STIFFNESS INDEX DERIVED FROM OVERNIGHT PHOTOPLETHYSMOGRAPHY, AND ITS RELATIONS TO AMBULATORY ARTERIAL STIFFNESS INDEX, BLOOD PRESSURE AND CARDIOVASCULAR RISK

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Objective: Stiffness index (SI) derived from photoplethysmography (PPG) is a non-invasive method to estimate arterial stiffness. We investigated how overnight SI relates to indices of arterial stiffness derived from ambulatory blood pressure monitoring (ABPM), blood pressure (BP) levels, and cardiovascular risk. Design and method: 79 individuals with confirmed or suspected hypertension (56 males, mean age 58 ± 11 years and body mass index 28.1 ± 4.0 kg/m2; 28 smokers) performed simultaneous 24-hour ABPM (Spacelabs ABP monitors Ultralight) and an overnight sleep polygraphy with PPG recording from the index finger (Somnocheck MicroCardio, Weinmann). Ambulatory arterial stiffness index (AASI) was calculated as 1– the regression slope of diastolic on systolic pressure during ABPM. Pulse propagation time (PPT, time from systolic to reflected peak of the pulse-wave) was calculated beat-by-beat and averaged for the whole sleep recording and used to calculate overnight SI (m/s) as height (m)/PPT (ms). Patients were divided into low respectively high SI (below and above median; 11 m/s). SCORE and Framingham risk scores were calculated to estimate cardiovascular risk. Results: Mean office BP and ABPM were 147.1 ± 19.2/89.7 ± 10.6 respectively 134.6 ± 15.1/81.5 ± 8.3 mmHg in all. The high SI group, as compared to the low SI group, was older (63.2 ± 8.6 vs 53.8 ± 10.7 years, P < 0.001) and had lower renal function (eGFR 95 ± 26 vs 113 ± 31 ml/min, P < 0.01). Indices of arterial stiffness were increased in the high SI group (office and 24-h ABPM pulse pressure 62.4 ± 17.9 vs 53.1 ± 14.2 mmHg, P < 0.05, and 58.0 ± 15.0 vs 49.0 ± 7.3 mmHg, P < 0.01, and AASI 0.49 ± 0.13 vs 0.37 ± 0.10, P < 0.001), and all showed correlations with SI (r = 0.34, 0.44, and 0.39, respectively; all P < 0.01). SI correlated with systolic ABPM (day r = 0.43, night r = 0.54; both P < 0.001), diastolic ABPM (night r = 0.36, P < 0.01) and systolic and diastolic dipping (r = −0.31 and −0.28, respectively; both P < 0.05). SI correlated with SCORE and with Framingham risk (r = 0.32 and 0.36; both P < 0.01). Most relations remained significant after adjustments for age, sex and BMI. Conclusions: Overnight SI correlates with ABPM indices of arterial stiffness, blood pressure levels and cardiovascular risk. This PPG-based measure is easy to use and may provide information for cardiovascular risk assessment beyond conventional variables acquired during overnight sleep studies.

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