Abstract:
A medical system includes an implantable sensor; and a medical device having a communication link with the sensor. The medical device further including a controller, wherein operating parameters for controlling sampling of a physiological characteristic or other parameter by the implantable sensor are calculated by the controller and communicated to the sensor dynamically.

Description:
RELATED APPLICATION DATA 
     The present application claims the benefit of priority under 35 U.S.C §119 to U.S. Provisional Application 60/630,910, filed Nov. 24, 2004. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to the field of diagnostic and therapeutic medical implants and, more particularly implanted biological sensor systems. 
     BACKGROUND 
     Inplantable devices for sensing physiological parameters (e.g., pressure or flow rate in a blood lumen) are known, e.g., an implanted sensor system configured to sample a physiological signal at an implantation site in a body. In order to retrieve clinic information from the sampled physiological signal, the original signal has to be sampled with sufficient accuracy and rate. The key operating parameters for sampling include: the sampling time, i.e., when is the signal sampled, the time span between the first and last measurement, whether the signal is quasi-periodic, when (within the sample period) the signal measured, the sampling rate, the measurement range (e.g. the minimum and maximum signal values that the sensor may sense), and the signal resolution (e.g. the minimal change in signal magnitude that would alter the resulting reading). 
     If the sensor includes a trigger mechanism, e.g. start to sample when the signal level goes below a threshold, then the parameters of the trigger mechanism also affect the quality of the measurements and the resources needed to generate them. When operated with the most stringent values for these parameters, the sensor may provide clinically relevant information, but the resources required to obtain such information may be too high, especially power consumption, but also in terms of the size, complexity, and throughput. 
     If the sensor is embedded in an implant, and especially if this implant operates using an acoustic switch, then sampling may take a significant proportion of the power budget of the implant. Since the sensor consumes practically no power when idle, most of its power budget is allocated to taking measurements. The more measurements it takes per exam, the fewer exams it can perform. An exam is a term used to measure a physiologically relevant value at a given point of time, for instance, systemic blood pressure. The resolution relates to the number of sampled bits. Sampling at higher resolution (for example, 1000 distinct signal levels) would require more power than sampling at lower resolution. When the signal range is diminished, one can reduce the number of distinct signal levels and keep the resolution, and thus save power. 
     Saving power is only one part of the problem. When optimizing the sampling parameters, one can sample fewer measurements, and thus require a smaller buffer to hold them. If the samples are transmitted to a medical system, either extracorporeal or implanted, the transmission bandwidth may be limited. Sampling fewer measurements may enable transmission that would otherwise not be possible. Reducing the signal range that the sensor needs to sample may reduce the size, cost, and complexity of the sensor. 
     SUMMARY OF EMBODIMENTS OF THE INVENTION 
     Embodiments of the invention are directed to an implantable sensor system that samples a physiological signal at an implantation site in a body. In one embodiment, a system is provided where the implanted sensor is controlled by another device. The other device may be another implanted medical device, or an external system. The control of, and communication with, the implanted sensor may be via a wired or wireless link. If the control device has additional information that is dynamically changing and which may affect the operation of the implanted sensor, optimized operation of the implanted sensor may be accomplished by transferring this information, or derivatives of this information, to the implanted sensor. 
     The sensor may be, by way of non-limiting example, a pressure sensor, and more specifically a sensor configured to measure hemodynamic pressure in a blood vessel. Other sensors may include, by way of further non-limiting examples, an electrocardiogram (ECG) sensor or an echo signal, an accelerometer, and sensors that measure physiological characteristics such as heart contractility, cardiac output, blood flow, oxygen saturation, glucose concentration, position of an orthopedic implant, radiation or temperature. 
