Anyone who’s ever had a blood pressure test will know that it normally involves an inflatable cuff that squeezes your arm until it cuts off circulation, then gradually deflates it, measuring the point at which circulation first resumes and then returns to normal.
The basic idea is that if you can detect a heartbeat, and then detect the pulse created by it at your wrist, you can time how long it takes to get from your heart to your wrist. This is known as the pulse transit time (PTT), and can be used to calculate blood pressure.
Wrist-worn devices and related methods measure a pulse transit time non-invasively and calculate a blood pressure value using the pulse transit time. A wrist-worn device includes an accelerometer, a photo-plethysmogram (PPG) or a pulse pressure sensor, and a controller. The PPG or the pulse pressure sensor coupled to the wrist-worn device for detecting an arrival of a blood pressure pulse at the user’s wrist. The controller is configured to process output signals from the accelerometer to detect when the blood pressure pulse is propagated from the left ventricle of the user’s heart, process a signal from the PPG or the pulse pressure sensor to detect when the blood pressure pulse arrives at the wrist, calculate a pulse transit time (PTT) for propagation of the blood pressure pulse from the left ventricle to the wrist, and generate one or more blood pressure values for the user based on the PTT.
The accelerometer would, when held against your chest, detect the heartbeat, and the existing heart-rate sensor would detect your pulse.
The patent notes that high blood pressure is a major risk factor for cardiovascular disease, and that the earlier it is detected, the more likely it is that measures can be taken to prevent damage. Routine monitoring would increase the chances of early detection.
Back in 2015, Apple CEO Tim Cook indicated that Apple was wary of including too many health-related sensors in the Watch itself, as the slow process of achieving FDA approval could hamper the pace of development. For example, the company even appears to have decided not to use the heart-rate monitor to measure oxygen saturation even though it contains the necessary hardware.
However, earlier this year the FDA said that it was aiming to streamline innovation of digital health and medical technology through a new approach to regulating devices.
Via Patently Apple