What's the difference?
The heart is a muscular pump. It keeps the blood circulating through the body through its continuous pumping action, allowing the blood to supply the organs with nutrients and oxygen.
The left side of the heart pumps blood that is rich in oxygen, while the right side receives blood that is low in oxygen from the veins and pumps it to the lungs to be re-oxygenated. The left and right sides of the heart each consist of two chambers: the upper chamber or atrium, and the lower, main chamber, the ventricle. The atria are thin-walled and serve primarily as reservoirs for blood received from the body and lung, while the two ventricles each have a thick, muscular wall that is responsible for most of the pumping action.
What the four heart valves do?
The four heart valves stand guard at the entry and exit points of the chambers, and keep the blood flowing in one direction. Oxygen-depleted venous blood from the body arrives at the right atrium via the inferior and superior vena cava, passes through the tricuspid valve into the right ventricle, and then from there via the pulmonal valve into the pulmonary artery and the lung, where it is enriched with oxygen and passed back to the left atrium. From the left atrium it flows through the mitral valve into the left ventricle and is pumped through the aortic valve into the aorta, and hence through the systemic circulation.
In a lifetime these valves will open and close more than two billion times.
The four heart valves act like doors which open and close in concert to keep the blood moving in one direction. They consist of flexible, thin but extremely robust flaps of tissue which have to withstand stretching and pressure with every heartbeat.
Each day the human heart beats approximately 100,000 times. In a lifetime this amounts to more than two billion heartbeats.
One or more heart valves can be affected by disease to the point where the flow of blood through the heart is compromised. In the advanced stage of valvular disease, intervention with a cardiac catheter or a heart operation is the only way of remedying this situation.
Mechanical heart valves are minor miracles
Prosthetic heart valves differ from one another in their properties, including their durability, their thrombogenicity (formation of small numbers of thrombi), their haemodynamic profile (the way blood behaves as it passes through the heart valve) and the amount of noise they generate. Since most implanted heart valves are of the mechanical variety, we will deal only with this type of valves.
Mechanical replacement heart valves can be divided into those with peripheral blood flow (where blood flows along the inside edge of the valve) and those with central blood flow (where blood flows through the middle).
Mechanical prostheses with a central closing mechanism and peripheral blood flow are divided into two groups: the caged-ball prosthesis (Starr-Edwards, Smeloff-Cutter) and the caged-disk prosthesis, which is now no longer manufactured.
The caged-ball prosthesis – a first-generation mechanical heart valve with peripheral blood flow – consists of a metal ring and 3 or 4 symmetrically arranged metal struts forming the cage. The ring and struts consist of a polished chromium / nickel / cobalt / molybdenum alloy (stellite). The sewing ring consists of a teflon-polypropylene mesh enveloping the metal ring except for its inner surface which is exposed to the flow of blood. The poppet is a silicone rubber ball impregnated with barium sulphate (2 % by weight).
Mechanical prostheses with central blood flow have tilting disks or bileaflets as the closing system.
Construction of a bileaflet prosthesis; here from St. Jude Medical heart (with central blood flow):
The closing mechanism consists of a graphite core impregnated with tungsten (5-10 % by weight) in order to afford some contrast on x-rays. The graphite core is completely enveloped in pyrolite (carbon). The mounting consists of pyrolite. The closing mechanism is formed by two biplanar semicircular leaflets which, in the closed position, form an obtuse angle of 120° and, when fully opened, are at an angle of 85% to the ring plane. The sewing ring by which the valve is attached in the appropriate position consists of dacron-velours.
Construction of a mono-tilting disk prosthesis, or simply tilting-disk prosthesis; here from Medtronic Hall (with central blood flow):
The circular, biplanar tilting disk consists of pyrolite with a circular central opening and x-ray contrasting impregnation – similar to the tilting disks from St. Jude Medical. The housing and mounting mechanism for the tilting disk are made of titanium (from a single cast). The mounting allows the tilting disk to rotate freely. As it opens, the closing mechanism moves along a raised hook passing through the centre of the disk. In the mitral position this tilting disk has an opening angle of 70° and, in the aortic position, of 75° to the ring plane.
Mechanical heart valves come in different sizes
The valves described come in different sizes. This is all to do with the fact that people of different sizes have variously sized heart valves. Prosthetic heart valves are manufactured in sizes ranging from 19 mm to a maximum size of 31 or 33 mm.
How is valve size measured?
The size quoted for a valve is its external diameter with the sewing ring compressed. An over-size sewing ring is to the detriment of the internal diameter determining the absolute flow rate. Manufacturers of mechanical heart valves are interested, therefore, in achieving maximum blood flow with each size of valve, in order to simulate as closely as possible the situation that existed prior to surgery.
When a heart surgeon replaces a diseased heart valve, he usually removes the diseased areas of tissue and replaces it with an implant as described above.
When choosing the size of a mechanical or biological heart valve, the heart surgeon looks at the size of the annulus (ring) which is created when the human heart valve is excised – i.e. the size of the patient’s natural heart valve.
Manufacturers of prosthetic heart valves have instruments that reproduce the dimensions of the valve, so that it is possible during the operation to determine the size of valve to be implanted. Valve sizes are in mm.
What effect does the size of the heart valve have on the subsequent course of events?
The heart’s pumping action is measured in terms of the difference in pressure that exists ahead of the prosthetic heart valve compared to the pressure behind it. This pressure gradient is on average between 10 and 20 mmHg. A natural heart valve does not, however, produce any significant pressure gradient ahead of and after the valve. Based on the size of the heart valve it is possible to calculate the pumping force of the heart.
- The larger the internal diameter of the heart valve, the smaller is the force needed to pump the blood.
- The smaller the annulus, the greater is the necessary pumping force of the heart.
- Raised blood pressure causes an increase in the rate of blood flow.
Why patients hear heart valve noises
The annulus of the prosthetic heart valve forms an unnatural barrier to blood flow. This is the reason why there are always slight turbulences behind the mechanical valve in aortic or mitral position. In the same way as with a swollen stream, greater turbulences occur as the flow of blood increases through the artificial valve. Not only the cardiologist can pick this up on his stethoscope; sometimes the wearer himself can hear the blood flowing and the tilting disks opening and closing – even without a stethoscope.
Virtually every mechanical heart valve is responsible for creating a certain level of noise. Often the noise decreases as the size of heart valve increases. In addition to noise actually produced by the valve, the human body’s so-called “resonance board” itself plays a role in propagating the noises produced in the mechanical heart valve. It is understandable, therefore, that patients perceive the noises differently.Based on life quality data obtained in the ESCAT study, it was found that approximately 10% of patients are bothered immediately post-operatively by the heart valve noise. Of this patient collective 80% become accustomed to the noise; only 20% of the collective, i.e. 2% of all patients who have undergone replacement mechanical heart valve surgery, find the noise bothersome. Women suffer more from the valve noise than men. In particular the 20-to-30 age group and the 40-to-50 age group develops a strong aversion to the heart valve noise.
What are the advantages of a mechanical heart valve?
The advantage of a mechanical heart valve is the fact that it will last almost forever and that it has flow characteristics that closely approximate those of natural heart valves. With long-term oral anticoagulant medication, thrombogenesis (formation of thrombi) by the heart valves no longer has any role to play.
Other ESCAT study results showed that, with anticoagulation self-management, the incidence of thrombus formation in the first 2 years is just 0.2% per patient year. This is a very low complication rate.
PD Dr. med. Heinrich Koertke, Heart- and Diabetes-Center North-Rhine Westphalia, Georgstr. 11, 32545 Bad Oyenhausen (Germany) (2004).