Saeb F. Khoury

In Vivo Echocardiographic Detection of Enhanced Left Ventricular Function in Gene-Targeted Mice With Phospholamban Deficiency

We evaluated the ability of M-mode and Doppler echocardiography to assess left ventricular (LV) function reliably and repeatedly in mice and tested whether these techniques could detect physiological alterations in phospholamban (PLB)-deficient mice. Anesthetized wild-type mice (n = 7) and mice deficient in PLB (n = 8) were studied with two-dimensional guided M-mode and Doppler echocardiography using a 9-MHz imaging and 5- to 7.5-MHz Doppler transducer. Data were acquired in the baseline state and after intraperitoneal isoproterenol administration (2.0 micrograms/g IP). Interobserver and intraobserver variability and reproducibility were excellent. PLB-deficient mice were associated with significant (P < .05) increases in several physiological parameters (mean +/- SD) compared with wild-type control mice: normalized mean velocity of circumferential shortening (7.7 +/- 2.1 versus 5.5 +/- 1.0 circ/sec), peak aortic velocity (105 +/- 13 versus 75 +/- 9.2 cm/s), mean aortic acceleration (57 +/- 16 versus 31 +/- 4 m/s2), and peak early-diastolic transmitral velocity (80.0 +/- 7.2 versus 66.9 +/- 7.7 cm/s). LV dimensions, shortening fractions, heart rates, late diastolic transmitral (A) velocities, and early to late (E/A) diastolic velocity ratios were similar in both groups. Isoproterenol administration resulted in significant increases in Doppler indices of ventricular function in control but not PLB-deficient mice. These findings indicate that assessment of LV function can be performed noninvasively in mice under varying physiological conditions and that PLB regulates basal LV function in vivo.

Effects of thyroid hormone on cardiac β-adrenergic responsiveness in conscious baboons

BACKGROUND Many of the cardiovascular manifestations of thyroid hormone excess resemble those produced by sympathoadrenal stimulation. The objective of this study was to determine the effects of thyroid hormone excess on myocardial beta-adrenergic expression and responsiveness to infused agonists in the primate heart. METHODS AND RESULTS The responses of left ventricular isovolumic contraction (dP/dt(max)) and relaxation (tau) during graded dobutamine infusion were studied both before and after 4 weeks of thyroid hormone administration in 8 chronically instrumented baboons. At matched (atrially paced) heart rates, thyroid hormone significantly increased resting dP/dt(max) (3073+/-1034 versus 2318+/-829 mm Hg/s, P<.05) and decreased tau (24.0+/-5.5 versus 28.2+/-5.4 ms, P<.05). The change from baseline for dP/dt(max) and tau in response to beta1-adrenergic stimulation was significant at each dobutamine dose (2.5 to 10 microg x kg(-1) x min(-1)), but when expressed as a percent change, it was similar before versus after thyroid hormone. Similar changes were found when beta2-adrenergic stimulation was produced by terbutaline infusion in three additional baboons. beta-Adrenergic receptor (betaAR) expression was higher in five thyroxine-treated than in five control baboons (37.4+/-1.2 versus 15.7+/-3.2 fmol/mg, P<.001), and this was due to a greater increase in the beta2AR (5.9+/-1.5 to 20.6+/-1.2 fmol/mg, P<.001) than the beta1AR (9.7+/-1.7 to 16.8+/-0.1 fmol/mg, P<.01) subtype. CONCLUSIONS In the primate heart, thyroid hormone produces positive inotropic and lusitropic effects in the resting state and upregulates both beta1AR and beta2AR, with the beta2AR increase predominating. At equivalent rates, however, thyroid hormone excess does not appear to enhance the sensitivity of left ventricular contractility and relaxation to either beta1- or beta2-adrenergic stimulation.

Effects of thyroid hormone on left ventricular performance and regulation of contractile and Ca(2+)-cycling proteins in the baboon. Implications for the force-frequency and relaxation-frequency relationships.

The transcriptional, posttranscriptional, and related functional effects of thyroid hormone on primate myocardium are poorly understood. Therefore, we studied the effects of thyroid hormone on sarcoplasmic reticulum (SR) Ca(2+)-cycling proteins and myosin heavy chain (MHC) composition at the steady state mRNA and protein level and associated alterations of left ventricular (LV) performance in 8 chronically instrumented baboons. The force-frequency and relaxation-frequency relations were assessed as the response of LV isovolumic contraction (dP/dtmax) and relaxation (Tau), respectively, to incremental atrial pacing. Both the heart rate at which dP/dtmax was maximal and Tau was minimal (critical heart rates) in response to pacing were increased significantly after thyroid hormone. Postmortem LV tissue from 5 thyroid-treated and 4 additional control baboons was assayed for steady state mRNA levels with cDNA probes to MHC isoforms and SR Ca(2+)-cycling proteins. Steady state SR Ca(2+)-ATPase and phospholamban mRNA increased in the hyperthyroid state, and alpha-MHC mRNA appeared de novo, whereas beta-MHC mRNA decreased. Western analysis (4 thyroid-treated and 4 control baboons) showed directionally similar changes in MHC isoforms and a slight increase in SR Ca(2+)-ATPase. In contrast, there was a statistically nonsignificant decrease in phospholamban protein, which resulted in a significant 40% decrease in the ratio of phospholamban to SR Ca(2+)-ATPase. Thus, thyroid hormone increases the transcription of Ca(2+)-cycling proteins and shifts MHC isoform expression in the primate LV. Our data suggest that both transcriptional and posttranslational mechanisms determine the levels of these proteins in the hyperthyroid primate heart and mediate, in part, the observed enhanced basal and frequency-dependent LV performance.

