Assesment of LV diastolic filling using color M-mode Doppler echocardiography: validation in new hydoraulic model.

The effect of LV properties on v(p) and the E/v(p) ratio remains a matter of debate. Therefore,the objective of this study is to explore - in a new hydraulic model - the individual contributions of LV relaxation, filling pressure and compliance in changes of E, v(p) and E/v(p) for different stages of diastolic function. A new hydraulic model, consisting of an open cylindrical LA connected to an ellipsoidal LV, is designed. E and v(p) are measured for varying values of tau (45-60-90 ms), LV compliance (0.45-1.35 ml/mmHg) and filling pressure (3-10-30 mmHg). The results are used for predicting the evolution of E, v(p) and E/v(p) during different stages of diastolic function. An increase in compliance decreases E, whereas it augments v(p). v(p) is less load-dependent than E. E decreases with delayed relaxation, increases for the case of pseudonormalisation, and becomes higher than the reference values during restrictive filling. The v(p) value is lower for the case of delayed relaxation than for the reference situation. During pseudonormalisation, the value of v(p) remains lower than the reference value but higher than the value for delayed relaxation. v(p) further decreases during restrictive filling. In conclusion, the effect of simultaneous changes in compliance and loading counterbalance changes in v(p). Therefore, under normal physiologic conditions where load and compliance are coupled, v(p) is apparently load-intensive and E/v(p) increases as filling pressure increases. Moreover, in the different stages of diastolic dysfunction, due to the interference of the co-varying relaxation, the increase in E/v(p) is more pronounced.

Validation of planar and tomographic radionuclide ventriculography by a dynamic ventricular phantom.

Although there is increasing interest in the automatic processing of tomographic radionuclide ventriculography (TRV) studies, validation is mainly limited to a comparison of TRV results with data from planar radionuclide ventriculography (PRV) or gated perfusion single photon emission computed tomography (SPECT). The aim of this study was to use a dynamic physical cardiac phantom to validate the ejection fraction (EF) and volumes from PRV and TRV studies. A new dynamic left ventricular phantom was constructed and used to obtain 21 acquisitions in the planar and tomographic mode. The directly measured volumes and EFs of the phantom during the acquisitions were considered as the gold standard for comparison with TRV and PRV. EFs were calculated from PRV by background-corrected end-diastolic and end-systolic frames. Volumes and EFs were calculated from TRV by region growing with different lower thresholds to search for the optimal threshold. EF from PRV correlated significantly with the real EF (r=0.94, P=0.00). The optimal threshold value for volume calculation from TRV in 336 cases was 50% (r=0.98, P=0.00) yielding the best slope after linear regression. When considering these calculated end-diastolic and end-systolic volumes, EF correlated well (r=0.99, P=0.00) with the real EF, and this correlation was significantly (P=0.04) higher than that of the EF from PRV. Our experiments prove that EF measured by TRV yields more accurate results compared with PRV in dynamic cardiac phantom studies.

Limitations of Doppler echocardiography for the post-operative evaluation of aortic coarctation.

Doppler blood flow measurements and derived pressure differences, through the Bernoulli equation, are used in the diagnosis of aortic coarctation, a congenital stenosis distal to the left subclavian artery. Doppler velocities remain elevated at the coarctation site after successful repair of coarctation, leading to high Doppler derived pressure differences without significant arm-leg pressure differences. We studied this apparent contradiction of two diagnostic methods, in vivo using patient and control data, and in vitro using a hydraulic model. Clinical and echocardiographic data from 31 patients, aged 13.0 +/- 4.0, 10.5 +/- 4.7 yr after coarctectomy by end-to-end anastomosis, and 18 age-matched healthy subjects were reviewed. Doppler peak velocities at the aortic isthmus were elevated in patients (2.2 +/- 0.4 vs. 1.2 +/- 0.2m/s, P < 0.001), corresponding to significant Doppler differences (20 +/- 7 mmHg), however, without significant arm-leg pressure differences. In all patients, a mild anatomic stenosis could still be observed. Local stiffness was increased. The hypothesis that the less distensible surgical scar in post-coarctectomy patients leads to a significant dynamic obstruction in systole was validated in a latex model of the aorta. Rigid rings (0.5-1.5 cm), matching the unloaded aortic diameter, were mounted around the aorta. Under loading conditions, Doppler peak velocities increased by 40 +/-7%, yielding Doppler differences of 21 +/- 3 mmHg, without a significant pressure drop. An alternative expression to calculate pressure differences, using both velocity and geometric information, was validated in the model. In conclusion, post-operatively, Doppler velocities remain elevated due to a mild anatomical and significant dynamic narrowing, but the specific geometry, resembling a tubular hypoplasia rather than an abrupt stenosis, permits an almost complete pressure recovery explaining the occurrence of Doppler differences in disagreement with the negligible arm-leg pressure difference.

