Cardiac surgery
Early clinical outcomes of the surgical treatment of patients with aortic stenosis and small aortic annuli

Introduction

Aortic valve stenosis is one of the most frequent diseases of the cardiovascular system encountered by surgeons during the course of their work. In recent years, the number of aortic valve implantation procedures has risen progressively in comparison to the decreasing number of coronary interventions.

In patients with small aortic annuli, after the aortic valve is replaced with a prosthetic valve, the phenomenon of patient-prosthesis mismatch (PPM) may occur. It may negatively affect hemodynamic parameters, result in transvalvular gradient increase, and prevent the reduction of left ventricular hypertrophy despite the normal functioning of the valve [1-3]. Consequently, this may lead to increased early and long-term mortality among the operated patients [4]. PPM is not a common problem, but, according to some studies, it may occur in as many as 70% of patients undergoing aortic valve replacement [5].

According to the original definition by Rahimtoola [6], PPM occurs when ‘the effective prosthetic valve area, after insertion into the patient, is less than that of a normal human valve’. This stems from the fact that the implanted valve is placed intra-aortically and is equipped with its own annulus, which significantly reduces the diameter of the valve orifice area after implantation [7, 8]. It is currently believed that the problem pertains to patients with indexed effective orifice area (iEOA) < 0.85 cm2/m2 [5, 9].

Depending on the iEOA value, PPM is considered to be moderate for 0.65 cm2/m2 < iEOA ≤ 0.85 cm2/m2 and severe for iEOA ≤ 0.65 cm2/m2. In different studies, iEOA > 0.85 cm2/m2 is considered either as mild PPM or insignificant PPM [10-12].

The data above indicate that surgeons can directly influence the improvement of the iEOA value by selecting appropriate prosthetic valves, which consequently leads to the improvement of hemodynamic parameters and helps avoid PPM [13-15].

Selecting prosthetic valves of appropriate sizes is particularly important in the case of patients with small aortic annuli, whose EOA is relatively small [12].

Aim of the study

The aim of the study was the evaluation of PPM incidence in relation to the size of the prosthetic valves implanted in patients undergoing aortic valve replacement, as well as the perioperative assessment of the treatment results of PPM patients.

Material and methods

The study encompassed 92 patients undergoing surgery, in whom mechanical St. Jude Medical valves of different sizes were implanted. The studied group consisted of 44 women (48%) and 48 men (52%), aged 21-74; mean age: 53.6 years (±SD 9.8 years).

All patients were qualified for aortic valve replacement due to severe valvular stenosis of the left arterial orifice. The patients were monitored perioperatively until their discharge from the hospital. The incidence of complications during the perioperative period was assessed, along with hemodynamic parameters after the implantation of prosthetic valves, by transthoracic echocardiography.

The patients were divided into 3 groups, based on the size of the implanted valves. The first group consisted of 15 patients who were implanted with valves of small

aortic annulus diameter: 17 mm (3 patients) and 19 mm

(12 pa­tients). The second, largest group comprised 56 patients who were implanted with valves of medium aortic annulus diameter: 21 mm (29 patients) and 23 mm (27 patients). The third group consisted of 21 patients, who were implanted with valves of large aortic annulus diameter: 25 mm (16 patients), 27 mm (2 patients), and 29 mm (3 patients).

Results

Clinically significant PPM was revealed in all patients with small aortic annuli. Severe PPM was revealed in 87% of patients, moderate in 13% (iEOA ranged from 0.38 cm2/m2 to 0.8 cm2/m2; mean: 0.53 cm2/m2, ±SD 0.12 cm2/m2). The range of EOA was from 0.65 cm2 to 1.13 cm2 (mean: 0.88 cm2 ±SD 0.13 cm2). The body surface area ranged from 1.42 m2 to 2.05 m2 (mean: 1.70 m2 ±SD 0.17 m2).

Among the patients with medium aortic annulus size, 48.2% suffered from severe PPM, 44.6% suffered from moderate PPM, and only 7.1% had no significant PPM (iEOA ranged from 0.4 cm2/m2 to 1 cm2/m2; mean: 0.68 cm2/m2 ±SD 0.12 cm2/m2). The range of EOA was from 0.91 cm2 to 1.58 cm2 (mean: 1.23 cm2 ±SD 0.16 cm2). The body surface area ranged from 1.51 m2 to 2.25 m2 (mean: 1.83 m2 ±SD 0.15 m2).

Among the patients with large aortic annulus size, 9.5% suffered from severe PPM, 42.5% suffered from moderate PPM, and only 47.6% had no significant PPM (iEOA ranged from 0.61 cm2/m2 to 1.37 cm2/m2; mean: 0.88 cm2/m2 ±SD 0.2 cm2/m2). The range of EOA was from 1.35 cm2 to 2.43 cm2 (mean: 1.69 cm2 ±SD 0.3 cm2). The body surface area ranged from 1.64 m2 to 2.20 m2 (mean: 1.94 m2 ±SD

0.14 m2) (Fig. 1).

Perioperative mortality of all patients undergoing surgery amounted to 5.4%. The largest percentage of perioperative deaths (20%) concerned the group with small aortic annuli. It appears significant that all 3 persons who died in this group were diagnosed with severe PPM (iEOA ≤ 0.65 cm2/m2). In the group with medium valves, the perioperative mortality was 1.8% (1 patient with severe PPM).

Mean and peak transvalvular gradient, as well as the change in ejection fraction during the early postoperative period, were also assessed in relation to valve size.

