Patent No. 4958638 Non-contact vital signs monitor

 

Patent No. 4958638  Non-contact vital signs monitor (Sharpe et al., Sep 25, 1990)

Abstract

An apparatus for measuring simultaneous physiological parameters such as heart rate and respiration without physically connecting electrodes or other sensors to the body. A beam of frequency modulated continuous wave radio frequency energy is directed towards the body of a subject. The reflected signal contains phase information representing the movement of the surface of the body, from which respiration and heartbeat information can be obtained. The reflected phase modulated energy is received and demodulated by the apparatus using synchronous quadrature detection. The quadrature signals so obtained are then signal processed to obtain the heartbeat and respiratory information of interest.

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GOVERNMENT INTEREST

This invention was made with Government support under contract No. N 00014-82C-0930 awarded by the Department of the Navy and under Contract No. F 33615-83D-0601 awarded by the Department of the Air Force. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

This invention relates in general to the use of radar techniques to detect minute body movements which are associated with cardiac and respiratory activity. The invention is based on the principle that breathing and heartbeat produce measurable phase changes in electromagnetic waves as they reflect off of a living person. The invention offers significant advantages over other similar and earlier approaches, including greater sensitivity, lower radiated power, improved reliability and lower cost.

Functionally, the non-invasive, electromagnetically-based Vital Signs Monitor (VSM) is an extremely sensitive motion detection system capable of detecting small body motions produced by respiratory and cardiac functioning. Motion detection is achieved by transmitting an interrogating electromagnetic field at the target of interest, and then measuring the time-delay of the return signal reflected back from the surface of the target. When the target surface is moving, as does the surface of the chest in conjunction with respiratory and cardiac activities, corresponding variations will be observed in the measured time delay. The observed variations can be used to determine motion-related target parameters such as displacement and velocity.

In the medical field, it is essential that a subject's respiration and heartbeat be capable of being measured. The medical profession is accustomed to voltage-derived electrocardiogram waveforms for monitoring heartbeat. Most respiration monitors also require physical connection to the subject's body. Many commercially-available devices are available for measuring heart and respiration rates, but most of them are electrode-based requiring physical contact with the subject. Devices requiring physical contact, however, are difficult to use on children susceptible to sudden infant death syndrome (SIDS) or burn patients who cannot tolerate the touch of electrodes. Many infants wear sensors while they sleep that trigger an alarm if their breathing stops, but electrodes attached to the child can be jarred loose as the infant tosses and turns.

The invention has similarities with motion-detection systems based on ultrasonic or optical techniques. However, an electromagnetically-based approach offers several advantages for monitoring of vital signs-related motions. For example, with proper antenna design, an interrogating electromagnetic field will suffer minimal attenuation while propagating in air (unlike ultrasonic signals which propagate poorly in air). Thus, the electromagnetically-based Vital Signs Monitor can easily be used in a completely non-contacting mode and can, in fact, be placed an appreciable distance from the test subject if required. Electromagnetic signals in the microwave band are also capable of penetrating through heavy clothing. This offers advantages over optical techniques which would have a difficult time of detecting motion through even thin clothing. Another feature of an electromagnetically-based approach is that the system could be designed to simultaneously interrogate the entire chest surface and provide information pertaining to any respiratory or cardiac function manifested as chest wall motions. Conversely, by modifying the antenna design, a localized region of the chest surface could be interrogated to obtain information about some specific aspect of respiratory or cardiac function. Such versatility would be difficult to achieve with other motion detection techniques.

In the prior art the patent to Allen, U.S. Pat. No. 4,085,740 discloses a method for measuring physiological parameters such as pulse rate and respiration without electrodes or other sensors being connected to the body. A beam of electromagnetic energy is directed at the region of interest which undergoes physical displacement representing variations in the parameter to be measured. The phase of the reflected energy when compared with the transmitted energy indicates the amount of actual physical movement of the body region concerned. The method does disclose simultaneous detection and processing of respiration and heart beat; however, frequency modulation is not used, therefore and the subject must be reasonably still. The receiver includes two channels and in one of them the received signal is mixed with a signal substantially in quadrature with the transmitted signal to maximize amplitude output in those cases in which the received signal is 180.degree. out of phase with the transmitted signal.

The patent to Kaplan, et al., U.S. Pat. No. 3,993,995 discloses an apparatus for monitoring the respiration of a patient without making physical contact. A portion of the patient's body is illuminated by a transmitted probe signal with the reflected echo signal detected by a monitor. The phase difference between the transmitted and reflected signals is determined in a quadrature mixer which generates outputs indicative of the sine and cosine of the difference signal. These two outputs are coupled to differentiators and when both time derivatives are substantially zero an x-ray unit is triggered since it represents an instant of respiration extrema (apnea). The outputs of the quadrature mixer are also coupled to a direction of motion detector which indicates inhalation or exhalation.

