Randall Y. Grimes

Experimental analysis of fluid mechanical energy losses in aortic valve stenosis: Importance of pressure recovery

Current methods for assessing the severity of aortic stenosis depend primarily on measures of maximum systolic pressure drop at the aortic valve orifice and related calculations such as valve area. It is becoming increasingly obvious, however, that the impact of the obstruction on the left ventricle is equally important in assessing its severity and could potentially be influenced by geometric factors of the valve, causing variable degrees of downstream pressure recovery. The goal of this study was to develop a method for measuring fluid mechanical energy losses in aortic stenosis that could then be directly related to the hemodynamic load placed on the left ventricle. A control volume form of conservation of energy was theoretically analyzed and modified for application to aortic valve stenosis measurements.In vitro physiological pulsatile flow experiments were conducted with different types of aortic stenosis models, including a venturi meter, a nozzle, and 21-mm Medtronic-Hall tilting disc and St. Jude bileaflet mechanical valves. The energy loss created by each model was measured for a wide range of experimental conditions, simulating physiological variation. In all cases, there was more energy lost for the nozzle (mean=0.27 J) than for any other model for a given stroke volume. The two prosthetic valves generated approximately the same energy losses (mean=0.18 J), which were not statistically different, whereas the venturi meter had the lowest energy loss for all conditions (mean=0.037 J). Energy loss correlated poorly with orifice pressure drop (r2=0.34) but correlated well with recovered pressure drop (r2=0.94). However, when the valves were considered separately, orifice and recovered pressure drop were both strongly correlated with energy loss (r2=0.99, 0.96). The results show that recovered pressure drop, not orfice pressure drop, is directly related to the energy loss that determines pump work and therefore is a more accurate measure of the hemodynamic significance of aortic stenosis.

Three-dimensional reconstruction of the flow in a human left heart by using magnetic resonance phase velocity encoding

Intraventricular flows have been correlated with disease and are of interest to cardiologists as a possible means of diagnosis. This study extends a method that use magnetic resonance (MR) to measure the three-dimensional nature of these flows. Four coplanar sagittal MR slices were located that spanned the left ventricle of a healthy human. All three velocity components were measured in each slice and 18 phases were obtained per beat. With use of the MR magnitude images, masks were created to isolate the velocity data within the heart. These data were read into the software package. Data Visualizer, and the data from the four slices were aligned so as to reconstruct the three-dimensional volume of the left ventricle and atrium. By representing the velocity in vectorial form, the three-dimensional intraventricular flow field was visualized. This revealed the presence of one large line vortex in the ventricle during late diastole but a more ordered flow during early diastole and systole. In conclusion, the use of MR velocity acquisition is a suitable method to obtain the complex intraventricular flow fields in humans and may lead to a better understanding of the importance of these flows.

Atrial inflow can alter regurgitant jet size: In vitro studies

Recent studies have attempted to predict the severity of regurgitant lesions from color Doppler jet size, which is a function of orifice momentum for free jets. Jets of mitral and tricuspid regurgitation, however, are opposed by flows entering the atria. Despite their low velocities, these counterflows may have considerable momentum that can limit jet penetration. The purpose of this study was to address the hypothesis that such counterflow fields influence regurgitant jet size. Steady flow was driven through 2.4- and 5.1-mm-diameter circular orifices at 2 to 6 m/s. At a constant orifice velocity and flow rate, the velocity of a uniform counterflow field was varied from 5 to 30 cm/s. Jet dimensions were measured by both fluorescent dye visualization and Doppler color flow mapping. The results showed that despite its relatively low velocities, counterflow dramatically curtailed jet length and area. Jet dimensions were functions of the ratio of jet to counterflow momentum. Thus, atrial inflow may participate in determining jet size and can alter the relation between jet size and lesion severity in mitral and tricuspid regurgitation.

Quantification of cardiac jets: theory and limitations.

Jet flows are consequences of many cardiac lesions. With the advent of color Doppler flow mapping, these jet flows can be visualized noninvasively. Currently, an intense effort is underway to quantify cardiac jet flows as a means to assess the severity of jet forming lesions. Two techniques, PISA and jet centerline decay, have been suggested as methods to quantify jet flow volume. Although both techniques are theoretically sound, both formulations are based on ideal flow conditions that may not be completely realized in cardiac chambers. Thus, the complex dynamics of cardiac jet flows must be considered as they may diminish the accuracy of flow rate calculations. However, realistic in vitro experiments that mimic the impact of cardiac flow conditions on converging flows and jets, combined with carefully controlled in vivo testing ofbothPISA and centerline techniques, may eventually produce clinically useful quantification formulations. (ECHOCARDIOGRAPHY, Volume 11, May 1994)

Quantification of Mitral and Tricuspid Regurgitation Using Jet Centerline Velocities: An In Vitro Study of Jets in an Ambient Counterflow.

