Cardiac Effort Provides a Reproducible Remote Assessment of 6-Minute Walk Test

Abstract

Rationale: 'Cardiac Effort' (CE), the total number of heart beats used during the 6-minute walk test (6MWT) divided by walk distance (beats/m), improves reproducibility in the 6MWT and correlates with right ventricular function in pulmonary arterial hypertension (PAH). The SARS-CoV-2 pandemic made in-office 6MWT challenging. We aimed to determine 1) whether a chestbased accelerometer could estimate 6MWT distance in the clinic and remotely;2) the reproducibility of CE measured during a remote 6MWT;and 3) the safety of remote 6MWT. We also compared measures of heart rate (HR) derived from electrocardiogram (ECG) and wrist-based photoplethysmography (PPG) during the 6MWT in PAH. Methods: This was a singlecenter, prospective observational study with IRB approval completed October 2020-April 2021. Group 1 PAH subjects on stable therapy for >90 days completed 4-6 total 6MWT during a 2 week period to assess reproducibility;we anticipated no clinical change during this short interval. The first and last 6MWT were performed in the clinic;2-4 remote 6MWT were completed at participant's discretion. Participants did not wear masks but did wear the MC10 Biostamp nPoint sensors to measure ECG HR and accelerometry. Two blinded readers estimated 6MWT distance using raw accelerometry data. We measured PPG HR with a wrist Nonin 3150 pulse oximeter during clinic 6MWT only. Averages of clinic variables and remote variables were used for paired Student's t test, Bland-Altman Plot, or Pearson correlation. Results: We enrolled 20 participants: 80% female;60% connective tissue disease;and 65% on initial combination therapy with ambrisentan and tadalafil. There was a wide range in baseline, clinicperformed 6MWT distance (220 -570 m). The median length of the remote 'hallway' was 40 ft. For clinic walks, there was 0.10% average difference between the directly observed and Biostamp accelerometry-estimated 6MWT distance with a strong correlation of r=0.99, p<0.0001 (figure 1). The 6MWT distance estimated using Biostamp in the clinic was greater than what was estimated remotely, 405 m vs. 389 m, p=0.007. There was no clear difference between clinic or remote CE, 1.83 beats/m vs 1.93 beats/m, p=0.14, or Borg Dyspnea Index, 3.5 vs 3.4, p=0.35. There were no safety concerns. PPG undercounted total HR expenditure during 6MWT compared to Biostamp (629 vs 719, p<0.0001). Conclusion: Remote 6MWT was feasible, appeared safe, and 6MWT distance was shorter than clinic distance. CE calculated by ECG HR and accelerometer-estimated distance provides a reproducible remote assessment of exercise tolerance, comparable to the clinic measured value. (Figure Presented).