Material Characteristics Ideal for Providing Either Partial or Total Mechanical Support to the Failing or Arrested Heart and Method for Developing Ideal Characteristics for Underlying Cardiac Disorders

Abstract

A system and method for determining the proper dynamic strain profile of an elastomeric construct. The strain characteristics of a deficient heart are determined and compared to the normal strain characteristics of a healthy heart. A construct having elastomeric elements is provided that can expand along multiple axes. In an unloaded condition remote from the deficient heart, the elastomeric elements are pressurized to determine the pressure differential being experienced. Furthermore, optimal strain characteristics are calculated along a first axis and a second axis as a function of the pressure differential. The first optimal strain characteristic and the second optimal strain characteristic are used to estimate the dynamic strain characteristics that will be applied to the heart. The dynamic strain characteristics are compared to the optimal strain characteristics required by the heart to determine if the construct is proper using an automated drive.

Claims

1. A method of determining the optimal strain characteristics for a construct being applied to a heart, said method including: determining a set of strain characteristics required to be applied to the heart to assist the heart in pumping; providing a construct with elastomeric elements, wherein each of said elastomeric elements expands along multiple axes when internally pressurized, and wherein said multiple axes include a first axis and a second axis; in an unloaded condition remote from the heart, internally pressurizing said elastomeric elements to determine a pressure differential experienced by said elastomeric elements within said construct; in said unloaded condition, calculating a first strain characteristic along said first axis as a function of said pressure differential by multiplying a log of said pressure differential times a first constant and subtracting a second constant; in said unloaded condition, calculating a second strain characteristic along said second axis as a function of said pressure differential; utilizing said first strain characteristic and said second strain characteristic to estimate dynamic strain characteristics to be applied by said construct; comparing said dynamic strain characteristics estimated for said construct to said set of strain characteristics to determine if said construct will assist the heart in pumping; placing said construct into contact with the heart in vivo; and operating said construct to determine if said construct assists the heart in pumping.

2. (canceled)

3. The method according to claim 1, wherein calculating said second strain characteristic along said second axis includes multiplying a log of said pressure differential times a first constant and subtracting a second constant.

4. The method according to claim 1, where said first axis is angled relative to said second axis.

5. The method according to claim 1, wherein said first strain characteristic is a function of a first peak strain measured along said first axis.

6. The method according to claim 5, wherein said second strain characteristic is a function of a second peak strain measured along said second axis.

7. The method according to claim 6, wherein said first peak strain in said first axis (?peak-LA) is calculated using the formula
?.sub.peak-LA=11.676 ln(P)?5.2073 where (P) is said pressure differential.

8. The method according to claim 7, wherein said first strain characteristic (?x) is calculated using the formula $?\left(x\right)=\frac{{?}_{\mathrm{peak}-\mathrm{LA}}}{40.8}\left(4?{10}^{-6}{x}^4-0.0008x^3+0.0324x^2+0.6868x-0.4757\right)$ where (?peak-LA) is said first peak strain in said first axis.

9. The method according to claim 6, wherein said second peak strain in said second axis (?peak-SA) is calculated using the formula
?.sub.peak-SA=6.5167 ln(P)?5.1506 where (P) is said pressure differential.

10. The method according to claim 9, wherein said second strain characteristic (?x.sub.SA) is calculated using the formula $?\left(x_{SA}\right)=\frac{{?}_{peak-\mathrm{SA}}}{20.1}\left(-0.00005x^3+0.0004x^2+0.5171x+0.0052\right)$ where (?peak-SA) is said second peak strain in said second axis.

11. A method of determining if a pumping construct assists a heart in pumping, said method including: determining required dynamic forces that need to be applied to the heart to assist the heart in pumping; providing said pumping construct, wherein said pumping construct has elastomeric elements that expands when internally pressurized, and wherein each of said elastomeric elements has a first axis and a second axis; internally pressurizing said elastomeric elements to determine a pressure differential experienced by said elastomeric elements within said pumping construct when said elastomeric elements are not in contact with the heart; measuring said elastomeric elements to obtain deformation measurements as elastomeric elements are internally pressurized, wherein said deformation measurements include a first peak strain measurement along said first axis and a second peak strain measurement along said second axis, and wherein said first peak strain measurement is a log of said pressure differential times a first constant and subtracting a second constant; utilizing said deformation measurements and said pressure differential to estimate applied dynamic forces; comparing said applied dynamic forces estimated for said pumping construct to said required dynamic forces to determine if said pumping construct can assist the heart in pumping; placing said pumping construct into contact with the heart in vivo; and operating said construct to determine if said construct assists the heart in pumping.

