Historically, hypopneas (decreased airflow) have been used to characterize OSA and studies have suggested that hypopneas may have the same clinical significance as apneas in many patients (63). However, the standard method of measuring airflow with a thermistor may leave many hypopneas unrecognized by this technique (13). In addition, partial upper airway obstruction that leads to increased amplitude of intrathoracic pressure can trigger an arousal (i.e., a RERA) and such arousals may produce daytime sleepiness (13,64).
Methods to capture more subtle hypopneas and measure airflow more quantitatively have become available. These currently focus around nasal pressure measurement which is an indirect measure of airflow and more sensitive than thermistors (13). Nasal pressure has been favorably compared against pneumotachograph airflow in OSA and appears more accurate than thermistor airflow (65,66). In addition, the use of an esophageal balloon or tube to measure intrathoracic pressure swings is recommended to determine the presence of RERAs (13).
Based on this newer technology, definitions of hypopnea and respiratory events for research purposes have been proposed including syndrome definition using a composite AHI > 5 for confirmation of OSA (13). However, almost all previous OSA studies used thermistors and none of the new definitions have been adequately validated against thermistors in patients with OSA or against non-OSA controls. Given the newer, more sensitive technology to detect respiratory events, it is possible, even likely, that a diagnostic AHI will be much higher than previously observed and many individuals who were considered with a combination of clinical evaluation and PSG results not to have OSA will now have an AHI in the OSA range of at least five and possibly much higher.
PSG is potentially capable of capturing all of the currently recommended respiratory events whereas portable monitors, in general, capture only disturbances in airflow and saturation leading to a RDI that frequently underestimates the number of potential respiratory disturbances during sleep (i.e., apneas, hypopneas, desaturations, arousals, and RERAs). Depending on the technology and definitions used, RDI may vary considerably on the same night in the same patient.
To confuse the matter further, Medicare as mentioned, has published criteria for scoring hypopneas on PSG for purposes of qualifying for CPAP (21). These require a > 30% decrease in airflow associated with a 4% desaturation from baseline during recorded sleep > 2 hours duration. The PSG must be performed in a facility-based sleep study laboratory and not in the home or in a mobile facility. Without the sleep requirement, it is likely that a portable monitor could more readily replicate this definition. Of note, several Local Medical Review Policies (LMRP) may have substituted recording time for sleep time (e.g., http://www.tricenturion.com). Medicare criteria require an AHI of at least five patients with symptoms of OSA such as daytime sleepiness or an AHI of 15, irrespective of symptoms.
The user of a portable monitor should be aware of the operating characteristics of the monitor and not rely on computer-generated scoring. In addition, since the portable monitor does not measure a number of events that may be recorded on the PSG and does not usually measure sleep and may not measure position, a negative study should not be accepted to exclude a diagnosis of OSA. On the other hand, since the portable monitor is generally less sensitive than the PSG, a positive study with a properly validated monitor, if technically adequate, should generally be accepted as confirmatory in the appropriate clinical setting.
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