The current American Heart Association and American College of Cardiology (AHA/ACC) guidelines recommend that biologic valves are reasonable in patients aged 70 or above (Class IIa recommendation), whereas a mechanical valve is reasonable for patients below 60 years of age (Class IIa recommendation) ¹. While the guidelines make clear that patient choice and willingness to take anticoagulation should be the major factor in deciding valve choice, these two recommendations reflect the dominating dichotomy in surgical valve replacement – young patients preferentially receive mechanical valves while older patients receive biologic valves. We provide the rationale for consideration of biologic valves as a primary choice for aortic valve replacement in adults below 70 years of age. Majority of data on biologic aortic valve replacement derives from studies of stented porcine and pericardial valves – we limit this discussion to these valve types. Separate considerations may apply to other biologic valve choices including stentless xenografts, and human valves (autografts and allografts).

1. Randomized trials show survival with biologic valves is equivalent to mechanical valves
   
   There are three published randomized trials comparing biologic and mechanical valves in patients undergoing aortic valve replacement^2-4. Critical analysis of these trials supports equivalence of long-term survival outcomes in patients receiving biological compared to mechanical valves. Although two of the trials evaluated historical early generation prosthesis in patients randomized in the 1970s to early 1980s, a more recent trial randomized 310 patients to receive a biological or mechanical valve between 1995 and 2003 and found no survival difference at twelve-year follow-up⁴. Of note analysis of large multicenter datasets (with propensity matching to partially balance section bias in choosing mechanical versus biological prosthesis) also show no survival difference up to 15 years post valve replacement^{5, 6}.
   
   
2. Randomized trials show more midterm morbidity with mechanical valves
   
   A major drawback of mechanical prosthesis is the higher mid-term morbidity which is due primarily to increased rate of bleeding and thromboembolic events. Biologic valves have a low valve-related morbidity rate within the first decade of implantation. However, in the second decade, the event rate with biologic valves equals and then surpasses that of mechanical valves because of structural valve degeneration. For the first decade, therefore, patients receiving biologic valves derive better health benefit compared to mechanical cohorts. The differential effect is seen because the complications of mechanical valves – bleeding, thrombosis, and stroke – occur in a linear fashion with constant risk for the entire duration one has a valve, whereas the hazard rate for biological valve degeneration is low for the first decade and then high in the second decade.
   
   While over a life time, such benefit of biological valves is eroded by morbidity related to failure of biological valves, in terms of value this may not be equivalent to the early morbidity of mechanical valves, as most people tend to discount long-term benefits favoring instead short and mid-term gains. This is particularly important to young patients as earlier years of life are generally valued more than latter years. It may be more valuable, for example, spending the fourth and fifth decades of life, when most adults are in their prime, in excellent health free from anticoagulation, even if it meant having fair or poor health or reoperation when they are 60, as opposed to being in a fair state of health from age 40, with requirement for daily anticoagulation and attendant risks, and remaining so for the rest of their life. Most young people would rather spend their prime years free from illness, obligatory medications, and hospitals. The low event rates with biologic valves therefore make them desirable for the young patient wishing to maintain health and independence for another decade or two.
   
   In contrast, mechanical valves are associated with a reoperation rate of 0.6% per year, bleeding rate of about 1.5% per year and thromboembolism rate of 0.6% per year⁴ – in a young patient this amounts to over 20% risk of major complication in the first decade post valve replacement. Several approaches to improved anticoagulation, such as use of novel anticoagulants, home International Normalized Ratio (INR) testing, and lower INR targets have not transformed to reduction in morbidity with mechanical valves^{7, 8}. In one recent trial a bleeding rate of 3.3% per year was noted in patients receiving conventional anticoagulation⁹.
   
   One aspect that has not been studied is potential economic impact of prosthesis choice over the first decade. Mechanical valves have constant stream of direct costs in terms of medication cost, INR testing and costs of hospitalizations related to bleeding and thromboembolism; this has to be compared to the costs of reoperations on early bioprosthetic valves. In addition mechanical valves have indirect costs of lost income during periods of hospitalization. It is probable that the costs of mechanical valves will exceed those of bioprosthesis in the mid-term, but this needs to be confirmed by formal study.
   
