Medtronic Hall valve replacement in a third-world population group

J

THoRAc CARDIOVASC SURG

1988;95:980-93

Medtronic Hall valve replacement in a third-world population group A review of the performance of 1000 prostheses Between May 1980 and December 1984, 1000 Medtronic Hall prostheses (538 mitral and 462 aortic) were implanted in 852 patients (mean age 30 ± 15 years) of a third-world type of population group. The most common valve lesions were aortic and mitral regurgitation, and 32 % of the patients were in New York Heart Association class IV or greater. Double valve (aortic and mitral) operations were performed in 209 patients. The 782 operative survivors were followed up for 2 to 6.5 years (mean 3.5 years) for a cumulative 2676 patient-years. Valve performance was analyzed in each position separately for valve-related complications. Results for the whole group for the following events are expressed in both linearized (percent/patient-year) and actuarial (percent ± standard error of the mean freedom at 5 years) terms, respectively: valve-related mortality, 2.9 and 92 ± 2; reoperation, 1.9 and 90 ± 2; systemic thromboembolism, 3.3 and 85 ± 2; thrombotic obstruction, 1.2 and 95 ± 1; prosthetic valve endocarditis, 0.7 and 97 ± 1; anticoagulant-related hemorrhage, 0.7 and 98 ± 1; and periprosthetic leak, 0.7 and 97 ± 1. Corresponding composite figures for valve failure were 4.3 and 85 ± 2, and for all valve-related morbidity and mortality were 6.7 and 75 ± 3. Thrombotic obstruction (fatal in 71 % of the cases), anticoagulant-related hemorrhage (61 % fatal), and prosthetic endocarditis (44 % fatal) were the most lethal complications. Although the differences did not reach statistical significance, aortic prostheses appeared to have a higher incidence of complications than the mitral prostheses. When analyzed against the background of an essentially noncompliant population group, the Medtronic Hall prosthesis proved to be reliable and remarkably free from structural failure. The high incidence of thromboembolism and thrombotic obstruction still mandates anticoagulation.

Manuel J. Antunes, MD, MMed, PhD, Andre Wessels, MB, BCh, Romuald G. Sadowski, MD, James G. Schutz, MB, BCh, Katharina M. Vanderdonck, MD, Jose M. Oliveira, MD, and Luis E. Fernandes, MD, Johannesburg, Republic of South Africa

Since the introduction of the caged-ball prosthesis in 1960, I. 2 the results of valve replacement have improved progressively because of better preoperative selection, surgical techniques, perioperative and postoperative care, and prosthetic designs. On the other hand, the new generation of tilting-disc prostheses have superior From the Division of Cardiothoracic Surgery, Johannesburg Group of Teaching Hospitals and the University of the Witwatersrand, Johannesburg, Republic of South Africa. Received for publication April 27, 1987. Accepted for publication July 8, 1987. Address for correspondence: Prof. M. J. Antunes, Div. of Cardiothoracic Surgery, University of the Witwatersrand, Medical School, 7, York Rd. 2193 Parktown, Republic of South Africa. Reprints are not available from the author.

980

hydraulic performance while maintaining the favorable characteristics of durability of their predecessors." However, thrombogenicity continues to defeat the imagination of the designers of new mechanical valves. The two-phase movement of the disc on a central pivot provides the Medtronic Hall valve with greater clearance between the occluder and the housing and reduces the areas of stasis. A larger small orifice and wider opening of the disc to 70 to 75 degrees also improve flow characteristics. Lower thromboembolic rates were, therefore, keenly anticipated," Our initial experience with this prosthesis in a third-world population group appeared to confirm these expectations, because the results obtained were better than those previously observed with other mechanical prostheses in the same population.'

Volume 95 Number 6 June 1988

Medtronic Hall valve replacement

98 1

Table I. Selected clinical and operative data Patients (No.) Valves (No.) Sex (M:F) Mean age (yr) Atrial fibrillation (%) Mean CTR (%) NYHA class IV (%) Emergency (%) Lesion (%) Regurgitation Stenosis Mixed Previous operation (%) ARC (%) IE (%) Associated operation (%) Pressures (mm Hg) Mean PAP Mean LVEDP

MVR

AVR

DVS

Total

386 386 1:2 28 ± 14 36 65 ± 7 24 10

257 257 2: 1 38 ± 16 2 63 ± 11 34 29

209 357' 1.5:I 25 ± 12 II 64 ± 8 45 30

852 1000 1:1 30 ± 15 20 64 ± 9 32 21

24 28 30 18 7 10 13

69 8 11 12 3 23 28

18 14 21 40

59 ± II 13 ± 8

40 ± 14 17 ± 13

56 ± 16 16 ± 8

16 7

16 24

CTR. Cardiothoracic ratio; ARC, acute rheumatic carditis; IE, infective endocarditis. PAP, systolic pulmonary artery pressure; LVEDP, left ventricular end-diastolic pressure. • Includes 152 mitral and 205 aortic valves.

In this demanding group of patients, we find a wide spectrum of causes of valvular lesions but with a predominance of rheumatic disease." Characteristically, the patients are young and geographically dispersed, Other adverse patient-related factors, such as the relatively poor educational and socioeconomic status, greatly influence postoperative valve performance, Compliance to anticoagulation and other forms of therapy is generally erratic and often nonexistent." Since 1980, more than 1600 Medtronic Hall valves were implanted by our unit in Johannesburg, Republic of South Africa, The performance of the first 1000 of these valves implanted in this population group, at the Baragwanath Hospital, is analyzed in this report in accordance with the comprehensive, stringent, and conservative definitions of valve-related complications proposed by the Stanford group and now widely accepted.'?

Methods Patients. From May 1980 through December 1984, 1000 Medtronic Hall valve prostheses were implanted in 852 patients (Table I). Isolated mitral valve replacement (MVR) and aortic valve replacement (AVR) were performed in 386 patients and in 257 patients, respectively, Double (mitral and aortic) valve surgery (DVS) was done in 209 patients, including 57 who had AVR and mitral valvuloplasty. The mean age of the patients in the MVR, AVR, and DVS groups was 28.2 ± 14,3,37.9 ± 16,1, and 25.4 ± 12,1 years, respectively. There was a female predominance in the MVR group, but there were more male patients in the AVR and DVS

groups. Because of their young age, only 36% of the patients in the MVR group and II % of those in the DVS group were in atrial fibrillation. The mean cardiothoracic ratio of the whole group was 0.64 ± 0.08. Valve regurgitation, pure or associated with a degree of stenosis, was the predominant lesion and was found in 65% and 80%, respectively, of the patients who underwent isolated MVR and AVR for the first time, Associated procedures were performed in 24% of the patients; tricuspid valve replacement or repair was required in 13% of the MVR group and in 7% of the DVS group. Bioprostheses were used for tricuspid valve replacement. In the AVR group, 34 patients (13%) had repair of ascending aortic aneurysms or dissection and 39 (15%) had patch widening of the aortic root. The latter was also done in 32% of the patients in the DVS group, Emergency valve replacement was performed in 21% of the patients. Patients in New York Heart Association (NYHA) functional class IV made up 32% of the group, and the remainder were in class III. Infective endocarditis necessitated valve replacement in 140 patients (16%), whereas 60 (7%) had acute rheumatic carditis. Previous operations had been done in 137 (16%) patients. Operative techniques. All operations were performed with standard cardiopulmonary bypass techniques and moderate hypothermia (25 0 to 280 C). The ascending aorta was cannulated in all patients except in 20 with ascending aortic aneurysm or dissection in whom femoral artery cannulation was used. Cold cardioplegic solution (K+ 24 mliq/L) for myocardial protection was usually administered in the aortic root. Where there was aortic regurgitation, or in administration of subsequent doses when the aortic root was open, the coronary orifices were perfused directly. A single dose was usually sufficient for isolated AVR and MVR, but in other cases the solution was readministered every 20 to 25 minutes. Additional protection was obtained by topical cooling of the

The Journal of

982

Thoracic and Cardiovascular Surgery

Antunes et al.

