Time-domain HRV parameter values in the severity groups based on the Ev and the DesSev in the 5-min segments
| Severity group | Mean RR (ms) | SDNN (ms) | RMSSD (ms) | pRR50 (%) |
| Events per 5-min segment | ||||
| Ev0 | 915.3 (815.3–1033.1) | 42.8 (26.6–67.9) | 33.3 (18.8–59.5) | 7.0 (0.8–28.0) |
| EvQ1 | 913.2 (814.6–1028.4) | 53.8 (33.6–84.4)** | 35.2 (19.2–63.8) | 7.8 (1.0–27.8) |
| EvQ2 | 907.7 (807.4–1026.3) | 58.9 (38.2–90.4)** | 36.5 (20.4–66.4) | 8.4 (1.5–27.8) |
| EvQ3 | 898.9 (801.4–1019.1) | 66.7 (43.3–98.0)** | 40.7 (22.8–70.2)** | 9.6 (2.1–27.6) |
| EvQ4 | 868.5 (784.2–973.9)** | 73.9 (49.3–104.6)** | 47.4 (27.1–80.8)** | 14.1 (4.1–32.3)** |
| Desaturation severity | ||||
| DesSev0 | 925.2 (824.3–1044.3)## | 45.7 (28.3–72.1)## | 34.7 (19.8–61.2)## | 8.2 (1.0–29.5)## |
| DesSevQ1 | 898.9 (797.0–1010.3)## | 53.3 (31.4–85.4)** | 34.1 (17.8–63.2) | 6.7 (0.8–25.6) |
| DesSevQ2 | 897.8 (801.3–1010.6) | 56.5 (35.0–88.5)**,## | 35.8 (19.5–64.7) | 8.2 (1.2–26.4) |
| DesSevQ3 | 878.6 (785.0–994.3)**,## | 60.3 (39.2–91.3)**,## | 38.5 (21.0–68.2)** | 8.3 (1.5–26.7) |
| DesSevQ4 | 853.6 (771.4–952.5)**,## | 75.5 (51.8–106.6)**,## | 46.7 (25.9–82.3)** | 12.2 (3.6–30.6)**,## |
The median of time-domain HRV parameter values was calculated from each 5-min RR interval segment. Data are presented as median (interquartile range). Statistical significance of differences between 1) Ev groups, 2) DesSev groups and 3) the corresponding Ev and DesSev groups was assessed using the Wilcoxon signed-rank test. The bolded values denote a statistically significant (p<0.01) difference compared to the corresponding reference group. HRV: heart rate variability; Ev: respiratory event rate; Ev0: reference group with 0 respiratory events starting during the 5-min segment; EvQ1–4: severity groups pooled based on the number of respiratory events (apnoeas/hypopnoeas) starting during the 5-min segment (EvQ1=1<EvQ2≤3<EvQ3≤5<EvQ4); DesSev: desaturation severity; DesSev0: reference group with desaturation severity of 0% during the 5-min segment; DesSevQ1–4: severity groups pooled based on the desaturation severity during the 5-min segment (0%<DesSevQ1≤0.305%<DesSevQ2≤0.906%<DesSevQ3≤2.463%<DesSevQ4); SDNN: standard deviation of RR intervals (artefacts corrected); RMSSD: root mean square of successive differences; pRR50: the number of adjacent RR intervals differing more than 50 ms. **: statistically significant difference (p<0.01) compared to all other Ev or DesSev groups; ##: statistically significant difference (p<0.01) compared to the corresponding Ev group.