In a patient with severe bullous emphysema, the most appropriate method for measuring lung volumes is
1.Body plethysmography
2.Diffusing capacity of carbon monoxide
3.Spirometry
4.Helium dilution
sol:
Spirometry does not measure total lung capacity because it cannot account for residual volume.
The most frequently used and accurate measures of lung volumes are steady-state helium dilution lung volumes and body plethysmography.
In helium dilution the patient inspires a known concentration of helium from a closed circuit of known volume.
After the patient rebreathes in the closed circuit for a period of time, the concentration of helium equilibrates, and subsequently the lung volumes can be calculated by using Avogadro’s law.
This calculation assumes that gas in the circuit will rapidly equilibrate with the ventilated portions of the lung. However, if there are slowly emptying areas of the lung, as in cystic fibrosis patients, or parts of the lung that do not participate in gas exchange at all, as in bullous emphysema patients, helium dilution will underestimate true lung volumes.
Subsequently, body plethysmography is the preferred method for lung volume measurement in these disease states.
To perform body plethysmography, the patient sits in a sealed box and pants against a closed mouth- piece.
Panting results in changes in the pressure of the box that, when compared with changes at the mouthpiece, can be used to calculate lung volumes.
This method measures total thoracic gas volume and is more accurate than helium dilution. Helium lung volumes are easier to perform for patients and staff and give reliable results in most circum- stances.
Many centers measure a single-breath helium dilution lung volume when measuring the diffusing capacity of carbon monoxide, which has the same or greater limitations as the rebreathing method.
Transdiaphragmatic pressure is used to measure respiratory muscle strength, not lung volumes.