Contactless Measurement of Breathing
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04-10-2010, 03:38 PM
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Changes in the lung air volume change the conductivity distribution in the thorax. Based on this effect, we describe a noncontact instrument to measure lung volume which is suitable for applications such as monitoring of breathing in sleeping subjects for sleep apnea. The instrument is based on a magnetic coil is positionned in a flexible mattress underneath the subject. AM frequency current is applied to the coil to generates Eddy currents in nearby conductive tissues, which in turn generate a magnetic field opposed to the transmitted field. This effect makes changes in the thoracic conductivity distribution due to breathing and blood flow change the effective inductance of the coil. We describe a new design to measure this effect using a single transmit and receive coil. The coil forms part of a Colpitts oscillator tuned to be at its stability margin, in order to give large frequency changes for given changes in conductivity. The oscillator output is then demodulated using a frequency counter. This device was tested in healthy volunteers and showed a good correlation with pneumotachograph measurements.
The radar capable to detect and perform diagnostics of a state of the human being behind obstacles and in conditions of bad visibility could be used for the manifold applications. The potential consumers of the radar are: rescue services, anti terrorist detachments and law-enforcement bodies. It is possible also to use this device in medicine. In this paper, the experiments with using of continuous wave subsurface radar are described. The recorded oscillograms and their frequency spectrums for heartbeat, respiration and articulation of a man, which is taking place behind an obstacle (wall), are presented.
There is significant interest in non-contact monitoring of lung activity, for applications such as monitoring of patients in intensive care for the onset of critical clinical conditions or for monitoring sleeping subjects for apnea. Apnea in infants is associated with sudden infant death syndrome, while adult apnea causes significant difficulties with sleep. While large medical imaging technologies, such as CT, MRI and PET scanning allow accurate non-invasive imaging of the thorax, they are not suitable for use in monitoring applications, due to the bulkyness and expense of the equipment.
Many instrumentation technologies also exist for lung function measurement. However, very few of these technologies are non-invasive. Perhaps the most widely accepted of these is inductive plethysmography, in which inductive bands are placed around the chest and abdomen and the change in diameter of each measured. Inductive plethysmography is currently considered the gold standard for non-invasive lung monitoring in sleep, but has several well known disadvantages over longer times, the monitoring bands tend to change position as the patient moves, resulting in measurement inaccuracies. Additionally, the placement of wires and bands on a sleeping patient is unconfortable and can induce feelings of claustrophobia.
The requirements of various breathing monitoring applications differ significantly. Some applications, such as infant apnea monitoring, require primarily breath timing. Any significant changes in breathing rate must be detected by the system and result in an alarm. The primary concern is respiratory arrest, or apnea, but other timing changes, such as tachypnea or rapid breathing, should also be detected. Adult sleep apnea monitoring is a more complex application in which ventilatory volumes and flows must be measured in addition to breath timing. This allows apneic events to be classified as to whether they are of obstructive or central origin. Obstructive events arise due to increases in airway resistance, and result in reduced flows for a given pressure generated by the airway muscles. In extreme cases, the airway is blocked and
paradoxal ventilation results as the chest and abdomical cavities move in opposite directions.
Central apnea, on the other hand, results from a depressed control of ventilation. Thus
obstructive apnea typically results in decreased tidal volume, while central apnea results in decreased breathing frequency. We are interested in exploring technology for completely
non-contact monitoring based on inductive measurement of thoracic conductivity for lung function measurement. Such an approach has the advantage that it has no cables and straps
and can have a more rigid mechanical housing.
This technology can monitor heart and lung activity by measuring the conductivity changes caused by the movement of air and blood in the thorax. In order to image the conductivity distribution, techniques such as magnetic impedance tomography (MIT), and electrical capacitanc tomography (ECT) have been developed . A series of coils are positioned near the chest, and AM radio frequency signals (in the range 1-10 MHz) are emitted from one coil while other coils measure the signals produced. The RF signals produce Eddy currents in conductive substances in the body (such as the heart and lungs), which, in turn, produce a signal which can be measured. Unfortunately, it appears that such imaging systems are inherently difficult to build, largely because the Eddy current signals are so small compared to the driving signals. Any small movement of the measurement coils produces an artifact much larger than the signals.
Thus, according to Griffiths, for MIT “ no convincing in-vivo images have been successfully produced”. We are interested in building a system which is less sensitive to sensor position. In order to accomplish this, we do not attempt to calculate an image of the conductivity distribution, but rather a single parameter which correlates to breathing and heart activity. This is somewhat similar to the original Eddy current system of Targan and McFee and extensions to it. The difference between our system and these previous ones is it uses a single coil for both signal transmission and detection, and that this measurement system does not need to be rigid. Specifically, these measurement coils can be embedded into mattress cover which could be placed between the subject and the mattress. The flexible coils are able to tolerate bending due to the subjects weight and movement, and provide a continuous monitoring of the patient breathing pattern.
For long time, radars intended for sounding of opaque mediums were developed for detection of motionless objects, as a rule, in the ground. It is accepted in the scientific literature to name such radars by surface-penetrating radar. The main fields of the surface-penetrating radars applications are much wider now:
- Ground sounding for inspection of subsurface communications (pipes, cables etc.)
- Detection of mines and unexploded ordnances
- Sounding of building designs for detection of built-in details, defects and latent objects
(for example, overhearing devices)
- Detection of the material evidence in criminalistics .
Traditional surface-penetrating radars are usual region of engineering, and the time-domain impulse subsurface radars are being produced in lots by many countries (the USA, Canada, Russia and so on). There is now keen interest to use of methods and equipment of surface-penetrating radars for detection and diagnostics of the live persons, which are taking place under rubble or behind walls of buildings. This task could be solved by radars that operate in the wavelength range of 3 - 30 cm (frequencies correspond to 1 - 10 GHz).
In this case, by subtraction of signals reflected from motionless objects it is possible to achieve high sensitivity at detection of objects, borders of which are subjected to mechanical fluctuations. According to estimation available in the literature, the sensitivity of the method could achieve 10-9M. Let's name the method as vibro-electromagnetic sounding. Though the objects subjected to mechanical fluctuations could have a various nature, in the present research we are limited only to detection and diagnostics of a live person. Objects in man’s body that are subjected more or less periodic fluctuations are cardiac muscle and lungs. Their reductions have frequencies in range of 0.8 – 2.5 Hz for heart and 0.2 – 0.5 Hz for lungs. Physical activity and medical state of the examinee determines the values of these frequencies. The remote or contactless measurement of breathing and pulse rates of the man behind an obstacle or in open space at some distance is the basic task of this experimental research. The task may be solved under condition of creation of enough sensitive radar and development of selection algorithms of background reflections that can mask a valid signal.
Last circumstance is a major factor constraining application of vibro-electromagnetic sounding for practical using. Background signals could be connected with reflections from the operator carrying out researches or other people, which are taking place in a zone of measurements. Besides that, working machines and mechanisms, vibrations of foliage and branches of trees, animal and other mobile objects can create interferences and noises also. This requires creation of the antenna with the minimal side and back lobe of the directional pattern or development of methods of their removal.
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Joined: Sep 2010
04-10-2010, 03:47 PM
It is a completely non-contact monitoring of lungs breathing measurement, based on inductive measurement of thoracic conductivity for lung function having no cables and straps and having more rigid mechanical housing.
contactless measurement of breathing.ppt.pptx (Size: 5.43 MB / Downloads: 107)
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