak velocity: r=0.97, y=0.94x + 0.69; systolic peak velocities: r=0.97, y=0.91x + 0.87; diastolic peak velocity: r=0.98, y=0.95x + 1.10). Coronary flow velocity reserve from TTDE correlated highly with those from DGW examinations (r=0.94, y=0.95x + 0.21).

CONCLUSIONS

Noninvasive measurement of CFV and CFVR in the distal LAD using TTDE accurately reflects invasive measurement of CFV and CFVR by DGW method.

Coronary bypass flow during use of intraaortic balloon pumping and left ventricular assist device

BACKGROUND

Intraaortic balloon pumping (IABP) and left ventricular assist device (LVAD) are used for left ventricular support when low cardiac output occurs after a coronary bypass operation for serious coronary artery disease. There are hemodynamic differences in blood flow in various kinds of coronary artery bypass grafts, caused by their inherent physiologic characteristics. The hemodynamic effects of left ventricular assistance with IABP and LVAD on blood flow through various coronary artery bypass grafts were investigated.

METHODS

An ascending aorta-coronary bypass graft (ACB), an internal thoracic artery, and a descending aorta-coronary bypass graft were anastomosed to the left anterior descending coronary artery in a canine model. In this experimental model, the blood flow to the same coronary bed in the three types of grafts could be evaluated. Blood flow in the left anterior descending coronary artery through the three types of coronary bypass grafts was studied in this model during or in the absence of ventricular assistance.

RESULTS

In the control study, the systolic blood flow did not differ among the three types of grafts, but the diastolic flow decreased in the following order: with the ACB, the internal thoracic artery, and the descending aorta-coronary bypass graft. The systolic flow during IABP and LVAD was similar to the control flows. Use of IABP increased the diastolic flow by 75.3%+/-12.4% of the control value in the ACB, 37.9%+/-25.0% in the internal thoracic artery, and 21.2%+/-11.4% in the descending aorta-coronary bypass graft. The LVAD increased the diastolic flow by 97.7%+/-18.7% of the control value in the ACB, 64.5%+/-25.7% in the internal thoracic artery, and 63.0%+/-27.9% in the descending aorta-coronary bypass graft. The diastolic blood flows in the left anterior descending coronary artery and the three types of grafts were significantly greater with IABP than the control values, and significantly greater with LVAD than with IABP and the control values. The degrees of increase of diastolic flows in the left anterior descending coronary artery and the ACB with IABP and LVAD were significantly greater than in the arterial grafts (p < 0.01).

CONCLUSIONS

The diastolic flows in the internal thoracic artery and descending aorta-coronary bypass graft increased less than in the native left anterior descending coronary artery and ACB during left ventricular assistance, particularly with IABP. It is important for the selection of tactics for the management of catastrophic status after coronary bypass grafting to consider the hemodynamic characteristics of the graft.

Ergonovine-induced alterations in coronary flow velocity preceding onset of occlusive spasm in patients without significant coronary artery stenoses

This study examined serial changes in coronary flow velocity to elucidate the dynamic change of coronary circulation during coronary spasm. Twenty patients with variant angina and 27 control patients were studied. Coronary flow velocity was monitored using a Doppler guidewire following intracoronary ergonovine administration. In the control group, diastolic flow velocity either did not change or increased slightly in response to ergonovine. However, in patients with variant angina, 2 patterns of flow velocity alterations were observed. In the first pattern, flow initially increased and then suddenly decreased (16 of 20 patients). In the second pattern, flow gradually decreased (3 of 20 patients). In the remaining patient, the coronary flow alteration could not be detected because of branch spasm. When abnormally high flow velocity was defined as a 100% increase in flow after ergonovine administration within 1 minute, and abnormally low flow velocity was defined as a 50% decrease in flow to diagnose variant angina, sensitivities of 35%, 75%, and 85% were noted if flow was measured 1.0, 2.0, and 3.0 minutes after ergonovine administration, respectively. These abnormal flow velocities were observed before ischemic ST changes appeared. In conclusion, in patients with variant angina, characteristic serial changes in coronary flow velocity occur before occlusive spasm. Variant angina may be diagnosed earlier by monitoring flow velocity rather than by monitoring for ischemic electrocardiographic changes.

