25-10-2012, 02:02 PM
Body Sensors Applied in Pacemakers
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Abstract—
This paper presents a survey of the body sensors applied in pacemakers and recent advances in modern pacemaker systems. New features of modern pacemakers that
are commercially available are briefly described. Body sensors incorporated in pacemakers are illustrated with their rationales,sensing signals, advantages, limitations and applications. Their further improvements are needed to better serve patients.
Introduction:
Cardiovascular diseases are major causes of morbidity and mortality in the developed countries. Early diagnosis and medical treatment of heart diseases can effectively prevent the sudden death of a patient. It is well known that implantable cardiac devices such as pacemaker are widely used nowadays. It has become a therapeutic tool used worldwide with more than 250, 000 pacemaker implants
every year [1]. A pacemaker is a medical device that uses electrical impulses, delivered by electrodes contacting the heart muscles, to regulate the beating of the heart. Its primary purpose is to treat bradycardia due to sinus node or atrioventricular conduction disorders and to maintain an adequate heart rate, either because the heart’s native
pacemaker is not fast enough, or there is a block in its electrical conduction system. It can help a person who has an abnormal heart rhythm resume a more active lifestyle [2].
Modern pacemakers are externally programmable and allow cardiologists to select the optimal pacing modes for individual patients. The complexity and reliability of modern
pacemakers have increased significantly, mainly due to developments in and use of sensing technologies. Therefore, modern pacemakers with sensors are applied not only for pacing but also for other functions such as obtaining diagnostic data and providing continuous cardiac monitoring and long-term trended clinical information.
Many problems exist regarding pacemakers in the treatment of a number of conditions. One of them is the diagnosis of cardiac abnormalities. This is because serious but infrequently occurring arrhythmias can be difficult to detect and analyze [3]. To solve them, various types of sensors, such as activity, metabolic and dual sensors, have been used to detect body activity, and measure some consequence of a physiological
change during exercise or facing environmental or emotional changes (temperature and posture).
Sensors have four main components: sensing, processing, communication, and energy/power units. Body sensor networks, owing to the sensing units’ proximity to each other, might have a base station that handles all the processing, communication, and power delivery. Body sensors fall into two main categories, implantable and wearable. The former measure parameters inside the body and mostly operate as interfaces to relatively small software components attached to or implanted into human bodies. They provide bidirectional communication interfaces between a person and a remote information system that provides healthcare services, diagnosis, or upgrades [4]. Wearable sensors, although not as invasive as their implantable counterparts, nevertheless must withstand the human body’s normal movements and infringe on them as little as possible.
Next, we provide a brief introduction to the recent advances and features in modern pacemakers. Our focus is on the review of various body sensors applied in them. The
contribution of this survey is to present a summary research and comparison of various sensors with their rationales, features and applications. By comparing different types of
body sensors, we conclude the signals they can sense, advantages, limitations and application conditions. This survey intends to benefit the readers interested in biomedical
sensing technology. Section II briefly describes the new features of modern pacemakers. Section III illustrates the main categories of body sensors in pacemakers and compares
their features. Finally, the last section presents conclusions and future research directions.
MODERN PACEMAKERS:
A. Basic Functions
Pacemakers are used to treat arrhythmias that are problems with the rate or rhythm of a heartbeat, maintain an adequate heart rate by delivering electrical stimuli (paces) to the
chambers of the heart, and prevent human from being harmed by low heart rate. During an arrhythmia, a heart can beat too fast (tachycardia), too slow (bradycardia), or with an irregular rhythm and may not be able to pump enough blood to the rhythmic electric impulse to the heart muscle in order to restore an effective heart’s rhythm to meet the oxygen needs of the body. A pacemaker can determine when stimuli must be delivered by calculating the timing of incoming contraction events.
A modern pacing system consists of at least three main parts, a pacemaker with body sensors, pacing leads carrying pacing impulses, and a programmer. Its programming
normally includes demand pacing and rate-responsive one.The former monitors the heart rhythm and sends only electrical pulses to the heart if it is beating too slowly or if it
misses a beat. The latter speeds up or slow down the heart rate depending on how active the patient is. It monitors the sinoatrial node rate, breathing, blood temperature, and other
factors to determine the activity level. A variety of sensors appropriate for rate-responsive pacing have been developed.
B. New Features
Modern pacemakers have many technological advances of functions, including various modes of dual-chamber pacing, rate-responsive algorithms with dual sensors for optimum
physiological response, cardiac resynchronization therapy, arrhythmia-prevention algorithms, antitachycardia pacing, hemodynamic monitoring, rest rate and sleep rate limits, and remote monitoring [5]. They automatically self-adjust the energy output required to pace the heart per the needs of each individual patient. Through actively monitoring the heart on a beat-by-beat basis, they provide pacing only when needed to
allow the patient’s own heart rhythm to prevail whenever possible, which is beneficial to patient’s cardiac health. The feature of rate response automatically adjusts the heart rate to match the level of activity. Special sensors detect changes in the body other than heart rate and allow the pacemaker controllers to increase or decrease the heart rate accordingly.
