Analysis of the association of each index of low HRV in preoperative patients with major cardiovascular events followed up to 6 months (Table 3.32) showed that the decrease in ASDNN, rMSSD and pNN 50 index increased the frequency of major cardiovascular events by 3.68 - 4.48 times when followed up to 6 months after surgery (95% CI: 1.11 - 15.74; p < 0.05).