Table 2.

Summary of the 9 studies that correlate changes in heart rate variability (HRV) indexes with COVID-19 using wearable devices.

Study	Study population	Demographic characteristics	Wearable device and applied technology	COVID-19 diagnostic test	Interest and comparison of studies
Natarajan et al [39], 2020	2745 COVID-19 positive	Mean age 41.2 (SD 12.8) years; 76% female	Fitbit; PPGa	RT-PCRb	HRV changes before and after RT-PCR test
Hijazi et al [40], 2021	186 COVID-19 positive	Mean age 40.2 (SD 17.2) years; 64% female	Fitbit, Garmin, and Apple watches; PPG	Self-reported by study participants	Tried to identify infection before SOc
Hasty et al [41], 2021	16 COVID-19 positive	Mean age 60.5 (SD 13.4) years; 71% male	Warfighter Monitor; ECGd	RT-PCR	HRV variation analysis during infection
Lonini et al [42], 2021	14 COVID-19 positive	Mean age 52 (SD 15.2) years; 50% female	Device developed by Northwestern University; mechanoacoustic	NRe	HRV variation analysis during infection
Hirten et al [43], 2021	297; 13/297 COVID-19 positive	Mean age 36.3 (SD 9.8) years; 69.4% female	Apple Watch; PPG	RT-PCR	Tried to predict infection before SO
Gutiérrez et al [44], 2022	1 COVID-19 positive	Aged 52 years; male	Polar H7; electrode (chest strap) ECG	RT-PCR	HRV variation analysis during infection
Natarajan et al [45], 2022	7200 COVID-19 positive	Mean age 41.7 (SD 13.3) years; 78.2% female	Fitbit; PPG	Self-reported by study participant	RMSSDf changes before and after SO
Jimah et al [46], 2022	2 COVID-19 positive	Aged 24/25 years; 100% female; pregnant	Oura Ring; PPG	RT-PCR	RMSSD variation during infection
Risch et al [47], 2022	66 COVID-19 positive	Mean age 42.9 (SD 5.6) years; 72% female	Ava bracelet; PPG	72% RT-PCR; 28% antibody test	Tried to predict infection before SO

^aPPG: photoplethysmography.

^bRT-PCR: real-time polymerase chain reaction.

^cSO: symptom onset.

^dECG: electrocardiogram.

^eNR: not reported.

^fRMSSD: root mean square of the successive differences.