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Eur J Cardiothorac Surg 2001;19:431-437
© 2001 Elsevier Science NL

Edge-to-edge mitral repair: gradients and three-dimensional annular dynamics in vivo during inotropic stimulation

Tomasz A. Timeka, Sten L. Nielsenb, David Liangc, David T. Laia, Paul Daguma, George T. Daughtersa,d, Neil B. Ingels, Jr.a,d, D. Craig Millera

a Department of Cardiovascular and Thoracic Surgery, Stanford University School of Medicine, Stanford, CA, USA
b Department of Cardiothoracic and Vascular Surgery, Aarhus University, Aarhus, Denmark
c Division of Cardiovascular Medicine, Stanford University School of Medicine, Stanford, CA, USA
d Laboratory of Cardiovascular Physiology and Biophysics, Research Institute of the Palo Alto Medical Foundation, Palo Alto, CA, USA

Received 18 September 2000; received in revised form 5 January 2001; accepted 9 January 2001.

Corresponding author. Tel.: +1-650-725-3826; fax: +1-650-725-3846
e-mail: dcm{at}stanford.edu


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Limitations
 References
 
Objective: The edge-to-edge (Alfieri) mitral repair technique appears to be clinically promising, but the potential for functional mitral stenosis, especially with exercise, remains a concern. We used the myocardial marker method combined with Doppler echocardiography to evaluate mitral annular (MA) three-dimensional (3-D) dynamics and transvalvular gradients after leaflet approximation before and during dobutamine infusion. Methods: Eight adult sheep underwent implantation of eight myocardial markers around the MA and nine in the left ventricle. Mitral leaflet edges were approximated at the valve center and micromanometers were placed in the left ventricle and atrium. The animals were studied with biplane videofluoroscopy to determine 3-D marker coordinates for computation of precise 3-D MA area and left ventricular (LV) volume. Epicardial Doppler echocardiography measured peak and mean diastolic mitral valve gradients at baseline and during dobutamine infusion (10 µg/kg per min). Results: During dobutamine stimulation, left ventricular dP/dt increased from 1776±712 to 3390±618 mmHg/s (P=0.002), and cardiac output (CO) increased from 2.7±1.1 to 5.1±1.2 l/min (P=0.009). Mitral annular area (MAA) at end-diastole (ED) fell from 8.6±1.4 to 7.0±1.8 cm2 (P=0.001) with inotropic stimulation, but only a modest increase was observed in mean (1.4±0.4 vs. 2.4±1.0 mmHg, P=0.046) and peak (2.7±0.8 vs. 4.9±2.5 mmHg, P=0.03) diastolic mitral valve gradients. MAA changed dynamically throughout the cardiac cycle, reflecting normal physiology, but the magnitude of MAA change was augmented during inotropic stimulation (18±5% and 27±4% for control and dobutamine, respectively; P=0.004). Conclusion: Dobutamine increased CO by 89% and decreased ED annular area by 19% after edge-to-edge repair, yet only a small increase in valve gradient occurred. Marker analysis showed enhanced dynamic motion of the mitral annulus. Thus, the edge-to-edge mitral valve repair was not associated with substantial transvalvular obstruction during high flow conditions and did not perturb normal MA 3-D dynamics in normal ovine hearts.

Key Words: Mitral valve • Mitral valve repair • Valvular gradient


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 5. Limitations
 References
 
Mitral valve repair has become the preferred method for correcting mitral regurgitation and provides durable and predictable results in most patients [1,2], and probably superior clinical outcomes relative to mitral valve replacement [3]. Since the introduction of mitral repair by Carpentier [4], surgical techniques have evolved to address a wider spectrum of mitral pathology. Recently, the innovative technique of edge-to-edge, or double orifice, repair introduced by Alfieri and colleagues [5] seems to be a valuable addition to the surgeon's armamentarium for selected challenging patients who otherwise would require overly-complex repair techniques. This procedure, in conjunction with mitral annuloplasty, has been utilized to treat mitral insufficiency secondary to degenerative pathology [6], Barlow's syndrome [7], endocarditis [8], and myocardial ischemia/infarction [6,9]. In addition, mitral regurgitation in patients with end-stage cardiomyopathy has been treated with a double orifice repair, including those undergoing partial left ventriculectomy [10,11]. The edge-to-edge technique offers a simple approach to complex mitral lesions involving either anterior leaflet prolapse or restriction or extensive posterior annular calcification, thereby permitting expedient repair with short cardiopulmonary bypass and aortic cross clamp times [6]. The procedure is reproducible and reported to provide predictable medium-term results for certain types of mitral pathology [8]. Furthermore, the site of leaflet approximation can be customized to the location of the regurgitant jet [6].

