Maximal blood flow velocity in severe coronary stenoses measured with a Doppler guidewire. Limitations for the application of the continuity equation in the assessment of stenosis severity

Assessment of the Severity of Left Anterior Descending Coronary Artery Stenoses by Enhanced Transthoracic Doppler Echocardiography: Validation of a Method Based on the Continuity Equation

BACKGROUND

To verify whether the severity of coronary stenosis could be non-invasively assessed by enhanced transthoracic coronary echo Doppler in convergent color Doppler mode (E-Doppler TTE) over a wide range of values (from mild to severe).

METHODS

Color-guided pulsed wave Doppler sampling in the left anterior descending coronary artery (LAD) was performed in 103 diseased LAD segments (corresponding to 94 patients examined) as assessed by quantitative coronary angiography (QCA) or intracoronary ultrasound (IVUS). The E-Doppler TTE examinations consisted of measuring the velocity (vel) at the stenosis site and a reference adjacent segment. Then the continuity equation (C-Eq) was applied to calculate the percent cross-sectional area reduction (%CSA) at the stenosis site. The applied formula was: %CSA = 100 × (1 - [TVIref × 0.5]/TVIs). TVI = the time velocity integral at the stenosis [s] and the reference site [ref], respectively); 0.5 = the correcting factor for a parabolic profile was used only when the % accelerated stenotic flow was >122% (AsF = diastolic peak vel at first site - diastolic peak vel at second site/diastolic peak vel at second site × 100).

RESULTS

E-Doppler TTE feasibility was 100%. Doppler and QCA/IVUS-derived %CSA stenosis showed very good agreement over a large range of values (from mild to severe), with no significant bias; the maximum difference between QCA/IVUS and transthoracic Doppler %CSA was mostly around 20% with a few patients exceeding this limit (limits of agreement = -27.53 to 23.5%). The scattering was slightly larger for the non-significant stenoses. The correlation was strong (r = 0.89, p < 0.001).

CONCLUSION

E-Doppler TTE is a feasible and reliable method for assessing the severity of LAD stenosis by applying the C-Eq.

Angiography derived assessment of the coronary microcirculation: is it ready for prime time?

INTRODUCTION

Non-obstructive coronary arteries (NOCA) are present in 39.7% to 62.4% of patients who undergo elective angiography. Coronary microcirculation (<400 µm) is not visible on angiography therefore functional assessment, invasive or non-invasive plays a prior role to help provide a more personalized diagnosis of angina.

AREA COVERED

In this review, we revise the pathophysiology, clinical importance and invasive assessment of the coronary microcirculation, and discuss angiography-derived indices of microvascular resistance. A comprehensive literature review over four decades is also undertaken.

EXPERT OPINION

The coronary microvasculature plays an important role in flow autoregulation and metabolic regulation. Invasive assessment of microvascular resistance is a validated modality with independent prognostic value, nevertheless, its routine application is hampered by the requirement of intravascular instrumentation and hyperaemic agents. The angiography-derived index of microvascular resistance has emerged as a promising surrogate in pilot studies, however, more data are needed to validate and compare the diagnostic and prognostic accuracy of different equations as well as to illustrate the relationship between angiography-derived parameters for epicardial coronary arteries and those for the microvasculature.

Nanoparticles eluting stents for coronary intervention: Formulation, characterization and in vitro evaluation

Coronary artery disease (CAD) is currently a leading cause of death worldwide. In the history of percutaneous coronary intervention for the treatment of CAD, a drug-eluting stent (DES) is recognized as a revolutionary technology that has the unique ability to significantly reduce restenosis and provide both mechanical and biological solutions simultaneously to the target lesion. The aim of the research work was to design and fabricate DES coated with a nanoparticulate drug formulation. Sirolimus, an inhibitor of the smooth muscle cell (SMC) proliferation and migration, was encapsulated in polymeric nanoparticles. The nanoparticle formulation was characterized for various physicochemical parameters. Cell viability and cell uptake studies were performed using human coronary artery smooth muscle cells (HCASMCs). The developed nanoparticle formulation showed enhanced efficacy compared to plain drug solution and exhibited time-dependent uptake into the HCASMCs. The developed nanoparticle formulation was coated on the FlexinniumTM ultra-thin cobalt-chromium alloy coronary stent platform. The nanoparticle coated stents were characterized for morphology and residual solvent analysis. In-vitro drug release was also evaluated. Ex-vivo arterial permeation was carried out to evaluate the nanoparticle uptake from the surface of the stents. The characterization studies together corroborated that the developed nanoparticle coated stent can be a promising replacement of the current drug-eluting stents.

Influence of Dissolution Vessel Geometry and Dissolution Medium on In Vitro Dissolution Behaviour of Triamterene-Coated Model Stents in Different Test Setups

The aim of this study was to investigate if the geometry of the dissolution vessel, the dissolution medium volume and composition might contribute to the variation in drug release from drug-eluting stents (DES) in different test setups, which has been observed in previous in vitro studies. Therefore, DES containing triamterene as model substance were produced via fluidised-bed technology. Dissolution testing was carried out using different incubation setups, the reciprocating holder (USP Apparatus 7) and two flow-through methods, a method similar to the USP Apparatus 4 (FTC) and the vessel-simulating flow-through cell (vFTC) equipped with a hydrogel as a second compartment simulating the blood vessel wall. The results indicate that dissolution vessel geometry and medium volume had no influence on the release behaviour and only the flow-through cell methods yielded a lower dissolution rate than the incubation setups (80.6 ± 2.0% released in the FTC after 14 days compared to > 90% for all incubation setups). The composition of the hydrogel used in the vFTC also affected the dissolution rate (53.9 ± 4.5% within 14 days with a hydrogel based on phosphate-buffered saline compared to 78.2 ± 1.2% obtained with a hydrogel based on water) possibly due to different solubility of triamterene in the release media as well as interactions between the coating polymer and the release medium. Hence, the introduction of a hydrogel as a second compartment might lead to a more biorelevant test setup.

