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Ann Thorac Surg 1996;61:1418-1422
© 1996 The Society of Thoracic Surgeons

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Original Articles: Cardiovascular

Early In Vivo Experience With the Hemodynamic Plus St. Jude Medical Heart Valves in Patients With Narrowed Aortic Annulus

Clinic for Cardiovascular Surgery and Department of Cardiology, University Hospital, Zürich, and Clinic for Thoracic and Cardiovascular Surgery and Department of Cardiology, University Hospital, Berne, Switzerland

Accepted for publication February 2, 1996.

	Abstract

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Background. Small aortic orifice primarily resulted in heart prosthesis mismatch in a significant number of patients. The Hemodynamic Plus (HP) series of St. Jude Medical heart valves represents an interesting innovation, allowing a larger valve orifice area with an equivalent tissue annulus diameter.

Methods. Hemodynamic characteristics of the 21-mm HP St. Jude Medical valve were prospectively compared with those of the standard 21-mm and 23-mm St. Jude Medical valves in three groups of 22 patients. Patients were selected from a database to be rigorously matched for age, sex, body surface area, functional class, underlying lesion, native valve opening area, left ventricular function, and preoperative peak and mean valve gradients. Postoperative evaluation (follow-up ranging from 3 to 24 months; mean, 11.5 months) included clinical examination and echocardiographic studies.

Results. There was no operative mortality or significant perioperative complications. Short-term clinical follow-up was marked by a complete absence of valve-related complications. Presently, all but 1 patient in the 21-mm HP group and 2 in the 21-mm standard group are in New York Heart Association functional class I. Doppler echocardiography-derived mean and maximal pressure gradients were significantly lower in the 21-mm HP group (8.1 ± 1.9 and 16.4 ± 3.4 mm Hg) than in the 21-mm standard group (13.4 ± 3.9 and 21.2 ± 4.3 mm Hg; p = 0.002 and p = 0.0004, respectively), confirming the better hemodynamic performance already described in in vitro studies. Pressure gradients did not differ significantly between the 21-mm HP and the 23-mm standard groups. The 21-mm HP valve demonstrated the highest performance index: 0.66 ± 0.08, compared with 0.49 ± 0.09 for the 21-mm standard valve (p < 0.001) and 0.59 ± 0.07 for the 23-mm standard valve (p < 0.001).

Conclusions. In vivo hemodynamic performance of the 21-mm HP valve corresponds closely to that of the 23-mm standard valve and is substantially better than that of the 21-mm standard valve. The 21-mm HP St. Jude Medical valve demonstrates excellent hemodynamic characteristics and can be recommended in normal-sized adult patients with narrow aortic root. This valve will minimize the need for aortic annulus enlargement.

	Introduction

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
The challenge with valve operation in the small aortic root is to obtain the most favorable hemodynamic performance with the shortest cross-clamping time. Options include root-enlarging techniques and using smaller prostheses. Therefore, surgical treatment varies with the patient's characteristics (age, lifestyle, body surface area), the surgeon's preference for various root-enlarging procedures, and the surgeon's personal opinion about smaller prostheses.

The Hemodynamic Plus (HP) series of St. Jude Medical (SJM) heart valves represents an interesting innovation in valve technology: the sewing cuff of the valve and its attachment to the valve were redesigned to achieve an effectively larger valve orifice area with an equivalent tissue annulus diameter. Identical technical parameters between 21-mm HP and 23-mm standard valves have been reported by the manufacturers. This prospective study analyses the in vivo performance of the 21-mm HP SJM prosthesis and compares the implantability and hemodynamic characteristics of this valve with those of 21-mm and 23-mm standard SJM valves in patients with narrowed aortic annulus.

	Material and Methods

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Patients and Procedures
In vivo hemodynamic performance of the 21-mm HP SJM valve was compared with that of the 21-mm and 23-mm standard valves in three groups of 22 patients each. Patients were selected from a database to be rigorously matched for age, sex, body surface, functional class, underlying lesion, native valve opening area, left ventricular function, and preoperative peak and mean valvular gradients. The matching process used only preoperative data and valve sizes, with the researcher being blinded to any intraoperative or postoperative data. Data of implantation extended from March 1993 to June 1995. Nine SJM

21 HP valves were implanted in Zürich and 13 in Berne. The matching process was realized with patients from the respective institution.

All operations were performed using similar cardiopulmonary bypass techniques and myocardial protection (intermittent antegrade cold blood cardioplegia) in the majority of cases. Continuous cold blood retrograde cardioplegia was used very rarely as an alternative. Valve suture technique was performed with interrupted, pledget-reinforced mattress sutures, stitched in a ventriculo-aortic fashion.

Postoperative evaluation included clinical examination, echocardiographic studies and a brief questionnaire about lifestyle criterias. Registration of any complication was done according to The Society of Thoracic Surgeons guidelines for valve operations. Postoperative echocardiography was performed before hospital discharge in each patient. The interval between surgical intervention and follow-up echocardiography ranged from 3 to 24 months (mean, 11.5 months).

