Study of the relationship between ventilation parameters: monitored expiratory time constant – τedyn and breathing – trigger frequency (ftrig) and time of breathing cycle (Tcy) are main goals of this article. Parameters were analyzed during last 4±2 h before weaning from ventilation in 66 patients ventilated in pressure support mode (PSV). We have found out, that there exist mathematical relationships, observed during adequate gas exchange, yet not described. Monitored parameters are represented by τedyn, ftrig and Tcy. The analysis showed close negative correlation between Tcy and ftrig (R2 =0.903). This implies that each increasing of τedyn causes decreasing of ftrig and vice versa. The calculation of regression equation between τedyn and Tcy outlined that Tcy = 5.2625 * τedyn + 0.1242 (R2 =0.85). Regulation of respiratory cycles by the respiratory center in the brain is probably based on evaluation of τedyn as the τedyn probably represents a regulatory element and Tcy regulated element. It can be assumed, that respiratory center can optimize the work of breathing in order to minimize energy in system patient + ventilator. The unique relationship, described above could be useful in clinical practice for development of new ventilation modes.