Accurate, wrist-based blood-pressure monitoring
So, we can now measure blood pressure from a smart watch. How accurate are the readings? Accuracy has been a challenge in terms of blood-pressure monitoring from the wrist because the arteries in the wrist are narrower and not as deep under the skin as upper arm arteries. Also, the arm and wrist must be at heart level to capture a correct reading.
One method of measuring blood pressure is via pulse transit time (PTT), which denotes the time that it takes a pulse pressure waveform to propagate through the length of the arterial tree. This pulse pressure waveform occurs when blood is ejected from the left ventricle, and it moves with a greater velocity than the forward movement of the blood itself. In addition to a measurement of blood pressure, PTT also provides an indicator of arterial stiffness.
PTT can be derived from calculations on electrocardiogram (ECG) and photoplethysmography (PPG) signals; it’s based on tightly defined characteristics of the shape of the pressure pulse waves in blood vessels. ECG provides a measurement as well as graphical representation, in terms of time, of the electrical signals associated with heart muscles. PPG provides an optical measurement of the volumetric change of blood in tissue during the cardiac cycle. Optical sensors that utilize PPG and ECG are used in wearables to measure heart rate. Measuring PTT involves calculating the time between the R-peak of the ECG and a reference point on the pressure pulse wave measured using PPG.
Studies have shown, however, that measuring PTT alone to assess blood pressure may not be sufficient “because the regulation of blood pressure within the human body is a complex, multivariate physiological process,” notes a team of university and industry researchers in a paper published by the National Institutes of Health in its U.S. National Library of Medicine.6 In their study, the researchers found that integrating PTT, heart rate, and a previous blood-pressure estimate results in a more accurate current blood-pressure value. In addition to general blood-pressure monitoring, PTT has been examined for use in other applications, such as a means to detect sleep ailments via measurement of respiratory effort or as an indicator of the onset of low blood pressure during spinal anesthesia.