An explanation for failure of impedance apnoea alarm systems.

24-hour recordings of the ECG and respiration, the latter from an impedance technique, have shown a phenomenon which could account for hitherto unexplained failures of impedance apnoea alarm systems. Whenever apnoea is accompanied by bradycardia there is a pronounced increase in the amplitude of the cardiac impulse on the respiration carrier. This imitates the respiration signal and prevents the alarm from sounding. Conversely, apnoea unaccompanied by bradycardia does not present this problem and is detected by the alarm. If impedance alarm systems are to be used to detect apnoea they must be accompanied by a heart rate (ECG) detector.