© 2002 American Thoracic Society
Breathing Flow Disturbances during SleepCan They Be Accurately Assessed by Nasal Prongs?
a Institut Clínic de Pneumologia i Cirurgia Toràcica Hospital Clínic Barcelona, Spain The assessment of breathing flow plays a key role in diagnosing respiratory disturbances during sleep. The pneumotachograph is the gold standard for accurately measuring breathing flow. It goes without saying that there is full agreement on these two statements. But the most appropriate way to measure breathing flow during sleep in the clinical arena has not yet been established. A pneumotachograph requires the patient to wear a cumbersome nasal or face mask, which limits routine application. To overcome this disadvantage, different sensors, such as thermistors, thoracoabdominal bands and, more recently, nasal prongs have been proposed for the routine assessment of respiratory flow (1, 2). Nevertheless, these sensors do not measure actual flow but a surrogate signal of this physiologic variable. The accuracy of these sensors should therefore be validated because breathing flow is essential for quantifying the severity of respiratory sleep disturbances. The importance of finding a suitable method for measuring flow during sleep was highlighted by a recent Task Force (3) and was reflected in two reports published last year in AJRCCM (4, 5). In this issue of AJRCCM, the work of Heitman and coworkers (6) (pp. 386–391) provides additional data on the clinical application of indirect measurements of respiratory flow during sleep. The investigators compared the performance of nasal prongs and thoracoabdominal bands in detecting obstructive respiratory events in a routine setting; the pneumotachograph was used as the reference. Heitman and coworkers (6) confirm earlier findings showing that the nasal prongs signal is suitable for assessing the total number of respiratory events (apneas plus hypopneas). Traditionally the only type of obstructive event considered was an apnea. The spectrum of sleep disordered breathing, however, is much broader, hypopneas cause the same disturbances as complete apneas (7), and the upper airway resistance syndrome is now considered a distinct entity (8). Identification of the different types of respiratory flow abnormalities requires tools to accurately measure the magnitude and shape of the flow waveform. Obstructive apneas result from static upper airway collapse. The corresponding absence of flow makes apneas particularly easy to detect, even with a thermistor. By contrast, hypopneas or flow limitation leading to an arousal related to respiratory effort are due to dynamic upper airway obstruction. The tools to detect this event should have a good dynamic response, as has been demonstrated over the years in the assessment of respiratory sleep disturbances (7, 8). In agreement with earlier findings (2, 4, 5), Heitman and coworkers (6) conclude that the pressure signal from the nasal prongs is suitable for determining the apnea–hypopnea index. The authors found no improvement when linearizing the nasal prongs pressure by computing its square root, which contrasts with recent reports both from bench (4) and patient (5) studies. Indeed, the authors of these reports demonstrated that the relationship between the nasal prongs signal and flow was quadratic and that assessment of the apnea–hypopnea index was improved by linearizing the signal either by computing the square root of the raw pressure or by simply changing the threshold of amplitude flow reduction from 50% to 25% (4, 5). The discrepancy concerning the usefulness of linearizing the nasal prongs signal could be attributed to two factors. First, because detecting apneas is insensitive to linearization, analyzing apneas combined with hypopneas might have decreased the sensitivity of the linearized nasal prongs. Different results would probably be obtained if apneas and hypopneas were analyzed separately. Second, visual scoring of the respiratory events could limit the accuracy when identifying the most critical events, i.e., those exhibiting a flow amplitude reduction around the 50% threshold. We agree with Heitman and coworkers (6) in that the raw nasal prongs pressure is useful for conventional visual scoring in routine sleep studies. Nevertheless, published data show that the signal should be linearized and analyzed quantitatively to take maximal advantage of the nasal prongs capabilities. The trend toward progressive quantification has been reinforced by the most recently issued recommendations (3). Improved quantification of flow events will help in standardizing the assessment of respiratory events, thereby minimizing the variability in the detection and classification of respiratory sleep disorders (9). In addition, better quantification of flow recording would facilitate both the detection of inspiratory flow limitation and the study of its clinical impact. As Heitman and coworkers (6) and other authors have stated (10), the detection of respiratory sleep disturbances by using nasal prongs suffers from a potential drawback. The sensor fails in the case of mouth breathing with the result that false apneas can be detected in some patients. This artifact, however, which is characterized by a loss of flow signal, can be easily detected and the erroneous data can be discarded. It should be pointed out that a satisfactory performance of nasal prongs in detecting sleep disturbances does not necessarily mean that this simple device ensures the accurate measurement of patient ventilation throughout the night (4, 5). Because small changes in the placement of the nasal prongs could modify the gain of the pressure–flow relationship (1), the flow assessed by nasal prongs in a given respiratory sleep event must always be compared with the normal flow in the preceding minutes. This check prevents the device from measuring the absolute value of ventilation. However, accurate detection of relative changes in flow amplitude within short time periods is possible. Taken together, the data from Heitman and coworkers (6) and all the previous data evaluating nasal prongs offer strong evidence in support of the use of this simple device in detecting respiratory flow events in routine sleep studies. REFERENCES
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