Insights from three-dimensional echocardiography into the mechanism of functional mitral regurgitation: direct in vivo demonstration of altered leaflet tethering geometry

BACKGROUND

Recent advances in three-dimensional (3D) echocardiography allow us to address uniquely 3D scientific questions, such as the mechanism of functional mitral regurgitation (MR) in patients with left ventricular (LV) dysfunction and its relation to the 3D geometry of mitral leaflet attachments. Competing hypotheses include global LV dysfunction with inadequate leaflet closing force versus geometric distortion of the mitral apparatus by LV dilatation, which increases leaflet tethering and restricts closure. Because geometric changes generally accompany dysfunction, these possibilities have been difficult to separate.

METHODS AND RESULTS

We created a model of global LV dysfunction by esmolol and phenylephrine infusion in six dogs. initially with LV expansion limited by increasing pericardial restraint and then with the pericardium opened. The mid-systolic 3D relations of the papillary muscle (PM) tips and mitral valve were reconstructed. Despite severe LV dysfunction (ejection fraction, 18+/-6%), only trace MR developed when pericardial restraint limited LV dilatation; with the pericardium opened, moderate MR accompanied LV dilatation (end-systolic volume, 44+/-5 mL versus 12+/-5 mL control, P<.001). Mitral regurgitant volume and orifice area did not correlate with LV ejection fraction and dP/dt (global function) but did correlate with changes in the tethering distance from the PMs to the anterior annulus derived from the 3D reconstructions, especially PM shifts in the posterior and mediolateral directions, as well as with annular area (P<.0005). By multiple regression, only changes in the PM-to-annulus distance independently predicted MR volume and orifice area (R2=.82 to .85, P=2x10(-7) to 6x10(-8)).

CONCLUSIONS

LV dysfunction without dilatation fails to produce important MR. Functional MR relates strongly to changes in the 3D geometry of the mitral valve attachments at the PM and annular levels, with practical implications for approaches that would restore a more favorable configuration.

Three-dimensional echocardiographic reconstruction of the mitral valve, with implications for the diagnosis of mitral valve prolapse

Mitral valve prolapse has been diagnosed by two-dimensional echocardiographic criteria with surprising frequency in the general population, even when preselected normal subjects are examined. In most of these individuals, however, prolapse appears in the apical four-chamber view and is absent in roughly orthogonal long-axis views. Previous studies of in vitro models with nonplanar rings have shown that systolic mitral annular nonplanarity can potentially produce this discrepancy. However, to prove directly that apparent leaflet displacement in a two-dimensional view does not constitute true displacement above the three-dimensional annulus requires reconstruction of the entire mitral valve, including leaflets and annulus. Such reconstruction would also be necessary to explore the complex geometry of the valve and to derive volumetric measures of superior leaflet displacement. A technique was therefore developed and validated in vitro for three-dimensional reconstruction of the entire mitral valve. In this technique, simultaneous real-time acquisition of images and their spatial locations permits reconstruction of a localized structure by minimizing the effects of patient motion and respiration. By applying this method to 15 normal subjects, a coherent mitral valve surface could be reconstructed from intersecting scans. The results confirm mitral annular nonplanarity in systole, with a maximum deviation of 1.4 +/- 0.3 cm from planarity. They directly show that leaflets can appear to ascend above the mitral annulus in the apical four-chamber view, as they did in at least one view in all subjects, without actual leaflet displacement above the entire mitral valve in three dimensions, thereby challenging the diagnosis of prolapse by isolated four-chamber view displacement in otherwise normal individuals. This technique allows us to address a uniquely three-dimensional problem with high resolution and provide new information previously unavailable from the two-dimensional images. This new appreciation should enhance our ability to ask appropriate clinical questions relating mitral valve shape and leaflet displacement to clinical and pathologic consequences.

Mechanism of ischemic mitral regurgitation with segmental left ventricular dysfunction: three-dimensional echocardiographic studies in models of acute and chronic progressive regurgitation

OBJECTIVES

This study aimed to separate proposed mechanisms for segmental ischemic mitral regurgitation (MR), including left ventricular (LV) dysfunction versus geometric distortion by LV dilation, using models of acute and chronic segmental ischemic LV dysfunction evaluated by three-dimensional (3D) echocardiography.

BACKGROUND

Dysfunction and dilation-both mechanisms with practical therapeutic implications-are difficult to separate in patients.

METHODS

In seven dogs with acute left circumflex (LCX) coronary ligation, LV expansion was initially restricted and then permitted to occur. In seven sheep with LCX branch ligation, LV expansion was also initially limited but became prominent with remodeling over eight weeks. Three-dimensional echo reconstruction quantified mitral apparatus geometry and MR volume.

RESULTS

In the acute model, despite LV dysfunction with ejection fraction = 23 +/- 8%, MR was initially trace with limited LV dilation, but it became moderate with subsequent prominent dilation. In the chronic model, MR was also initially trace, but it became moderate over eight weeks as the LV dilated and changed shape. In both models, the only independent predictor of MR volume was increased tethering distance from the papillary muscles (PMs) to the anterior annulus, especially medial and posterior shift of the ischemic medial PM, measured by 3D reconstruction (r2 = 0.75 and 0.86, respectively). Mitral regurgitation volume did not correlate with LV ejection fraction or dP/dt.

CONCLUSIONS

Segmental ischemic LV contractile dysfunction without dilation, even in the PM territory, fails to produce important MR. The development of MR relates strongly to changes in the 3D geometry of the mitral apparatus, with implications for approaches to restore a more favorable configuration.

