Continuous Peripheral Blood Pressure Measurement with ECG and PPG Signals at Fingertips

A good habit of measuring blood pressure (BP) daily is helpful for us to stay healthy or to monitor hypertensive conditions. However, the conventional method of measuring BP using a pressure cuff has many weaknesses. In order to eliminate the use of this pressure cuff, we proposed a system using the pulse arrival time (PAT) to measure BP. This PAT can be measured using time difference between the R-peaks of electrocardiogram (ECG) and photoplethysmography (PPG) signals. In our system, we obtained these two signals by using our self-designed ECG and PPG sensors. Our sensors were fabricated in 0.18 μm CMOS technology with a small recording area of about 2.53 mm ² and 6.25 mm ² , respectively. Our ECG sensor has variable amplifying gains and can achieve a total maximum gain of 60 dB. Besides that, it has a high pass filter with wide cutoff frequencies between 0.1-200 Hz, and low pass filter with cutoff frequencies of 0.2-10 kHz. The design of our ECG circuit allows us to obtain the ECG signals using fingertips and without using a ground electrode. This compact system has the potential to become a wireless wearable in the future. The measured PAT was fitted into a mathematical model and cuff-less BP readings were obtained. A plot of reference BP using oscillometric cuff and cuff-less BP showed a good correlation of r = 0.83 for systolic blood pressure (SBP). The SBP and diastolic blood pressure (DBP) mean absolute difference for the system are 6.75 mmHg and 6.08 mmHg respectively, which fairly passed the strict standard set by IEEE. In the future, our system will be compared with the use of sphygmomanometer, which is the gold standard, to further evaluate its accuracies.