50

40 -

~

aort ic mit ra I r-t-r-

-

30r--

./.

-

20-

W:

10

mil 21

23

25

i

27

I~

29

.......

31

Valve size (mm)

Fig. 1. Distribution of valve sizes for aortic (shaded) and mitral positions. Percent sign indicates percent of number of valves implanted. pericardium. For reoperations the right side of the heart only was freed from adhesions, as previously described. I I Primary MVR was performed with a continuous suture technique. Aortic valve prostheses and most subsequent mitral prostheses were implanted with interrupted figure-of-eight sutures. Widening of the aortic root, to allow insertion of the largest valve possible in a small aortic anulus, was often done by a technique similar to that described elsewhere." The position of the disc and respective main orifice was reorientated to avoid impingement by the ventricular and aortic walls and the intermittent aortic regurgitation previously reported." The distribution of valve sizes is shown in Fig. I. The mitral prostheses were made temporarily incompetent with a Foley catheter so that the ventricle could be vented through a partially open left atrial suture line. In A VR, the ventricle was vented through the right superior pulmonary vein. Postoperative management and follow-up. Postoperatively, anticoagulation with subcutaneous heparin (5000 U, every 12 hours) was begun as soon as pericardial and mediastinal drainage subsided. During the first 14 months of this series, warfarin was not used in patients in sinus rhythm; these patients received dipyridamole (100 mg three times a day) and aspirin (300 mg daily). After that time, warfarin was given to all patients beginning on postoperative day I, and heparin was discontinued when the prothrombin index reached 35% to 50% of normal value, usually within 3 to 5 days after the operation. After discharge, the patients were advised to check the prothrombin index at monthly intervals, or more frequently if required. Most patients are still receiving dipyridamole, but the use of salicylates has since been discontinued. Oral penicillin V potassium was given routinely to all patients younger than 30 years old who had rheumatic disease. All patients surviving the operation were followed up for a minimum of 2 and a maximum of 6.5 years (median 4.3 years). The cumulative follow-up was 2676 patient-years (MVR 1223, AVR 793, and DVS 661) and the mean

follow-up for all survivors, up to the time of death or lastest assessment, was 3.5 years. All patients were asked to return to the outpatient clinic I, 3, and 6 months after opreation and biannually thereafter. However, follow-up was hampered by the wide geographic distribution of the patients' locale of residence. One social worker was employed to visit defaulters and traveled up to 500 krn to bring the patients to the clinic, whenever possible. Despite these efforts, 13% of the patients were lost to follow-up and were censored when last seen or contacted. The majority of these patients originate from neighboring countries from which accurate information is not easily obtainable. Data analysis and definitions. All deaths within 30 days of operation or during the same hospital admission, whichever was longer, were included in the early mortality figures. All other deaths were included in the late mortality figures. Data regarding survival and valve-related events were obtained from review of the clinical files and from the information obtained by the social worker during visits to the patients and to the regional clinics and hospitals. Valve performance was evaluated by the following hazardincidence end points defined according to the general guidelines established by Miller and associates": Valve-related mortality. Valve-related mortality included all valve complications, as defined below, that resulted in the late death of the patient. Because sudden, unexpected deaths and deaths of unknown cause constituted 38% of the total number of known deaths, these were reclassified to the different groups in the same ratio as for the known causes of death. Reoperation. Reoperations included those performed for repair or replacement of previously implanted Medtronic Hall prostheses. Thromboembolism. Thromboembolism was defined as any new focal, neurologic, or peripheral deficit, either transient or permanent, occurring after postoperative day 7 (to avoid inclusion of episodes of embolism by air or valve debris), unless another source of embolism could be proved. Thrombotic obstruction. Thrombotic obstruction included all confirmed cases diagnosed at reoperation or autopsy and all clinically suspected cases in the absence of autopsy confirmation. Anticoagulant-related hemorrhage. Anticoagulant-related hemorrhage encompassed all confirmed cases of spontaneous internal or external hemorrhage sufficiently severe to warrant hospitalization and that necessitated operation or resulted in the death of the patient. Prosthetic valve endocarditis. Prosthetic valve endocarditis was recorded only for patients in whom the diagnosis was confirmed at reoperation or postmortem examination; thus patients treated medically were excluded. Bland periprosthetic leak. Bland periprosthetic leak was determined as all cases diagnosed clinically and that required reoperation because of hemodynamic imbalance, excluding cases of prosthetic valve endocarditis. Valve failure. Valve failure included all valve-related complications that necessitated reoperation or resulted in the death of the patient. Mortality index. The mortality index was calculated for each type of complication as a ratio of the number of fatal events against the total number of known episodes of that specific complication.

Volume 95 Number 6 June 1988

Medtronic Hall valve replacement

Table II. Early mortality according to indication for operation and functional class of the patients Total

MVR AVR DVS Total

Elective

Emergency/ class IV

No.

%

(%)

(%)

p Value

30 24 .!.§. 70

7.8 9.3 7.7 8.2

4.1 3.0 2.0 3.2

22.7 22.9 17.1 20.9

<0.01 <0.01 <0.01 <0.01

Statistical analysis. All episodes were included in the calculation of the linearized incidences, equivalent to the number of events per 100 patient-years (percent per patientyear). The actuarial probabilities of survival were calculated according to the method of Grunkemeier and Starr," but no patient wasconsidered at risk for more than one similar event. Percentages wereexpressed as percent ± I standard error, and continuous data as mean ± I standard error, for 68% confidence limits. Statistical significance was established by the x' test with the use of 2 X 2 contingency tables and Yates' correction for continuity for small numbers. Results Mortality. The early mortality rate was 8.2% ± 0.9% (70 patients). The mortality rates for MVR, AVR, and DVS were 7.8% ± 1.4%, 9.3% ± 1.8%, and 7.7% ± 1.8%, respectively (p = NS*) (Table II). The mortality rate for elective valve replacement was 3.2% ± 0.7%, and that for patients in class IV or having an operation on an emergency basis was 20.9% ± 3.1% (p < 0.01). Fourteen patients died at operation because it was impossible to wean them from cardiopulmonary bypass. All of these had extremely low cardiac output and were operated on under emergency conditions. Another 14 patients died in the intensive care unit of low output syndrome. Multiple organ failure (12 patients) and neurologic deficits (seven patients) were the other most common causes of early death. Late death occurred in 169 patients (6.3% ± 0.5%/ pt-yr). The linearized mortality rates for MVR, AVR, and DVS were 5.7% ± 0.7%/pt-yr,6.9% ± 0.9%/pt-yr, and 6.7% ± 1.0%/pt-yr, respectively (p = NS). The causes of late death were indicated in Table III. Sudden death or death of unknown cause occurred in 64 (38%) patients. Long-term actuarial survival rates for the 782 operative survivors are shown in Fig. 2. At 5 years, the estimated survival rates were 75% ± 2% for the whole group, 76% ± 3% for MVR, 72% ± 5% for AVR, and 72% ± 5% for DVS.