Chordal cutting: a new therapeutic approach for ischemic mitral regurgitation

BACKGROUND

Mitral regurgitation (MR) conveys adverse prognosis in ischemic heart disease. Because such MR is related to increased leaflet tethering by displaced attachments to the papillary muscles (PMs), it is incompletely treated by annular reduction. We therefore addressed the hypothesis that such MR can be reduced by cutting a limited number of critically positioned chordae to the leaflet base that most restrict closure but are not required to prevent prolapse. This was tested in 8 mitral valves: a porcine in vitro pilot with PM displacement and 7 sheep with acute inferobasal infarcts studied in vivo with three-dimensional (3D) echo to quantify MR in relation to 3D valve geometry.

METHODS AND RESULTS

In all 8 valves, PM displacement restricted leaflet closure, with anterior leaflet angulation at the basal chord insertion, and mild-to-moderate MR. Cutting the 2 central basal chordae reversed this without prolapse. In vivo, MR increased from 0.8+/-0.2 to 7.1+/-0.5 mL/beat after infarction and then decreased to 0.9+/-0.1 mL/beat with chordal cutting (P<0.0001); this paralleled changes in the 3D leaflet area required to cover the orifice as dictated by chordal tethering (r(2)=0.76).

CONCLUSIONS

Cutting a minimum number of basal chordae can improve coaptation and reduce ischemic MR. Such an approach also suggests the potential for future minimally invasive implementation.

Refined structure of alkaline phosphatase from Escherichia coli at 2.8 A resolution

The structure of alkaline phosphatase from Escherichia coli has been determined to 2.8 A resolution. The multiple isomorphous replacement electron density map of the dimer at 3.4 A was substantially improved by molecular symmetry averaging and solvent flattening. From these maps, polypeptide chains of the dimer were built using the published amino acid sequence. Stereochemically restrained least-squares refinement of this model against native data, starting with 3.4 A data and extending in steps to 2.8 A resolution, proceeded to a final overall crystallographic R factor of 0.256. Alkaline phosphatase-phosphomonoester hydrolase (EC 3.1.3.1) is a metalloenzyme that forms an isologous dimer with two reactive centers 32 A apart. The topology of the polypeptide fold of the subunit is of the alpha/beta class of proteins. Despite the similarities in the overall alpha/beta fold with other proteins, alkaline phosphatase does not have a characteristic binding cleft formed at the carboxyl end of the parallel sheet, but rather an active pocket that contains a cluster of three functional metal sites located off the plane of the central ten-stranded sheet. This active pocket is located near the carboxyl ends of four strands and the amino end of the antiparallel strand, between the plane of the sheet and two helices on the same side. Alkaline phosphatase is a non-specific phosphomonoesterase that hydrolyzes small phosphomonoesters as well as the phosphate termini of DNA. The accessibility calculations based on the refined co-ordinates of the enzyme show that the active pocket barely accommodates inorganic phosphate. Thus, the alcoholic or phenolic portion of the substrate would have to be exposed on the surface of the enzyme. Two metal sites, M1 and M2, 3.9 A apart, are occupied by zinc. The third site, M3, 5 A from site M2 and 7 A from site M1, is occupied by magnesium or, in the absence of magnesium, by zinc. As with other zinc-containing enzymes, histidine residues are ligands to zinc site M1 (three) and to zinc site M2 (one). Ligand assignment and metal preference indicate that the crystallographically found metal sites M1, M2 and M3 correspond to the spectroscopically deduced metal sites A, B and C, respectively. Arsenate, a product analog and enzyme inhibitor, binds between Ser102 and zinc sites M1 and M2. The position of the guanidinium group of Arg 166 is within hydrogen-bonding distance from the arsenate site.(ABSTRACT TRUNCATED AT 400 WORDS)

Design of a new surgical approach for ventricular remodeling to relieve ischemic mitral regurgitation: insights from 3-dimensional echocardiography

BACKGROUND

Mechanistic insights from 3D echocardiography (echo) can guide therapy. In particular, ischemic mitral regurgitation (MR) is difficult to repair, often persisting despite annular reduction. We hypothesized that (1) in a chronic infarct model of progressive MR, regurgitation parallels 3D changes in the geometry of mitral leaflet attachments, causing increased leaflet tethering and restricting closure; therefore, (2) MR can be reduced by restoring tethering geometry toward normal, using a new ventricular remodeling approach based on 3D echo findings.

METHODS AND RESULTS

We studied 10 sheep by 3D echo just after circumflex marginal ligation and 8 weeks later. MR, at first absent, became moderate as the left ventricle (LV) dilated and the papillary muscles shifted posteriorly and mediolaterally, increasing the leaflet tethering distance from papillary muscle tips to the anterior mitral annulus (P<0.0001). To counteract these shifts, the LV was remodeled by plication of the infarct region to reduce myocardial bulging, without muscle excision or cardiopulmonary bypass. Immediately and up to 2 months after plication, MR was reduced to trace-to-mild as tethering distance was decreased (P<0.0001). LV ejection fraction, global LV end-systolic volume, and mitral annular area were relatively unchanged. By multiple regression, the only independent predictor of MR was tethering distance (r(2)=0.81).

CONCLUSIONS

Ischemic MR in this model relates strongly to changes in 3D mitral leaflet attachment geometry. These insights from quantitative 3D echo allowed us to design an effective LV remodeling approach to reduce MR by relieving tethering.

