Patent Application
20080045779
The invention will now be described, as a simple example and without limiting intent, with reference to the attached drawings, in which:
Although the present description makes reference to a VAD with electromechanical operation, those of skill in the art will immediately appreciate that the invention applies in general also to VADs with different actuation systems, such as those to which reference was made in the introductory part of this description.
Indeed, the present invention principally concentrates on the alternating gaseous flow that occurs in the conduit 7, which—as was described above—is produced, and thus is caused, by the contraction and expansion movements of the pumping sac, regardless of the ways and means adopted to produce the contraction movement and enable expansion of the variable-volume chamber comprising the sac 3.
The proposed solution, similarly to known solutions developed for the systems Novacor and HeartMate described above, is based on detecting the filling of the blood sac 3 of the VAD. This detection is not however achieved directly through sensors applied inside the blood pump, on the sac itself or on the actuator, but rather indirectly by measuring the flow of air coming out, through the conduit 7, from the casing 2 of the VAD to compensate for the reduction in free volume inside it caused by the filling of the sac with blood.
As is represented in diagram form in
The sensor 10 is thus capable of generating a signal that is indicative of the flow in the conduit 7 and thus of the cyclic inflow and expulsion of blood into and out of the sac/variable-volume chamber, that is in practice, of the functioning of the VAD as a pumping device. In particular, the gaseous flow leaving the conduit 7 is indicative of the filling of the sac 3 of the VAD and thus of the ejection of blood from the left ventricle of the heart, that is of its systole.
Typically, the sensor 10 generates a signal indicative of the rate of flow (volume per unit time) or of the speed of flow of the gaseous matter in the conduit 7 and consequently indicative of the flow of blood ejected from the left ventricle of the heart towards the flexible sac 3 of the VAD.
The sensor 10 may consist of a flow meter of known type, such as a flow meter for gases or a device comprising a pressure transducer (for example piezoelectric) situated upstream of a constriction or fluid resistance.
For example, the flow meter 10, whose output signal is provided to the electronic unit (controller) 8 that supervises operation of the VAD 1 overall, may to advantage consist of the flow meter produced by the company Honeywell and available on the market under the code number AWM43600V. It may if desired be mounted in parallel to a small bypass conduit useful to adapt the interval of flow characteristic of the commercial flow meter to that of interest for the specific application (for example between 0 and 40 litres/minute).
It will be appreciated that the sensor 10 lends itself to being situated external to the blood pump (connected to the circulatory system and if desired implantable inside the human body), associating it in very diverse manners, for example introducing the sensor 10 into the conduit 7 or integrating it into the filter unit 9 (
In any case, the most critical part of the VAD system, the pump, is not needlessly made more complex by the addition of the sensor or sensors needed for its operation in the “synchronous counterpulsation” modality and at the same time these sensors, here represented by the sensor 10, are situated in positions easily reachable for any replacement or repair action.
Measurement of the flow of air leaving the conduit 7, see curve F in
This signal may be acquired and processed with simple techniques, of themselves known, by the electronic control unit 8, which is therefore capable of commanding the pumping unit 3-6 in function of the signal generated by the sensor 10. Just as it is incidentally clear to technicians who are experts in the sector, that in stating that the electronically controlled unit 8 is capable of commanding the pumping unit 3-6 as a function of the signal generated by the sensor 10 it is not in any way intended to imply that control of the pumping unit is only carried out as a function of the signal of the sensor 10: in reality, the unit 8 receives the flow signal from the sensor 10 and utilises that signal to control the functioning of the pumping unit 3-6, together with other parameters/signals available to the unit 8. For example, in the case of actuators operated by an electric motor, the electronic control unit 8 generally has available to it signals relating to the motion of the motor 6 and/or of the pusher plate 5 provided by specific encoders.
This situation may easily be verified in a VAD 1 of the type described here by observing the fact that, other conditions being equal, the device behaves in a different way, that is the unit 8 commands the pumping unit 3-6 in a different manner, depending upon whether the sensor 10 is connected or is not connected to the unit 8.
