The physician usually orders the lower and upper limits of positive pressure. The APAP device then delivers the lowest effective pressure between these limits.
The upper and lower pressure limits could be placed as wide as possible (4-18 cm H2O) or narrowed based on information from a previous CPAP titration or previous nights of APAP use. Some patients find starting at 4 cm H2O uncomfortable and it may take some APAP machines several minutes to reach a pressure level that they find comfortable. In this case the lower pressure could be increased to 6-10 cm H2O. Awakening with the feeling of insufficient pressure could be another situation in which the lower pressure limit should be increased. Bloating or evidence of excessive mouth leak might be an indication to lower the upper pressure limit. Alternatively, if the 90th percentile pressure essentially equals the upper pressure limit, then a higher upper pressure limit is likely needed (especially if the residual AHI is high).
Information on delivered pressure is not the only data stored in the APAP device that is potentially useful for tailoring treatment. High leak may indicate the need for another mask or a full-face mask (mouth leak). A high-residual AHI might indicate a need for an increase in either the lower or upper pressure. A high number of residual apneas might suggest that central apneas could be present.
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