A new integrated system for three-dimensional echocardiographic reconstruction: development and validation for ventricular volume with application in human subjects

OBJECTIVES

The purpose of this study was to improve three-dimensional echocardiographic reconstruction by developing an automated mechanism for integrating spark gap locating data with corresponding images in real time and to validate use of this mechanism for the measurement of left ventricular volume.

BACKGROUND

Initial approaches to three-dimensional echocardiographic reconstruction were often limited by inefficient reconstructive processes requiring manual coordination of two-dimensional images and corresponding spatial locating data.

METHODS

In this system, a single computer overlays the binary-encoded positional data on the two-dimensional echocardiographic image, which is then recorded on videotape. The same system allows images to be digitized, traced, analyzed and displayed in three dimensions. This system was validated by using it to reconstruct 11 ventricular phantoms (19 to 271 ml) and 11 gel-filled excised ventricles (21 to 236 ml) imaged in intersecting long- and short-axis views and by apical rotation. To measure cavity volume, a surface was generated by an algorithm that takes advantage of the full three-dimensional data set.

RESULTS

Reconstructed cavity volumes agreed well with actual values: y = 0.96x + 2.2 for the ventricular phantoms in long- and short-axis views (r = 0.99, SEE = 2.7 ml); y = 0.95x + 2.9 for the phantoms, reconstructed by apical rotation (r = 0.99, SEE = 2.7 ml); and y = 0.99x + 0.11 ml for the excised ventricles (reconstructed in long- and short-axis views; r = 0.99, SEE = 5.9 ml). The mean difference between three-dimensional and actual volumes was 3% of the mean (3.0 ml) for the phantoms and 6% (4.6 ml) for the excised ventricles. Observer variability was 2.3% for the phantoms and 5.6% for the excised ventricles. Application to 14 normal subjects demonstrated feasibility of left ventricular reconstruction, which provided values for stroke volume that agreed well with an independent Doppler measure (y = 0.97x + 0.94; r = 0.95, SEE = 3.2 ml), with an observer variability of 4.9% (2.4 ml).

CONCLUSIONS

A system has therefore been developed that automatically integrates locating and imaging data in no more time than the component two-dimensional echocardiographic scans. This system can accurately reconstruct ventricular volumes in vitro over a wide range and is feasible in vivo, thus laying the foundation for further applications. It has increased the efficiency of three-dimensional reconstruction and enhanced our ability to address clinical and research questions with this technique.

Three-dimensional echocardiography. In vivo validation for left ventricular volume and function

BACKGROUND

Current two-dimensional quantitative echocardiographic methods of volume assessment require image acquisition from standardized scanning planes. Left ventricular volume and ejection fraction are then calculated by assuming ventricular symmetry and geometry. These assumptions may not be valid in distorted ventricles. Three-dimensional echocardiography can quantify left ventricular volume without the limitations imposed by the assumptions of two-dimensional methods. We have developed a three-dimensional system that automatically integrates two-dimensional echocardiographic images and their positions in real time and calculates left ventricular volume directly from traced endocardial contours without geometric assumptions.

METHODS AND RESULTS

To study the accuracy of this method in quantifying left ventricular volume and performance in vivo, a canine model was developed in which instantaneous left ventricular volume can be measured directly with an intracavitary balloon connected to an external column. Ten dogs were studied at 84 different cavity volumes (4 to 85 cm3) and in conditions of altered left ventricular shape produced by either coronary occlusion or right ventricular volume overload. To demonstrate clinical feasibility, 19 adult human subjects were then studied by this method for quantification of stroke volume. Left ventricular volume, stroke volume, and ejection fraction calculated by three-dimensional echocardiography correlated well with directly measured values (r = .98, .96, .96 for volume, stroke volume, and ejection fraction, respectively) and agreed closely with them (mean difference, -0.78 cm3, -0.60 cm3, -0.32%). In humans, there was a good correlation (r = .94, SEE = 4.29 cm3) and agreement (mean difference, -0.98 +/- 4.2 cm3) between three-dimensional echocardiography and Doppler-derived stroke volumes.

CONCLUSIONS

Three-dimensional echocardiography allows accurate assessment of left ventricular volume and systolic function.

Three-dimensional echocardiography. In vivo validation for right ventricular volume and function

BACKGROUND

Current two-dimensional echocardiographic measures of right ventricular volume are limited by the asymmetrical and crescentic shape of the ventricle and by difficulty in obtaining standardized views. Three-dimensional echocardiographic reconstruction, which does not require geometric assumptions or standardized views, may therefore have potential advantages for determining right ventricular volume. Three-dimensional techniques, however, have not been applied to the right ventricle in vivo, where cardiac motion and contraction could affect accuracy. The purpose of this study was to determine the feasibility and accuracy of three-dimensional echocardiographic reconstruction for quantifying right ventricular volume and function in vivo. In particular, it was designed to test the accuracy of a newly developed system that provides rapid, efficient, and automated three-dimensional data collection (minimizing motion effects) and takes advantage of the full three-dimensional data set to obtain volume.

METHODS AND RESULTS

The three-dimensional system was applied to reconstruct the right ventricle and measure its volume and function during 20 hemodynamic stages created in five dogs. Actual instantaneous volumes were measured continuously by an intracavitary balloon connected to an external column. Hemodynamics were varied by volume loading and induction of ischemia. Three-dimensional reconstruction successfully reproduced right ventricular volume compared with actual values at end diastole (y = 1.0 chi-3.4, r = .99, SEE = 1.8 mL) and end systole (y = 1.0 chi+ 2.0, 4 = .98, SEE = 2.5 mL). The mean difference between calculated and actual volumes throughout the cycle was 2.1 mL, or 4.9% of the mean. Ejection fraction also correlated well with actual values (y = 0.96 chi-0.3, r = .98, SEE = 3.3%).

