What is a mechanical valve in the context of heart valve replacement?

What is a Mechanical Valve?

A mechanical valve is a prosthetic heart valve constructed entirely from artificial materials—primarily pyrolytic carbon, titanium, and other synthetic components—designed to permanently replace diseased native heart valves with exceptional structural durability but requiring lifelong anticoagulation therapy. 1, 2

Structural Design and Components

Mechanical valves consist of three main design categories that have evolved since the 1960s 1:

• Ball-and-cage valves feature a silicone ball (poppet) that moves within a metallic cage structure, demonstrating excellent mechanical stability at follow-up intervals exceeding 30 years 2, 1

• Single tilting-disc valves (monoleaflet) contain one disc that tilts open at angles of 60-80° relative to the valve annulus, creating two orifices of different sizes 1

• Bileaflet mechanical valves (the most commonly used modern design) have two separate discs that open nearly synchronously at 75-90° angles, creating three orifices: two larger semi-circular lateral orifices and one smaller central slit-like orifice 1

The construction materials include a titanium or cobalt-chromium alloy ring with leaflets made of pyrolytic carbon, often with a graphite substrate or tungsten-loaded graphite components 1, 3.

Key Functional Characteristics

Modern bileaflet valves are mechanically stable and relatively hemodynamically efficient, with the structural stability translating to freedom from reoperation due to valve deterioration—the primary advantage over bioprosthetic valves. 2

Normal Physiologic Features:

• Built-in regurgitation of 5-10 mL per beat is normal and necessary, creating "washing jets" that prevent blood stasis and thrombus formation through small leakage backflow at pivot points and closure rims 1

• Disc motion should be brisk and consistent with each heartbeat, with specific opening angles depending on valve type and position 1

• On echocardiography, mechanical valves produce highly reflective echoes from metallic components with characteristic shadowing 1

Mandatory Anticoagulation Requirements

All patients with mechanical valves require lifelong anticoagulation with vitamin K antagonists (VKA) to prevent valve thrombosis and thromboembolic events, as the thrombogenicity stems from intravascular prosthetic material and abnormal flow conditions causing platelet activation. 2, 1

Specific INR Targets:

• Mechanical aortic valve (bileaflet or current-generation): INR 2.5 without additional risk factors 1

• Mechanical aortic valve with risk factors (atrial fibrillation, prior thromboembolism, LV dysfunction, hypercoagulable state, or older ball-in-cage design): INR 3.0 1

• Mechanical mitral valve: INR 2.5-3.5 indefinitely 4, 1

• Aspirin 75-100 mg daily should be added to warfarin for all mechanical valves 1, 2

VKA therapy reduces the incidence of valve thrombosis or embolism from 8.6 to 1.8 per 100 patient-years, with protective effects against valve thrombosis (OR: 0.11) and thromboembolic events (OR: 0.21) 2.

Clinical Trade-offs and Risks

The annual bleeding risk with appropriate anticoagulation is 1-2%, with persistent thromboembolism risk despite warfarin therapy, representing the fundamental trade-off for mechanical valve durability. 2

Additional Complications:

• Prosthetic valve endocarditis 2
• Hemodynamic inefficiency in smaller valve sizes 2
• Structural valve deterioration (rare—includes wear, fracture, poppet escape) 1
• Non-structural dysfunction from pannus ingrowth or tissue entrapment 1

Importantly, trials attempting to diminish or eliminate warfarin have demonstrated high rates of thromboembolism, confirming that the goal of producing a mechanical valve not requiring anticoagulation has not been achieved. 2

Age-Based Selection Criteria

• Patients <50-60 years: Mechanical valves are reasonable as bioprosthetic valves have 50% failure rates by 10 years in patients under 40, with predicted 15-year reoperation risks of 22% at age 50,30% at age 40, and 50% at age 20 1, 2, 1

• Patients 50-70 years: Either valve type is reasonable, though uncertainty exists in this age range 1

• Patients >65-70 years: Bioprosthetic valves are generally preferred as mechanical valve durability exceeds life expectancy and older patients face higher bleeding risks with anticoagulation 1

1, 2, 4, 3, 5, 6, 7