Noninvasive analysis of coronary artery poststenotic flow characteristics by using transthoracic echocardiography

This study was performed (1) to test whether transthoracic echocardiography may detect coronary flow velocity in the left anterior descending coronary artery distal to stenoses; and (2) to noninvasively assess coronary artery hemodynamics distal to coronary artery stenoses. High-frequency transthoracic echocardiography was used to assess blood velocity patterns in the distal segment of the left anterior descending coronary artery of 128 consecutive patients (mean age, 58 +/- 9 years; 97 men and 31 women) who underwent cardiac catheterization for investigation of angina. Biphasic, diastolic predominant Doppler velocity patterns were obtained in 67 patients (52%). There was no significant difference in any measurements of systolic blood velocity between patients with unobstructed (less than 30% stenosis) left anterior descending coronary artery, moderate stenosis (30% to 70% obstruction), or severe stenosis (more than 70% obstruction). Patients with severe stenosis demonstrated a reduction in the diastolic component of blood flow velocity in the distal left anterior descending coronary artery compared with patients in the other two groups. This technique may be useful for the noninvasive assessment of the significance of stenotic left anterior descending coronary artery disease or the outcome of interventional procedures.

Alpha-adrenergic vasoconstriction reduces systolic retrograde coronary blood flow

There is a paradoxical alpha-adrenoceptor-mediated coronary vasoconstriction whenever there is adrenergic activation of the heart, as during cardiovascular reflexes or exercise. A previous study demonstrated that this paradoxical vasoconstriction helps maintain blood flow to the vulnerable inner layer of the left ventricular wall during exercise, but the mechanism for this effect was not elucidated. The purpose of the present investigation was to test the hypothesis that alpha-adrenoceptor-mediated vasoconstriction lessens the to-and-fro oscillation of blood flow that occurs in the coronary arterial tree during systole and diastole. Septal coronary artery blood velocity was measured in anesthetized open-chest dogs with a 20-MHz pulsed Doppler velocimeter. Systolic retrograde velocity and diastolic forward velocity were compared during norepinephrine infusion before and after alpha-adrenoceptor blockade with phenoxybenzamine. Systolic aortic pressure was held constant by aortic banding; heart rate was controlled by pacing at 80, 140, and 200 beats/min; and maximum left ventricular dP/dt was unchanged by alpha-blockade. At each pacing rate, systolic retrograde velocity was significantly greater after alpha-blockade, indicating that alpha-vasoconstriction reduced systolic retrograde flow by changing coronary vascular impedance. Transmural blood flow was measured with microspheres in a second group of dogs during the same experimental conditions, and flow to the inner layer of the left ventricle was diminished by alpha-adrenoceptor blockade at a heart rate of 250 beats/min, demonstrating a beneficial effect of alpha-vasoconstriction. In conclusion, adrenergic alpha-adrenoceptor-mediated coronary vasoconstriction reduces systolic retrograde coronary flow during norepinephrine infusion. This lessens to-and-fro flow oscillation in the coronary circulation and probably is the mechanism whereby alpha-vasoconstriction helps maintain blood flow to the inner layer of the left ventricle during exercise.

Analysis of phasic flow velocity dynamics in the left anterior descending coronary artery before and after angioplasty using transthoracic echocardiography in patients with stable angina pectoris

High-frequency transthoracic Doppler echocardiography was used to determine the effects of significant stenosis on distal coronary blood flow velocity profiles. Before coronary angioplasty there was a reduction in diastolic and systolic flow and diastolic/systolic peak velocity ratio. After successful angioplasty velocity ratios returned to normal.

