 Recently the investigators described the double-loop gain of the respiratory control system as a measure of periodic breathing. This is a frequency-dependent variable which describes 1 the tendency of the respiratory system to oscillate at a given frequency and 2 the degree to which the relation between oscillations in ventilation and oscillations in arterial blood gas values is linear. The underlying hypothesis is that periodic breathing results from negative feedback regulation of arterial O2 and CO2 pressure through the chemoreflexes. The double-loop gain describes the gain in the negative feedback loop under the assumption that accidental changes occur in both ventilation and arterial blood gas pressures. A simple version of the double-loop gain is derived from nasal pressure changes and arterial O2 saturation. From all-night recordings, the double-loop index DLI can be derived, which is determined by the time during which the double-loop gain exceeds a given threshold. 