sRAW correlated with changes of intrathoracic gas volume ( ITGV ) ( r = 0.61 ; p < 0.001 ) , residual volume ( RV ) ( r = 0.60 ; p < 0.001 ) , FVC ( r = 0.44 ; p = 0.001 ) , and VAS ( r = 0.73 ; p < 0.001 ) , while FEV1 correlated only with FVC ( r = 0.34 ; p = 0.008 ) .