Low total lung index can be seen as a result of a number of clinical pathologies. Total lung index is mainly the amount of gas in the lungs from deep inspiration to maximum exhalation called total lung volume, total lung TLC, which refers to the total amount of gas in the lungs measured at the end of forceful inspiration, i.e., the amount of deep inspiration plus the amount of functional residual gas is total lung volume. It is also a very important indicator of the severity of restrictive lung disease. A low total lung index can be seen in severe myasthenia gravis, pneumothorax from various causes and pulmonary pleural effusion, as well as in some solid lung diseases such as atelectasis, lobar pneumonia, diffuse interstitial lung fibrosis, pulmonary edema, and fluid pneumothorax. It is also seen in congenital thoracic and spinal deformities and diseases such as chronic obstructive pulmonary disease.