In vitro evaluation of paclitaxel coatings for delivery via drug-coated balloons

Lately, drug-coated balloons have been introduced in interventional cardiology as an approach to treat occluded blood vessel. They were developed for the rapid transfer of antiproliferative drugs during the angioplasty procedure in stenosed vessels with the intent to reduce the risk of restenosis. In this study five different paclitaxel (PTX) balloon coatings were tested in vitro in order to examine how solvents and additives influence coating stability and drug transfer rates. PTX-coated balloons were advanced through a guiding catheter and a simulated coronary artery pathway under perfusion and were then inflated in a hydrogel acceptor compartment. The fractions transferred to the gel, remaining on the balloon and the PTX lost in the simulated coronary pathway were then analysed. The results obtained suggest that the solvent used for the coating process strongly influences the surface structure and the stability of the coating. Ethanol/water and acetone based PTX coatings showed the lowest drug transfer rates to the simulated vessel wall (both <1%) due to their high drug losses during the prior passage through the coronary artery model (more than 95%). Balloons coated with PTX from ethyl acetate-solutions showed smaller drug loss (83%±9%), but most of the remaining PTX was not transferred (mean balloon residue approximately 15%). Beside the solvent, the use of additives seemed to have a great impact on transfer properties. The balloon pre-treatment with a crosslinked polyvinylpyrrolidone (PVP) film was able to increase the PTX transfer rate from less than 1% (without PVP) to approximately 6%. The best results in this study were obtained for balloon coatings with commercially available SeQuent© Please balloons containing the contrast agent iopromide. For this formulation drug transfer rates of approximately 17% were determined. Fluorescence microscopic imaging could visualize the particulate transfer of labelled PTX from the balloon surface during dilatation. The findings of this study underline the importance of drug adhesion and coating stability for the efficiency of PTX transfer.

Usage of different vessel models in a flow-through cell: in vitro study of a novel coated balloon catheter

Calcium alginate and polyacrylamide hydrogels were used as tissue models for the simulated anatomic implantation process.

Development of hydrophobized alginate hydrogels for the vessel-simulating flow-through cell and their usage for biorelevant drug-eluting stent testing

The vessel-simulating flow-through cell (vFTC) has been used to examine release and distribution from drug-eluting stents in an in vitro model adapted to the stent placement in vivo. The aim of this study was to examine the effect of the admixture of different hydrophobic additives to the vessel wall simulating hydrogel compartment on release and distribution from model substance-coated stents. Four alginate-based gel formulations containing reversed-phase column microparticles LiChroprep® RP-18 or medium-chain triglycerides in form of preprocessed oil-in-water emulsions Lipofundin® MCT in different concentrations were successfully developed. Alginate and modified gels were characterized regarding the distribution coefficient for the fluorescent model substances, fluorescein and triamterene, and release as well as distribution of model substances from coated stents were investigated in the vFTC. Distribution coefficients for the hydrophobic model substance triamterene and the hydrophobized gel formulations were up to four times higher than for the reference gel. However, comparison of the obtained release profiles yielded no major differences in dissolution and distribution behavior for both fluorescent model substances (fluorescein, triamterene). Comparison of the test results with mathematically modeled data acquired using finite element methods demonstrated a good agreement between modeled data and experimental results indicating that gel hydrophobicity will only influence release in cases of fast releasing stent coatings.

Particle depositions and related hemodynamic parameters in the multiple stenosed right coronary artery

BACKGROUND

Blood flow analysis of the human right coronary artery (RCA) has been carried out to investigate the effects of serial stenosis on coronary hemodynamics. A 3-D model of a serial stenosed RCA was reconstructed based on multislice computerized tomography images.

METHODS

A velocity waveform in the proximal RCA and a pressure waveform in the distal RCA of a patient with a severe stenosis were acquired with a catheter delivered wire probe and applied as boundary conditions. The numerical analysis examines closely the effect of a multiple serial stenosis on the hemodynamic characteristics such as flow separation, wall shear stress (WSS) and particle depositions.

RESULTS AND CONCLUSIONS

Energy loss associated with such flow expansion after each constriction will be large and consequently the pressure drop will be higher. Overall pressure drop increased from 1700 Pa (12.75 mmHg) at the end diastole to 11000 Pa (82.5 mmHg) at the peak systole. At the peak systole the WSS values reached 110 Pa in the stenosis with 28% diameter reduction and 210 Pa in the stenosis with 54% diameter reduction, which is high enough to damage the endothelial cells. However at the end of one cardiac cycle a percent of 1.4% (15 from 1063 particles release at the inlet section) remain inside the stenosed RCA.

In-vitro dissolution methods for controlled release parenterals and their applicability to drug-eluting stent testing

Objectives

Dissolution testing is a powerful tool for the characterization of dosage form performance in vitro under standardized conditions. In spite of the increasing number of parenterally administered medicinal products, currently there are no compendial dissolution test methods designed especially for these types of dosage forms. In addition to classical drug delivery systems, drug/device combination products, such as drug-eluting stents, are being used increasingly.

