What is Extracorporeal Therapy?

Extracorporeal circulation is a life support technology that uses a series of special artificial devices to drain the venous blood from the body to the outside of the body, performs gas exchange through artificial methods, adjusts the temperature and filters, and then returns it to the arterial system in the body. During the extracorporeal circulation, since the artificial device replaces the human body function, it is also called cardiopulmonary bypass, and the extracorporeal circulation machine is also called artificial heart-lung machine. The purpose of extracorporeal circulation is to maintain blood supply to tissues and organs throughout the body during open heart surgery. With the development of clinical medicine, the scope of cardiopulmonary bypass applications has been continuously expanded, not only in heart, liver, kidney, lung and other large blood vessel operations, but also in the treatment of tumors, and the life support of patients with heart and lung failure. Performance has become an important technology in clinical medicine.

Basic Information

nickname

Cardiopulmonary bypass

English name

extracorporeal circulation

Visiting department

Cardiac Surgery

Cardiopulmonary bypass

Extracorporeal circulation device (artificial heart-lung machine) consists of artificial heart (blood pump), artificial lung (oxygenator), thermostat, pipeline, filter, console and electronic equipment.

Artificial lung

It is the part of blood that stays longer through the machine and has a greater impact on the blood. Artificial lungs directly contact blood with gas (bubble type, turntable type, roller type, and vertical screen type), or gas exchange through semi-permeable membrane, so that the blood oxygen saturation after the transfer reaches 90% or more. There are three types of artificial lungs: bubble type, blood type and membrane type.

(1) Bubbling oxygenator blood enters the container that blows oxygen and carbon dioxide through sieve plate, thin tube to form oxygen-containing blood bubbles, oxygenates, and then blood passes through stainless steel wire, plastic wire or polyurethane sponge. The silicone oil defoaming agent removes the air bubbles in the blood after oxygenation, and then filters through the nylon cloth filter into the blood storage tank, and drives the blood into the aorta of the body through an arterial pump. The advantages are simple structure, low cost, good oxygenation performance, reliable disinfection and convenient operation. The disadvantage is that oxygen and blood are not in physiological contact, and the general flow time cannot be too long.

(2) The blood film type oxygenator is composed of a blood tank and a rotating stainless steel plate or a rotating plastic cylinder, or a blood tank and a plurality of vertical stainless steel wire screens. Dozens to hundreds of discs are connected in series by a shaft with a certain distance between them. The shaft is supported in a blood tank. Half of the disc is immersed in blood. The shaft rotates and the blood forms a film attached to both sides of the disc. The blood tank is semi-cylindrical, covered with a transparent plastic cover, and also semi-cylindrical. It is filled with oxygen and carbon dioxide to allow the blood film to exchange gases. Rotary dish type, rotary type and vertical screen type are all blood film type oxygenators. The oxygenation capacity is related to the total blood film area, rotation speed, and blood film thickness. The advantage is that there is no need to form foam and less blood damage. The disadvantage is that after each work, manual cleaning is required to remove the protein material attached to the surface of the disc tube, and the discs and rollers need to be siliconized regularly to increase the surface finish.

(3) Membrane lung type In order to avoid direct contact with blood gas, a semi-permeable membrane separates the running blood from oxygen, and performs gas exchange through the membrane to approximate a physiological state. The semi-permeable membrane is made of silicone rubber, polytetrafluoroethylene or polyacetone, and the structure can be a roll membrane type, a folded membrane type, a micro tube type or a hollow fiber tube type. The membranous lung type has the advantages of less damage to blood constituents and fibrinogen, etc. At present, it is mostly used in respiratory distress syndrome. Infants and young children can be transferred for one week. The disadvantage is that the carbon dioxide is slightly worse.

Artificial heart

At present, most roll-type pumps are used. The central column is connected to the horizontal axis that rotates clockwise. The shaft is a semi-circular groove. The groove is built into an elastic tube (pump tube). The most commonly used pump tubing is silicone tubing. The length of the horizontal axis can be adjusted so that it can just press the pump tube. When the horizontal axis rotates clockwise, squeeze the pump tube to make the blood in the tube flow forward in one direction. Increasing or decreasing the horizontal axis speed can increase or decrease the flow. Roller pumps have the advantages of simple structure, reliable performance, no intravalvular valve, less blood damage, and large range of rotary flow. Generally, the artificial heart and lung are equipped with 4 identical roller pumps, which are respectively used to drain blood into the aorta, recover left heart blood, recover blood in the surgical field and pericardial cavity, and sometimes can also be used for coronary perfusion. In recent years, roller pumps are mostly pulsating, which is closer to physiology.