12. (canceled)

13. (canceled)

14. The method according to claim 11, further including calculating a second peak strain measurement along said second axis by multiplying a log of said pressure differential times a third constant and subtracting a fourth constant.

15. The method according to claim 14, wherein said first peak strain measurement and said second peak strain measurement are used to estimate said applied dynamic forces.

16. The method according to claim 15, wherein said first peak strain measurement in said first axis (?peak-LA) is calculated using the formula
?.sub.peak-LA=11.676 ln(P)?5.2073 where (P) is said pressure differential.

17. The method according to claim 16, wherein said first strain measurement (?x) is calculated using the formula $?\left(x\right)=\frac{{?}_{\mathrm{peak}-\mathrm{LA}}}{40.8}\left(4?{10}^{-6}{x}^4-0.0008x^3+0.0324x^2+0.6868x-0.4757\right)$ where (?peak-LA) is said first peak strain measurement in said first axis.

18. The method according to claim 17, wherein said second peak strain measurement in said second axis (?peak-SA) is calculated using the formula
?.sub.peak-SA=6.5167 ln(P)?5.1506 where (P) is said pressure differential.

19. The method according to claim 18, further including calculating a short axis strain (?x.sub.SA) using the formula $?\left(x_{SA}\right)=\frac{{?}_{peak-\mathrm{SA}}}{20.1}\left(-0.00005x^3+0.0004x^2+0.5171x+0.0052\right)$ where (?peak-SA) is said second peak strain measurement in said second axis.

20.-27. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] For a better understanding of the present invention, reference is made to the following description of exemplary embodiments thereof, considered in conjunction with the accompanying drawings, in which:

[0023] FIG. 1 is a schematic of an exemplary embodiment of a heart pump system showing a heart, a heart cuff, and an external automated drive;

[0024] FIG. 2 shows the heart cuff of FIG. 1 in cross-section and in an unloaded condition with indicators of maximum and minimum deflections due to applied pressure differentials;

[0025] FIG. 3 shows the heart cuff of FIG. 1 from above, unloaded and in a state of maximum positive pressure inflation and producing inflated compartments;

[0026] FIG. 4 is a heart cuff illustrated by a transection of the long axis and the short axis on a compartment membrane this serves to illustrate how partial cuffs can be used to support a more limited select area;

[0027] FIG. 5 is a graph that illustrates the change in volume in a compartment as a function of pressure change from the external automated drive;

[0028] FIG. 6 is a graph that illustrates an example strain profile of an unloaded cuff across one beating cycle;

[0029] FIG. 7 is a graph of material peak strain verses surface area with results for an unloaded condition, an in vivo loaded condition and a mock loaded condition;

[0030] FIG. 8 is a graph that illustrates the relationship between active membrane surface area in relation to the maximal diameter of the heart at the end of the diastolic cycle;

[0031] FIG. 9 is a graph that illustrates the changes in pressure experienced by the heart cuff in operation; and

[0032] FIG. 10 is a schematic of an exemplary embodiment of a heart pump system showing a heart in conjunction with a partial heart cuff.

DETAILED DESCRIPTION OF THE DRAWINGS

[0033] Although the present invention system and methodology can be embodied in many ways, only two examples are illustrated and described. The exemplary embodiments being shown are for the purposes of explanation and description. The exemplary embodiments are selected in order to set forth some of the best modes contemplated for the invention. The illustrated embodiments, however, are merely exemplary and should not be considered as limiting when interpreting the scope of the appended claims.

