Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
2000-09-07
2004-05-25
Lewis, Aaron J. (Department: 3761)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S204210
Reexamination Certificate
active
06739335
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and apparatus for monitoring respiration to determine whether there is breathing obstruction and for titrating nasal continuous positive airway pressure based thereon.
BACKGROUND OF THE INVENTION
Obstructive sleep apnea syndrome (OSAS) is a well recognized disorder which may affect as many as 5% of the adult population. OSAS is one of the most common causes of excessive daytime somnolence. OSAS is most frequent in obese males, and it is the single most frequent reason for referral to sleep disorder clinics. OSAS is associated with all conditions in which there is anatomic or functional narrowing of the patient's upper airway occurring during sleep. The obstruction results in a spectrum of respiratory disturbances ranging from the total absence of airflow (apnea) to significant obstruction with or without reduced airflow (hypopnea and snoring), despite continued respiratory efforts. The morbidity of the syndrome arises from hypoxemia, hypercapnia, bradycardia, and sleep disruption associated with the apneas and arousal from sleep.
While monitoring respiration, it is frequently necessary to determine whether there is obstruction of breathing, whether the obstruction be manifested as apnea, hypopnea, or simply periods of high resistance which need not be accompanied by frank reduction in airflow. Identification of these events is used in both diagnosis of sleep disordered breathing and in the feedback control of automatically adjusting nasal CPAP therapy for obstructive sleep apnea syndrome. Whereas apnea and hypopnea are defined by absolute airflow, the recognition of partial obstruction can only be determined by calculating resistance. However, this generally requires invasive measurement of flow and respiratory effort, e.g., by intraesophageal pressure monitoring.
During therapeutic titration of nasal continuous positive airway pressure (CPAP) in patients with obstructive sleep apnea syndrome, residual apneas may occur that can be either obstructive or “central.” Differentiating these two types of apnea, which have different physiologic as well as therapeutic implications, may impact on the adjustments made to the CPAP pressure. On the one hand, if events are obstructive it is generally assumed that a higher pressure is needed. In contrast, if the events are central, the optimal response is not clearly defined at the present time, as these apneas may be transient irregularities of breathing such as those that occur after arousal (Marrone et al (1991)) or during REM. Not increasing the pressure in the presence of these central events has been recommended as desirable as part of the titration protocol (Teschler et al (1996); Series et al (1997)). In addition, clinical experience suggests that central apneas may even occur as a reaction to excessive CPAP (Berthon-Jones et al (1996); Boudewyns et al (1998)), and this suggests the need to lower the therapeutic pressure. Alternatively, some central apneas may respond to further increases in CPAP (Issa et al (1996)). Irrespective of the decision to raise, lower or maintain the CPAP when central apnea is detected, this decision can only be made, and the impact of the decisions tested, if the apneas are correctly classified.
In addition, during treatments of other breathing disorders in sleep (hypoventilation syndromes), positive pressure is applied to the airway in such a way as to both ventilate a patient (high pressure during inspiration) and maintain the airway free of obstruction, i.e., maintain patency (low pressure during expiration). Such bilevel devices are available, for example, under the trade names BiPap (Respironics), VPAP (Resmed), and MALLINKRODT 335 (Mallinkrodt). Adjustment of this lower expiratory pressure may be dictated by decisions similar to those dictated by setting CPAP in OSAS.
By definition, both types of apnea are identified by the absence of airflow. Differentiation between them is based on analysis of respiratory effort during the apneic period. This can be done either by non-invasive methods (e.g., impedance band) or from direct but invasive measurement of intrathoracic effort (e.g., esophageal balloon). Both of these approaches rely on more than the detection of airflow alone.
Prior methods for monitoring respiration are based on a mathematical analysis technique applied to the flow signal alone during nasal CPAP therapy. This technique is illustrated in U.S. Pat. Nos. 5,335,654, 5,490,502 and 5,803,066, the entire contents of each of which being hereby incorporated by reference. Thus, it has been demonstrated that one can recognize a surrogate of high resistance in the shape of the inspiratory airflow alone. This shape is known as a “flow limitation contour” and is a characteristic flattened contour seen on the inspiratory airflow curve. An example is shown in the top curve of FIG. 8 of U.S. Pat. No. 5,803,066, in the section marked “flow limitation”. It correlates highly with an elevated resistance and can be used in applications which rely on detecting abnormal behavior of the upper airway. Prior methods were based on recognition of this contour as a feedback variable for adjusting CPAP therapy.
Whereas clearly abnormal (flattened) and normal (sinusoidal) contours are readily identified, a significant number of breaths occur which are of ambiguous contour. In some individuals, these are merely variants of normal breath shapes and are not associated with partial obstruction. In other individuals, these intermediate shaped breaths are the only indication available that there is an abnormal resistance which results in clinical consequences (sleep disruption) and which requires treatment (e.g., raising the level of therapeutic CPAP). Misclassifying these breaths as to their resistance in either direction impedes optimal adjustment of CPAP therapy by either manual or automatic means.
A frequent incidental finding seen during monitoring of respiratory signals is the presence of cardiogenic oscillations (West et al (1961). These have been observed and reported during expiration as well as during apnea. Visible oscillations on the airflow signal during quiet exhalation are frequently seen during measurements made of pulmonary physiology, e.g., single breath nitrogen (Lauzon et al (1998) and diffusion studies (Brenner et al (1995). Detection of small movements at the cardiac frequency on inductive plethysmography or expired carbon dioxide signal (Kryger et al (1994)) during apnea has been suggested as an index of their “central” nature. More recently, similar oscillations have been observed on the airflow signal in adults and neonates during central apneas (Morrell et al (1995); Lemke et al (1996); Lemke et al (1998); Milner et al (1990); Shepard (1991)). Whereas Lemke et al. suggested that the presence of cardiogenic oscillations always correlated with a directly visualized open airway, Morrell et al. showed that similar oscillations were observed during central apneas regardless of the airway patency. Thus, there is no consensus on whether the presence of cardiogenic oscillations transmitted to the flow signal is dependent on patency of the airway, which can be compromised during the course of a “central” event, or on lack of respiratory effort.
In U.S. Pat. No. 5,803,066, a technique was identified to classify periods of apnea as being either obstructive or central. That patent discloses that if cardiac frequency pulsations (cardiogenic oscillations) can be detected on the airflow signal during such a period of apnea, the apnea is always classified as central. This implies that the apnea is never obstructive if cardiogenic oscillations are present.
SUMMARY OF THE INVENTION
The present invention provides a method for optimizing the controlled positive pressure in treating sleep disordered breathing by using the appearance or disappearance of cardiogenic oscillation in the airway signal as an additional parameter useful for classifying the level of resistance.
The present invention also provides a method for resolving an ambiguity in breath
Norman Robert G.
Rapport David M.
Browdy and Neimark , P.L.L.C.
Lewis Aaron J.
New York University School of Medicine
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