Evaluation of successful PTCA by transstenotic flow velocity ratios

Doppler probes mounted on the tip of a guidewire allow the measurement of coronary blood flow velocities, not only proximal but also distal to stenoses eligible for percutaneous transluminal coronary angioplasty (PTCA). The objective of this study was to determine the improvement of transstenotic Doppler flow velocity ratios following PTCA and to investigate the possible impact on restenosis during follow-up control angiography three months later. Doppler flow velocity measurements were performed in 29 patients with 29 stenoses eligible for PTCA. Results of PTCA were morphologically evaluated by computer-assisted quantitative coronary angiography (QCA) and measured hemodynamically by determining transstenotic Doppler flow velocity ratios. Successful PTCA according to QCA was present in all cases with a reduction of mean diameter stenosis from 66 +/- 8% to 35 +/- 7%. Resting spectral peak velocities and velocity integrals were markedly reduced distal to lesions (all P < 0.001), resulting in mean transstenotic flow velocity and velocity integral ratios of less than 0.60 prior to PTCA. Owing to endoluminal enlargement, significant improvement of transstenotic Doppler ratios was observed in mean ratios greater than 0.90 (all P < 0.0001). In patients with restenosis, transstenotic ratios following PTCA demonstrated a tendency to be smaller than in patients without restenosis. Transstenotic Doppler flow velocity ratios are diminished in severe coronary stenoses. Improvement of these ratios provides information on hemodynamic success of interventional procedures. Thus, the determination of intracoronary Doppler flow velocity ratios contributes, in addition to angiographic estimation, to the evaluation of stenoses severity and success of interventional procedures.

Practical coronary physiology. Clinical application of the Doppler flow velocity guide wire

Coronary angiography incompletely delineates the physiologic consequences of many epicardial stenoses. Intracoronary translesional flow velocity measurements using the Doppler flow wire during cardiac catheterization provide immediate data discriminating the physiologic significance of coronary stenoses. The validity and accuracy of the flow wire for analyzing lesion hemodynamic significance have been confirmed in multiple studies. Flow velocity analysis provides objective criteria for refining the selection of cases for revascularization, and prospective clinical data have confirmed the safety of deferring intervention on lesions with normal physiologic assessment. Translesional and distal coronary flow velocity dynamics during procedures also provide immediate data assessing the physiologic adequacy of intervention. Impaired postintervention distal coronary flow velocity and vasodilator reserve can predict subsequent clinical events, and comparisons of flow velocity indices prestenting and poststenting suggest that physiologically inadequate results of angioplasty may be improved by additional intervention. Flow velocity assessment may also have utility in profiling the adequacy of infarct artery reperfusion following acute myocardial infarction. Evidence has been accumulated to support use of Doppler flow velocity analysis as a clinically relevant technique for improving both diagnostic and therapeutic aspects of cardiovascular medicine.

Hemodynamic consequences of stenosis remodeling during coronary angioplasty

Quantitative hemodynamic assessment during various endovascular interventions including balloon angioplasty is lacking. Translesional pressure drops measured by angioplasty catheters can cause flow blockage and thus lead to inaccurate estimates of preintervention and postintervention flow rates. A new analytical model of the flow rate-pressure drop relation across vascular stenoses is utilized that is nonlinear yet relatively simple in principle, easily applicable in vivo, and compatible with the presence of catheters. The model incorporates in vitro experimental evidence, angiographic data on the dimensions and shapes of coronary arterial stenoses before and after balloon angioplasty, reported translesional pressure gradients, and measurements of coronary flow reserve. Reasonable estimates of mean coronary artery flow rates and translesional pressure drops in the absence of angioplasty catheters are obtained. Prior to angioplasty significant flow restriction across a 68% diameter stenosis exists during hyperemic flow conditions. Following successful balloon dilation, increased minimal cross-sectional area (residual 40% diameter stenosis) results in an improved flow rate-pressure drop relation. Despite minimal flow restriction during hyperemic conditions following angioplasty remodeling, residual luminal constriction leads to elevated wall shear stress levels within the entry region of the stenosis. The flow analysis described may be of clinical utility in evaluating the hemodynamic significance of the anatomic severity of stenoses in coronary and peripheral arteries before and after endovascular therapeutic interventions.

