Abstract:
There is provided a system and method for continually monitoring the occurrence of contraction during stimulation of skeletal muscle which is employed in a cardiac assist-type system. During delivery of a periodic burst, or train of stimulus pulses, the impedance of the muscle between the electrodes through which the pulses are delivered is monitored, and evaluated to determine whether or not stimulation has been achieved. In a particular embodiment, the impedance and impedance derivative values are accumulated throughout the burst, and assessed to determine whether the impedance change corresponded to a full muscle contraction. In the event of failure to stimulate the muscle to contraction, the system can automatically adjust the pulse output parameters to achieve reliable contraction.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to systems for stimulating skeletal muscle, e.g., cardiac assist systems, and more particularly augmenting such systems to provide an ongoing determination of whether the skeletal muscle is being stimulated. 
     BACKGROUND OF THE INVENTION 
     Cardiac assist systems are designed to aid patients with chronically and unacceptably low output who cannot have such cardiac output raised to an acceptable level by traditional treatments, e.g., pacing or drug therapy. A specific type of cardiac assist system to which this invention is addressed, as a preferred example, is cardiomyoplasty. Cardiomyoplasty is surgical procedure for treating chronic heart failure (CHF), whereby a patient&#39;s latissimus dorsi (LD) muscle is elevated, dissected, passed into the thoracic cavity, and then wrapped and secured around the failing heart. The LD muscle is first gradually stimulated with electrical impulses for a period of time up to about twelve weeks, to convert the muscle to a fatigue-resistant state. Following this conditioning, the LD muscle is chronically stimulated to contract in synchrony with the heart, in order to provide hemodynamic assistance. During the chronic muscle stimulation, an implantable pulse generator senses contractions of the heart via one or more sensing leads and controls generation of stimulation of the appropriate nerves of the muscle tissue with burst signals adapted to cause the muscle tissue to contract in synchrony with the heart. As a result, the heart is assisted in its contractions, thereby raising the stroke volume, and thus the output. 
     Following transposition of the LD muscle into the thoracic cavity, there is currently no reliable method or device for determining if the muscle is in fact contracting when stimulated. Further, contractions may be weak or strong, and there is no reliable method of determining the strength of the contractions that do take place. In the prior art, implantable sensors have been proposed and tested, with varying degrees of as of yet unfulfilled promise. The most basic sensors provide a yes/no indication of muscle contraction, while more sophisticated sensors are designed to provide a quantitative indication of relative strength of the contraction or contractility. However, such more sophisticated sensors are bulky and generally not yet proven. There thus remains a need in the art for a method and subsystem for monitoring skeletal muscle contraction on a beat-by-beat basis, so as to provide information for controlling the output level of the pulses in each stimulation pulse burst. 
     As is known, a unique characteristic of cardiac muscle is that it is able to be paced and fully captured by the application of a single electrical impulse. Skeletal muscle, on the other hand, will yield only a twitch contraction to a single electrical impulse. To effect a full, tetanic contraction in skeletal muscle requires delivering a train of electrical pulses to that muscle. Electrical pulses are typically delivered via a pair of leads, e.g., intramuscular, epimysial, etc., or between one lead electrode placed within or on the muscle and the pulse generator case for unipolar stimulation. It has been noted that during stimulation of the skeletal muscle, when the muscle does contract, the lead electrodes move relative to each other. As the distance between the electrodes carried by the leads changes, so does the impedance between the leads. This invention takes advantage of the relationship between muscle length and inter-lead impedance, which is known to be a substantially linear relationship. Consequently, the basis of this invention is to measure the impedance between the stimulation electrodes during each pulse in a stimulation train, and from such impedance information determine whether a contraction has occurred, and the quality of the muscle contraction. 
     Although the preferred embodiment of this invention is set forth with the illustration of cardiomyoplasty, it is equally applicable to other procedures, including aorta myoplasty, incontinence treatment, and any other muscle-activated system. 