Key findings

This review describes the current methods that are used most often for in-vitro dissolution testing of parenteral dosage forms, i.e. the ‘sample and separate’ methods, the ‘dialysis’ methods, and the ‘flow-through’ methods, with a special emphasis on whether these methods can be used for drug-eluting stent testing. In the light of current regulatory requirements and with the exploding costs of preclinical and clinical development, test systems that include biorelevant parameters and are predictive of in-vivo performance are increasingly important. Published attempts to take biorelevant conditions into consideration in the design of dissolution test apparatus developed for parenteral dosage forms, including a method that was designed to emulate the embedding and flow-conditions at the site of stent implantation, have been outlined in this review.

Summary

In spite of the large quantity of highly potent controlled release parenteral products marketed today, there is still a lack of suitable methods for in vitro dissolution testing for these dosage forms especially with regard to biorelevant testing conditions. For dosage forms implanted into tissues it seems of major importance to reproduce the transport forces which are predominant in vivo (diffusive versus convective) in the in-vitro experimental setup.

Resting coronary flow velocity in the functional evaluation of coronary artery stenosis: study on sequential use of computed tomography angiography and transthoracic Doppler echocardiography

AIMS

Accelerated flow at the site of flow-limiting stenosis can be detected by transthoracic Doppler echocardiography (TTDE). We studied feasibility and accuracy of sequential coronary computed tomography angiography (CTA) and TTDE in detection of haemodynamically significant coronary artery disease (CAD).

METHODS AND RESULTS

We prospectively enrolled 107 patients with intermediate (30-70%) pre-test likelihood of CAD. All patients underwent CTA using a 64-slice scanner. Using TTDE, the ratio of maximal diastolic flow velocity to pre-stenotic flow velocity (M/P ratio) was measured in the coronary segments with stenosis in CTA. In all patients, the results were compared with invasive coronary angiography, including measurement of fractional flow reserve when appropriate. All analyses were done blinded. TTDE was feasible in 276 of 285 evaluated coronary segments. Significant coronary stenoses were associated with a higher M/P ratio than non-significant stenoses (3.59 ± 1.82 vs. 1.28 ± 0.60, P < 0.001). The optimal M/P ratio for detection of significant stenosis was 2.2 (area under receiver operating characteristic curve 0.92, P < 0.001). Compared with the strategy of CTA alone, sequential CTA and focused TTDE had a better positive predictive value (PPV; 61 vs. 78%) and diagnostic accuracy (93 vs. 96%, P = 0.006) without impairment of the negative predictive value (97 vs. 97%).

CONCLUSION

Sequential use of CTA and TTDE is feasible for combined anatomic and functional evaluation of coronary stenoses. Compared with coronary CTA alone, addition of TTDE improved PPV for detection of significant CAD.

Examination of drug release and distribution from drug-eluting stents with a vessel-simulating flow-through cell

The recently introduced vessel-simulating flow-through cell offers new possibilities to examine the release from drug-eluting stents in vitro. In comparison with standard dissolution methods, the additional compartment allows for the examination of distribution processes and creates dissolution conditions which simulate the physiological situation at the site of implantation. It was shown previously that these conditions have a distinct influence on the release rate from the stent coating. In this work, different preparation techniques were developed to examine the spatial distribution within the compartment simulating the vessel wall. These methods allowed for the examination of diffusion depth and the distribution resulting in the innermost layer of the compartment simulating the vessel wall. Furthermore, the in vitro release and distribution examined experimentally were modelled mathematically using finite element (FE) methods to gain further insight into the release and distribution behaviour. The FE modelling employing the experimentally determined diffusion coefficients yielded a good general description of the experimental data. The results of the modelling also provided important indications that inhomogeneous coating layer thicknesses around the strut may result from the coating process which influence release and distribution behaviour. Taken together, the vessel-simulating flow-through cell in combination with FE modelling represents a unique method to analyse drug release and distribution from drug-eluting stents in vitro with particular opportunities regarding the examination of spatial distributions within the vessel-simulating compartment.

Percutaneous, Catheter-Based Coil Embolization of Coronary Fistula:

A 55-year-old male with angina-like chest pain and positive thallium-scintigraphy was admitted to our hospital. Cardiac catheterization was performed for suspected coronary artery disease. The coronary angiogram showed no significant epicardial stenosis, but a large coronary fistula, connecting the left anterior descending artery with the pulmonary artery. Swan-Ganz catheter measurements, intracoronary Doppler, and quantitative coronary angiography were used to determine cardiac output, coronary blood flow, and coronary-to-pulmonary artery shunt fraction. These measurements showed a hyperdynamic cardiac output of 17 L/min, a coronary blood flow of 140 mL/min in the left anterior descending coronary artery with an estimated shunt fraction of 58% into the pulmonary circulation. Percutaneous, catheter-based coil embolization was performed to occlude the fistula. After embolization of one coil, coronary angiography showed the fistula's stump only. Cardiac output (9 L/min) and coronary blood flow (48 mL/min) were almost normalized. The patient was discharged from the hospital the day after the procedure. After a 6-month follow-up, there were still no complaints, angina-like symptoms or signs of myocardial ischemia in stress tests.

