Hypopnea, a floating metric: Implications for prevalence, morbidity estimates, and case finding

The respiratory disturbance index (RDI) is the most frequently used metric to characterize sleep-disordered breathing. Clinically, the RDI is used to classify disease status and guide treatment decisions. For research purposes, the RDI is used to describe population distributions of sleep-disordered breathing. Its popularity as a cardinal disease-defining measure, however, may not be justified given that standardized criteria do not exist for defining hypopnea, a key component of the RDI. This paper reviews sources of variability in identifying hypopneas, including: the magnitude of changes in breathing amplitude necessary to describe breathing as "reduced" (from "discernible" to >50%), variations in the utilization of sensors with different sensitivities to detect airflow/ventilation (i.e. thermocouples, thermistors, and pressure transducers), and differential use of data on oxygen saturation and arousals to discriminate normal breathing from hypopneas. The extent to which disparate approaches influence the overall RDI and population estimates of disease also is discussed.