Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
2000-02-08
2002-12-24
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
Reexamination Certificate
active
06498652
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to monitors for detecting cardiac and pulmonary motions such as heartbeat, respiration, physical movement, and other body activities of a patient.
2. Description of Related Art
For detecting vital signs of a patient, there are a number of monitors available with a variety of uses and applications. These vital sign monitors and other advanced monitors range from simple sound monitors such as “baby monitors” used at home, to sophisticated apnea monitors and electro-cardiograms (EKGs) used by physicians and hospitals.
Off-the-shelf low cost, non-prescription baby monitors are currently in wide use to monitor infants at home to verify an infant's well-being. These monitors generally employ a microphone sensor that sends a signal to a remote unit with visual and audio displays. For example, the microphone sensor can be placed close to the crib on a table or hooked up to the crib with a bracket while the remote unit is carried around in the house by parent or other caregiver. Although these units are suitable to monitor low risk infants when they are awake and moving around or crying in the crib, they are not reliable for monitoring them during sleep periods. Parents may be concerned that their child is in jeopardy if they can not hear their child's cries or other normal sounds over the monitor. To address this concern, a parent may make many visits to the room where the infant is asleep to either wait patiently until the infant moves in the crib or attempt to touch the infant in an effort to detect child's breathing.
An alternative system for monitoring infants on the market today uses a rocker switch or pressure/capacitive sensor placed under a mattress to detect the rocking action of the mattress when the infant or other patient moves. To work, such a system requires a hard surface under the mattress and a rocking mechanism placed between the mattress and the hard surface. When the patient moves, the mattress rocks back and forth across a fulcrum causing the sensor to detect the movement. The presence of the right amount of movement might indicate that the patient is healthy, while the lack of movement or too much movement could suggest that the patient is in trouble; although lack of movement does not always reflect that the patient is in trouble. In the case of a small infant who normally does not move much, the sensor may not detect any movement, and accordingly it may produce a large number of false alarms. A significant number of false alarms will undoubtedly cause the parent or caregiver to stop using the device and render it effectively useless. Additionally, the system is not sensitive enough to accurately detect the small-scale acousto-mechanical signals generated by the heart beating, which may provide a complimentary signal that could be used to reduce false alarms.
Neonatologists use apnea monitors for detecting sleep apneic events in premature or high-risk infants, which if not detected can result in Sudden Infant Death Syndrome (SIDS). Generally the apnea monitors are used to detect and measure respiration rate of an infant. In the absence of respiration for about 20 seconds, the monitor sounds an alarm indicating an urgent need for nurse intervention. These type of monitors typically utilize impedance pneumography techniques, in which the electrical sensors are placed on infant's body to detect variations in resistance caused by respiration and physiologic changes when a 10-100 kHz electrical current of about 50 micro amperes is passed through the patient. The voltage developed and measured is proportional to the transthoracic impedance, which varies during each breath. These sensors are designed to measure changes in this resistance, on the order of 0.1-1 ohms for shallow breather infants to normal ones. One problem with these sensors is that they do not provide high enough sensitivity to distinguish between shallow breathing and no breathing. As a result, they generate a large number of false alarms, requiring constant attention of nurses and frequent re-setting of the instrument. To better detect the apneic event, a separate heart rate sensor is often added to apnea monitors. Monitoring the heart rate in conjunction with respiration can lower the false alarm rates because prolonged apnea is frequently followed by bradycardia (i.e. slowing of the heart rate), which is an event that can be measured using the heart rate sensor. Further improvements can be made by incorporating a saturated oxygen monitoring sensor to an apnea monitor. Such a sensor is affixed at the finger tip of the patient using separate electrical leads. Monitoring the changes in oxygen level in conjunction with respiration and heartbeat rates is useful because it can more accurately detect the onset of an apenic event so that appropriate decisions can be made to administer the correct treatment.
Even though addition of both a heart rate sensor and an oxygen sensor to an apnea monitor is useful to reduce false alarms, and better detect apneic events, such monitors still have a number of shortcomings. For example, 1) with the addition of these sensors and associated electronics, the system's cost increases; 2) electrical leads can come loose or break; 3) if the sensors are inadvertently pulled due to body motion or lead tangling, the infant's skin can peel off; 4) the infant's skin can develop dermatitis from electrode creams, gels and adhesives 5) because the sensors are electrical in nature, they are susceptible to interference from electromagnetic sources, static discharge, radio frequencies, and shocks; 6) they do not function properly in the presence of conductive liquids; 7) they utilize a number of sensors and leads to pick up signals from the patients, which can impede the development of mother/infant relationship; 8) unless the respiration sensors are appropriately placed and the controls adjusted properly, the monitor can be sensitive to artifacts generated by heartbeat; 9) patients can not sleep well because they are forced to lie in a certain position determined by the sensors, and cannot move around freely once the sensors and leads are hooked up; 10) if X-rays are required, the sensors must be removed; and 11) the output signals are susceptible to body motion artifacts.
Another type of monitor is a motion sensing
on-impedance type, such as an air mattress that is designed and developed for the detection of sleep apnea. The motion-sensing sensors are based on the principle of air displacement from one segment to another through a manifold, which is measured by a heated thermistor in the manifold. The moving within the mattress air cools the heated thermistor in the manifold, and this temperature change is detected as a breath. Unfortunately, these pads are very sensitive to body motion and vibration artifact, and therefore the respiration signals are not easily detected from the artifacts. In addition, these sensor pads are not rugged enough to fold and flex, and therefore, are difficult to use and store. Furthermore, because the sensor includes an electrically active thermistor, it is prone to electromagnetic interference.
Other monitor types, including sensors with capacitive mattresses, and sensors with magnetic, thermistor, and pressure transducer pads have similar disadvantages. Because most devices used to date are electrical in nature, they have similar shortcomings as described for the impedance or air mattress sensors. None are totally suitable for reliably detecting apneic conditions or near-miss SIDS.
U.S. Pat. No. 5,241,300 discloses a fiber optic breathing sensor that monitors the air movement localized at the nasal cavities. By placing this fiber optic sensor at the base of the nasal cavity, it measures the change in temperature due to air flow. This sensor does not provide the cardiac signal necessary in the clinical diagnosis of the apnea. Although this system may be useful as a supplementary sensor in the detection of central apnea in adul
Maida, Jr. John L.
Varshneya Deepak
Connolly Patrick
Turner Samuel A.
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