The invention provides a multi-
sensor system that uses an
algorithm based on adaptive filtering to monitor a patient's
respiratory rate. The
system features a first sensor selected from the following group: i) an impedance pneumography sensor featuring at least two electrodes and a
processing circuit configured to measure an impedance pneumography
signal; ii) an ECG sensor featuring at least two electrodes and an ECG
processing circuit configured to measure an
ECG signal; and iii) a PPG sensor featuring a
light source,
photodetector, and PPG
processing circuit configured to measure a PPG
signal. Each of these sensors measures a time-dependent
signal which is sensitive to
respiratory rate and, during operation, is processed to determine an initial
respiratory rate value. An adaptive
digital filter is determined from the initial respiratory rate. The
system features a second sensor (e.g. a digital 3-axis
accelerometer) that attaches to the patient's
torso and measures an ACC signal indicating movement of the chest or
abdomen that is also sensitive to respiratory rate. This second signal is processed with the
adaptive filter to determine a final value for respiratory rate.