*NS = Not significant.

Table

983

m. Causes of late death Total

Cardiac Valve-related Non-valve-related Noncardiac Unknown Sudden Total

MVR

AVR

DVS

No.

%/pt-yr

23 16 6 17

23 2 10 15

10 10 5 13

56 28 21 45

2.1 1.0 0.8

~ 70

...2

....Q

-.!2.

0.7 6.3

55

44

1.7

169

Table IV. Causes of valve-related mortality and

respective mortality index* Deaths

Thrombotic obstruction Thromboembolism Hemorrhage Prosthetic endocarditis Periprosthetic leak Dysfunction Rcclassifiedt

Total

No.

%/pt-yr

22 9 II 8 4 2 23 79

0.8 0.3 0.4 0.3 0.1 0.1 0.9 2.9

Index (%) 71 10 61 44 7 40

± ± ± ± ± ±

8 3 II 12 3 22

48 ± 4

'Ratio between valve-related mortality and total number of valve-related incidents.

tSee text for explanation.

Valve-related mortality. Fifty-six patients (33%) died of confirmed valve-related complications (Table IV). To this figure were added 23 of the 64 (36%) sudden or cause-known deaths (same ratio as for the known causes), which resulted in a linearized incidence of 2.9% ± 0.3%/pt-yr. The most common known causes of valve-related mortality were thrombotic obstruction (0.8% ± 0.2%/pt-yr), anticoagulant-related hemorrhage (0.4% ± 0.1%/pt-yr), thromboembolism (0.3% ± O.1%/pt-yr), and prosthetic valve endocarditis (0.3% ± O.1%/pt-yr). In actuarial terms, 92% ± 2% of all patients were free from valve-related mortality at 5 years (Fig. 3). Rates of freedom from valve-related mortality after MVR, A VR, and DVS were 93% ± 2%, 88% ± 3%, and 94% ± 3%, respectively. Reoperation. Forty-eight patients required 52 reoperations on the same valve or valves (1.9% ± O.3%/pt-yr). The linearized incidences of reoperations for MVR, AVR, and DVS were 1.5% ± 0.2%/pt-yr, 2.1% ± 0.5%/pt-yr, and 2.6% ± 0.6%/pt-yr, respectively (p < 0.10). Another three patients required reoperation for replacement of a valve other than that originally operated on. The indications for reoperation are shown in

The Journal of

984

Antunes et

at.

Thoracic and Cardiovascular

Surgery

"0

.~80

...::J

If)

•,.!

70 183

12

36

24

48 Months

60

72

Fig. 2. Actuarial survival for the whole group of patients.

o Free from VRM • Free from VF o Free from all VR complications

598

727

12

24

536 36

48 Months

407

117

243

60

72

Fig. 3. Actuarial estimates of freedom from valve-related mortality (VRM). valve failure (VF). and all valve-related (VR) complications after a maximum follow-up of 72 months.

Table V. The most common were periprosthetic leak (0.7%/pt-yr, 20 cases), thrombotic obstruction (0.6%/ pt-yr, 15 cases), and prosthetic valve endocarditis (O.4%/pt-yr, 12 cases). The mortality rate for reoperation was 23% ± 5% (12 cases). Actuarially, 90% ± 2% of all patients were free from reoperations at 5 years (Fig. 4). Rates of freedom from reoperation after MVR, AVR, and DVS were 93% ± 3%, 91% ± 3%, and 88% ± 4%, respectively. Thromboembolism. Eighty-two patients had 88 episodes of systemic thromboembolism (3.3% ± 0.3%/ pt-yr) at a mean of 17 ± 12 months after operation (Table VI). The incidences for MVR, AVR, and DVS were 3.1% ± 0.5%/pt-yr, 3.5% ± 0.7%/pt-yr, and

3.3% ± 0.7%/pt-yr, respectively (p = NS). The embolic episodes were fatal in nine patients (0.3% ± 0.1%/pt-yr, mortality index 16%), and 37 patients (42%) had residual sequelae (1.4% ± 0.2%/pt-yr). The overall incidence for patients in atrial fibrillation was similar to that for patients in sinus rhythm. On an actuarial basis, 91% ± 1% of the patients were free from thromboembolism at 2 years and 85% ± 2% at 5 years (Fig. 5). For the latter period, the rates of freedom from thromboembolism for MVR, AVR, and DVS were 89% ± 3%, 82% ± 4%, and 84% ± 5%, respectively. Thrombotic obstruction. Thirty-one thrombosed valves were identified in 29 patients (two patients had both aortic and mitral valve thrombosis) 19 ± 10

Volume 95 Number 6

Medtronic Hall valve replacement

June 1988

985

ioo ~ 98

0

~96

~ 94

~92

~ 90 ;;!

t

i

727

12

i

593

24

529

I

36

i

404

48

I

241

60

Months

i

117

72

Fig. 4. Actuarial probability of freedom from reoperation for valve-related reasons.

100

95

90 85

oFree from ACH .Free from TO o Free from TE

714 12

594 24

514 36

381 48

Months

215 60

96

72

Fig. 5. Actuarial curves depicting freedom from anticoagulant-related hemorrhage (ACH). thrombotic obstruction of the prosthesis (TO). and systemic thromboembolism (TE).

months after operation, for a linearized incidence of 1.2% ± 0.2%/pt-yr (Table VI). There was a history of erratic or deficient anticoagulation in all instances. Whereas the incidence in the MVR and A VR groups was similar (I.l % ± 0.3%/pt-yr and 1.1 % ± 0.4%/ pt-yr), that for DVR was slightly higher (1.4% ± 0.5%/ pt-yr, p = NS). Fifteen patients underwent reoperation with a mortality rate of 40% ± 13% (6/15). The remainder died before an operation could be performed, and the diagnosis was made either clinically or by postmortem examination. Thus fatal thrombotic obstruction occurred at a rate of 0.8% ± 0.2%/pt-yr (mortality index 71%). Actuarially, 95% ± 1% of the patients were free from thrombotic obstruction at 5 years (Fig. 5). The corresponding figures for MVR, AVR, and DVS were 96% ± 2%, 94% ± 2%, and 93% ± 3%, respectively.

