In 1931, Paul Dudley White stated “There is no treatment for aortic stenosis.” Even today the medical therapy of aortic stenosis has not significantly advanced.¹ Conversely, patients may tolerate aortic insufficiency for many years, but as the ventricle starts to dilate, a progressive downhill course begins and early operation is warranted.² Definitive therapy for aortic valve disease was unavailable until the advent of cardiopulmonary bypass. Innovative cardiovascular surgeons then began to develop cardiac valve prostheses. Over the subsequent 60 years,³ the variety of prostheses that have become available for use have expanded greatly. Available aortic valve substitutes include mechanical valve prostheses, stented biologic valve prostheses, stentless biologic valve prostheses, human homograft tissue (both as isolated valve replacement and aortic root replacement), and a combination of a biologic valve utilizing a pulmonary autograft and pulmonary outflow tract replacement with heterograft prostheses (Ross procedure). Most recently, innovative transarterial/apical aortic valve replacement (TAVR) has gained approval in Europe and North America with acceptable intermediate term results.⁴ Reports of the use of novel sutureless bioprosthetic valves are appearing.⁵ This chapter focuses on the use of mechanical valve replacement in the aortic position.

In 1952, Hufnagel used an aortic valve ball and cage prosthesis heterotopically in the descending thoracic aorta to treat aortic insufficiency.⁶ After the advent of cardiopulmonary bypass, initial attempts at aortic valve replacement (AVR) consisted of replacement of the individual aortic cusps with Ivalon gussets or fascialata sewn to the annulus or repair of aortic valve by bicuspidization.⁷ When successful, these prostheses often calcified and results were short-lived. Shortly thereafter, surgical pioneers Starr, Braunwald, and Harkin began replacement of the aortic valve in the orthotopic position. First-generation aortic valve prostheses, the ball and cage, became the standard for AVR for over a decade (Fig. 27-1). Many of these prostheses have remained durable for up to 40 years.^8,9 Multiple modifications ensued including changing the material of the ball from Silastic to Stellite, changes in the shape of the cage, depression of the ball occluder, the addition of cloth coating to the sewing ring and the cage, and changes in the sewing ring itself. These valves, however, required intense anticoagulation.¹⁰ Hemodynamic performance was compromised, as there were three areas of potential outflow obstruction: the annular size of the sewing ring (the effective orifice area of the valve), the distance between the cage and the walls of the ascending aorta (particularly in the small aortic root), and obstruction to outflow by the ball itself distal to the aortic annulus. Flow patterns were also abnormal (Fig. 27-2). These problems led to the development of the next generation of aortic valve prostheses: the tilting disc valve. Innovators such as Bjork, Hall, Kastor, and Lillehei developed three models of tilting disc prostheses that became the second generation of commonly implanted aortic valve replacement devices between 1968 and 1980. The low-profile configuration simplified surgical implantation (Fig.27-3). Problems with the tilting disc valve included stasis and eddy current formation at the minor flow orifice (see Fig. 27-2), and sticking or embolization of the leaflet, the latter leading to discontinuation of the Bjork prosthesis in spite of otherwise good long-term results.¹¹ The Lillehei-Kastor prosthesis evolved into the Omniscience valve, now discontinued. The Medtronic Hall valve, the third tilting disc prosthesis, is also now discontinued (Fig. 27-4A).

Kalke and Lillehei developed the first rigid bileaflet valve, but it had very limited clinical use. In 1977, the St. Jude Medical (SJM) prosthesis was developed and implanted by Nicoloff and associates (Fig. 27-4C).^3,12,13 Over the following decades, the dramatic step of a bileaflet prosthesis nearly obviated the use of all other kinds of mechanical prosthetic valves in the United States and to a large extent elsewhere. The SJM valve demonstrated low aortic gradients, minimal aortic insufficiency, and low rates of thromboembolism (TE).^12,14 Anticoagulation continued to be necessary but to a lesser extent than with previous design models.¹⁵ Because of the low-profile design and lesser need for orientation, surgical implant was further simplified. Following the introduction of the SJM valve, several other third-generation models of bileaflet prostheses were introduced, including the Sulzer CarboMedics valve (Fig. 27-4B), the ATS Medical prosthesis (Fig. 27-4D), and the On-X prosthesis (Fig. 27-4E). Since the introduction of the bileaflet valve, over 2 million implants on a global basis have been accomplished and extensive literature has developed. Surgeons have become more confident in earlier aortic valve replacement and guidelines for anticoagulation necessary for all mechanical valves have been developed for each generation of prosthesis at progressively decreasing target levels.^15,16

Over the past 25 years, design and configurational changes have been made in bileaflet prostheses. The ATS Medical valve changed the “rabbit ears” pivot style of other bileaflet prostheses, incorporating a convex or open-pivot design allowing more complete washing of the moving parts of the valve and possibly a quieter valve closing.^17,18 The sewing ring of the SJM valve has changed (SJM HP) to allow a larger valve size implantation for any given tissue annulus, as has ATS Medical with its AP design. The sewing ring of the Sulzer CarboMedics valve has been modified such that this valve is implanted in a supravalvular position (top hat model). The On-X valve incorporates advanced pyrolytic carbon technology using a purer, more flexible coating to allow flanging of the inflow portion of the valve housing, better mimicking the normal flow pattern.