Is internal thoracic artery grafting suitable for a moderately stenotic coronary artery?

Grafting an internal thoracic artery to a coronary artery with moderate stenosis remains controversial. Competitive flow from the native coronary artery has been proposed as the cause of distal narrowing and ultimate failure of the internal thoracic artery graft. We investigated intraoperative phasic blood flow in internal thoracic arteries grafted to coronary arteries with various degrees of stenosis and the influence of stenosis on postoperative angiographic findings. One hundred patients who underwent coronary artery bypass grafting of an internal thoracic artery to the left anterior descending coronary artery were divided into three groups according to degree of coronary stenosis. Group 1 included 39 patients who had 75% or less stenosis, group 2 included 34 patients with stenosis from 76% to 90%, and group 3 included 27 patients with stenosis greater than 90%. Mean flow and peak systolic flow of internal thoracic artery graft in group 1 were lower than those in group 2 (p < 0.01, p < 0.05). Peak diastolic flow in group 1 showed no difference from flows in groups 2 and 3. In eight patients in group 1, internal thoracic artery flow showed a predominant diastolic peak with characteristic systolic reversal as a result of competitive flow from the native coronary artery. Angiography at 1 month showed that the internal thoracic artery graft was patent in every case. Relative contributions of native coronary artery and internal thoracic artery flow to distal perfusion differed among the three groups (p < 0.001). In group 1, 15% of patients showed native-dominant flow, 62% showed balanced flow, and 23% showed internal thoracic artery-dependent flow. In group 2, 9% of patients showed native-dominant flow, 29% showed balanced flow, and 62% showed internal thoracic artery-dependent flow. In group 3, 96% of patients showed internal thoracic artery-dependent flow. String sign of the internal thoracic artery graft developed in only three patients; in two of these patients internal thoracic arteries were grafted to coronary arteries with stenosis of 50% or less and in the other patient there was competitive flow from a diagonal vein graft. Eleven of 13 internal thoracic arteries grafted to coronary arteries with stenosis of 50% or less did not show string sign. Competitive flow from a moderately stenotic coronary artery did not predispose the patient toward string sign of the internal thoracic artery graft in the presence of substantial diastolic internal thoracic artery flow. We conclude that internal thoracic artery grafting is acceptable for a moderately stenotic coronary artery.

Numerical analysis of steady flow in aorto-coronary bypass 3-D model

Intimal hyperplasia and atherosclerosis have a predominant role in the failure of coronary artery bypass procedures. Theoretical studies and in vivo observations have shown that these pathologies are much more likely to occur in the proximity of end-to-side anastomosis, thus indicating that fluid dynamic conditions may be included in the pathogenic causes of the initiation, progression and complication of intimal hyperplasia. In order to study the fluid dynamics at the anastomosis of an aortocoronary bypass, a three-dimensional mathematical model based on a FEM approach was developed. Steady-state simulations were studied in two different geometrical models of anastomosis which differ in their insertion angles (45 and 60 degree). Flow fields with three-dimensional helical patterns, secondary flows, and shear stresses were also investigated. The results show the presence of low shear stresses on the top wall just beyond the toe of the anastomosis and in the region of the coronary artery before the junction. A high wall shear stress region is present on the lateral wall of the coronary artery immediately downstream from the anastomosis. The influence of flow rate distribution on the secondary flows is also illustrated. These results confirm the sensitivity of flow behavior to the model's geometrical parameters and enhance the importance of reproducing the anastomosis junction as closely as possible in order to evaluate the effective shear stress distribution.

Internal thoracic artery graft function during exercise assessed by transthoracic Doppler echography

BACKGROUND

Noninvasive quantitative assessment of internal thoracic artery (ITA) graft function at rest and during exercise is important in patients who have undergone coronary artery bypass grafting.