Hemodynamic wall shear stress profiles influence the magnitude and pattern of stenosis in a pig AV fistula

Venous stenosis is a significant problem in arteriovenous fistulae, likely due to anatomical configuration and wall shear stress profiles. To identify linkages between wall shear stress and the magnitude and pattern of vascular stenosis, we produced curved and straight fistulae in a pig model. A complete wall stress profile was calculated for the curved configuration and correlated with luminal stenosis. Computer modeling techniques were then used to derive a wall shear stress profile for the straight arteriovenous fistula. Differences in the wall shear stress profile of the curved and straight fistula were then related to histological findings. There was a marked inverse correlation between the magnitude of wall shear stress within different regions of the curved arteriovenous fistula and luminal stenosis in these same regions. There were also significantly greater differences in wall shear stress between the outer and inner walls of the straight as compared to curved arteriovenous fistula, which translated into a more eccentric histological pattern of intima-media thickening. Our results suggest a clear linkage between anatomical configuration, wall shear stress profiles, and the pattern of luminal stenosis and intima-media thickening in a pig model of arteriovenous fistula stenosis. These results suggest that fistula failure could be reduced by using computer modeling prior to surgical placement to alter the anatomical and, consequently, the wall shear stress profiles in an arteriovenous fistula.

Hemodynamic diagnostics of epicardial coronary stenoses: in-vitro experimental and computational study

BackgroundThe severity of epicardial coronary stenosis can be assessed by invasive measurements of trans-stenotic pressure drop and flow. A pressure or flow sensor-tipped guidewire inserted across the coronary stenosis causes an overestimation in true trans-stenotic pressure drop and reduction in coronary flow. This may mask the true severity of coronary stenosis. In order to unmask the true severity of epicardial stenosis, we evaluate a diagnostic parameter, which is obtained from fundamental fluid dynamics principles. This experimental and numerical study focuses on the characterization of the diagnostic parameter, pressure drop coefficient, and also evaluates the pressure recovery downstream of stenoses.MethodsThree models of coronary stenosis namely, moderate, intermediate and severe stenosis, were manufactured and tested in the in-vitro set-up simulating the epicardial coronary network. The trans-stenotic pressure drop and flow distal to stenosis models were measured by non-invasive method, using external pressure and flow sensors, and by invasive method, following guidewire insertion across the stenosis. The viscous and momentum-change components of the pressure drop for various flow rates were evaluated from quadratic relation between pressure drop and flow. Finally, the pressure drop coefficient (CDPe) was calculated as the ratio of pressure drop and distal dynamic pressure. The pressure recovery factor (η) was calculated as the ratio of pressure recovery coefficient and the area blockage.ResultsThe mean pressure drop-flow characteristics before and during guidewire insertion indicated that increasing stenosis causes a shift in dominance from viscous pressure to momentum forces. However, for intermediate (~80%) area stenosis, which is between moderate (~65%) and severe (~90%) area stenoses, both losses were similar in magnitude. Therefore, guidewire insertion plays a critical role in evaluating the hemodynamic severity of coronary stenosis. More importantly, mean CDPe increased (17 ± 3.3 to 287 ± 52, n = 3, p < 0.01) and mean η decreased (0.54 ± 0.04 to 0.37 ± 0.05, p < 0.01) from moderate to severe stenosis during guidewire insertion.ConclusionThe wide range of CDPe is not affected that much by the presence of guidewire. CDPe can be used in clinical practice to evaluate the true severity of coronary stenosis due to its significant difference between values measured at moderate and severe stenoses.

Functional and anatomical diagnosis of coronary artery stenoses.