Although the double orifice repair is gaining clinical acceptance by some, many questions remain unanswered. The durability of the repair and the potential for creating functional mitral stenosis, especially with exercise, remain major concerns. Low mitral gradients have been measured at rest in patients at short- to medium-term follow-up [6–8]. No data exist, however, on the hemodynamics of the double orifice valve under stress conditions. In addition, the effects of leaflet approximation on mitral annular dynamics are unknown. To shed some light on these questions, we implanted radiopaque markers in normal sheep and performed an edge-to-edge mitral repair to investigate mitral annular dynamics and diastolic valve gradients at baseline and during inotropic stimulation.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 5. Limitations
 References
 
2.1. Surgical preparation
The general procedures used have been previously described [12]. In eight adult, castrated sheep, eight subepicardial tantalum helices (inner diameter 0.8 mm, outer diameter 1.3 mm, length 1.5–3.0 mm) were inserted into the left ventricle along four equally spaced longitudinal meridians with two levels between left ventricular (LV) apex and base (Fig. 1A) . After establishment of cardiopulmonary bypass and cardioplegic arrest, eight additional markers were sutured equidistantly (approximately every 45°) around the circumference of the MA (one near each commissure and three along the anterior and posterior annulus). Subsequently, the centers of the anterior and posterior mitral leaflets were approximated at their edges with a 5-0 polypropylene suture reinforced with two small Teflon-felt pledgets. The approximating suture or Alfieri stitch was placed approximately 5 mm from each leaflet edge, and also secured a miniature force transducer, which served as another myocardial marker. Four markers were placed on the posterior and anterior leaflet edges at the center of each of the two newly created valve orifices. The complete mitral annular and leaflet marker array is shown in Fig. 1B.



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  		Fig. 1. (A) Schematic of the left ventricular and mitral annular marker array used in this experimental preparation. (B) Diagram of the mitral valve after the Alfieri repair. Mitral annular and leaflet edge markers are shown schematically as solid circles. Dashed double line represents an area bounded by four markers used to calculate one orifice area of the double orifice valve.

 
Following completion of marker implantation, the heart was defibrillated, the animal weaned from cardiopulmonary bypass, and transferred immediately to the experimental animal catheterization laboratory where they were studied intubated, open-chest, and anesthetized with ketamine (1–4 mg/kg per h i.v. infusion) and diazepam (5 mg i.v. bolus as needed). Intravenous esmolol infusion (20–50 µg/kg per min) was utilized to minimize reflex sympathetic responses. Simultaneous biplane videofluoroscopy, hemodynamic data recordings, and epicardial color Doppler echocardiography were conducted under two experimental conditions: (1) control and (2) inotropic stimulation with dobutamine (10 µg/kg per min).

All animals received humane care in compliance with the ‘Principles of Laboratory Animal Care’ formulated by the National Society for Medical Research and the ‘Guide for Care and Use of Laboratory Animals’ prepared by the National Academy of Sciences and published by the National Institutes of Health (DHEW NIHG publication 85-23, revised 1985). This study was approved by the Stanford Medical Center Laboratory Research Animal Review committee and conducted according to Stanford University policy.

2.2. Data acquisition
Images were acquired with the animal in the right lateral decubitus position with the chest open using a Philips Optimus 2000 biplane Lateral ARC 2/Poly DIAGNOST C2 system (Phillips Medical Systems, North America Company, Pleasanton, CA) with the image intensifier in the 9-inch fluoroscopic mode. Data from the two radiographic views were digitized and merged using custom-designed software [13,14] to yield the three-dimensional (3-D) x, y, z coordinates for each of the radiopaque markers every 16.7 s throughout the cardiac cycle. Ascending aortic pressure, LV pressure, LA pressure, and ECG voltage signals were digitized and recorded simultaneously on the marker images during data acquisition.

