IntroductionLong covid (LC), also known as Post-COVID-19 syndrome, refers to symptoms persisting 12 weeks after COVID-19 infection. It affects up to 1 in 7 people contracting the illness and causes a wide range of symptoms, including fatigue, breathlessness, palpitations, dizziness, pain and brain fog. Many of these symptoms can be linked to dysautonomia or dysregulation of the autonomic nervous system after SARS-CoV2 infection. This study aims to test the feasibility and estimate the efficacy, of the Heart Rate Variability Biofeedback (HRV-B) technique via a standardised slow diaphragmatic breathing programme in individuals with LC.Methods and Analysis30 adult LC patients with symptoms of palpitations or dizziness and an abnormal NASA Lean Test (NLT) will be selected from a specialist Long COVID rehabilitation service. They will undergo a 4-week HRV-B intervention using a Polar chest strap device linked to the Elite HRV phone application while undertaking the breathing exercise technique for two 10-min periods every day for at least 5 days a week. Quantitative data will be gathered during the study period using: HRV data from the chest strap and wrist-worn Fitbit, the modified COVID-19 Yorkshire Rehabilitation Scale (C19-YRSm), composite autonomic symptom score (COMPASS 31), World Health Organisation Disability Assessment Schedule (WHODAS 2.0) and EQ-5D-5L health related quality of life measures. Qualitative feedback on user experience and feasibility of using the technology in a home setting will also be gathered. Standard statistical tests for correlation and significant difference will be used to analyse the quantitate data.Ethics and DisseminationThe study has received ethical approval from Health Research Authority (HRA) Leicester South Research Ethics Committee (21/EM/0271). Dissemination plans include academic and lay publications.Study RegistrationClinicaltrials.gov No: NCT05228665Strengths and limitations of the studyTo our knowledge, this is the first study of HRVB in long covid and will provide new information regarding the feasibility of the technology-based intervention in this condition.The estimation of efficacy will determine the scope and sample size for a larger controlled trial in the condition that currently has no definitive treatmentsThe study will provide preliminary evidence on the correlation between long covid symptoms and dysautonomia.The limitation of this study is the small sample size of 30 participants which might not give an accurate estimate of efficacy.HRV-B is a technology-based intervention, therefore its take-up could be limited in those with a lack of experience in using digital technology in daily life, particularly those from less privileged backgrounds.