In an emphysematous patient with hullous lesion which is the best investigation to measure lung volumes

Body plethysmography During pulmonary function testing, lung volumes are usually measured by the spirometer. But ceain lung volumes e.g., RV, FRC and TLC cannot be measured by spirometer because they include the volume of the gas left in the lung (i.e., this air cannot come out of the lung, however.forcefully one may expire). Two techniques are commonly used to measure the lung volumes. - Helium dilution method. - Body plethysmography. Helium dilution method In the helium dilution method, the subject repeatedly breathes in and out from a reservoir with a known volume of gas containing a trace amount of helium. The helium is diluted by the gas previously present in the lungs and very little is absorbed into the pulmonary circulation. From knowledge of the reservoir volume and the initial and final helium concentrations, the volume of the gas present in the lungs can he calculated. The helium dilution method may underestimate the volume of gas in lungs if there are slowly communicating airspaces such as bullae. In this situation, lung volumes can be measured more accurately with a body plethysmography a sealed box in which patient sits while panting against a closed mouthpiece. Because there is no airflow into or out of the plethysmography the pressure changes in the thorax during panting cause compression and rarefaction of gas in the lungs and simultaneous rarefaction and compression of gas in the plethysmograph. By measuring the pressure changes in the plethysmograph and at the mouthpiece the volume of gas in the thorax can he calculated using Boyle's law.