     The dynamic side information, by which the medical device makes “decisions” regarding the operating parameters of the implanted sensor, can be of many sources. For example, the side information can come not only from a sensor but also from an operation performed by the medical device, such as neuro-activation, electrical signaling, pacing, and/or releasing or pumping of a drug into the body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows pressure signals as seen by the implant for external barometric pressure signals at sea level, 1 km, and 2 km height 
         FIG. 2  illustrates a pressure signal during diastole 
         FIG. 3  illustrates performing an accurate measurement on sensed pressure 
         FIG. 4  describes a system having a control unit with sensors for measuring physiological signals 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Knowledge of the Pressure Range 
     In hemodynamic monitoring, the pressure measured by the implanted pressure sensor is referenced to the barometric pressure. If the controlling unit knows the barometric pressure, it can direct the implant to measure the pressure at the vicinity of this pressure. By way of examples, the controlling unit may be an external unit with access to a barometer, or an internal unit that has access to the barometric measurement information. This reduces the pressure range that the implant needs to be able to measure, and for a given resolution, may save on power consumption. 
       FIG. 1  shows the pressure signal as seen by the implant in three different cases, designated as A, B and C. The three cases differ in the external barometric pressure, for instance at sea level, 1 km height, and 2 km height. If the barometric pressure is known to be close to signal B, the pressure range described by the arrow “PR” can be conveyed to the pressure sensor implant. This will only have to sample at this pressure range, improving its accuracy for a given power, or improving power consumption for a given accuracy. 
     Knowledge of the Change Rate 
     It is well known that a rapidly changing signal has to be sampled at higher rate than a slowly changing signal. In hemodynamic pressure measurement application, if the control unit has access to the heart rate, the pressure sensor can be provided with the heart rate information, or its derivative, and adjust the sampling rate accordingly. The control unit may estimate the rate of change of the heart signal using electro cardio gram signal (ECG). 
     Knowledge of the Signal Phase 
     Further optimization is possible when the phase of the pressure signal is known. For example, if the system is interested in the pressure value of the diastole, if the time of the diastole can be estimated from an ECG signal, then the time when the diastole will be sensed by the pressure sensor may be estimated. This is true for many locations where the pressure sensor may be positioned, for instance, the pulmonary artery, iliac artery, femoral artery, renal artery, aorta, subclavian artery, carotid artery. The control unit then can provide the pressure sensor with information when to sample, as well as the other operating parameters for most efficient measurement. 
       FIG. 2  depicts a schematic illustration of a pressure signal going through the diastole. The arrows show that the time span, sampling rate, pressure range may all be adjusted to the signal at hand. 
     Triggering 
     If the coarse level of the signal is known, the sensor may be optimized by dynamically providing it with trigger parameters. For example, if the device measures pressure using a coarse sensor, it may estimate the level and direction where a trigger should be set at the implanted pressure sensor. By dynamically providing the pressure sensor with such information, the pressure sensor may then perform an accurate measurement at the exact spot, as is shown in  FIG. 3 . 
     Other Signals 
     With reference to  FIG. 4 , a system is disclosed in which the control unit may is an implanted medical device that has access to a physiological signal. One example is an electro cardiac signal. Other possible signals include an echo signal, accelero-metric signal, a pressure signal taken at a different location, optical signal, and others. The information extracted from the primary signal is transferred directly, or using a derivative, to the pressure sensor. 
     Specific Example 
     A specific example may be where the ECG and control unit are part of an implantable pulse generator (“IPG”), or cardiac resynchronization therapy (“CRT”) device, and the pressure sensor is distinct. The pressure sensor may be connected to an IPG using a wired or wireless (e.g., acoustic) communication link. The sampling time may be passed to the pressure sensor as a trigger. Alternatively, it may be passed as a time signal, when both implants keep synchronized clocks. This alleviates possible problems when the communication link between the implants may drop a message. The heart rate may be passed directly, or may be passed as a parameter, indicating the required sampling rate. The pressure range and resolution may be passed directly, or as a parameter 
     Other Communication Methods 
     The communication between the pressure sensor and the other medical device could be acoustic. However, if reasonable electromagnetic link exists between the modules, the communication can also be using electric field or magnetic field. This is especially true of both devices are close to the skin. The communication can be directly between the devices or through one or more devices, forwarding the messages between them. 
     Example with Thermal Therapy and a Thermal Sensor 
     During thermal therapy, the external system knows the expected thermal profile at the location of the sensor, and can inform the implant regarding the range. 
     Example with Oxygen Saturation 
     Oxygen saturation can be measured during the systole, where the diameter of the artery is maximal. This improves the quality of the measurement. By providing this information to the oxygenation meter (when to sample, and what expected range), the power and accuracy may be optimized.