Optimisation of the pressure and flow ratio under extracorporeal circulation
Pulsator (half section and prototype)
Exemplary result of a measurement from an experiment with the Pulsator
The project presented here examines the research cooperation between the Institute for Product Development, Technische Universität München, and a cardiology centre in Munich, Deutsches Herzzentrum München. The main aim of the project was the optimization of the interaction of the heart lung machine (HLM) and the human organism. The extracorporeal circulation (ECC) - where the HLM takes over the blood circulation, gas exchange and thermoregulation of the blood outside of the body - belongs to the routine procedures of open heart surgery. Nowadays the control in clinical application takes place depending on the evaluation of hemodynamic pressure and flow ratio and chemical data from the laboratory. The control is based on the intuition and experiences of the surgeons, anaesthesiologist and cordiotechnician essentially. Until today no direct feedback between the actual values of the HLM and the reactions in the organism caused thereby exists. Target of the research project is the development of a system, which enables a more physiologic ECC through an improved adjustment to the human organism. In this way the risks and the after-effects of open heart surgery can be permanently reduced.
The extracorporeal circulation (ECC) - where the HLM takes over the blood circulation, gas exchange and thermoregulation of the blood outside of the body - belongs to the routine procedures of open heart surgery. Nowadays the control in clinical application takes place depending on the evaluation of haemodynamic pressure, flow ratio and chemical data from the laboratory. The control is based on the intuition and experiences of the surgeons, anaesthesiologist and cardiotechnician essentially. Because of intern control mechanisms, which affect the heart and the vascular system, the organism is able to adjust optimal perfusion pattern in every situation normally. While an ECC there is no direct feedback between HLM and the (patient). During the surgery the HLM and important physiological parameters are monitored and controlled by the cardiotechnician. For this it is necessary to make time-consuming laboratory analyses of blood values (oxygen saturation, ph value etc.). That leads to the fact that deviations from the ideal condition can be recognized only with a quite large time delay of roundabout 10 minutes. Because of this it occurs rather often, that a reaction is introduced too late and can hardly be controlled.
In addition it comes that conventional HLM generate nearly constant flow and pressure, which is not comparable with the physiological gradient of flow and pressure. As a result of this the resistance of peripheral blood vessels rises and the capillary perfusion worsens. The generation of tissue hormones is an aftereffect of this less blood circulation. Tissue hormones are responsible for a generalized inflammation reaction. In the consequence of an inflammation reaction sometimes organs fail after surgery. So far there is no information whether a current online modification and adjustment of the perfusion pattern generated by the HLM improves the coupling of HLM and patient, and whether it generates a more physiological perfusion.
Definition of an ideal heart-lung-machine
How does an „ideal heart-lung-machine” look like? As described in the beginning of this article, the extracorporeal circulation - and related to this the interaction between the heart-lung-machine and the patient’s organism - are responsible for the postoperative trauma. The time for reconvalescence after e.g. a heart attack does not depend on the disease itself, but of the surgery and in this particular case especially of the extracorporeal circulation. In time of restricted budgets in the social section, especially in the health care system, this is an unacceptable situation.
Expressed in an abstract way, an “ideal heart-lung-machine” is therefore a system which is taking over the function of the heart (and lung), without being noticeable to the patient’s organism at all. This target can be divided into two subordinated targets: On the one hand, the patient’s blood should not be affected by the system, on the other hand the system should guarantee a supply with blood in a naturally and physiological way.
The first target is already pursued by existing systems. Surfaces coated with Heparin, and transitions decreasing flow resistance between different compounds which are streamed by blood, are the actual technical solutions for his target. Though the demand for a physiological perfusion is far from being fulfilled. Today’s blood-pumps, taking over the function of the heart, are roller pumps generating a more or less constant flow of the blood. Most of these common pumps are also able to create a so called “pulsatile” flow of the blood. Indeed this type of flow isn’t physiologic in the least: Neither the frequency of this pulsation, nor the velocity of the increase of the blood flow and pressure, nor the flow rate correspond to the natural parameters of the human heart.
The ideal physiological supply for the patient in the clinical service could look like this: At the beginning of the surgery, the relevant parameters of the patient are recorded. These parameters will be transferred to the HLM subsequently. The HLM processes the received data and generates a pulse similar to the one of the patient’s heart. Can things like this be realized at all? Besides the challenging hardware development, difficulties exist especially for the required controller. A mathematical relation between the operational characteristics of the HLM and the generated pulse in the patient’s body seems to be quite difficult. It will be necessary to apply an intelligent concept for the controller: During the first minutes of the surgery, the patient receives a blood supply, which is approximated to his own. In the further course of time this blood flow will be optimized by a self-learning controller. This might be possible by using neuronal networks. Apart from this it has to be ensured, that the damage of the blood doesn’t increase - a surely challenging demand for the design of the pulsator.