CONCLUSIONS

Despite the irregular crescentic shape of the right ventricle, this newly developed three-dimensional system and surfacing algorithm can accurately reconstruct its shape and quantitate its volume and function in vivo without geometric assumptions. The increased efficiency of the system should increase applicability to issues of clinical and research interest.

Mechanism of dynamic regurgitant orifice area variation in functional mitral regurgitation: physiologic insights from the proximal flow convergence technique

OBJECTIVES

We used the Doppler proximal flow convergence technique as a physiologic tool to explore the effects of the time courses of mitral annular area and transmitral pressure on dynamic changes in regurgitant orifice area.

BACKGROUND

In functional mitral regurgitation (MR), regurgitant flow rate and orifice area display a unique pattern, with peaks in early and late systole and a midsystolic decrease. Phasic changes in both mitral annular area and the transmitral pressure acting to close the leaflets, which equals left ventricular-left atrial pressure, have been proposed to explain this dynamic pattern.

METHODS

In 30 patients with functional MR, regurgitant orifice area was obtained as flow (from M-mode proximal flow convergence traces) divided by orifice velocity (v) from the continuous wave Doppler trace of MR, transmitral pressure as 4v(2), and mitral annular area from two apical diameters.

RESULTS

All patients had midsystolic decreases in regurgitant orifice area that mirrored increases in transmitral pressure, while mitral annular area changed more gradually. By stepwise multiple regression analysis, both mitral annular area and transmitral pressure significantly affected regurgitant orifice area; however, transmitral pressure made a stronger contribution (r2 = 0.441) than mitral annular area (added r2 = 0.008). Similarly, the rate of change of regurgitant orifice area more strongly related to that of transmitral pressure (r2 = 0.638) than to that of mitral annular area (added r2 = 0.003). A similar regurgitant orifice area time course was observed in four patients with fixed mitral annuli due to Carpentier ring insertion.

CONCLUSIONS

In summary, the time course and rate of change of regurgitant orifice area in patients with functional MR are predominantly determined by dynamic changes in the transmitral pressure acting to close the valve. Thus, although mitral annular area helps determine the potential for MR, transmitral pressure appears important in driving the leaflets toward closure, and would be of value to consider in interventions aimed at reducing the severity of MR.

Paradoxic decrease in ischemic mitral regurgitation with papillary muscle dysfunction: insights from three-dimensional and contrast echocardiography with strain rate measurement

BACKGROUND

Ischemic mitral regurgitation (MR) was first ascribed to papillary muscle (PM) contractile dysfunction. Current theories include apical leaflet tethering caused by left ventricular (LV) distortion, but PM dysfunction is still postulated and commonly diagnosed. PM contraction, however, parallels apical tethering, suggesting the hypothesis that PM contractile dysfunction can actually diminish MR due to ischemic distortion of the inferior base alone.

METHODS AND RESULTS

We therefore occluded the proximal circumflex circulation in 7 sheep while maintaining PM perfusion, confirmed by contrast echocardiography. By 3D echocardiography, we measured the tethering distance between the ischemic medial PM tip and anterior annulus and LV ejection volume to give MR (by subtracting flowmeter LV outflow). In 6 sheep without initial MR, inferior ischemia alone produced PM tip retraction with restricted leaflet closure and mild-to-moderate MR (regurgitant fraction, 25.2+/-2.8%). Adding PM ischemia consistently decreased MR and tethering distance (5.2+/-0.3 to 1.4+/-0.3 mL; +3.8+/-0.5 mm to -2.2+/-0.7 mm axially relative to baseline; P<0.001) as PM strain rate decreased from +0.78+/-0.07 per second (contraction) to -0.42+/-0.06 per second (elongation, P<0.001) and leaflet tenting decreased. In one sheep, prolapse and MR resolved with inferior ischemia and recurred with PM ischemia.

CONCLUSIONS

PM contractile dysfunction can paradoxically decrease MR from inferobasal ischemia by reducing leaflet tethering to improve coaptation. This emphasizes the role of geometric factors in ischemic MR mechanism and potential therapy.

Quantitative three-dimensional reconstruction of aneurysmal left ventricles. In vitro and in vivo validation

BACKGROUND

Current two-dimensional (2D) echocardiographic measures of left ventricular (LV) volume are most limited by aneurysmal distortion, which restricts application of simple geometric models that assume symmetrical shape. 2D methods also fail to provide separate volumes of the aneurysm and nonaneurysmal residual LV cavity, which could help assess the stroke volume wasted by dyskinesis and the potential residual LV body to guide surgical approaches and predict their outcome. Three-dimensional (3D) echocardiographic reconstruction has potential advantages for assessing aneurysmal left ventricles because it is not dependent on geometric assumptions, does not require standardized views that may exclude portions of the aneurysm, and can potentially measure separate aneurysm and nonaneurysm cavity volumes of any shape. The purpose of this study was first, to validate the accuracy of 3D echocardiographic reconstruction for quantifying total LV and separate LV body and aneurysm volumes in vitro so as to provide direct standards for the separate volumes; and second, to determine the feasibility and accuracy of 3D echocardiographic reconstruction for quantifying the total volume and function of aneurysmal left ventricles in an animal model, providing a reference standard for instantaneous LV volume.

