Cardiogenic pulse waves that travel along the airway wall are captured as airflow variations synchronized with the electrocardiogram at the airway exit. We have used airflow variations caused by cardiogenic pulse waves (hereinafter referred to as cardiogenic oscillation) to classify types of sleep apnea. Pulse waves do not travel downstream if there is an airway obstruction or choke point (wave speed theory). Thus, cardiogenic oscillation was detectable in central apnea or hypopnea, but not in obstructive apnea. In mixed-type sleep apnea, cardiogenic oscillation disappeared, and thus airway obstruction was judged to have occurred, during central sleep apnea. It was thought that obstructive sleep apnea followed central sleep apnea because there was an airway obstruction, although respiratory effort resumed after the end of central sleep apnea. The pattern of mixed-type sleep apnea was understood from the observation of cardiogenic oscillation. Cardiogenic oscillation is useful for not only classifying types of sleep apnea but also for detecting an airway obstruction.