     SUMMARY OF THE INVENTION 
     There is provided a system and method for stimulating skeletal muscle as part of a therapeutic procedure, which includes determining when delivered stimulation has evoked contraction of the skeletal muscle. The system provides a controllable pulse generator for periodically generating a burst of stimulus pulses and leads for delivering the bursts to electrodes in the skeletal muscle, and impedance measuring circuitry for measuring the impedance between the electrodes at the time of each pulse of the series. The impedance measurements are processed, preferably including obtaining the derivative or change in impedance for each pulse burst, and the impedance data is analyzed to determine whether it reflects a full muscle contraction. In the event of determinations of no contraction, the system includes a feedback mechanism for adjusting the power output of the burst pulses, so as to provide cyclical stimulation sufficiently above threshold to provide reliable muscle contraction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic representation of a system for performing chronic stimulation of a skeletal muscle for cardiac assistance using systolic augmentation, as well as direct electrical stimulation of the heart, in accordance with the present invention. 
     FIG. 2 is a block diagram of a skeletal muscle stimulation system in accordance with this invention, providing for continued measurement of skeletal muscle impedance during delivery of stimulation bursts. 
     FIG. 3A is a timing diagram showing a first pulse burst wherein each pulse has an output level above the skeletal muscle threshold, and a second following burst of pulses wherein each pulse has an output level below the muscle threshold; 
     FIG. 3B is a pair of curves representing variations of impedance(Z) and dZ/dt corresponding to delivery of the stimulation burst. 
     FIG. 4 is a flow diagram illustrating the primary steps carried out by the implantable system of this invention for determining when skeletal muscle has been stimulated to full contraction, and for ongoing data storage concerning muscle contractions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an example of a system  1  for performing long-term stimulation of skeletal muscles for cardiac assistance using systolic augmentation as well as direct electrical stimulation of a heart  2 . While the invention is illustrated in the environment of a cardiac assist system, a preferred embodiment, it is to be understood that it is equally applicable to other treatment systems, as noted above. As shown in FIG. 1, skeletal muscle graft  3  is positioned about the heart  2 . In a preferred embodiment the latissimus dorsi muscle is used for the skeletal muscle graft, as is well known in the art. The longitudinal fibers of the muscle graft  3  are oriented generally perpendicular to the longitudinal axes of the right ventricle  4 , left ventricle  5  and interventricular septum  10  of the heart. Muscle graft  3  is positioned in this manner so that when it is stimulated, muscle graft  3  compresses ventricles  4  and  5 , and particularly left ventricle  5 , to thereby improve the force of right and left ventricular contraction. In such a manner the overall hemodynamic output of heart  2  is increased. 
     In a preferred configuration, muscle graft  3  is wrapped around the heart  2  and fixedly attached to itself to form a cup-shaped “sling,” using running sutures  12 . Alternatively, muscle graft  3  may be attached to heart  2  using running sutures  13  as illustrated. 
     In the illustrated system, device  6  includes a pacemaker portion of standard form, for generating pacing pulses for delivery to the patient&#39;s heart. For applications other than cardiac assist, the device  6  is another suitable pulse generator programmed to deliver appropriate pulses. As seen, electrical stimulation and sensing of heart  2  is accomplished through lead  15 . In particular, lead  15  electrically couples pacing pulses from generator  6  to heart  2 , specifically to the right ventricle. Although not illustrated, the system may embody a dual chamber pacemaker subsystem, or a or 4 chamber pacemaker as is used frequently for CHF patients. Generator  6  may also provide defibrillation and/or cardioversion pulse therapies, and lead  15  has appropriate electrodes for providing cardiac pacing as well as defibrillation therapies. In the preferred embodiment lead  15  is the model 6936 tripolar TRANSVENE lead from Medtronic, Inc., Minneapolis, Minn. As illustrated, lead  15  is implanted in right ventricle  4  such that bipolar pacing electrode assembly  16  is in the right ventricular apex and defibrillation coil  17  is within the right ventricle  4 . Although in the preferred embodiment a single lead is provided for pacing as well as defibrillation therapies, other types of lead configurations, such as multiple transvenous or subcutaneous or any combination thereof, may be used. 
     Muscle graft  3  is electrically stimulated through a pair of leads  21 ,  22 . In particular, leads  21 ,  22  couple pulse generator  6  to skeletal muscle graft  3 . In the preferred embodiment leads  21 ,  22  are the model 4750 intramuscular lead from Medtronic, Inc., Minneapolis, Minn. As seen, each lead  21 ,  22  extends from pulse generator  6  to latissimus dorsi muscle graft  3 . The electrodes (not shown) of each lead  21 ,  22  are placed to cause muscle graft  3  to contract when electrically stimulated, as is well known in the art. Other types of leads or electrodes, however, may be used, such as epimysial or neuromuscular leads. 
