Univariate analyses were first conducted on factors including BMI, age, and AHI during the diagnostic study, AHI on CPAP titration (CPAP AHI), bicarbonate levels (since most patients did not have arterial blood gas analysis to identify PaCO2 levels, we assessed bicarbonate levels close to the time of their PSGs), presence or absence of CSR, presence or absence of CSA, presence or absence of atrial