Respiratory movements of the thoracic cage with the corresponding change in the volume of the thoracic cavity lead to changes in the intra-thoracic pressure. Since the changes in the intra-thoracic pressure are the driving force for lung expansion/compression, the intra-thoracic pressure is considered a direct measure of respiratory effort, i.e. the effort exerted in order to inhale and/or exhale. However, intra-thoracic pressure affects not only lungs but also the other organs in the thoracic cavity, particularly those with those with wide lumens and thin walls containing little elastic elements such as large veins and esophagus. Thus, the gold standard for quantitatively measuring respiratory effort is using an esophageal catheter with a pressure transducer. Due to invasiveness of the method and its discomfort, esophageal pressure monitoring is rarely used in the routine clinical practice. Instead, indirect measures of respiratory effort are used such as measuring the movements of the thorax and abdomen with piezo-resistive belts, or changes in the volume of the thorax and abdomen with respiratory inductance plethysmography (RIP).