Wearables Cardiovascular Monitoring: Effects of Cold Pressor Test on Heart Rates Estimated From ECG, PPG and IPG Signals

As the demand for wearable technologies rises, the precision, reliability and user-friendly operation of wearable devices become increasingly critical. This study systematically investigated the effects of cold pressor test (CPT) on heart rate (HR) and heart rate variability (HRV), which are clinically useful parameters for the assessment and monitoring of autonomic nerve function and cardiovascular activities. Especially, HR data obtained from Electrocardiography (ECG), Photoplethysmography (PPG) and Impedance Plethysmography (IPG) were compared under the same temperature conditions. The CPTs were conducted on 22 subjects during baseline phase (Rest1), cold stimulus phase, recovery phase and another baseline phase (Rest2). It was found that cold water exposure would result in significant increased HR (p<0.001) and decreased HRV. Notably, a unique response was observed in one hypertensive subject that his HR decreased during cold stimulus phase. Furthermore, the results of comparative analysis demonstrated that HR from IPG exhibited better alignment with ECG across four phases while PPG showed poorer performance. However, under cold stimulus conditions, decrease in correlation was observed. This suggests that, compared to PPG, IPG may serve as a more reliable alternative to ECG for HR estimation. It should be pointed out that wearable devices incorporating IPG sensors can offer an efficient and gesture-free or hands-free alternative for HR estimation under diverse environmental conditions. Therefore, considering the estimation accuracy and user-friendly aspects, the IPG seems to be an optimal choice for wearable HR monitoring in comparison with the commonly used ECG and PPG methods, subject to further tests under a large database.