Wearable hemodynamic sensor
A wearable device and the accompanying method for the determination of continuous pulsatile BP are described. The absolute values can be obtained in the initial phase and how a transfer function can transform the BP-signal obtain at the finger or wrist to correct BP-values corresponding to the brachial artery and at heart level. The wearable device contains an orthostatic level-correcting element, which can measure the vertical distance between heart level and finger/wrist level, where the actual measurement takes places. The wearable device may be in the form of a ring, a watch, or a bracelet. Further, the wearable device has elements for wirelessly transmitting signals to host devices such as a smart phone, tablet or other computers.
1. A method for the continuous non-invasive measurement of hemodynamic parameters within a wearable device comprising:
measuring a photoplethysmographic signal disposed on a body part containing an artery using a photoplethysmographic system having:
at least one light source; and
at least one light detector generating the photoplethysmographic signal;
wherein the contact pressure of the photoplethysmographic system can be varied by an actuator;
wherein the movement of the actuator and the contact pressure is controlled by a control system using at least a part of the photoplethysmographic signal;
wherein the true pulsatile BP-signal and further calculated hemodynamic parameters are derived from the contact pressure and the photoplethysmographic signal;
wherein the hemodynamic parameters are further derived from a transfer function; and
wherein at least one of different modes of operation A to C is implemented:
a mode A for continuous measurement of hemodynamic parameters, wherein the contact pressure is adjusted by the control system and the actuator according to mean arterial pressure multiplied by a constant factor, and wherein the transfer function is applied to the photoplethysmographic signal and the contact pressure;
a mode B for the initialization of the continuous measurement of hemodynamic parameters,
wherein the contact pressure is adjusted by the control system and the actuator to different pressure levels,
wherein the amplitude of the photoplethysmographic signal is determined,
wherein mean blood pressure is determined at the contact pressure, where coevally the highest amplitude of the photoplethysmographic signal is measured, and
wherein the mean blood pressure multiplied by a constant factor is used as the starting point for the measurement modus; and
an idle mode C wherein the contact pressure is adjusted to a constant pressure around 30-40 mmHg,
wherein the system waits for user interaction in order to bring the system in initialization and further into the measuring mode,
wherein the photoplethysmographic signal is measured for obtaining heart rate, and
wherein the system can be further brought in initialization and thereafter into the measuring mode if heart rate increases over or decreases under a predefined threshold.
2. The method of claim 1 wherein contact pressure is measured by a pressure sensor.
3. The method of claim 1 wherein the hemodynamic parameters are corrected to heart level using an orthostatic level-correcting element.
4. The method of claim 3 wherein the orthostatic level-correcting element comprises a fluid filled hose between the photoplethysmographic system and heart level;
wherein the hose is filled with a fluid having a density similar to that of blood;
wherein the hose is closed on one end with a freely floating membrane;
wherein the hose is closed on the other end with a pressure sensor; and
wherein the pressure obtained from this heart level pressure sensor is used for heart level correction.
5. The method of claim 3 wherein the orthostatic level-correcting element comprises an accelerometer or motion sensor;
wherein the accelerometer or motion sensor is placed next to the photoplethysmographic system;
wherein the movement in the same direction as gravity is used for calculating orthostatic distance between heart level and the location of the photoplethysmographic system; and
wherein the correcting procedure can be zeroed by simultaneously placing the wearable device on heart level.
6. The method according to claim 1 wherein the transfer function is a general transfer function valid for humans.
7. The method according to claim 1 wherein the transfer function is personalized by using the individual systolic and diastolic blood pressure of a patient.
8. The method of claim 1 wherein systolic and diastolic blood pressure is estimated using the distribution of the amplitude of the photoplethysmographic signal versus the applied contact pressure; and
wherein systolic and diastolic blood pressure is used for the calibration of the transfer function.
9. The method according to claim 1 wherein the accelerator detects falls of the patient and initiates or re-initiates the initialization mode.
10. The method according to claim 1 wherein some of the hemodynamic parameters are calculated by using biomedical algorithms applied to the photoplethysmographic signal, the contact pressure and the transfer function.