STROKE VOLUME

This invention broadly relates to heart assistance systems based upon the aortic counterpulsation mechanism and more particularly it concerns an aortic counterpulsation system adapted to monitor the effectiveness level of a clinical treatment by measuring one of the critical parameters of the cardiocirculatory functionality of a patient, namely the stroke volume.

The aortic counterpulsation (ACP) performed by means of an intra-aortic balloon (IAB) can be considered as the presently most diffusely employed technique for mechanical circulation assistance (MCA) in the treatment of the left ventricular insufficiency.

This kind of assistance, which is carried out in series with respect to the cardiovascular system, is performed by means of a small inflatable balloon of synthetic non-trombogenic material, attached to a suitable catheter, which enables it to be introduced and placed into the ascending aorta, by means of already known and largely tested techniques.

The catheter upon which the intra-aortic inflatable balloon is mounted has first and second lumens. A first lumen is initially used to let the catheter slide along a metal guide wire preliminarly inserted into the vase and, upon placing the balloon and withdrawing the guide wire, to record the arterial pressure values; a second lumen is connected to a pneumatic system which enables the balloon to be inflated (inflation) during the filling phase of the heart (diastole) and to be deflated (deflation) during the ejection phase of the heart (systole) .

The volume changes of the concerned balloon result into consequences similar to the consequences of an

2 equivalent blood volume when it is alternatively subjected to suction and re-infusion cycles: but without requiring the intervention and the work of the heart. As a consequence thereof, the external work of the heart is effectively reduced, thereby lowering the metabolic requirements of the cardiac muscle and particularly the oxygen demand, as it has been evidenced both by means of experimental data and by means of clinical observations.

Since, in protodiastole phase, the intra-aortic balloon is inflated, it promotes a noticeable increase of the systemic diastolic pressure, with resulting increase of the coronaric perfusion gradient (which is defined as the difference between the systemic diastolic pressure and the left ventricular telediastolic pressure) which in combination with the left ventricle filling time, with the anatomical characteristics (lumen width and elasticity) of the coronaric arteries and with self-adjusting capability of the coronaric circulation, determines the cardiac muscle perfusion.

The hemadynamic and metabolic effects of the aortic counterpulsation can be summarized as follows: - the pre-load and the post-load as well as the telediastolic volume of the left ventricle are reduced,

- the coronaric flow both in the normal and in the side circulation paths is increased,

- the production and the build-up of the lactates, caused by activation of the anaerobic glycolyse in low oxygen condition, are reduced.

A system for aortic counterpulsation, therefore, comprises two sections: a first section consisting of an aortic inflatable balloon, inserted into the vascular tree of the patient, and a second section which is externally placed in order to perform control and supply functions for the first section.

The intra-aortic balloon is made of extendible, non-trombogenic polyurethane and, in its standard sizes, it has a length of 15 cm and a width of 2 cm with an external volume of about 40 cm ³ . It usually 5 consists of a single chamber which, in some embodiments, is divided by means of internal separation walls into two or three independent compartments (refer to Figures 1A and IB) .

As above mentioned, the concerned intra-aortic

10 balloon is attached to a vascular catheter having two lumens. The first lumen is connected, at one end, directly to the chamber or the chambers of the intra- aortic balloon itself and, at the other end, to a gas source (helium or C0 ₂ ) , thereby enabling the

15 introduction and the withdrawal (inflation-deflation) of an adjustable amount of gas. The second lumen, on the contrary, is arranged so as to receive the metal guide wire which is needed for percutaneous introduction of the intra-aortic balloon. After the

20 balloon is inserted and the guide wire is withdrawn, the above said lumen can be connected to a pressure transducer, by which the arterial pressure of the patient can be continuously monitored. If necessary, it can also be employed for infusion of pharmaceutical : ^"• 25 preparations.

A second section of the already known system for aortic counterpulsation comprises a control board including all necessary systems for controlling and timing the inflation-deflation phases of the balloon as

30. well as the gas source.

