Clinical and Hemodynamic Evaluation of the 19 mm Björk-Shiley Aortic Valve Prosthesis

Clinical and Hemodynamic Evaluation of the 19 mm Bjork-Shiley Aortic Valve Prosthesis Hartzell V. Schaff, M.D., A. Michael Borkon, M.D., Clifford Hughes, M.D., Stephen Achuff, M.D., James S. Donahoo, M.D., Timothy J. Gardner, M.D., Levi Watkins, [r., M.D., Vincent 1. Gott, M.D., Andrew G. Morrow, M.D., and Robert K. Brawley, M.D. ABSTRACT Between November, 1973, and March, 1980,43 patients underwent isolated aortic valve replacement with 19 mm Bjork-Shiley prostheses at the Johns Hopkins Hospital. There were 4 male and 39 female patients ranging from 12 to 75 years old (mean, 54.5 years). Average weight was 62 ± 2 kg and average body surface area, 1.64 ± 0.3 m". Five patients died within thirty days of operation; however, since 1975, hospital mortality has been 5.9%. The 38 survivors have been followed up for as long as 85 months (mean, 40 months). There were 4 late deaths, and actuarial survival in patients discharged from the hospital was 81 % at five years. All longterm survivors were in New York Heart Association Functional Class I (29 patients) or Class II (5 patients). Preoperative and postoperative echocardiograms in 17 patients demonstrated significant decreases in mean left ventricular wall thickness (12.9 ± 1.8 mm vs 10.3 ± 1.4 mm; p < 0.001) and in left ventricular mass (262 ± 95 gm vs 188 ± 50 gm; p < 0.02). Postoperative cardiac catheterization data were obtained from an additional 24 patients undergoing aortic valve replacement with the 19 mm BjorkShiley prosthesis at the National Heart Institute. Average peak systolic gradient at rest was 16 mm Hg (range, 0 to 45 mm Hg) and was found to be directly related to body surface area (r = 0.60, P < 0.002). Average effective valve orifice area was 1.06 cm 2 (range, 0.63 to 2.02 ems). For patients with small aortic roots, aortic valve From the Departments of Surgery and Medicine, the Johns Hopkins University School of Medicine, Baltimore, and the Clinic of Surgery, National Heart Institute, Bethesda, MD. Most of the patients from the National Heart Institute were operated on by Drs. Charles L. Mcintosh and Michael Jones. The postoperative hemodynamic studies in these patients were carried out by Drs. Kenneth M. Kent, Douglas R. Rosing, and Lewis C. Lipson. Presented at the Twenty-seventh Annual Meeting of the Southern Thoracic Surgical Association, Nov 13-15, 1980, White Sulphur Springs, WV. Address reprint requests to Dr. Brawley, The Johns Hopkins Hospital, 600 N Wolfe St, Baltimore, MD 21205.

50

0003-4975/81/070050-08$01.25

replacement with the 19 mm Bjork-Shiley valve is a satisfactory and, perhaps, preferable alternative to aortic annuloplasty to accommodate larger sized prostheses.

Small annulus size complicates aortic valve replacement in 3 to 15% of patients [1-3]. Both caged-ball prostheses (Starr-Edwards) and porcine heterograft valves (Hancock) have reduced effective orifices in the smaller sizes (sewing ring :s; 22 mm) and are not recommended for patients with narrow aortic roots [1-9]. To accommodate larger sized valves, enlargement of the aortic annulus has been advocated [2, 3, 5, 6, 10-12]. Another approach is selection of a central-flow tilting-disc valve such as the Bjork-Shiley prosthesis. The 21 mm BjorkShiley valve has an orifice diameter of 16 mm and a theoretical orifice area of 2.01 em", Studies by Bjork and colleagues [1] demonstrated that this prosthesis is satisfactory both in regard to postoperative patient survival and reduction of left ventricular outflow obstruction. Clinical experience with the 19 mm Bjork-Shiley valve has not been reported. This prosthesis has a theoretical orifice area of 1.50 em- and, therefore, should provide satisfactory relief of left ventricular outflow obstruction in smaller patients [13]. The present study reviews the clinical and echocardiographic data in 43 consecutive patients undergoing isolated aortic valve replacement using the 19 mm BjorkShiley valve at the Johns Hopkins Hospital. In addition, hemodynamic data for 24 patients having aortic valve replacement with the 19 mm Bjork-Shiley prosthesis at the National Heart Institute are presented. Material and Methods From November, 1973, through February, 1980, 43 patients underwent isolated aortic valve replacement using the 19 mm Bjork-Shiley pros-

