Disclosed are devices and methods for measuring lung respiration volume including processor means for receiving a detected series of heart beats, measuring variability between a period of successive beats, identifying the start and finish of successive breaths by the maxima and minima in the period, identifying the amplitude of variability of period between successive breaths, and thereby determining a value for a measurement of an extent of lung respiration, and output means for generating the value for the measurement of the extent of lung respiration. The disclosed devices and methods have applications in different medical fields. The disclosed devices can be utilised as wearable devices, wherein the signals are generated and may be processed remotely or locally.