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.