[0034] Referring to FIG. 1, a heart pump system 10 is shown that helps a heart 11 pump blood. The heart pump system 10 has an external automated drive 14 that selectively applies positive and negative pneumatic pressure to a heart cuff 12. The heart 11 has measurable dimensions that are unique for a particular individual or condition. One of the measurable dimensions is the maximal diameter of the heart in its short axis (D.sub.max). The maximal diameter corresponds to the diameter of the myocardium as measured at the end of the diastolic cycle. Likewise, the heart 11 has a measurable length L.sub.(ED) that corresponds to the length of the ventricles at the end of the diastolic cycle. These dimensions of the heart 11 can be readily obtained from various medical scanning equipment, such as x-rays, ultrasounds, MRIs, and the like. Furthermore, it is understood that the heart 11 has a heartbeat, wherein the heart 11 contracts with a regular rhythm. Although the rhythm can be irregular, it has a general average rate of contraction over increments of time. The rhythm of the heart 11 can also be quantified using heart monitoring equipment, such as a blood pressure monitor or an ECG unit.

[0035] The heart cuff 12 fits outside the ventricles of the heart 11. In the shown embodiment, the heart cuff 12 covers the ventricles of the heart 11. However, other cuff and cup designs can be used that only contact specific areas of the ventricles. The selection of a full cuff or a partial cuff depends upon the needs of the patient. A full cuff, as shown, can apply forces to both the left and right ventricles. A partial cuff or similar device, such as a contact bladder, may only apply forces to one ventricle or part of one ventricle.

[0036] Referring to FIG. 2 in conjunction with FIG. 1, it will be understood that the exemplary heart cuff 12 being illustrated is made, at least in part, from elastomeric material. The heart cuff 12 is connected to the external automated drive 14 that can cause the heart cuff 12 to selectively contract and expand. The heart cuff 12 has an elastomeric shell 13. Within the shell 13 are a plurality of elastomeric compartment membranes 20. The expansion and contraction actions of the heart cuff 12 are typically created by applying positive and negative pneumatic pressure to various elastomeric compartment membranes 20 embodied or attached within the shell 13.

[0037] When the heart cuff 12 is applied to the heart 11, it is considered to be in a loaded condition. That is, the heart cuff 12 is being contacted by the heart 11 and is effected by the various forces applied by the heart 11. In an unloaded condition, the heart cuff 12 is not in contact with the heart 11 or any model of the heart 11. Rather, the heart cuff 12 is free to expand and contract as determined only by the design of the heart cuff 12 and the pressures applied to the heart cuff 12 by the external automated drive 14.

[0038] In FIG. 2 the heart cuff 12 is shown in an unloaded condition. As will be explained, the heart cuff 12 is designed, tested, and customized in this unloaded condition. Both the shell 12 of the heart cuff 12 and the elastomeric compartment membranes 20 within the shell 13 have the ability to expand and contract. Due to the design of the heart cuff 12, when the elastomeric compartment membranes 20 are pressurized, they tend to expand inwardly and fold in into relatively defined compartments 18. The compartments 18 are defined between the shell 13 and the compartment membranes 20. Constructs of a heart cuff 12 can be designed such that they comprise only one or multiple compartments 18 depending on how much surface area of the heart 11 in which the cuff 12 intents to act upon.

[0039] A basal attachment seam 22 is located at the top of each compartment membrane 20 where the compartment membrane 20 is anchored to the shell 13 of the heart cuff 12. Likewise, an apical attachment seam 24 is located at the bottom of each compartment membrane 20 where the compartment membrane 20 is anchored to the shell 13 of the heart cuff 12. Between the basal attachment seam 22 and the apical attachment seam 24, the compartment membrane 20 is free to expand and contract in an elastic manner. For a heart cuff sized for an average person, the surface area of compartment membranes that is free to expand and/or contract over the majority of the entire heart's outer surface is approximately 220 square centimeters, +/?55 square centimeters.

[0040] The compartment membrane 20 is made of elastomeric material that expands and contracts as different pressures are applied. Accordingly, the volume of the underlying compartment 18 dynamically changes with changes in pneumatic pressure. Referring to FIG. 4 and FIG. 5 in conjunction with FIG. 2 and FIG. 3, it can be understood that each compartment 18 has a given volume at ambient pressure. The volume increases significantly when the compartment 18 experiences a positive pressure and decreases slightly in volume when a negative pressure is applied. The changes in volume over time correspond to the change in shape of the compartment 18. The change in shape over time of the compartment 18 can be expressed as the dynamic strain ?(t) of the compartment.

