By definition, the apnea-hypopnea index (AHI) in patients with UARS is smaller than five. The lowest oxygen desaturation in our initial report was greater than 92% (2).
However, based on the AHI criterion of < 5, it is conceivable that some patients with UARS may present with lowest SaO2 values < 92%. AHI alone is not sufficient to diagnose UARS. It simply means that if the AHI criterion of OSA is met, UARS can be excluded and a diagnosis of OSA must be made with additional information from the clinical history (27). For the diagnosis of UARS, a clear presence of RERAs must be established. Oronasal thermistors lack the sensitivity to detect this respiratory abnormality (28). In some sleep clinics it has become common practice to use direct or indirect assessment of esophageal pressure or more sensitive respiratory flow measurement techniques such as a nasal cannula connected to a pressure transducer, calibrated thoracic/abdominal inductive plethysmography, or use of a pneumotachograph. While there is agreement regarding the type of sensors used to identify RERAs, there is little consensus in the definition of RERAs. Some investigators using nasal cannulas (28-30) and respiratory inductive plethysmography (31) require a "flattening" of the inspiratory flow curve in conjunction with an arousal according to AASM criteria to identify RERAs. While the term "flattening" is rarely defined, our own experience with the use of nasal cannulas suggests that the signal quality is often compromised especially in the presence of mouth breathing. Respiratory inductive plethysmography, however, is problematic because of position changes. Data from a 2005 study show that inspiratory flow limitation from nasal cannula recording was superior to esophageal pressure recording for the detection of RERAs (32). In this study of normal subjects, alcohol [which is known to depress arousal responses and ventilatory drive (33)] was given to induce a breathing abnormality. Taken together, it is fair to conclude that a nasal cannula flow signal and esophageal pressure signal are reliable methods to identify RERAs if waveform changes resulting in event detection are clearly defined.
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