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CARDIAC SURGER

Cardiac Surgery

Surgical Anatomy of the Heart

Shape of the heart is that of a three-sided pyramid located in the middle

mediastinum, enclosed by the serous and fibrous pericardium. The right border

consists of the right atrium, the inferior border is made up mostly by the right

ventricle, with a small portion of the left ventricle which forms the apex. The

left border formed mostly by the left ventricle & partially by left atrium, The

anterior (sternocostal) border is made mainly by the right ventricle, the

diaphragmatic surface is 1/3 by RV & in 2/3 by LV and the posterior the (base)

of the heart is mainly by the left atrium.

Cardiac chambers and valves:

The right atrium is an elongated chamber lies between the opening of the

superior and inferior venae cavae while the left atrium lies behind it. It drains

the venous return form heart itself through the coronary sinus. Its main

communication with the RV is through the tricuspid valve. The left atrium

receives pulmonary venous drainage via pulmonary veins which drain into the

posterior portion of the left atrium. The bicuspid mitral valve(anterior &

posterior cusps) guards the left side atrioventricular connection.

The right ventricle is located anteriorly. The inflow is via the tricuspid valve.

Flow enters the right ventricle into a large sinus portion and a smaller

infundibulum or outlet portion just proximal to the pulmonary valve. A septum

lies between the inflow and outflow portions of the right ventricle and thus lies

adjacent to the pulmonary valve (3 cusps: left ant, right ant & posterior) which

forms the outlet. The LV is characterised by its muscular wall which is three

times thicker than that of the RV. The aortic valve is composed of three

smilunar cusps (non-coronary, left coronary & right coronary), forming the

outlet of the LV.

Blood supply of the heart: These consist of a left main coronary artery (LMS ~

1.5 cm ) which bifurcates to the circumflex coronary (CX) artery and left

anterior descending coronary artery (LAD). The orifice of the left main coronary

artery lies in the left coronary sinus. The orifice of the right coronary (RCA)

artery arises from the right coronary sinus. The right coronary artery has a first

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branch called an acute marginal artery which supplies the free ventricular

surface and may also give off a conal branch which supplies the infundibulum of

the right ventricle. The right coronary artery continues to bifurcate into a

posterior descending artery (PDA) as well as a

continuing posterior ventricular branch, also

called a posterolateral branch or LV extension

branch. The dominance of the coronary arteries is

determined by which side, i.e. right or left,

supplies the posterior descending artery. In 90%

of people, the posterior descending artery is a

continuation of the right coronary artery; in 10%

it is a continuation of the circumflex coronary

artery or the left anterior descending coronary artery.

Basic Principles of Cardiopulmonary Bypass (CPB) & Myocardial

protection:

Cardiopulmonary bypass is a process by which systemic venous blood is taken

from the patient, transferred to a pump oxygenator and delivered back to the

arterial circulation of the patient. Cardiac

surgery is unique in that an

extracorporeal circulation system is

required for open cardiac. The bypass

circuit consists of a single venous

cannula in two stages or two different

venous canulas according to the type of

surgery, the venous line drains down to a

reservoir the. The reservoir’s blood then

enters a hollow fiber membrane pump

oxygenator (Fig) with a temperature regulating device in the proximal portion of

the system and the oxygenator just distal to this. Once the blood passes through

the membrane where the CO2/O2 exchange takes place, the blood travels

through a 40 micron filter and then back into the arterial circuit of the patient

and then to the ascending aorta via the arterial canula. The filter serves to

remove particulate and gaseous emboli. The arterial circuit has a purge line

which can remove gross air. Cooling in cardiopulmonary bypass is done at

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approximately 1°C per minute. The advantages of cooling are that it decreases

the metabolic requirement of the body organs, in particular, the brain and the

heart. However, disadvantages are that it may increase bleeding after coming off

bypass because of stunning of the coagulation enzyme systems, and it may

induce myocardial edema by impairment of enzyme system

Alternative uses of CPB machine:

1. Rewarming from profound hypothermia,

2. Resuscitation in severe respiratory failure,

3. As an adjunct in pulmonary embolectomy,

4. In single and double lung transplantation

5. In cardiopulmonary trauma

6. Resection of highly vascular tumours

7. Tumours invading large blood vessels (e.g. renal or hepatic tumours

extending into inferior vena cava, right atrium or even pulmonary arteries

Methods of Myocardial protection:

Principle: To obtain a bloodless operative field, the ascending aorta is usually

cross-clamped once CPB has been established and blood is diverted away from

the heart. The heart ceases to eject and, as a result of inhibition of coronary

blood flow, becomes anoxic. Permanent myocardial damage will develop within

30-45 min. Therefore, most cardiac operations require some form of myocardial

protection.

1. Cardioplegic arrest

Most methods now involve combinations of topical cooling by ice appilcation

and intracoronary infusions of cardioplegic solutions. Most solutions contain

potassium as the arresting agent. Potassium arrests the heart in diastole by

depolarisation of the membrane. Cold (4-10 ° C) isotonic crystalloid or chilled

blood solutions aid myocardial protection by reducing metabolic requirements

through local hypothermia.

2. Intermittent cross-clamp fibrillation

Ventricular fibrillation is induced by a small electrical charge. The heart does

not eject and is relatively still, but not bloodless. To perform an operative

procedure such as coronary artery bypass grafts, the aorta is cross-clamped to

render the heart ischaemic. The heart can tolerate short periods (10-20 minutes)

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of intermittent ischaemia, providing the heart is reperfused and allowed to beat

in between.

3. Total circulatory arrest

The metabolic rate of all organs of the body is reduced by 50% with every 7°C

drop in temperature. So, with the pump switched off at 18°C, circulatory arrest

can be tolerated for 20-30 minutes.

Common incisions used to approach the Heart

1)Median Sternotomy

The most common approach for operations on the heart and aortic arch is the

median sternotomy. The skin incision is made from 1-2 cm below jugular notch

to just below the xiphoid process.

2) Bilateral Transverse Thoracosternotomy (Clamshell Incision)

The bilateral transverse thoracosternotomy (clamshell incision) is an alternative

incision for exposure of the pleural spaces and heart.

3) Anterolateral Thoracotomy

The right side of the heart can be exposed through a right anterolateral

thoracotomy. The patient is positioned supine, with the right chest elevated to

approximately 30 degrees by a roll beneath the shoulder.

4)Posterolateral Thoracotom

A left posterolateral thoracotomy is used for procedures involving the distal

aortic arch and descending thoracic aorta. With left thoracotomy, cannulation for