Fixed region of nondistensibility after coarctation repair: in vitro validation of its influence on Doppler peak velocities.

After coarctectomy, local loss of distensibility is noted in addition to mild anatomic narrowing. We hypothesize that the increased Doppler peak velocities measured at the aortic isthmus in these patients partly reflect obstruction secondary to the stiff surgical scar. The hypothesis was studied in a pulsatile hydraulic model. Thirty-one patients (13.0 +/- 4.0 years of age), 10.5 +/- 4.7 years after coarctectomy by end-to-end anastomosis, were studied clinically and echocardiographically. Indexes of distensibility were calculated. The effect of isolated increased stiffness was studied in vitro with a stiff and a compliant 1:1 scale latex model of the aorta mounted in a pulsatile full-scale circulation loop. Local stiffening was obtained by a rigid ring mounted around the aorta, fitted to the dimension of the unloaded aorta. For different pressure and flow regimens, pressures and Doppler velocities were measured across the ring. Mean peak velocities at the surgical scar were 2.2 +/- 0.4 m/s. Mild anatomic stenosis was present. All distensibility indexes indicated locally increased stiffness (P <.001). In the stiff latex model, Doppler peak velocities increased from 1.89 +/- 0.04 m/s to 2.32 +/- 0.06 m/s (P <.03); in the compliant model, from 1.15 +/- 0.03 m/s to 1.79 +/- 0.05 m/s (P <.001). The increase of Doppler peak velocities depends on model compliance only and is independent of flow rate, length of the noncompliant segment, and viscosity of the perfusion fluid. Velocities do not change when semicircular stiffening is applied. We have demonstrated in vitro that isolated local nondistensibility leads to vessel narrowing during vascular distension. The relative contribution of local scar stiffness in the increase of Doppler peak velocities after coarctectomy was hereby assessed.

Assessment of the time constant of relaxation: insights from simulations and hemodynamic measurements.

The objective of this study was to use high-fidelity animal data and numerical simulations to gain more insight into the reliability of the estimated relaxation constant derived from left ventricular pressure decays, assuming a monoexponential model with either a fixed zero or free moving pressure asymptote. Comparison of the experimental data with the results of the simulations demonstrated a trade off between the fixed zero and the free moving asymptote approach. The latter method more closely fits the pressure curves and has the advantage of producing an extra coefficient with potential diagnostic information. On the other hand, this method suffers from larger standard errors on the estimated coefficients. The method with fixed zero asymptote produces values of the time constant of isovolumetric relaxation (tau) within a narrow confidence interval. However, if the pressure curve is actually decaying to a nonzero pressure asymptote, this method results in an inferior fit of the pressure curve and a biased estimation of tau.

Diastolic filling and pressure imaging: taking advantage of the information in a colour M-mode Doppler image.

The ability to derive non-invasively information on left ventricular diastolic function on one hand and pressure gradients on the other hand, makes Doppler ultrasound a very attractive tool in clinical practice. However, the limitations of the standard Doppler approaches in differentiating between normal and pseudonormal filling patterns, together with the limitations of the simplified Bernoulli equation for assessing pressure gradients, are well described. In this manuscript the role of colour M-mode Doppler echocardiography as a tool that can overcome these limitations is discussed. Relevant key concepts of the haemodynamics of left ventricular filling and its relationship with colour M-mode Doppler echocardiography are introduced.