© 1981 by The Society of Thoracic Surgeons

51 Schaff et al: Evaluation of Bjork-Shiley Prosthesis

thesis at the Johns Hopkins Hospital. In 10 patients concomitant procedures were performed: aortic valve replacement and coronary revascularization, 8 patients; replacement and excision of a subvalvular ring and myectomy, 1patient; and replacement and closure of a fistula between the aorta and right ventricle, 1 patient. Patients having multiple valve replacement were excluded. Seven prior operations had been performed in 5 patients (12%).The 4 male and 39 female patients ranged in age from 12 to 75 years (mean, 54.5 years). In general, patients were small with body surface areas ranging from 1.13 to 2.22 m2 (mean, 1.64 mz). Pure aortic stenosis was present in 35 patients, and predominant aortic regurgitation was demonstrated in 5; 3 patients had mixed aortic stenosis and regurgitation. The major symptoms were congestive heart failure (33 patients), angina pectoris (22), and syncope (5). Twenty-seven patients (63%) were in New York Heart Association (NYHA) Functional Class I11 (23 patients) or IV (4). There was 1 patient in Class I and 15 were in Class 11. The 1 patient who was asymptomatic had increasing cardiomegaly secondary to traumatic aortic insufficiency and a fistula between the aorta and right ventricle. All patients were operated on using ischemic arrest during hypothermic systemic perfusion. Systemic hypothermia (28" to 30°C) plus profound topical cooling (4°C Ringer's lactate solution) was used for myocardial protection prior to 1976. Since 1976, multidose hypothermic potassium cardioplegia administered by direct coronary perfusion has been our preferred method for myocardial preservation during aortic valve replacement. Surgical techniques varied little during the years of this study. Since 1974, the aortic prosthesis has been secured with individual 0 Tevdek mattress sutures reinforced with Teflon pledgets. Postoperatively, all patients were anticoagulated with Coumadin (sodium warfarin) unless bleeding complications developed. Hospital survivors and their physicians were interviewed by telephone. Twenty-seven patients returned for clinical evaluation, routine chest roentgenogram, and 12-lead electrocardiogram. M-mode echocardiographic examinations were performed in patients returning

for follow-up using a Smith-Kline Ekoline ultrasonoscope with a 2.25 MHz internally focused transducer. End-diastolic measurements were taken at the onset of the QRS complex; end-systolic measurements were made at the smallest ventricular diameter. Left ventricular end-diastolic and end-systolic transverse diameters were quantitated as well as posterior wall and interventricular septa1 thicknesses. Ejection fraction and estimated left ventricular mass were calculated from the primary measurements [14, 151. Twenty-four additional patients who had had aortic valve replacement with a 19 mm BjorkShiley valve at the National Heart Institute were studied in the basal state by cardiac catheterization an average of 7 months following operation. There were 6 men and 18 women 28 to 67 years old (mean, 51 years). The average body surface area was 1.62 m2 (range, 1.29 to 2.07 m2). Sixteen patients underwent isolated aortic valve replacement, and 8 received aortic and mitral valves. Nineteen patients were asymptomatic, and 5 were in NYI-IA Class I1 at the time of postoperative catheterization. Right heart catheterization was performed using the Seldinger technique. Left ventricular pressure was obtained either by the transseptal approach or by direct left ventricular puncture. Simultaneous brachial artery pressure was recorded. Effective valve orifice area was computed according to the Gorlin formula [16], when the peak systolic gradient was greater than 15 mm Hg-

Results Clinical Follow-up Five of the 43 patients died within thirty days of operation, a hospital mortality of 11.6%. Since January, 1975, however, there have been 2 hospital deaths among 34 patients (5.9%). During this latter period there was 1 hospital death among 26 patients who had aortic valve replacement only, a mortality of 3.8%. This figure is comparable to our overall mortality for patients undergoing aortic valve replacement with larger diameter prostheses. At the termination of the follow-up period, March, 1980, the status of 37 hospital survivors was known for intervals from 1 to 85 months

52 The Annals of Thoracic Surgery Vol 32 No 1 July 1981

100

-

.

