THE AIR IN THERE – LUNG VOLUMES

WHAT ARE THE OPTIONS?

Plethysmography incorporates Boyle’s Law to determine the volume of air inside the thorax. It does this by having a patient pant inside a body box with a closed shutter as it measures the pressure at the mouth compared to the pressure in the alveoli, while maintaining a constant temperature inside the body box and a constant volume. Problems can occur if a patient pants too quickly or if they do not maintain a consistent seal around the mouthpiece, among others. The benefit would be that rapid sequential testing could be performed without having to wait for gases inside the body to return to normal. 

Nitrogen washout involves a patient breathing in 100% oxygen through an open circuit to allow for nitrogen to be washed out of the lungs. The most likely problem to occur during this method is for a leak at the mouthpiece to occur, as obstructed patients will spend a longer time performing the test. In patients with an obstructive lung disease, the clearance of N2 from the lungs may take longer as gas exchange in obstructed spaces are often poorly ventilated. The therapist would also need to wait for the additional oxygen to clear the lungs prior to testing again. Ideally this would be at least 2 times longer than it took to perform the test. The test should be stopped after 10 minutes, but may not have met the criteria to end the test and should be noted on the report.

Helium dilution involves a patient breathing in a helium mixture from an external source into a closed circuit until equilibrium has been achieved between the spirometer and the lungs. As with the other two methods, this method is also at risk for errors from leaks at the mouthpiece, as well as leaks in the eardrums which can lead to interfering with pressure changes. The test may take up to 10 minutes to perform. And the patient should wait for the time it took to perform the test before repeating the test, if a second attempt is warranted for accuracy purposes.

WHICH IS BEST?

All three methods for measuring TLC during the PFT, along with CT scans, when compared, have shown similar results in normal and restricted patients. The disparity between tests, as well as studies, lies in obstructed patients. When we look at this set of patients, the comparisons and determination of what is best is hard to distinguish for various reasons.

Since the 1950s, body plethysmography has been regarded as being the premier choice. First refined at the University of Pennsylvania by Arthur B DuBois, along with colleagues, having patients hold their cheeks while panting. And a 1956 study by Tierney and Nadel of 13 patients showed that the FRC was nearly 1 L larger using plethysmography than N2 washout. The European Respiratory Society recommended this method for patients with airflow limitations, as well as air trapping. It has since removed this recommendation. Plethysmography has also been the standard measurement for research. But this may not be the premier choice.

There is an argument that N2 washout may not be ideal in obstructed patients. From the Tierney and Nadel study, it was concluded that N2 washout underestimated TLC. But poor patient technique may have led to the underestimation of changes in pressure at the alveolar level, which would lead to overestimation of TLC in these patients. There have been studies, reported by Darryl Sue, showing that increasing the time of the N2 washout up to 15 minutes would increase the FRC measured.  This would allow for a closer TLC as reported by plethysmography.

Severely obstructed patients whose FEV1 is less than 1L are rarely studied in comparison studies. A study reported in Chest in 2013 by Carl O’Donnell et. al. was performed on 132 patients over three sites. This study found that TLC was closer in these patients between the CT scan and the Helium dilution. Plethysmography resulted in over 1 L difference, resulting in an overestimation. This would negate the belief that Helium dilution resulted in underestimation. This study did not limit patients to 7 minutes to meet equilibrium, which may account for a closer representation to the results of the CT scan. Patients were supine for the CT, which could potentially lead to a lower value for TLC when compared to being in a seated position.

Another study reported in 2016 by Tantucci et. al. showed similarities in plethysmography and CT scans. During this study they accounted for the differences in supine vs seated in both spirometry and radiographic imaging including CT scans by substituting the vital capacity (VC) in the lung volume for the VC performed supine. These factors were plugged into their equation to more accurately determine the lung volumes. This novel approach to measuring FVC at supine could account for the differences seen in other studies. They found that the plethysmography and radiographic methods were nearly the same, while the helium dilution was significantly lower. They concluded that when performed correctly, body plethysmography does not overestimate TLC in obstructed patients.

Though there is no clear choice for the ideal test to measure TLC, FRC, and RV, there continues to be improvements in finding the answer for obstructive lung disease patients. And with further work to improve the patients’ technique and therapists’ guidance, errors due to the therapist and patient can be cut down. There is also the potential to use several tests to help determine the amount of air that is not ventilated during other tests such as the N2 washout and Helium dilution, as suggested by Tantucci. The time limits set by N2 washout and Helium dilution studies may be too short for these patients, resulting in decreased values. With more studies to refine testing procedures, all three PFT testing procedures could eventually result in similar results for all patients.