Mechanisms of ventilatory periodicities.

The study of ventilatory periodicities is relevant to the problem of obstructive sleep apnea. Apneas occur at the nadirs of periodicities during sleep. Periodicities can be caused by chemical instability, related to unstable action of the closed loop feedback system for the chemical regulation of breathing. Such instability occurs when overall loop gain is greater than or equal to unity and the phase lag around the loop is 180 degrees. Periodic breathing during hypoxia and in patients with congestive heart failure is likely to be explained by this mechanism. Periodic breathing can also be the result of state instability. Here ventilation declines at sleep onset and the resultant changes in blood gases trigger an arousal, i.e., sudden transition to a lighter stage of sleep. With arousal, ventilation increases. Thus, periodic breathing is secondary to these changes in sleep state. These processes, chemical instability and state instability, can interact and produce complex patterns of oscillation in ventilation.