The automaticity features of pacemakers with body sensors enable continuous or intermittent monitoring of various pacemaker parameters including pacing impedance, sensing levels, pacing thresholds, and daily activity log. The benefits include increased patient safety, improved quality of life, increased battery longevity, cost effectiveness, and remote device interrogation including data monitoring as well as patient alert functions for device malfunction.
To illustrate the details of the new features in modern pacemakers, several representative ones are introduced next.The Adapta pacemaker offers the Medtronic-exclusive pacing
mode called Managed Ventricular Pacing (MVP), which enables it to be programmed to deliver pacing pulses to the heart’s lower right chamber (ventricle) only when necessary.
Accent RF pacemaker features daily wireless remote monitoring, providing timely notification of actionable events and flexible remote follow-up scheduling through
Merlin.net® Patient Care Network (PCN). Victory pacemakers offer an important combination of features, including optimized settings to save time at implant,Ventricular Intrinsic Preference (VIP) technology to minimize ventricular pacing, the FastPath summary screen to speed follow-up exams, and advanced technologies to extend the life of the device in patients. The company provides a suite of algorithms designed to make it easier for physicians to manage patients with atrial fibrillation (AF). AF is the world’s
most common cardiac arrhythmia that results in a very fast,uncontrolled heart rhythm caused when the upper chambers of the heart (atrial) quiver instead of beating. An Evia pacemaker system integrates wireless remote monitoring with small size. It is able to provide home monitoring for patients with some sensors. With Closed Loop Stimulation (CLS), Evia responds to changes in the autonomic nervous system on a beat-by-beat
basis. CLS is one of the most advanced and physiologic rate regulation sensors. For standard motion-based rate-adaptation,the Evia is also equipped with an accelerometer located within the pulse generator. This sensor produces an electric signal during physical activity of the patient.
Although modern pacemakers offer a range of advanced pacing features mentioned above, serious but infrequently occurring arrhythmias can be difficult to detect and diagnose. Hence, a body sensor system incorporating physiological information to aid diagnosis is also an important research field for implantable pacemakers. In rate-responsive pacemakers, some of new physiological parameters are sensed and utilized for diagnosis, such as body vibration or movement, respiratory rate, electrocardiograph (ECG), heart rate, physiological impedance, temperature, and venous oxygen saturation.
III. BODY SENSORS
A body sensor is a device for the detection of an analyte that combines a biological component with a physicochemical detector component. It normally consists of three parts:
(a) the sensitive biological element (biological material, a biologically derived material or biomimic);
(b) the transducer or detector element works in a physicochemical way that transforms the signal resulting from the interaction of the analyte with the biological element into
another signal (i.e., transducers) that can be more easily measured and quantified;
© the associated electronics or signal processors that are primarily responsible for the display of the results in a userfriendly way. This sometimes accounts for the most expensive part of a sensor device. As the sensing technology advances, pacemakers have been able to detect various kinds of physiological variables as well as cardiac signals. Now body sensors are incorporated in most pacemakers as a programmable option. In addition, the role of sensors has been expanded to include functions other than rate
augmentation such as the detection of atrial and ventricular capture, and monitoring of heart failure, sleep apnoea, and haemodynamic status [6]. Through the utilization of sensors to monitor cardiac haemodynamics, right ventricular pressure has been found to be a good estimate of pulmonary arterial diastolic and capillary wedge pressure. A fully implanted device has been used to reduce heart failure hospitalization [7].
Body sensors fall into those that detect only body activity and so react only to movement (accelerometers) and those that measure some consequence of physiological change
during exercise or other conditions (QT interval, respiration, temperature, and venous oxygen saturation). Table I illustrates the categories of sensors for adaptive pacemaker
systems. We conclude specifications, sensed signals, advantages, limitations and application conditions for activity sensors, metabolic sensors, blended sensors, closed loop stimulation (CLS), dual sensors, and new diagnostic sensing systems, as shown in Table II [8].
A. Activity Sensor
Chronotropic incompetence is defined as the inability of a sinus node to react adequately with an increase in heart rate to exercise or other movement. For patients suffering from this disease, rate-response pacemakers were invented [9, 10]. It represents a significant advance over constant rate demand ventricular pacing when first introduced in the 1980s, which relies on sensors to detect patient’s activity [11]. The key element of such pacemakers is their activity sensors. Such sensors have been almost universally applied because of their technical simplicity and relative lack of incorrect responses.
Activity sensors, which offer rapid response to exercise by assessing body vibrations or movements, are old and widely used. The working modality is based on the relationship
between activity and heart rate. Activity may be recognized by an accelerometer that identifies the postural changes and the body movements related to physical activity [12].
A simple but robust solution for activity sensing is the use of an accelerometer to register body movement. An accelerometer placed in a pacemaker detects movement and
patient’s physical activity and generates an electronic signal that is proportional to physical activity. Because it is noninvasive (the sensing device is placed inside the pacemaker without direct contact with the human body), this is the preferred technique used in most rate-responsive pacemakers sometimes complimented with sensors for other parameters such as ventilation rate, venous 2 O saturation, or body impedance [13]. An accelerometer evaluates amplitude representing a movement force and also a signal frequency, which is a rate scale factor of movement. It responds to a particular range of vibration frequencies, reducing unwanted external vibrations.