Photoplethysmography (PPG) is used in many wearable devices and it is becoming the most commonly measured cardiovascular signal, but its association with cardiovascular events is undetermined. This study uses data from the UK Biobank to assess the association between PPG morphological features and risk of cardiovascular (CV) events. N=175,284 individuals without CV disease were included (44.6% male, 56.4 ±8.1 years old). A single finger PPG waveform of 101 data points, evenly sampled over the cycle length was available. The PPG waveforms were normalized between 0 and 1 and the maximum of the first derivative during the pulse's upslope was measured (𝒙 𝑴𝑨𝑿 ′ ). Cox regressions were used to assess the association between 𝒙 𝑴𝑨𝑿 ′ and mortality and cardiovascular events. After a median follow-up period of 11.2 years, incidence of all-cause mortality (ACM), myocardial infarction (MI), heart failure (HF), atrial fibrillation (AF) and stroke (STR), ranged between 2.1% and 5.2%. A reduction of 1 standard deviation in 𝒙 𝑴𝑨𝑿 ′ was associated with increased risk of all outcomes with hazard ratio between 1.20 and 1.30. After adjusting for sex, age, and body mass index, associations remained significant for all outcomes except AF.

Wearable devices enable continuous heart rate (HR) monitoring through photoplethysmography (PPG). The impact of wrist-worn devices' sensor contact pressure and sweat, and of their interaction with movement, on HR monitoring is unclear. HR was recorded in 17 healthy individuals using two smartwatches, Garmin Vivoactive 4 (GV) and Fitbit Sense (FS), concurrently with ECG at rest and during controlled arm movement at three increasing intensities. Recordings were repeated after reducing contact pressure by loosening the wristband by one or two notches and using one or two drops of saline solution to simulate sweating. In optimal conditions, the mean absolute percentage error (MAPE) was (median [interquartile range]) 4.3% (1.4%, 7.7%) and 3.1% (1.6%, 5.0%) (p=0.58), for GV and FS, respectively. Loosening the wristband by 1 notch increased MAPE for FS during rest (p=0.021), moderate (p=0.004) and vigorous (p=0.002) movement, but not for GV, for which loosening the wristband by 2 notches increased MAPE during moderate (p=0.015) and vigorous (p=0.008) movement. Simulated sweat increased MAPE during moderate movement using FS (p=0.002), and during vigorous movement for both devices. In conclusion, contact pressure and sweating can increase HR inaccuracy even during rest and moderate movement.

Heart rate (HR) variability (HRV) is a non-invasive cardiac autonomic marker, which, in normal conditions, is inversely associated with the underlying HR. This study investigates the hypothesis that uncoupling between HRV and HR during exercise and recovery may indicate increased cardiovascular risk. UK Biobank participants without underlying cardiovascular disease (n=48,671, 46.3% male 56.3±8.2 years old) underwent an ECG exercise stress test. Uncoupling between HR and HRV was measured as 𝜐 = 1 − 𝑟 𝐻𝑅,𝐻𝑅𝑉 , where r indicates the Spearman's correlation coefficients between the HR profile and the instantaneous HRV power. Cox regressions were used to assess the association between the uncoupling index, 𝜐, and major adverse cardiovascular events (MACE). Models were adjusted for age, sex, body mass index, blood pressure, resting HR, HR increase and decrease during exercise and recovery, respectively. During a median follow-up of 10 years, incidence of MACE was 2.9%. In the adjusted model, 1 standard deviation increase in log-transformed 𝜐 was associated with MACE, with hazard ratio (95% confidence interval) = 1. 09 (1.03, 1.15), p=0.004. In conclusion, in middleaged man and women without underlying cardiovascular disease, the uncoupling between HR and HRV during exercise and recovery was associated with MACE.

Wearable devices enable continuous heart rate (HR) monitoring through photoplethysmography (PPG). The impact of wrist-worn devices' sensor contact pressure and sweat, and of their interaction with movement, on HR monitoring is unclear. HR was recorded in 17 healthy individuals using two smartwatches, Garmin Vivoactive 4 (GV) and Fitbit Sense (FS), concurrently with ECG at rest and during controlled arm movement at three increasing intensities. Recordings were repeated after reducing contact pressure by loosening the wristband by one or two notches and using one or two drops of saline solution to simulate sweating. In optimal conditions, the mean absolute percentage error (MAPE) was (median [interquartile range]) 4.3% (1.4%, 7.7%) and 3.1% (1.6%, 5.0%) (p=0.58), for GV and FS, respectively. Loosening the wristband by 1 notch increased MAPE for FS during rest (p=0.021), moderate (p=0.004) and vigorous (p=0.002) movement, but not for GV, for which loosening the wristband by 2 notches increased MAPE during moderate (p=0.015) and vigorous (p=0.008) movement. Simulated sweat increased MAPE during moderate movement using FS (p=0.002), and during vigorous movement for both devices. In conclusion, contact pressure and sweating can increase HR inaccuracy even during rest and moderate movement.