Long-term angiotensin-converting enzyme inhibition reduces plasma asymmetric dimethylarginine and improves endothelial nitric oxide bioavailability and coronary microvascular function in patients with syndrome X

Angiotensin-converting enzyme (ACE) inhibition has been shown to improve clinical myocardial ischemia in patients with syndrome X (angina pectoris, positive treadmill exercise test, normal coronary angiograms, and no evidence of coronary spasm). This study was conducted to investigate the effects of long-term ACE inhibitors on endothelial nitric oxide (NO) metabolism and coronary microvascular function in patients with syndrome X. After a 2-week washout period, 20 patients with syndrome X were randomized to receive either enalapril, an ACE inhibitor, 5 mg twice daily (n = 10) or placebo (n = 10) in a double-blind design for 8 weeks. Another 6 age- and gender-matched subjects with negative treadmill exercise tests were also studied as controls. Compared with control subjects, patients with syndrome X had significantly reduced coronary flow reserve, reduced plasma levels of nitrate and nitrite (NOx), and a reduced plasma L-arginine to asymmetric dimethylarginine (ADMA) ratio (an index of systemic NO metabolism), as well as reduced endothelial function. These patients also had increased plasma levels of ADMA, which is an endogenous inhibitor of NO synthase and of von Willebrand factor, a marker of endothelial injury. Baseline characteristics including exercise performance and coronary flow reserve were similar between enalapril and placebo groups. After an 8-week treatment period, exercise duration (p = 0.001) and coronary flow reserve (p = 0.001) significantly improved with enalapril but not with placebo. Enalapril treatment, but not placebo, reduced plasma von Willebrand factor (p = 0.03) and ADMA levels (p = 0.01) and increased NOx levels (p = 0.01) and the ratio of L-arginine to ADMA (p <0.01). In patients with syndrome X, the plasma NOx level was positively and ADMA level inversely correlated with coronary flow reserve before and after the treatment. In conclusion, long-term ACE inhibitor treatment with enalapril improved coronary microvascular function as well as myocardial ischemia in patients with syndrome X. This may be related to the improvement of endothelial NO bioavailability with the reduction of plasma ADMA levels.

Does the application of gadolinium-DTPA have an impact on magnetic resonance phase contrast velocity measurements? Results from an in vitro study

INTRODUCTION/OBJECTIVE

To evaluate the potential influence of various concentrations of gadolinium (Gd)-DTPA on magnetic resonance phase contrast (MR PC) velocimetry.

MATERIAL AND METHODS

Imaging was done with a 1.0 T scanner using a standard Flash 2D sequence and a circular polarized extremity coil. In a validated flow phantom with a defined 75% area stenosis different concentrations of Gd-DTPA, diluted in a 10:1 water-yogurt mixture, MR PC measurements were correlated with a Doppler guide wire as gold standard.

RESULTS

MR PC measurements correlated well with the Doppler derived data (r=0.99; P<0.01; maximum pre-stenotic velocity: 21.6+/-0.5 cm/s; maximum intra-stenotic velocity: 81.7+/-0.6 cm/s). Following Gd-DTPA administration no significant (P>0.05; Student's t-test) flow measurement changes were noted (maximum pre-stenotic velocity: 21.3+/-1.3 cm/s; maximum intra-stenotic velocity: 84.0+/-3.6 cm/s). However, delineation of the perfused lumen was enhanced after the application of Gd-DTPA.

DISCUSSIONS AND CONCLUSION

The application of Gd-DTPA does not affect MR PC velocimetry. However, the application of contrast media allowed a more accurate vessel segmentation. MR PC measurements can be reliably carried out after application of Gd-DTPA.

Non-invasive assessment of coronary flow velocity reserve: a new method using transthoracic Doppler echocardiography

Transthoracic Doppler echocardiography (TTDE) allows noninvasive flow measurement in the distal left anterior descending artery (LAD). The feasibility of detecting coronary flow by contrast-enhanced TTDE with second harmonic technique was assessed, the coronary flow velocity reserve (CFVR) was evaluated in comparison to intracoronary Doppler flow (ICD) analysis and the CFVR after PTCA in LAD was investigated. In 77 (96%) of 80 patients, CFVR was successfully determined with intravenous adenosine infusion. Doppler signal quality was evaluated in the first 46 patients by use of intravenous Levovist infusion and second harmonic technique. The Doppler flow was not visible in 1. patient only. CFVR determined from TTDE (2.77 +/- 0.65) was correlated closely with those from ICD (2.88 +/- 0.78) measurements (y = 0.73x + 0.67, r = 0.87, P < 0.001). In conclusion, TTDE is a feasible method and provides reliable data on CFVR which can be used for follow-up after PTCA.

Physiological transport forces govern drug distribution for stent-based delivery

BACKGROUND

The first compounds considered for stent-based delivery, such as heparin, were chosen on the basis of promising tissue culture and animal experiments, and yet they have failed to stop restenosis clinically. More recent compounds, such as paclitaxel, are of a different sort, being hydrophobic in nature, and their effects after local release seem far more profound. This dichotomy raises the question of whether drugs that have an effect when released from a stent do so because of differences in biology or differences in physicochemical properties and targeting.

METHODS AND RESULTS

We applied continuum pharmacokinetics to examine the effects of transport forces and device geometry on the distribution of stent-delivered hydrophilic and hydrophobic drugs. We found that stent-based delivery invariably leads to large concentration gradients, with drug concentrations ranging from nil to several times the mean tissue concentration over a few micrometers. Concentration variations were a function of the Peclet number (Pe), the ratio of convective to diffusive forces. Although hydrophobic drugs exhibited greater variability than hydrophilic drugs, they achieved higher mean concentrations and remained closer to the intima. Inhomogeneous strut placement influenced hydrophilic drugs more negatively than hydrophobic drugs, dramatically affecting local concentrations without changing mean concentrations.

