Effects of Pulmonary Venous Flow Direction on Mitral Regurgitation Jet Area as Imaged by Color Doppler Flow Mapping
An In Vitro Study
Background Although the effects of adjacent walls and left atrial pressure on mitral regurgitation (MR) jet area imaged by color Doppler have been examined, few data exist regarding the influence of pulmonary venous (PV) filling flow on regurgitant jets. Therefore, we designed a left atrial model to examine the relation between PV flow direction and MR jet area.
Methods and Results The left atrial chamber (7.6 cm in diameter) was built with a PV inflow (1.0 cm in diameter) and mitral valve regurgitant orifice in the same plane. The MR jet was simulated as fixed in volume and velocity (3.5 m/s) and directed with a pulsatile pump into the left atrial model. PV flow with a constant velocity (30 cm/s) was driven by gravity (83 cm H2O). With left atrial mean pressure at either 10, 30, or 50 mm Hg, four flow patterns were examined: (1) PV flow away from the mitral valve, MR jet toward the pulmonary vein; (2) PV flow toward the mitral valve, MR jet toward the pulmonary vein; (3) PV flow away from the mitral valve, MR jet away from the pulmonary vein; and (4) PV flow toward the mitral valve, MR jet away from the pulmonary vein. MR color Doppler images were recorded with a 3.5-mHz frequency transducer and at 7-kHz pulse repetition frequency. For each condition, we compared jet area, length, and width of the MR signal. MR jet areas for conditions 3 and 4 were larger at 10 mm Hg than at 30 or 50 mm Hg left atrial pressure. Especially at the lower pressures, PV flow diminished the MR jet area in condition 4 compared with that in condition 3, such that MR jets were smaller in condition 4. In conditions 1 and 2, the jets were imaged at an oblique angle and were smaller than in conditions 3 and 4 (P<.001), but they were not significantly different from each other as imaged.
Conclusions In this model, factors including the direction of PV flow, the direction of MR as relates to the angle of interrogation, and the level of left atrial pressure influenced the size of MR jets. The effect of PV flow direction was diminished by increased left atrial pressure. PV flow directed away from the mitral valve was associated with larger MR jets than when PV flow was directed toward it (condition 4), probably because of jet distortion and flattening.
Color Doppler flow mapping has been used to measure regurgitant jet flow areas1 2 3 to assess the clinical severity of both mitral4 5 6 and aortic7 8 regurgitation. Although regurgitant jet area generally reflects the volume of regurgitation,9 many factors have been shown to influence the size of a jet imaged by color Doppler. Receiving chamber size, chamber compliance and pressure, regurgitant orifice size, driving pressure, and the presence of adjacent walls or surfaces have been shown as physiological factors that can influence the area of regurgitant jets by color flow.10 11 12 13 14 In addition, instrument settings (pulse repetition frequency, frame rate, color gain, and Nyquist limit) can alter imaged regurgitant jet size.15 16 17 18
Mitral regurgitation (MR) jets may influence the blood flow dynamics in pulmonary venous (PV) channels or left atrial chamber.19 Transesophageal Doppler echocardiographic studies have demonstrated that increases in the severity of MR were associated with systolic reversals of PV flow pattern.20 Although the interaction between MR and PV flow has been examined related to the PV flow patterns,21 22 23 few data exist regarding the influence of PV flow patterns on the appearance of the MR jet.
The interaction of PV flow with the regurgitant jet depends on the driving pressure of regurgitation, the type of regurgitant orifice, the directional relation between the MR jet and PV flow, and the dynamics and timing of the PV flow. Thus, in this study we designed a left atrial model to evaluate the effects of PV flow direction and also left atrial pressure on MR jet size as imaged by color Doppler.
Flow Model of Left Atrium
The left atrial model consists of a rigid acrylic chamber equipped with a PV duct and inflow orifice and a mitral valve regurgitant orifice in the same plane. The chamber was in the shape of a cylinder 7.6 cm in diameter, 13 cm in height, and 320 mL in volume. The size of the PV orifice was 1.0 cm in diameter. PV flow was oriented at an angle of 30° from the chamber wall, and the flow could be directed toward or away from the mitral valve. PV flow was driven by gravity by placing a reservoir 83 cm above the PV orifice. A steady-flow pump was used to maintain a constant solution level of 15 cm in the reservoir, and the PV flow velocity was kept constant at 30 cm/s.
