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Aortic valve replacement with small-size disc prostheses (Medtronic-Hall)

António J Brazão MD, David Prieto MD, J Ferrão de Oliveira MD, Luís Eugénio MD, Manuel J Antunes MD, PhD, DSc.

Cardiothoracic Surgery, University Hospital
Coimbra, Portugal

Abstract
Introduction and Objectives
Material and Methods
Results
Discussion
Conclusions

Abstract
Background and aim of the study: Several works have demonstrated worse performance of small prostheses in the narrow aortic root. However, modern low profile mechanical prostheses are easy to implant and have been increasingly used in elderly patients, where narrow roots are frequently observed. We describe our experience in patients with aortic annuli < 21mm with Medtronic-Hall prostheses.
Patients and Methods: From April 1988 through December 1997, 332 Medtronic-Hall size 20 (140 patients; 42.2%), 21 (96; 28.9%), 22 (96; 28.9%) prostheses were used. There were 218 females (65.7%) and the mean age was 59.3 + 9.8 years (29 - 75 years). Mean body surface area was 1.59 + 0.12 m2 (1.27 - 2.01m2). NYHA classes III/IV were present in 140 patients (42,2%). There were no significant differences in the clinical characteristics of the 3 groups.
Results: Mean pre-operative systolic LV/Aorta gradient was 64.5 + 24.8mmHg. After cardio-pulmonary bypass, peak gradients through the aortic prostheses were 13.9 + 8.0mmHg in size 20, 14.0 + 8.1mmHg in size 21 and 10.1 + 8.9mmHg in size 22. Four patients (1.2%) died in hospital. There were no significant differences in hospital morbidity among the patients in the 3 groups. The follow-up was complete for 96.2% of the patients (mean 4.2 years; range 1-11 years). Late mortality was 9.3% (31 patients; 2.4%/pt.yr). Twenty patients (14.3%) had size 20 prostheses, 7 (7.3%) size 21 and 4 (4.2%) size 22 (p=ns). Sixteen patients (4.8%) died of cardiac causes, but only 7 (2.1%) of prosthetic-related causes, including sudden death. Preoperative older age and aortic regurgitation were the only independent predictors of late mortality. Eight patients had systemic T-E events (0.6%/pt.yr), 2 had prosthetic thrombosis (0.2%/pt.yr) and 5 had haemorrhagic episodes (0.4%/pt.yr). Seven patients had PVE (0.6%/pt.yr). Ninety percent of the survivors are in NYHA class I/II.
Conclusions: The small (20, 21 and 22) Medtronic-Hall prostheses have good haemodynamic performance and are an excellent option as valve substitutes in patients with narrow aortic roots.

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Introduction and objectives:

Several works have demonstrated worse performance of small prostheses in the narrow aortic root. Aortic root enlargement, to permit the implantation of a larger prosthesis, is a common practice with good results, but is technically demanding and prolongs myocardial ischaemic time with consequences in the operative morbidity and mortality. However, modern low profile mechanical prostheses have improved haemodynamic performance, are easy to implant and have been increasingly used in elderly patients, where narrow roots are most frequently observed. We describe our experience in patients with aortic annuli < 21 mm with the use of Medtronic-Hall prostheses.

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Material and Methods:

