Artificial valves and circulatory support

Cardiac Anatomy and Physiology

• The human heart has two pumping chambers, namely ventricles, and two filling chambers referred to as atria
• The heart functions as a pulsatile pump
• Muscular contraction of ventricles and atria drives the heart
• The heart is designed for positive blood displacement through two circulatory systems

Function of Ventricles

• The right ventricle pumps blood into the pulmonary circulation
• In pulmonary circulation, blood becomes oxygenated
• The left ventricle pumps blood into the systemic circulation
• Systemic circulation ensures that oxygenated blood reaches tissues throughout the body, enabling oxygen transport

Heart Valves

• Heart valves ensure that blood flows correctly from the ventricles, from each atrium and to the associated ventricle
• These valves prevent backflow

Specific Heart Valves

• The tricuspid valve regulates blood flow between the right atrium and right ventricle
• The pulmonary valve regulates blood flow from the right ventricle to the pulmonary arteries, which take blood to the lungs for oxygen
• The mitral valve allows oxygen-rich blood from the lungs to flow from the left atrium to the left ventricle
• The aortic valve allows oxygen-rich blood to flow from the left ventricle to the aorta
• All heart valves operate by means of a pressure gradient

Circulatory System

• Blood travels from the heart to the body through a network of arteries, arterioles, and capillaries
• Blood returns to the heart via venules and veins
• Vessels that carry oxygen-rich blood are shown in red, while those carrying oxygen-poor blood are in blue

Cardiac Output

• Cardiac output (CO) indicates the amount of blood ejected by each ventricle per minute
• CO = Heart Rate (HR) x Stroke Volume (SV)
• Resting CO averages 75 beats/min x 70 ml/beat, which equals 5250 ml/min, generally about 4 to 6L/min
• During intense exercise, CO can climb to 21 L/min for a fit person and up to 35 L/min for an elite athlete

Stenosis and Angioplasty

• Stenosis, or the narrowing of an artery, significantly decreases blood flow
• Angioplasty is frequently required to relieve the blockage caused by stenosis

Angiography

• Angiography uses medical imaging techniques to view the inside of blood vessels and organs, especially arteries, veins, and heart chambers
• Traditionally, this involves injecting a radio-opaque agent into a blood vessel, followed by X-ray imaging
• Blood vessels are typically accessed via the femoral, radial, or brachial artery

Angioplasty Procedure

• Angioplasty uses a balloon to widen constricted or blocked arteries
• Arteries get blocked or narrowed due to atherosclerotic plaque composed of fat accumulation on artery walls
• Angioplasty uses a balloon catheter: a small, flexible tube that contains a balloon near its end
• The balloon catheter goes into or near the blocked area, then the balloon inflates in order to open the vessel and restore blood flow
• Patients are sedated to prevent blood clot formation during angioplasty

Stents

• Stents—hollow metal mesh tubes—are often implanted at the site of narrowing or blockage to keep the artery open and prevent it from narrowing again
• When a stent is placed, it is collapsed to a small size and positioned over the balloon catheter, then moved to the blocked area
• Once the stent is in place, the balloon is inflated and the stent expands, remaining in the artery to hold it open
• Stent recipients must use blood thinners to lower risk of blood clots which can block the stent and artery

Aneurysm

• Blood moving through the systemic circulation can encounter widened sections as a result of aneurysms
• An aneurysm refers to a localized bulge that stems from the weakening of an arterial wall
• Aneurysms frequently develops in the abdominal aorta and thoracic aorta

Surgical Aneurysm Repair

• Compared to a normal aorta, the aortic media is notably thinner in aneurysms
• Surgical intervention is advising when an abdominal aortic aneurysm (AAA) reaches around 5 cm in diameter to prevent rupture
• Traditionally, surgical repair involves open surgery to reconsturct the aorta with a synthetic graft and 3 months to recover

EVAR (Endovascular Aneurysm Repair)

