FIG 9: What is the pathophysiology of periodic breathing in heart failure patients?  Central sleep apnea relates only to the apnea threshold, which indicates a certain level of CO₂ in the blood.  As long as the CO₂ is above the apnea threshold, the patient breathes normally (upper left tracing).  However, if the CO₂ falls below the apnea threshold, respiration stops (lower, middle tracing).  In this case the patient produces a central apnea and does not get the signal from the brainstem to inspire and expire. 

The interesting case occurs when the CO₂ level moves through the apnea threshold (upper right tracing).  In this case there are more shifts of ventilation, with only one or two deep breaths, which reduces CO₂ below the apnea threshold, at which point if the patient stops breathing then the CO₂ level rises above the apnea threshold.  The result is a continuous shift between CO₂ levels above and below the apnea threshold, which characterizes the periodic breathing seen in this patient’s polysomnographic study.