The mean gradient through the aortic valve differed among the three groups during postoperative follow-up. The largest mean gradient was observed among the patients implanted with valves of small aortic annulus diameter; it was 20.25 mm Hg (±SD = 7.37). In turn, in the patients with medium aortic annuli, the mean gradient was 13.42 mm Hg (±SD = 5.94), and in the patients with large aortic annuli it was 11 mm Hg (±SD = 4). It appears important that the comparison between the mean gradient values of patients with small annuli and patients with medium

(p = 0.009) and large annuli (p = 0.001) indicated, in both cases, statistically significant differences, while the comparison between the gradient values of patients with medium and large annuli showed no such difference (p > 0.05). The peak transvalvular gradient among the patients with small, medium, and large valve annuli was, respectively: 34.75 mm Hg (±SD = 11.34); 22.85 mm Hg (±SD = 9.25); 18.18 mm Hg (±SD = 6.35). In all three groups, statistically significant differences were found.

Changes in ejection fraction (EF) during the early postoperative period were noted in all three groups. In the group of patients with small aortic annuli, EF decreased in 67% of patients; the mean EF change within the whole group was –7.58% (±SD = 10.59). In the group of patients with medium aortic annuli, EF decreased in more than 80% of patients; the mean EF change within the whole group was –10.68% (±SD = 11.22). Different results were noted in the group of patients with large aortic annuli, as the EF parameters in this group increased in 75% of patients, and the mean EF change was +11.55% (±SD = 11.97). These changes, however, are not statistically significant (p < 0.05).

The study also analyzed the characteristics of the implemented valve size distribution in dependence on gender, age, and BMI.

Valves with small aortic annuli were most often used in women (93% women vs. 7% men), while valves with large aortic annuli were most frequently employed in men (90.5% men vs. 9.5% women). Valves with medium annuli were used in the same number of men (50%) and women (50%). The distribution was statistically significant (p < 0.05) (Fig. 2).

No statistically significant changes were noted in any of the three groups with regard to patient age or BMI.

Conclusions

The occurrence of PPM among patients with small aortic annuli was relatively more frequent (100% of iEOA results ≤ 0.85 cm2/m2) in comparison to other patients.

Female patients are particularly susceptible to the occurrence of severe PPM and high transvalvular gradient values.

The occurrence of PPM appears to be a factor increasing the risk of perioperative death in the studied group.

Discussion

Decreasing the transvalvular gradient is the primary objective of aortic valve replacement. Surgeons always prefer implanting the largest possible prosthetic valve if the aortic annulus is small, as it influences cardiac hemodynamic parameters and surgical treatment results. Nonetheless, the reduction of the gradient during the early postoperative period is often unsatisfactory due to the size of the employed valvular prosthesis [13, 17, 18]. Patient-prosthesis mismatch (PPM) is a frequently occurring problem which leads to the deterioration of hemodynamic function, results in a smaller postoperative reduction of left ventricular hypertrophy, increases the number of complications, and influences mortality.

Our study results provide evidence that the chance for PPM occurrence among patients with small aortic annuli (diameters 17 mm and 19 mm) is higher (100% of iEOA results ≤ 0.85 cm2/m2) than among patients in whom the diameter of the aortic annulus is more than 21 mm. Women constitute a group that is particularly susceptible to the occurrence of severe PPM, which results from the fact that the body surface area and aortic annuli in women are significantly smaller in comparison to men [19]. These findings are supported by earlier publications from other centers [20].

According to some reports, the occurrence of PPM, in particular severe PPM, appears to be a factor which increases mortality during the early postoperative period [12]. In our study, almost all perioperative deaths occurred in patients with small aortic annuli, among whom 87% suffered from severe PPM.

The implantation of 17 mm and 19 mm valves was inextricably associated with significantly higher postoperative average and maximal transvalvular gradient values in the study group. Not only can this lead to enhancing the abovementioned risks related to PPM, but it can also result in faster degeneration of the implanted valve, provided that a biological valve was employed [19].

The development of cardiac surgery allows for a significant reduction of PPM, if not avoiding it altogether. This requires approaching each patient individually before the operation, calculating the desired EOA, and selecting appropriate valves for implantation.

The slight change in EF observed during the early postoperative period appears to be caused by the use of extracorporeal circulation during surgery. However, in-depth analysis of this phenomenon would require long-term EF observation, which was not the subject of this study.

The possibility of symptomatic PPM occurrence in patients with small aortic annuli may be significantly reduced, owing to the rich variety of valve prostheses and the availability of supra-annular prostheses providing better hemodynamic parameters.

Stentless valves may be used instead of mechanical prostheses and biological stent valves, as their different structure (no frame) allows for a larger orifice area, therefore providing better hemodynamic parameters [20-23]. Due to the larger iEOA of stentless valves, the risk of PPM occurrence is smaller than in the case of stent valves [24].

One of the more recent options is the use of sutureless valves [25]; their structure and hemodynamic properties allow for a significant improvement of the EOA size and hemodynamic parameters, particularly with regard to the transvalvular gradient.

Other methods of avoiding PPM include valve implantation with aortic annulus enlargement [22]. This procedure should be considered if iEOA < 0.65 cm2/m2. Its aim is the implantation of a larger prosthesis and achieving the assumed index [26]. Unfortunately, the database of the Society of Thoracic Surgeons indicates that performing this procedure is associated with a significant risk increase, which is why it is rarely performed [STS database].

Employing homografts and autografts (the Ross procedure) may also improve the hemodynamic parameters and reduce the possibility of PPM occurrence [27-30]. The use of these valves may, however, be constrained by their limited availability, technical difficulties, and greater complexity of the surgical procedure [23].

The results obtained in this study are supported by the conclusions of reports published by other centers. However, some reports indicate no PPM influence on perioperative mortality [24]. Therefore, the results concerning mortality should be interpreted with caution.

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