Mechanical and Tissue Heart Valves
Tissue vs Mechanical Heart Valve?
When heart valve disease progresses to the point that treatment by medicines does not provide relief from a patient’s symptoms, surgery to repair or replace the valves becomes the best alternative. If the surgery is not a repair, homograft or autograft, the choice is most likely between carbon-based mechanical valves and biological tissue valves.
The physician and patient will choose the type of valve, taking into account the patients’ overall condition and preference. Some of the patient factors considered are:
- Age and life expectancy: Although age is not a rigid indicator, the American College of Cardiology and American Medical Association guidelines indicate that mechanical valves are generally appropriate for a majority of patients 65 years old and younger, or patients already on coumadin.
- Other diseases: Lung, liver and kidney disease, diabetes, cancer and other chronic conditions can affect life expectancy and alter the age criterion.
- Heart and vascular system condition: Coronary, carotid and peripheral artery diseases and heart rhythm disturbances can affect valve selection.
- Patient lifestyle and preference: Life situations or intangible factors often influence valve selection.
Tissue Heart Valves
Tissue valves are harvested from pig heart valves (porcine) or cow heart sac (bovine). These tissues are treated and neutralized so that the body will not reject them. Some are mounted on a frame or stent; others are used directly (stentless).
The lifetime of a tissue valve is typically 10 to 15 years, often less in younger patients. Over this time the valve will likely be degenerating to the point of requiring replacement. Because valve replacement surgery carries a significant risk of death, patient life expectancy is a major criterion in considering a tissue valve.
With relatively high pressure gradients, stented tissue valves do not perform as well as the native valve in terms of blood flow. Tissue valves without frames (stentless) improve blood flow, although they are more difficult to put in place and are not usable in all cases.
The primary advantage of tissue valves is their lower requirement for anticoagulation therapy, which reduces the incidence of bleeding. For the majority of tissue valve patients, taking an aspirin a day is sufficient anticoagulation therapy. Many patients with tissue valves, however, do not enjoy this benefit due to anticoagulation requirements for other heart or vascular conditions.
Mechanical Heart Valves
The most widely used mechanical valves are made from pyrolytic carbon, which has been used for over 30 years. Most are bileaflet designs, meaning that they employ two carbon “leaflets” to regulate flow to a single direction.
The primary advantage of mechanical valves is that they will last a patient’s lifetime. Mechanical valves are preferred for patients with life expectancies beyond 10-15 years because they eliminate the mortality risk inherent in the replacement of a worn out tissue valve. The best mechanical valves, such as the On-X valve, have excellent flow performance, rivaling that of the native valve even in the small sizes.
The main drawback of mechanical valves has been their requirement for warfarin anticoagulation therapy, with its accompanying risk of bleeding. Bleeding events are rare but are potentially fatal. With properly managed anticoagulation therapy, rates are low for both bleeding and clotting.
Mechanical valves can sometimes be audible when opening and closing. The sound level varies with the patient, but among those noticing it, a small number of patients find the sound disturbing, while others find it reassuring.
Valve selection is a balancing act between the major positives and negatives of both valve types:
- The limited durability of tissue valves (performance degradation and the stress or mortality of reoperation).
- The requirement for lifelong anticoagulation therapy of mechanical valves.
The decision is reached differently for each patient as many factors are considered and weighed but the goal is always to improve quality of life while minimizing risk.