Bland periprosthetic leak. Of the 54 episodes of periprosthetic leak diagnosed clinically, only 20 (in 19 patients) were considered significant enough to warrant reoperation (0.7% ± 0.2%/pt-yr). The mitral prosthesis was involved in eight cases and the aortic in ten cases. One patient had periprosthetic leakage of both the mitral and aortic valves. Four patients died during or after reoperation (21% ± 9% or 0.1% ± O.I%/pt-yr). In actuarial terms, 97% ± 1% of all patients were free from significant periprosthetic leakage after 5 years. Prosthetic valve endocarditis. Eighteen patients had prosthetic valve endocarditis (0.7% ± 0.2%/pt-yr). Thirteen of these had originally been operated on for infective endocarditis. Six died before treatment could be instituted, and 12 underwent reoperation (two deaths, 17% ± 11%). Thus prosthetic valve endocarditis was fatal in eight patients (0.3% ± 0.1%/pt-yr, mortality

The Journal of

986

Thoracic and Cardiovascular

Antunes et al.

Surgery

Table V. Indications for and mortality of reoperation

Table YD. Causes and incidence of valve failure* Incidence

MVR AVR DVS Total Mortality (%) Prosthetic endocarditis Periprosthetic leak Thrombotic obstruction Dysfunction Total

5 4 6

3 7 5

-.l

-.1

18

17

4 9 4

12 20 15

--

-.l

17

52

17 20 40 40 23

± ± ± ± ±

11 9 13 22 5

Table VI. Rates of thrombotic obstruction and

thromboembolism Thrombosis

MVR AVR DVS Total

Embolism

No.

%/pt-yr

No.

%/pt-yr

13 9

1.1 1.1 1.4 1.2

38 28 22 88

3.1 3.5 3.3 3.3

...2 31

index 44%). Actuarially, 97% ± 1% of all patients were free from prosthetic valve endocarditis at 5 years (Fig. 6). Anticoagulant-related hemorrhage. Eighteen serious episodes of anticoagulant-related hemorrhage occurred, for a linearized incidence of 0.7% ± 0.2%/ pt-yr. Eleven of these episodes were fatal (0.4% ± 0.1%/pt-yr, mortality index 61%). The incidence was twice as high in patients with AVR (1.0% ± 0.4%/ pt-yr) as in those in the MVR (0.5% ± 0.2%/pt-yr) and DVS (0.6% ± O.3%/pt-yr) groups (p < 0.05). The actuarial estimate of freedom from anticoagulant-related hemorrhage for all patients after 5 years was 98% ± 1% (Fig. 5). Valve failure. According to the broad-based criteria previously cited, valve failure occurred in 115 patients (4.3% ± O.4%/pt-yr) and was fatal in 69% ± 4% of the cases. This total is made up by the valve-related mortality (including the 23 sudden or cause-unknown deaths) and the 36 survivors of reoperation. Table VII shows the causes of valve failure and respective timerelated incidence. Thrombotic obstruction (1.1% ± 0.2%/pt-yr), periprosthetic leak (0.7% ± 0.2%/pt-yr), prosthetic valve endocarditis (0.7% ± 0.2%/pt-yr), and anticoagulant-related hemorrhage (0.4% ± 0.1%/pt-yr) were the main causes. Hemodynamic dysfunction occurred on five occasions (0.2% ± 0.1%/pt-yr). Considering the different groups, mitral valve failure rate was 3.6% ± 0.5%/pt-yr and aortic valve failure rate was 4.8% ± 0.8%/pt-yr. The rate of valve failure in the DVS group was 5.0% ± 0.8%/pt-yr (p < 0.10). The

MVR Thromboembolism Thrombotic obstruction Hemorrhage Prosthetic endocarditis Periprosthetic leak Valve dysfunction and other Reclassified Total

AVR DVS

4 13 5 6 4 3

4 9 5 4 7 2

...2

...J..

44

38

I

7 1 8 9

7 33

Total

(%/pt-yrj

9 29 11 18 20 5

OJ 1.1 0.4 0.7 0.7 0.2

...n 115

0.9 4J

'See text for definition.

vm. Linearized incidence of composite valve-related morbidity and mortality

Table

Incidence MVR Thromboembolism Thrombotic obstruction Hemorrhage Prosthetic endocarditis Periprosthetic leak Dysfunction Total

AVR DVS

38 13 6 6 4

28 9 8 4 7

-.l

-.1

70

58

22 7 4 8 9

--

50

Total

(%/pt-yrj

88 29 18 18 20 _5 178

3.3 1.1 0.7 0.7 0.7 0.2 6.7

actuarial estimates of freedom from valve failure after 5 years were 85% ± 2% for the whole group (Fig. 3), 87 ± 3% for MVR, 82% ± 4% for A VR, and 85% ± 4% for DVS. Mortality index. The mortality index is the ratio between valve-related mortality and the total number of events for each complication (Table IV). In this series, the most lethal modes of valve failure were thrombotic obstruction (71% ± 8% fatal, 22/31 cases), anticoagulant-related hemorrhage (61% ± 11% fatal, 11/18), and prosthetic valve endocarditis (44% ± 12% fatal, 8/18). Conversely, thromboembolism (lO% ± 3% fatal) and periprosthetic leak (7% ± 3% fatal) were relatively benign complications. Overall, 48% ± 4% of all valverelated complications were fatal. Valve-related complications. A total of 165 patients had 178 valve-related complications for a linearized incidence of 6.7% ± 0.5%/pt-yr (Table VIII). This figure combines the cases of valve failure and of valve-related complications that did not result in reoperation or death (thromboembolism, anticoagulant-related hemorrhage, periprosthetic leak). The incidence for each of the three groups was as follows: MVR 5.7% ± 0.7%/pt-yr, A VR 7.3% ± 0.9%/pt-yr, and DVS

Volume 95 Number 6

Medtronic Hall valve replacement· 987

June 1988

100

~ 99 0E 98

--a: 97 0....

....

IJ...

~

0

96

t

I

12

714

I

544

605

24

36

I

48

Months

420

I

60

253

123 72

Fig. 6. Actuarial depiction of freedom from prosthetic valve endocarditis (PVE).

7.6% ± 1.0%jpt-yr (p = NS). Calculated actuarially, 75% ± 3% of all patients were free from all valverelated mortality and morbidity at 5 years. The estimates for MVR, A VR, and DVS were 78% ± 4%, 70% ± 5%, and 71% ± 6%, respectively (Fig. 3). Valve performance. Table IX shows a resume of the indices of valve performance as previously defined. Although there were no statistically significant differences, the mitral valve appeared to perform better than the aortic valve. Some higher values in the DVS group were because two valves were at risk in the patient-time equation. Discussion Poor compliance with postoperative therapy, especially anticoagulation, constitutes a significant hazard for patients with mechanical prostheses," On the other hand, in this characteristically young population group, the rate of biodegradation and calcification of the bioprostheses is unacceptably high." In this country, a large number of patients with these characteristics have free access to the advanced technology of heart surgery. In our unit alone, more than 1500 such patients underwent prosthetic replacement of cardiac valves in the past 6 years. This figure does not include more than 600 patients who had conservative closed or open mitral valve operations. Almost all of those having MVR and a large majority of those having AVR had rheumatic valvular lesions, often with superadded infective endocarditis. Acute rheumatic fever is still endemic, particularly among those living in urban centers, and increasing numbers of elderly patients have degenerative valve conditions. In the late 19608, valve replacement was performed with homografts and with Starr-Edwards prostheses. An insufficient supply of homografts, which could not keep pace with the increasing demand, and poor durability, which resulted mainly from the methods of preservation