2.3. Data analysis
2.3.1. Hemodynamic and cardiac cycle timing markers
Two to three consecutive steady-state beats during baseline and after dobutamine infusion were averaged and defined as ‘control’ and ‘dobutamine’ data for each animal, respectively. For each cardiac cycle, end-systole (ES) was defined as the frame containing peak negative LV rate of pressure fall (-dP/dt). End-diastole (ED) was defined as the videofluoroscopic frame containing the peak of the ECG R-wave. Instantaneous LV volume was computed from the epicardial LV markers using a space-filling multiple tetrahedral volume method [15]. Although myocardial volume is included in this calculation of LV volume, it accurately reflects relative changes in LV chamber size [16]. Such epicardial calculations over-estimate LV chamber volume (since they include the LV myocardium) and also do not take into account systolic LV wall thickening, thereby overestimating ES chamber volume; this combination of artifacts means that marker-derived measurements of stroke volume (SV) and ejection fraction (EF) are underestimates. These artificially low ejection fractions, however, correspond to ejection fractions of 0.5–0.7 as determined by ventriculography (unpublished data). Stroke volume was calculated as the difference between end-diastolic LV volume (EDV) and end-systolic LV volume (ESV). Cardiac output (CO) was calculated as CO=SVxHR using the marker-derived volumes.

2.3.2. Mitral annular dynamics
Mitral annular area (MAA) was computed from the 3-D coordinates of the eight markers sutured to the mitral annulus. After determining the annular centroid, the annular area was divided into eight individual ‘pie slices’ which were summed to yield total annular 3-D area. MAA was calculated at end-diastole and end-systole, while annular area reduction was determined as the percent reduction from maximal MAA in late diastole to minimal MAA in early systole. Geometric valve orifice area was calculated from leaflet edge and commissural markers. Individual geometric orifice areas of the double orifice valve were calculated as the diamond shaped area between the respective commissural marker, the central Alfieri stitch (strain gauge), and the two leaflet edge markers, as illustrated by the dashed double line in Fig. 1B. Total mitral geometric valve orifice area was determined by summing the areas of the two geometric orifices created by leaflet approximation; calculated at end-diastole, and at its maximum value when the mitral valve was maximally open. The septal–lateral (S-L) annular diameter was calculated as the distance in 3-D space between markers placed on the mid-anterior and mid-posterior mitral annulus.

2.4. Statistical analysis
All data are reported as mean±1 standard deviation (±1SD), unless otherwise shown. Hemodynamic and marker-derived data from consecutive steady-state beats from each heart were aligned at end-diastole. Marker data were calculated over 20 frames before and after end-diastole, thus allowing evaluation over a time period of 650 ms. The mean and SD for each variable at each sampling instant were computed for control and dobutamine conditions. Data were compared using Student's t-test for paired comparisons.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 5. Limitations
 References
 
The average weight of the animals was 68±5 kg. The total duration of cardiopulmonary bypass was 82±9 min, with a mean aortic cross-clamp time of 61±7 min. Proper marker position and integrity of the leaflet approximating stitch were confirmed in all animals by post-mortem examination.

3.1. Hemodynamics
Hemodynamic variables after the edge-to-edge repair during control and dobutamine conditions are shown in Table 1. As expected, dobutamine infusion significantly increased heart rate, LV dP/dt, maximum LVP, and cardiac output. The inotropic effect was quite marked; cardiac output and peak LV dP/dt almost doubled with dobutamine infusion, simulating high stress conditions. The leaflet approximating stitch divided the valve into two orifices of almost equal size, as the antero-lateral and postero-medial orifices had areas of 1.54±0.33 and 1.72±0.26 cm2, respectively. The control end-diastolic and maximal total mitral geometric orifice areas (calculated as sum of the two individual geometric orifice areas) were 3.26±0.33 and 3.59±0.39 cm2, respectively. Maximal mitral orifice area during dobutamine infusion was 3.68 cm2 and was statistically unchanged from the control value (P=0.15).


