Patent ID: 8652065

Claim:
A system for determining the transition between inspiration and expiration of a patient's respiratory cycle comprising: a flow sensor for measuring a patient's respiratory flow to obtain a flow waveform indicative of the respiratory flow; and an electronic device to determine a second derivative of the flow waveform or part thereof, and determine an inspiration/expiration transition of the patient's respiratory flow by identifying the steepest descent in the flow waveform using the second derivative of the flow waveform; wherein to determine an inspiration/expiration transition of the patient's respiratory flow using the second derivative the electronic device uses a negative to positive zero crossing in the second derivative; and wherein the flow waveform has an offset and the electronic device is adapted to detect a zero crossing of the flow waveform without an offset, wherein to use a negative to positive zero crossing in the second derivative the electronic device uses a negative to positive zero crossing in the second derivative that corresponds to the zero crossing in the flow waveform.