used at the time, led to the homograft option being abandoned. The Starr-Edwards valves used from 1970 to 1974 were the cloth-covered models, which were plagued by a significant incidence of thromboembolic complications. From 1974, the Bjork-Shiley tilting spherical-disc valve was used in all patients and in all positions. Although these prostheses had improved hydrodynamic characteristics, they were soon found to have a high incidence of thrombotic obstruction, often with catastrophic consequences, especially in the mitral position and in patients receiving inadequate anticoagulation." Naturally, the porcine bioprostheses were at that time believed to be ideally suited for these patients and were used exclusively for replacement of the mitral valve from 1976 to 1980. That experience cnfirmed the extremely poor durability of bioprostheses in young patients. Valve failure rates exceeded 23%jpt-yr for patients less than 20 years of age, and only 19% of the patients were alive with the original prostheses at 6 years." The durability in older patients appeared then to be significantly better, but still, because the majority of the remaining patients were in the third and fourth decades of life, most have since required valve replacement. Unfortunately, this information became known too late: Although the porcine valves had been in clinical use since 1970, it was not until 1978 that the first reports of accelerated biodegradation and calcification in children were published.": 19 Our experience contrasts with those of others in that calcification of mitral valve bioprostheses was often as catastrophic as was thrombotic obstruction of mechanical prostheses and resulted in the death of the patient. Besides, reoperation, even when done electively, carried a mortality rate significantly higher than that of primary valve replacement." Therefore, it was assumed that mechanical prostheses offered the best probability of survival in this population. The introduction of a new generation of prostheses, of

988

The Journal of Thoracic and Cardiovascular

Antunes et al.

Surgery

Table IX. Indices of valve performance

Valve-related mortality" Reoperation Thromboembolism Thrombotic obstruction Prosthetic endocarditis Hemorrhage Valve failure Valve-related mortality and morbidity

MVR

AVR

(%/pt-yr)

(%/pt-yr)

2.5 1.5

3.7 2.1 3.5 l.l 0.5 1.0 4.8 7.3

3.1 1.1 0.5

0.5 3.6 5.7

DVS (%/pt-yr)

Actuarial"

2.4

92 ± 2

2.6 3.3 1.4

90 ± 2

1.2 0.6 5.0 7.0

(%)

85 ± 2

95 ± I 97 ± 1 98 ± 1 85 ± 2 75 ± 3

'Event-free estimate for the whole group at 5 years. tCorrected (see text).

which the Medtronic Hall valve (originally Hall-Kaster) is a fine example, offered the ideal opportunity to test this assumption. This prosthesis was developed ostensibly to improve on existing tilting-disc valves by reducing the risk of valvular thrombosis. The two-phase motion of the disc on a central guide rod increases the clearance between the disc and the housing in the open position. Also, the augmented size of the small orifice, combined with a wide angle of opening (70 to 75 degrees), reduces the areas of stasis behind the disc. In vitro testing showed low resistance to forward flow in combination with minimal regurgitation and other favorable hydrodynamic characteristics that compare well with those of other mechanical prostheses in current clinical use.v " In our unit, this prosthesis was used routinely in all patients requiring valve replacement since 1980, unless there was an absolute contraindication to anticoagulation. Since then, more than 1600 prostheses have been implanted. The results obtained must be considered against the background of this particular population. For any type of prosthesis, the patients' compliance to anticoagulation is the most important determinant of thrombosis and thromboembolism.> 22 The incidence of these two complications can be minimized, but probably not abolished, by well-controlled anticoagulation only possible in a sophisticated, socioeconomically well-developed population group. Compliance is usually poor in our relatively underdeveloped, third-world group of patients. The deficient socioeconomic conditions and educational status lead to erratic warfarin intake and unsatisfactory control of prothrombin levels. Consequently, thromboembolic and thrombotic obstruction rates were expected to be higher than those generally reported in other population groups. Although this proved to be the case, the rates were lower than anticipated. Two factors may have affected considerably the results obtained. First, the lack of anticoagulant therapy in the patients operated on in the first 14 months of this

series must have had an as yet undetermined negative influence on the incidence of systemic thromboembolism and thrombotic obstruction. The initial rates of thromboembolic complications in patients not receiving anticoagulants? were significantly higher (systemic thromboembolism 6.8%jpt-yr, thrombotic obstruction 2.0%/ pt-yr) than those now obtained for the whole series including the original group of patients. Second, the wide geographic dispersion of the patients, in the absence of a sophisticated medical-care infrastructure, resulted in a significant loss to follow-up. The influence of such a factor on the results of any statistical study is unknown and the subject of controversy. Possibly, the most balanced view is that those lost to follow-up would have the same incidence of complications as those who attend the clinic regularly. Indeed, as this series has been reviewed periodically, the percentage of patients lost to follow-up has remained static. There has been a constant recovery of patients previously lost (± 5%), while a similar number of patients is lost de novo. Nevertheless, the absenteeism of these patients raises the question of the long-term consequences of their not receiving any form of anticoagulation. Another aspect that warrants further consideration is the large percentage of sudden and cause-unknown deaths (34% of the total number of known deaths). Their influence on the final results is, again, largely speculative. Arrhythmias and thromboembolism are generally considered the most probable causes of sudden death. In the particular circumstances of our patients, generally operated on with advanced disease, left ventricular dysfunction is also an important cause of late mortality. Hence it appears excessive to consider all sudden deaths as valve related, as suggested by some. In all probability, the most balanced method of dealing with deaths of unknown cause is to reclassify them into the different categories in a ratio similar to that observed for the known causes of death. Within these constraints, we have analyzed the per-