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  		Table 1. Hemodynamicsa

 
3.2. Mitral valve gradients
Valvular gradients were measured by means of epicardial color Doppler echocardiography prior to institution of cardiopulmonary bypass and under control and dobutamine conditions after completion of the Alfieri repair. The peak and mean diastolic mitral gradients were 1.4±0.6 and 0.7±0.4 mmHg prior to the edge-to-edge repair, respectively. The peak and mean mitral flow velocities and gradients after leaflet approximation during control and dobutamine conditions are shown in Table 2. The mean mitral gradient increased only slightly after creation of the double-orifice valve to 1.4±0.4 mmHg; even with an 89% increase in cardiac output after dobutamine, the mean gradient was less than 3 mmHg. Similarly, although the maximum mitral gradient after inotropic stimulation almost doubled relative to control, it was still quite low. Micromanometer-measured LV and left atrial pressure tracings did not reveal the presence of substantial diastolic mitral gradients either during control or dobutamine conditions (Fig. 2) .


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  		Table 2. Valvular gradientsa

 


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  		Fig. 2. Left ventricular (squares) and left atrial (circles) pressure (mmHg) throughout the cardiac cycle after the edge-to-edge repair during control (solid symbols) and dobutamine (open symbols) conditions. A 650-ms time window centered at end-diastole (t=0) is illustrated.

 
3.3. Mitral annular dynamics
Mitral annular area measurements under the two conditions are summarized in Table 3. The dynamic nature of the changes in geometric valve orifice area throughout the cardiac cycle are illustrated in Fig. 3 . Mitral annular area was maximum in late diastole and minimum in early systole during both conditions. Relative percent mitral annular area reduction was greater during dobutamine infusion. This would be expected due to the enhanced inotropic state leading to greater ‘sphincteric’ action of the mitral annulus. At end-diastole, mitral annular area with dobutamine was 19% smaller than during control conditions. Annular size reduction with dobutamine was also observed at end-systole and at the times of maximal and minimal annular area. As the Alfieri stitch approximates the leaflets at their central edge, excessive leaflet tension here might limit annular motion in the S-L direction due to a transannular tethering effect. Our data, however, revealed that S-L mitral annular diameter remained dynamic throughout the cardiac cycle (Fig. 4) , closely paralleling mitral annular area.


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  		Table 3. Mitral annular dynamicsa

 


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  		Fig. 3. Average mitral annular area (cm2) throughout the cardiac cycle after the Alfieri repair during control ({blacksquare}) and dobutamine ({circ}) conditions for all animals. A 650-ms time window centered at end-diastole (t=0) is shown. Error bars represent one standard error of the mean.

 


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  		Fig. 4. Average septal–lateral (S-L) mitral annular diameter (cm) throughout the cardiac cycle for all animals after the Alfieri repair during control ({blacksquare}) and dobutamine ({square}) conditions. A 650-ms time window centered at end-diastole (t=0) is shown. Error bars represent one standard error of the mean.

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
5. Limitations
 References
 
Mitral valve reconstruction has become the procedure of choice for patients with most forms of mitral regurgitation. Introduction of the Alfieri ‘edge-to-edge’ technique has further expanded the indications for repair in some patients with complex lesions [6], but the resultant fixed mitral orifice area, particularly during exercise, could lead to functional mitral stenosis. We investigated the hemodynamic performance of the double orifice valve in normal sheep and found that no clinically important mitral valve gradients were present either at baseline or during inotropic stimulation with dobutamine. The dynamic changes in mitral annular area and dimensions throughout the cardiac cycle remained normal following edge-to-edge repair.

