Figure 4. Examples of apnea detection errors by different signals.

(A) Obstructive apnea missed by the RIPsum signal. Sometimes during obstructive apnea, the RIPsum signal is not reduced more than 90% as it should during apneas because the thoracic and abdominal signals are not necessary in paradoxical movements. Thus, the thorax and abdominal belt signals do not exactly sum up to zero. (B) Central apnea missed by the thermistor signal. Based on the thermistor signal, there is flow and an apnea should not be scored. However, when examining the esophageal pressure, there was no respiratory effort which is interpreted as a presence of central apnea. Note that the event is clearly identified by the nasal pressure, tracheal sound and RIPsum signals. This discrepancy could be due to high sensitivity of the thermal flow sensor. (C) Oral breathing mistaken for apnea by the nasal pressure signal. In the absence of the thermistor and based solely on the nasal pressure, one could score an apnea given that the nasal flow amplitude is reduced more that 90%. Note that respiratory cycles persist on the thermistor, tracheal sound and RIPsum signals. NP = nasal pressure, RIP = respiratory inductance plethysmography.