Volume 95 Number 6 June 1988

formance of the Medtronic Hall prosthesis according to nine parameters: valve-related mortality, reoperation, systemic thromboembolism, thrombotic obstruction, anticoagulant-related hemorrhage, bland periprosthetic leaks, prosthetic valve endocarditis, valve failure, and composite valve-related mortality and morbidity. Each highlights the advantages and disadvantages of the valve when inserted into the population under study. In this analysis, special consideration was given to the evaluation of valve performance rather than to conducting a patient-based study, an approach recommended by Bodnar, Wain, and Haberman" and applied clinically by Miller and associates 10.24 in their analysis of the performance of the Starr-Edwards prosthesis. This valve has generally been accepted as the gold standard against which all other valves must be compared. Hence we have followed the comprehensive definition of valve failure proposed by these authors. Thromboembolism. Systemic thromboembolism remains the major hazard of mechanical valves, particularly with poor anticoagulation. However, even the tissue valves are not exempt from this complication, and systemic thromboembolic rates of 2% to 4%/pt-yr have been reported. 6. 25-27 The rates for mechanical prostheses (1% to lO%/pt-yr) are almost consistently higher for the mitral valve than for the aortic. 10. 24. 28-30 Reports dealing with other large series of patients with Medtronic Hall prostheses have been relatively rare." Recently, Hall, Nitter-Hauge, and Abelnoor-? reported thromboembolic rates of l.7%/pt-yr and 1.4%/pt-yr, respectively, for MVR and AVR in a population clearly more compliant than ours. These rates compare favorably with those reported recently by other authors for other prostheses. In our own experience in a population with wellcontrolled anticoagulation, the incidence of thromboembolism for MVR and AVR with the St. Jude Medical prosthesis was 2.7%/pt-yr and 2.5%/pt-yr, respectively." Miller and associates'P" reported a linearized rate (all events included) of 9.5%/pt-yr for the StarrEdwards prosthesis in the mitral position and 3.8%/ pt-yr in the aortic position. Horstkotte and colleagues" reported an incidence of 2.8%/pt-yr after MVR and 1.9%/pt-yr after AVR with the Bjork-Shiley prosthesis. The incidence of 3.1%/pt-yr for the MVR group in this series compares well with those previously cited, even without the differences in compliance being considered. But our experience is at variance with those of most other authors in that a higher incidence of thromboembolism was recorded in patients having AVR (3.5%/ pt-yr). The low percentage of patients in atrial fibrillation in the MVR group may have contributed to attenuation of the differences between the MVR and AVR groups. However, some authors have found no

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correlation between atrial fibrillation and an increased risk of thromboembolism in patients with mechanical prosthesis. 10. 34 Rabago and associates" also found a higher incidence of thromboembolism for AVR (4.7%/ pt-yr) than for MVR (2.0%/pt-yr) in their experience with the Omniscience valve. Thromboembolism was fatal in 10% of the episodes in this series, in 11.5% of patients in our experience with the St. Jude Medical prosthesis," and in 12% to 15% of the patients of Miller and associates 10.24; but thromboembolism constituted only 11% of all our known valverelated late deaths, whereas it comprised up to 35% of valve-related mortality in the experience of these authors. Thrombotic obstruction. In this series, thrombotic obstruction occurred at a rate of l.l %/pt-yr in both the MVR and AVR groups. At 5 years, 96% and 94% of patients, respectively, were free from thrombotic obstruction. In the experience of Hall, Nitter-Hauge, and Abelnoor," the incidence of thrombotic obstruction was 0.2%/pt-yr for MVR and 0.1%/pt-yr for AYR. The higher incidence of thrombotic obstruction in our patients is the natural consequence of their poor compliance to anticoagulation and contrasts with the incidence of 0.8%/pt-yr and 0.1%/pt-yr in the St. Jude Medical series of well-controlled patients." Nevertheless, it is much lower than that observed with the Bjork-Shiley prosthesis in the same population group, estimated at 4%/pt-yr for the 148 patients who underwent MVR in our unit from 1974 to 1976, after a similar length of follow-up. This incidence compares well with that reported by Karp and associates" for the same prosthesis (3.3%/pt-yr). The lower thrombogenicity of the Medtronic Hall prosthesis, when compared with previous tilting-disc prostheses, appears to be related to the improved flow characteristics. The valve is partricularly well-suited to cinefiuoroscopic examination, which makes the diagnosis of restriction of the movements of the disc easy and renders cardiac catheterization unnecessary. The echocardiographic patterns of the normal and abnormal disc motions are also characteristic and have been the subject of a recent report from our institution." In contrast with our experience, most authors report a much higher rate of thrombotic obstruction for the mitral than for the aortic valve, but Bjork and Henze' reported a 27-fold increase after AVR with Bjork-Shiley prostheses in patients not receiving anticoagulation. Others have also found a direct correlation between deficient anticoagulation and increased risk of thrombotic obstruction.F: 38 Pregnancy, frequent in our patients, also appears to increase thrombogenicity." In the past, we have reoperated on several pregnant patients for removal of throm-

990

Antunes et al.

bosed prostheses." For this reason, a special antenatal clinic was recently established to deal with patients with cardiac valve prostheses. The results have thus far been encouraging." In this series thrombotic obstruction was fatal in 71% of the cases. Other authors have reported similarly high mortality.P" Fourteen of the 29 patients with thrombotic obstruction never reached the operating table and six of the 15 (40%) who were reoperated on died. However, this high operative mortality rate does not conform with the 13% operative risk we have previously reported for patients with thrombosed prostheses and is probably related to the small numbers involved.v" Contrary to the opinion expressed by Venugopal and co-workers," we believe that thrombectomy of mechanical valves is seldom justified, because of the high incidence of rethrombosis. Some years ago we performed seven thrombectomies in patients with spherical-disc Bjork-Shiley prostheses. Three of these prostheses subsequently thrombosed and necessitated replacement. Three of the remaining four patients have since been lost to follow-up. Anticoagulant-related hemorrhage. The incidence of anticoagulant-related hemorrhage is a function of the intensity of anticoagulation. 10. 43 The low rate of anticoagulant-related hemorrhage in this series (0.7%/pt-yr) is thus clearly related to the lower level of compliance. Others have reported incidences of up to 4.3%/ptyr.'? 24. 28. 35. 44 We must emphasize that only the episodes severe enough to result in the death of the patient or to require hospitalization were entered in this series. Often, bleeding occurred in patients who took an overdose of warfarin to compensate for a period of deficient intake. Nevertheless, anticoagulant-related hemorrhage still contributed more to value-related mortality than did thromboembolism, a well-recognized trend whenever the intensity of anticoagulation is stepped up. In our previously reported well-controlled population, there were also more deaths caused by anticoagulant-related hemorrhage than by thromboembolism." Anticoagulant-related hemorrhage was fatal in 61% of the cases reported here. Therefore, adequate anticoagulation still lies in the delicate balance between thrombosis and bleeding. Other complications. Surprisingly, prosthetic valve endocarditis was extremely rare in this series (0.7%/ pt-yr), especially if one considers the high pervalence of preoperative infective endocarditis. Again, only cases entering the classification of valve failure, that is, resulting in the death of the patients or necessitating reoperation, were considered. Only a few suspected cases were cured medically and the diagnosis of prosthetic valve endocarditis was never confirmed. The low

