Acquired Cardiac Surgery

Surgery for Acquired Cardiac Surgical DiseaseHistory

Surgery for aquired heart disease was initially performed on a “closed heart” for Mitral Stenosis. Other closed operations such as pulmonary valvotomy were also performed. Initially attempts at open surgery for simple procedures such as ASD using hypothermia were attempted The advent of cardiopulmonary bypass in 1953 initially with a crude form of apparatus and over the years with much more developed and safer equipment has led to surgery for valve disease, coronary artery disese and myocardial dysfunction in the form of ventriculoplasty and transplantaion.

The Principles of Cardiopulmonary Bypass

Cardiopulmonay bypass literally means Heart-Lung bypass. The “Heart-Lung machine” performs the action of the heart by pumping blood, usally by means of a roller pump around the body at a rate assessed from a normogram calcualated from tables of patient height and weight. A centrifugal pump (similar in principle to the water pump of a car or washing machine) may also be used and has advantages in patients requiring long term support. Usually the blood is pumped continuosly but most bypass machines are now able to mimic the heart and produce pulsatile perfusion.

The function of the lung is taken over by an oxygenator, initially this was done with an air blood direct interface, but now using a membrane oxygenator which has a membrane separating blood from an oxygen-air-anaesthetic gas mixture allowing respiratory exchange to take place.

The heart is usually protected during periods of bypass, when the heart is separated from its blood supply by a clamp on the aorta, by being cooled and paralysed by cardioplegia. Cardioplegia literally means heart paralysing. By perfusing the heart with a fluid of compatable PH with high levels of potassium either as a crystalloid solution or mixed with oxygenated blood, the heart can be protected and its function preserved for many hours while intricate procedures can be performed. Similar solutions are used to preserve hearts fro translantaion so that they can be trasported relatively long distances to reach suitable donors.

Coronary artery surgery where diseased vessels are bypassed is often performed without the use of bypass or with the heart beating to minimise some of the deleterious effects which cardioplegia may have on cardiac muscle

The ability of a heat exchanger in the bypass circuit to change the patient’s temperature can be used to produce moderate to profound hypothermia. This gives a greater margin of safety during periods of bypass and at temperatures around 18-22C, the heart lung machine can be turned off for periods of over 30 minutes so that operations on the aortic arch or other critical vessels can be performed in a relatively safe manner. This is called hypothermic circulatory arrest. (This principle has also been used to resuscitate children who have fallen into cold water and ice covered lakes in winter with rapid body cooling, rather then drowning and apparently dead children have been brought back to normothermia using bypass and survive without major brain injury)

Surgery for valve disease

Surgery may be by valve repair or valve replacement.

Valve Repair

The mitral valve is the commonest valve to repair. Reported results for degenerative mitral disease such as Barlow’s Disease are very good. However results for rheumatic valve disease are less good in most centres. The main reason for this appears to be the fact that rheumatic heart disease is ongoing even without further episodes of acute rheumatic fever. Further thickening of the valve continues after the surgery which frequently gives less satisfactory results. The advantages of repair are that the ventricular morphology is maintained with a better ventricular function, there is usually no need for anticoagulation and there is no risk of acute dysfunction due to clotting or mechanical failure of the valve. It is usually less expensive that replacement.

Valve Replacement

Replacement valves are either mechanical or biological.

Mechanical valves are generally made out of pyrolytic carbon leaflets, either single or bileaflet with a metallic ring housing the leaflets with a cloth sewing ring around the outside of the valve. Pyrolytic carbon is very smooth and has only a small tendency to form clots on its surface. These usually start around the hinge mechanism of the valve and require that the patient is kept on long term anticoagulation with an INR of 2-4. These valves sometimes get infected and also sometimes develop a leak around the sewing ring. Their function may also become impaired by tissue growth arond the annulus (the ring housing the valve with itscloth sewing ring) which may also lead to valve dysfunction.

Surgery for coronary artery disease

Elective coronary artery bypass grafting (CABG) has become one of the most frequently performeed surgical procedures in the world with elective operative mortality between 1 & 4 % according to the type of disease.

Emergency surgery carries a much higher morbidity and mortality.

CABG improves the blood flow to the coronary arteries by placing vascular grafts using the internal mammary artery, reversed long saphenous vein and other arteries such as radial artery to take more blood flow beyond narrowings in the coronary arteries to increase blood supply to the myocardium. This is an additional blood supply and does not affect the nature of coronary artery disease directly. The treatment of coronary artery disease remains diet, cessation of smoking, strict management of diabetes, management of dyslipidaemia and exercise to help with maintaining general fitness and weight control.