Parasternal long-axis views were obtained and the systolic diameter of the left ventricular outflow tract was evaluated just below the prosthetic valve. Determination of flow velocities at the subvalvular and valvular levels was performed respecting the distribution of areas with different velocity. Recording of velocity in the subaortic area was made using color flow mapping to position the pulsed sample in line with flow. Mean velocity was assessed by using planimetry of the systolic tracing, and the area under the tracing was calculated by the echocardiography machine. This area was then divided by the time interval of forward flow. The modified Bernouilli equation was used to calculate peak and mean pressure drop (gradient) across the valve: pressure gradient = 4 (V_cw² - V_pw²), where V_cw and V_pw are the velocities (peak and mean, respectively) across the valve, using continuous-wave Doppler and pulsed-wave Doppler echocardiography in the left ventricular outflow tract.

Postoperatively, patients with normal and elevated blood pressure were put on a long-term regimen of converting enzyme inhibitors to promote or accelerate regression of left ventricular hypertrophy. All patients received subcutaneous heparin and oral warfarin on the second postoperative day.

Statistical analysis of postoperative data consisted of a comparison of the means of each valve size group and a Wilcoxon paired rank test for the comparison of each matched paired group of patients.

Definitions
Effective orifice area was calculated with the continuity equation by the simplified peak velocity method as CSA (Pk V_LVOT/Pk V_jet), where CSA is the subvalvular crosssectional area, and Pk V_LVOT and Pk V_jet are the maximal velocity in the left ventricular outflow tract and across the valve, respectively. This simplified method has shown a good correlation with that of the original continuity equation [1, 2]. Effective orifice area is an index of how well a valve design utilizes its geometric orifice area.

The effective area index is a measure of how well the effective orifice area (or flow area) of the valve matches the body surface area and is calculated as effective orifice area/body surface area. This index is used to detect mismatch between valve size and body surface area. Index greater than 1.0 indicates a good match between valve size and body surface area.

The performance index is a measure of how well a valve utilizes the valve annulus area; it is defined as the effective orifice area divided by the sewing area. Most studies provide performance index, making possible the comparison with other valve types.

	Results

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
The main technical parameters reported by the manufacturers are presented in Table 1. There was no perioperative mortality in the three groups of patients; short-term clinical follow-up was marked by a complete absence of valve thrombosis, thromboembolic events, and anticoagulation-related hemorrhage. Presently, all but 1 patient in the 21-mm HP group and 2 in the 21-mm standard group are in New York Heart Association functional class I.

The main demographic parameters did not present any significant difference between the three groups of patients. Preoperative data are presented in Table 2. Although clinical examination and questionnaire did not allow us to reveal any difference between the groups (mainly due to the short follow-up interval), there was a significant difference in Doppler echocardiography-derived pressure gradients and performance index between the three examined valve sizes.

	Comment

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
In narrowed aortic annulus, decision-making is based on careful judgment and the following factors: patient's symptoms, hemodynamic characteristics, age, lifestyle, and body surface area, as well as the types of prostheses available and the familiarity of the surgeon with root-enlarging techniques. Each method has its own advantages and disadvantages. Obviously, the main goal is to obtain the best hemodynamic results with the shortest aortic cross-clamping time. Nowadays, smaller prostheses generally allow for larger orifice area in annuli that may have traditionally called for root enlargement, but there is no general agreement on when a small prosthesis is effectively too small. Discussing this issue, one may consider that even in experienced hands, the risk of aortic root enlargement increases the perioperative risk with longer cardiopulmonary bypass time, cross-clamp time, and increased chance of bleeding.

From a surgical point of view, pledgets placed beneath the aortic annulus were never associated with malfunction of the leaflets; one has to respect the fact that the valve should be carefully pushed down before the sutures are tied. In our experience, this technique leads to less eversion of the aortic annulus itself and allows better positioning of the valve into or just over the annulus.

In the present study, a majority of patients who presented with narrowed aortic root were elderly patients scheduled for aortic valve replacement with severely calcified valve leading to severe stenosis. The percentage of such patients is currently increasing in most practices.

The main valve design objective is to maximize the effective orifice to tissue annulus to minimize pressure gradients. Therefore, the ratio of flow orifice to valve mounting is one of the most important determinants of a heart valve's hemodynamic potential, besides occluder opening angle. When the sewing flange is reduced, one could be worried about the suture stitches interfering with the moving parts of the valve or the impingment of the pivot guards into the left ventricular outflow tract. This never appeared in our series, and the new available rotatable Masters series will definitively eliminate this potential problem.

Several studies have examined the impact of small aortic prostheses on long-term results. In a series of 254 patients, Kratz and others [3] implanted 19- and 21-mm valves in 115 patients. In comparison with the overall results, the implantation of a small aortic valve was followed by a somewhat higher incidence of late sudden death in patients with a body surface area of 1.9 m².

