Table 1.
Characteristics of the included studies.
| Author | Country | Comparison | Population (Mean Age) | Assessment Time Point | Assessment Duration (Device) | HRV Parameters |
|---|---|---|---|---|---|---|
| Adler 2021 [37] | USA | G1: Previous COVID-19 infection (at 3- and 6-months post-discharge) (n = 18) G2: Matched controls (n = 7) |
G1: 50 ± 16 G2: 50 ± 14 |
3 months (12 weeks) or more | 1-min HRV (unclear) responses to orthostatic stress (3-min active standing) | 1. SDNN (ms); 2. RMSSD (ms); 3. pNN50 (%) |
| Acanfora 2022 [38] | Italy | G1: Long COVID patients (n = 30) G2: No-COVID-19 patients (n = 20) |
G1: 58.6 ± 17.6 G2: 56.3 ± 14.7 |
unclear | 24-h ECG monitoring (portable twelve-channel tape recorder) | 1. SDNN (ms); 2. SDANN (ms); 3. RMSSD (ms); 4. SDNN Index (ms); 5. pNN50 (%); 6. total power (ms2); 7. VLF (ms2); 8. LF (ms2); 9. HF (ms2); 10. LF/HF ratio |
| Aranyó 2022 [39] | Spain | G1: Long COVID patients with IST) (n = 40) G2: Fully recovered COVID-19 patients (n = 19) G3: Uninfected controls (n = 17) |
G1: 40.1 ± 10 G2: 42.2 ± 11 G3: 39.5 ± 13 |
3 months (12 weeks) or more | 24-h ECG monitoring (AFT 1000 + B recorder) | 1. Daytime SD (ms); 2. Daytime pNN50 (%); 3. Nighttime SD (ms); 4. Nighttime pNN50 (%); 5. VLF (Hz); 6. LF (Hz); 7. HF (Hz); 8. LF/HF ratio |
| Asarcikli 2022 [40] | Turkiye | G1: Previous COVID-19 infection (>12 weeks) and no current clinical symptoms (n = 60) G2: Matched healthy controls (n = 33) |
G1: 39 (range 31–49) G2: 30 (range 26–42) |
3 months (12 weeks) or more | 24-h ECG monitoring (DMS300-4A Holter ECG recorder) | 1. SDNN (ms); 2. SDANN (ms); 3. RMSSD (ms); 4. SDNN Index (ms); 5. pNN50 (%); 6. total power (ms2); 7. LF (ms2); 8. HF (ms2); 9. LF/HF ratio; 10. SDNN > 60 ms; 11. RMSSD > 40 ms |
| Freire 2022 [41] | Brazil | G1: Previous COVID-19 infection (at 15–180 days) (n = 20) G2: Matched healthy controls (n = 18) |
G1: 29.17 ± 6.32 G2: 26.22 ± 5.22 |
15–180 days | 5-min HRV (Polar RS800CX) | 1. SDNN (ms); 2. RMSSD (ms); 3. pNN50 (%); 4. LF (nu); 5. HF (nu); 6. LF/HF ratio; 7. Triangular index; 8. TINN (ms) |
| Kurtoğlu 2022 [42] | Turkiye | G1: Patients with a confirmed history of COVID-19 (at 20.0 ± 11.4 weeks) (n = 50) G2: Healthy controls without a history of COVID-19 and vaccination (n = 50) |
G1: 40.82 ± 10.31 G2: 38.24 ± 12.02 |
3 months (12 weeks) or more | 24-h ECG monitoring (iH-12Plus Holter System) | 1. SDNN (ms); 2. SDANN (ms); 3. RMSSD (ms); 4. SDNN Index (ms); 5. pNN50 (%); 6. total power (ms2); 7. VLF (ms2); 8. LF (ms2); 9. HF (ms2); 10. LF (nu); 11. HF (nu); 12. Triangular index (HRVI) |
| Liu 2021 [43] | China | G1: Discharged COVID-19 patients (n = 186 → 164 analyzed) G2: Matched healthy controls (n = 186 → 166 analyzed) |
Not reported | unclear | more than 10-h recording (ballistocardiography-based internet-of-medical-things system) | 1. SDNN (ms); 2. SDANN (ms); 3. LF (ms2); 4. HF (ms2) |
| Marques 2022 [44] | Brazil | G1: Long COVID clinical group (n = 155 → 81 analyzed) G2: Uninfected controls (n = 94) |
G1: 43.88 ± 10.03 G2: 40.69 ± 6.35 |
3 months (12 weeks) or more | 5-min HRV (Polar RS800CX) | 1. SDNN (ms); 2. RMSSD (ms); 3. LF (nu); 4. HF (nu); 5. LF/HF ratio; 6. SD1 (ms); 7. SD2 (ms) |
| Mekhael 2022 [45]; Dagher 2022 [48] |
USA | G1: Previous COVID-19 infection (at 171 ± 114 days) (n = 122) G2: Controls who were not diagnosed with COVID-19 (n = 588) |
G1: 41.32 ± 15.7 G2: 45.99 ± 14.0 |
171 ± 114 days | 5-min HRV (PPG-based smartband) | 1. Mean HRV day/person (ms) |
| Shah 2022 [46] | India | G1: Previous COVID-19 infection (recovered within 30–45 days) (n = 92) G2: Healthy volunteer controls (n = 120) |
G1: 50.6 ± 12.1 G2: 51.8 ± 4.2 |
unclear (recovered within 30–45 days) | 1-min HRV (VESTA 301i) responses to orthostatic stress (3-min active standing) | 1. RMSSD (ms) |
| Zanoli 2022 [47] | Italy | G1: Previous COVID-19 infection (>12 weeks) (n = 92) G2: Matched controls (n = 180) |
G1: 55 ± 12 G2: 55 ± 13 |
3 months (12 weeks) or more | 5-min HRV (Finometer Midi device) | 1. LF/HF ratio; 2. Triangular index |
Abbreviations. COVID-19, Coronavirus disease 2019; ECG, electrocardiogram; G, group; HF, high-frequency band; HRV, heart rate variability; IST, inappropriate sinus tachycardia; LF, low-frequency band; nu, normalized unit; pNN50, proportion of the number of pairs of successive normal-to-normal RR intervals that differ by more than 50 milliseconds divided by the total number of normal-to-normal RR intervals; RMSSD, root mean square of the successive differences; SD, standard deviation of the interbeat interval; SDANN, standard deviation of the averages of normal-to-normal RR intervals; SDNN, standard deviation of normal-to-normal RR intervals; SDNN index, mean of the standard deviations of all normal-to-normal RR intervals for all 5 min segments of the entire recording; TINN, triangular interpolation of normal-to-normal RR intervals; VLF, very low-frequency band.