Abstract
A key metric utilized by coaches and athletes to track athlete performance is heart rate variability (HRV). HRV calculated via electrocardiogram (ECG) has been shown to track autonomic function. However, many wearable devices utilize photoplethysmography (PPG) to calculate pulse rate variability (PRV). Our study investigated the agreement between PRV and HRV in a beat-to-beat analysis in a sample of American football players. Data from 103 male, Division I collegiate American football athletes, collected over three seasons, were analyzed. Heart rate (HR), pulse rate (PR), and two time-domain indices for PRV/HRV were measured (rMSSD and SDNN). Agreement between PRV and HRV was assessed using Bland-Altman analysis (bias, limits of agreement, and confidence intervals) with supporting error metrics (MAE, RMSE, Pearson r, and Lin's concordance correlation coefficient). To evaluate whether PRV detected autonomic deviations of the same magnitude to HRV on the same day, a sliding-window z-score threshold-crossing analysis (0.5-2.0 SD) quantified PRV detection rates and PRV delay.
HR and PR were similar at (59.4 (10.3) bpm vs. 59.7 (10.3) bpm). In contrast, PPG-PRV values were lower than ECG-HRV for both rMSSD and SDNN (80.9 (23.1) ms vs. 103.9 (22.0) ms, 141.3 (41.7) ms vs. 167.9 (40.0) ms). Bland-Altman analysis showed negative bias for rMSSD and SDNN across PPG wavelengths, race, and obesity, while HR/PR agreement was high with small bias (0.24-0.44 bpm). In the deviation analysis, PRV detected fewer autonomic deterioration events on the same day as HRV, capturing 16.7% to 56.7% of HRV-identified rMSSD events and 16.8%-52.0% of SDNN events across thresholds, and exhibited an average delay of 1.8-5.5 days in detecting changes already identified by ECG-HRV.
Our findings refute PPG-PRV as an equivalent surrogate for ECG-HRV for tracking autonomic function over time. Specifically, the delay in response when using PPG-PRV to track athlete autonomic function will result in missed opportunities for coaches to prevent autonomic deterioration. Finally, the PRV calculated with PPG should not be called HRV as it confuses scientists and consumers.