METHODS AND RESULTS

A recently developed 3D system that automatically combines 2D images and their locations was applied (1) to reconstruct 10 aneurysmal ventricular phantoms and 12 gel-filled autopsied human hearts with aneurysms, comparing cavity volumes (total and aneurysm) to those measured by fluid displacement; and (2) to reconstruct the left ventricle during 19 hemodynamic stages in four dogs with surgically created LV aneurysms, comparing total volumes with actual instantaneous values measured by an intracavitary balloon attached to an external column for validation and also calculating the stroke volume wasted by aneurysmal dyskinesis. 3D reconstruction reproduced the distorted aneurysmal LV shapes. In vitro, calculated volumes (aneurysm, nonaneurysm, and total) agreed well with actual values, with correlation coefficients of .99 and SEEs of 3.2 to 6.1 cm3 for phantoms and 3.4 to 4.2 cm3 for autopsied hearts (mean error, < 4% for both). In vivo, LV end-diastolic, end-systolic, and stroke volumes as well as ejection fraction calculated by 3D echocardiography correlated well with actual values (r = .99, .99, .95, and .99, respectively) and agreed closely with them (SEE = 4.3 cm3, 3.5 cm3, 1.7 cm3, and 2%, respectively). The stroke volumes wasted by the aneurysm were -20.1 +/- 19.3% of LV body (nonaneurysm) stroke volume.

CONCLUSIONS

Despite distorted ventricular shapes, a recently developed 3D echocardiographic system and surfacing algorithm can accurately reconstruct aneurysmal left ventricles and quantify total LV volume (validated in vivo and in vitro) as well as the separate volumes of the aneurysm and residual LV body (validated in vitro). This should improve our ability to evaluate such ventricles and guide surgical approaches.

Three-dimensional echocardiography: techniques and applications

Current echocardiographic devices provide only 2-dimensional views of the heart. To appreciate 3-dimensional structural relations, therefore, requires mental reconstruction of 2-dimensional views by an experienced observer. Our ability to answer new questions about the heart could be increased if 2-dimensional images could be combined to display 3-dimensional relations. Such 3-dimensional reconstruction would permit analysis of structures of unknown or complex shape and the noninvasive quantification of cardiac chamber size and function without making geometric assumptions. To overcome previous limitations, mechanisms have been developed for automated integration of images and positional data during routine echocardiographic scanning, thereby greatly enhancing the efficiency and application of image reconstruction. Refining the diagnosis of mitral valve prolapse has presented a uniquely 3-dimensional problem requiring information previously unavailable from the 2-dimensional technique. To date, 3-dimensional studies have demonstrated that the mitral valve is saddle-shaped in systole, so that apparent superior leaflet displacement in the mediolateral 4-chamber view, often seen in otherwise normal individuals, lies entirely within the bounds defined by the mitral annulus and occurs without leaflet distortion or actual displacement above the entire mitral valve. Other applications of 3-dimensional image reconstruction include calculation of ventricular volume and ejection fraction by transthoracic or transesophageal scanning without geometric assumptions; improving the standardization and accuracy of 2-dimensional measurements by improving spatial appreciation; and 3-dimensional reconstruction of vascular walls to guide interventions. In the future, systems for acquiring multiple views more rapidly by parallel processing and improving endocardial border extraction should allow more routine application of 3-dimensional methods as the next stage in the evolution of cardiac ultrasound, thereby expanding the range of questions that can be answered. Achieving these goals will depend, in large measure, on persistence in developing the necessary technology.

Location of antigenic epitopes on antibody molecules

Using X-ray crystallographic co-ordinates of immunoglobulins, surface regions accessible to a large spherical probe, comparable in size to an antibody domain, were computed. Locations of these exposed regions were compared with those of experimentally determined antigenic sites, i.e. idiotypic, allotypic and isotypic serological markers. In all cases, an excellent agreement was found. The most prominent computed epitopes correspond to convex parts of antibody surface made by reverse turn segments of the polypeptide chain. The computed epitopes occur in homologous positions in all the immunoglobulin domains, and most of the beta-sheet surfaces on the domains are poorly antigenic. The CH2 domain (Fc fragment) has many more antigenic sites than the Fab fragments (antigen-binding fragments). Variable domain epitopes (idiotypes) involve both hypervariable and framework residues, and only about 25% of the hypervariable residues are strongly antigenic. The results indicate that, in a vertebrate body, each antibody molecule may be recognized, and its concentration regulated, by at least 40 complementary anti-immunoglobulin antibodies; therefore, a possibility of an "immune network" with much higher connectivity than is generally assumed should be seriously contemplated.

Three-dimensional echocardiographic reconstruction of right ventricular volume: in vitro comparison with two-dimensional methods

Two-dimensional echocardiographic measures of right ventricular volume are limited by the asymmetric and crescentic shape of that ventricle and the difficulty in obtaining standardized views. We have developed a three-dimensional echocardiographic system that automatically integrates images and positional data and calculates right ventricular volume without the need for geometric assumptions or standardized views and a surfacing algorithm that takes advantage of the full three-dimensional data set. The accuracy of this system was studied and compared with two-dimensional methods in 12 gel-filled excised human right ventricles (18 to 74 ml). Volumes calculated by three-dimensional echocardiography correlated well with actual values (r = 0.99) and agreed more closely with them than did those obtained by two-dimensional methods (p < 0.02).