The most recent development in bileaflet valve design was the introduction of the SJM Regent valve (Fig. 27-4F). This valve model not only modified the sewing ring, but also redefined the external profile in a nonintrinsic structural portion of the valve, increasing the effective flow orifice area. Thus, a larger prosthesis could be implanted for any given tissue annulus diameter. This was the first mechanical prosthesis to demonstrate left ventricular mass regression across all valve sizes.^19,20 The Regent valve is seated supra-annular with only the pivot guards protruding into the aortic annulus.²¹

In spite of advances in the design and performance of AVR with a mechanical valve prosthesis, the use of mechanical aortic valves has decreased over the last decade. This is due to improvements in the longevity of bioprosthetic aortic valves, the introduction of TAVR, and the theoretical potential of valve-in-valve TAVR for failed bioprosthetic valves. More importantly, however, is the fear of chronic anticoagulation with warfarin by both patient and physician essential over the long term in patients having mechanical aortic valve prostheses.

As with any medical therapy, AVR with a mechanical prosthesis is not indicated for all patients. Several prospective randomized studies have shown no difference in survival in patients having biologic or mechanical valve prostheses or among mechanical prostheses per se.^22-27 However, follow-up was limited to less than 15 years. Conversely, in other nonrandomized studies of patients followed over longer time frames, freedom from all valve-related events and from reoperation were improved in patients with mechanical valve prostheses as compared to patients with biologic prostheses.^11,28

Most recently several publications have shown improved survival in patients having bileaflet mechanical valve prosthesis, most likely related to improved anticoagulation regimens and longer patient follow-up.^29,30 Importantly quality of life was similar to that of a biologic prosthesis, even in the elderly.³⁰

While the advantages of large effective flow orifice and durability with a mechanical valve are paramount, the confounding effects resulting from the necessity for anticoagulation continue. Patients that are transient, noncompliant, or incapable of managing medications are not good candidates for long-term chronic anticoagulation, nor are those with dangerous lifestyles or hobbies.³¹ Patients with higher levels of education, and those from geographic areas with a sophisticated medical infrastructure and a static population have better compliance with necessary medication, anticoagulant monitoring, and those with fewer risk factors for TE are good candidate for AVR with a mechanical valve prosthesis.³²

Home monitoring of anticoagulation has become an important adjunct in managing international normalized ratios (INRs) in Europe, but is unfortunately lesser used in North America.

A mechanical valve prosthesis is recommended to patients having second valve reoperations regardless of the nature of the first procedure, as re-reoperative risks increase substantially.^33,34 Some studies report low mortality for reoperation of patients with failed biologic valves, but failures can occur abruptly, creating more risk.³⁵ Reoperative risk is also higher in those patients having combined procedures^33,36 or after prior coronary bypass.

Many surgeons have opted for an age of >70 years as the indication for bioprosthetic AVR, based on data by Akins.³³ In patients younger than 60 years of age, most would opt for a mechanical prosthesis based on prosthesis durability.³⁷ In the decade between 60 and 70 years of age, other factors have to be taken into account.^38,39

Implantation of mechanical valve prostheses has been previously described and is straightforward.¹⁴ Historically, high-profile aortic valve prostheses could be difficult to implant, particularly in small aortic roots. In such cases, a hockey-stick aortotomy is used to “unroll” the aorta and expose the annulus. While the implantation of low-profile bileaflet prostheses is simpler, problems can still arise in the small aortic root. If a tilting disc prosthesis is utilized, orienting the major flow orifice toward the greater curve of the aorta is necessary. Because bileaflet prostheses are the most commonly utilized, the surgical technique for implantation of these devices is described: A midline incision and sternotomy is made and a pericardial well created. Alternatively, a right anterior thoracotomy approach with femoral cannulation may be used. A partial sternotomy is also an alternative in thin patients, creating a sternal “T” at the fourth inter-space.⁴⁰

These alternative techniques are particularly amenable to the implantation of low-profile aortic valve prostheses. The patient is cannulated via the aorta and a single atrial venous cannula. Most commonly, retrograde cardioplegic solution is utilized and a left ventricular vent is placed via the right superior pulmonary vein to maintain a dry operative field. After cross-clamping of the aorta, a transverse aortotomy is made approximately 1 cm above the take off of the right coronary artery, slightly above the level of the sinotubular ridge (Fig. 27-5). The incision is extended three-quarters of the way around the aorta, leaving the posterior one-quarter of the aorta intact allowing excellent visualization of the native aortic valve and annulus. The leaflets of the aortic valve are excised to the level of the annulus and the annulus is thoroughly débrided of any calcium. Extensive de-calcification will minimize the risk of paravalvular leak, particularly in newer-generation prostheses with thinner sewing rings, and allows for better seating of the valve prosthesis. Braided 2-0 sutures with pledgets are utilized. Beginning at the noncoronary commissure, the annulus is encircled with interrupted mattress sutures (Fig. 27-6) extending from the aortic to the ventricular surface (everting). Recently, a technique of placing sutures from the ventricle to the aorta has been used to place the prosthesis in a supra-annular position. Alternatively, multiple single interrupted sutures may be placed. After placement, the suture bundles are divided into two equal portions and two individual sutures placed into the sewing ring at the level of the pivot guards, orienting the pivot guard toward the ostia of the left and right coronary artery (Fig. 27-7). Next, each half of the suture bundles are inserted through the sewing ring and the prosthesis seated (Fig. 27-8).