CONCLUSIONS

Because local concentrations and gradients are inextricably linked to biological effect, our results provide a potential explanation for the variable success of stent-based delivery. We conclude that mere proximity of delivery devices to tissues does not ensure adequate targeting, because physiological transport forces cause local concentrations to deviate significantly from mean concentrations.

Prospective comparison of MR phase-contrast velocimetry with intravascular doppler US during infrainguinal artery angioplasty

PURPOSE

To evaluate the accuracy of magnetic resonance (MR) velocimetry for quantitative assessment of stenosis in patients undergoing percutaneous transluminal angioplasty (PTA).

MATERIALS AND METHODS

Thirty patients underwent PTA of the infrainguinal region. To assess hemodynamic parameters of lesions, MR phase-contrast velocimetry with a circular-polarized extremity receiver coil and a cardiac gated gradient echo sequence was conducted before and 1 day after PTA. Additionally, all lesions were examined by means of intravascular Doppler flow measurements (0.018-inch wire, 12 MHz). From these data, the degree of stenosis was calculated and a comparison of MR velocimetry with intravascular Doppler US was undertaken.

RESULTS

Correlation between calculated grade of stenosis for MR velocimetry and intravascular Doppler US was good and significant (r = 0.74; P <.001). Calculated luminal stenosis grade were similar for both methods before PTA (intravascular Doppler US: 0.62 +/- 0.18, MR velocimetry: 0.54 +/- 0.19; P =.17 with paired Student t-test) and after PTA (0.25 +/- 0.23 and 0.3 +/- 0.2, respectively; P =.56).

CONCLUSION

MR velocimetry results in reliable noninvasive in vivo flow measurements and allows accurate assessment of stenosis in a clinical setting.

Measuring maximal percent area stenosis poststent placement with intracoronary Doppler and the continuity equation and correlation with intracoronary ultrasound and angiography

Quantitative coronary angiography (QCA) and intracoronary ultrasound (ICUS) are methods for anatomic assessment of stent deployment. Intracoronary Doppler is primarily a method for the physiologic assessment of coronary stenoses. It correlates well with traditional noninvasive measurements of lesion significance. Intracoronary Doppler was used for the anatomic assessment of de novo coronary artery stenosis with variable success; however, its use for anatomic assessment of adequate stent deployment is unavailable. A rapid, automated software program was developed based on a modified continuity equation to calculate the maximal in-stent percent area stenosis by comparing the maximal in-stent velocity to an average reference velocity (proximal and distal). This study was designed to compare the Doppler method of an anatomic assessment with QCA and ICUS in 15 patients. Physiologic success of stent deployment was determined by the distal coronary flow reserve to 24 to 36 microg of intracoronary adenosine. Following successful stent deployment, distal coronary flow reserve increased significantly from a baseline of 1.6 +/- 0.5 to 2.9 +/- 1.1. There was a significant correlation between the maximal in-stent percent area stenosis as measured by Doppler and both QCA (r = 0.78, p <0.01) and ICUS (r = 0.84, p <0.01). This study demonstrates that maximal in-stent percent area stenosis can be measured by intracoronary Doppler and a novel software program. The intracoronary Doppler guidewire method can assess the adequacy of stent deployment using both anatomic and physiologic principles and may supplement other quantitative methodologies.

Improved high-frequency transthoracic flow velocity measurement in the left anterior descending coronary artery after intravenous peripheral injection of levovist

New ultrasonic technology allows noninvasive measurement of the flow in the distal left anterior descending coronary artery. The goal of this study was to validate transthoracic determination of coronary flow velocity with the intracoronary Doppler flow wire technique. In 20 patients with normal coronary arteries, 2 intracoronary and 2 comparative transthoracic Doppler measurements (TTDMs) of the average peak velocity (APV) and the mean systolic and diastolic velocities were performed. The diastolic/systolic ratio was calculated. Blood flow velocity was determined in the distal left anterior descending coronary artery with a Doppler guide wire. Color Doppler and subsequent pulsed wave Doppler readings in an optimal left lateral position were available within 1 hour after completion of the invasive examinations. TTDM were performed during continuous administration of 2.0 g of contrast agent. A modified apical view was obtained from the fourth or fifth intercostal space, and a high-frequency transducer was used (7 MHz for 2-dimensional and 6 MHz for color Doppler imaging; 3.5 MHz for pulsed wave Doppler readings). The Doppler flow signal quality was graded from I to III (I = no flow mapping obtainable, II = poor quality, III = Doppler signals with a well-defined outline). In 13 (65%) patients, 26 TTDMs revealed signal quality of grade III. APV was calculated to be within normal limits (APVecho = 19.96 +/- 7.62 cm/s vs APVinvasive = 20.77 +/- 7.87 cm/s). APVecho correlated well with APVinvasive (r = 0.85, y = 0.82x + 2.85, P <.001). The mean difference between APVecho and APVinvasive (Bias) was -0.81 +/- 4.23 cm/s. No correlation was found between invasive and noninvasive measurements of diastolic/systolic velocity ratios (P >.05). High-frequency TTDM provides reliable data on APV in the majority of patients. It has the potential to be introduced as a relevant screening test for follow-up of patients after interventional treatment.

