Optoelectronic plethysmography (OEP) is based on the analysis during breathing of the trajectories of a series of markers positioned on the thoracic-abdominal surface of the patient. The positions in space of these markers are recorded and processed with mathematical models and algorithms by a computing unit that can accurately measure not only volume variations of the whole thoracic-abdominal wall, but also the variations of the various compartments. This detects, for example, asymmetries in the action of respiratory muscles. The system can accurately measure current volume, vital capacity, respiratory frequency, duration of the phases of inspiration and exhalation, the average inspiration and exhalation flux and the volume variations at the end of exhalation.