• EVAR involves deploying grafts through cardiac catheterization in a minimally invasive way
• Catheters guide and deliver a stent-graft made of fabric supported by metal mesh through blood vessels to the aneurysm site
• The stent-graft stays collapsed during delivery, expanding upon reaching the aneurysm and attaching to the aortic wall
• The graft allows blood to flow through it, which avoids the aneurysm.
• The aneurysm decreases because blood pressure affects the graft wall

Heart Valve Failure Types

• There are two potential origins of heart valve failure, each requiring years to develop
• One mechanism involves valve stenosis, or narrowing; It often links to atherosclerosis, creating outflow area
• 2-D ultrasound and Doppler flow probe-based echocardiograms can diagnose this condition, which may show no symptoms until the orifice is reduced up to 50% of its area

Valvular Stenosis

• Assessment of valvular stenosis involves measuring the pressure gradient and flow through the affected valve

Heart Valve Failure: Prolapse

• Valve failure can be from weakening valve leaflets, called valve prolapse
• Valve prolapse involves abnormal displacement of thickened mitral valve leaflet into the left atrium during systole
• Backflow turns turbulent, easily heard through stethoscope or seen through Doppler ultrasound

Prosthetic Heart Valves

• The mitral and aortic valves of the left heart receive the most replacements because the left heart withstands higher pressures
• Artificial heart valves have been available for over 50 years; earlier models contain either a caged ball or tilting disk

Caged Ball Valve

• The ball valve consists of silicone in a stainless steel cage and annular ring
• The ball valve is durable and strong
• The ball valve may cause hemolysis and thrombosis as it closes with significant force

Tilting Disk Valve

• The tilting-disk valve was created to restore central blood flow, which the ball valve lacked
• These valve has a single circular disc restrained by two metal struts and a metal ring

Function of Tilting Disk Valve

• Struts keep disk from leaving the device
• Disk movement depends on the same idea as the ball valve, using a disc instead of a ball
• Tilting disc valves reach an angle of 60° and a rate of 70 beats per minute.
• The angular opening of this valve reduces blood cell damage
• Though the struts are an advancement over the ball design, struts of tilting disc valves tend to crack over time

Bileaflet Valve

• Many modern prosthetic heart valves are by nature bileaflet
• The bileaflet valves reach 80° and use pins to pivot and connect the valve leaflets to the annular ring

Bileaflet Valve Properties

• The ring is made of stainless steel or titanium
• The leaflets tend to consist of pyrolytic carbon offering strength and durability
• The design has two leaflets. Its closing force is less than valves with a single tilting disk and significantly less than that of a ball valve.
• The best prosthetic heart valves minimize hemolysis (red cell destruction) and thrombosis
• The best prosthetic heart valves provide a life span of 30–40 years

Surgical Implantation of Prosthetic Heart Valves

• Surgical implantation of prosthetic heart valves is now efficient
• The diameter of the patient is evaluated with echography
• The right-sized prosthetic valve gets placed in a valve holder
• The patient receives cardiopulmonary bypass (heart-lung machine) while the original valve gets removed
• A surgeon places the prosthetic valve inside of the orifice and sows the sutures and curved needle around the valve
• The area get check for potential leaks and get additional sutures
• Mechanical valve replacement recipients remain on continuous warfarin anticoagulation therapy for the lifetime of the valve, averaging at 30-40 years

Transcatheter Heart Valves

• Transcatheter valves mark the latest advancement in heart valve engineering
• Current investigated designs include(A) the Edwards SAPIEN and (B) the Medtronic CorValve heart valves.
• The SAPIEN valve presents as a bovine prosthesis atop a balloon-expandable stent
• The CorValve is a valve from a pig atop a self-expanding stent
• Both get deployed via catheter

Transcatheter Valves

• Transcatheter valve was designed for patients needing surgical valve replacement
• Transcatheter valve replacement replaces a narrow valve invasively, without eliminating damaged valves
• The TVR approach, the fully collapsible replacement valve goes through a canula
• Once the valve gets place and expanded, it pushes the old valve leaflets out of the way and tissues takeover regulating blood flow