A. Kulik, A. Zaderihin, V. Shulgin, and A. Martynenko, “INDICATORS OF HEART RATE VARIABILITY IN HEALTHY VOLUNTEERS DEPENDING ON THE RESPIRATION RATE AND INHALATION-EXHALATION RATIO”, J. V. N. Karazin Khark. Nat. Univ., Ser. Med., no. 18, pp. 14-19, 1.