CT Volumetry Versus Body Plethysmography in Measuring TLC in Lung Transplantation

Abstract

Purpose Though Body plethysmography (BP) is currently the gold standard to measure total lung capacity (TLC) of patients awaiting lung transplantation (LT), it is unreliable in patients with end stage lung disease. CT lung volumetry (CTV) is a validated technique unlikely to be affected by the state of the lung. We investigated the degree of agreement between CTV and BP in patients awaiting LT. Methods and Materials CTV was first validated in an experiment using isolated sheep lung blocks (n=7) ventilated with a fixed tidal volume (700 ml). TLC from CTV was compared to the value from Fluid displacement method (FD), which provided accurate measure of volume of the lung block. Patients listed for LT (13 COPD & 12 lung fibrosis) underwent CTV following a protocol which measured TLC at maximal inspiration. This was compared to TLC measured by BP. Bland-Altman plots evaluated the degree of agreement between these two measures of TLC. This method evaluates the limits between which 95% of the differences between methods are expected to lie. Results Ex-vivo sheep study demonstrated strong correlation between TLC measured by CTV and FD with an r value of 0.81 (p value = 0.02, 95% CI = 0.14 to 0.97). Mean TLC in the COPD group measured by BP and CTV were 8.32 L and 6.72 L respectively. Mean TLC in the lung fibrosis group measured by BP and CTV were 3.61 L and 3.19 L respectively. Bland-Altman plots showed that the limits of agreement between CTV and BP were -3.65 to 0.45 in the COPD group (mean bias -1.60) and -1.01 to 0.18 in the lung fibrosis group (mean bias -0.42). Conclusions There are significant differences in the measurement of lung volumes by BP and CTV in patients awaiting LT. In animal experiments CTV correlates well with FD. These data suggest that lung size matching may be improved by employing CTV as an adjunct to determine appropriate TLC for recipients. BP might not represent TLC accurately in patients awaiting LT as there are significant areas of hypoventilation, air-trapping and consolidation. In such patients, CTV might be more accurate in estimating TLC.