METHODS

Blood flow in the ITA graft was measured using transthoracic color Doppler echography before and after operation in 50 patients who underwent coronary artery grafting using an ITA to the left anterior descending artery. The patients were divided into three groups according to the degree of coronary stenosis and previous anterior myocardial infarction: Group 1 included 12 patients with severe (90% or more) coronary stenosis accompanied by anterior infarction. Group 2 included 26 patients with severe coronary stenosis without anterior infarction. Group 3 included 12 patients with moderate (75% or less) coronary stenosis without anterior infarction. Transthoracic echographic images of the ITA were obtained through the first intercostal space using a 7.5-MHz probe, and the diameter and cross-sectional area of the ITA were measured on B-mode imaging. Systolic, diastolic, and mean blood flow velocity and volume were measured by the Doppler method.

RESULTS

Internal thoracic artery diameter increased significantly from 2.2 mm to 2.4 mm after operation. The ITA flow patterns in both flow velocity and volume changed from systolic-dominant to diastolic-dominant after operation. Postoperative ITA graft flow was 82 +/- 24 mL/min, 53 +/- 30 mL/min, and 31 +/- 15 mL/min (p < 0.01, group 1 versus 3; p < 0.05, group 1 versus 2) and percent diastolic fraction of ITA flow was 72%, 68%, and 62% (not significant) in groups 1, 2, and 3, respectively. Compared with intraoperative ITA flow, which was measured using an ultrasound transit-time flowmeter, postoperative ITA graft flow was increased in group 1, but not changed in group 2 or 3. The ITA graft flow was measured before and after exercise in 19 patients and was compared with ITA flow in 10 normal control subjects. The ITA graft flow increased significantly with exercise in all patients in the three groups. Percent diastolic fraction of ITA flow increased significantly with exercise in patients with severe coronary stenosis (groups 1 and 2), but decreased significantly in patients with moderate stenosis (group 3).

CONCLUSIONS

Changes in native coronary artery and ITA graft may be predicted by assessing ITA flow pattern during exercise. Transthoracic color Doppler echography is a clinically useful noninvasive method of assessing ITA graft function at rest and during exercise.

Assessing the physiologic significance of coronary artery disease: role of Doppler methodology

It is important to define both anatomic and functional significance of coronary artery stenoses. Quantitative angiography has decreased the inter- and intraobserver variability in interpreting the coronary angiogram, but it is less clinically applicable in assessing functional significance. The coronary Doppler catheter and guidewire can provide considerable information regarding the functional effects and pathophysiology of coronary stenosis in humans at the time of cardiac catheterization. Clinically, it is a simple and safe technique which makes it feasible in a clinical setting to use it as a tool to assess the physiologic significance of an intermediate stenosis or the functional result of an interventional procedure. Other uses for the intravascular Doppler method, such as the evaluation of cardiac transplant vasculopathy and rejection and evaluation of patients with chest pain syndromes and normal coronary angiograms, are being studied. However, the usefulness of this technique in decision-making has yet to be fully clarified. Future clinical studies should be directed toward comparing this method with noninvasive methods, that is, exercise treadmill test and thallium studies, and attempt to answer questions regarding its prognostic value.

Flow rate-pressure drop relation in coronary angioplasty: catheter obstruction effect

Quantitative methods to measure the hemodynamic consequences of various endovascular interventions including balloon angioplasty are limited. Catheters measuring translesional pressure drops during balloon angioplasty procedures can cause flow blockage and thus inaccurate estimates of pre- and post-intervention flow rates. The purpose of this investigation was to examine the influence of the presence and size of an angioplasty catheter on measured mean pressure gradients across human coronary artery stenoses. Analytical flow modeling and in vitro experimental evidence, coupled with angiographic data on the dimensions and shape of stenotic vessel segments before and after angioplasty, indicated significant flow blockage effects with the catheter present.

