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東北大学加齢医学研究所 心臓病電子医学分野
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Making of the feedback loop in circulation by an artificial heart system

Is the information of the biological system necessary for artificial heart?

It may be needless if doesn't think about QOL. A patient who implanted artificial heart will must return to the Society in future. Various kinds of stimulation from external is given to a patient after having returned to the Society. Artificial heart must respond to this condition. Even if Homeostasis is disturbed by various stimulation, it must be returned. If these requirements isn't filled, circulation can't be maintained, and of course homeostasis cannot be able to be maintained. Accordingly, stimulation from external needs to be responded. Thus, some sensor is necessary to respond to turbulence of circulation. A representative of turbulence of circulation is the alteration of a blood pressure. If blood pressure can't be maintained, circulation can't be maintained. There is arterial blood pressure reflex mechanism to maintain blood pressure in the circulatory regulatory system in the biological system. This system is considered to become necessary to maintain blood pressure in an artificial heart circulation.

In 1994, Abe et al. designed 1/R control algorithm for an artificial heart [3, 22]. By this system, vascular resistance is calculated. As for this information, it is fed back to the artificial heart control algorithm. Accordingly, this algorithm can be able to become a control mechanism same as arterial baroreflex system of biological system.

However, we must maintain the optimal drive point of the artificial heart to prevent thrombus formation, and left and right heart flow balances were important. These concept had thought to be having the priority.

In this research, optimal control of the artificial heart, right and left flow balance, and automatic control which maintained hemodynamics within normal range was produced experimentally. And, in addition to this, we fed back vascular resistance every 1 heart rate. In other words, we created control algorithm to have arterial blood pressure reflex while maintaining circulation. We examined this algorithm in Moc circulation and with the acute and chronic animal experiments.

And the next important issue of this study is shown below. Several investigators suggest that hemodynamic fluctuations like heart rate variability (HRV) were originated from some biological feedback mechanisms. Especially, Mayer wave around 0.04 - 0.15 Hz in HRV were suggested to be originated from the arterial baroreflex system. Thus, it was very interesting problem to confirm these thesis and investigate the spectral analysis of this artificial baroreflex system.

Material and method

Before we start the animal experiments, we tested these automatic control algorithm in Moc circulation model during changes of the peripheral vascular resistances. After the confirmation of these control algorithm, we started the acute animal experiments, firstly.

Three goats were kept fasting two days before the experiments. After the anesthesia inhalation with halothane, left chest cavity were opened through the fourth intercostal space. Inflow and outflow polyvinyl chloride (PVC) cannulae were inserted into the left atrium and descending aorta, respectively. And then, right Inflow and outflow polyvinyl chloride (PVC) cannulae were inserted into the right atrium and pulmonary artery, respectively. Both sets of cannulae were connected to our TH-7 pneumatically driven sac type blood pump by the connector built-in silicone ball valves. Electrocardiogram (ECG), central venous pressure, left atrial pressure, arterial pressure, and both sides of pump flow were recorded during the experiments. Both pump flow were measured with electromagnetic flow meter on the cannulae. After animal preparation, ascending aorta and pulmonary artery were completely ligated to constitute the total biventricular bypass with the artificial heart.

TAH automatic control algorithm were based upon these concept. Firstly, optimal drive point of the inner sac were maintained by the automatic control algorithm for the pneumatic driver. It was needed to prevent the thrombus formation in the inner sac. Secondary, by the alteration of the stroke volume, left and right flow balances were maintained. Stroke volume changes were maintained within an optimal operating point. And at last, maintain the hemodynamics within normal range and an automatic TAH control algorithm based upon artificial baroreflex concept were added to these basic control algorithm.

By the drug administration, arterial blood pressure were altered to observe the behavior of the automatic TAH control algorithm.

Chronic animal experiments using three healthy adult goats were performed for the confirmation of the usefulness of these automatic control algorithm. After the anesthesia were induced with halothane inhalation in the three adult goats weighing 46+5 kg, the left pleural cavity were opened through the fourth intercostal space under mechanical ventilation. Electrodes for electrocardiogram were put on the pericardium. Biventricular bypass were implanted as the acute animal experiments. Before we closed the chest cavity, pulmonary artery were completely ligated to constitute the total right heart bypass and ascending aorta was almost ligated to prevent the myocardial ischemia. Only coronary flow were maintained with natural heart. Left and right atrial pressures and arterial blood pressures were measured from the side hole of the cannulae, and pump flow were measured with electromagnetic flow meter on the cannulae.

After the chest were closed, the goats were placed in the cage and extubated after waking. The Hemodynamic derivatives was recorded to data recorder. The data was input to the personal computer system through A-D converter.

Results

All hemodynamics was maintained within normal range by this automatic control algorithm. Right and left balance, too was able to be maintained. Reaction at having administered agent with acute animal experiment is shown in fig.21. By administration of vasoconstrictive medicine, blood pressure rises. It is methotrexate that was administered here. As for this phenomenon, a thing by elevation of vascular resistance is shown in figure 1. Vascular resistance is calculated every one beat for the input in this automatic control algorithm. Accordingly, the reaction of artificial heart was immediate. Beat / min of artificial heart falls after thing of elevation of vascular resistance immediately. Blood pressure falls hereby, and homeostasis is maintained.

Discussion

The most important results of the study was that Artificial baroreflex system were embodied by these basic TAH automatic control algorithm having optimal TAH drive and left - right balances and keep hemodynamic parameters within normal range and at last, 1/R control were added to these basic concept. Maintain the optimal TAH drive were of course important to prevent thrombus formation and endurance of the inner sac. Left and right heart balances were of course important to keep peripheral and pulmonary circulation and prevent the pulmonary congestion. And basic hemodynamic parameters were of course needed to keep peripheral circulation and blood supply for each organs. And as last Baroreflex system were of course important , too, to keep homeostasis in a patient with artificial heart. In this study these stepwise automatic control were embodied.

Various investigators researched the circulatory regulatory system during artificial circulation. However, previous reports may be all altered by the automatic control algorithms. Thus, for the physiologically optimal TAH drive, further study may be needed in future.

Thus, in the next step, we performed chronic animal experiments and nonlinear dynamic analysis.

Last modified:2006/04/10 22:46:38
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References:[Welcome Dr. Yambe's Home Page!! (eng)] [Origin of Chaos]

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