The extracorporeal circulation is the method that allows to compensate for
the cardiopulmonary function, excluding the heart and the lungs from the
circulation, while the heart is in a pharmacological arrest for being subjected
to cardiac surgery. This method allows the same to keep the whole organism alive
and to intervene inside the cardiac cavities.
Extracorporeal circulation serves to replace cardiac and pulmonary function in
order to stop the heart during cardiac surgery. When working on the heart, a
cardiac surgery team should take care of assisting the patient during open heart
surgery. These are highly specialized technicians who control the heart-lung
machine.
The figures involved are:
1. Cardiologist
2. Cardiac surgeon
3. Cardioanesthesia
4. Technician of body perfusion
5. Professional nurse
6. Rehabilitative physiotherapist.
The cardiologist knows how to intervene and recognizes what information the cardiac surgeon needs; the cardioanestesista is specialized for the management of the cardiopathic patient. During the most important phases of the operation, he exchanges information with the cardiac surgeon and performs controls under trans-oesophageal echographic monitoring, being competent for the reading and execution of the transesophageal echo.
These procedures are important during a
valve repair. The heart-lung machine technician manages the complicated
mechanism of the heart-lung machine whenever it is necessary to intervene on the
heart for major surgery. Not all cardiac surgical operations require
extracorporeal circulation. This implies that extracorporeal circulation is not
required to perform operations on the coronary arteries. The first intervention
with extracorporeal circulation dates back to 26/3/54 when the first heart
surgery was performed at the University of Minnesota. After this date the method
was developed that allows the heart to be stopped and to replace cardiac and
pulmonary function. Another big step was accomplished thanks to the use of
disposable material. Previously, in fact, the surgical instruments and the
surgical material was reused, after having been cleaned and sterilized. This was
a great risk to the patient, because some parts of the heart-lung machine
remained contaminated with proteins from the various patients, causing dangerous
allergic lung reactions. For this reason, later, when this was sensed, the tubes
of the circuit and the other parts where the blood flows are disposable.
The first interventions were performed on congenital heart disease and they
presented respiratory complications and died. Finally a boy named Gregory a year,
as the heart-lung machine was not working, a parent (in this case the mother)
was used as a heart lung machine, carrying out a cross-circulation with one of
the two parents. As you know the father or the mother have a blood group that is
equal to that of the child and therefore under this point of view there are no
particular reactions. In this case there was a good response and this encouraged
then to develop the heart-lung machine and solve the problems associated with
the heterologous proteins that contaminated the circuits.
The heart lung machine for extracorporeal circulation is obviously made up of:
- a pump to propel blood,
- a reservoir that is nothing but a blood collection chamber
- a heat exchanger that regulates the temperature of the blood. In fact, some
interventions are made at a body temperature of 18 ° C, and the patient must be
cooled to reduce the metabolisms and consume less oxygen.
- The oxygenator that allows to oxygenate the venous blood and make it arterial
and obviously to support and replace pulmonary function. Today this mechanism is
changed with every intervention
- the filters so that there are no particles that can create embolisms
- the connection cannulae: even these, like the oxygenator that I initially said
was not disposable, for several decades the cannulae are disposable and are made
of PVC.
During the operation the blood is taken through a venous cannula, usually positioned in the right sections of the heart, arrives in the reservoir and a centrifugal pump that is nothing more than a rotor with electromagnetic control turns and pushes the blood. Then the oxygenator charges the blood with oxygen and the blood flows into an arterial chamber which is positioned in the ascending aorta. The pumps are of various types: there are rollers that compress the tubes to give propulsion, or there are the blades that rotate propulsion to the blood, or finally a centrifugal pump that also rotating it gives propulsion to the blood. The oxygenator, as I told you, is very simple because it is made up of membranes, through which there is the exchange between oxygen and venous blood, or they are hollow fibers, ie inside the fiber the blood flows, and outside the fibers there is oxygen, so while the blood flowing inside these hollow fibers has hematosis and venous blood becomes arterial. The blood flows through the whole extra-corporeal circuit and is put back into the ascending aorta, so the heart is completely excluded from the circulation and so is the pulmonary circuit. In short the upper vena cava and the inferior vena cava are cannulated, which are excluded from the right atrium through two tourniquet, which are two laces tight around the veins so that the blood goes all in the cannulae, this allows to open the atrium right without having the blood that enters the atrial chamber, so if I have to go to intervene on the interatrial septum I will have to cannulate the two quarries separately and exclude them from the atrium.
