Open-heart surgery is considered as one of the most significant advances in medicine of 20th centuiy. Establishment of safe cardio pullnonary bypass (CPB) by extra corporeal circulation (ECC) has helped the surgeons to stop the heart,open it, c m y out intricate corrections or do cardiac transplantation.
Equipment
To maintain effective circulation and carry out functions of the lungs during an open heart operation the equipment should have:
1) Blood Pump
2) Oxygenator
3) Heat Exchangers.
1) Blood Pump
Ideal Characteristics
1) I should be able to pump up to seven litres of blood per minute.
2) It should not damage cellular and acellular components of blood.
3) All parts coming in contact with blood should have a smooth surface.
4) Assuring no turbulence and no stagnation.
5) Parts like tubing, oxygenators and heat exchangers, which come in contact with blood, should be disposable wilhout contaminating permanent parts of the pump.
6 ) It should be able to monitor blood flow accurately.
7)Availabilily of manual operation facility at times of power failure.
Types of Pumps
a) Roller Pump: This is the type most coinmo~ily used in clinical perfusion. It has a circular metal housing inside which there is a rotating 'urn with two rollers fixed at 180 degrees apart. Polyvinyl chloride (PVC) tubing which is resilient, passing through the inner perimeter of the housing is compressed by the rollers effecting non-pulsatile forward flow. The occlusion of the rollers are checked routinely and adjusted to optimum level. Too inuch occlusioil will cause haemolysis; too less will reduce forward flow.
b) Centrifugal Pumg: This is available for clinical perfusion from 1976 (Bio Medicus Pump). It is disposable, causes less blood trauma a d reduces the risk of massirk air ernboiism. It is useful for prolonged mechanical circulatory support like ventricular assist devices (VAD), percutaneous cardiopulmon~y support and extra corporeal membrane oxygenation(ECMO) . '
c) Pulsatile Perfusion: Conventional pumps give continuous flow with very little pulsatile property. Pulsatile perfi~sioii is likely to result in better post operative renal and cardiac function, A simple method of achieving pulsatile perfusion is by adding intra aortic balloon pump (IABP) to the bypass circuit.
They serve the function of lungs during extra corporeal circulation (ECC) -Oxygenation, removal of carbon dioxide and transport of gaseous anaesthetic agents:The oxygenator will have a reservoir for venous blood and the blood sucked from cardiac chambers as well as pericardium (cardiotomy suckers). It will have facility to cool and rewarm blood (heat exch'mger).
1)Ideal Characteristics
2) Maximize gas transfer (oxygen, carbon dioxide and anaesthetic gases)
3) Good heat transfer efficiency
4) Minimize priming volume (amount of fluid required to fill the oxygenator and the tubing).
Minimize blood trauma
Types of Oxygenators
a) Film Oxygenators
b) Disc Oxygenators
c) Bubble Oxygenators
d) Membrane Oxygenators.
Film and Disc Oxygenators are not used for clinical perfusion now.
a)Bzrbhb Oxygenators: These have a mixing chamber where venous blood is collected and from the bottom end micro bubbles of oxygen are passed and as they rise to the top, gas exchange takes place. The oxygenated blood is then defoamed using silicon antifoam A (Dimethyl Polysiloxane) before pumping into the circulation. Bubble oxygenators are used rarely and that too for only short by pass procedures.
b)Membrane Oxygenators: They are more physiological and are similar to natural lungs. There is separation of blood and gas by membrane across which gas exchange takes place. There are two types. (1) True membrane,
(2) Micro porous membrane oxygenators:
i) True Menzbrane Oxygenator: There is no contact between blood and gas at any time. It is more expensive and needs larger priming
volume.
ii) Micro Porous Membrane Oxygenator: At the beginning of pelfusion,there is contaigbetween blood and gas across the micro pores. In a
short lime, protein coaling of inicro pores occurs, severing direct contact. However after several hours of use, he functional capacity of the membrane decreases. So they are not useful for prolonged use as in extra corporeal membrane oxygenator (ECMO),
3) Heat Exchangers
This is an integral part of cardjo pulmonary bypass and is designed to cool and warm the perfusate. ~011-sterile water from ice bath or warm water passed through the tubing made of stainless steel or aluminium is immersed in blood flowing in opposite directions. (counter current heat exchange). Rapid cooling is desirable at the beginning of perfusion with the circulating water kept at 0 degree C. Rewarming has to be done -sldwly to avoid dawake to blood elements and to avoid bubble formation in the perfusate. The temperature gradient between water and blood should always be less than 1 O°C, while rewidng .The maximuill temperature of the bath should be less than 42OC.