The Journal of Thoracic and Cardiovascular Surgery

incidence may be related to the characteristics of the sewing ring, although Rossiter and colleagues" have indicated that the risk of prosthetic valve endocarditis is independent of the type of prosthesis implanted. The mortality index was 44%, lower than those reported by Kinsley, Antunes, and Colsen" (100%) and Miller and associates," (53%) and may be due to a younger, more resilient population and to improved methods of diagnosis and stricter indication for reoperation." Bland periprosthetic leaks were suspected in 54 of our patients but only 20 were significant enough to necessitate reoperation (0.7%/pt-yr). The incidence of periprosthetic leak did not appear to be related to the technique of implantation of the prosthesis, because it was identical in the MVR group (continuous suture technique) and in the AVR group (interrupted sutures). Some periprosthetic leaks may, however, have been the result of a cured infective process not identifiable during reoperation. Hemodynamic dysfunction, as opposed to structural failure, occurred in five patients (0.2%/pt-yr) and resulted in the death of two. There was, however, one case of true structural failure in a patient who had severe mitral regurgitation 5 months after MVR. An immobilized fractured disc was found at reoperation. Mishandling during implantation was not suspected and the thickness of the coating of pyrolytic carbon was found to be at the lower limits of acceptability by the Medtronic Inc. laboratories. To the best of our knowledge, this is the first reported case of structural failure of the Medtronic Hall prosthesis. After more than 40,000 implants world-wide, * this constitutes an enviable record that is only paralleled by the reported structural integrity of the Starr-Edwards models 6120 and 1260. Hence a total of 178 valve-related complications occurred in 165 patients for a linearized incidence of 6.7%/pt-yr. After 5 years, 75% of all patients were free from all valve-related complications. The high incidence, identical for all three groups of patients, highlights the assertion that valve replacement substitutes one disease for another." This is further emphasized by the fact that valve-related complications were fatal in 48% of the cases. Similar experiences were reported by Miller and associates. 10. 24 In their series of MVR with the StarrEdwards prosthesis model 6120, only 54% of the patients were free from valve-related mortality and morbidity after 5 years. The figure for the aortic prosthesis model 1260 was 66%. Reoperation. A total of 52 reoperations were performed in 48 patients (1.9/pt-yr). The slightly higher rate of reoperation in the AVR and DVS groups than in 'Information supplied by Medtronic Inc., Minneapolis, Minn.

Volume 95 Number 6 June 1988

the MVR group was related to the greater incidence of some of the valve-related complications discussed previously in this paper. The mortality rate for reoperations was 23%, higher than those we have recently reported in a large series including all types of prostheses. 19. 42 This high rate reflects the increased mortality in cases of prosthetic valve endocarditis and thrombotic observation, an experience also reported by the Stanford group.10 Our mortality rate for periprosthetic leak (20%) was also inordinately high and may reflect the presence of undiagnosed prosthetic valve endocarditis. Valve failure. As defined previously in this paper, valve failure occurred in 115 patients (4.3%/pt-yr) and slightly more often in AVR than in MVR. However, this rate is considerably lower than the rate we have reported for porcine bioprostheses implanted in the same population group (11.1%/pt-yr). Five years after operation, 85% of all patients were free from valve failure. The causes of valve failure were discussed in the context of each individual complication. In this population, valve failure was fatal in 69% of the cases (2.9%/pt-yr). Although both the linearized rates and the actuarial estimates may have been underestimated because of the high rate of loss to follow-up and the number of sudden and cause-unknown deaths, as discussed previously, the inclusion of a proportion of these deaths tended to minimize the influence of these factors. A valve-related mortality of 2.9%/pt-yr is still much too high a price to pay. It compares unfavorably with a l.3%/pt-yr incidence for our St. Jude Medical series," but is only slightly higher than the 2.5%/pt-yr reported by Miller and associates" in their StarrEdwards 6120 mitral valve series. Conclusion Valve performance is determined by a magnitude of factors of which the prosthesis is but one. The characteristics of the patient population play an important role. Other factors include the surgeon, the period of implantation, the cardiologist, and the reviewer. In our circumstances, an extremely unfavorable population background has had a negative influence on the rates of thrombotic obstruction and systemic thromboembolism. These two complications remain the main hazard of this valve, as well as of all other mechanical prostheses. Also, the high prevalence of preoperative infective endocarditis almost certainly added to the incidence of prosthetic valve endocarditis, a lethal complication of valve replacement. In the differing incidence of these three complications lie the major differences between our series and those of other authors quoted in this paper. The supreme challenge for any heart valve prosthesis is its successful application in a third-world population

Medtronic Hall valve replacement

99 I

group in which minor qualitative differences are likely to be highlighted. The Medtronic Hall prosthesis has excellent hemodynamic characteristics and a remarkably low incidence of structural failure. When implanted in this population, it proved to be a reliable valve substitute and confirmed the results obtained clinically by other authors in much better controlled and more compliant population groups. Although it compared most favorably with other substitute valves used in this population group, its performance cannot be interpreted as optimal. Clearly, the ideal prostheses has not yet been found. We acknowledge and extend our thanks for the contributions given to this work by the following members of this department: Mrs. Sophie Mokgoko for the follow-up of these patients, Mrs. Anne Betts for the gathering of the data, Miss Annette Hagoort and Miss Aviva Block for the typing and computerization of the manuscript, and Dr. Peter Colsen and other surgeons who operated on many of the patients included in this series. REFERENCES I. Harken DE, Taylor WJ, Lefemine AA, et al. Aortic valve replacement with caged-ball valve. Am J Cardiol 1962; 9:292-9. 2. Starr A, Edwards ML. Mitral replacement: clinical experience with a ball-valve prosthesis. Ann Surg 1961; 154:726-40. 3. Nicoloff OM, Emery RW, Arorn KY, et al. Clinical and hemodynamic results with the St. Jude Medical cardiac valve prosthesis. J THORAC CARDIOVASC SURG 1981; 82:674-83. 4. Semb BK, Nitter-Hauge S, Hall KY. Intraoperative and postoperative hemodynamic studies in patients undergoing aortic valve replacement with the Hall-Kaster cardiac disc valve prosthesis. Thorac Cardiovasc Surg 1979;27: 92-7. 5. Bjork YO, Henze AH. Ten years' experience with the Bjork-Shiley tilting disc valve. J THORAC CARDIOVASC SURG 1979;78:331-42. 6. Hall KY, Kaster RL, Woien A. An improved pivotal disc-type prosthetic heart valve. J Oslo City Hosp 1979;29:3-21. 7. Kinsley RH, Colsen PR, Antunes MJ. Medtronic-Hall valve replacement in a third world population group. Thorac Cardiovasc Surg 1983;31:69-72. 8. Kinsley RH, Girdwood RW, Milner S. Surgical treatment during the acute phase of rheumatic carditis. In: Nyhus LM, ed. Surgery annual; vol 13. New York: Appleton-Century-Crofts, 1981:299-323. 9. Antunes MJ. Thromboembolism and anticoagulation: compliant versus noncompliant population groups. In: Rabago G, ed. Heart valve replacement. Mt. Kisco, New York: Futura Publishing Co., Inc., 1987:307-17. 10. Miller DC, Oyer PE, Stinson EB, et al. Ten to fifteen year reassessment of the performance characteristics of the