   
3. Reoperation rates are understated with mechanical and overstated with biological valves
   
   The major argument against use of biologic valves in young and middle aged patients is the inevitability of reintervention for structural valve failure. In contrast, mechanical valves are often presented as a life-long solution. Are these views supported by evidence? A critical review of the literature will show that most patients receiving biologic valves will not have reoperations. Aortic stenosis, its cardiac sequalae, and its treatment all result in reduced life expectancy, regardless of therapy. The life expectancy after valve replacement varies with age, but life-table analyses of large datasets suggest the average life-expectancy of a 60 year old after aortic valve replacement is about 12 years¹⁰. Because death and structural valve degeneration are competing outcomes, and most biological valves have a median valve survival (time to structural degeneration) in the 10 to 15 year range, many patients will die before the valve degenerates. For those patients, there would be no incremental benefit of a mechanical valve in terms of freedom from reoperation. Indeed only 45% of patients aged 50, and 25% of patients aged 60, who receive biological valves are likely to have a reoperation in their life-time¹⁰. This is validated by prospective studies which report that less than 20% of patients aged below 70 years have reoperations within 15 years of initial biological valve implantation^{4, 6}.
   
   Also important to note is that mechanical valves do not guarantee freedom from reoperation – prospective studies show a 0.6 to 1.8% reoperation rate per years due to valve-related complications, such as thrombosis, infection and paravalvar leak^{3, 4, 9}. At 15 years the absolute benefit of a mechanical valve compared to a biological valve in terms of freedom from reoperation is probably a reduction from about 25% with a biological valve to about 10% with a mechanical valve^{3, 4, 6}; the majority of patients will not have a reoperation, regardless of prosthetic choice and at least 1 in 10 patients will be reoperated even if they opt for mechanical valves.
   
   
4. Redo AVR is as safe as primary AVR
   
   It may seem implausible that the long-term survival curves for biological and mechanical valves are superimposed and similar in the second decade post-surgery, despite the former having more reoperations. This is explained largely by the low mortality associated with reoperations. Most patients having surgical replacement of degenerated bioprosthesis still remain with similar survival compared to patients who do not have reoperations. This low operative risk of a reoperative AVR does not seem to exceed the baseline valve and cardiac related mortality hazard of mechanical aortic valve replacement. Most series from experienced valve centers report a mortality rate of reoperative AVR below 5%, even though a substantial number of these operations have been for acute complications such as endocarditis. Therefore, most young patients who opt for tissue valves could predictably undergo reoperation for valve failure (when required) with a low operative mortality, provided surgery is done electively in an experienced valve center. It is likely that in the future transcatheter valve replacement will also become an established option for some patients with structural degeneration of biologic valves. These transcatheter procedures are, however, currently limited to high-risk or inoperable patients because of higher procedural risk, high residual gradients in patients with smaller surgical valves, and unknown long-term outcomes.
   
   
5. Complications specific to mechanical valves are more devastating than complications specific to biological valves
   
   Structural valve degeneration of biological valves is rarely an acute emergency. In many patients degeneration is detected by periodic surveillance before onset of severe symptoms, in others symptoms of heart failure may develop and prompt the diagnosis. The onset of symptoms may be insidious, especially where mode of failure is valve stenosis, though can sometimes be acute due to sudden leaflet tear. Regardless of presentation, the patients are usually well compensated, allowing planned elective or semi-elective surgery. These procedures are associated with low mortality and morbidity as described above if patients are referred to an experienced valve center. However, for mechanical prosthesis, even though the rate of valve related complications may be low, these may be devastating, and sometimes irretrievable, and typically require emergency surgery. These patients generally do not have the option of referral to an experienced valve center, so outcome is variable depending on surgeon experience and other local factors. The most potentially devastating complications of mechanical valves are probably embolic stoke and hemorrhagic stroke which can result in permanent neurological injury; such injury will dramatically change the life of a young patient. Valve thrombosis can be a life threatening surgical emergency during pregnancy. For younger females contemplating pregnancy, a bioprosthesis seems to offer lower risk of cardiac and obstetric complications during pregnancy¹¹
   
   
6. The negative impact of mechanical valves on day to day quality of life is significant but understated
   
   The impact of a mechanical valve on day to day quality of life rarely features in discussions and decisions on prosthesis choice. Mechanical valves have three major constraints on quality of life: firstly all mechanical valves have an audible click; secondly mechanical valves require strict compliance with anticoagulation medication including frequent blood testing; thirdly warfarin therapy makes patients more prone to bruising and hematomas during day to day activities. One qualitative study provides great insight into what patients deal with in living with a mechanical valve and how these factors can result in significant impairment of quality of life¹². The requirement for anticoagulant therapy was the most troublesome consequence of living with a mechanical valve, followed by the closing click¹². Several patients are disturbed by the clicking sound to the extent that it keeps them awake at night, interferes with their sex life and in some cases has resulted in psychiatric illness such as post-traumatic stress disorder. The requirement to take warfarin religiously may not be conducive to many young patients with active or travelling life-style. The lifestyle implications of warfarin therapy are understated. Some patients note they bruise easily to the extent of having to wear protective clothing and gloves for relatively mundane activities. Other patients report not being able to wear shorts and bikinis as they can expose areas of bruising. These issues appear to be more pronounced with younger patients than older patients.
   
