Determination of Apnea type

Details Determination of Apnea type Determination of Apnea type

2000-10-06

2002-04-23

Shaver, Kevin (Department: 3736)

Surgery

Diagnostic testing

Respiratory

C600S529000, C600S586000, C600S595000

Reexamination Certificate

active

06375623

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of determination of breathing disorders and more particularly to the determination of Apnea and Hypopnea and in particular of the type of Apnea.
BACKGROUND OF THE INVENTION
Apnea is defined as cessation of breathing activity for a certain duration (6-20 seconds). Hypopnea is either slow breathing or shallow breathing or both. Detection of apnea/hypopnea (AH) is important in the diagnosis of the sleep apnea syndrome (SAS), a condition that afflicts 2% of adult females and 4% of adult male population in western countries. Detection of AH is also important in the diagnosis of breathing irregularity conditions in children and elderly patients. The diagnosis of SAS is traditionally performed in a sleep lab, but in recent years devices for home analysis of performance have been developed to offset the substantial cost and inconvenience of a full sleep lab study.
Detection of AH in a sleeping subject requires identification of actual air flow into and out of the thorax. Thus, mere monitoring of chest motion as performed with impedance pneumography is not sufficient. The existing technology is based on the use of thermal sensors in the nostrils and/or the use of chest and abdomen volume monitors (for example, inductive plethysmography which is available under the trade name Respitrace™. These methods either require instrumentation of the patient's face or use of circumferential belts around the chest and abdomen. Both approaches impose substantial inconvenience and are prone to data corruption.
One aspect of apnea detection is the prevention of sudden infant death syndrome (SIDS), the leading cause of death in infants 1-12 months old in western societies. While the etiology of SIDS is obscure, it is broadly believed that if the cessation of breathing is detected in time and resuscitate measures are taken the baby can be saved. Based on this notion many monitoring devices for home use have been introduced into the market. Unfortunately, the technology of many such devices results in many false alarms being activated, often wearing the mental stability of the parents to the point where use of the device is discontinued.
Most of the existing devices rely on monitoring the electrical impedance of the chest However, the chest impedance shows continued substantial oscillations when the breath is held due to the action of the heart within the chest. To prevent this cardiac activity from being detected as breathing activity, the detection algorithm parameters and threshold must be set such that false detection of apnea is almost inevitable. In practice, chest impedance measurements appear to be incapable of providing a clear indication of apnea without at the same time causing an undesirably high number of false alarms.
In PCT application PCT/IL97/00318, filed Sep. 30, 1997 and published as WO 98/144116 on Apr. 9, 1998, the disclosure of which is incorporated herein by reference, the present inventor described a breath sounds analysis system which includes a methodology for the detection of breath sounds in which at least one breath related sensor is placed around the respiratory system of a patient for measuring breath sound data signals; a breath analyzer which continuously matches the breath sound data signals produced by the at least one breath related sensor to at least a regular breath sound template to determine the presence of breathing and which provides an alert indication when no breathing is present for a time period longer than a given period.
In addition, the PCT application describes a method of determining the state of breathing of a patient, the method comprising determining the inspiration/expiration phase of a breath from chest movement data and defining a breath phase variable therefrom; if the tracheal breath sound data are significant and if the external noise is low determining if the tracheal breath sound data has a generally normal shape; and if not determining if the lack of flow indicates the presence of apnea and, if so, setting an apnea alarm. Preferably, the method includes generating a loud noise indication if the breath shape is not normal.
In a preferred embodiment of the invention described in the PCT application, the amplitude of the level of breath sounds are compared with an adaptive breath sounds threshold during consecutive periods of time. If the sound level is below a given level or if the detected sound does not match the spectral characteristic of authentic tracheal breath sounds, for a number of consecutive time periods an alarm may be sounded.
In preferred embodiments of the invention described in the PCT application, the breath sounds are identified as authentic breath sounds by matching the spectrum of the sounds to a spectrum which is determined during a reference period and/or by determining whether the spectral shape of the sounds, as characterized by certain parameters, is normal.
SUMMARY OF THE INVENTION
One aspect of the present invention presents an improved method and apparatus for the determination of apnea and hypopnea.
In a preferred embodiment of a method of the present invention, spectral power is used to determine the presence or absence of breath sounds. In a preferred embodiment of the invention, the average or integral of the sound power amplitude over a particular frequency range is used as a measure of the total breath flow. If the measure is below a threshold, an hypopnea alarm is preferably activated. If the measure is below a second, lower value, an apnea alarm is preferably activated.
In accordance with a further aspect of the invention, a running time average of the spectral power is used in the determination of apnea and/or hypopnea. The averaging time is made relatively short when it is desired to test for apnea in SIDS and relatively longer when sleep testing of adults is performed.
A third aspect of the invention is related to the measurement of breath sounds in the presence of noise. In the above referenced PCT application, an ambient sound sensor was used in order to determine the level of ambient sound. If the level of sound was too high, then no measurements were taken. In critical testing, such as SIDS testing, it is important to continue the testing even in the presence of some noise, as might be present in a household, such as a television in an adjoining room or the like.
In a preferred embodiment of the invention, the ambient sound is treated in the same manner as the breath sound. For frequencies at which the ambient sound level is higher than a certain level, the breath sound spectrum at that frequency is ignored in determining the breath sound power and flow. If the power of the ambient sound is above the certain level for too great a portion of the relevant spectrum, the measurement of breath power is aborted and an ambient noise indication is activated.
A fourth aspect of the invention involves the differentiation between Obstructive Sleep Apnea (OSA) and Central Sleep Apnea (CSA). While both these conditions involve cessation of breathing, with potentially lethal implications, the treatment for the two conditions is different. In particular, OSA is generally treated surgically or by a pressure device that overcomes the obstruction and CSA is not helped by either of these therapies and generally requires a neurological evaluation to determine treatment.
In accordance with a preferred embodiment of the invention, the two conditions are differentiated by determining if there is chest motion (expansion and contraction). When there is expansion and contraction of the chest, but no tracheal breath sounds, this absence of sound is presumed to be caused by an obstruction and the Apnea is classified as of the OSA type. If there are neither breathing movements nor sound, then the apnea is presumed to be centrally caused and the apnea is presumed to be of the CSA type.
There is thus provided, in accordance with a preferred embodiment of the invention a method of differentiating between OSA and CSA comprising:
determining if chest motion above a

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