The patent to Kearns, U.S. Pat. No. 4,289,142 discloses a respiration monitor and x-ray triggering apparatus in which a carrier signal is injected into the patient's thorax which is indicative of the transthoracic impedance of the patient. This impedance changes as a function of the respiration cycle. The carrier signal is injected through electrodes coupled to the patient's thorax. The transthoracic impedance has an alternating current component having a respiratory component between 0.2 to 5 ohms and a cardiac component varying between 0.02 to 0.2 ohms.

The patent to Robertson et al., U.S. Pat. No. 3,524,058 discloses a respiration monitor which uses body electrodes to direct an electric current to a particular part of the patient's body where changes in electrical impedance provide output signals that vary with respiration.

The patent to Bloice, U.S. Pat. No. 3,796,208 discloses an apparatus for monitoring movements of a patient including a microwave scanner (doppler radar) which creates a movement sensitive field surrounding part of the patient. Movements of the patient create disturbances in the field which are monitored and which trigger alarm circuitry.

Also in the prior art, apexcardiograms (ACG), which represent a contact technique for measuring small chest surface motions overlying the cardiac apex, have been used to estimate cardiac contractility, left ventricular end-diastolic pressure, pressure changes during atrial systole and cardiac ejection fraction, in addition to diagnosing myocardial wall abnormalities and dysfunction. One of the problems associated with the use of an ACG for the estimation of cardiac function is that the motions recorded are indicative only of activity at the apex of the heart and not of the heart as a whole. Analysis of the VSM waveform is potentially a better choice for estimation of cardiac function since the larger beamwidth of the VSM antenna actually integrates motion over a certain area of the chest. In addition, since the VSM waveform appears to contain information related to aortic and other vascular pulses, it can be used to measure pulse transit times directly out of the heart into the aorta. This measurement can potentially be used as a non-invasive, non-contact means of estimating blood pressure as discussed by L. A. Geddes, M. Voelz, C. F. Babbs, J. D. Bourland and W. A. Tucker in "Pulse Transit Time as Indicator of Arterial Blood Pressure," Psychophysiology, Vol. 18, No. 1, pp. 71-74, 1981. This paper showed that the pulse-wave velocity in the dog aorta increased linearly with increasing diastolic pressure. Similarly, pulse pressures may be related to either the magnitude of the aortic peak in the VSM waveform, or possibly to the rate of rise of this peak.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to Provide an electromagnetic vital signs monitor that can reliably measure simultaneously both heart and respiration rates.

It is a further object of this invention to provide a device for measuring physiological parameters without physically contacting the subject with sensors or like attachments.

It is a further object of this invention to provide a device for measuring physiological parameters of subjects remotely at distances up to approximately 20 feet.

It is a further object of this invention to provide a device for non-contact and non-invasive diagnosis and monitoring capabilities of cardiac, pulmonary, and thoracic mechanical functions resulting from normal or induced physiological responses, trauma, disease or response to therapy.

It is a further object of this invention to provide a device for measuring remotely the physiological parameters of subjects that are fully clothed and that can be either stationary or moving while sitting or standing.

It is a still further object of this invention to provide a device which can be used as an apnea monitor for patients in hospital or clinic intensive care units, or as a patient monitor in burn or trauma clinics or in nursing homes.

It is a still further object of this invention to provide a portable device that can be taken into patient areas for the purpose of measuring heart beat and respiration rates.

It is a still further object of this invention to detect the presence of persons in visually obstructed areas or under debris resulting from certain disasters.

The non-contact electromagnetic vital signs monitor is comprised of a coherent, linear, frequency modulated continuous wave radar with refinements to optimize the detection of small body movements. The transmitter of the device is frequency modulated by a linear ramp derived from a master clock. The transmitted signal is fed to the radio frequency (RF) network which routes a portion of the energy to the antenna which then interrogates the subject. Signals reflected by the subject containing motion-related phase modulation are intercepted by the antenna and applied to the RF network where they are mixed with a portion of the original signal.

The mixing process produces a difference signal which contains harmonics of the original modulating ramp. Each harmonic line is surrounded by sidebands which are related to the body movements. The relative levels of these sidebands are a function of target range, transmitter frequency deviation, and harmonic number. By properly choosing these latter two parameters, signals from a desired range can be detected while others are suppressed. The process is further refined to result in more ideal range discrimination by multiplication by a weighting function synchronized to the ramp which reduces the range sidelobes.

The final synchronous demodulation is accomplished by mixing the received signal (after weighting) with both the in-phase and quadrature components of the desired harmonic of the modulating ramp which is generated by a synthesizer. After recovery of the in-phase and quadrature components of the received signal, sophisticated digital signal processing can be economically applied since the bandwidths are relatively low. In the preferred embodiment, a high order linear phase finite impulse response digital filter is used on each channel to reduce the dominance of the strong respiratory signal. A complex autocorrelation is performed from which the rates of interest may be calculated.

Still other objects, features and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings.