Body Sensors Applied in Pacemakers: A Survey
By Wei Vivien Shi and MengChu Zhou
NOTE: This is an abstract of the entire article, which appeared in the June 2012 issue of the IEEE Sensors Journal.
Click here to read the entire article.
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.
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 pacemakers are widely used nowadays. They have become a therapeutic tool used worldwide with more than 250, 000 pacemaker implants every year. 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.
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. The paper provides a brief introduction to the recent advances and features in modern pacemakers, with a focus 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.
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 slows 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.
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. Table I illustrates the categories of sensors for adaptive pacemaker systems.
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. A simple but robust solution for activity sensing is the use of an accelerometer to register body movement. With a 3-axis accelerometer sensor, the acceleration can be measured in x, y, and z-axis directions in a three-dimensional space. Based on the acceleration of gravity, the tilt angle increase, represent their main limitations.
Activity-controlled pacing with vibration detection remains the most widely used form of rate adaptation because it is simple, easy to apply clinically, and rapid in onset of rate response. The piezoelectric crystal is bonded to the inside wall of the pulse generator “can”.
Metabolic Sensor
Metabolic sensors, based on minute ventilation, QT interval or peak endocardial acceleration, provide pacing rates more closely and specifically related to physical and mental stress requirements. Minute ventilation, the product of respiratory rate (the number of breaths per minute a person is taking) and tidal volume, is one such sensor that has an excellent correlation with metabolic demand, including body oxygen consumption and cardiac output.
Closed Loop Stimulation
Future devices may provide the opportunity to use physiologic sensors to monitor a cardiac function and to adapt the pacemaker function to assist therapy for associated disorders. The Closed Loop Stimulation (CLS) is a physiological impedance-based pacemaker rate-response sensor, which relies on changes in intra-ventricular impedance to dictate heart rate.
Venous Oxygen Saturation
Mixed venous oxygen saturation (Sv O2) measured in the pulmonary artery is an average of the venous oxygen saturations of the body. It reflects the balance between oxygen supply and demand and might be used for diagnostic decisions, therapeutic guidance, prognostic prediction and, in combination with oxygen uptake and arterial oxygen saturation to determine cardiac output. It is demonstrated that an Sv O2 sensor is a physiologically acceptable sensor.
Other Body Sensors
Temperature of right ventricular blood is affected by physical activity and emotional stress, and increases with workload because about 80 percent of the energy expended in skeletal muscles is converted to heat.
Blood pressure is a clinically important measurement. This kind of sensors measures the rate of change of right ventricular blood pressure (dP/dt) as an indicator of the force of contraction.
At present, many of pacemakers relay stored information to a server, which then makes the distilled data available to clinicians, in some cases via web browsers. They communicate with PCs to upload stored information and may soon communicate with devices such as smartphones. All these conveniences may come with possibility that hackers could break into the pacemakers’ communications and either send harmful commands to the devices or steal private patient information and even reprogram their devices. Hence, researchers and manufacturers need to design a sensor with security features that protect a patient’s data.
ABOUT THE AUTHORS
Wei Vivien Shi (S’11) received a B.S. degree in electrical engineering from the Nanjing University of Aeronautics and Astronautics, Nanjing, China, in 2005, and a M.S. degree in electrical engineering from the Hebei University of Technology, Tianjin, China, in 2008. She is an Assistant Professor with the Engineering and Technology Department, University of Wisconsin-Stout, Menomonie, WI, while completing her Ph.D. study in the Department of Electrical and Computer Engineering, New Jersey Institute of Technology, Newark. Her current research interests include control systems, robotics, automation, dynamic systems, and biomedical applications.
MengChu Zhou (S’88, M’90, SM’93, F’03) received a B.S. degree in electrical engineering from the Nanjing University of Science and Technology, Nanjing, China, in 1983, a M.S. degree in automatic control from the Beijing Institute of Technology, Beijing, China, in 1986, and a Ph.D. degree in computer and systems engineering from Rensselaer Polytechnic Institute, Troy, NY, in 1990. He joined the New Jersey Institute of Technology (NJIT), Newark, in 1990, and is currently a Professor of electrical and computer engineering and the Director of the Discrete-Event Systems Laboratory. He is presently a Professor with the MoE Key Laboratory of Embedded System and Service Computing, Tongji University, Shanghai, China.
