The guidelines acknowledge that there is little experience with mechanical valves in this setting, raising concerns about adequate anticoagulation.
The use of bioprosthetic valves as the primary choice for right-sided valve replacement is widespread and the inclination for bioprosthetic valves over mechanical valves stems from several practical considerations. This primarily includes avoidance of life-long commitment to vitamin K antagonist and its implications on patients with CHD who are often still relatively young. Major deterrents include strict adherence to the drug and the need for frequent international normalised ratio (INR) monitoring, the cumulative risk of bleeding, contraindications during pregnancy and precautions when choosing sport activities. The low pressure on the right side of the heart has dual effects on valve performance. While it increases the risk of thrombosis for mechanical valves, it also contributes to the greater durability of right-sided bioprosthetic valves. As a result, the anticipated life span of bioprosthetic valves implanted on the right side is currently considered to be 10–20 years or even more. Furthermore, percutaneous options for treating bioprosthetic valve failure are available, but they are not applicable when a mechanical prosthesis is implanted. The proposed advantages of mechanical valves in terms of longevity and freedom from re-intervention are less evident.
Although bioprosthetic valves are overwhelmingly favoured in current clinical practice, there are specific cases where mechanical prostheses might be preferred as they are considered to offer the best chance for individuals to avoid the need for repeated surgery and/or intervention. These decisions are often based on expert opinion rather than being supported by clear evidence, and there is a scarcity of long-term outcome data, especially regarding mechanical valve durability and patient outcomes.
A current paper by Abozied and coauthors from the Mayo Clinic provides valuable insights into the performance of mechanical valves in the right heart (pulmonary and/or tricuspid position) compared with bioprosthetic valves. The authors conducted a retrospective analysis of 48 consecutive patients with CHD with an average age of 35 years who received mechanical prostheses on the right side between 2003 and 2021. For each patient with a mechanical prosthesis, two patients who received a bioprosthesis (1:2 matching) in the same position were used as controls. The majority of patients had underlying diagnosis of tetralogy of Fallot or had previously undergone a Ross operation for aortic valve disease. Prior to the implantation of a right-sided mechanical prosthesis, 61% of the patients were already on vitamin K antagonist. The study had long follow-up period with a median time of 82 and 94 months for the pulmonary and tricuspid groups, respectively. Regarding mechanical prostheses, the main findings of the study include lower incidence of valve dysfunction and lower need for reoperations, but higher incidence of major bleeding in comparison with the bioprosthesis group. Furthermore, within the mechanical prostheses group, patients who received mechanical prosthesis in the pulmonary position had significantly lower incidence of valve dysfunction, reoperations and major bleeding in comparison with those who received mechanical prosthesis in the tricuspid position.
Historically, there has been a lot of caution when considering mechanical valves on the right side. This caution may be a legacy from early trials where patients received older generation of mechanical valves without adequate anticoagulation and, therefore, had high incidence of valve dysfunction due to thrombosis. Later, it became clear that all mechanical valves require the use of adequate vitamin K antagonist dose, and right-sided valves need to be treated with a higher INR range than left ones. With this knowledge and improved mechanical prostheses, there was a new optimism in the field, particularly in the pulmonary position. In 2010, the Mayo Clinic published an important paper entitled ‘The increasing use of mechanical pulmonary valve replacement over a 40-year period’ that did not have a single case of valve thrombosis out of the 54 patients who had this procedure. A smaller retrospective study from a similar period also had no cases of thrombosis out of 28 patients with pulmonary mechanical prosthesis. While the reported incidence varies, it is worth noting that the risk of thrombosis and valve failure is significantly lower than in early trials, although still higher compared with left-sided mechanical valves. Comparing the performance of mechanical valves with bioprosthetic valves in this population is difficult due to the very small number of mechanical valves inserted compared with bioprosthetic valves as well as a significant selection bias. Mechanical valves traditionally are reserved for patients who are not candidates for future operations due to unacceptably high operative risk.
In line with previous study from the Mayo Clinic, current study by Abozied and coauthors on mechanical right heart prostheses showed a substantially lower incidence of valve thrombosis compared with the previous studies, approximately 6% vs 10%, respectively. The rate of thrombosis with mechanical prostheses in pulmonary position was particularly encouraging, with just one patient suffering this complication. There were two patients who experienced mechanical tricuspid valve thrombosis. Importantly, it was observed that all cases of valve thrombosis had subtherapeutic INR levels. Of note, the remaining cases of valve dysfunction were associated with infective endocarditis. Conversely, among the patient population with a mechanical prosthesis, 13% experienced a major bleeding event. All cases of spontaneous bleeding were associated with an inadequately high INR >4, while two cases were linked to heparin bridging during non-cardiac surgery. Overall, the annual rate of bleeding was understandably higher in the mechanical prosthesis group (2.2% vs 0.1% per year). The work by Abozied and coauthors provides a strong contribution to a cohort that has been notoriously difficult to investigate. A prospective randomised trial of bioprosthetic versus mechanical right-sided valve replacement is not feasible, highlighting the importance of observational analyses despite inherent shortcomings. Although appropriate attempts were made to match candidates with similar controls, it is very difficult to account for factors such as surgical risk and underlying anatomy which likely render the mechanical group a higher-risk population.
For the vast majority of patients, a bioprosthetic pulmonary valve replacement followed by transcatheter valve implantation remains the most appropriate choice. This is especially true for women of childbearing age or patients with a high bleeding risk. Recently, there has been a growing trend towards the use of bioprosthetic valves in general. Research efforts have focused on enhancing the percutaneous approach for repeated insertion of bioprosthetic valves. However, there are potential long-term concerns regarding the impact of repeated insertions on the right ventricular haemodynamics, particularly as the outflow tract becomes narrower, and repeated surgery, especially if the initial bioprosthetic valve has been implanted at young age, may not be avoidable. Additionally, in case of interventional complication, there is a risk of requiring conversion to open heart surgery.
Hence, a subset of highly selected patients may benefit from the use of a mechanical prosthesis. This option is best reserved for patients who are likely to face very high risk and operative difficulty with future sternotomy or percutaneous interventions. Patients with a concurrent indication for warfarin may also be candidates for mechanical option. Additionally, patients may have their own preferences, and these data will help both patients and clinicians determine the optimal strategy through shared decision-making. On the occasion that a mechanical valve is preferred as a definite option to avoid a further sternotomy to the greatest extent possible, this research provides reassurance that there is a reasonable expectation of safety and increased valve longevity compared with a bioprosthesis. This is especially relevant for mechanical prosthesis in pulmonary position.
Given the limitations for conducting large randomised controlled trials on the right side, research concerning mechanical valves in this context should focus on observing the safety and efficacy of advancements seen in left-sided valves and anticoagulation management. Perhaps the strongest message out of this work of Mayo Clinic team is the importance of having meticulous anticoagulation protocol in place, as both thrombotic and bleeding complications have been associated with inadequate level of INR. Encouragingly, newer mechanical valves have shown possible decreased thrombogenicity and enhanced durability. Ongoing active research is exploring the use of new oral anticoagulants in the setting of mechanical valves, although evidence demonstrating their safety and non-inferiority to warfarin is still lacking. Future research on right-sided mechanical valves is likely to be guided by advancements in valve design and a focus on achieving safer and more convenient anticoagulation management.