Clinical data: From April 1988 through December 1997, 332 Medtronic-Hall size 20 (140 patients; 42.2%), 21 (96; 28.9%) and 22 (96; 28.9%) prostheses were used. There were 218 females (65.7%) and the mean age was 59.3 + 9.8 years (29 - 75 years). Mean body surface area was 1.59 + 0.12 m2 (1.27 - 2.01 m2). One hundred and fifty-one patients (45.5%) had angina, 59 (17.8%) had a history of syncope and 324 (97.6%) had congestive cardiac failure, 42.2% of whom were in NYHA class III or IV. One hundred and fifty-one patients (45.5%) had angina, 59 (17.8%) had a history of syncope and 324 (97.6%) had congestive cardiac failure, 42.2% of whom were in NYHA class III or IV. There were no significant differences in the clinical characteristics of the 3 groups, with the exception of a predominant use of size 20 and 21 prostheses in women.
The Medtronic-Hall Prostheses 20 and 22:
Like most other types of prostheses, classical Medtronic-Hall valves are available in different sizes, represented by uneven figures (21,23,25...), corresponding to annular diameter in millimetres. However, a size 20 model was launched some years ago, which consisted of a size 21 mechanism inserted into an attenuated sewing ring. Hence, it can be implanted within an aortic annulus 19 mm in diameter, which results in a much improved haemodynamic performance. Subsequently, a size 22 and a size 24 were added, with similar characteristics and based on the same principle of a larger mechanism within a smaller skirt. The internal orifice areas are 2.01 cm
2 for sizes 20 and 21 and 2.54 cm2 for size 22 prostheses.
Follow-up and statistical analysis:
Follow-up was achieved by sending a written questionnaire to all patients. The follow-up was complete for 97.4% of the patients (total 1,210 patient.years; mean 4.3 years; range 1-11 years). Statistical analysis of discontinued data was carried out by 3x2  c 2 test.

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Results:

The mean peak systolic LV/Aorta gradients, measured by direct catheterisation in the operating room before surgical correction, were 64.5 + 24.8 mmHg. There was no significant difference between the 3 groups (Table II). After cardio-pulmonary bypass, peak gradients through the aortic prostheses were 13.9 + 8.0 mmHg in size 20, 14.0 + 8.1 mmHg in size 21 and 10.1 + 8.9 mmHg in size 22. One hundred and thirty patients (39.2%) had associated surgery One hundred and thirty patients (39.2%) had associated surgery (table III). Four patients (1.2%) died in hospital (table IV). There were no significant differences in hospital morbidity among the patients in the 3 groups (table V). Late mortality was 9.3% (31 patients; 2.56%/pt.yr). Twenty patients (14.3%) had size 20 prostheses, 7 (7.3%) size 21 and 4 (4.2%) size 22 (p=0.01). Fourteen patients (4.2%) died of cardiac causes, but only 6 (1.8%) of prosthetic-related causes (table VI). Preoperative older age and aortic regurgitation were the only independent predictors of late mortality. Eight patients had systemic T-E events (0.66%/pt.yr), 2 had prosthetic thrombosis (0.17%/pt.yr) and 5 had haemorrhagic episodes (0.41%/pt.yr). Seven patients had PVE (0.58%/pt.yr). Ninety seven percent of the survivors are in NYHA class I/II (table VII).

Conclusions:

The small (20, 21 and 22) Medtronic-Hall prostheses have good haemodynamic performance and are an excellent option as valve substitutes in patients with narrow aortic roots.

Table I

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Table II

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Table III

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Table IV

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Table V

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Table VI

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Table VII

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Discussion:

The surgical attitude towards the narrow aortic root has motivated an intense debate. Several works have demonstrated worse performance of small prostheses with prognostic implications. The principal issue here is the eventual patient-prosthesis mismatch, with high trans-prosthetic gradients. In these cases, ventricular arrhythmias and sudden death occur more frequently. Concentric hypertrophy of the left ventricle, when severe, is also an important risk factor, as it may condition dynamic gradients so high that, besides their medium to long- term significance, may result in serious intra-operative difficulties with the interruption of cardio-pulmonary bypass and also in the immediate post-operative period. For this reason, we routinely perform an extensive septal myectomy, in order to eliminate this factor responsible for excessive left ventricular afterload. Obstruction of the left ventricular outlet may also be minimised by enlargement of the aortic root, and several techniques have been described. The objective is the insertion of a prosthesis with an area adequate to the patient’s body surface area, which guarantees a minimum residual gradient and may be implanted with no additional operative risk. However, narrow and calcified aortic roots are increasingly encountered, due to the growing number of aged patients submitted to aortic valve surgery, which makes this procedure longer and with added risks, including haemorrhagic complications, damage to the mitral valve, coronary arteries and conduction system, and peri-prosthetic leakage. The narrow aortic roots thus represent a surgical challenge.

