Doppler Radar Physiological Sensing

Olga Boric-Lubecke, Victor M. Lubecke, Amy D. Droitcour, Byung-Kwon Park, Aditya Singh

• Publisher: Wiley-IEEE Press
• Year: 2015
• Print ISBN: 9781118024027
• eBook ISBN: 9781119078432
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biotechnology
• Index: Biological Sciences

Presents a comprehensive description of the theory and practical implementation of Doppler radar-based physiological monitoring

This book includes an overview of current physiological monitoring techniques and explains the fundamental technology used in remote non-contact monitoring methods.  Basic radio wave propagation and radar principles are introduced along with the fundamentals of physiological motion and measurement. Specific design and implementation considerations for physiological monitoring radar systems are then discussed in detail. The authors address current research and commercial development of Doppler radar based physiological monitoring for healthcare and other applications.

• Explains pros and cons of different Doppler radar architectures, including CW, FMCW, and pulsed Doppler radar
• Discusses nonlinear demodulation methods, explaining dc offset, dc information, center tracking, and demodulation enabled by dc cancellation
• Reviews advanced system architectures that address issues of dc offset, spectrum folding, motion interference, and range resolution
• Covers Doppler radar physiological measurements demonstrated to date, from basic cardiopulmonary rate extractions to more involved volume assessments

Doppler Radar Physiological Sensing serves as a fundamental reference for radar, biomedical, and microwave engineers as well as healthcare professionals interested in remote physiological monitoring methods.

Chapter 1
Introduction

Amy D. Droitcour¹, Olga Boric-Lubecke², Shuhei Yamada², and Victor M. Lubecke²

¹Wave 80 Biosciences, Inc., San Francisco, California, United States

²Department of Electrical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii, United States

Noncontact detection and monitoring of human cardiopulmonary activity is one of the most promising solutions for sleep monitoring, postsurgery monitoring, home health care, and search and rescue applications. Without contact or subject preparation (special clothing, attachments, etc.), this could facilitate health monitoring to the chronically ill, enable sleep monitoring outside of sleep laboratories, detect survivors under rubble, and deliver warnings of emergencies or changes in conditions of patients. Doppler radar remote sensing of physiological signatures has shown promise to this end. The development of Doppler radar for remote sensing of vital signs, with proof of concept demonstrated for various applications [Li et al., 2013], could offer a platform for unobtrusive, noncontact, yet continuous physiological monitoring systems.

Cardiopulmonary monitoring is typically carried out with contact sensors such as electrocardiogram (ECG) electrodes. The use of contact sensors is neither possible nor desirable in many situations, due to, for example, skin irritation, or simply lack of access for direct contact. The long-term, continuous use of contact sensors is also limited by degradation in contact quality over time. Some examples of long-term health-care monitoring that would benefit from noncontact sensing include monitoring postsurgery patients, chronic and elderly patients, and patients with sleep disorders. Premature infants and burn victims will also clearly benefit from noncontact sensing due to compromised skin integrity. A failure to respond to patient deterioration promptly and appropriately can lead to increased morbidity and mortality, increased requirement for intensive care, and elevated costs [Tarassenko et al., 2006]. Early identification of patient deterioration is important, as it can prevent subsequent cardiopulmonary arrest and reduce mortality. Early recognition of physiological abnormalities coupled with the rapid intervention of suitably trained staff may result in an improvement in the functional outcome or mortality rate. Early recognition relies on the physiological observations being measured accurately and at intervals appropriate to the condition of the patient. However, many patients are not monitored regularly, and some vital signs such as respiratory rate are measured significantly less frequently than other vital signs. There is a need for straightforward, automated, continuous physiological monitoring technology.

Noncontact physiological monitoring may make a significant impact beyond health-care applications, especially in situations where direct access to the subject is not available. Such situations include, for example, occupancy sensors for energy efficiency [Yavari et al., 2014], search and rescue operations for survivor detection under rubble [Chuang et al., 1990], and detection of adversaries through walls [Lubecke et al., 2007].

1.1 Current Methods of Physiological Monitoring

Assessment of cardiopulmonary functions is most often performed with contact sensors when direct access to the subject is available. ECG is the gold standard for heart monitoring that is often used in hospital and ambulatory settings, whereas there is no equivalent gold standard for respiratory monitoring. There are many different approaches used for respiratory monitoring; however, none of them are easily applied. Even though respiratory rate is a key early indicator of physiological instability that may lead to a critical event, respiratory rate is measured significantly less frequently than other vital signs, such as blood pressure, pulse rate, and arterial oxygen saturation. Among the vital signs, respiratory rate is the only sign that is typically measured manually, via visual assessment, with a nurse counting chest excursions.