A superior medical question exists: Are the above mentioned hypotheses correct at all? Does man need a pulse? Has it to be physiological? Experiments with calves demonstrated that these animals were able to survive for months without showing any complications or major restrictions even though their hearts had been exchanged by artificial ones generating a constant blood flow. Obviously the design of an „ideal heart-lung-machine” can only be successful if it’s possible to clarify whether a physiologic circulation offers any advantage to the patient.
In accordance with the considerations above prototypes were developed, which avoid the described weak points. Regarding the perfusion during the ECC the system (Pulsator) should fulfill the following requirements:
- Closed loop control of:
- aortic blood pressure (65 – 120 mmHg)
- blood flow (0 – 8 lpm, peak flow 30 lpm)
- “pulse” frequency (50 – 95 bpm)
- slew rate of pressure (1000 mmHg/s).
- No traumatization of blood caused by the pulsator.
- In vivo data of the patient can be read into the system over appropriate sensors.
- The cardiotechnician can intervene at any time in the system.
The developed pulsator is a simple piston pump, consisting of a cylinder and a piston with a flexible actuation and a programmable logic controller. A constant flow generated by the roller pump of a conventional HLM passes a cylinder. At the inlet of the pulsator is a non-return valve. If the piston is stopped, the blood still can flow through the pulsator to the patient. In this case there are the same haemodynamic pressure and flow ratio, as known from conventional ECC.
Generating a pulsation takes place in two phases, which are described subsequent:
- Phase 1: If the piston is pulled back, the volume of the cylinder increases and must be filled by the flowing fluid of the HLM.
- Phase 2: If the piston is moved forward again, the volume in the cylinder decreases and the fluid leaks out from the cylinder towards the patient. Because of the non-return-valve the blood can not exhaust to the HLM. Compared to the ECC without pulsator this leads to an increase of the flow rate and the blood pressure to the patient.
Changes of the pressure and the flow rate can be set very individually by a variation of the speed, the stroke and the frequency of the piston’s motion. In this way and with an adequate control and high-dynamic mechanics a pulse can be copied.
After completion and technical verification of the prototype ten laboratory tests with young pigs were carried out. These first tests were necessary to improve the prototype and to gain experiences in handling the system. In consequence of the tests some constructional changes had to be made to reduce the pressure loss between pulsator and laboratory animal and the maximum pressure within the cylinder of the pulsator.
After this first test phase twelve standardized attempts were accomplished. All experiments ran according to the same pattern.
The evaluation of the measured data showed that it is possible to reproduce the pressure ratio with the pulsator to the greatest possible extent. The use of this was confirmed by the evaluation of first tissue specimen, which showed a improved blood circulation in the organs in relation to values of former experiments without pulsation.
To make a well-founded statement, actually experiments with conventional ECC are carried out. The results of these tests have to be compared with the results of the Pulsator tests.
In the context of this project it could be shown that the pulsators effects a clear improvement to the hemodynamic pressure and flow ratio. To verify this cognition, the results from the attempts with pulsation and from the attempts without pulsation must be compared. If the positive estimate could be confirmed, following measures must be taken, in order to improve the current design of the pulsator.
- A modular integration of the Pulsator into existing heart lung machines must be possible. That means that the entire aggregate, consisting of control, PLC and drive is assembled in a module of the size of today’s HLM components.
- The parts, that pipeline blood, consist of high-grade steel today. They must be arranged for costing and sterility reasons as one-way parts.
- Development of different cylinder/piston combinations, which make a flexible use for different patient sizes possible.
- The valve consisting of an expensive artificial heart valve must be developed as one-way part with the same functionality.
- The large pressure losses must be reduced.
- The mechanical damage of the blood corpuscles by the piston must be decreased.
- The communication of the control with existing HLM systems must be possible.
- Measurement of the aortic pressure and flow of the patient must suffice for control of the pulsator.
The system-costs, especially all the one-way parts, must be reduced.
Structure, test and certifying of an close-to-production prototype.
The development of the necessary mechanics was already continued. In the following pictures the developed new prototype is shown. It operates with a flexible membrane and a separation liquid (NaCl), which separates the blood of the piston. The unit corresponds in its dimensions with conventional HLM.
In this way the described project can be a contribution for the reduction of the consequential damage and the convalescence times after a heart surgery. This does not only lead to an improvement of the situation of the patient. This development can help to reduce the costs of the health insurance companies and thus for the public also.
Dipl.-Ing. Christoph Jung | TU Munich