Patent Application
20090036938
External counterpulsation (ECP) therapy is a non-invasive, outpatient therapy used in the treatment of cardiac disease. As it has been conventionally applied, a set of cuffs is wrapped around the calves, thighs and buttocks of a patient. These cuffs attach to air hoses that connect to valves that inflate and deflate the cuffs. Electrodes are applied to the skin of the chest and connected to an electrocardiograph (ECG) machine. Blood pressure may also be monitored. Inflation and deflation of the cuffs are then electronically synchronized with the heartbeat and blood pressure using the ECG and blood pressure monitors. The ECP treatment compresses the blood vessels in the lower limbs to increase blood flow to the heart. Each wave of pressure is electronically timed to the heartbeat, so that the increased blood flow is delivered to the heart at the time when it is relaxing during diastole. When the heart pumps again during systole, the cuff pressure is released. This lowers resistance in the blood vessels in the legs so that blood may be pumped more easily from the heart. ECP may also encourage the development of collateral blood flow to the heart and thus contribute to the relief of angina symptoms.
Described herein is an improved system and method for delivering external counterpulsation therapy that involves direct stimulation of skeletal muscle and/or vascular smooth muscle in synchronization with the cardiac cycle in a manner that increases the fluid pressure within veins and/or arteries during cardiac diastole. Contraction of the skeletal muscle compresses the arteries and veins that course through the muscle to increase fluid pressure within the vessels, while contraction of venous and/or arterial smooth muscle causes venous and/or arterial constriction to also increase fluid pressure. As described above, the basic idea of counterpulsation therapy is to increase fluid pressure within blood vessels during cardiac diastole. Stimulation of skeletal and vascular smooth muscle in synchronization with the cardiac cycle efficiently raises the pressure in both arteries and veins during cardiac diastole and, as the skeletal and smooth muscle relaxes, lowers the arterial pressure during the subsequent systolic phase of the cardiac cycle. Periodic application of counterpulsation therapy is useful in the treatment of patients having a number of cardiac conditions. For example, coronary blood flow that supplies the heart is greatest during diastole, and increased aortic pressure during diastole will increase that flow to benefit patients suffering from coronary artery disease. It has also been found that increased aortic pressure brought about by counterpulsation therapy induces the formation of coronary collateral vessels. Increased venous pressure during diastole increases ventricular filling and cardiac output, and the subsequent decrease in arterial pressure during systole decreases the workload of the heart. Both of these effects are beneficial for patients having some degree of heart failure and may help to prevent or reverse deleterious cardiac remodeling.
Counterpulsation therapy via muscular stimulation has a number of advantages over the conventional manner of delivering counterpulsation therapy by external compression of extremities. Contraction of skeletal muscle applies compressive force directly to the vessels that run through the muscle to more efficiently compress both veins and arteries than external compression Muscular stimulation has the further advantage of producing a training effect for the patient that mimics physical exercise. Another advantage is that, unlike conventional external counterpulsation therapy, pressure can be increased in blood vessels located in body regions other than extremities. Although it may be preferable and most convenient to stimulate muscle tissue in the legs or arms, stimulation could also be applied to muscle tissue in the abdomen and buttocks, for example.
An exemplary system is equipped with one or more muscle stimulation transducers and a control unit that actuates the muscle stimulation transducers during the diastolic phase of the cardiac cycle as determined by detecting cardiac electrical activity or by detecting another physiological variable reflective of the cardiac cycle such as blood pressure or heart sounds. A muscle stimulation transducer may be any device that delivers energy to muscular tissue in a manner that causes contraction of the tissue such as a unipolar electrode, bipolar or multi-polar electrode set of electrodes, a radio-frequency transducer, a magnetic transducer, or an ultrasonic transducer. Although such muscle stimulation transducers could be of an implantable type, they are preferably adapted for transcutaneous stimulation of muscle. The muscle stimulation transducers may be adapted for cutaneous placement near the muscular tissue selected for stimulation in a number of different ways. For example, muscle stimulation transducers may be directly affixed to the skin by adhesive or other means. In another embodiment, the muscle stimulation transducers are incorporated into a garment or structure that is worn by the patient such as a cuff, sock, glove or pants that dispose the muscle stimulation electrodes near the muscular tissue to be stimulated. In another embodiment, the muscle stimulation transducers are incorporated into a patient-support structure such as a mat, a chair, or a recliner that dispose the muscular stimulation electrodes near the muscles of an extremity when the patient is supported thereon.
