Definition
The heart–lung machine is a device for shifting the heart and lung function outside the body. It takes over the pumping function of the heart and the oxygenation function (= oxygenation) of the lungs while the heart is being operated on. A heart–lung machine (HLM for short) is subject to a number of high medical-technical demands, since it is involved in decisive processes in the body.
Functions
The heart-lung machine has several functions. Firstly, its main task is to tap the venous blood return to the heart at the vena cava and release it again at the aorta in enriched form (i.e. rich in oxygen). The organs heart and lungs are excluded from the bloodstream, which makes operations on these organs possible.
However, a distinction must be made between two blood circulations: On the one hand, 5-6 liters of blood per minute move through both organs (the so-called circulation volume), on the other hand, heart and lung tissue are also supplied with blood themselves, and finally they must be supplied with oxygen. Only the circulation volume is cut off, the blood supply to the organs themselves remains, otherwise they would die. In the heart-lung machine, the oxygen-depleted, used blood arrives from the body and is first cleaned of CO2 (carbon dioxide).
This is produced as a normal “decomposition product” in the cells, similar to the way CO2 is produced in a car by burning energy (petrol). Once this has taken place, the blood is supplied with O2 (oxygen), i.e. oxygenated. At this point it is also possible to heat or cool the blood, which may be necessary during longer heart operations.
The enriched oxygen-rich blood is then pumped back into the body, more precisely into the aorta, through tubes from the heart-lung machine. From there, it is distributed as usual through the arteries to all areas of the body, such as the brain, kidneys, muscles, etc. During operations, the heart-lung machine is usually located a few meters away from the patient, the operating table and the surgeon, and is operated by a specially trained cardiotechnician.
The cardiotechnician monitors the function of the heart-lung machine during the entire operation, and changes its parameters in consultation with the surgeon and anesthesiologist. Secondly, the heart-lung machine offers the anaesthetist the possibility of introducing anaesthetic gas into the blood, thus achieving relatively precise control of the anaesthesia. There are extra devices and valves on the machine for this purpose.
Third, the filter function. For patients with terminal kidney failure, filtration can be performed in the heart-lung machine. It works on the same principle as a dialysis machine, i.e. it washes the blood.
For this purpose, filters and membranes are interposed in the blood circulation. To a certain extent, this is also necessary because plastic parts can always come loose from the tubes, or blood clots from the vessels, and embolisms (blood clots) can form in the body. Of course, both the device and the tubes are subject to the highest medical requirements and standards, but microscopic particles are sufficient to cause vascular occlusion.
(For more information, see ? Risks and side effects )In addition, undissolved gas bubbles can be filtered out of the circulating blood, thus preventing air embolism, in which an air bubble collects in the vessels. Fourthly, in the heart-lung machine, blood can be added or diverted and stored.
This is particularly useful during operations with increased blood loss. Thus, at the beginning of operations, the circulating volume can be reduced and blood can be stored in a bag in the HLM. If there is increased blood loss later, this blood is replenished and, if necessary, supplemented with donor blood of the same blood group.