One of the main determinants of the long-term results in this type of surgery is the regression of left ventricular wall mass that depends on the residual pressure gradient. High post-surgical pressure gradients inhibit this regression and may be responsible for higher mortality and morbidity rates. The obstructive characteristics and the thrombogenic potential of the different valve prostheses oblige to continuous efforts to perfect these devices, both in terms of the design and of the materials used for their construction. The haemodynamically more efficient design of modern mechanical valves permits the implantation of smaller sizes, with good haemodynamic and, consequently, clinical results. On the other hand, the lower thrombogenicity makes it possible to use them in all patients, including the elderly, especially in the aortic position, where lower levels of anti-coagulation are currently suggested. The hospital mortality observed in this series (1.2%) is lower than that reported by many surgical teams for aortic valve surgery in general and identical to our global mortality rates. Although this mortality affected only patients with size 20 prostheses, we believe that there is no direct relationship with the size of the prosthesis, as one patient died of haemorrhage and another of acute renal failure. Late mortality was higher in patients with size 20 prostheses than in those with size 21 and 22 valves. This is difficult to understand, especially because sizes 20 and 21 prostheses bear the same mechanism, as was also reflected by similar postoperative gradients. However, information on trans-prosthetic gradients in the late follow-up is not available at this stage and this is, probably, the most important drawback of this work. The explanation for the difference may be the predominance of older patients and women in the size 20 group. Older age at operation was, together with aortic regurgitation, an independent predictor of late mortality. Of interest, when differentiated between valve-related and non valve-related mortality there were no statistically significant differences. In any case, the rates of late mortality compare favourably with those reported by other groups. Other types of mechanical prostheses with a 19 mm diameter have been widely used with good results, although with higher risk in patients with a body surface area greater than 1.9 m2 . Most 19mm prostheses have internal orifices smaller than 1.7 cm2. By contrast, Medtronic-Hall prostheses sizes 21 and 22 have larger orifices (2.01 cm2 and 2.54 cm2, respectively), which represents a major haemodynamic benefit. The greatest advantage of the size 20 Medtronic-Hall prosthesis is that its internal orifice is the same as that of the size 21, as it bears of the same mechanical device, the difference being in the lower thickness of the sewing ring. In this way, it is possible to implant it in aortic roots that cannot accommodate a 21 mm prosthesis and which would otherwise only permit the use of a 19 mm prosthesis. Although other factors may condition left ventricular performance in patients subjected to aortic valve replacement, in what concerns the type of valve substitute Medtronic-Hall prostheses have shown excellent haemodynamic performances expressed by the low trans-prosthetic peak to peak systolic gradients, even in the smaller size 20 prostheses. Of interest, the gradients in sizes 20 and 21 were identical, despite the difference in body surface between the two groups of patients, which obviously reflects the fact that these two valves integrate the same mechanical device.

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Conclusions:

The small (20, 21 and 22) Medtronic-Hall prostheses have good haemodynamic performance and are an excellent option as valve substitutes in patients with narrow aortic roots. Fortunately, the great majority of our patients with a narrow aortic root are women with a small body surface area. Hence, it is possible to perform a rapid and low risk surgery by simple valve replacement with a small prosthesis but with a haemodynamic performance that guarantees good long-term results. Nonetheless, the choice of size should have in mind the body surface area and, when absolutely necessary, the aortic root should be widened in order to accommodate a larger prosthesis that better matches the size of the patient.

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Questions, contributions and commentaries to the Authors: send an e-mail message (up to 15 lines, without attachments) to surgery-pcvc@pcvc.sminter.com.ar , written either in English, Spanish, or Portuguese.

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© CETIFAC
Bioengineering
UNER
Update
Dic/04/1999