Sleep apnea risk evaluation

Details Sleep apnea risk evaluation Sleep apnea risk evaluation

2001-12-28

2004-11-02

Jones, Mary Beth (Department: 3736)

Surgery

Diagnostic testing

Respiratory

C600S300000, C600S301000, C600S323000

Reexamination Certificate

active

06811538

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the acquisition of physiological data for health signs monitoring and, more particularly, for the diagnosis and treatment of sleep disorders.
2. Description of the Related Art
Sleep apnea (SA) is the most common disorder observed in the practice of sleep medicine and is responsible for more mortality and morbidity than any other sleep disorder. SA is characterized by recurrent failures to breathe adequately during sleep (termed apneas or hypopneas) as a result of obstructions in the upper airway.
Apnea is typically defined as a complete cessation of airflow. Hypopnea is typically defined as a reduction in airflow disproportionate to the amount of respiratory effort-expended and/or insufficient to meet the individual's metabolic needs. During an apnea or hypopnea, commonly referred to as a respiratory event, oxygen levels in the brain decrease, while the carbon dioxide (CO2) levels rise, causing the sleeper to awaken. The heart beats rapidly and blood pressure rises to high levels (up to 300 mm Hg). The brief arousals to breathe are followed by a return to sleep, but the apneas may recur over 60 times per hour in severe cases.
SA is a serious, yet treatable health problem for individuals worldwide. Published reports indicate that untreated SA patients are three to five times more likely to be involved in industrial and motor vehicle accidents and have impaired vigilance and memory. Studies show that more than 15% of men and 5% of women over the age of 30 and up to 30% of men and women over the age of 65 suffer from SA. SA during pregnancy is associated with hypertension and a risk of growth retardation in the fetus. Current estimates reveal that over 90% of individuals with moderate to severe SA remain undiagnosed.
A. Polysomnography
The current “gold standard” for the diagnosis of SA is an expensive (up to $2,000) overnight sleep study, called polysomnography (PSG), that is administered and analyzed by a trained technician and reviewed by a Board Certified Sleep Specialist. The limited availability of sleep centers coupled with the high capital expense to add capacity has resulted in a growing number of patients awaiting their PSG.
i. Data Recording
A conventional full overnight PSG includes recording of the following signals: electroencephalogram (EEG), submental electromyogram (EMG), electrooculogram (EOG), respiratory airflow (oronasal flow monitors), respiratory effort (plethysmography), oxygen saturation (oximetry), electrocardiography (ECG), snoring sounds, and body position. These signals are considered the “gold standard” for the diagnosis of sleep disorders in that they offer a relatively complete collection of parameters from which respiratory events may be identified and SA may be reliably diagnosed. The RR interval, commonly referred to as beats per minute, is derived from the ECG. Body position is normally classified as: right side, left side, supine, prone, or up (or sitting erect). Typically, the microphone and the body position sensor are taped over the pharynx. Each signal provides some information to assist in the visual observation and recognition of respiratory events.
Collapse of the upper airway is identified when the amplitude of the respiratory airflow and effort signals decrease by at least 50%, snoring sounds either crescendo or cease, and oxygen desaturation occurs. A respiratory event is confirmed (i.e., desaturation not a result of artifact) by the recognition of an arousal (i.e., the person awakens to breathe), typically identified by an increase in the frequency of the EEG, an increase in heart rate, or change in snoring pattern. The remaining signals assist in determining specific types of respiratory events. For example, the EEG and EOG signals are used to determine if a respiratory event occurred in non-rapid eye movement (NREM) or rapid eye movement (REM) sleep. The position sensor is used to determine if an airway collapse occurs only or mostly in just one position (typically supine).
ii. Identifying Respiratory Events
A reduction or absence of airflow at the airway opening defines sleep-disordered breathing. Absent airflow for 10 seconds in an adult is an apnea, and airflow reduced below a certain amount is a hypopnea. Ideally one would measure actual flow with a pneumotachygraph of some sort, but in clinical practice this is impractical, and devices that are comfortable and easy to use are substituted. The most widely used are thermistors placed in front of the nose and mouth that detect heating (due to expired gas) and cooling (due to inspired air) of a thermally sensitive resistor. They provide recordings of changes in airflow, but as typically employed are not quantitative instruments. Currently available thermistors are sensitive, but frequently overestimate flow. Also, if they touch the skin, they cease being flow sensors. Measurement of expired carbon dioxide partial pressure is used in some laboratories to detect expiration, but it is not a quantitative measure of flow.
An alternative method is to measure changes in pressure in the nasal airway that occur with breathing. This approach provides an excellent reflection of true nasal flow. A simple nasal cannula attached to a pressure transducer can be used to generate a signal that resembles that obtained with a pneumotachygraph. It allows detection of the characteristic plateau of pressure due to inspiratory flow limitation that occurs in subtle obstructive hypopneas.
An obstructive apnea or hypopnea is defined as an absence or reduction in airflow, in spite of continued effort to breathe, due to obstruction in the upper airway. Typical polysomnography includes some recording of respiratory effort. The most accurate measure of effort is a change in pleural pressure as reflected by an esophageal pressure monitor. Progressively more negative pleural pressure swings leading to an arousal have been used to define a “Respiratory Effort Related Arousal” (RERA), the event associated with the so-called “Upper Airway Resistance Syndrome”. However the technology of measuring esophageal pressure is uncomfortable and expensive, and rarely used clinically. Most estimates of respiratory effort during polysomnography depend on measures of rib cage and/or abdominal motion. The methods include inductance or impedance plethysmography, or simple strain gages. Properly used and calibrated, any of these devices can provide quantitative estimates of lung volume change and abdominal-rib cage paradox. However calibrating these devices and keeping them accurately calibrated during an overnight recording is very difficult and as a practical matter is almost never done. Thus the signals provided by respiratory system motion monitors are typically just qualitative estimates of respiratory effort.
B. Measuring Oxyhemoglobin Desaturation during Sleep
One of the functions of the lungs is to maintain a normal partial pressure (tension) of oxygen and carbon dioxide in the arterial blood. Various dynamic processes, such as ventilation, diffusion, and the matching of ventilation and perfusion within the lung support this function. Ventilation or breathing, for example, continuously replenishes the oxygen (O
2
) in the gas-exchanging units of the lung, the alveoli, and removes carbon dioxide (CO
2
). An apnea or hypopnea occurring during sleep, however, temporarily decreases alveolar ventilation, causing a drop in arterial oxygen tension (pO
2
) and an increase in arterial carbon dioxide tension (pCO
2
). Because there is currently no accurate non-invasive method for continuously monitoring arterial pO
2
or pCO
2
, non-invasive measures of oxyhemoglobin percent saturation are instead used today to determine apneas or hypopneas.
Blood transports oxygen both as dissolved O
2
and in chemical combination with hemoglobin. The amount of dissolved O
2
is directly proportional to the partial pressure of O
2
. At atmospheric pressure, the amount of dissolved O
2
accounts for only a trivial amount of the blood oxygen content, not

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