In order to properly time the delivery of muscle stimulation in relation to the cardiac cycle, the control unit either incorporates, or is interfaced to, a cardiac sensor for detecting cardiac activity. Such a cardiac sensor may be, for example, a surface ECG apparatus that generates electrical signals reflective of the depolarization corresponding to cardiac contraction and the electrical repolarization corresponding to cardiac relaxation. Alternatively, the control unit could communicate via wireless telemetry with a cardiac device implanted in the patient having cardiac sensing capability such as a pacemaker or ICD. Such implantable devices generate electrogram signals analogous to surface ECG signals via internally disposed electrodes. Both ECG and electrogram signals reflect the electrical activity of the heart and contain cardiac activity markers such as T waves and R waves indicative of the phases of the cardiac cycle. The objective is to actuate the one or more muscular stimulation transducers during the diastolic phase of the cardiac cycle as determined from the detected cardiac activity. For this purpose, the control unit and/or cardiac sensor may incorporate filtering and other signal processing circuitry for detecting R waves and/or T waves that correspond to the beginning of systole and diastole, respectively. Alternatively, the cardiac sensor may detect cardiac activity from measurements or detection of physiological variables reflective of the mechanical activity of the heart such as blood pressure, heart sounds, or blood flow. The control unit could also employ a combination of different types of cardiac sensors as described above for synchronizing counterpulsation therapy with the cardiac cycle.
The timing of muscular stimulation may be controlled in a number of different ways. For example, muscular stimulation may be initiated after some specified delay (e.g., 20-30 milliseconds) following detection of an R wave, where the delay is estimated to coincide with the start of mechanical diastole. Alternatively, detection of a T wave could be used as a marker to initiate muscular stimulation after a shorter delay. The start of mechanical diastole may be represented as the dichrotic notch in an aortic pressure waveform and is caused by closure of the aortic valve. The sound of aortic valve closure could also be used as a marker for the start of diastole. Once initiated, the muscular stimulation may then be delivered for a specified stimulation duration selected to lapse before the start of systole in the next cardiac cycle (e.g., 20-300 milliseconds). Alternatively, detection of an R wave or the sound of mitral valve closure may be used to terminate the stimulation. Also, multiple stimulation transducers may be disposed on an extremity and then actuated sequentially during diastole in a distal-to-proximal direction according to specified sequence parameters that specify the sequence. When multiple stimulation transducers are disposed on different body regions, more complicated stimulation sequences are also possible.
The control unit may also be configured to automatically adjust one or more stimulation parameters in closed-loop fashion based upon one or more measured physiological variables related to the patient's hemodynamics and that are affected by the counterpulsation therapy. Examples of such physiological variables include cardiac output, blood pressure, peripheral blood flow, and blood oxygen concentration. Among the stimulation parameters that may be adjusted in this manner are those that relate to the timing of the stimulation in relation to the cardiac cycle. As aforesaid, muscular stimulation may be initiated after some specified delay following detection of an R wave and then ceased after some specified stimulation duration. The specified delay and/or the specified stimulation duration in this embodiment could be automatically optimized by the control unit. In embodiments utilizing other markers of cardiac activity to initiate delivery of muscular stimulation (e.g., T waves, heart sounds), a specified delay and stimulation duration may be similarly automatically optimized. Sequence parameters used for sequential multi-transducer stimulation could also be automatically optimized. Another stimulation parameter that could be automatically optimized relates to the energy delivered to the muscular tissue. For safety and comfort reasons, it is desirable to minimize this parameter while still providing the desired therapeutic effect.
The invention has been described in conjunction with the foregoing specific embodiments. It should be appreciated that those embodiments may also be combined in any manner considered to be advantageous. Also, many alternatives, variations, and modifications will be apparent to those of ordinary skill in the art. Other such alternatives, variations, and modifications are intended to fall within the scope of the following appended claims.