Correlation of flow velocity measurements by magnetic resonance phase contrast imaging and intravascular Doppler ultrasound

RATIONALE AND OBJECTIVES

The authors compare the magnetic resonance (MR) phase contrast flow velocity measurements in varying concentric stenoses with invasive measurements obtained with a Doppler guidewire.

METHODS

Flow velocity measurements were obtained using a calibrated 0.018 inch 12 MHz Doppler guidewire and a 1.0 T MR imaging system in a pulsatile hydraulic model with variable arterial stenoses. Velocity measurements were performed proximal, intrastenotic, and distal to the stenoses. The cross-sectional area of stenosis was calculated from the data of both methods. For MR imaging measurements, fast low-angle shot two-dimensional phase contrast sequences with different velocity encodings were used.

RESULTS

Phase contrast flow measurements correlated well (r = 0.95, Pearson) with Doppler guidewire-based flow velocity data. Generally, flow velocities obtained with MR imaging were lower when compared with the Doppler-based data (P < or = 0.001, Wilcoxon matched pairs test). However, the ratios and the calculated cross-sectional area of stenoses showed a high correlation (r = 0.96) with the predefined area of stenoses.

CONCLUSIONS

The assessment of flow alterations in vitro due to variable stenoses using MR phase contrast flow measurements is very well correlated with the Doppler guidewire. Consequently, these results required in vivo measurements of atherosclerotic lesions to evaluate the clinical impact.

Use of the continuity equation for transesophageal Doppler assessment of severity of proximal left coronary artery stenosis: a quantitative coronary angiography validation study

OBJECTIVES

We tested the value of transesophageal Doppler echocardiography (TEDE) for quantitating proximal left coronary artery (LCA) stenosis by using the continuity equation.

BACKGROUND

The continuity equation applied to a stenosis states that the ratio of the time-velocity integral (TVI) of prestenotic to stenotic flow velocities is equal to the ratio of stenotic to prestenotic cross-sectional areas. TEDE allows the measurement of coronary blood flow velocities within the proximal part of the LCA. METHODS; Forty-one patients with a stenosis of the proximal or mid left anterior descending coronary artery or with a nonostial stenosis of the left main coronary artery were studied. Coronary flow velocities were recorded by TEDE guided by color flow imaging. Prestenotic velocities were recorded by pulsed Doppler echocardiography and transstenotic velocities were recorded by pulsed or high pulse repetition frequency or continuous wave Doppler echocardiography. The prestenotic and transstenotic diastolic TVIs were calculated and the TEDE-derived percent area stenosis was calculated as (1 - TVI ratio) x 100. Quantitative angiography lesion analysis was performed using a computer-assisted automated edge-detection system.

RESULTS

TEDE recordings were successful in 35 of the 41 patients. A good linear correlation was found between TEDE and quantitative angiographically derived percent area stenosis (r = 0.89, p = 0.0001, SEE 5.7). However, TEDE measurements underestimated the actual percent area stenosis (slope of regression 0.54). A better agreement (slope 1.08) was obtained after dividing prestenotic velocity by 2 in the continuity equation, based on the assumption of a parabolic cross-sectional velocity profile in the prestenotic segment.

CONCLUSIONS

TEDE may be used for quantitating stenosis of the proximal part of the LCA with the use of a modified continuity equation that takes into account the parabolic velocity profile in the normal prestenotic segment.

Disturbed intracoronary hemodynamics in myocardial bridging: early normalization by intracoronary stent placement

BACKGROUND

The purpose of this study was to evaluate the hemodynamic mechanisms leading to myocardial ischemia in patients with myocardial bridging. Myocardial bridging is known to induce angina and even severe myocardial ischemia.

METHODS AND RESULTS

In 12 symptomatic patients with myocardial bridges, quantitative coronary angiography was performed to obtain systolic/diastolic vessel diameters within the bridged segments. Coronary flow velocities, flow reserve, and pressures were determined with a 0.014-in Doppler and a 0.014-in pressure microtransducer. In 3 symptomatic patients, coronary stents were implanted and hemodynamic measurements were repeated immediately and after 7 weeks. An in vitro validation of the pressure measurements was performed. Angiography revealed a systolic diameter reduction of 80.6+/-9.2% and a persistent diastolic reduction of 35.3+/-11% within the bridged segment. Diastolic flow velocities (cm/s) were increased (31.5+/-14.3 within versus 17.3+/-5.7 proximal and 15.2+/-6.3 distal, P<.001). Coronary flow reserve distal to the bridge was 2.5+/-0.5. There was an increased peak systolic pressure within the bridged segment (171+/-48 versus 113+/-10 mm Hg proximal, P<.001). Stent placement abolished the phasic lumen compression, the diastolic flow abnormalities, the intracoronary peak systolic pressure, and clinical symptoms. Coronary flow reserve improved to 3.8+/-0.3.

CONCLUSIONS

Coronary hemodynamics in myocardial bridges are characterized by a phasic systolic vessel compression with a localized peak pressure, persistent diastolic diameter reduction, increased blood flow velocities, retrograde flow, and a reduced flow reserve. These alterations may explain the occurrence of symptoms and ischemia in these patients. Intracoronary stent placement abolished all hemodynamic abnormalities and may improve clinical symptoms in otherwise unsuccessfully treated patients with myocardial bridges.

Coronary vasomotion after percutaneous transluminal coronary angioplasty depends on the severity of the culprit lesion

OBJECTIVES

This study sought to evaluate coronary vasomotor response to percutaneous transluminal coronary angioplasty (PTCA) and its influence on proximal and distal vessel diameters with regard to stenosis severity and coronary blood flow.

