Differences in plethysmographic lung volumes. Effects of linked vs unlinked spirometry

Area of Science:

• Pulmonary Function Testing
• Respiratory Physiology

Background:

• Body plethysmography is the gold standard for absolute lung volume measurement.
• Laboratories often use spirometry data with plethysmography, calculating total lung capacity (TLC) and residual volume (RV) from thoracic gas volume (Vtg) at functional residual capacity (FRC).
• The accuracy of this method relies on FRC stability between separate devices.

Purpose of the Study:

• To compare TLC and RV values derived from linked versus unlinked spirometry and plethysmography.
• To assess the clinical significance of discrepancies in lung volume measurements.

Main Methods:

• Compared TLC and RV calculations using linked and unlinked spirometers in 220 patients.
• Analyzed statistically significant differences and deviations beyond 95% confidence intervals.
• Investigated potential shifts in functional residual capacity (FRC).

Main Results:

• Statistically significant differences (p < 0.05) were found in TLC and RV between linked and unlinked spirometer methods.
• Over 5% of patients showed differences exceeding 95% confidence intervals for repeated linked measurements.
• Large TLC differences often correlated with opposite expiratory reserve volume (ERV) changes, indicating FRC shifts.

Conclusions:

• While statistically significant, differences in lung volumes calculated using unlinked spirometry are generally small.
• Clinical diagnoses were rarely altered by these measurement discrepancies.
• Awareness of potential FRC shifts can further minimize errors, making the unlinked method acceptable for clinical use.