thus more hemoglobin and other light-absorbing proteins ; during the diastolic or relaxation phase , it will be at its minimum . The algorithm relates these measurements to blood pressure .
This is the same method used by the Apple Watch and other devices to measure heart rate , and by the pulse oximeters that became popular during the Covid-19 pandemic to record the level of saturation , or oxygenation , of the blood . It is also the method used by the Swiss company Aktiia ’ s wristband , available only in Europe for now . This device automatically records blood pressure values over 24 hours , even when someone is sleeping , averaging the results every two hours and displaying the results through an app on a smartphone .
There are also electrical sensors , which are modified versions of the electrocardiogram that measures the electrical activity of the heart ; mechanical sensors , used in ballistocardiography and seismocardiography , which attach to the surface of the skin to capture small variations in pressure ; and bioimpedance sensors , similar to the instruments that analyze body composition by measuring the body ’ s resistance to the flow of electrical current .
Calibrated and uncalibrated There are two broad categories into which these new cuffless blood pressure measurement devices can be grouped : those that require calibration — periodic comparison of the recordings with those obtained with a manual or digital sphygmomanometer — and those that do not .
Both types of devices capture signals from the body noninvasively , from the fingertip , ear or wrist , to name the most common sites . The estimated blood pressure is then displayed or transmitted to nearby devices , such as smartphones or tablets .
One of the measurement methods that require calibration is the pulse transit time , or PTT , which represents the time in milliseconds that the pulse takes to travel between two arterial points : The stiffer an artery is , the higher the arterial pressure will be ( because the pulse travels faster ) and the lower the PTT will be . This method is the one with the most scientific evidence to date .
Another is based on analysis of the shape and amplitude of the pulse wave , which is the pressure wave depicting the propagation of the blood pumped by the heart through the entire arterial tree , and whose characteristics depend in part on the rigidity of the artery walls . In people with hypertension , the amplitude of the pulse wave is greater because the heart must exert more force to overcome the resistance of the arteries .
More recently , other devices have emerged that use images captured with a camera — like selfies — to detect changes in the PTT or subtle modifications in the color of the face , imperceptible to the eye , that accompany each heartbeat , thus reconstructing the flow of blood under the skin and the shape of the pulse waves .
Uncalibrated cuffless blood pressure measurement methods aim to eliminate the need to crosscheck the device ’ s measurements with those captured by a classic sphygmomanometer or digital sphygmomanometer . They use only machine learning and artificial intelligence to establish , from the signals captured by the sensors , the person ’ s blood pressure values .
Just as a jet of water can exert more or less force on the walls of a hose if one changes the height or opening of a faucet , the analysis of oscillations or fluctuations in blood volume can be measured when a ring is worn and the arm is lowered , because the finger ’ s internal blood pressure increases as it receives more blood flow due to gravity . Alternatively , a ring can also obtain measurements of the oscillations in blood volume by periodically applying gentle pressure on the finger . A sensor in a smartphone can also do this analysis when it is pressed following the instructions given by the device .
Other methods for uncalibrated devices use ultrasound waves to visualize variables such as artery dimensions and blood flow velocity , which are also related to blood pressure .
The road to clinical application The development of these devices for measuring blood pressure without a blood pressure cuff is progressing rapidly , but that doesn ’ t mean they are ready for use in the medical world . “ Unfortunately , the pace of evidence , regulation and validation testing has lagged behind the pace of innovation and direct consumer marketing ,” write Stephen P . Juraschek , physician investigator of Beth Israel Deaconess Medical Center in Boston , and colleagues in
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