We observed a low mean mitral gradient at baseline following creation of the double orifice valve – the peak gradient did not exceed 3 mmHg. These values reflect the similarly low gradients seen in patients with the Alfieri repair when performed for anterior leaflet prolapse [8], ischemic mitral regurgitation [9], and in those undergoing partial left ventriculectomy for end-stage heart disease [11]. With postoperative mitral valve area reported to be greater than 2.5 cm2, no mitral stenosis, at least at rest, has been appreciated [6–8,11]. The calculated orifice area was 3.6 cm2 in this experiment, and no baseline gradient would be expected. During dobutamine infusion, the peak mitral gradient was still less than 5 mmHg, which should alleviate potential concern that functional mitral obstruction may arise with exercise. At the time of writing, there are no clinical data available describing the hemodynamic performance of the double orifice valve under stress conditions in humans which would support or refute these experimental findings. A computational model using finite element analysis of the Alfieri repair, however, corroborates these implications, at least in theory [17]. This computer model revealed that for an effective orifice area of 3 cm2, the peak mitral gradient does not exceed 5 mmHg at flows of 11 l/min. When mitral area falls below 2.5 cm2, however, the gradient rises sharply. Based on this model, mitral stenosis would not be expected to occur in the current experiment, even at high flow rates.

Mitral annular area changed dynamically throughout the cardiac cycle reaching a maximum in late diastole and minimum in early systole during both control and dobutamine conditions. This dynamic pattern of annular motion mirrors normal annular dynamics, and the extent of MAA reduction approximates the 12% value measured in normal, closed-chest, conscious sheep [12]. It is noteworthy that annular area reduction as measured from end-diastole to end-systole was less during dobutamine infusion (7.0±1.8 and 6.8 cm2 at ED and ES, respectively) than at baseline (8.6±1.4 and 7.8 cm2 at ED and ES, respectively). Although this result appears to suggest limited annular excursion under stress condition, it is not unexpected as MAA reduction in sheep occurs mainly prior to end-diastole [12], and increased MAA reduction with inotropic stimulation is therefore not reflected when end-diastole and end-systole are used to define MAA reduction. With inotropic stimulation, the force of atrial contraction can be expected to increase and pre-systolic annular reduction increases, with relatively little subsequent annular contraction during systole (Fig. 3). Augmented MAA reduction during dobutamine infusion is, however, clearly seen when differences between diastolic maximum and systolic minimum MAA are compared as in Table 3. Enhanced MAA reduction during inotropic stimulation is supported by the findings of other investigators [18,19]. Using 3-D sonomicrometry, Umaña [20] and co-workers from Columbia University reported a 21% annular area contraction in sheep with chronic mitral regurgitation, prior to undergoing Alfieri repair. They found no difference in annular area reduction before and after edge-to-edge repair. In normal human subjects, MAA reduction during the cardiac cycle has been reported to range between 12 and 25% [21,22].

Although cardiac output almost doubled and calculated MAA decreased by 19% with dobutamine infusion, the mitral gradient after the edge-to-edge repair increased only slightly. This may be related to normal annular dynamics which allows for annular expansion in diastole, but such is not compromised by the Alfieri repair. It is important to emphasize that although MAA decreased during inotropic stimulation, mitral orifice area did not change and a large increase in the diastolic trans-valvular gradient would not be expected. The use of an annuloplasty ring in conjunction with a double orifice valve, as is standard clinical practice of those who champion the Alfieri technique [5–8,11], may, however, increase the valvular gradient as both rigid and flexible rings decrease mitral annular size and abolish normal annular dynamics, at least in normal sheep hearts [23]. In addition, ring annuloplasty ‘freezes’ the posterior mitral leaflet in a semi-open configuration [24]. Anecdotal clinical experience confirms dynamic annular motion in patients with the Alfieri repair alone versus those with a concomitant ring annuloplasty [9]; indeed, the durability of the edge-to-edge repair may be jeopardized if ring annuloplasty is not performed. McCarthy et al. [11] reported on four patients undergoing partial ventriculectomy and the Alfieri repair alone who, despite minimal initial mitral regurgitation, developed substantial mitral insufficiency within several months. This may have been due to progressive annular dilatation if diastolic annular size is the key determinant of Alfieri stitch tension as postulated by Umaña et al. [9]. Most subsequent patients received an undersized (26 mm) Cosgrove–Edwards ring, which resulted in a more durable repair which was associated with a mitral valve gradient of only 2.9±1.4 mmHg at rest. This low gradient must be interpreted with caution, however, because these patients had advanced heart failure and low cardiac output, which could lead to abnormally low gradients. Although small partial rings were used, the pronounced annular dilatation in these patients with dilated cardiomyopathy made for a mitral valve orifice area that was still in the normal range (3.9±1.1 cm2).

