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Division of Cardiothoracic Surgery, Lynn Heart Institute, Boca Raton Community Hospital, Boca Raton, Florida
* Address correspondence to Dr Kulik, Lynn Heart Institute, Boca Raton Community Hospital, 801 Meadows Rd, Ste 104, Boca Raton, FL 33486 (Email: alex_kulik{at}yahoo.com).
In patients with advanced aortic valve disease, the goals of aortic valve replacement (AVR) are to reduce pressure and volume overload on the left ventricle, relieve symptoms, and improve long-term survival. Ideally, transprosthetic pressure gradients after surgery should be minimal. Despite normally functioning prostheses, however, high pressure gradients are occasionally seen after surgery, particularly in patients who have received small-sized prosthetic valves. This results in persistent postoperative left ventricular outflow tract obstruction, a condition termed prosthesis–patient mismatch (PPM). First described by Rahimtoola [1] in 1978, PPM is described as an "effective prosthetic valve area, after insertion into the patient, that is less than that of a normal valve." Pibarot and Dumesnil [2] have since defined aortic PPM as a prosthetic valve effective orifice area (EOA) indexed to body surface area of less than 0.85 cm2/m2.
Considerable controversy exists in literature regarding the significance of PPM and its impact on postoperative outcomes after valve replacement. Although some studies have suggested that there is no risk associated with PPM [3–5], others have demonstrated PPM to be a strong and independent risk factor for adverse events after AVR [2, 6–9]. Several groups have shown that PPM is associated with reduced early and late survival after AVR [2, 6, 7]. With residual postoperative gradients, PPM has also been linked to reduced left ventricular mass regression after surgery [8], as well as lower functional class and exercise tolerance [9]. Mismatch after mitral replacement has also been shown to have a detrimental impact on postoperative outcomes. Defined as an indexed EOA of less than 1.2 cm2/m2, mitral PPM is associated with persistent pulmonary hypertension, congestive heart failure, and reduced survival after surgery [10].
The EOA of a prosthetic valve, indexed for the patient's body surface area, is the only factor that has been found to consistently correlate with transprosthetic gradients after AVR. To avoid the potential sequelae of PPM, current recommendations advocate for the implantation of an aortic prosthesis that will yield an indexed EOA in excess of 0.85 cm2/m2 [2]. Prosthesis–patient mismatch can be prevented by applying the reference values for different types and sizes of prostheses. Before surgery, multiplying the patient's body surface area by 0.85 will determine the minimum EOA necessary to avoid postoperative PPM [2]. Thus, based on preoperative calculations and known reference values, the minimum size of a specific prosthesis type can be determined before surgery to prevent mismatch. During surgery, when the aortic annulus is sized, if PPM is projected with a particular size of the prosthesis originally intended, the surgeon has several options. One option is to accept PPM, a conservative approach that may be very reasonable for a frail or elderly patient. A second option is to implant another type of prosthesis with a larger EOA. For example, implanting a newer generation mechanical valve may yield a larger EOA and lower postoperative gradients. Alternatively, a stentless bioprosthesis or an aortic homograft can be used. Finally, an aortic root enlargement (ARE) procedure may be considered.
Several surgical techniques have been developed to enlarge the aortic root to fit a larger prosthesis. In 1970, Nicks and colleagues [11] described an aortic annular enlargement using a posterior incision through the noncoronary sinus of Valsalva. Subsequently, Konno and associates [12] demonstrated in 1975 their surgical technique involving anterior annular enlargement for congenital aortic stenosis. In the late 1970s, Manouguian and Seybold-Epting [13] reported an enlargement technique through the commissure between the left and noncoronary sinuses of Valsalva onto the anterior mitral valve leaflet. Nunez and associates [14] presented a modification of that technique in which the commissure between the left and noncoronary sinuses was resected and replaced with a Dacron patch. These surgical strategies were developed with the objectives of implanting prostheses as large as possible to minimize postoperative gradients.
A number of reports exist in the literature describing favorable outcomes using the root enlargement procedure at the time of AVR [15, 16]. However, only a few studies have discussed the technique of enlarging the aortic annulus and the mitral annulus during concurrent aortic and mitral valve replacement. In this issue of the Annals of Thoracic Surgery, Zhong and colleagues [17] describe a series of 78 consecutive patients who underwent ARE and double-valve replacement between 1999 and 2008. The average patient age was 38.5 ± 9.4 years, and the mean body surface area was 1.4 ± 0.2 m2. The average aortic annulus diameter before surgery was 18.3 ± 1.3 mm. Enlargement of the aortic root was performed with a pericardial-lined Dacron patch using the technique of Manouguian and Seybold-Epting for 42 patients. A modification of the technique of Nunez and colleagues was applied for 36 patients in whom the commissure between the left and noncoronary sinuses was incised but not resected. Using these techniques, Zhong and associates [17] implanted aortic and mitral prostheses averaging 20.5 and 25.9 mm in size, respectively. No patient had postoperative PPM, and after surgery the mean aortic and mitral indexed EOA were 1.13 ± 0.14 cm2/m2 and 1.56 ± 0.17 cm2/m2, respectively. Postoperative mean aortic and mitral transprosthetic gradients were 10.7 ± 2.3 mm Hg and 3.7 ± 1.6 mm Hg, respectively [17].