3. Thermostat

The blood circulating outside the body is cooled, and the low temperature is combined with an artificial heart-lung machine. The structure is mostly sleeve type. Two stainless steel cylinders with different diameters. The inner cylinder is connected to the water pump box and passes the water flow; the outer cylinder passes the blood and can be lowered or raised by the temperature of the water flow in the inner cylinder. The volume of blood in the thermostat is 60 to 200 ml. An effective thermostat can reduce the body temperature of an adult from 37 ° C to 30 ° C at a rate of 0.7 ° C to 1.5 ° C per minute; the temperature rise is slower than the temperature drop, generally 0.2 ° C to 0.5 ° C per minute. The water temperature should not exceed 40 ° C when warming up. Excessively high temperatures will denature plasma proteins. The difference between the water temperature and the blood temperature cannot be greater than 14 ° C. Excessive temperature difference will promote the release of dissolved gas and form micro-bubbles.

4.Filter

In the past, artificial heart-lung machines used stainless steel wire mesh filters, which had large holes and could only filter out large blood clots, while small blood clots could cause pulmonary complications, such as perfusion of the lungs. Recently, microporous filters have been commonly used, which are made of nylon, nylon, polyurethane foam, and the like. The microthrombus in the bloodstream mainly comes from platelet agglomerates, cellulose agglomerates, free silicone oil, tube wall desquamation, and microbubbles. Of the blood recovered from the surgical field, these microthrombuses are the most numerous and should be filtered.

5. Pipeline section

The body and the blood tank, the blood tank and the oxygenator, the oxygenator and the arterial blood pump must be connected by pipes. Except for the intubation part, the general venous gravity siphon suction uses a 12.7mm (1/2) inner diameter tube, the aorta uses 9.5mm (3/8) inner diameter tubes for blood supply, and the inner diameter is 6.4 mm (1/4 hour) PVC pipe. It is required that the inner wall of the pipeline is smooth and siliconized, and the diameter changing place is required to be gradually tapered, no flute angle, and no rough edges to reduce resistance, pressure difference and eddy current. Children use thin pipes accordingly.

6. Power

With safety fuse device. In the event of a sudden power outage, some machines are equipped with hand-operated handles that can temporarily maintain flow until power is repaired.

7. Electronic monitoring part

It can continuously display the working conditions of the machine, such as blood pressure, flow rate, and oxygen concentration, by fluorescent numbers, so that the perfusionist can easily understand and grasp the working conditions of the flow.

Extracorporeal circulation

1. Preparation and connection of pipeline before transfer

The perfusionist should participate in the preoperative discussion in advance to fully understand the pathological anatomy and cardiac function of the patient's circulatory system, understand the surgeon's surgical plan and special requirements for extracorporeal circulation. Check the extracorporeal circulation equipment such as power supply, artificial heart-lung machine, temperature-changing water tank, etc. before operation to ensure that it is in good working condition. All items that come into contact with the surgical field and the patient's blood should be thoroughly sterilized and treated under strict aseptic conditions Install pipes and oxygenators.

2. Pre-filling and blood dilution of artificial heart-lung machine

(1) The purpose of pre-charging the extracorporeal circulation device Exhaust the gas in the pipeline to avoid air embolism; Proper blood dilution; Adjust the imbalance between acid and alkali and water electrolyte in the body.

Prefilled fluids include: basal fluids, electrolytes, alkaline fluids, colloidal fluids, mannitol, and blood cell protectants.

(2) Blood thinning Extracorporeal circulation uses blood thinning to reduce blood viscosity and vascular resistance, improve microcirculation, reduce blood cell destruction, and reduce the accumulation of blood constituents in capillaries.

3. Detection of physiological indicators in patients with extracorporeal circulation

It mainly includes arterial pressure, central venous pressure, electrocardiogram, nasopharyngeal temperature and rectal temperature, blood gas analysis, blood electrolytes, whole blood activation and coagulation time, and hematocrit.

4. Anticoagulation

The use of large-dose heparin anticoagulation during extracorporeal circulation is called heparinization. In order to avoid excessive heparin dose or insufficient anticoagulation, the whole blood activated clotting time should be closely monitored during the circulation to keep it at 480 to 600 seconds.

5. Myocardial protection

For all open heart surgery requiring temporary interruption of coronary circulation, how to protect myocardium and reduce myocardial ischemia, hypoxia and reperfusion injury is a key issue related to the success of the operation and the prognosis of the patient. Research and solve problems.

At present, the most widely used methods of myocardial protection are: whole body low temperature, local deep hypothermia of the heart, ascending aorta clamp, coronary arteries perfused with cardioplegia.

Cardioplegic fluid promotes cardiac arrest and quickly stops all electrical and mechanical activities of the heart, which is conducive to preserving the heart's energy reserve, supplemented by the deep hypothermia of the heart, which can further reduce myocardial energy and oxygen consumption, and reduce carbon dioxide, hydrogen ions and oxygen radical Accumulation of harmful substances.

Cardiopulmonary bypass

In open-heart and open-heart surgery with extracorporeal circulation, a sternal midline split incision is generally used, and most are inserted into the donor blood vessel via the ascending aorta and into the vena cava drainage tube through the right atrium. Only in special cases is it necessary to use other sites for intubation.

Arterial supply vessel

(1) Ascending aorta cannulation The ascending aorta is the most commonly used intubation site. The advantages are easy operation, safe intubation, and fewer complications.

