This invention pertains to implantable medical devices such as cardiac pacemakers and implantable cardioverter/defibrillators. In particular, the invention relates to a system and method enabling an implantable medical device to communicate with and control an external drug delivery device.
Implantable medical devices are commonplace today, particularly for treating cardiac dysfunction. Cardiac pacemakers, for example, are implantable medical devices that replace or supplement a heart""s compromised ability to pace itself (i.e., bradycardia) due to chronotropic incompetence or a conduction system defect by delivering electrical pacing pulses to the heart. Implantable cardioverter/defibrillators (ICD""s) are devices that deliver electrical energy to the heart in order to reverse excessively rapid heart rates (tachycardia) including life threatening cardiac arrhythmias such as ventricular fibrillation. Since some patients have conditions that necessitate pacing and also render them vulnerable to life-threatening arrhythmias, implantable cardiac devices have been developed that combine both functions in a single device.
Most pacemakers today are operated in some sort of synchronous mode where the pacing pulses are delivered in a manner that is dependent upon the intrinsic depolarizations of the heart as sensed by the pacemaker. ICD""s must also sense the electrical activity of the heart in order to detect an arrhythmia that will trigger delivery of the shock pulse in an attempt to reverse the condition. Such sensing information can be used to initiate another mode of therapy, and efforts have been made in the past to combine automatic drug delivery by an implantable drug delivery system with either pacemakers, ICD""s, or both in order to treat cardiac arrhythmias.
Implantable drug delivery systems suffer from a number of disadvantages, however, when compared with an external drug delivery device. Although the drug reservoir of an implantable delivery device can be replenished, it is difficult to change the drug once it is put into the reservoir, making patient management difficult in cases where a patient""s condition either changes or otherwise requires a change of medication. In addition, drugs degrade over time. Finally, there is the risk of leakage from the implanted reservoir, the consequences of which can range from an annoyance to a medical emergency. For these reasons, drug delivery from an external device is preferred in many situations.
In order to control the delivery of drugs or other therapies by an external device, the implanted device must be capable of transmitting command and control information to the external device. This is the primary problem with which the present invention is concerned.
The present invention relates to a drug delivery system and method in which an implantable medical device communicates with and controls an external drug delivery device with low energy potential signals. The implantable device generates potential signals by operating a current source to cause corresponding electrical potentials that can be sensed at the skin surface by the external drug delivery device. The current source is operated so as to generate a carrier waveform that can be modulated with digitally encoded command information. The external drug delivery device includes electrodes at the skin surface for sensing potentials and circuitry for demodulating the sensed carrier waveform. The drug delivery device may then decode the digital data to extract command information therefrom and deliver a drug accordingly.
In accordance with the invention, the carrier waveform is digitally modulated with the digitally encoded information by varying the amplitude or frequency of the carrier waveform using, for example, amplitude shift-keying or frequency shift-keying. In a particular embodiment, a digital pulse train is modulated with the digitally encoded information by varying the frequency, width, or position of the pulses. The pulse train is then used to amplitude modulate the carrier waveform.
Certain implantable medical devices, such as rate-adaptive pacemakers, may use an impedance technique for measuring minute ventilation and/or cardiac stroke volume. In that technique, an oscillating current is made to flow between two electrodes located within the thorax, and the impedance between the electrodes is measured. In accordance with the invention, the impedance measuring current may be used as the carrier waveform and modulated with digitally encoded information by the implantable device for transmission to the external drug delivery device.
The present invention may be incorporated into a system where the implantable device is a cardiac device such as an implantable cardioverter/defibrillator, cardiac pacemaker, or combination device. A dose of a drug is then delivered by the drug delivery device to a patient upon detection of a particular medical condition such as an arrhythmia. In one embodiment, the drug delivery device is an electrically modulated transdermal drug delivery device designed for external affixation to a patient""s skin surface at a suitable location. The electrically modulated transdermal delivery injector may have one electrode with a drug reservoir in contact with the skin, another electrode also contacting the skin, and a voltage source for imposing a voltage between the electrodes. The drug delivery device also has a data communications interface for receiving command signals from the implantable cardiac device, and circuitry for controlling the delivery of the drug in accordance with the command signal.