   

As current guidelines suggest, the choice of valve prosthesis should be based on discussion between the clinician and the patient. However, the strong differences in patterns of use of one prosthetic choice over another in different clinics suggest that physician bias remains a strong factor, and that patients may not be well- informed participants in the choice. All practitioners involved in guiding patients on choice of prosthesis should be aware of the above six factors and discuss them with young patients faced with the decision of prosthesis type.

Biological Vs Mechanical Valves – Key Points Long-term survival is equivalent Mid-term morbidity is worse with mechanical valves Reoperation rates are low with biological valves and are not insignificant with mechanical valves Reoperative aortic valve replacement has similar mortality to primary valve replacement Complications of mechanical prosthesis are more devastating than those of biological valves Mechanical valves can have substantial negative impact on daily quality of life

References

1. Nishimura RA, Otto CM, Bonow RO, et al. 2014 AHA/ACC guideline for the management of patients with valvular heart disease: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol 2014; 63:e57-185.
2. Oxenham H, Bloomfield P, Wheatley DJ, et al. Twenty year comparison of a Bjork-Shiley mechanical heart valve with porcine bioprostheses. Heart 2003; 89:715-21.
3. Hammermeister K, Sethi GK, Henderson WG, Grover FL, Oprian C, Rahimtoola SH. Outcomes 15 years after valve replacement with a mechanical versus a biologic valve: final report of the Veterans Affairs randomized trial. J Am Coll Cardiol 2000; 36:1152-8.
4. Stassano P, Di TL, Monaco M, et al. Aortic valve replacement: a prospective randomized evaluation of mechanical versus biological valves in patients ages 55 to 70 years. J Am Coll Cardiol 2009; 54:1862-8.
5. Brennan JM, Edwards FH, Zhao Y, et al. Long-term safety and effectiveness of mechanical versus biologic aortic valve prostheses in older patients: results from the Society of Thoracic Surgeons Adult Cardiac Surgery National Database. Circulation 2013; 127:1647-55.
6. Chiang YP, Chikwe J, Moskowitz AJ, Itagaki S, Adams DH, Egorova NN. Survival and long-term outcomes following biologic vs mechanical aortic valve replacement in patients aged 50 to 69 years. JAMA 2014; 312:1323-9.
7. Matchar DB, Jacobson A, Dolor R, et al. Effect of home testing of international normalized ratio on clinical events. N Engl J Med 2010; 363:1608-20.
8. Eikelboom JW, Connolly SJ, Brueckmann M, et al. Dabigatran versus warfarin in patients with mechanical heart valves. N Engl J Med 2013; 369:1206-14.
9. Puskas J, Gerdisch M, Nichols D, et al. Reduced anticoagulation after mechanical aortic valve replacement: interim results from the prospective randomized on-X valve anticoagulation clinical trial randomized Food and Drug Administration investigational device exemption trial. J Thorac Cardiovasc Surg 2014; 147:1202-10.
10. van Geldorp MW, Eric Jamieson WR, Kappetein AP, et al. Patient outcome after aortic valve replacement with a mechanical or biological prosthesis: weighing lifetime anticoagulant-related event risk against reoperation risk. J Thorac Cardiovasc Surg 2009; 137:881-5.
11. Bouhout I, Poirier N, Mazine A, et al. Cardiac, obstetric, and fetal outcomes during pregnancy after biological or mechanical aortic valve replacement. Can J Cardiol 2014; 30:801-7.
12. Oterhals K, Fridlund B, Nordrehaug JE, Haaverstad R, Norekval TM. Adapting to living with a mechanical aortic heart valve: a phenomenographic study. J Adv Nurs 2013; 69:2088-98.

Keywords: Allografts, American Heart Association, Anticoagulants, Aortic Valve, Aortic Valve Stenosis, Autografts, Biological Products, Bioprosthesis, Constriction, Pathologic, Contusions, Embolism, Endocarditis, Heart Failure, Heart Valve Prosthesis, Hematoma, Hemorrhage, Heterografts, Life Expectancy, Life Style, Pregnancy, Quality of Life, Referral and Consultation, Reoperation, Stress Disorders, Post-Traumatic, Stroke, Surgeons, Thromboembolism, Warfarin, Heart Valve Diseases

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