Occurrence of relative bradycardia and relative tachycardia in individuals diagnosed with COVID-19
Authors: Natarajan Aravind , Su Hao-Wei , Heneghan Conor 
Published: February 2022

AbstractBackgroundThe COVID-19 disease caused by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has become one of the worst global pandemics of the century causing tremendous human and economic suffering worldwide. While considered a respiratory disease, COVID-19 is known to cause cardiac complications. Wearable devices are well equipped to measure heart rate continuously and their popularity makes them valuable devices in the field of digital health. In this article, we use Fitbit devices to examine resting heart rate from individuals diagnosed with COVID-19 aMethodsThe Fitbit COVID-19 survey was conducted from May 2020 - June 2021. We collected resting heart rate data from 7,200 individuals (6,606 symptomatic, 594 asymptomatic) diagnosed with COVID-19 between March 2020 - December 2020, as well as from 463 individuals diagnosed with influenza between January 2020 - December 2020. Data from healthy individuals served as a control, in order to model the seasonal variation. We also computed heart rate variability and respiratory rate data for symptomatic COVID-19.FindingsResting Heart Rate is elevated during COVID-19 symptom onset, with average peak increases relative to the baseline of 1.8%±0.1% (3.4%±0.2%) for females (males), where the quoted numbers are mean and standard error of the mean. After the initial peak, the resting heart rate decreased and reached a minimum on average ≈ 13 days after symptom onset. The minimum value relative to the baseline is more negative for females (−1.75% ± 0.1%) compared to males (0.08% ± 0.2%). The resting heart rate then increased, reaching a second peak on average ≈ 28 days from symptom onset, before falling back to the baseline ≈ 112 days from symptom onset. All estimates vary with disease severity.InterpretationThe resting heart rate is modified for several months following a COVID-19 diagnosis. Interestingly, this effect is seen with seasonal influenza also, although the bradycardia minimum and the second tachycardia peak are often more pronounced in the case of symptomatic COVID-19. By computing resting heart rate daily, wearable devices can contribute to monitoring wellness during recovery from COVID-19, and seasonal influenza.FundingA.N., H.-W.S., and C.H. are supported by Fitbit Research, Google LLC.Research in ContextEvidence before this studyWe searched PubMed, Google, and Google Scholar for research articles published in English up to Oct 31, 2021, using common search terms such as “bradycardia and COVID-19”, “cardiac complications and COVID-19”, etc. Articles were also retrieved by searching through citations of known literature. It is known that COVID-19 can cause cardiac complications such as bradycardia and arrhythmias. Using data from commercially available wearable devices, it has been shown previously that the resting heart is elevated during symptom onset, then decreases reaching a minimum, before rising again to attain a second peak, before finally returning to the baseline.Added value of this studyWe present results from the largest (to our knowledge) dataset considered to-date, involving 7200 participants (6606 symptomatic and 594 asymptomatic) diagnosed with COVID-19. We also present results from 463 individuals diagnosed with influenza. Our large dataset allows us to perform more detailed examinations by age, disease severity, and sex. We also discuss the time evolution of heart rate variability and respiratory rate. The heart rate variability shows a similar time evolution as the resting heart rate but with opposite phase, while the respiratory rate decreases monotonously following the peak at symptom onset.Implications of all the available evidenceThe results presented in this work show that commercially available trackers and smart-watches can help in monitoring heart health in the weeks and months following a COVID-19 diagnosis. An estimate of the amplitude of the bradycardia dip may provide information valuable to critical care.