According to the results of Czer and associates [4], implantation of smaller valves did not affect short-term hospital results. However, in this series, the results were limited to patients with body surface area less than 1.7 m². Burckhardt and colleagues [5, 6] suggested that peak aortic pressure gradient of up to 40 mm Hg did not bear any clinical significance for up to a decade in the majority of patients. Fortunately, no patient in our series (even those with a 21-mm standard valve) was demonstrated to have a transvalvular gradient of more than 31 mm Hg. The recent literature suggests that a conventional 19-mm bileaflet prosthesis is adequate for patients with a body surface area up to 1.7 m². Several authors have demonstrated that despite mismatch between flow and body surface area, the SJM bileaflet valve remains an acceptable device with favorable hemodynamics, even during exercise [7–10].

The transvalvular gradient is still the most frequently used parameter to characterize a prosthetic heart valve. The most important gradient is defined by ventricular pressure minus aortic pressure; this gradient represents the total pressure loss through the valve. Isolated or localized pressure gradients within the valve have no effect on ventricular workload.

Doppler echocardiography has shown to be a dependable method to measure flow velocity and to estimate pressure gradients across the valve. Some authors have found a significant correlation between mean Doppler and catheter gradients [11, 12]. Ihlen and associates [13] found that the catheter mean systolic gradient was consistently smaller than the ultrasonographic gradient, but regression analysis demonstrated a significant association between the two methods. The effective orifice area corrected for body surface area increased with increasing valve size, thus demonstrating a moderate valve-patient mismatch [14]. Using the HP valve may logically contribute to eliminate this mismatch.

Baumgartner and associates [15] have shown that the correlation between Doppler echocardiography- and catheter-derived pressure gradients may not be true for bileaflet mechanical valves. Localized areas of high velocities in SJM valves (and in other bileaflet prosthetic valves) violate the condition of the Bernouilli equation, making Doppler echocardiography an inappropriate means of determining the absolute gradient across the valve or of comparing it with other valves. The greater the difference between the highest localized gradient and the average of all localized gradients, the less representative the Doppler-derived gradients will be of ventricular pressure minus aortic pressure. However, Doppler echocardiography remains of great value in comparing different sizes of the same valve model or in tracking changes in pressure gradients in an individual SJM valve patient.

In the SJM valve, Baumgartner and associates demonstrated that Doppler gradients were generally higher than catheter-derived gradients because of the presence of localized high-pressure gradients across the valve. Therefore, we should also consider that Doppler gradients may be a slight overestimation of invasively measured pressure gradients in our study.

Hemodynamic performance of the SJM HP valve (19 and 21 mm) has already been studied in vitro in a pulsatile model. This valve was demonstrated to have significantly lower forward flow pressure drops and lower total energy loss than the CarboMedics, Duromedics, and Björk-Shiley monostrut valves [16].

Our data are consistent with available Doppler echocardiographic evaluation of the standard SJM valves. Most studies evaluating SJM or CarboMedics valves reported catheterization mean pressure gradients of 8 to 20 mm Hg [1, 17–20]; similar values are reported here. Interestingly, although the hemodynamic characteristics are comparable, the performance index found in this study is significantly greater that those reported by Ihlen and others [13] and de Paulis and associates [1]; in our study, 21-mm HP valves had the greatest performance index.

Orifice to annulus ratio (based on purely theoretical calculations) is much more favorable in the 21-mm HP SJM valve than in other bileaflet valves. Considering the value of an ideal orifice to annulus ratio is 1.0, this value is 0.74 in an SJM 21-mm HP valve, 0.58 in the standard 21-mm SJM valve, 0.55 in the 21-mm CarboMedics standard valve, and 0.61 in the 21-mm CarboMedics reduced series.

The relatively small number of patients and the inclination of surgeons to select HP series valves for specific patients, removing them from the randomization pool, would result in an enrollment period for a randomized study being much longer than desired. The alternative selected in this study was a matched paired comparison, where groups were formed to be as rigorously similar as possible in age, body surface area, clinical symptoms, functional class, and preoperative valvular characteristics. The definition of a small aortic root is not yet clear in the literature; because the 19-mm mechanical prosthesis has never been implanted routinely in our institution, we consider an aortic root to be small when sizing indicates that a valve less than 23 mm will be implanted.

In conclusion, the HP valve achieves a reduction in external value diameter without any reduction of the internal orifice diameter; this development improves hemodynamic performance of the 21-mm HP valve, getting similar characteristics to those of the 23-mm standard valve. Our results demonstrate that in vivo hemodynamic performances of the 21-mm HP valve correspond closely to those of the standard 23-mm valve and are substantially better than those of the 21-mm standard valve. However, the short follow-up does not allow comparison by classic cut-off points (mortality, valve-related complications). The 21-mm HP SJM valve can be recommended in patients with narrow aortic root and will help minimize the need for aortic annulus enlargement. In terms of pressure drops and total energy loss, the HP series of SJM heart valves compares very favorably with other mechanical prostheses such as the CarboMedics, Duromedics, and Björk-Shiley monostruts [1, 11, 20].

	Footnotes

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Address reprint requests to Dr Carrel, Clinic for Thoracic and Cardiovascular Surgery, University Hospital, Freiburgstrasse, CH-3010 Berne, Switzerland.

	References

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 

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