Three-dimensional echocardiography improves noninvasive assessment of left ventricular volume and performance

To calculate left ventricular (LV) volume by two-dimensional echocardiography (2DE), assumptions must be made about ventricular symmetry and geometry. Three-dimensional echocardiography (3DE) can quantitate LV volume without these limitations, yet its incremental value over 2DE is unknown. The purpose of this study was to compare the accuracy of LV volume determination by 3DE to standard 2DE methods. To compare the accuracy of 3DE with standard 2DE algorithms for quantitating LV volume, 28 excised canine ventricles of known volume and varying shapes (15 symmetric and 13 aneurysmal) and 10 instrumented dogs prepared so that instantaneous ventricular volume could be measured were examined by 2DE (bullet and biplane Simpson's formulas) and again by 3DE. In both excised and beating hearts, 3DE was more accurate in quantitating volume than either 2DE method (excised: error = 0.6 +/- 3.2, 2.5 +/- 10.7, and 4.0 +/- 8.5 ml by 3D, bullet, and Simpson's, respectively; beating: error = -0.5 +/- 3.5, -0.3 +/- 9.6, and -7.6 +/- 8.0 ml by 3DE, bullet, and Simpson's, respectively). This difference in accuracy between 3DE and 2DE methods was especially apparent in asymmetric ventricles distorted by ischemia or right ventricular volume overload. Stroke volume and ejection fraction calculated by 3DE also demonstrated better agreement with actual values than the bullet or Simpson methods with less variability (ejection fraction: error = -2.0% +/- 5.1%, 7.7% +/- 8.5%, and 6.8% +/- 12.3% by 3DE, bullet, and Simpson's, respectively). In both in vitro and in vivo settings, 3DE provides improved accuracy for LV volume and performance than current 2DE algorithms.

Three-dimensional reconstruction of ventricular septal defects: validation studies and in vivo feasibility

OBJECTIVES

The purpose of this study was to demonstrate the feasibility of in vivo three-dimensional reconstruction of ventricular septal defects and to validate its quantitative accuracy for defect localization in excised hearts (used to permit comparison of three-dimensional and direct measurements without cardiac contraction).

BACKGROUND

Appreciating the three-dimensional spatial relations of ventricular septal defects could be useful in planning surgical and catheter approaches. Currently, however, echocardiography provides only two-dimensional views, requiring mental integration. A recently developed system automatically combines two-dimensional echocardiographic images with their spatial locations to produce a three-dimensional construct.

METHODS

Surgically created ventricular septal defects of varying size and location were imaged and reconstructed, along with the left and right ventricles, in the beating heart of six dogs to demonstrate the in vivo feasibility of producing a coherent image of the defect that portrays its relation to surrounding structures. Two additional gel-filled excised hearts with defects were completely reconstructed. Quantitative localization of the defects relative to other structures (ventricular apexes and valve insertions) was then validated for seven defects in excised hearts. The right septal margins of the exposed defects were also traced and compared with their reconstructed areas and circumferences.

RESULTS

The three-dimensional images provided coherent images and correct spatial appreciation of the defects (two inlet, two trabecular, one outlet and one membranous Gerbode in vivo; one inlet and one apical in excised hearts). The distances between defects and other structures in the excised hearts agreed well with direct measures (y = 1.05x-0.18, r = 0.98, SEE = 0.30 cm), as did reconstructed areas (y = 1.0x-0.23, r = 0.98, SEE = 0.21 cm2) and circumferences (y = 0.97x + 0.13, r = 0.97, SEE = 0.3 cm).

CONCLUSIONS

Three-dimensional reconstruction of ventricular septal defects can be achieved in the beating heart and provides an accurate appreciation of defect size and location that could be of value in planning interventions.

Three-dimensional echocardiography: in vivo validation for right ventricular free wall mass as an index of hypertrophy

OBJECTIVES

This study tested the ability of three-dimensional echocardiography to reconstruct the right ventricular free wall and determine its mass in vivo using a system that automatically combines two-dimensional images with their spatial locations.

BACKGROUND

Right ventricular free wall thickness is limited as an index of right ventricular hypertrophy because right ventricular mass may increase by dilation without increased thickness and because trabeculations and oblique views can exaggerate thickness in individual M-mode and two-dimensional scans. Three-dimensional echocardiography may have potential advantages because it can integrate the entire free wall mass, uninfluenced by oblique views or geometric assumptions.

METHODS

The three-dimensional system was applied to 12 beating canine hearts to reconstruct the right ventricular free wall in intersecting views. The corresponding mass was compared with actual weights of the excised right ventricular free wall (15.5 to 78 g). For comparison, right ventricular sinus and outflow tract thickness were also measured by two-dimensional echocardiography, and the ability to predict mass from these values was determined.

RESULTS

The three-dimensional algorithm successfully reproduced right ventricular free wall mass, which agreed well with actual values: y = 1.04x + 0.02, r = 0.985, SEE = 2.7 g (5.7% of the mean value). The two-dimensional predictions showed increased scatter: The variance of mass estimation, based on thickness, was 9.5 to 12.5 (average 11) times higher than the three-dimensional method (p < 0.02).

CONCLUSIONS

Despite the irregular crescentic shape of the right ventricle, its free wall mass can be accurately measured by three-dimensional echocardiography in vivo, providing closer agreement with actual mass than predictions based on wall thickness. This method, with the increased efficiency of the three-dimensional system, can potentially improve our ability to evaluate the presence and progression of right ventricular hypertrophy.