Procedures
Heparinisation
The patient should be fully heparinised before the start of cardio pulmonary bypass. Baseline activated clotting time is measured (ACT). 3 ing (300 units) per kg body weight of heparin is administered. After five minutes, ACT is checked.It should be above 400 seconds for safe cardio pulmonary bypass. Heparin is added to the priming fluid in the pump, at the rate of 3 units per ml of fluid.During perfusion, ACT is checked every 15 minutes, to keep it above 400 seconds at 30°C and at 480 seconds, if the temperature is below 30°C. At the end of surgery, when the pump is switched off and the venous cannulae are removed, protsunine sulphate is given to neutralize the effect of heparin. The usual dose is 1 to 1.5 milligrams of protarnine sulphate for each milligram of beparin administered.
Circuitry and Priming
The cardio pulmonary bypass circuit consists of oxygenator, tubings, c:u~nulae,cardiotomy reservoirs and cardioplegia attachments. The first step is to pi-iine the circuit with either clear fluid or with addition of blood. The clear prime collsists of crystalloids, colloids, albumin or plasma. Most of the adult patients do not need addition of blood in the prime. On bypass, the haematoclit is brou g ht down to 18 or 20 (haemo dilution). In babies and children, blood and albuilli~l are added to the piime. By sophistication of the oxygenators, priming volumes have been brought down to a rninim~uln (mini-prime).
Cannulation
Vpically blood is drained by gravity through two canllillae inserted illto the superior and inferior vena cavae. During bypass, if the SVC and IVC are snared,the entire venous return to the heart is diverted to the pump (total cardio pulmonary bypass). Whenever the right side of the heiurt has to be opened, the patient has to be on total bypass. Otherwise, when a chamber is opened, air will enter the tubing, causing air lock and cessation of venous retu1-n. For an aortic valve replacement or coronuy iuleiy bypass, the right side need not be opened. So the operation can be done using a single, two stage venous cannula passing through right atiium into the inferior vena cava (atrio-venous cannula). The oxygenated blood is returned through an arterial cannula inserted into the ascending aorta or femoral artery.
Conduct of Perfusion
At the beginning of the bypass, the pump output is usually kept at 2.4 litres per meter per minute. On coming off bypass, the flow is gradually reduced till the patient's heart takes over completely. Only when fully satisfied, is the pump switched off and decannulations done.
Equipment
To maintain effective circulation and carry out functions of the lungs during an open heart operation the equipment should have:
1) Blood Pump
2) Oxygenator
3) Heat Exchangers.
1) Blood Pump
Ideal Characteristics
1) I should be able to pump up to seven litres of blood per minute.
2) It should not damage cellular and acellular components of blood.
3) All parts coming in contact with blood should have a smooth surface.
4) Assuring no turbulence and no stagnation.
5) Parts like tubing, oxygenators and heat exchangers, which come in contact with blood, should be disposable wilhout contaminating permanent parts of the pump.
6 ) It should be able to monitor blood flow accurately.
7)Availabilily of manual operation facility at times of power failure.
Types of Pumps
a) Roller Pump: This is the type most coinmo~ily used in clinical perfusion. It has a circular metal housing inside which there is a rotating 'urn with two rollers fixed at 180 degrees apart. Polyvinyl chloride (PVC) tubing which is resilient, passing through the inner perimeter of the housing is compressed by the rollers effecting non-pulsatile forward flow. The occlusion of the rollers are checked routinely and adjusted to optimum level. Too inuch occlusioil will cause haemolysis; too less will reduce forward flow.
b) Centrifugal Pumg: This is available for clinical perfusion from 1976 (Bio Medicus Pump). It is disposable, causes less blood trauma a d reduces the risk of massirk air ernboiism. It is useful for prolonged mechanical circulatory support like ventricular assist devices (VAD), percutaneous cardiopulmon~y support and extra corporeal membrane oxygenation(ECMO) . '
c) Pulsatile Perfusion: Conventional pumps give continuous flow with very little pulsatile property. Pulsatile perfi~sioii is likely to result in better post operative renal and cardiac function, A simple method of achieving pulsatile perfusion is by adding intra aortic balloon pump (IABP) to the bypass circuit.
They serve the function of lungs during extra corporeal circulation (ECC) -Oxygenation, removal of carbon dioxide and transport of gaseous anaesthetic agents:The oxygenator will have a reservoir for venous blood and the blood sucked from cardiac chambers as well as pericardium (cardiotomy suckers). It will have facility to cool and rewarm blood (heat exch'mger).