For example, the unit 8 may detect the maximum MF in the curve of the flow leaving the conduit 7 and wait for the flow to drop below a threshold T, defined as a given percentage fraction of the maximum flow MF, substantially corresponding to the end of the filling peak of the sac 3 and thus to the end of the systole of the natural heart. The electronic control unit 8 may at that instant command the actuator 6 such as to start a cycle of movement of the pressure plate 5, such as that illustrated in
It must be stressed that, in general, analysis of the flow signal may be performed on the basis of the shape of said signal without the need to have available absolute values of the gaseous flow measured. This leads to intrinsic system safety since it makes the control system independent of any variations in sensitivity of the flow meter consequent on changes in temperature or on ageing.
For further reference, the scale of the abscissa in
with I, the instant at which the flow drops below the threshold T defined above and at which the control unit 8 commands the start of the movement of the pusher plate 5 aimed at producing contraction of the sac 3,
with II, the instant at which the VAD begins ejection (crossover point of zero flow in the conduit detected by the sensor 10),
with III, the interval of ejection of the VAD, and
with IV, the amplitude of the movement squeezing the sac 3 by the pusher plate 5 (representative in practice of the charge of blood delivered) after contact with the sac 3.
It will be appreciated that the instants I and II do not coincide. Indeed, it is usual to operate such that (according to known criteria) in the release movement of the sac 3 that leads the sac 3 to fill with blood, the pusher plate 5 disengages from the wall of the sac in order to leave the sac 3 completely free to expand and receive blood. In consequence, in commanding contraction of the sac, the pusher plate 5 must initially make a brief movement to regain contact with the sac 3 and to begin ejection. From observation of this fact it appears that, in an independent manner from any other consideration, the action of monitoring the gaseous flow in the conduit (percutaneous line) 7, achieved through the sensor 10, also makes it possible to detect the movement of expulsion of the blood with regard to the sac 3 in a much more precise and faithful fashion than could be done, for example, by monitoring movement of the pusher plate 5. This precisely because the gaseous flow in the conduit 7 is caused by the contraction/expansion of the sac 3 which acts as a flexible pumping chamber for the blood.
Other control logics are naturally possible; it is for example possible to detect the start of the systole of the natural ventricle in correspondence with the maximum slope of the rising part of the air flow leaving the conduit 7, and control operation of the VAD starting from detection of this instant, which is closely correlated, in the cardiac cycle, to the start of the systole.
The signal corresponding to the air flow leaving the conduit 7 is appropriate to be acquired and processed by the electronic unit 8 of the VAD in such a manner as to control operation of the VAD in a synchronous manner and in counterpulsation with the heart. This result conforms to what is illustrated in
In programming the electronic unit 8 that controls the VAD, as was said above, an operational cycle is defined that comprises, for example:
compression of the sac (ejection of blood by the VAD),
return to the retracted position (sac free to fill again),
wait to detect the end of a subsequent cardiac systole before starting the next cycle.
In programming the electronic unit 8, a maximum value may be set for the above wait time. In this case, should no systole be detected, due to a period of cardiac asystolia or a malfunctioning of the flow sensor 10, the VAD 1 is activated at a fixed frequency. This ensures that the system operates in a condition of safety, without allowing cardiocirculatory assistance to lack, either in the case of an irregular cardiac cycle or in the case of malfunctioning of the cycle detection system. Should the cause of a failure to detect the cardiac systole be an arrhythmia (such as for example an asystolia) followed by a return to a regular cardiac rhythm, the system to detect and process the air flow leaving the conduit 7 enables the control unit 8 to become aware of the return to a regular rhythm and to re-synchronise operation of the VAD with the heart beat.
This is illustrated in
It may be noted that, during the asystolia, the VAD is activated at a fixed frequency (movement of the pusher plate with long and constant wait times) whereas on return to the regular cardiac rhythm, operation of the VAD, during a small number of cycles, returns to being synchronous and in counterpulsation.
Naturally, without prejudice to the underlying principles of the invention, construction details and embodiments may vary, even significantly, with regard to what is described here as a simple example without limiting intent, without thereby departing from the scope of the invention, as defined by the attached claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06425592.0 | Aug 2006 | EP | regional |