The experimental model used a mitral valve regurgitant orifice of 0.15 cm2. The MR jet was produced by a pulsatile Harvard piston pump (model 680). The regurgitant flow volume (8 mL), peak flow velocity (3.5 m/s measured by color Doppler guided continuous wave Doppler), and flow frequency (68 beats per minute) were kept constant throughout the experiments. The MR jet was directed parallel to the chamber wall, but its direction was also variable toward or away from the PV orifice.
We examined four different conditions of PV flow and MR jet spatial orientations as follows (Fig 1⇓): (1) The PV flow was directed away from the mitral valve, and the MR jet was directed toward the PV orifice; (2) the PV flow was toward the mitral valve, and the MR jet was toward the PV orifice; (3) the PV flow was away from the mitral valve, and the MR jet was directed away from the PV orifice; and (4) the PV flow was toward the mitral valve, and the MR jet was directed away from the PV orifice.
A circular window made of polycarbonate in the model allowed for flow imaging by Doppler echocardiography from a site opposite the entry of the mitral flows and the pulmonary veins as it would be to some extent for transesophageal studies. Angles between color Doppler interrogation and MR jet direction were 0° for conditions 3 and 4 and between 60° and 70° for conditions 1 and 2. The outflow orifice of 2.2 cm in diameter was located on the top of the left atrial chamber so as not to interfere with the PV flow and MR jet interactions. The flexible outflow tubing could be variably constricted to vary the outflow resistance and to change the ambient left atrial mean chamber pressure from 10 to 30 and 50 mm Hg as measured through a pressure tap. For filling the model we used a 1% cornstarch particle/water solution that produces physiological ultrasound Doppler reflections.
Color Doppler Scanning and Measurement of MR Jet Area
We obtained MR color Doppler images using an Aloka model SSD-870 system with a 3.5-mHz frequency transducer and a 7-kHz pulse repetition frequency (yielding a 76 cm/s Nyquist limit) at a frame rate of 10/s to achieve a wide enough color sector to image both jets and PV flows simultaneously (Fig 1B⇑), and we used a fixed low color Doppler gain. The instrument settings were maintained constant throughout all experiments. All color Doppler images were obtained in a central plane that encompassed both the PV entry and the MR jets. The ultrasonic scanning was projected so that the MR jet was coming toward the transducer in conditions 3 and 4 (red coding) (Fig 1B⇑); however, for the scanning window this meant that for conditions 1 and 2 the MR jets were color coded as a blue signal, and scanning direction was oriented somewhat more perpendicular to the MR jets. A subset of data at 10 mm Hg of left atrial pressure was obtained at an alternative window rotated 60° around the wall of the model so as to be more parallel to the jets in conditions 1 and 2. All MR color flow images were recorded on a 1/2-in. videotape for later playback and analysis.
We compared maximal area (centimeters squared), length (centimeters), and width (centimeters) of MR jet images for conditions 1 and 2 or 3 and 4. In each condition we also compared these measurements between left atrial pressures of 50, 30, and 10 mm Hg (LAP50, LAP30, LAP10). At least 15 peak systolic maximal MR color jet images were measured from videotaped records in each condition; the data are expressed as mean±SD.
To obtain optical images of MR jets, especially to explain the changes visualized on ultrasound for conditions 3 and 4, at LAP10 we injected a 0.01% blue dye into the system by filling the pump inlet. Through a Y-type connection, the dye was injected through the MR orifice by the pump coincident with its stroke volume. A Sony CCD-TR 75 video camera was set up perpendicular to the jet, but imaging encompassed the same plane as the ultrasound to image the PV inflow–MR entry plane. The first MR jet image as it was pulsed into the chamber was recorded on videotape for subsequent off-line measurements of the maximal jet area by dye labeling at peak ejection.
To evaluate interobserver variability, two independent observers who had no knowledge of the other observer’s data measured color jet area, jet length, and jet width from 20 randomly selected recordings.
Differences in MR jet area, length, and width for each condition were analyzed by ANOVA. Statistical significance was defined at a value of P<.05.