1)Ideal Characteristics
2) Maximize gas transfer (oxygen, carbon dioxide and anaesthetic gases)
3) Good heat transfer efficiency
4) Minimize priming volume (amount of fluid required to fill the oxygenator and the tubing).
Minimize blood trauma
Types of Oxygenators
a) Film Oxygenators
b) Disc Oxygenators
c) Bubble Oxygenators
d) Membrane Oxygenators.
Film and Disc Oxygenators are not used for clinical perfusion now.
a)Bzrbhb Oxygenators: These have a mixing chamber where venous blood is collected and from the bottom end micro bubbles of oxygen are passed and as they rise to the top, gas exchange takes place. The oxygenated blood is then defoamed using silicon antifoam A (Dimethyl Polysiloxane) before pumping into the circulation. Bubble oxygenators are used rarely and that too for only short by pass procedures.
b)Membrane Oxygenators: They are more physiological and are similar to natural lungs. There is separation of blood and gas by membrane across which gas exchange takes place. There are two types. (1) True membrane,
(2) Micro porous membrane oxygenators:
i) True Menzbrane Oxygenator: There is no contact between blood and gas at any time. It is more expensive and needs larger priming
volume.
ii) Micro Porous Membrane Oxygenator: At the beginning of pelfusion,there is contaigbetween blood and gas across the micro pores. In a
short lime, protein coaling of inicro pores occurs, severing direct contact. However after several hours of use, he functional capacity of the membrane decreases. So they are not useful for prolonged use as in extra corporeal membrane oxygenator (ECMO),
3) Heat Exchangers
This is an integral part of cardjo pulmonary bypass and is designed to cool and warm the perfusate. ~011-sterile water from ice bath or warm water passed through the tubing made of stainless steel or aluminium is immersed in blood flowing in opposite directions. (counter current heat exchange). Rapid cooling is desirable at the beginning of perfusion with the circulating water kept at 0 degree C. Rewarming has to be done -sldwly to avoid dawake to blood elements and to avoid bubble formation in the perfusate. The temperature gradient between water and blood should always be less than 1 O°C, while rewidng .The maximuill temperature of the bath should be less than 42OC.
Procedures
Heparinisation
The patient should be fully heparinised before the start of cardio pulmonary bypass. Baseline activated clotting time is measured (ACT). 3 ing (300 units) per kg body weight of heparin is administered. After five minutes, ACT is checked.It should be above 400 seconds for safe cardio pulmonary bypass. Heparin is added to the priming fluid in the pump, at the rate of 3 units per ml of fluid.During perfusion, ACT is checked every 15 minutes, to keep it above 400 seconds at 30°C and at 480 seconds, if the temperature is below 30°C. At the end of surgery, when the pump is switched off and the venous cannulae are removed, protsunine sulphate is given to neutralize the effect of heparin. The usual dose is 1 to 1.5 milligrams of protarnine sulphate for each milligram of beparin administered.
Circuitry and Priming
The cardio pulmonary bypass circuit consists of oxygenator, tubings, c:u~nulae,cardiotomy reservoirs and cardioplegia attachments. The first step is to pi-iine the circuit with either clear fluid or with addition of blood. The clear prime collsists of crystalloids, colloids, albumin or plasma. Most of the adult patients do not need addition of blood in the prime. On bypass, the haematoclit is brou g ht down to 18 or 20 (haemo dilution). In babies and children, blood and albuilli~l are added to the piime. By sophistication of the oxygenators, priming volumes have been brought down to a rninim~uln (mini-prime).
Cannulation
Vpically blood is drained by gravity through two canllillae inserted illto the superior and inferior vena cavae. During bypass, if the SVC and IVC are snared,the entire venous return to the heart is diverted to the pump (total cardio pulmonary bypass). Whenever the right side of the heiurt has to be opened, the patient has to be on total bypass. Otherwise, when a chamber is opened, air will enter the tubing, causing air lock and cessation of venous retu1-n. For an aortic valve replacement or coronuy iuleiy bypass, the right side need not be opened. So the operation can be done using a single, two stage venous cannula passing through right atiium into the inferior vena cava (atrio-venous cannula). The oxygenated blood is returned through an arterial cannula inserted into the ascending aorta or femoral artery.
Conduct of Perfusion
At the beginning of the bypass, the pump output is usually kept at 2.4 litres per meter per minute. On coming off bypass, the flow is gradually reduced till the patient's heart takes over completely. Only when fully satisfied, is the pump switched off and decannulations done.
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