For the sake of completeness, it is to be mentioned that the alternated inflation-deflation phases of the concerned intra-aortic balloon are not carried out directly, but, on the contrary, they are

35 carried out by means of a pneumatic mechanism adapted to generate positive and negative pressures within a small container included in said control board (defined

as a buffer chamber) and comprising an auxiliary balloon of identical volume with respect to said IAB and connected thereto through one of said two lumens of the catheter filled with gas (refer to Figures 2A and 2B) .

The pneumatic system included in the control board of the counterpulsation apparatus alternatively generates a positive pressure and a negative pressure within the buffer chamber. The inflatable balloon housed therein is caused to inflate and to collapse thereby causing the gas contained therein to pass into the main intra-aortic balloon, so that there is no need that this balloon be directly connected to the gas source itself. The gas introduced into the circuit can be C0 ₂ or helium, as above mentioned, the latter being preferred because it is characterized by a lower viscosity and is adapted to guarantee a fast transit time even through catheters having a restricted cross- section. As it can be easily understood, the inflation- deflation phases should be performed in strict synchronization with the cardiac cycle (deflation in protodiastole and inflation in telediastole) . A correct timing can be guaranteed by automatic mechanisms, included in the control board, which enable to time the operation of the intra-aortic balloon with respect to the electrocardiographic pattern or to the pressure pulsation at the wrist of a patient, which is picked up through the lumen of the vascular catheter upon which the intra-aortic balloon is mounted.

In particular, the inflation phase is set on the base of the R wave of the electrocardiographic signal, corresponding to the telediastole phase. In this way, the concerned balloon is inflated exactly in coincidence with the closure of the aortic crescent shaped valves. This event is indicated on the systemic arterial pressure wave by the dicrotic incisure and

this repere can be used for timing the intra-aortic balloon in stead _, of the electrocardiographic signal, should this signal be of bad quality.

In all this operation mode, the life parameters of the patient should be carefully monitored.

In brief summary, four situations can be focused for a patient in need to be assisted by an aortic counterpulsation treatment:

- before any surgical operation, as a continuation of an already begun assistance course,

- during a surgical operation, before a cardio- pulmonar by-pass,

- during a surgical operation, as a help for all those patients having difficulties in exiting from a cardio-pulmonar by-pass, after a surgical operation as a support treatment, during a cardiac surgery post-operatory intensive therapy, in patients having a remarkable hemadynamic instability, low stroke volume or in the case of a perioperatory acute infarct of the myocardium.

Also in these circumstances, the characteristics of an aortic counterpulsation treatment are utilized to reduce the telediastolic pressure in the left ventricle, the left atrial pressure as well as the

^" post-load value, by exploiting, the possibility to achieve an increase ^' of the systolic stroke volume, of the cardiac rate and of the diastolic pressure, together with a consistent decrease of the total peripheric drags or resistances as well as of the left ventricular telediastolic pressure.

An aortic counterpulsation system can be additionally employed in patients having cardiac insufficiency in terminal stage as a "bridge to transplant" and, after a heart transplant, should difficulties be met in adapting the organ to the

hemadynamic requirement of the receptor subject or, in addition, in the case of acute rejection.

It has already been mentioned that, for an aortic counterpulsation treatment to be successfully carried 5 out, it is absolutely necessary to pick up a set of biologic signals adapted to enable the effects of the administered assistance to be evaluated, in particular in respect of the functional and hemadynamic parameters which can be indicative that the contracting and

10 pumping actions of the heart have been re-started.

In addition to the surface electrocardiogram already utilized for timing the inflation-deflation phases and in addition to the systemic arterial pressure values, it is possible to measure the pressure

15 establishing in the various heart chambers, by means of a special catheter, the so-called Swan-Ganz catheter, the operation of which is based upon a thermodiluition principle. The reliability and repeatability of the measurements carried out by the thermodiluition method ^■ -20 - have, however, noticeable limitations, only partially depending on the accuracy of the performed procedures.

The most noticeable limitations are as follows: the procedural error is 15%; in effect the pulmonary

^" ' flow rate is measured and it can be considered as

^• 25 equivalent to the systemic flow rate only in absence of

...intracardiac shunts; the cold solution should be at a temperature clearly lower than the patient's temperature and it -should be injected as rapidly as possible, in order to prevent it from being influenced

30 by the environmental temperature; as a consequence of the noticeable measurement differences, it is necessary to carry out at least five rapid successive measurements, to eliminate the extreme values and to calculate the average value of the three intermediate

35 measurements; the reliability of the measures depends on the cardiac rate and on the respiratory events; the

reliability of the method decreases as the cardiac flow rate lowers.

