Cardiac Surgery د.محمد جلال Lec: 1 5/10/2015
Cardiopulmonary Bypass (CPB)
Is the method by which the physiological function of the heart and lung is replaced by a machine in anesthetized patient for a limited period of time during cardiac operation, it is required for most of cardiac operations.
The CPB machine is composed mainly of three parts:-
Part one: act as a mechanical pump to pump the blood instead of the heart during the period of cardiac stand still.
Part two: Called the Oxygenator which acts instead of patient's lung to provide O2 and remove CO2 i.e. to do the gas exchange function during cardiac operation.
Part Three: Called heat exchanger which control body temperature by cooling the blood at the beginning and during the cardiac operation and then by re-warm the blood after the competence of operation to return to normal body temperature. The benefit of cooling the body during the operation is to decrease the basal metabolic rate and O2 requirement this allows reduction of blood flow rate (cardiac output) of the mechanical heart and when blood flow rate is decreased the surgeon will get a bloodless field in human heart to facilitate the doing of intended procedure.
The reduction of temperature during cardiac operation is done according to the length and complexity of operation, the longer and more complex the operation the more the reduction in temperature is needed.
Hypothermia is categorized to 4 Levels:-
Mild Hypothermia: - the body temperature is decreased to 32C at which machine flow rate can be decreased to 2 Litre/minute/Square meter.Moderate Hypothermia: - the temperature is decreased to 28C and flow rate decreased to 1.5 Litre / minute/ m2.
Deep Hypothermia: -the temperature is decreased to 23C and flow rate decreased to 1 Litre/minute/m2.
Profound Hypothermia: - the temperature is decreased to 18C and the flow rate decreased to 0.5 Litre /minute/m2.
HOW CPB IS STARTED:-
After the patient is anesthetized in supine position the chest is opened by Median Sternotomy which is the classical route for cardiac operation
The sternum is divided by a mechanical saw.
The pericardium is opened.
The patient is heparinised (given Heparin) the dose is 4 mg/Kg.
The Ascending Aorta is cannulated by special size cannula according to body weight and surface area, the cannula is fixed to aorta by a purse-string suture.
The venous cannula is inserted to the venous circulation through the right atrial appendage, after the insertion of the arterial and venous cannulae both are connected to CPB machine by special plastic tubes which are designed to decrease damage to blood element.
When the circuit is completed CPB is started by an order from surgeon, the blood is cooled to the temperature necessary for operation.
A cross clamp is put on the Ascending Aorta proximal to the site of aortic cannula and a cold cardioplegic solution is infused to the aortic root by a small needle, the cardioplegic solution contains high potassium concentration and will induce diastolic arrest of the heart ,then the surgeon can do the operation.
When the operation is finished, the blood is re-warmed, Aortic clamp is removed and the heart will return to beat again either spontaneously or by D.C shock.
When the general condition of the patient becomes stable, Protamine is given to reverse the heparin effect, the dose is 1 mg Protamine for each 1 mg of heparin.
CPB is stopped, the Aortic and venous cannulae are removed and the sternum is closed by steel wires.
Surgical Management of Ischemic heart Disease:-
The Coronary circulation to the heart is composed of Left and Right coronary arteries.
Left Coronary artery arises from Left coronary sinus and supplies most of the anterior wall, the apex and the interventricular septum, it is divided to left anterior descending and circumflex artery.
Right Coronary artery arises from Right coronary sinus and supply SA and AV nodes and part of the interventricular septum.
The coronary arteries are liable to develop atherosclerotic changes, this will narrow them and when the obstruction reaches the critical level it will prevent the transition of blood to myocardium and this will produce ischemic symptoms.
Indications of Coronary Artery By-pass Graft:
1. Disease involves Left Main Stem Coronary Artery.
2. Two vessel disease including Left anterior descending artery.
3. Three vessel disease not including Left anterior descending artery.
4. Symptoms refractory to medical treatment.
5. Complex lesion not amenable for PTC.
During CABG usually 3 to 4 grafts are used to transmit blood directly from Ascending Aorta to coronary arteries distal to site of atherosclerotic obstruction, the grafts may be arterial or venous
Arterial graft like Left internal mammary artery which is harvested from chest wall, the proximal end remain attached to subclavian artery and the distal end is anastomosed with left anterior descending coronary artery.
Venous graft like Long Saphenous Vein Which is harvested from the leg and used as free graft.
After surgery most patients will get benefit by relief of symptoms in 97% of cases ,5 to 10 years later symptoms may develop again because of the development of atherosclerosis in the grafts which may obstruct them, the internal mammary artery is more resistant to atherosclerosis than saphenous vein graft .
The restenosis can be treated nowadays by Redo-CABG.
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