     It is to be noted that although FIG. 1 shows one specific configuration, other skeletal muscle-powered cardiac assist systems may be configured in other ways. Thus, the present invention may be used in any system providing cardiac augmentation using skeletal muscles, such as aortic counter pulsation, or a skeletal muscle ventricle. 
     Referring now to FIG. 2, there is shown a simplified block diagram of the primary components of pulse generator device  6 , which are functional providing the stimulus bursts and in measuring skeletal muscle impedance and thus determining the presence or absence of skeletal muscle contraction. Reference is made to U.S. Pat. No. 5,697,952, assigned to Medtronic, Inc., for a more detailed description of a device which provides cardiac pacing and defibrillation, as well as stimulation of a skeletal muscle wrap. 
     Referring to the details of FIG. 2, a skeletal muscle generator circuit  30  is controlled by control block  32 , which suitably incorporates a microprocessor and associated software, as discussed more fully in the referenced application. Generator  30  provides periodic bursts of pulses, which are synchronized with the cardiac contractions, which are sensed by electrodes  16  or coordinated with delivered pacing pulses. The generator  30  preferably provides constant voltage pulses, and measuring circuit  34  suitably uses a current measuring transistor and a sample hold circuit to obtain a measure of current flow during each pulse. The pulses of the burst are delivered on leads  21  and  22 , to electrodes positioned in the skeletal muscle (SM) and identified at  35 A and  35 B. For each pulse of the burst, the current measure obtained by circuit  34  is processed in circuitry  36 . Such processing may include normal signal processing, e.g., amplification, and also includes calculating Z by dividing the pulse voltage V by the measured current I; a measure of Z can be obtained simply by getting the inverse of I. As seen from the diagram of FIG. 2, Z represents the impedance between the electrodes, as well as the lead impedance, as is discussed further below. The processing also includes obtaining the difference ΔZ in impedance compared to the last pulse, to get a measure of dZ/dt. In a preferred embodiment, these calculations are preferably made with a microprocessor, although dedicated circuitry may also be used. Where available, e.g., at time of the surgery for providing the cardiac assist system, the measured impedance is outputted, or indicated as illustrated at  37 . The impedance and dZ/dt values are also suitably stored at  38 , and can be transmitted through T/R circuit  39  to an external receiver for analysis. Circuitry for telemetry transmission and receiving, shown at T/R block  39 , is used to receive program commands from an external source, and connect them through to control block  32 . 
     The processed impedance data from block  36  is also analyzed at feedback block  31 , which determines whether conditions require adjustments of the power output of the stimulating pulses. Thus, if a burst fails to produce a muscle contraction, or x out of n+x bursts fail to result in full contractions, a signal is outputted to controllable generator  30 , to increase one of the pulse parameters so as to augment the power out, to secure the output power level above the chronic stimulus threshold of the skeletal muscle. Block  31  suitably includes microprocessor analysis of the Z and dZ/dt data for determining whether or not the heart has contracted in response to a stimulus burst, and whether a contraction is weak or strong. 
     Referring to FIGS. 3A and 3B, there is shown in juxtaposition, a pulse train of pulses delivered to the skeletal muscle, as well as a graph of variations of impedance and the derivative of impedance with time. As set forth above, when the stimulation burst is delivered and the muscle begins to contract, the distance between the lead electrodes changes, and so does the impedance between the lead electrodes. The impedance relationship is substantially linear with muscle length, in accordance with, for example, the following equation: 
     
       
           Z= 2.8+0.65 L   
       
     
     where Z is impedance and L is the length between the electrodes. This equation illustrates that in the physiological range of contraction during stimulus, which is approximately 10-20 mm, an impedance change in the order of 6.5-13 ohms is expected. Assuming a coil and connector impedance of 250 ohms, this represents a 2.2-4.3% impedance change. This change in impedance is illustrated in FIG.  3 B. It is noted that from time t=0 until the first pulse of the stimulus burst, impedance is substantially constant, at a level in the range of 250-300 ohms. As the pulse train continues, impedance drops, and dZ/dt increases in negative amplitude. Toward the end of the pulse train, the impedance drop diminishes, such that dZ/dt starts to rise. In the illustrated curves of FIG. 3B, a fairly sharp in Z and a pronounced detectable drop in dZ/dt is found, indicating that contraction has been achieved. 