A further starting point to design the device according to this invention is the idea to exploit in novel manner the Doppler effect, which is already used in clinical field in evaluating the hematic flow speed; in particular by means of Doppler transducers arranged upon the patient's skin along the vase to be analyzed and by using the particulate components of the blood, in particular the red cells, as moving reflecting elements.

This theory ^" is based upon the concept that the sounds are mechanical vibrations capable to cause a rarefaction or a condensation (elastic waves) of the particles contained in the fluid medium wherein they are propagated. In particular, ultrasounds have been largely employed in recent years in clinical diagnostics, for instance to generate images (echography) . The ultrasounds utilized in clinical procedures are generated by using particular crystals (piezoelectric crystals) adapted to act as transducers, capable to convert an energy form to another one, in this case to convert mechanical energy to electric energy and vice versa. In fact, when an electric potential difference is applied to the two ends of a piezoelectric crystal, it

^■ changes its size, by restricting or extending its dimensions, as a function of the sign of the potential difference applied thereto and in this manner compression and rarefaction waves are generated. On the other hand, the concerned crystal will react to a shock wave with a temporary deformation and by generating a measurable potential difference between its end surfaces. The same piezoelectric crystal can be used both for emitting ultrasounds and for detecting the effects of the interactions between the emitted ultrasounds and

the structures they pass through as they are propagated.

The ultrasounds interact with the structures they pass through while they are propagated by generating absorption, divergence, reflection and refraction phenomena. The refraction effects taking place upon the interface separating two propagation media characterized by different acoustic impedances are particularly relevant. The case of a mobile interface is particularly important because it involves a frequency difference between the incident and reflected waves: this phenomenon is known as Doppler effect.

From the above explanation, the importance of the aortic counterpulsation in clinical practice, as well as its unreplaceable role in treatment of patients suffering of severe left ventricular insufficiency and/or in critical cardiac surgery procedures,

It has also been argumented that, for a correct assistance to be offered, it will be necessary of course to have a system available adapted to evaluate the efficiency of the treatment, as it is expressed by changes in the cardio-circulatory parameters of the assisted patient.

This invention is based upon the realization that the stroke volume, which is defined as the blood amount output from the left ventricle at each systole, is the most significant parameter to represent the contraction

- and pumping capability of the heart. On the base of such value, it is possible: - to graduate the assistance, by evaluating the indications and the .benefits for the patients; - to evaluate the effects of a concomitant pharmacological therapy; - to evaluate the interactions between pharmacological therapy and aortic counte-rpulsation; - to ascertain the most convenient time to- start a weaning procedure from the .assistance.

In view of all above observations, it is a specific object of this invention to propose an aortic counterpulsation system wherein the stroke volume is continuously measured, so as to modulate the aortic counterpulsation as a function of this continuous survey and wherein such measurement is locally carried out by means of a Doppler effect transducer.

As it is known from fluidodynamics, the quantity

(Q) of a fluid passing through the cross-section of a conduct can be calculated as the product of the flow cross-section area (CSA) and of the flow velocity integral (FVI) according to the following relationship:

Q = FVI x CSA

When this relationship is applied to the aorta during an aortic counterpulsation treatment, the Q value can be substituted for the stroke volume. The flow cross-section corresponds to the area of the aorta and it can be calculated starting from the aorta diameter (measured by means of echocardiographic procedures) .