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Starr-Edwards model 6120 mitral valve prosthesis. J THORAC CARDIOVASC SURG 1983:85:1-20. II. Antunes MJ, Magalhaes MP, Azevedo MG, Baptista AL, Kinsley RH. Reoperations of the mitral and aortic valves. Z Kardiol 1986;75: 163-7. 12. Kinsley RH, Antunes MJ, McKibbin JK. Enlargement of the narrow aortic root and oblique insertion of a St. Jude prosthesis. Br Heart J 1983;50:330-2. 13. Antunes MJ, Colsen PR, Kinsley RH. Intermittent aortic regurgitation following aortic valve replacement with the Hall-Kaster prosthesis. J THoRAc CARDIOVASC SURG 1982;84:751-4. 14. Grunkemeier G L, Starr A. Actuarial analysis of surgical results: rationale and method. Ann Thorac Surg 1977; 24:404-8. 15. Antunes MJ, Santos LP. Performance of glutaraldehydepreserved porcine bioprosthesis as a mitral valve substitute in a young population group. Ann Thorac Surg 1984; 37:387-92. 16. Copans H, Lakier JB, Colsen PR, Kinsley RH, Fritz VU, Barlow JB. Thrombosed Bjork-Shiley mitral prostheses. Circulation 1980;61: 169-74. 17. Antunes MJ. Bioprosthetic valve replacement in children: long-term follow-up of 135 isolated mitral valve implantations. Eur Heart J 1984;5:913-8. 18. Kutsche LM. Oyer PE, Shumway N, Baum D. An important complication of Hancock mitral valve replacement in children. Circulation 1979;60(Pt 1):98-103. 19. Silver MM, Pollok J, Silver MD, Williams WG, Trusler GA. Calcification in porcine xenograft valves in children. Am J Cardiol 1980;45:685-9. 20. Antunes MJ, Magalhaes MP. Isolated rereplacement of a prosthesis or bioprosthesis in the mitral valve position. Am J Cardiol 1987;59:346-9. 21. Semb BK, Hall KV. The Hall-Kaster cardiac disc valve prosthesis: experimental and clinical results. Vase Surg 1981;15:193-217. 22. Moggio RD, Hammond GL, Stansel HC Jr, Glenn WWL. Incidence of emboli with cloth-covered StarrEdwards valve without anticoagulation and varying forms of anticoagulation. J THoRAc CARDIOVASC SURG 1978; 75:296-9. 23. Bodnar E, Wain WH, Haberman S. Assessment and comparison of the performance of cardiac valves. Ann Thorac Surg 1982;34: 146-56. 24. Miller DC, Oyer PE, Mitchell RC, et al. Performance characteristics of the Starr-Edwards Model 1260 aortic valve prosthesis beyond ten years. J THoRAc CARDIOVASC SURG 1984;88: 193-207. 25. Oyer PE, Stinson EB, Reitz BA, Miller DC, Rossiter SJ, Shumway NE. Long-term evaluation of the porcine xenograft bioprosthesis. J THoRAc CARDIOVASC SURG 1979;78:343-50. 26. Jamieson WRE, Janusz MT, Miyagishima RT, et al. Embolic complications of porcine heterograft cardiac valves. J THoRAc CARDIOVASC SURG 1981;81:626-31.

Thoracic and Cardiovascular Surgery

27. Jamieson WRE, Pelletier LC, Janusz MT, Chaitman BR, Tyers GFO, Miyagishima RT. Five-year evaluation of the Carpentier-Edwards porcine bioprosthesis. J THoRAe CARDIOVASC SLRG 1984;88:324-33. 28. Teply JF, Grunkemeier GL, Sutherland HD, Lambert LE, Johnson VA, Starr A. The ultimate prognosis after valve replacement: an assessment at twenty years. Ann Thorac Surg 1981;32:111-9. 29. Horstkotte D, Korfer R, Seipel L, Birks W, Loogen F. Late complications in patients with Bjork-Shiley and St. Jude Medical heart valve replacement. Circulation 1983;68:175-84. 30. Rabago G, Martinell J, Fraile J, Andrade IG, Montenegro R. Results and complications with the Omniscience prosthesis. J THoRAc CARDIOVASC SURG 1984;87:13640. 31. Nitter-Hauge S, Semb B, Abdelnoor M, Hall KY. A 5-year experience with the Medtronic- Hall disc valve prosthesis. Circulation 1983;68(Pt 2):11169-74. 32. Hall KV, Nitter-Hauge S, Abelnoor M. Seven and one-half years' experience with the Mcdtronic-Hall valve. J Am Coil Cardiol 1985;6:1417-21. 33. Kinsley RH, Antunes MJ, Colsen PRo St. Jude Medical valve replacement: an evaluation of valve performance. J THoRAc CARDIOVASC SURG 1986;92:349-60. 34. Barnhorst DA, Oxman HA, Connolly DC, et al. Longterm follow-up of isolated replacement of the aortic and mitral valve with the Starr-Edwards prosthesis. Am J Cardiol 1975;35:228-33. 35. Karp RB, Cyrus RJ, Blackstone EH, Kirklin JW, Kouchoukos NT, Pacifico AD. The Bjork-Shiley valve. Intermediate-term follow-up. J THoRAc CARDIOVASC SLRG 1981;81:602-14. 36. Klein HO, Schamroth CL, Marcus BD, Hummel D, Antunes M, Sareli P. Echophonocardiographic assessment of the Medtronic-Hall mitral valve prosthesis: observation on normal and abnormal function. J Cardiavase Ultrasonography 1986;5:115-27. 37. Wright JO, Hiratzka LF, Brandt B, Doty DB. Thrombosis of the Bjork-Shiley prosthesis: illustrative cases and review of the literature. J THoRAc CARDIOVASC SLRG 1982;84: 138-44. 38. Metzdorff MT, Grunkemeier GS. Pinson W, Starr A. Thrombosis of mechanical cardiac valves: a qualitative comparison of the Silastic ball valve and the tilting disc valve. J Am Coil Cardiol 1984;4:50-3. 39. Taguchi K. Pregnancy in patients with a prosthetic heart valve. Surg Gynecol Obstet 1977;145:206-8. 40. Antunes MJ, Myer IG, Santos LP. Thrombosis of mitral valve prosthesis in pregnancy: management by simultaneous caesarian section and mitral valve replacement. Br J Obstet Gynaecol 1984;91:716-8. 41. Berk MR, Epstein M, Driscoll J, et al. Anticoagulation during pregnancy in patients with mechanical valve prostheses: maternal and fetal sequelae [Abstract]. Circulation 1986;74(Pt 2):11452.

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42. Antunes MJ. Isolated replacement of a prosthesis or bioprosthesis in the aortic valve position. Am J Cardiol 1987;59:351-3. 43. Venugopal P, Kaul L, Iyer KS, et al. Fate of thrombectomized Bjork-Shiley valves: a long-term cinefluoroscopic, echocardiographic, and hemodynamic evaluation. J THORAC CARDIOVASC SURG 1986;91:168-73. 44. Macmanus Q, Grunkemeier GL, Lambert LE, Teply JF, Harlan BJ, Starr A. Year of operation as a risk factor in the late results of valve replacement. J THORAC CARDIOv A<;{' SURG 1980;80:834-41.

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45. Rossiter SJ, Stinson EB, Oyer PE, et al. Prosthetic valve endocarditis: comparison of heterograft tissue valves and mechanical valves. J THORAC CARDIOVASC SURG 1978; 76:795-803. 46. Lewis BS, Agathangelou NE, Colsen PR, Antunes MJ, Kinsley RH. Cardiac operation during active infective endocarditis. J THORAC CARDIOVASC SURG 1982;84:57984. 47. Kinsley RH. Valve replacement. Ann Life Ins Med 1980;6:185-201.