Preop

Postop

a, 90

.-> m

5 80 Y

a,

a 70 60 0

1

3

2

4

5

Years Fig 1. Actuarial survival following aortic valve replacement with the 19 mm Bjork-Shiley prosthesis. Operative deaths are excluded, and vertical bars represent the 70% confidence limits.

postoperatively (mean, 40 months). One patient was lost to follow-up but had been asymptomatic 42 months postoperatively. Four late deaths occurred, 3 of which were clearly due to noncardiac causes. One patient died 54 months postoperatively of jaundice and hepatic failure. Another patient, 45 months following aortic valve replacement and double coronary artery bypass, died of sepsis secondary to Salmonella gastroenteritis complicated by intestinal perforation and peritonitis. Pancreatic carcinoma was responsible for the death of a third patient 30 months after aortic valve replacement. A fourth patient suffered a cerebrovascular accident two years postoperatively. Although a postmortem examination was not performed, this death was considered valve related. Evaluation was complete for all other survivors. Actuarial analysis of postoperative survival is seen in Figure 1. Projected three-year survival for patients with a 19 mm Bjork-Shiley aortic prosthesis is 93'/0, and five-year survival is 81%. Including operative deaths, the overall estimated survival at five years is 71%. All patients with symptoms preoperatively were very much improved following operation and had increased their functional capacity by an average of 1.4 Classes (Fig 2). The single patient who was in NYHA Class I preoperatively remained asymptomatic after valve replacement. Eighty-five percent of survivors were in Class I and the remaining 5, Class 11. Information regarding thromboembolic events or complications of anticoagulation was

Fig 2 . N e w York Heart Association functional status of 34 patients before and after aortic valve replacement with the 19 mm Bjork-Shiley prosthesis.

Table 1. Late Complications in Patients Receiving a 19 mm Bjork-Shiley Aortic Valve Patients Complication

Rate

(percent/ Percent- patientNumber age year)

Thromboembolism Death 1 Transient episode 1

2.6 2.6

2.1 2.1

13.1 13.1

10.4 10.4

Hemorrhagic complications Major

Minor

5 5

obtained from hospital records, patient interviews, and referring physicians' records (Table 1). One patient died of a major stroke, presumed to be valve related. Transient visual symptoms developed in another patient, and emboli could not be excluded. Bleeding complications were considered major if hospitalization was required for treatment; this was true for 5 patients. Upper gastrointestinal bleeding necessitated transfusion in 2 patients, and rectal bleeding developed in another patient. Hospital evaluation did not identify specific etiologies, and in all patients bleeding stopped after readjustment of Coumadin dosage. Another patient underwent hysterectomy due to menometrorrhagia. Coumadin was discontinued due to retroperitoneal hemorrhage in a 62-year-old woman who subsequently was found to have pancreatic carcinoma. Minor bleeding difficulties developed in 5 patients and included transient hematuria, menorrhagia, and gingival bleeding.