Further details and advantages of this invention will be evident from the following specification by referring to the enclosed drawings

, wherein the preferred embodiment is shown by way of ^" illustration and not by way of limitation. In the drawings:

Figures 1A and IB schematically show two types of single chamber intra-aortic balloon, with and without separation diaphragms, respectively; Figures 2A and 2B respectively show the diastole and ventricular systole phases in aortic counterpulsation carried out by means of a safety buffer chamber;

Figure 3 schematically shows a catheter for aortic counterpulsation according to this invention, in combination with a Doppler transducer;

Figure 4 shows a detailed cross-section view of the catheter end point of Figure 3;

Figure 5 shows and end from view of the catheter of Figure 3. Referring now to the Figures and particularly to Figures 3, 4 and 5, it can be seen that the composite catheter according to this invention is a coaxial double-lumen catheter and comprises an outer lumen 16 and an inner lumen 17. The outer lumen 16 has a balloon 18 that can be inflated and deflated through the outer lumen 16 by means of holes 19 in its wall 20. The inner lumen 17 has the function to let the catheter slide along a thin metal wire preliminarly inserted into the concerned vase and then to record the values of the arterial pressure. The catheter additionally comprises, as an essential and characterizing feature, a piezoelectric crystal 10 having a circular shape and a diameter for instance of 2 mm, for a conventional type of catheter, placed at the end point of the aortic counterpulsation catheter. Furthermore, it has a central hole of 1 mm diameter for enabling the insertion of the guide metal wire which is needed to insert and to position the catheter itself into the vase. The micro-crystal 10 is conventionally excited so as to generate an ultrasound beam with a frequency of 20 MHz and a focal length of 3 mm. As for any already commercially available transducer, the excitation of the micro-crystal can be carried out by pulses, so as to utilize the intra-pulse gaps for receiving the echo signals from the fluid to be analyzed. It should be understood, however, that the principles of this invention can be advantageously exploited also when the micro-crystal is continuously excited, provided that the receiving system is suitably modified in a manner that those skilled in the art can easily conceive, or

provided that other commercially available transducers already designed for continuous excitation be employed. The external surface of the piezoelectric micro- crystal 10 is protected by a small cup 11 of a 5 suitable, non-trombogenic plastic material, having a neutral behavior with respect to ultrasounds.

Positioning the catheter within aorta guarantees an optimum alignment of the ultrasound beam and the direction of the blood flow, thereby enhancing the

10 Doppler effect (the ultrasound beam and the flow direction are parallel) .

The excitation voltage which is needed to generate the ultrasound beam is externally generated with respect to the patient and is coupled to micro-crystal

15 10 by means of connecting wires 12 and 13 extending within the wall of the inner lumen of the catheter. The electric signal generated by said crystal when it is hit by the reflection echo from the particulate elements of the blood (in particular the red cells) is

20 externally led by means of the same conductor wires 12 and 13. This output signal is applied to a Doppler flowmeter 14 which is adapted to process it both in sound form and in spectral form, in order to display it on a suitable monitor 15, in combination with the

- 25 electrocardiographic (ECG) signal and with the system

.. arterial pressure (AP) , as measured through the central lumen of the aortic counterpulsation catheter.

^■ - The device according to this invention can be considered as the sole aortic counterpulsation device

30 adapted to continuously measure, beat by beat, the systolic stroke volume during the counterpulsation process. In this manner, any further catheter up to now usually used to this effect becomes superfluous, thereby exploiting to the maximum extent the

35 capabilities of an already installed catheter to carry out an assistance treatment and so avoiding any further

action that intrinsically entails a certain risk to the patient.

The measurement carried out in real time is extremely accurate and is the sole measurement that can be directly carried out intravascularly.

Since a device according to this invention makes any change in the contractility of the left ventricle ^■ immediately evident, it enables the general performance of a mechanical assistance treatment of the circulation to be noticeably improved and, from a research view point, it offers an extremely useful help in defining which weaning technique is to be preferred in any individual circumstance.

Furthermore, from an operatory technique view point, at the time the catheter is installed, it enables to analyze the flow characteristics, so as to determine whether the common femoral artery has been

.entered (which is the traditional introduction channel for the catheter) and to ascertain that the introduced - catheter has not been installed in an other vase or in

. a dummy lumen, as in the case of an aneurysm and/or an

. aorta dissection, with possible consequent perforation

..-of the aorta itself: this means that the flow characteristics themselves can be utilized as a _. positioning guide for the wire.

..- The preferred embodiments of this invention

.have . been described and a number of variations have

- - been suggested hereinbefore, but it should expressly be understood that those skilled in the art can make other variations and changes to the details and construction particulars, without so departing from the scop,e thereof.