BACKGROUND Functional/physiological evaluation of coronary artery stenoses may be more important than anatomical measurements of severity. Optimization of thresholds for stenosis intervention and treatment endpoints depend on coupling functional hemodynamic and anatomical data. We sought to develop a single prognostic parameter correlating stenosis-specific anatomy, pressure gradient, and velocities that could be measured during catheterization. MATERIALS AND METHODS In vivo Experiments were performed in six swine (41 +/- 3 kg). The lumen area of the left anterior descending coronary artery was measured with intravascular ultrasound. An angioplasty balloon was inflated to create the desired intraluminal area obstructions. Fractional flow reserve (FFR), coronary flow reserve (CFR), and hyperemic-stenosis-resistance index were measured distal to the balloon at peak hyperemia with 10 mg intracoronary papaverine. A functional index:pressure drop coefficient (CDP) and a combined functional and anatomical index:lesion flow coefficient (LFC) were calculated from measured hyperemic pressure gradient, velocity, and percentage area stenosis. P < 0.05 was considered statistically significant. RESULTS The CDP and LFC correlated linearly and significantly with FFR and CFR. The CDP (R(2) = 0.72, P < 0.0001) correlated better than LFC (R(2) = 0.19, P < 0.003) with hyperemic-stenosis-resistance index. When LFC was correlated simultaneously with FFR and CFR, R(2) improved to 0.82 (P < 0.0001). Inclusion of percentage area stenoses concurrently with FFR and CFR marginally improved the correlation with LFC. CONCLUSIONS A dimensionless parameter combining measured pressure gradient, velocity, and area reduction data can optimally define the severity of coronary stenoses based on our preliminary results and could prove useful clinically.

In Vitro Quantification of Guidewire Flow-Obstruction Effect in Model Coronary Stenoses for Interventional Diagnostic Procedure

The objective is to quantify the guidewire (diameter of 0.35 mm) flow-obstruction effect in the in vitro model coronary stenoses in relation to trans-stenotic pressure drop, p, fractional flow reserve (gFFR; “g” represents FFR measurement with guidewire insertion) and coronary flow reserve (gCFR) for steady and pulsatile physiological flows. The sensor tipped pressure or flow measuring guidewire insertion through stenotic lumen increases the trans-stenotic pressure drop or reduces the pharmacologically induced hyperemic flow in the coronary arteries with plaques. These hemodynamic changes may cause error in true FFR and CFR measurements, especially for intermediate coronary stenosis. To quantify guidewire flow-obstruction effect, simultaneous measurements of trans-stenotic pressures and flow were performed by two methods: (a) guidewire based measurements (gCFR and gFFR by inserting sensor tipped guidewire) and (b) true physiological measurements (CFR by in-line Doppler flow cuff and FFR by the radially drilled pressure ports in three epicardial coronary stenotic test sections, postangioplasty, intermediate, and preangioplasty). The diagnostic parameters measured before guidewire insertion (CFR and FFR) and during guidewire insertion (gCFR and gFFR) were validated numerically and correlated with the new diagnostic parameter “lesion flow coefficient (LFC).” There was significant flow reduction with increased trans-stenotic pressure drop due to guidewire insertion. The FFR-gFFR and CFR-gCFR correlations were FFR=0.92gFFR+0.097 R 2 =0.99 and CFR=0.91gCFR+0.44 R 2 =0.99, respectively, where gCFR is reported from clinical pressure-flow data. Similar highly regressed R 2 0.9 correlations were obtained for LFC and gLFC with flow ratios and pressure ratios. There was significant difference between steady and pulsatile pressure drops for the same mean flow with and without guidewire insertion. The trans-stenotic hemodynamics was altered due to guidewire insertion. The true FFR and CFR were underestimated because of guidewire insertion. Hence, the FFR-gFFR and CFR-gCFR correlations can be used to find out true FFR and CFR from clinically measured values (i.e., gFFR and gCFR). In addition, the gLFC-gCFR and gLFC-gFFR were correlated significantly for post- and preangioplasty conditions. DOI: 10.1115/1.2776336

In Vitro Pressure Flow Relationship in Model of Significant Coronary Artery Stenosis

Simultaneous measurement of pressure and flow rate has been found to be helpful in evaluating the physiologic significance of obstructive coronary artery disease and in the diagnosis of microvascular disease. This experimental study seeks to find important pressure-flow relationship in an in-vitro model of significant coronary artery stenoses using a non-Newtonian liquid, similar to blood showing a shear thinning behavior, using significant stenotic in-vitro model (minimal area stenosis = 90%). The geometry for the stenotic model is based on data provided in an in vivo study by Wilson et al., (1988). For 90% area stenosis, the maximum recorded pressure drop for steady flow rate of 55, 79 and 89 are 14, ~24 and ~32 mmHg respectively. The maximum pressure drop at flow rate of 115 ml/min (the physiological limit) is 50.3 mmHg respectively. Using a power law curve fit, the maximum pressure drop (in mmHg) related with flow rate (in ml/min) provided a power law index of 1.72. Shorter distal length than required in the in-vitro model did not allow the recording of complete pressure recovery. This preliminary data provides reference values for further experimentation both in vitro with pulsatile flow as in physiological conditions, and in vivo.Copyright