     As shown in FIG. 3A, the burst suitably comprises six pulses, each separated by about 30 ms and having a pulse width of about 200 μs. The output voltage is suitably in the range of 5-6 volts, although chronic stimulation may vary from case to case. Also shown in FIG. 3A is a second shorter series of sub-threshold pulses suitably delivered at about 250-300 ms from time=0. These sub-threshold pulses are used to monitor muscle length during the relaxation phase of the contraction, and the impedance measurements taken during these sub-threshold pulses is also processed to determine the quality of the relaxation phase. 
     Referring now to FIG. 4, there is shown a flow diagram representing the primary steps taken in carrying out the impedance measurements and determination of contraction, in accordance with a cardiac-assist embodiment of this invention. At block  42 , the cardiac sync signal is obtained, as discussed above in relation to FIG.  2 . Of course, for treatment applications other than cardiac assist, the sync is provided from another source, e.g., from a sync generator within device  6 . At  43 , in timed relation to the cardiac sync signal, a burst of n pulses is generated and delivered to the skeletal muscle. Although the invention has been illustrated with a burst of six such pulses, n is a variable which can be adjusted at time of implant, or programmed after implant. As the burst of impulses is being delivered, the impedance Z for each burst is measured and stored at  44 , and dZ/dt is calculated and stored following each burst pulse, suitably by obtaining the differential impedance between pulses. Note that each impedance measurement contains a measure of the contraction of the muscle concurrent with the pulse, and the collective impedance data provides information from which the degree of contraction can be determined. At  48 , following the burst delivery and obtaining the impedance data, contraction is determined as a function of the impedance data. The determination of whether or not there has been contraction, and the relative strength of the contraction, is preferably a software task carried out by a microprocessor subsystem. The invention embraces any algorithm, simple or complex, that operates on the impedance data. For example, in many cases it may be sufficient to determine how sharply dZ/dt changes following initiation of the burst, or simply the maximum drop in dZ/dt corresponding to a burst. Thus, if the maximum negative value of dZ/dt exceeds a predetermined threshold, contraction is indicated; if the negative value is within a given range below the threshold, a weak contraction is indicated, but it exceeds a second, greater threshold, a strong contraction is indicated. It is to understood that other more complex determinations as a function of the measured impedance values may be utilized. If, at  48 , it is determined that a contraction has taken place, then at  50  data reflecting this is stored, suitably by incrementing a counter designated C-YES, to obtain a running count of contractions. If it is determined that there has not been a contraction, or that the contraction is weak, then at  52  a C-NO counter is incremented. The routine then goes to  53  and determines whether an adjustment to the output level of the burst pulses should be made to raise them safely above contraction threshold. Here too, the decision may be based upon a simple or complex algorithm, e.g., a single failure to achieve contraction may warrant an adjustment or x failures out of n+x cycles may be required. If no adjustment is called for, the routine branches to block  56 ; if an adjustment is called for, one or more parameters of the burst pulses are adjusted at block  54 . 
     Referring to block  56 , a shorter burst of sub-threshold pulses is delivered following the stimulation burst. As indicated at  58 , for each pulse of the sub-threshold burst, a measure of Z and dZ/dt is obtained. At  60 , the sub-threshold impedance data is analyzed to determine whether or not there has been a physiologic relaxation. If yes, at  62  the R-YES counter is incremented; if no the R-NO counter is incremented. This data may be monitored for use in adjusting therapy. 
     There has thus been disclosed a system and method for making serial lead impedance measurements in a system for applying therapy by stimulating skeletal muscle. The measurements are made during each stimulation burst which is delivered to the skeletal muscle, for determining whether or not the burst achieved a satisfactory contraction. It is to be understood that variations with respect to the illustrated system and method are within the scope of the invention. For example, any convenient circuitry for determining muscle impedance can be used, e.g., constant current pulses can be used together with measurements of the differential output voltage. Of course, muscle threshold will differ from case to case, and the burst pulse and sub-threshold pulse parameters will be programmed accordingly. It is to be noted that while the preferred embodiment incorporates the simplicity of using the stimulus pulses for determining impedance, a separate impedance detection circuit using either pulses or a continuous signal may be used for monitoring impedance changes relative to the stimulus burst.