(2) Femoral arterial intubation can be used when left ventricular bypass, ascending aortic aneurysm, ascending aortic dissection aneurysm, and other emergency situations are not safe and rapid.

2. Intravenous drainage cannula

(1) A single cannula through the right atrium is suitable for ascending aorta, aortic valve, mitral valve, and coronary surgery. Blood from the inferior vena cava and right atrium was drained through a single cannula.

(2) The superior and inferior vena cava cannulas are free of the superior and inferior vena cava, cover the blocking band, cut in the right atrial appendage suture, and insert the superior vena cava drainage tube. A purse suture is usually placed on the outer wall of the right atrium near the entrance of the inferior vena cava, and the inferior vena cava drainage tube is inserted. If it is not necessary to cut the right atrium during surgery, two purse sutures can be placed anywhere on the right atrium wall. When inserting the tube through the pouch suture, cut out the trabeculae and inner ear trabeculae, and use two vascular forceps to lift the inner and outer edges of the auricle and insert the drainage tube. When intubating the atrial wall purse suture, a small knife was used to puncture the purse suture with a sharp knife. The assistant enlarged the incision with vascular forceps, and the operator inserted the drainage tube into the vena cava. Tighten the purse suture, and tie the rubber tube with the drainage tube with thick silk thread to prevent slipping off.

The purse sutures of the superior and inferior vena cava cannula can also be sutured directly on the anterior wall of the vena cava. For infants or children with more complicated operations in the right atrium, right-angle venous intubation should be selected to avoid affecting the operation and exposure.

(3) Pulmonary arterial cannula In special circumstances, such as when the mitral valve is exposed through a left thoracotomy, a pulmonary vein cannula can be used for drainage. A purse suture or cushioned suture with a gasket is placed in the right ventricular outflow tract, and a drainage tube is inserted and sent to the common pulmonary artery through the pulmonary valve orifice.

(4) Femoral vein cannulation The femoral vein is the most commonly used site for peripheral vein cannulation. The femoral vein on the inside of the femoral artery was exposed, and the blocking band was free and sheathed. A longitudinal incision is made on the femoral vein, and it is gently inserted into the cannula toward the heart, and as far as possible, it can be delivered to the lower segment of the inferior vena cava. Adult femoral veins can be inserted into catheters over 24F, but generally thinner catheters (such as 20F) are preferred.

3. Left heart drainage tube

During extracorporeal circulation, blood is returned to the left atrium from the bronchial artery, the sinusoidal vasculature, the coronary venous system, the atria and the vena cava. Therefore, left ventricular drainage and decompression should be performed to prevent left ventricular inflation.

The upper right pulmonary vein cannula is usually connected with the left atrium to handle the purse suture, insert a 18F or 24F catheter with a lateral hole, and send it to the left chamber through the mitral valve port.

Extracorporeal circulation complications

After decades of clinical application and improvement, the artificial heart-lung machine has been able to provide surgeons with a relatively good safety time and safety level, and can be used for many kinds of heart surgery. However, the effects of perfusion on the body are still unavoidable. The main points are as follows:

1. Metabolic acidosis and respiratory alkalosis

During perfusion, there is a certain degree of tissue ischemia, hypoxia, and increased lactic acid, which leads to metabolic acidosis. High-concentration oxygen is blown into the machine, and carbon dioxide is easily discharged, resulting in excessive ventilation, which causes respiratory alkalosis.

2. There is damage to blood constituents

After perfusion, the number and function of platelets decreased, fibrinogen decreased, and there may be hemoglobinuria, hemolytic jaundice, and progressive anemia. Coagulation dysfunction takes 1 to 7 days to recover.

3. electrolyte imbalance

Perfusion reduced blood potassium, increased capillary endothelial permeability, and tissue edema.

4. Decreased renal function

Hypotension, decreased renal blood flow, metabolic acidosis, and vascular reactions during perfusion can all lead to decreased renal function, decreased urine output, decreased sodium in urine, and increased increased potassium in urine.

5. Reduced circulatory system function

After perfusion, the resistance of peripheral blood vessels increases, and the blood output after myocardial trauma decreases, resulting in hypotension and decreased tissue perfusion.

6. Impaired respiratory function

After perfusion, pulmonary interstitial edema, decreased lung compliance, increased airway resistance, pulmonary parenchyma may have bleeding and alveolar ridges, which can lead to perfused lungs.

7. Brain damage

Intracranial edema and bleeding.

8. Other damage

After perfusion, it also has certain effects on endocrine and complement [1-3].

References

1. Edited by Xu Guangya and Wu Shuming. Graphic Cardiac Surgery: Science Press, 2010: pp. 69-80. Pp. 18-30.

2. Translated by Liu Zhongmin and Wu Qingyu. 2007. Adult Cardiac Surgery. Beijing: People's Medical Publishing House

3. Edited by the Chinese Medical Encyclopedia Editorial Board; edited by Luo Zhicheng. Chinese Medical Encyclopedia · 26 Biomedical Engineering: Shanghai Science and Technology Press, 1989: 126-128.