53 Schaff et al: Evaluation of Bjork-Shiley Prosthesis

60

LV enddiastolic dimension (mm)

100

50

LV ejection fraction

80

(%)

60

40

40

30

I 20

P=NS

Preop

20

Postop

.* Preop

Postop

cardiomegaly. As seen in Figure 3 , the mean left ventricular end-diastolic dimension prior to operation was 44 k 2 mm (mean f standard deviation). Postoperatively, this dimension in these patients was virtually unchanged (45 f 2 mm). Preoperative left ventricular ejection fraction by echocardiogram was 74 f 4%, indicatEchocardiographic Data Preoperative and postoperative echocardio- ing that patients had preserved ventricular grams of sufficient quality for measurement of function. In postoperative studies, 3 patients chamber dimensions were available in 17 pa- demonstrated paradoxical septal motion. tients. Follow-up studies were performed an Paradoxical septal movement by echocardiogaverage of 46 months postoperatively (range, 19 raphy is common after open-heart operation to 85 months), and data were interpreted with- and may not necessarily reflect muscle injury out knowledge of the patient’s clinical status. [17]. It does, however, result in a marked rePreoperatively, the majority of patients had duction in calculated ejection fraction. Even concentric left ventricular hypertrophy without though data from 3 patients with paradoxical septal motion are included, postoperative ejection fraction was 70 f 6Y0, well within the Fig 4 . Preoperative and postoperative left ventricular (LV) wall thickness and mass by echocardiography. normal range and not significantly different Significant reductions in LV wall thickness and mass in- from the preoperative value. Values for left dicate that ventricular hypertrophy has regressed in ventricular thickness and mass are shown in patients after aortic valve replacement with the 19 mm Bjork-Shiley prosthesis. Figure 4. Mean left ventricular wall thickness, Fig 3 . Preoperative and postoperative left ventricular (LV) end-diastolic dimension and ejection fraction by echocardiography. After aortic valve replacement, both LV end-diastolic dimension and ejection fraction were within the normal range, which suggests that ventricular function is well preserved.

16

500

14

400

Mean LV wall 12 thickness (mm)

LV

. . I

8

t

mass

300

(9) 200

100

P < 001

6

Preop

Postop

0

Preop

Postop

54 The Annals of Thoracic Surgery Vol 32 No 1 July 1981

Table 2 . Hemodynamic Results in 24 Patients Receiving a 19 m m Bjork-Shiley Valve Hemodynamic Data Left atrial pressure (mm Hg) Left ventricular end-diastolic pressure (mm Hg) Peak left ventricularaortic gradient (mm Hg) Cardiac output (Llmin) Effective valve orifice area (cmz) Effective valve orifice index (cm*/m)

Mean (Range) 15 (2-39) 15 (5-30)

+ Z

wo a

50-

40-

U

0 0

?>

301

20{

16 (0-45)

4.6 (2.2-12.5) 1.06 (0.63-2.02) 0.62 (0.40-0.98)

I

'

1.10

'

L30

'

l(50 '

I:70

'

1.'90 '

2'.10

BODY S U R F A C E A R E A

Fig 5 . Dependency of peak left ventricular-aortic (LV-Ao) gradient on preoperative body surface area.

obtained by dividing the sum of the left ventricular posterior wall and the interventricular septum by 2, was 12.9 +_ 1.8 mm preoperatively and 10.3 k 1.4 mm postoperatively. This 20% reduction in left ventricular wall thickness following aortic valve replacement was statistically significant ( p < 0.001). Reduction in left ventricular wall thickness with maintenance of a normal end-diastolic diameter suggests that left ventricular hypertrophy regressed after operation. Left ventricular mass was 262 k 95 gm preoperatively and decreased to 188 f 50 gm postoperatively ( p < 0.02).

Hemodynamic Data The postoperative hemodynamic findings in 24 patients having aortic valve replacement with a 19 mm Bjork-Shiley aortic valve prosthesis at the National Heart Institute are shown in Table 2. Peak systolic gradient was found to be directly related to body surface area (Y = 0.60, p < 0.002) (Fig 5). Only 1of 18 patients with body surface areas less than 1.70 m2 had a postoperative peak systolic gradient greater than 25 mm Hg at rest. On the other hand, 4 of 6 patients with body surface areas greater than 1.70 m2 had postoperative gradients greater than 25 mm Hg. Comment Patients with small aortic roots undergoing aortic valve replacement present a special challenge to the surgeon both in regard to operative technique and selection of prosthesis. These