Insights from three-dimensional echocardiographic laser stereolithography. Effect of leaflet funnel geometry on the coefficient of orifice contraction, pressure loss, and the Gorlin formula in mitral stenosis

BACKGROUND

Three-dimensional echocardiography can allow us to address uniquely three-dimensional scientific questions, for example, the hypothesis that the impact of a stenotic valve depends not only on its limiting orifice area but also on its three-dimensional geometry proximal to the orifice. This can affect the coefficient of orifice contraction (Cc = effective/anatomic area), which is important because for a given flow rate and anatomic area, a lower Cc gives a higher velocity and pressure gradient, and Cc, routinely assumed constant in the Gorlin equation, may vary with valve shape (60% for a flat plate, 100% for a tube). To date, it has not been possible to study this with actual valve shapes in patients.

METHODS AND RESULTS

Three-dimensional echocardiography reconstructed valve geometries typical of the spectrum in patients with mitral stenosis: mobile doming, intermediate conical, and relatively flat immobile valves. Each geometry was constructed with orifice areas of 0.5, 1.0 and 1.5 cm2 by stereolithography (computerized laser polymerization) (total, nine valves) and studied at physiological flow rates. Cc varied prominently with shape and was larger for the longer, tapered dome (more gradual flow convergence proximal and distal to the limiting orifice): for an anatomic orifice of 1.5 cm2, Cc increased from 0.73 (flat) to 0.87 (dome), and for an area of 0.5 cm2, from 0.62 to 0.75. For each shape, Cc increased with increasing orifice size relative to the proximal funnel (more tubelike). These variations translated into important differences of up to 40% in pressure gradient for the same anatomic area and flow rate (greatest for the flattest valves), with a corresponding variation in calculated Gorlin area (an effective area) relative to anatomic values.

CONCLUSIONS

The coefficient of contraction and the related net pressure loss are importantly affected by the variations in leaflet geometry seen in patients with mitral stenosis. Three-dimensional echocardiography and stereolithography, with the use of actual information from patients, can address such uniquely three-dimensional questions to provide insight into the relations between cardiac structure, pressure, and flows.

The power-velocity integral at the vena contracta: A new method for direct quantification of regurgitant volume flow

BACKGROUND

Noninvasive quantification of regurgitation is limited because Doppler measures velocity, not flow. Because backscattered Doppler power is proportional to sonified blood volume, power times velocity should be proportional to flow rate. Early studies, however, suggested that this held only for laminar flow, not for regurgitant jets, in which turbulence and fluid entrainment augment scatter. We therefore hypothesized that this Doppler power principle can be applied at the proximal vena contracta, where flow is laminar before entrainment, so that the power-times-velocity integral should vary linearly with flow rate and its time integral with stroke volume (SV).

METHODS AND RESULTS

This was tested in vitro with steady and pulsatile flow through 0.07- to 0.8-cm(2) orifices and in 36 hemodynamic stages in vivo, replacing the left atrium with a rigid chamber and column for direct visual recording of mitral regurgitant SV (MRSV). In 12 patients, MRSV was compared with MRI mitral inflow minus aortic outflow and in 11 patients with 3D echo left ventricular ejection volume-Doppler aortic forward SV. Vena contracta power in the narrow high-velocity spectrum from a broad measuring beam was calibrated against that from a narrow reference beam of known area. Calculated and actual flow rates and SV correlated well in vitro (r=0.99, 0.99; error=-1.6+/-2.5 mL/s, -2. 4+/-2.9 mL), in vivo (MRSV: r=0.98, error=0.04+/-0.87 mL), and in patients (MRSV: r=0.98, error=-2.8+/-4.5 mL).

CONCLUSIONS

The power-velocity integral at the vena contracta provides an accurate direct measurement of regurgitant flow, overcoming the limitations of existing Doppler techniques.

Evolutionary conservation of both the hydrophilic and hydrophobic nature of transmembrane residues

An algorithm (HRG), developed to allow the pairwise comparisons of the aligned residues of several members of large gene families of polytopic integral membrane proteins is described. Using hydrophobicity scales, application of this algorithm allows the number and size of the membrane-spanning domains of bacteriorhodopsin, a polytopic protein whose structure has been partially determined, to be predicted with a high degree of accuracy (sensitivity 94%, specificity 82% for predicting the membrane embedded or extramembranous location of residues). As opposed to previously reported structure-prediction algorithms, delineation of putative transmembrane segments from connecting loops is also more clearly evident with the application of the HRG algorithm, even with proteins from widely divergent species. This indicates strong evolutionary pressure for the conservation of both the hydrophobic and hydrophilic character of residues in membrane-embedded regions of polytopic proteins, such as those of the G-protein-coupled receptor superfamily. These and other structural and functional implications evident from the application of the HRG algorithm are considered.

Three-dimensional echocardiography: the influence of number of component images on accuracy of left ventricular volume quantitation

One approach to three-dimensional echocardiography is to reconstruct the surface of cardiac structures from two-dimensional images positioned in three-dimensional space. This approach has yielded accurate measures; however, the relationship between the number of nonparallel images used in three-dimensional echocardiographic reconstruction to the accuracy of the volume calculated has not been determined. With a canine model in which instantaneous left ventricular volume could be measured in vivo, images were obtained from intersecting long- and short-axis scans and stored with their spatial coordinates. The left ventricle was reconstructed and its volume calculated. The difference between three-dimensional echocardiographic and true volume was determined in 84 different cavitary volumes (4 to 85 ml). In each case, long- and short-axis images were deleted serially from the original data set (maximum of 27) until there were only three images left in the reconstruction. After each set of deletions, left ventricular volume was recalculated with the remaining images. Three-dimensional echocardiography accurately quantified ventricular volume with eight to 12 intersecting images, with a mean error of less than 1 ml and an SD of 5 ml. With a reduction of component images below eight, there were progressive increases in both absolute and mean percentage error. Accurate assessment of stroke volume and ejection fraction in this beating heart model also required eight to 12 images. Left ventricular volume and systolic function can be quantitated by three-dimensional echocardiography with as few as eight to 12 intersecting or nonparallel images.

