MEASUREMENT OF TOTAL LUNG CAPACITY BY 2 PLANIMETRIC METHODS

We analyzed accuracy and reproductibility of a radiographic method for calculation of total lung capacity (TLC) from the sum of pulmonary area. Pulmonary area from a routine postero-anterior and lateral chest radiography was measured by two methods: using a planimeter (standard) and with a virgin radiographic plaque with lines separating squares of 1 cm per side. Plethismographic TLC was obtained adding the functional residual capacity to inspiratory capacity. We studied 13 healthy subjects (mean age of 30 years, eight males), 13 with interstitial lung disease (mean age of 45 years, two males) and 12 with chronic bronchitis or emphysema (mean age of 63 years, three males). Measured TLC varied from 1.9 to 7.2 liters. The linear regression equation found was: TLC = 0.007 total lung area (cm2) -0.572, R = 0.906, P < 0.001 which is very similar to that reported by Harris et al. 8 Interobserver variability in the measurement was very small taking into account that no efforts were made to uniformize the observers. Main interobserver differences were in the tracing of the lung limits, and not in the actual measurement of the lung area. We found no differences between the two methods for measuring lung area except that the planimeter is faster and more reproducible. Radiographic method for measuring TLC is accurate and available in most places.