We applied symbolic transfer entropy (STE) for the detection of directed couplings between pulmonary variables registered during repeated progressive and maximal cardiopulmonary exercise tests (CPET). We verified the hypothesis whether effort accumulation has an impact on the decrease of the level of coupling between ventilation (VEbtps), fraction of expired oxygen (FeO2) and carbon dioxide (FeCO2). A group of 10 volunteers performed two consecutive CPET (T1 and T2) on a cycle ergometer. STE values obtained for T1 are higher than for T2, which indicates that the interaction of these variables is sensitive to effort accumulation. The difference of the STE between signals corresponds to the dominating direction of the coupling and indicates that FeO2 and FeCO2 drives the VEbtps.