patients are predominantly older women with small body-surface areas and severe calcific aortic stenosis. Experience with the small Starr-Edwards valves (7-A and 8-A) in patients with narrow aortic roots has not been favorable due to high postoperative systolic gradients and increased perioperative mortality [l, 3, 7, 181. Najafi and co-workers [7] suggested that the narrowed aortic segment between the valve annulus and commissural attachments was responsible for outflow gradients following valve replacement with smaller Starr-Edwards prostheses, and recommended a patch angioplasty of the ascending aorta. In most instances, however, the primary and secondary orifices of the ball valve are responsible for the obstruction. Similarly, high transvalvular gradients have been demonstrated with the smaller Hancock heterograft aortic prostheses (19 mm and 21 mm) [4, 5, 191. Modification of the Hancock valve by replacement of the septa1 leaflet and muscular shelf with a larger noncoronary leaflet has improved hemodynamic performance in the 21 mm size, but gradients vary widely [191. To avoid residual left ventricular outflow obstruction, the annulus can be enlarged to accommodate a larger prosthesis, and several techniques have been described. An oblique aortotomy can be extended into the noncoronary sinus. Blank and colleagues [3] advocated continuing the incision 5 to 10 mm across the aortoventricular membrane and using a Dacron patch to fill the gap created by the annular enlargement. With this method, they were able to

55

Schaff et al: Evaluation of Bjork-Shiley Prosthesis

insert 21 and 23 mm porcine heterografts in patients with small aortic roots, and postoperative resting gradients ranged from 3 to 24 mm Hg. Manouguian and Seybold-Epting [6] enlarged the annulus by incision and patch repair in the commissure between the left and noncoronary cusps onto the anterior mitral leaflet. Another technique for enlarging the aortic annulus is the aortoventriculoplasty described by Konno [lo], Rastan [12], and their associates. With this technique, an incision between the right and left coronary arteries is extended across the septum and right ventricular free wall. Although aortoventriculoplasty can be used to enlarge the aortic annulus by several valve sizes, there are three resultant defects that must be patched-the free wall of the right ventricle, the ventricular septum, and the aorta. Each of these methods, however, may increase the risk of aortic valve replacement. Injury to the coronary arteries, the mitral valve, and the atrioventricular conduction bundle have been reported [6, 12, 201. Added incisions in the aorta (and ventricle) along with the requirement for patch enlargement can increase the risk of postoperative bleeding. Left ventricular obstruction associated with a narrow aortic annulus may also be treated by creating a double-outlet left ventricle with a valved conduit [21]. The present study demonstrates that the 19 mm Bjork-Shiley valve is a satisfactory prosthesis for most patients with small aortic roots. Hospital mortality and late mortality in this series are comparable to those following aortic valve replacement with larger sized BjorkShiley valves, porcine heterografts, and cagedball valves [4, 13, 17, 18, 22-25]. In the present series, actuarial survival following operation was 81% at five years (71% including hospital deaths). Three of the late deaths were due to noncardiac causes, and 1 patient died of a cerebrovascular accident. All long-term survivors experienced sustained improvement in functional capacity, again comparable to results with larger prostheses. Eighty-five percent of the patients were asymptomatic following operation. No patient experienced prosthetic valve dysfunction or progressive congestive heart failure. Echocardiographic data confirmed that ventricular function was well maintained