[0041] The compartment 18 is a three-dimensional construct that experiences strain in multiple directional planes that include a long axis plane 26 and a short axis plane 28. The long axis plane 26 would be the long axis of the heart's ventricular surface for which the cuff is intended to act upon. The short axis plane 28 would be the short axis plane of the heart's ventricular surface for with the heart cuff 12 is intended to act upon. Strains also occur in a radial axis plane. The radial axis plane would be the radial axis of the heart's ventricles for which the cuff is intended to act upon. Measurements of strains along the long axis plane 26 and the short axis plane 28 can be used as surrogates in estimating the overall three-dimensional strains. The long axis plane 26 extends generally vertically through the center of the compartment membrane 20. The short axis plane 28 extends generally horizontally through the center of the compartment membrane 20. Accordingly, the long axis plane 26 and the short axis plane 28 are perpendicular, or near perpendicular planes that are within five degrees of perpendicular. When in an unloaded condition, the compartment membrane 20 in the long axis plane 26 and in the short axis plane 28 have initial lengths at ambient pressure. As the compartment membrane 20 expands and contracts, there is a change in length (?L). This change in length translates to dynamic strain ?(t), as is later explained.

[0042] It will be understood that the compartment membrane 20 will be exposed to positive pressures and negative pressures in a manner that corresponds to the beating rhythm of a heart. Referring to FIG. 6 in conjunction with FIG. 1 and FIG. 2, a typical ventricular strain profile for an average human heart is shown, wherein the heart is average size and morphology for the adult human population. Such a ventricular strain profile can be generated for any heart of a given AV diameter. As can be seen, the dynamic strain ?(t) has a compression phase and a retraction phase 40 over time (t). As can also be seen, there is a peak strain ?.sub.(peak), a time to peak strain t.sub.(peak), and an overall cycle period t.sub.(cycle).

[0043] The elastomeric compartment membranes 20 of the heart cuff 12 apply direct forces to the heart 11 since the compartment membranes 20 physically contact with the heart 11. The shell 13 of the heart cuff 12 applies forces indirectly to the heart 11 since the shell 13 does not directly contact the heart 11 and forces are transferred through the elastomeric compartment membranes 20. The combined forces provided by the shell 13 and compartment membranes 20 and automated drive should optimally produce an ideal strain in the heart 11 that does not damage the heart and assists the heart in achieving an optimal heart pumping functionality. An ideal strain ?.sub.(t) characteristic can be estimated using the following equation:

[00001]$\begin{array}{cc}\mathrm{Strain}=?\left(t\right)=\frac{?L}{L_0}=\{\begin{array}{c}\frac{{?}_{\mathrm{peak}}}{2}\left(\sin \left(\frac{?}{t_{\mathrm{peak}}}?t-\frac{3}{2}?\right)-1\right)\\ \frac{{?}_{\mathrm{peak}}}{2}\left(\sin \left(\frac{?}{t_{\mathrm{cycle}}-t_{\mathrm{peak}}}?\left(t-t_{\mathrm{peak}}\right)-\frac{?}{2}\right)-1\right)\end{array}& \mathrm{Equation}2\end{array}$

This ideal strain characteristic can be converted into a practical value for validation or testing in a construct that is being used either ex vivo in an unloaded condition, in vivo in a loaded condition, and/or in a mock loaded condition. Referring to FIG. 7, it will be understood that the ideal strain ?(t) can be converted into peak strain ?.sub.(peak) for an unloaded condition using Equation 3 below:

?.sub.peak?0.0481(SA.sub.ED)+24.496Equation 3

This value can be adjusted by +/?10% depending upon variabilities pertaining to intended surface area of the heart or the number of cuff compartments being tested.

[0044] The ideal strain ?(t) can be converted into peak strain ?.sub.(peak) for an in vivo loaded condition using Equation 4 below:

?.sub.peak?0.0481(SA.sub.ED)+11.064Equation 4

This value can be adjusted by +/?10% depending upon testing variables.

[0045] Lastly, the ideal strain ?(t) can be converted into peak strain ?.sub.(peak) for a mock loaded condition, i.e. use on a model, using Equation 5 below:

?.sub.peak?0.0481(SA.sub.ED)+1.496Equation 5

This value can be adjusted by +/?10% depending upon physical variability of the mock system utilized for testing.