New concepts for interpretation of intracoronary velocity and pressure tracings

The development of quantitative angiography and the introduction of new imaging techniques cannot replace functional methods of assessing the severity of stenosis. Measurement of transstenotic pressure gradient and poststenotic flow velocity using miniaturised sensors with guidewire technology offers an alternative to the conventional non-invasive methods that is immediately applicable in the catheterisation laboratory during interventional procedures. The complexity of the coronary circulation, however, makes it difficult to establish simple cut-off criteria to identify the presence of a flow-limiting stenosis. For intermediate lesions or in the presence of variable haemodynamic conditions, the accuracy of the assessment can be improved by the application of more complex indices proposed and validated in the laboratory animals. Two of these indices are myocardial fractional flow reserve and the slope of the instantaneous relation between pressure or pressure gradient and flow velocity.

Estimation of severity of stenosis with a Doppler guide wire in the experimental models

Application of the continuity equation to Doppler catheter measurement of quantitative coronary flow velocity has been reported recently to be one of the accurate methods to evaluate mild to moderate coronary stenosis. This method, however, has not been validated in moderate to severe coronary stenosis. Furthermore, the ratio of prestenotic velocity/stenotic velocity may be influenced by side branches proximal to moderate to severe stenosis. Therefore we designed this study to evaluate the accuracy of the continuity equation method in the assessment of moderate to severe stenosis by an 0.018-inch (0.46 mm) Doppler guide wire (12 MHz) and the influence of a side branch on estimation of stenotic severity. Doppler spectra were recorded in the straight rigid tubes (4 mm diameter) with different severities of stenosis (50%, 62.5%, and 75% diameter stenosis) without a side branch with a Doppler guide wire. By the continuity equation, percent diameter stenosis was calculated from the proximal/stenotic and distal/stenotic velocity ratios in each model at different flow rates. In the model of 75% diameter stenosis with a side branch (1 mm diameter) proximal to the stenosis and the model of 75% diameter stenosis with a side branch (1 mm diameter) distal to the stenosis, percent diameter stenosis was calculated by the same method. Percent diameter stenosis derived from the proximal/stenotic velocity ratio in each model was in agreement with the true severity of stenosis in each model without a side branch (48% +/- 1%, 62% +/- 1%, and 75% +/- 1%, respectively). Percent diameter stenosis from the distal/stenotic velocity ratio was also in agreement with the true severity of stenosis in each model without a side branch (48% +/- 3%, 61% +/- 1%, and 75% +/- 1% respectively). In the model of 75% diameter stenosis with a side branch proximal to the stenotic site, however, percent diameter stenosis derived from the proximal/stenotic velocity ratio was underestimated compared with the real stenosis and significantly smaller than that derived from the distal/stenotic velocity ratio (65% +/- 1% versus 74% +/- 1%; p < 0.001). On the other hand, in the model of 75% diameter stenosis with a side branch distal to the stenotic site, the percent diameter stenosis derived from both the proximal/stenotic and distal/stenotic velocity ratios was in agreement with that derived from the distal/stenotic velocity ratio (75% +/- 2% and 77% +/- 1%). In the experimental models the estimation of stenotic severity by a Doppler guide wire is accurate even in moderate to severe stenosis unless a side branch exists just proximal to the stenosis. However, the distal/stenotic velocity ratio is applicable even if a side branch exists. This suggests that severity of stenosis derived from the distal/stenotic velocity ratio may be more useful than that from the proximal stenotic velocity ratio in human coronary stenosis with side branches.

Relation between diastolic perfusion time and coronary artery stenosis during stress-induced myocardial ischemia

BACKGROUND

Experimental studies have demonstrated that during stress-induced myocardial ischemia, coronary obstruction and diastolic perfusion time are factors that limit subendocardial perfusion and correlate to degree of myocardial dysfunction. The relation between these two factors has not yet been investigated in humans. The aim of the present study was to assess the relation between diastolic perfusion time and degree of coronary stenosis during different types of stress tests.