Zhong and colleagues [17] are to be congratulated for achieving outstanding results in a large series of complex cardiac patients. Whereas double-valve replacement is associated with 7% to 11% mortality in The Society of Thoracic Surgeons (STS) database, Zhong and associates [17] reported a perioperative mortality rate of 1.3% (1 patient), and no patient required reoperation for bleeding or pacemaker for heart block. Valve-related complications during the course of follow-up were extremely low in their series, and the 10-year survival was impressive at 97.4%. Based on their experience, the authors conclude that "aortic root enlargement in patients undergoing double-valve replacement can be performed safely to avoid postoperative aortic prosthesis–patient mismatch" [17]. Notwithstanding their excellent results, it is important to consider the limitations of the paper by Zhong and coworkers [17]. First and foremost, their study lacked a control group for comparison. Second, it is important to note that the population treated by Zhong and colleagues [17] is uniquely different from patients typically seen in North America. In contrast to the elderly and frequently obese patients with degenerative valve disease treated in North America, patients treated by Zhong and colleagues [17] were young, small in body size, and primarily had rheumatic valve disease. Of note, the youngest patient in their series was 10 years of age, and the smallest patient was 21 kg. Thus, the results achieved by Zhong and associates [17] cannot be generalized to contemporary cardiac surgery practice in all regions of the world, and certainly not to patients seen by North American adult cardiac surgeons.
It is interesting to speculate whether patients treated by Zhong and colleagues [17] actually benefited from the root enlargement procedure. Because the study was not a randomized trial and there was no control group in the study, it is impossible to predict what the counterfactual experience would have been had ARE not been performed. Would patients have suffered had they undergone double-valve replacement without ARE? Is an aortic valve indexed EOA of less than 0.85 cm2/m2 hazardous in a patient after double-valve replacement? No study to date has firmly established that ARE at the time of double-valve replacement is beneficial. Examining the patient population treated by Zhong and coworkers [17], the largest patient had a body surface area of 1.63 m2. Had this patient and all other patients in their study received a 17-mm St. Jude Medical Regent valve (EOA, 1.41 cm2), then the indexed EOA would have been 0.86 cm2/m2 or greater for all patients. In essence, if root enlargement had not been performed, most of the patients of Zhong and associates [17] would have avoided postoperative PPM. Mismatch would be expected only for those few patients (<10) with very small aortic annuli in whom a 17-mm valve could not be implanted. Although demonstrated to be safe and feasible in their hands, it is unclear whether any patient actually derived benefit from the addition of the ARE to the double-valve operation.
Several surgical groups have questioned the relevance of PPM, reporting little to no relationship between valve orifice size and outcome [3, 4]. Moreover, strategies proposed to avoid postoperative PPM appear to have minimal impact on postoperative outcomes. With the elimination of the sewing ring, stentless valves were designed to provide larger EOA and reduce transprosthetic gradients. Although more technically demanding to implant, it was anticipated that stentless valves would facilitate postoperative ventricular mass regression and improve postoperative outcomes compared with stented bioprostheses. However, a number of randomized clinical trials comparing the implantation of aortic stentless bioprostheses with conventional stented bioprostheses have failed to demonstrate an improvement in ventricular mass regression, functional outcomes, or clinical outcomes [18, 19]. A recent meta-analysis of ten randomized controlled trials involving 919 patients noted lower postoperative transprosthetic gradients and increased EOA after stentless valve implantation. Nevertheless, there was no difference in left ventricular mass regression 1 year after surgery, and the use of a stentless valve, on average, led to a 23-minute longer cross-clamp time and 29-minute longer cardiopulmonary bypass time [20].
Similarly, root enlargement and insertion of a larger prosthesis at the time of AVR has been recommended to reduce the risk of PPM and minimize postoperative gradients. Several cardiac surgery centers have reported their experience with ARE procedures, demonstrating these operations to be safe and feasible [15, 16]. In a recent observational study, the outcomes of 712 patients with small aortic roots (21 mm or less) were compared between patients who underwent AVR alone (n = 540) to those who underwent AVR plus ARE (n = 172). The addition of the ARE procedure led to a 10-minute longer cross-clamp time, but there were no increased risk of perioperative complications. Postoperative gradients were reduced (p < 0.01), and the incidence of postoperative PPM was lower in the ARE group (p < 0.0001). However, the presence of mismatch did not significantly impact long-term outcomes after surgery. Further, the root enlargement did not improve long-term survival (p = 0.81), and ARE was associated with only a slight trend toward fewer congestive heart failure symptoms (p = 0.19). Thus, in patients with small aortic roots, ARE does not appreciably improve long-term clinical outcomes [15].
Given the controversy associated with postoperative PPM, what role does the ARE procedure have in contemporary cardiac surgical practice? Focused subgroup analysis in mismatch studies suggest that PPM is most detrimental in patients younger than the age of 70 [21] and in patients with depressed left ventricular function [6, 22, 23]. Root enlargement, therefore, may be most relevant for young, physically active patients or for those with a poorly functioning ventricle, in which the increased hemodynamic burden associated with PPM may be poorly tolerated. An important skill in a surgeon's armamentarium, the root enlargement procedure may also be helpful at the time of AVR for a large patient with a small aortic root. This is not uncommon in North American cardiac surgery practice, particularly among young obese women. In contrast, for patients with small body size, several studies from Asia have demonstrated that small-size aortic prostheses are generally well tolerated [24, 25].
In summary, in their series of 78 patients undergoing double-valve replacement, Zhong and colleagues [17] achieved excellent outcomes in their routine application of the ARE procedure. However, small-size patients normally do well with small prosthetic valves, and very few patients in their cohort would have had PPM without enlargement of the aortic root. Thus, it is unclear whether any benefit was derived from routine application of root enlargement in their series. Moreover, multivalve operations are often complex procedures, with the occasional need for concomitant tricuspid repair or coronary bypass. Routine ARE is likely unnecessary, and potentially dangerous in inexperienced hands, and therefore in my view, cannot be recommended on the basis of the data presented.
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