A method and apparatus are disclosed for assessing a respiratory function from plethysmography data obtainable, for example, from a pulse oximeter. A pulsatile waveform obtained from pleth data is processed to obtain a second waveform extending over a over multiple respiratory cycles and representing respiration-related variations in a pulse-beat area defined by the pulsatile waveform. The second waveform is analyzed to obtain information indicative of respiratory distress. A respiratory index that provides a good indicator of respiratory distress may be computed based on a magnitude of respiratory peaks in the second waveform. Applications to asthma and sleep apnea are described.
