Certified Pulmonary Function Technologist (CPFT) Practice Exam

How does plethysmography measurements typically compare to gas dilutional measurements?

1. It is always smaller than gas dilution measurements

2. It measures only inhaled gas

3. It is often larger than gas dilutional measurements

4. It provides a less accurate measurement

The correct answer is: It is often larger than gas dilutional measurements

Plethysmography is a technique used to measure lung volumes and assess respiratory mechanics by determining the volume of gas in the lungs, including both ventilated and non-ventilated areas. In contrast, gas dilutional measurements estimate lung volumes based on the distribution and dilution of gas in the lungs, typically only accounting for the air that is actively involved in gas exchange. The reason that plethysmography measurements are often larger than gas dilutional measurements is due to the fact that plethysmography includes measurements from all lung compartments, including those areas that may not participate significantly in gas exchange, such as parts of the lung that are obstructed or poorly ventilated. Gas dilution methods may underestimate total lung capacity because they do not account for gas in these lung regions. Consequently, plethysmography is generally regarded as a more comprehensive method for measuring total lung capacity and is especially useful in conditions where lung volumes vary widely, such as obstructive and restrictive lung diseases. This distinction elucidates why plethysmography measurements tend to be larger. Understanding the variations in measurement techniques and what they represent is essential for interpreting pulmonary function tests accurately.