BACKGROUND

Coronary vasoconstriction of the distal vessel segment has been reported after PTCA. This vasoconstrictive effect was thought to be due to balloon-induced injury of the vessel wall, with release of local vasoconstrictors or stimulation of the sympathetic system with release of catecholamines, or both.

METHODS

Thirty-nine patients were prospectively studied before and after PTCA. Patients were classified into two groups according to the severity of the culprit lesion: group 1 = > or = 70% to < or = 85% diameter stenosis (n = 23); and group 2 = > 85% to < or = 95% diameter stenosis (n = 16). The coronary vessel diameter of the proximal and distal vessel segments as well as the minimal lumen diameter were determined by quantitative coronary angiography. In a subgroup of 16 patients, basal and maximal coronary flow velocity was measured before and after PTCA with the Doppler FloWire system.

RESULTS

The groups were comparable with regard to age, gender, serum cholesterol levels and medical therapy. The proximal vessel segment remained unchanged after PTCA in group 1 ([mean +/- SD] 0.9 +/- 3.5%, p = 0.8) but showed vasodilation in group 2 (+13.7 +/- 3.6%, p < 0.05). However, the distal segment showed vasoconstriction in group 1 (-6.7 +/- 2.0%, p < 0.01) and vasodilation in group 2 (+31 +/- 8.0%, p < 0.01). A significant correlation was found between the change in distal vessel diameter after PTCA and stenosis severity (r = 0.61, p < 0.0001). Changes in blood flow were directly correlated to stenosis severity (r = 0.85, p < 0.002); that is, rest flow increased after PTCA in narrow lesions but remained unchanged in moderate lesions. The diameter changes in the distal vessel segment after PTCA were significantly related to flow changes (r = 0.90, p < 0.0001). Coronary distending pressure of the distal vessel segment increased significantly in both groups; however, this increase was significantly greater in group 2 than in group 1 (55 +/- 4 vs. 14 +/- 3 mm Hg, p < 0.0001).

CONCLUSIONS

Coronary vasomotion of the proximal and distal vessel segments after PTCA depends on the severity of the culprit lesion; that is, vasoconstriction of the distal segment is found in patients with moderate lesions and vasodilation in those with severe lesions. Thus, vasomotion of the post-stenotic vessel segment depends on the severity of the culprit lesion and is influenced by changes in coronary flow or distending pressure, or both.

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.

Assessment of stenosis severity using a novel method to estimate spatial and temporal variations of blood flow velocity in biplane coronarography

The authors present a novel method to estimate absolute blood flow velocity in coronary arteries from biplane angiograms. Spatial and temporal velocity variations are derived giving simultaneously a direct geometric and an indirect functional index of stenosis severity, stenosis ratio and coronary flow reserve. No prior assumption concerning stenosis geometry is made. Deformable models are used to track a coronary artery segment dynamically in three dimensions. A densitometric map is obtained by summing densities across sections at every position along the previously calculated path and at every time of the cardiac cycle. An advection relationship between density and velocity is observed. The spatiotemporal velocity map is a solution of a nonlinear least-squares scheme. A simulation protocol based on simple geometric conformations and blood flow properties is used to assess numerical stability and immunity towards noise. Predicted results for temporal velocity variations are compared with the intracoronary Doppler recordings to test the model assumptions for basal state and hyperaemia examinations of the same patient. The stenosis ratio was accurate to within 3% for a simulated additive Gaussian noise with a standard deviation of 0.14. The limits of agreement between angiographic and Doppler velocities were -11.4 and 11.8 cm s-1 for a peak value of 23 cm s-1 (basal state) and -16.8 and 13.5 cm s-1 for a peak value of 52 cm s-1 (hyperaemia), corresponding to 18 and 3.5% errors on the average peak values and a 16% error on the coronary flow reserve. To summarize, the advection model derivation and its solution are presented. Simulated and experimental results corroborate the validity of the numerical schemes and support clinical applicability.

Assessment of the hemodynamic result of PTA with a Doppler guide wire: initial experience

PURPOSE

To determine the usefulness of a Doppler guide wire for assessing the hemodynamic results of balloon dilation of atherosclerotic arterial obstruction.

MATERIALS AND METHODS

A 0.018-inch 12-MHz Doppler guide wire was used to measure the maximum peak velocity and the time-averaged peak velocity (APV) proximal to, inside, and distal to lesions in 10 consecutively seen patients (six men, four women; mean age, 68 years +/- 11 [standard deviation]). Measurements were made before and after percutaneous transluminal angioplasty (PTA).

RESULTS

The proximal-to-intrastenotic APV ratio was the best predictor of the grade of residual stenosis and correlated well with angiographic measurements (r = .8). Balloon dilation resulted in a significant increase in mean APV distal to the obstruction (12.5 cm/sec before versus 27.6 cm/sec after PTA; P < .05). The mean proximal-to-distal APV ratio also decreased significantly (3.7 before versus 1.0 after PTA; P < .005).

CONCLUSION

The Doppler guide wire can be used for direct intravascular evaluation of the result of balloon dilation in atherosclerotic obstructions.