If I do not have to open the right atrium I will use a cannula only, which I will place in the right atrium with the tip in the inferior cavity and therefore will drain all the right blood and not having to open the right atrium will not have vision problems. So these things are to remember: these are curves that indicate the consumption of oxygen at various temperatures and various flows because the body needs a basic oxygen, which varies as you know with the temperature: at 18 degrees the consumption of oxygen is lower than at 36 ° C. As you can see here there are some temperature: this is the consumption at 37 ° C, this is the one at 15 ° C and the flow of perfusion must be these. As you can see 100% of the flow at 37 ° C and around 2.5 l / min * m2, this is what we know to be the basic requirement at 36 ° C. With the temperature as you see the percentage of that 100% is lowered, so we know that at 18 ° C we can also stop the circulation for some time because the oxygen consumption is very low and so we use this expedient to lower the temperature to reduce the consumption of oxygen, in order to perform some complex interventions on the aorta, especially when we have to intervene on the supraortic trunks, on the aortic arch and then we must cool the patient well and suspend the circulation for about 30 minutes, it is actually 45 minutes , but we have a quarter of an hour of security to be able to replace the whole aortic arch and the supraortic trunks, then the carotids.
+ So we use a low temperature in this case, this is called deep hypothermia with a circulation stop. This is the brain's oxygen consumption, depending on the temperature it can arrive I do not say zero, but almost. Be aware that when we lower the temperature to 18 ° C we have silence electroencephalographic, that is the electroencephalogram disappears, not in children because in children they have a somewhat more complex situation, but in adults most of the time at 18 ° C there is electroencephalographic silence and if we take blood from the brain, that is the jugular vein, we see that the blood extraction, that is the arterio-venous difference of blood at that temperature, is practically 1-2%. All these data are obviously verified, so the brain at that temperature extracts almost nothing, this allows us to have a safety time to be able to intervene on supraortic trunks and aorta.
During this procedure coagulation is activated, there is complement activation, platelet activation and a non-specific inflammatory reaction, because the blood passes through a PVC circuit, so obviously this can create some problems. For this reason, for some interventions the extracorporeal circulation can be avoided, above all interventions on the coronaries, when we do not have to enter inside the heart. Therefore the intervention can be done without extracorporeal circulation. Obviously the activation of the hemocoagulative cascade means that if we put some cannula inside the right atrium, in the aorta, and if we do not make some anticoagulative gesture, we risk causing a thrombosis or embolization, then before inserting the cannulae into the atrium right, in the aorta and so on, we have to completely block the coagulation cascade. The cardiac surgeon at the beginning of the extracorporeal use massive doses of heparin to the patient who obviously, after the intervention, is neutralized with an antagonist that is the protamine.
Heart and ischemia during the interventions
If we exclude the heart and the lungs from circulation, the organ that will be
ischemic by antonomasia is the heart, because clearly excluding the aorta and
all the rest of the organism downstream, we exclude the proximal portion of the
aorta and we must protect the myocardial ischemia. A solution called a
cardioplegic solution is used, which is a saline solution with water and
electrolytes with a small amount of blood, which allows to cool and keep the
heart alive during the central phase of the intervention, so that it is firm and
protected and does not go into severe intraoperative ischemia. For this reason
the consumption of myocardial oxygen is lowered, that is, the heart consumes
oxygen
Normally the heart consumes oxygen for:
- Electric activity: you know that depolarization requires energy consumption
and therefore also consumption of 02, up to 35%
- Mechanical activity: that is, contractility, 60% of oxygen consumption
- Metabolic activity: ie the basic metabolic activity of myocardial cells which
is 5%
To lower the consumption of oxygen the electric activity is stopped with
potassium at very high doses that give us the complete asystole, so electric
tracing is flat, so the electrical activity of the heart is stopped, obtaining a
saving of 35% in the oxygen consumption. Then the mechanical activity is also
stopped, the heart is emptied during the cardioplegia infusion phase, we keep it
decompressed, we do not contract it because there is also the potassium that
blocks the spontaneous depolarization of the myocardial cells and therefore the
function of pump, which is supported by the extracorporeal circulation, we do
not need it and therefore we earn another 60% of oxygen consumption. We are
already at 95%. Of this last 5% of the basic metabolic activity, we can still
obtain something if the heart temperature is lowered. The heart temperature is
brought to 8-10 ° C and therefore also this determinant of oxygen consumption is
lowered. This solution, which allows to stop the heart (cardioplegic solution),
is infused at a temperature of about 4 ° C, then we cool the heart, block the
electrical systole because we give potassium (about 20mEq per liter) and
obviously decompress it and not we contract, in this way we can have so-called
times of myocardial ischemia long enough to be able to intervene. You should
repeat the cardioplegic solution every 30 minutes or as soon as the heart
temperature, which we can measure intraoperatively, rises above 10 ° C, and then
we can make a sufficiently long ischemia time, which is about 2,5 hours and
satisfactory for the 99.9% for heart surgery. The infusion is carried out
directly in the aortic bulb, obviously the ascending aorta is closed, separated
from the rest of the circulation, and a cannula is inserted that infuses the
cardioplegic solution, or it is possible to infuse the solution directly into
the coronary hosts, if we have to intervene on the aortic valve, or we can do it
retrograde from the coronary sinus and then when we do the aortocoronary
by-pass.
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