Effect of Receiving Chamber Pressure
Measurements of MR jet area obtained at LAP10 were larger than those obtained at LAP30 or LAP50 for all conditions (Fig 2⇓). The Table⇓⇓ shows the results of MR jet area, length, and width measurements for each condition. For condition 1, the MR jet had larger area and length at LAP10 than LAP50 (area, P=.04; length, P=.0001). For condition 2, MR jet length at LAP10 was larger than at LAP30 (P=.01) and LAP50 (P=.0001). For condition 3, MR jet measurements at LAP10 were larger for area only than those at LAP30 (area, P=.01; length, P=NS; width, P=NS) and were larger for all three measurements than at LAP50 (area, P=.0001; length, P=.003; width, P=.004) (Fig 3⇓). For condition 4, MR jet area and length at LAP10 were larger than at LAP50 (area, P=.0002; length, P=.019).
Effect of PV Flow Direction
At low left atrial pressure (10 mm Hg), smaller MR jet areas were imaged with PV flow directed toward the mitral valve (condition 4) than when it was directed away (condition 3) (Fig 3⇑, bottom). At LAP10, MR jet area, length, and width in condition 3 were larger than those in condition 4 (area, P=.0001; length, P=.0001; width, P=.0007; Table⇑⇑).
The effect of PV flow direction on the MR color image was diminished by increasing left atrial pressure to 30 and 50 mm Hg. At LAP50, MR jet area in condition 3 was also larger than in condition 4 (P=.01), but the differences were less (Table⇑⇑).
Effect of Scanning Orientation
For the conditions in which the MR jet was directed toward the PV orifice (conditions 1 and 2), the orientation of the MR jets was more perpendicular to the echo scanning, producing smaller jet images, and additional changes in MR jet area induced by PV flow direction were not detectable (Fig 3⇑, top; Table⇑⇑). Thus, we used an alternative echo window to obtain MR jet images with better angles (10°) between color Doppler interrogation and MR jet direction for conditions 1 and 2. In this setting, for a subset of data at LAP10 at 30 cm/s of PV flow velocity, condition 1 showed jet area results of 8.33±0.46 cm2, and condition 2 showed 8.05±0.48 cm2 (P<.05). These jet areas from the new window were larger than those from the original window (P<.0001) and more comparable in size to those for conditions 3 and 4 as derived from the original window. At 50 cm/s of PV flow velocity, the difference between the two conditions became more pronounced (7.56±0.32 versus 7.02±0.38 cm2, P<.01), with condition 1 jets larger than those in condition 2. (The data set from the alternative window is not shown in the Table⇑⇑.)
As visualized at peak ejection by dye coding, MR jet areas in condition 3 were larger than those in condition 4 for LAP10 (8.4±0.7 versus 7.0±0.9 cm2, P<.001; Fig 4⇓). Also, the optical images showed more clearly how the PV flow deflected the MR toward the wall, flattening it.
There was good agreement between measurements of the color Doppler jet area, length, and width by two independent observers (area: r=.89, SEE=0.05 cm2, P<.001; length: r=.86, SEE=0.04 cm, P<.001; width: r=.85, SEE=0.03 cm, P<.01).
Color Doppler flow mapping is often used to assess valvular regurgitation.1 2 3 4 5 6 7 8 However, since color Doppler describes the velocity profile of the regurgitant flow as well as the entrained fluid, experimental and clinical studies have demonstrated that the color flow Doppler imaged area of disturbed high-velocity variance-encoded flow (while roughly reflecting regurgitant volume) is affected by a number of instrument and physiological factors. In our study, we could show that both the direction of PV flow and the level of left atrial pressure influenced the size of a simulated MR jet. Color Doppler jet size decreased as the receiving chamber pressure increased, and a smaller jet area was also shown when PV flow was directed toward the mitral valve (condition 4) rather than away from it (condition 3). Additionally, the effect of PV flow direction on the MR jet was diminished by increased left atrial pressure.
Concerning the effect of receiving chamber pressure on regurgitant jet color area, our group has previously reported12 that the spatial distribution of jets decreased significantly as the pressure within the receiving chamber increased. In that previous study, the regurgitant jets under constant-flow conditions were driven by a steady-flow pump, and regurgitant flow rate was varied from 10 to 270 mL/min. The decrease in jet area with pressure at the higher flow rates was substantially greater than at low flow rates. The mean color jet area changed from 10.2 to 7.25 cm2 as chamber pressure increased from 8 to 57 mm Hg at 270 mL/min.12 In our present study, we demonstrated a decrease in jet area from 8.86 to 7.57 cm2 as pressure was increased from 10 to 50 mm Hg at 8 mL per beat (68 beats per minute, 544 mL/min) regurgitant flow rate. Although the degree of decrease in MR color flow area in the present study was smaller than in the previous study despite higher flow rate, we observed the same pressure effect on color flow area now verified in a pulsatile model with a separate PV inflow.