The universal requirement for annuloplasty in the setting of the double orifice repair still needs to be clarified. The Alfieri repair always reduces mitral valve area, and a mitral annuloplasty ring can only be expected to compound this effect. Consequently, Alfieri's group does not advocate an edge-to-edge repair unless the annulus is dilated considerably [6]. There simply is not enough clinical information available at this time on the application of mitral annuloplasty with the edge-to-edge repair in patients without annular dilatation to make definitive statements. Perhaps an Alfieri repair alone is sufficient in patients without concomitant annular dilatation, and ring annuloplasty would only increase the risk of creating functional mitral stenosis. Conversely, in the absence of mitral annuloplasty, durability of the edge-to-edge repair may be limited. Obviously, further investigations on humans are needed to answer these questions.

In conclusion, the Alfieri repair does not cause any important degree of mitral valve obstruction either at baseline or with dobutamine infusion in normal sheep hearts; moreover, normal mitral annular dynamics are maintained. The hemodynamics of the double orifice valve along with concomitant ring annuloplasty, on the other hand, requires further investigation.


   5. Limitations
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Limitations
References
 
The major limitations of this study are inherent to the animal model. The hemodynamic performance of the Alfieri repair was evaluated in normal sheep hearts which departs from the clinical pathophysiology the surgeon confronts that usually includes annular dilatation. These animals were anesthetized, studied in open-chest conditions, and no annuloplasty rings were used, all of which limits extrapolation of these experimental results to humans. Furthermore, due to practical considerations, dobutamine rather than exercise was used to create high flow conditions. However, dobutamine does not precisely mimic the cardiovascular response of exercise as it has a smaller effect on systolic blood pressure and mitral orifice area enlargement [25], and does not duplicate the neural and humoral changes associated with stress of physical exercise. During exercise mitral orifice area has been reported to increase [26], whereas in our experiment this area was essentially unchanged. This disparity is perhaps due to the different effects of exercise and dobutamine on cardiac physiology, although both induce high flow states. These factors must therefore also be considered in extrapolating the data to human subjects. It is also feasible that the double orifice repair acts to restrict annular distensability by limiting excursion in the direction orthogonal to the line of leaflet coaptation, i.e. the annular septal–lateral diameter. Although this possibility cannot be excluded, annular septal–lateral diameter in our edge-to-edge repair animals remained quite dynamic throughout the cardiac cycle during both control and dobutamine infusion, but further studies are needed to address this important question. Because one of the goals of this study was to investigate annular dynamics after the edge-to-edge repair, ring annuloplasty was purposely avoided. The lack of a randomized control group, i.e. sheep without an Alfieri repair, for comparison of normal annular dynamics data presents a further limitation. We have previously described, however, annular dynamics in conscious sheep 7–10 days after myocardial marker implantation, and the current results are in accord with these [12] and other published data [27]. Finally, the cardiac outputs we calculated were based on LV marker motion and were not validated using other techniques; they should not be considered to be absolute values (since cardiac output was based on LV stroke volume, which in turn was calculated from epicardial marker coordinates). The reported relative changes in flow, however, should be accurate.


   Acknowledgments
 
We appreciate the technical assistance provided by Mary K. Zasio, B.A., Carol W. Mead, B.A., and Maggie Brophy, B.A.. This work was supported by Grant HL-29589 from the National Heart, Lung and Blood Institute. T.A.T., D.T.L., and P.D. are Carl and Leah McConnell Cardiovascular Surgical Research Fellows. T.A.T. is a recipient of the Thoracic Surgery Foundation Research Fellowship Award. T.A.T. and P.D. were also supported by NHLBI INRSA HL-10452-01 and HL-09569.


   Footnotes
 
Presented at the 14th Annual Meeting of the European Association for Cardio-thoracic Surgery, Frankfurt, Germany, October 7–11, 2000.


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Limitations
 References
 

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