Figure 1.

Difference between the movements of the chest and abdomen and the sum of the two movements during obstructive apnea (OA), central apnea (CA), and normal breathing. During normal breathing, chest and abdomen movements are in phase. During OA, breathing effort and airway blockage result in the out-of-phase movement of the chest and abdomen, and the sum of the two movement signals (respiratory sum) is close to zero. During CA, there is no movement in the chest or the abdomen. CA: central apnea; OA: obstructive apnea; Resp sum: respiratory sum.