Toward Ambulatory Baroreflex Sensitivity: Comparison Between Indices of Arterial Line and Photoplethysmography in Male Volunteers

Background: Baroreflex sensitivity (BRS) is the body's ability to adjust heart rate (HR) to control blood pressure. Traditionally, BRS is quantified by measuring HR changes (obtained via an electrocardiogram [ECG]) following alterations in arterial pressure (conventionally measured through an arterial line). However, the invasiveness of arterial line necessitates alternatives, such as the volume clamp method and the less invasive pulse photoplethysmography (PPG). Notably, the PPG method is also suitable for continuous and free-living conditions. Objective: This study aims to compare PPG-based features for BRS determination based on the volume clamp method and gold standard arterial line. Data from a previous study was used where the primary analysis focused on evaluating the accuracy of PPG-derived HR variability, while this analysis quantifies BRS by measuring HR changes following alterations in arterial line pressure or less invasive alternatives. In addition, we investigate the feasibility of assessing BRS patterns over 24 hours using data from a single volunteer. Methods: A total of 28 male volunteers (age 52, SD 7 y; BMI 27, SD 4 kg/m2) equipped with four sensing modalities: (1) arterial line [ABP], (2) infrared PPG, (3) volume clamp finger pressure (VCP), and (4) ECG, performed a protocol of 3 repetitive sessions in supine position. For the extended feasibility of continuous BRS measurement, ECG and PPG data were acquired for 24 hours in free-living conditions from a normotensive male volunteer (33 y). BRS index was calculated within the low-frequency window (0.04-0.15 Hz) averaging over all trials for each intervention and participant. A transfer function was estimated with systolic blood pressure (SBP) or its surrogate as input and HR (from the ECG) as output. Results: PPG-based BRS features, specifically the rise-decay time ratio (RDRatio) and pulse arrival time (PAT), demonstrate intraparticipant precision of 44% and 23%, respectively, with interparticipant variation of 91% and 53%. The correlation of BRSPAT,PPG and BRSRDRatio,PPG with the gold standard BRSSBP,ABP (SBP) is 0.66 and 0.56, respectively. During intervention, the correlations remain high for BRSRDRatio,PPG (rest: 0.75, paced-breathing: 0.50, and handgrip: 0.46) and BRSPAT,PPG (rest: 0.69, paced-breathing: 0.52, and handgrip: 0.62). In the 24-hour data, the BRSPAT,PPG and BRSRDRatio,PPG exhibit changes during the day corresponding to the activity levels and SBP variations. Notably, during the night, a cyclic rhythm is observed for both BRSPAT,PPG and BRSRDRatio,PPG. Conclusions: This study demonstrates that in male volunteers, PPG-based PAT and RDRatio BRS serve as suitable surrogates for gold-standard BRS derived from arterial line, showing the highest correlation and comparable intraparticipant coefficient variation. Furthermore, they show expected changes during controlled activities and a 24-hour feasibility test in free-living conditions.