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Surgery for valvular heart disease

HOW THE OPERATION IS DONE

The actual opening of the chest, exposure of the heart and insertion of the bypass pipes are effectively the same as for CABG. Cardio­pulmonary bypass and myocardial protection are needed, but conduits are not (unless you are having a combined valve and CABG operation, of course). Once the patient is ‘on bypass’, and myocardial protection instituted, the specific valve replacement part of the operation can be performed. Before the operation is even started, however, a very important decision needs to be made, and that is choosing which valve to put in.

Choice of valve

There is quite a wide range of artificial heart valves on the market, broadly divided into two categories:
1. Mechanical valves

2. Tissue valves (xenografts).

Within each of these categories there are several different designs. Within the mechanical valve subgroup, the most common types of valves found use single or double tilting discs (mono-leaflet and bi­leaflet, respectively). The ‘ball and cage’ design which was originally the most popular has been largely superseded by these ‘bi-leaflet’ valves. Tissue valves may be preserved whole valves from an animal (usually a pig), or may be constructed from the pericardium (the membrane surrounding the heart) of an animal (usually a cow).
The feature that all valves share is that they allow the passage of fluid only in one direction, and prevent it from going the other way. The choice of which valve to use is a combined decision between the surgeon and the patient, because there are many different factors to be considered, and each patient is different and has individual needs.

Broadly speaking, mechanical valves probably last at least 15 to 20 years, but quite often much longer. Tissue valves are a newer invention, and it is not known how long they last, but it is probably a shorter time of around 12 to 15 years. The newest generation of tissue valves may well last even longer than that according to laboratory tests, but unfortunately they haven’t been around long enough to measure how long they actually last. When a valve has worn out, a re-do operation is needed to take it out and replace it with yet another new one.

• Mechanical valves:

These consist of the valve itself mounted within a special ring which is the means by which it is sewn into the heart. The valve component is made of a strong, solid material which has been put through strict testing. The important thing is that it is an artificial material and, like any other artificial material introduced into the bloodstream, is prone to cause clots to develop upon its surface. Clots that form in the bloodstream can be very dangerous, because if they dislodge they can shoot off to distant places such as the brain, causing a stroke. Therefore, if you have a mechanical valve you need to take an anticoagulant drug to thin the blood. This is usually warfarin, and is taken as a small tablet once a day.
If you have a mechanical valve it is important to understand that this commits you to taking warfarin for life. The only difference the warfarin will make to you is that you may bleed a little longer if you cut yourself. This can be a nuisance to some people with physical jobs. It also means that you will need to attend your doctor’s surgery or a clinic at regular intervals to check the thinness of the blood. Many people having valve replacements have an irregular heart beat called atrial fibrillation as a result of enlargement of the heart over the years. These people will usually be taking warfarin anyway, and there­fore a mechanical valve is probably the best choice. The other potential disadvantage of mechanical valves is that they make a slight clicking noise, and this can be disconcerting to some people. However, most people can hear nothing at all, or perhaps only at times of complete silence.

• Tissue valves:

These are also called xenografts (xeno- comes from the Greek for ‘foreign’), and comprise a complete pig’s valve, or may be made from pericardium. Whichever of these the valve is made from, they are all mounted in a similar sewing ring to that of mechanical valves. Pigs have similarly sized hearts and valves to humans, and their valves have been used successfully for many years. All xenografts are specially treated to prevent the body from rejecting them, and to make them stronger. They do not stimulate clot formation, so you don’t have to take warfarin for the rest of your life, although you are usually put on it for about six weeks to allow the valve to settle in. Tissue valves are also completely silent.
The choice of valve depends on weighing up the longer durability of a mechanical valve with its disadvantages. In general, if you think your life expectancy is less than 10–15 years, and you will not get as far as a re-do operation, then it may be best to have a xenograft and its lack of disadvantages. Otherwise, it is a decision that is made between you and the surgeon, and is tailored to your individual needs.

Aortic valve replacement

Once the patient is on cardio­pulmonary bypass (CPB), cardio­plegia is given to stop the heart, and then the aorta can be opened to gain access to the valve. The old valve is carefully cut out and discarded. The aperture that is left is then measured with an instrument called a ‘sizer’, because each type of valve comes in a wide range of sizes. The biggest valve that will fit is chosen, and the surgeon then sews the valve ring into place exactly where the old one was removed, and closes the aorta over it. When the perfusionist has warmed the patient up, the heart is restarted and CPB stopped. The pipes can then be removed and the chest is closed as for CABG.

Mitral valve replacement

Mitral valve surgery is technically very similar to aortic valve surgery. The main difference is that the left atrium needs to be opened to expose and cut out the old valve, and there are a number of different approaches to this. The other feature of mitral valve surgery is that, when opening the atrium, there is a chance of disturbing the nerve conduction fibres from the natural pacemaker. This may cause rhythm problems when coming ‘off bypass’ or after the operation, so to guard against this the surgeon attaches some temporary ‘pacing wires’ to the heart. These come out of your skin just below the chest scar, and can be connected to a pacemaker at any time if required. Usually, they are not needed, and simply (and painlessly) pulled out by a nurse on the ward before you go home. It is not uncommon for temporary pacing wires to be needed in any of the other forms of heart surgery, and some surgeons attach them to nearly all patients. The important thing to realise is that they are not usually needed for more than a day or two, if at all, and they are simply a precaution while the heart recovers from the operation.

Valve repair

A technique that is becoming increasingly popular is valve repair and this is far more common in the mitral valve. Aortic valve repair is still very rare, mainly as a result of the nature of the disease process that causes aortic stenosis (narrow­ing). Mitral valve repair should always be considered in cases of mitral regurgitation, and is very often possible, depending on the experience of the surgeon. In these cases, a variety of repair techniques is used, including reattaching the cords to the leaflets, cutting out excessive floppy valve tissue, and reducing the overall size of the valve using a special ring. The ultimate aim is to reconstruct a functional valve, which is then tested during the operation. If it is not functional, and the repair has not worked, then it is still possible to replace the valve as above. For valve repair, you still need to undergo a similar operation to replacement and go on the heart– lung machine, but your own valve is repaired rather than replaced. This has obvious advantages, because it means that you preserve your own tissues, and do not have foreign materials implanted in your body.

Other valve surgery

The pulmonary and tricuspid valves are much less frequently operated on than the aortic and mitral ones, but the operative principles outlined above are the same.
There are other operations on your aortic valve, where the whole section of your aorta containing the valve is cut out and replaced with a synthetic tube containing a mech­anical valve or sometimes a whole section of animal or human aorta containing the natural valve. These are quite complex operations and involve some different principles to the ones we have discussed.