Wranne et al24 demonstrated in an experimental study that the distance of jet intrusion depended on the flow volume, driving pressure, and flow velocity. In our model, the peak flow velocity at the regurgitant orifice measured by continuous wave Doppler was constant on a beat-to-beat basis. Although the mechanisms for the receiving chamber pressure effect are not clearly defined, the pressure inside the receiving chamber can now be more firmly established as one additional important factor affecting color flow area on the basis of our documentation of the same effect in a different and more physiological model. As a mechanism of this effect, we believe that with higher pressure inside the receiving chamber, the MR jet could recover pressure more rapidly than at lower pressure, and accordingly, spatial velocity would fall more rapidly to a level below the minimum detectable by color flow Doppler.12
Effect of PV and MR Directions on MR Jet Area
Our data also showed a significant difference in jet area for differences in PV flow direction between condition 3 (away from the mitral valve) and condition 4 (toward the mitral valve). Although we could not observe directly by color Doppler imaging the nature of these interactions, some possible mechanisms can be considered. Interaction of the farthest extending end of the decelerating MR jet with the swirling PV flows may extend and spread MR jet turbulence for condition 3, whereas for condition 4 the PV flows may direct the MR jet toward the surrounding wall, resulting in a smaller jet in this plane by the usual effect of wall adjacency, the Coanda effect.13 14 This was supported by optical visualization studies that clarify the deflection and flattening of the MR jets by the PV flow, deflecting the MR flow toward the wall with the Coanda effect and leading to wall adherence, which diminishes jet size further (Fig 4⇑).
Effect of Scanning Orientation: Consistency of Results
The effect of PV flow direction on MR jet area would also have been expected to be observed in condition 1 versus 2 by color Doppler in our study; however, the acute angle between the interrogating Doppler beam and MR jet direction in our model resulted in smaller jet sizes that did not allow discrimination of potential alterations in color jet size by other factors. The fact that the interrogation angle diminishes the color area of even these turbulent flows has been underemphasized in previous studies. Our study includes a limited series of color Doppler imaging studies in this same model through an alternative window that imaged conditions 1 and 2 with the alignment of the MR jet more parallel to the direction of the interrogation. With this change of angle, jet sizes were larger and more comparable to those for conditions 3 and 4 observed in this study. With this new imaging window, albeit more dramatically with higher PV flows (50 cm/s) to correct for changes in the model that required repositioning of the inlets to achieve the new window, color Doppler jets for condition 1 were imaged as larger than those in condition 2 (P<.01). In condition 2, since PV flow is directed almost opposed to the propagation of MR jet flow, this interaction could decrease MR jet velocity more rapidly, whereas in condition 1 entrainment of moving PV flow into the jet occurred with less MR jet velocity loss and thus increased MR jet extension.
Our flows were imaged in one plane, and although our mitral jet could be moved toward or away from the pulmonary vein, the MR jet did not change direction during systole as it does in some patients. Our model has other limitations, such as an unphysiologically large left atrium and a single constant PV inflow. Thus, quantitative results induced from our in vitro study may not be directly applicable to clinical conditions. Also, it is not clear how important the effect of the spatial relation between the two flows, the MR jet versus the PV interaction, is on the color MR jet size compared with the effect of left atrial pressure.
Nonetheless, our study results reaffirm the chamber pressure effect and add both the angle of jet imaging and PV flow direction to the factors that may affect jet area as an indicator of the severity of MR. Relatively mild MR might appear to be slightly more severe if left atrial pressure is low and if the regurgitation is directed away from the most closely adjacent PV flows. These findings lend support for the continued development of new methods, such as flow convergence, which have evolved from an improved understanding of fluid dynamics and color Doppler and which are aimed at more direct quantification of regurgitant flow rates.25
This study was supported in part by grant HL-43287 from the National Heart, Lung, and Blood Institute. Dr Paik was supported by a grant from the Society for Pediatric Research.
- Received September 8, 1994.
- Accepted October 31, 1994.
- Copyright © 1995 by American Heart Association
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