and that left ventricular hypertrophy regressed markedly. Estimated left ventricular mass had decreased 20% at an average of 46 months following operation. Similar reductions in left ventricular hypertrophy following aortic valve replacement for aortic stenosis with a variety of aortic valve prostheses have been documented by others 124-261. Finally, results of cardiac catheterization data obtained postoperatively in patients having aortic valve replacement at the National Heart Institute show the 19 mm Bjork-Shiley valve to be acceptable hemodynamically. The average resting peak systolic gradient was 16 mm Hg in these patients and was less than 25 mm Hg in all patients but 1with body surface areas of 1.7 m2 or less. Postoperative survival and functional improvement in patients with the 19 mm Bjork-Shiley prosthesis demonstrate that severe and progressive left ventricular outflow obstruction has been relieved and that residual gradients are well tolerated. The risk of thromboembolism and the requirement for long-term anticoagulation continue to be serious considerations in the selection of any mechanical prosthesis for aortic valve replacement [27-291. Thromboembolic and bleeding complications were not excessive and prosthetic thrombosis, reported by others [27], did not occur in our patients with 19 mm Bjork-Shiley valves. To avoid the risk of thromboembolism and anticoagulation, some authors recommended annular enlargement specifically to accommodate porcine heterograft valves [3, 111. It should be noted, however, that Lakier and associates [30] reported a 9.5% cumulative incidence of thromboembolic events following mitral or aortic valve replacement with porcine heterografts. Furthermore, although porcine valve failure in the initial 48 postoperative months was infrequent, 10 of 42 patients (23.8%) with isolated aortic valve replacement required reoperation during the longer followup period of 60 to 89 months [191. This substantial incidence of valve failure must be considered when selecting a valve substitute and, in our opinion, argues against the routine use of porcine aortic valves, particularly if annular enlargement is required. In summary, the present study demonstrates

56 The Annals of Thoracic Surgery Vol 32 No 1 July 1981

that the 19 mm Bjork-Shiley valve is a satisfactory prosthesis for most patients with small aortic roots a s evidenced by (1) actuarial survival comparable to that for similar patients undergoing valve replacement with larger sized prostheses, (2) excellent postoperative functional improvement, (3) significant reductions in left ventricular hypertrophy w i t h preservation of left ventricular function, a n d (4) satisfactory hernodynamic performance provided body surface area is less t h a n 1.7 mz. Enlargement of t h e aortic a n n u l u s is considered necessary only for those patients with a n aortic root that will not accept the 19 mm Bjork-Shiley prosthesis and for patients in whom there is a specific contraindication to anticoagulation that requires use of a larger diameter bioprosthesis.

References 1. Bjork VO, Henze A, Holmgren A, Szamosi A: Evaluation of the 21 mm Bjork-Shiley tilting-disc valve in patients with narrow aortic roots. Scand J Thorac Cardiovasc Surg 7:203, 1973 2. Bjornstad PG, Rastan H, Keutel J, et al: Aortoventriculoplasty for tunnel subaortic stenosis and other obstructions of the left ventricular outflow tract. Circulation 60:59, 1979 3. Blank RH, Pupello DF, Bessone LN, et al: Method of managing the small aortic annulus during valve replacement. Ann Thorac Surg 22:356,1976 4. Cohn LH, Sanders JH Jr, Collins JJ Jr: Aortic valve replacement with the Hancock porcine xenograft. Ann Thorac Surg 22:221, 1976 5. Jones EL, Craver JM, Morris DC, et al: Hemodynamic and clinical evaluation of the Hancock xenograft bioprosthesis for aortic valve replacement (with emphasis on management of the small aortic root). J Thorac Cardiovasc Surg 75:300, 1977 6. Manouguian S, Seybold-Epting W: Patch enlargement of the aortic valve ring by extending the aortic incision into the anterior mitral leaflet. J Thorac Cardiovasc Surg 78:402, 1979 7. Najafi H, Ostermiller WE, Javid H, et al: Narrow aortic root complicating aortic valve replacement. Arch Surg 99:690, 1969 8. Rahimtoola SH: The problem of valve prosthesis-patient mismatch. Circulation 58:20, 1978 9. Roberts WC, Hammer WJ: Cardiac pathology after valve replacement with a tilting disc prosthesis (Bjork-Shiley type): a study of 46 necroscopy patients and 49 Bjork-Shiley prostheses. Am J Cardiol 37:1024, 1976 10. Konno S, Imai Y, Nakajima M, Tetsuno K: A new

