valves come in sizes from 19 mm to 29 mm. Stentless valves are directly sutured at the aortic root. The major advantage of stentless valves is that they limit patient–prosthesis mismatch (when the area of the prosthetic valve is too small in relation to the size of the patient, increasing the pressure inside the valve) and can be helpful when dealing with small aortic root. However, stentless valves take more time than stented valves to implant. Mechanical valves Mechanical valves are made from synthetic materials, such as titanium or pyrolytic carbon. They are more durable than tissue valves, typically lasting 20–30 years. However, the risk of blood clots forming is higher with mechanical valves than with tissue valves. As a result, people with mechanical heart valves must take anticoagulant (blood-thinning) drugs, such as warfarin, for the rest of their lives, making them more prone to bleeding. The sound of the valve can sometimes be heard, often as clicks, and can be disturbing. Valve selection Valve choice is a balance between the lower durability of tissue valves and the increased risk of blood clots and bleeding with mechanical valves. Guidelines suggest that patient age, lifestyle and medical history should all be considered when choosing a valve. Tissue valves deteriorate more rapidly in young patients and during pregnancy, but they are preferable for women who wish to have children because pregnancy increases the risk of blood clots. Typically, a mechanical valve is considered for patients under 60 years old, while a tissue valve is considered for patients over the age of 65 years. Surgical replacement Surgical aortic valve replacement is conventionally done through a median sternotomy, meaning the incision is made by cutting through the breastbone (sternum). Once the protective membrane around the heart (pericardium) has been opened, the patient is cannulated (aortic cannulation by a cannula placed on the aorta and a venous canulation by a single atrial venous cannula inserted through the right atrium). The patient is put on a cardiopulmonary bypass machine, also known as the heart–lung machine. This machine breathes for the patient and pumps their blood around their body while the surgeon replaces the heart valve. Once on cardiopulmonary bypass, the patient's heart is stopped (cardioplegia). This can be done with a Y-type cardioplegic infusion catheter placed on the aorta, de-aired and connected to the cardiopulmonary bypass machine. Alternatively, a retrograde cardioplegic cannula can be inserted at