METHODS AND RESULTS

Nine patients with isolated and proximal stenosis of the left anterior descending coronary artery were selected. Patients underwent three different randomized stress tests (upright, supine bicycle stress test, and transesophageal atrial pacing). Diastolic perfusion time, heart rate (RR interval), and systolic and diastolic pressures were measured during the test and at the ischemic threshold (0.1-mV ST-segment depression). Angiographic measurements of coronary stenosis were evaluated by quantitative coronary angiography. At the ischemic threshold, significant differences among tests were found in heart rate (P < .05), systolic pressure (P < .001), and diastolic pressure (P < .05). In each stress test, diastolic perfusion time at the ischemic threshold was closely correlated with minimal stenosis diameter (r = .97; P < .001) and percent diameter stenosis (r = .92; P < .001) with no difference among the tests. In contrast, heart rate, rate-pressure product, and time to ischemic threshold were not significantly correlated with percent diameter stenosis and minimal stenosis diameter. No significant correlation was observed at the ischemic threshold between diastolic perfusion time and corresponding values of heart rate, despite the close correlation at rest (r = .95; P < .001).

CONCLUSIONS

Despite differences in associated hemodynamic responses to various stress tests, a close relation exists between stenosis severity and diastolic perfusion time at the onset of stress-induced myocardial ischemia. Therefore, diastolic perfusion time at the ischemic threshold may be an indirect estimate of the hemodynamic significance of coronary stenosis.

Differential characterization of blood flow, velocity, and vascular resistance between proximal and distal normal epicardial human coronary arteries: analysis by intracoronary Doppler spectral flow velocity

To characterize coronary blood flow velocity parameters and to determine the relation among velocity, volumetric flow, and vascular resistance in awake human beings, we performed paired proximal and distal velocity measurements in 28 angiographically normal coronary arteries. Mean velocity, peak velocity, diastolic-to-systolic velocity ratio, and diameter and cross-sectional area of proximal and distal arteries were determined and coronary flow and vascular resistance computed. Mean velocity and coronary vasodilator reserve were similar for all three native arteries and were preserved from proximal to distal segments. Volumetric flow decreased from proximal to distal segments. The demonstrated inverse and curvilinear (polynomial) relation between volumetric flow and vascular resistance agrees with theoretical and animal models of coronary physiologic characteristics and suggests a nadir of coronary vascular resistance below which coronary flow no longer increases.

Simultaneous measurement of pulmonary venous flow by intravascular catheter Doppler velocimetry and transesophageal Doppler echocardiography: relation to left atrial pressure and left atrial and left ventricular function

OBJECTIVES

The aim of our study was to compare measurements of pulmonary venous flow velocity obtained either by transesophageal Doppler echocardiography or by intravascular catheter Doppler velocimetry. Furthermore, the relation among pulmonary venous flow velocity, left atrial compliance and left atrial pressure was evaluated.

BACKGROUND

Data about the relation between left atrial pressure and pulmonary venous flow velocity are controversial.

METHODS

A total of 32 patients undergoing elective open heart surgery for coronary artery bypass grafting were included prospectively in the study. Pulmonary venous flow velocity (Doppler catheter) and left atrial pressure (microtip pressure transducer) were recorded simultaneously with recordings of pulmonary venous flow velocity obtained by transesophageal Doppler echocardiography.

RESULTS

Agreement between Doppler catheter and Doppler echocardiographic measurements of pulmonary venous flow velocity (n = 18 patients) was analyzed using the Bland-Altmann technique. The 95% limits of agreement were -0.16 to +0.11 m/s for systolic peak velocity, -0.14 to +0.09 m/s for diastolic peak velocity and -0.12 to +0.10 m/s for atrial peak velocity. The closest agreement between both methods was found for the ratio of systolic to diastolic peak velocity, the ratio of systolic to diastolic flow duration and the time from Q deflection on the electrocardiogram to maximal flow velocity. Mean left atrial pressure was strongly correlated with the ratio of systolic to diastolic peak velocity (r = -0.829), systolic velocity-time integral (r = -0.653), time to maximal flow velocity (r = 0.844) and the ratio of systolic to diastolic flow duration (r = -0.556). The ratio of systolic to diastolic peak velocity and the time to maximal flow velocity were identified as strong independent predictors of mean left atrial pressure. Left atrial compliance was not found to be an independent predictor of mean left atrial pressure.