Crystallographic observations of the metal ion triple in the active site region of alkaline phosphatase

Diffraction analysis reveals three metal ion binding sites, M1, M2 and M3, in each of two symmetric active centers 32 A apart in alkaline phosphatase from Escherichia coli with intermediate distances within the center of 4, 5 and 7 A for M1-M2, M2-M3 and M1-M3, respectively. A fourth site, M4, has been reported 25 A away. Arsenate, a product analog, binds adjacent to M1 and M2. The active serine residue, 102, which is phosphorylated during normal enzymatic turnover, is also adjacent to M1 and M2 and arginine 166 is adjacent to the arsenate. The implication with respect to the mechanism is that M1, M2 and Arg 166 neutralize and redistribute charges within the phosphate group, activate the serine hydroxyl, and stabilize transition states during bond formation and breakage. Three sites, A, B and C, have been deduced from solution studies and defined specifically on the basis of nuclear magnetic resonance data, binding studies and activity data. The evidence suggests correspondence of A to M1, B to M2, and C to M3. Strong antagonism between binding at M1 and M2 is evidenced crystallographically by a pseudo-saturation, which is relieved by phosphate binding. Local destabilization of the protein, particularly residues 323 through 333, is produced by removal of metals from the crystal.

Quantification of pericardial effusions by three-dimensional echocardiography

OBJECTIVES

The purpose of this study was to examine the accuracy of three-dimensional echocardiography for the quantification of asymmetric pericardial effusion volume and to compare this new technique with two-dimensional echocardiography.

BACKGROUND

Quantification of pericardial effusion by two-dimensional echocardiography relies on a symmetric distribution of the fluid. Three-dimensional echocardiography can quantitate volume without these limitations, but its accuracy for pericardial effusion volume has not yet been assessed.

METHODS

In six open chest dogs, 41 different asymmetrically distributed pericardial effusions of known volume were created by serial infusions of fluid through a pericardial catheter. The hearts were imaged using an automated echocardiographic method that integrates three-dimensional spatial and imaging data. The surfaces of the pericardial sac and heart were then reconstructed, and the volumes of pericardial effusions were calculated. Two-dimensional echocardiography was performed simultaneously, and volumes were calculated using the prolate ellipsoid method. Asymmetric distribution of the fluid was obtained by applying localized hydrostatic pressure to the pericardium.

RESULTS

The volumes of pericardial effusion quantified using three-dimensional echocardiography correlated well with actual volumes (y = 1.0x - 1.4, SEE = 7.7 ml, r = 0.98). Two-dimensional echocardiography had an acceptable correlation (y = 1.0x + 2.3, SEE = 23 ml, r = 0.84), but a marked degree of variation from the true value was observed for any individual measurement.

CONCLUSIONS

Three-dimensional echocardiography accurately quantifies pericardial effusion volume in vivo, even when the fluid is distributed asymmetrically, whereas two-dimensional echocardiography is less reliable. This new technique may be of clinical value in quantitating pericardial effusion, especially in the serial evaluation of asymmetric or loculated effusions.

Quantitative three-dimensional reconstruction of left ventricular volume with complete borders detected by acoustic quantification underestimates volume

Recently a new acoustic-quantification (AQ) technique has been developed to provide on-line automated border detection with an integrated backscatter analysis. Prior studies have largely correlated AQ areas with volumes without direct comparison of volumes for agreement. By using complete AQ-detected borders as the input to a validated method for three-dimensional echocardiographic (3DE) reconstruction, we can compare an entire cavity volume measured with the aid of AQ against a directly measured volume. This would also explore the possibility of applying AQ to 3DE reconstruction to reduce tracing time and enhance routine applicability. To compare reconstructed volumes with actual values in a stable standard allowing direct volume measurement, the left ventricles of 13 excised animal hearts were studied with a 3DE system that automatically combines two-dimensional (2D) images and their locations. Intersecting 2D views were obtained with conventional scanning and AQ imaging, with gains optimized to permit 3D reconstruction by detecting the most continuous AQ borders for each view, with maximal cavity size. Reconstruction was performed with manually traced central endocardial reflections and AQ-detected borders visually reproduced the left ventricular shapes; the AQ reconstructions, however, were consistently smaller. The reconstructed left ventricular (LV) volumes correlated well with actual values by both manual and AQ techniques (r = 0.93 and 0.88, with standard errors of 2.3 cc and 2.0 cc, p = not significant [NS]). Agreement with actual values was relatively close for the manually traced borders (y = 0.93x + 0.68, mean difference = -0.8 +/-2.2 cc). AQ-derived reconstructions consistently underestimated LV volume by 39 +/- 10% (y = 0.62x-0.09, mean difference = -7.8 +/- 3.0 cc, different from manually traced and actual volumes by analysis of variance [ANOVA], F = 69, p<0.00001). The AQ-detected threshold signal was displaced into the cavity, and volume between walls and false tendons was excluded, leading to underestimation, which increased with increasing cavity volume (r = 0.76). The AQ technique can therefore be applied to 3DE reconstruction, providing volumes that correlate well with directly measured values in a stable in vitro standard, minimizing observer decisions regarding manual border placement after image acquisition. However, when the complete borders needed for 3D reconstruction are used, absolute volumes are underestimated with current algorithms that integrate backscatter and displace the detected threshold into the ventricular cavity.