[0046] For Equation 3, Equation 4 and Equation 5, the variable SA.sub.ED is the surface area of the compartment membrane 20 at the end of the diastolic cycle. To get a result from any of the equations, a value for SA.sub.ED must be obtained. As can be seen from FIG. 8, the surface area of the compartment membrane 20 at the end of the diastolic cycle SA.sub.ED is a function of the maximal diameter D.sub.max of the patient's heart at the end of the diastolic cycle. Accordingly, by measuring the maximal diameter D.sub.max of the patient's heart at the proper cycle time, the needed surface area SA.sub.ED of the compartment membrane 20 becomes known.

[0047] Referring back to FIG. 1, FIG. 3, and FIG. 4, it will be understood that good estimates of the dynamic strain profile of the heart cuff 12 can be accurately estimated by knowing the strains acting in the long axis plane 26 and in the short axis plane 28 of the compartment membrane 20. The strain in the long axis plane 26 is the longest vertical deformation of the compartment membrane 20 between the basal attachment seam 22 and the apical attachment seam 24. The short axis plane 28 is the longest horizontal path over the compartment membrane 20. The degree of deformation in the long axis plane 26 and in the short axis plane 28 both depend upon the pressures supplied by the external automated drive 14. However, the pressure supplied by the external automated drive 14 may be changed to a cycle that matches the rhythmic beat of the heart 11 depending on the functioning of the heart.

[0048] Referring to FIG. 9, it can be seen that the pressures supplied to the compartment membrane 20 by the external automated drive have a maximum pressure P.sub.max, a minimum pressure P.sub.min, a cycle time T.sub.cycle, and a pressure range P.sub.range, which is the difference between the maximum pressure P.sub.max and the minimum pressure P.sub.min during the cycle time T.sub.cycle. Given these parameters the pressure P(t) on the compartment membrane at any time (t) in the cycle time can be calculated using Equation 6 below:

[00002]$\begin{array}{cc}P\left(t\right)=\{\begin{array}{cc}\frac{P_{\mathrm{range}}}{2}\sin \left(\frac{?}{t_{\mathrm{peak}}}?t-\frac{?}{2}\right)-2\left(P_{\min }\right)& 0?t<t_{\mathrm{peak}}\\ \left(P_{\mathrm{range}}\right)\sin \left(\frac{?}{t_{\mathrm{cycle}}-t_{\mathrm{peak}}}?\left(t-t_{\mathrm{peak}}\right)-\frac{3}{2}?\right)-P_{\min }& t_{\mathrm{peak}}?t<t_{\mathrm{cycle}}\end{array}& \mathrm{Equation}6\end{array}$

Once the pressure (P.sub.t) is calculated, the stress experienced by an unloaded compartment membrane 20 in both the long axis plane 26 and the short axis plane 28 can be calculated.

[0049] The first step in calculating strain in the long axis plane 26 and in the short axis plane 28 is to use the pressure (P) to determine the maximum strain in the long axis plane 26 and in the short axis plane 28. The maximum strain ?.sub.(peak-LA) in the long axis plane 26 is determined by Equation 7 below:

?.sub.peak-LA=11.676 ln(P)?5.2073Equation 7

The maximum strain ?.sub.(peak-SA) in the short axis plane 28 is determined by Equation 8 below:

?.sub.peak-SA=6.5167 ln(P)?5.1506Equation 8

Again, the values generated by Equation 7 and Equation 8 can vary by up to 5% depending on testing platform and area of material and/or compartments used in the cuff. Once the peak strains are calculated in the long axis plane 26 and in the short axis plane 28, the dynamic strains can be calculated. The dynamic strain ?.sub.(x) for the long axis plane 26 is determined by Equation 9 below:

[00003]$\begin{array}{cc}?\left(x\right)=\frac{{?}_{\mathrm{peak}-\mathrm{LA}}}{40.8}\left(4?{10}^{-6}{x}^4-0.0008x^3+0.0324x^2+0.6868x-0.4757\right)& \mathrm{Equation}9\end{array}$

The dynamic strain ?.sub.(x) acting in the short axis plane 28 is determined by Equation 10 below:

[00004]$\begin{array}{cc}?\left(x_{SA}\right)=\frac{{?}_{peak-\mathrm{SA}}}{20.1}\left(-0.00005x^3+0.0004x^2+0.5171x+0.0052\right)& \mathrm{Equation}10\end{array}$

[0050] In view of the equations provided above, the strain profile for the compartment membrane 20 in the heart cuff 12 are calculated in both the long axis plane 26 and in the short axis plane 28.