CONCLUSIONS

Flow velocity in the left upper pulmonary vein can be reliably recorded by transesophageal pulsed wave Doppler echocardiography. Our data reveal further evidence that mean left atrial pressure can be estimated by the pattern of pulmonary venous flow velocity.

Doppler flow velocity measurements during coronary angioplasty

In addition to further studies using Doppler catheters to assess blood flow velocity during coronary angioplasty this study intends to evaluate the functional significance of coronary stenoses and to estimate their hemodynamic relevance prior to and after percutaneous transluminal coronary angioplasty (PTCA). Diameters of coronary artery stenoses were quantified by means of the cardiovascular angiographic analysis system (CAAS) both prior to and following successful PTCA in 37 patients. During coronary artery angioplasty a 12 M:Hz 0.018-in. Doppler-tipped guidewire was used to measure prestenotic and poststenotic parameters of coronary artery flow velocity both prior to and following PTCA. The minimal stenosis diameter was raised from 1.01 +/- 0.58 to 1.76 +/- 0.73 mm (P < 0.0001), the percent diameter stenosis decreased from 63 +/- 11 to 35 +/- 6% (P < 0.0001). Prestenotic average (APV) and maximum peak velocity (MPV), peak velocity integral (PVI), average systolic (ASPV) and diastolic (ADPV) peak velocity, systolic (SPVI) and diastolic (DPVI) peak velocity integral, and diastolic/systolic velocity ratio showed--in contrast to further studies--a considerably significant difference (P < 0.05), whereas poststenotic Doppler data (APV, MPV, PVI, ASPV, DSPV, SPVI, DPVI, DSVR) differed highly significantly (P < 0.0001) prior to and following PTCA. Prestenotic and poststenotic measurements of coronary artery flow velocity differed significantly before and after PTCA and offer the potential for estimating both the hemodynamic relevance of coronary artery stenoses and success of PTCA.

Intracoronary Doppler guide wire versus stress single-photon emission computed tomographic thallium-201 imaging in assessment of intermediate coronary stenoses

OBJECTIVES

The purpose of this study was to compare measures of coronary flow reserve by an intracoronary Doppler guide wire with results of stress single-photon emission computed tomographic (SPECT) thallium-201 imaging in patients with intermediate coronary artery disease (40% to 70% stenosis).

BACKGROUND

Visual assessment of the coronary arteriogram as a means of predicting the physiologic significance of intermediate coronary stenoses is inaccurate. Coronary flow reserve is a reliable marker of the functional importance of a coronary lesion. The recent development of an intracoronary Doppler guide wire permits routine assessment of coronary flow reserve distal to coronary artery stenoses.

METHODS

We prospectively evaluated coronary flow reserve in 30 subjects with intermediate stenoses using an intracoronary Doppler guide wire during elective coronary angiography. Patients subsequently underwent stress SPECT thallium-201 testing, and the blinded interpretations were correlated. Coronary flow reserve in a control group with normal coronary arteries classified our sample into group 1 (abnormal flow reserve, < 2.0) and group 2 (normal flow reserve, > or = 2.0).

RESULTS

As defined, the coronary flow reserve of 16 vessels in group 1 was diminished in comparison to that of 19 vessels in group 2 (p = 0.0001). Qualitative and quantitative analysis of stress SPECT thallium-201 images confirmed perfusion defects in 15 of 16 vascular territories in group 1 in contrast to 1 of 19 regions in group 2. The sensitivity, specificity and overall predictive accuracy of Doppler-determined coronary flow reserve for stress SPECT thallium-201 results were 94%, 95% and 94%, respectively.

CONCLUSIONS

In appropriately selected patients with intermediate coronary artery stenoses, Doppler guide wire determination of lesion significance provides equivalent data to those acquired by stress SPECT thallium-201 imaging.