The current practices to measure respiration are divided into three categories: measurement of oxygen saturation, measurement of airflow, and measurement of respiratory effort/movement [Webster, 2010]. Pulse oximetry measures the percentage of hemoglobin (Hb) that is saturated with oxygen. A source of light originates from the probe at two wavelengths (650 and 805 nm). The light is partly absorbed by hemoglobin, at amounts which differ depending on whether it is saturated or desaturated with oxygen. Direct measurement of airflow typically uses a spirometer with a mouth piece or a face mask. It contains a precision differential pressure transducer for the measurements of respiration flow rates. The spirometer records the volume and rate of air that is breathed in and out over a specified time. These spirometers are rarely used continuously because they have large dead volumes and high resistance, which make them unpleasant to use. Indirect measurement of airflow, such as with a thermocouple or capnograph, has less adverse effects, but still requires the placement of sensors in front of the nose and/or mouth. Respiratory effort/movement measurement can be monitored by measuring body volume changes; transthoracic inductance and impedance plethysmographs, strain gauge measurement of thoracic circumference, pneumatic respiration transducers, and whole-body plethysmographs are examples of indirect techniques. Each respiratory measurement method has unique advantages and disadvantages. Pulse oximetry measurements indicate that a respiratory disturbance has occurred, but do not provide respiratory rate. Airflow measurements are the most accurate, but interfere with normal respiration. The whole-body plethysmograph can be highly accurate and does not interfere with respiration, but requires immobilization of the patient. The performance of commonly used transducers (belts or electrodes) for ambulatory respiration monitoring significantly degrades over time with wear and tear. Impedance plethysmography, performed through ECG electrodes, is the most common method of continuously measuring respiratory rate in the hospital.

The electrocardiograph (ECG) is traditionally considered the standard way to measure the cardiac activity. It records the electrical activity of the heart over time. Electrical waves cause the heart muscle to contract. These waves pass through the body and can be measured at electrodes attached to the skin. Electrodes on different sides of the heart measure the activity of different parts of the heart muscle. An ECG displays the voltage between pairs of these electrodes, and the muscle activity that they measure, from different directions. This display indicates the overall rhythm of the heart, and weaknesses in different parts of the heart muscle. The other approach is pulse measurement of changes in blood volume in the skin. Pulse measurements, such as a photoplethysmograph (PPG) or piezoresistance, use optical or pressure sensors to identify pulses of blood driven by heartbeats. These are less invasive and simpler than ECG, yet both of these methods require patients to be tethered to the sensing devices.

1.2 Need for Noncontact Physiological Monitoring

The ability to remotely detect vital signs such as heart beat and respiration is particularly useful in situations where direct contact with the subject is either impossible or unwanted. Avoidance of problems such as skin irritation, restriction of breathing, and electrode contacts is desirable in a number of health-care applications, including monitoring of patients with compromised skin, and sleep monitoring. Beyond health care, the potential applications that could benefit from remote sensing of physiological signals include fatigue monitoring, border crossing monitoring, occupancy sensors, sense through the wall, and search and rescue operations.

1.2.1 Patients with Compromised Skin

Development of reliable noninvasive physiological monitoring is an important goal in modern health-care research. Knowledge of routinely monitored heart and respiratory patterns would be clinically useful in many situations. In neonatal intensive care units, infants often suffer skin damage from adhesive tape, electrocardiogram electrodes, electroencephalogram electrodes, and transcutaneous probes, with some lesions leaving scars [Colditz et al., 1999]. Monitoring the cardiac state of burn victims can be challenging because it is sometimes difficult to find enough skin on which to apply a...

Table of contents

• Cover
• Series Title
• Title Page
• Copyright
• Table of Contents
• List of Contributors
• Chapter 1: Introduction
• Chapter 2: Radar Principles
• Chapter 3: Physiological Motion and Measurement
• Chapter 4: Physiological Doppler Radar Overview
• Chapter 5: CW Homodyne Transceiver Challenges
• Chapter 6: Sources of Noise and Signal-to-Noise Ratio
• Chapter 7: Doppler Radar Physiological Assessments
• Chapter 8: Advanced Performance Architectures
• Chapter 9: Applications and Future Research
• Index
• End User License Agreement