The pivot guard sutures are tied first followed by the sutures beginning at the left coronary cusp extending to the mid-portion of the right coronary cusp. Lastly, the sutures of the noncoronary cusp are secured, seating the valve appropriately. In a small aortic root, should a valve not be able to be seated, paravalvular leak can be prevented if the unseated area of the valve is in the noncoronary cusp. External aortic sutures can be placed from outside the aorta to the valve sewing ring, securing the prosthesis and preventing paravalvular regurgitation. Because of the low-profile nature of the leaflets, opening and closing can still occur unimpeded. Leaflet motion should always be checked and the surgeon must be assured that the coronary arteries are not obstructed. The aortotomy is closed with a double layer of polypropylene suture consisting of an underlying mattress suture and a more superficial over-and-over suture. The patient is placed in the Trendelenburg position and the heart filled with blood and cardioplegic solution, vented, and the cross-clamp removed. After resuscitation and de-airing of the heart, the procedure is completed and the patient transferred to the intensive care unit. On the first postoperative day the chest tubes are removed if output is less than 125 mL in the previous 8 hours. Following the removal of the chest tube, the patient is begun on subcutaneous heparin (5000 U every 8 hours) or low-molecular-weight heparin (1 mg per kg twice a day), and warfarin therapy started. Valve implantation can usually be accomplished in under 40 minutes of aortic cross-clamping and with cardiopulmonary bypass times of approximately 1 hour, allowing limited coagulopathic and homeopathic alterations.

When coronary bypass grafting is indicated, the order of the operation changes. The diseased valve is excised, distal vein or free arterial grafts constructed, the valve is replaced, and the aortotomy closed. Proximal anastomoses are then completed, with one left untied for de-airing. The distal anastomoses of pedicled grafts (internal maxillary artery, IMA) are then completed. De-airing is accomplished through the untied proximal anastomosis.

The durability and function of mechanical valve prostheses, particularly those of the modern generation, is unquestioned.^28,37,40-44 It is the process of anticoagulation that is key and drives long-term success. INR is the standard to which anticoagulation levels should be targeted.^31,45 Anticoagulation is begun slowly following removal of the chest tubes, as the danger of overshooting target INR to dangerous levels is common.⁴⁶ Current data on anticoagulant regimens indicate that a one-size-fits-all recipe is inadequate to obtain excellent long-term results.^15,32,47 Horstkotte noted that complications occur during fluctuations in the INR, and less often during steady-state levels, be they high or low.⁴⁸ When levels of INR increase, bleeding episodes become more common, and when levels of INR decrease, thromboembolic episodes become more common, both on the slope of the change. These events are opposite ends of the continuum of anticoagulation-related complications. The presence of a mechanical valve prosthesis is also not the only risk factor for TE.^32,46

Traditional risk factors for TE listed in Table 27-1 predispose patients to thromboembolic episodes, and as such higher therapeutic INRs are warranted. Similarly, as shown in Table 27-2, nontraditional risk factors for TE will also predispose patients to embolic events.^32,46-49 Butchart has noted that the more of these risk factors patients have, the greater the incidence of events and the greater the need for a higher target INR (Fig. 27-9).^19,32 Thus, it is imperative in the modern era that patient risk factors be taken into account and the INR individualized for a given patient.^15,16,47 Recommendations for INR target levels in our practice are shown in Table 27-3. These levels are more liberal than those offered by the American College of Cardiology/American Heart Association and the American College of Chest Physicians (ACCP) guidelines, but more conservative than those recommended by the European self-anticoagulation trials.^50-52 These later reports are especially relevant, because they demonstrate that a lower INR is consistent with a lower incidence of TE if patients are maintained in the therapeutic target range.^50,53 Patients with home testing were maintained in the therapeutic range a substantially greater percentage of the time than those whose status was monitored at anticoagulation clinics.^50,53 Starting self-management early after mechanical valve replacement further reduced valve-related events.⁴⁶ Puskas et al in a prospective FDA approved randomized trial substantiated theses data and showed that in patients using home INR monitoring between target INR of 1.5 and 2.0 with the addition of low-dose aspirin in these without contraindication.¹⁶ This report documented a significantly lower risk of bleeding without a significant increase in thromboembolic events. In the United States, home testing has not become commonplace or popular. However, home testing can certainly be expected to lower the incidence of valve-related thromboembolic and bleeding events. It has been approved for reimbursement for weekly testing in patients with a mechanical valve prosthesis or atrial fibrillation, but only after a 3-month waiting period. Obtaining appropriate funding for access in the immediate postoperative period would be an important initiative that could acutely reduce the incidence of valve-related events.