[0051] Using the above, the strain profile for the compartment membrane 20 is calculated in both the long axis plane 26 and in the short axis plane 28 assuming an unloaded condition. This is highly useful in creating an elastomeric construct that will function very close to what is needed. However, once the elastomeric construct is loaded, that is applied to the heart, actual strain features can be determined by measurements during trial and error.

[0052] The strain dynamics embodied by the elastomeric material of the heart cuff 12 can be selectively altered by changing the thickness and/or material properties of the elastomeric material used in the heart cuff 12. The value for the dynamic strain applied by the external automated drive 14 can be controlled to some degree by active programming. However, certain variables, such as the rate that pressure can be increased and decreased is limited by the hardware being used. Accordingly, using the present invention methodology, a designer now has the ability to program the external automated drive 14 to settings that are the closest to ideal. The final changes in strain that are unachievable by programming can be achieved by altering the physical characteristics of the compartment membrane 20. By altering the elastomeric material used in the construction of the compartment membrane 20 and optimizing the programming of the external automated drive 14, the combined factors can create a dynamic strain profile that reinforces the heart 11 and enable the heart to pump in a more optimal manner.

[0053] Referring to all figures, it will be understood that the first step in determining what heart cuff 12 to use and what programming to use on the external automated drive 14 is to determine the dynamic strain profile for an average heart with generally normal morphology. The actual dynamic strain profile for the heart is measured using known techniques. The patient's heart may be in full or partial failure. The difference between the dynamic strain profile of the actual heart 11 and the dynamic strain profile of an average heart with generally normal morphology are determined. It is this difference that has to be compensated for using the heart cuff 12.

[0054] A heart cuff 12 is selected having a dynamic strain profile that was estimated and/or measured in an unloaded condition and extrapolated for the loaded condition. The heart cuff 12 is then loaded. That is, placed into contact with the heart 11. If the strain dynamics embodied by the heart cuff 12 does not provide the pumping assist needed, then the heart cuff 12 is altered or is replaced by another heart cuff that is better suited to the circumstances. The strain characteristics of the heart cuff 12 can be altered by altering the elastomeric material used in the shell 13 and compartment membranes 20 of the heart cuff 12. The elastomeric material can be made thicker and/or thinner in various places. Other variables, such as the hardness, tensile strength, tensile modulus, elongation, resilience, compression set, and specific gravity can also be altered to change the dynamic strain profile of the heart cuff.

[0055] Once the dynamic strain profile of the heart cuff 12 is altered, the altered heart cuff can again be tested. This cycle of alteration and checking can be repeated until the dynamic strain profile embodied by the heart cuff 12 matches what is needed by the actual heart in order to enable the heart to pump more in a more effective manner.

[0056] In the illustrated embodiment, the heart cuff 12 encircles the heart. It will be understood that there are many elastomeric constructs that are selectively inflated with pneumatic pressure, that do not encircle the heart. Rather, such devices typically press against only one area of the heart or are positioned internal of the heart. Referring to FIG. 10, a partial cuff 50 is shown. The partial cuff 50 can be positioned to apply forces to only the left ventricle, the right ventricle or portions thereof. Such partial cuffs can be used when scar tissue or other abnormalities prevent the application of a full cuff. Such partial cuffs are also easier to use during non-invasive procedures. Regardless, the partial cuff 50 contains one or more elastomeric constructs 52 that apply forces to the heart 11. The methodology previously described for use of a full cuff is still valid for use on a partial cuff. Accordingly, the system and method described for full cuffs apply equally to partial cuffs or even smaller inflatable constructs. All such constructs are intended to be covered within the scope of the claims.

[0057] It will be understood that the embodiments of the present invention that are illustrated and described are merely exemplary and that a person skilled in the art can make many variations to those embodiments. All such embodiments are intended to be included within the scope of the present invention as defined by the claims.