Fractional flow reserve. A useful index to evaluate the influence of an epicardial coronary stenosis on myocardial blood flow

BACKGROUND

Fractional flow reserve (FFR), defined as the ratio of maximum flow in the presence of a stenosis to normal maximum flow, is a lesion-specific index of stenosis severity that can be calculated by simultaneous measurement of mean arterial, distal coronary, and central venous pressure (Pa, Pd, and Pv, respectively), during pharmacological vasodilation. The aims of this study were to define ranges of FFR values, whether associated with inducible ischemia or not, and to investigate FFR in normal coronary arteries.

METHODS AND RESULTS

In 60 patients accepted for percutaneous transluminal coronary angioplasty (PTCA) of single-vessel disease, with a positive exercise test (ET) < 24 hours before PTCA, FFR was determined during adenosine-induced hyperemia just before and 15 minutes after angioplasty. Pa was measured by the guiding catheter, Pd by an 0.018-in fiber-optic pressure-monitoring wire, and Pv, by a multipurpose catheter. The ET was repeated after 5 to 7 days, and only if this second ET had reverted to normal was the pre-PTCA value of FFR definitely considered to be associated with inducible ischemia and the post-PTCA value not. Myocardial FFR (FFRmyo) increased from 0.53 +/- 0.15 before PTCA to 0.88 +/- 0.07 after PTCA. Coronary FFR increased from 0.38 +/- 0.19 to 0.83 +/- 0.12. In all patients, values of FFRmyo definitely associated with ischemia were < or = 0.74, whereas all except two values not associated with inducible ischemia exceeded 0.74. Moreover, FFRmyo in 18 coronary arteries in 5 normal patients equaled 0.98 +/- 0.03.

CONCLUSIONS

A value of FFRmyo of 0.74 reliably discriminates coronary stenosis, whether associated with inducible ischemia or not. Therefore, FFRmyo is a useful index to determine the functional significance of an epicardial coronary stenosis and may facilitate clinical decision making in patients with an equivocal coronary stenosis.

Improvement of coronary morphology and blood flow after stenting. Assessment by intravascular ultrasound and intracoronary Doppler

Intravascular ultrasound (IVUS) and intracoronary Doppler (ICD) were performed in eight patients (54.3 +/- 6.5 years, 6 male) immediately after PTCA and after stenting. ICD was also performed before PTCA. After PTCA, IVUS has demonstrated intimal rupture in all patients. After stenting, IVUS revealed wall wrapping of the intimal flap with a free lumen in all patients. The lumen diameter was 2.42 +/- 0.55 mm after PTCA and was 2.74 +/- 0.49 mm after stenting (p < 0.001). The cross-sectional area increased from 4.70 +/- 1.99 mm2 post-PTCA to 6.40 +/- 2.15 mm2 post-stent (p < 0.005). Coronary flow velocity reserve, calculated by the ratio of mean flow velocity at rest and after intracoronary papaverine administration, increased from 2.05 +/- 1.01 to 2.99 +/- 1.14 after PTCA (p = 0.015); and increased to 4.51 +/- 1.33 after stenting (p < 0.001). The morphological data derived from IVUS correlated with the functional information obtained with ICD. In addition to its established role in bail out situations, stent implantation may be considered when a suboptimal morphological and functional result has been demonstrated.

Hemodynamic significance of coronary jet velocity in patients: limitations of the Bernouilli equation in small conduits

The objective of this study was to assess the use of coronary stenosis velocity in the determination of translesional pressure gradients. In the physiologic assessment of coronary stenosis, the accelerated intracoronary flow velocity within a narrowing has correlated with minimal lesion cross-sectional area according to the continuity equation. In large conduits the jet velocity can determine pressure gradients when used in the Bernouilli equation. However, the use of intralesional flow velocity for calculation of translesional pressure gradients by the simplified Bernouilli equation (delta P = 4V2) may be inaccurate in small (< 5 mm diameter) conduits. Translesional pressure (2.2F catheter) and flow velocity (0.018-inch guidewire) were measured in a single coronary artery in 23 patients undergoing diagnostic angiography or angioplasty. The electronically determined mean of phasic proximal and distal pressure and planimetry of the instantaneous phasic pressure gradient were used and compared with the instantaneous velocity calculations of pressure by the simplified Bernouilli formula with both maximal jet velocity and a modified formula including proximal velocity. The mean measured translesional pressure gradient was 18 +/- 13 mm Hg (range 0 to 50 mm Hg) and was equivalent to the instantaneous average pressure gradient by planimetry. The maximal jet velocity was 125 +/- 40 cm/sec (range 63 to 250 cm/sec), yielding a calculated pressure gradient of 3 +/- 3 mm Hg. The calculated pressure gradient by the simplified Bernouilli equation correlated poorly with the measured translesional gradient (r = 0.27, F = 1.63, p = 0.21).(ABSTRACT TRUNCATED AT 250 WORDS)

Three dimensional reconstruction of cross sectional intracoronary ultrasound: clinical or research tool?

Images

The application of breath hold phase velocity mapping techniques to the measurement of coronary artery blood flow velocity: phantom data and initial in vivo results

A segmented kappa-space breath hold phase velocity mapping technique has been developed for the study of coronary artery blood flow velocity. In vitro validation has been performed using a number of pulsatile flow phantoms and the accuracy of the technique for determining the velocity increase at the site of a stenosis demonstrated in several phantom models. Examples of both in-plane and through-plane velocity maps of the left anterior descending and right coronary arteries of normal subjects in early diastole are presented. In one subject, through-plane velocity maps were obtained in the right and left anterior descending arteries throughout the cardiac cycle in order to generate flow velocity time curves. The problems associated with coronary artery velocity mapping are discussed.