method for prosthetic valve replacement in congenital aortic stenosis associated with hypoplasia of the aortic valve ring. J Thorac Cardiovasc Surg 70:920, 1976 11. Pupello DF, Blank RH, Bessone LN, et al: Surgical management of the small aortic annulus. Chest 74:163, 1978 12. Rastan H, Abu-Aishah N, Rastan D, et al: Results of aortoventriculoplasty in 21 consecutive patients with left ventricular outflow tract obstruction. J Thorac Cardiovasc Surg 75:659, 1978 13. Levang OW: Aortic valve replacement: a randomized study comparing the Bjork-Shiley and Lillehei-Kaster disc valves. Scand J Thorac Cardiovasc Surg 12:197, 1978 14. Devereux RB, Reichek N: Echocardiographic determination of left ventricular mass in man: anatomic validation of the method. Circulation 55:613, 1977 15. Fortuin NJ, Hood WP, Craig E: Evaluation of left ventricular function by echocardiography. Circulation 46:26, 1972 16. Gorlin R, Gorlin SG: Hydraulic formula for calculation of the area of the stenotic mitral valve, other cardiac valves and central circulatory shunts. Am Heart J 51:1, 1951 17. Henry WL, Bonow RO, Borer JS, et al: Observation of the optimum time for operative intervention for aortic regurgitation: evaluation of the results of valve replacement in symptomatic patients. Circulation 61:471, 1980 18. Bjork VO, Henze A, Holmgren A: Five years’ experience with the Bjork-Shiley tilting-disc valve in isolated aortic valvular disease. J Thorac Cardiovasc Surg 68:393, 1974 19. Craver JM, King SB, Douglas JS, et al: Late hemodynamic evaluation of Hancock modified orifice aortic bioprosthesis. Circulation 6O:Suppl 1:1-93, 1979 20. Baumgartner WA, Reitz BA, Oyer PE, et al: Cardiac homotransplantation. Curr Probl Surg 16:40, 1979 21. Norman JC, Cooley DA, Hallman GL, Nihill MR: Left ventricular apical-abdominal conduits for left ventricular outflow obstructions. Circulation 56:S~ppl2:II-62, 1977 22. Barnhorst DA, Oxman HA, Connolly DC, et al: Long-term follow-up of isolated replacement of the aortic or mitral valve with the Starr-Edwards prosthesis. Am J Cardiol 35:228, 1975 23. Bjork VO, Henze A: Ten years’ experience with the Bjork-Shiley tilting-disc valve. J Thorac Cardiovasc Surg 78:331, 1979 24. Henry WL, Bonow RO, Borer JS, et al: Evaluation of aortic valve replacement in patients with valvular aortic stenosis. Circulation 61:814, 1980 25. Pantely G, Morton M, Rahimtoola SH: Effects of successful, uncomplicated valve replacement on

57 Schaff et al: Evaluation of Bjork-Shiley Prosthesis

ventricular hypertrophy, volume, and performance in aortic stenosis and in aortic incompetence. J Thorac Cardiovasc Surg 75:383, 1978 26. Rothkopf M, Davidson T, Lipscomb K, et al: Hemodynamic evaluation of the CarpentierEdwards bioprosthesis in the aortic position. Am J Cardiol44:20, 1979 27. Byrd CL, Yahr WZ, Greenberg JJ: Long-term results of "simple" thrombectomy for thrombosed Bjork-Shiley aortic valve prostheses. Ann Thorac Surg 20:265, 1975 28. Moggio RA, Hammond GL, Stansel HC Jr, Glenn

WWL: Incidence of emboli with cloth-covered Starr-Edwards valve without anticoagulation and with varying forms of anticoagulation: analysis of 183 patients followed for 3% years. J Thorac Cardiovasc Surg 75:296, 1978 29. Moreno-Cabral RJ, McNamara JJ, Mamiya RT, et al: Acute thrombotic obstruction with BjorkShiley valves: diagnostic and surgical considerations. J Thorac Cardiovasc Surg 75:321, 1978 30. Lakier JB, Khaja F, Magilligan DJ, Goldstein S: Porcine xenograft valves: long-term (60-89 months) follow-up. Circulation 62:313, 1980

Notice from the American Board of Thoracic Surgery

The voluntary recertification examination scheduled for October 4,1981, by the American Board of Thoracic Surgery has been postponed. A poor response made it impractical to administer the examination. The examination may be administered at a later date.