A method for quantifying mitral and tricuspid regurgitant volume that utilizes a measure of jet orifice velocity U0 ‐ m/sec), a distal centerline velocity (Um ‐ m/sec), and the intervening distance (X ‐ cm) was recently developed; where jet flow rate (Qcal ‐ L/min) is calculated as Qcal= (UmX)2/(26.46U0). This method, however, modeled the regurgitant jet as a free jet, whereas many atrial jets are counterflowing jets because of jet opposing intra‐atrial flow fields (counterflows). This study concentrated on the feasibility of using the free jet quantification equation in the atrium where ambient flow fields may alter jet centerline velocities and reduce the accuracy of jet flow rate calculations. A 4‐cm wide chamber was used to pump counterflows of 0, 4, and 22 cm / sec against jets of 2.3, 4.8, and 6.4 m/sec originating from a 2‐mm diameter orifice. For each counterflow‐jet combination, jet centerline velocities were measured using laser Doppler anemometry. For free jets (no counterflow), flow rate was calculated with 98% mean accuracy. For all jets in counterflow, the calculation was less accurate as: (i) the ratio of jet orifice velocity to counterflow velocity decreased (U0/Uc where Uc is counterflow velocity), i.e., the counterflow was relatively more intense, and (ii) centerline measurements were made further from the orifice. But although counterflow lowered jet centerline velocities beneath free jet values, it did so only significantly in the jets distal portion (X/D > 16, i.e., > 16 orifice diameters from the origin of the jet). Thus, the initial portion (X/D < 16) of a jet in counterflow behaved essentially as a free jet. As a result, even in significant counterflow, jet flow rate was calculated with > 93% accuracy and > 85% for jets typical of mitral and tricuspid regurgitation, respectively. Counterflow lowers jet centerline velocities beneath equivalent free jet values. This effect, however, is most significant in the distal portion of the jet. Therefore, regurgitant jets, although not classically free because of systolic atrial inflow or jet‐induced intra‐atrial swirling flows, will decay in their initial portions as free jets and thus are candidates for quantification with the centerline technique.

HOW SENSITIVE ARE JET CENTERLINE VELOCITIES TO AN OPPOSING FLOW? IMPLICATIONS FOR USING THE CENTERLINE METHOD TO QUANTIFY REGURGITANT JET FLOW

A method for quantifying peak mitral and tricuspid regurgitant jet flow rate that utilizes a measure of jet orifice velocity (Uo, m s-1), a distal centerline velocity (Um, m s-1), and the intervening distance (X, cm) was recently developed. This method, however, modeled the regurgitant jet as a free jet, whereas many atrial jets are counterflowing jets because of jet opposing intra-atrial flow fields (counterflows). This study evaluated the feasibility of using the free jet quantification equation in the atrium where ambient flow fields may alter jet centerline velocities and therefore reduce the accuracy of jet flow rate calculations. A 4 cm wide chamber was used to pump counterflows of 0, 4, and 22 cm s-1 against jets of 2.3, 4.8, and 6.4 s-1 originating from a 2 mm diameter orifice. For each counterflow-jet combination, jet centerline velocities were measured using laser Doppler anemometry. For free jets (no counterflow), flow rate was calculated with 98% mean accuracy. For all jets in counterflow, the calculation was less accurate as (i) the ratio of jet orifice velocity to counterflow velocity decreased (Uo/Uc, where Uc is counterflow velocity), i.e. the counterflow was relatively more intense, an (ii) centerline measurements were mad further from the orifice. But although counterflow lowered jet centerline velocities beneath free jet values, it did so only significantly in the jets distal portion, while the initial portion (X/D < 16, where D is jet orifice diameter) of a jet in counterflow behaved essentially as a free jet. Therefore, regurgitant jets, although not classically free because of systolic atrial inflow, will decay in their initial portions as free jets and hence are candidates for quantification with the centerline technique.

Dimensional analysis applied to the evaluation of LV function in the presence of mitral regurgitation: computer simulations

Traditional indices of LV function do not accurately reflect the true contractile state in mitral regurgitation (MR) because the low impedance systolic MR flow allows the LV ejection fraction (EF) to remain normal despite LV contractile dysfunction. It would therefore be useful to have method for predicting the EF if MR is eliminated from a given ventricle assuming contractility and blood pressure do not change. A computer model was used to simulate ventricular hemodynamics in the presence and absence of MR. The data were then analyzed using dimensional analysis, a technique for combining multiple variables into a compact formulation. Dimensional analysis revealed that the data could be collapsed into a single equation. This formulation potentially permits ejection fraction calculations that are corrected for the degree of MR from a specified set of ventricular function indices. Validation in patients is the next step.