as already stated above , pwv measurement is a promising approach , which offers the opportunity to derive significant changes of arterial blood pressure at a heart - beat frequency without applying an external pressure . for that , the pulse arrival time is determined , which is defined as the time - delay between the r - peak of the qrs wave from the ecg and the arrival of the arterial pulse wave at the periphery . this comprises the sum of the pre - ejection period ( pep ) and the pulse transit time ( ptt ), both representing different underlying cardiovascular mechanisms : more precisely , the pep is a cardiac component covering the iso - volumic ventricular contraction stage while the ptt is a purely vascular component that can only be defined after the aortic valve opening . it was found that pep is a good marker for blood pressure changes induced by physical stress . however , pep is also sensitive to fluid shifts within the body induced by posture changes at constant blood pressure , which has to be taken into account for detecting accurately significant blood pressure changes . further , it has been found that the known pep effect causes a significant pat change that is not associated with a blood pressure change . the observed pat change for different postures is covered almost completely by the pep changes . in order to compensate this effect , according to an embodiment of invention , the posture of the patient is detected . in order to detect the posture of the patient , according to this preferred embodiment of the invention , an acceleration sensor 1 is attached to the patient 2 and connected to a monitoring device 3 , which is schematically depicted in fig1 . the connection of the acceleration sensor 1 with the monitoring device 3 is a wireless connection 6 . the patient &# 39 ; s 2 posture is inferred from the dc - components , i . e . averages , of the acceleration signals , which contain information of the acceleration sensor &# 39 ; s 1 orientation with respect to the earth gravity axis . since earth gravity creates a constant acceleration towards ground , its projection on the axis of the acceleration sensor 1 unveils the orientation of this axis and , hence , of the acceleration sensor 1 . thus , the patient 2 is considered to be standing when its trunk is upright , sitting when its trunk is slightly tilted backward , and lying when its trunk is roughly parallel to the ground . for a correct classification of the patient &# 39 ; s 2 posture it is preferred to calibrate the system with a known posture . further , for the actual blood pressure monitoring , according to the embodiment of the invention shown in fig1 , an ecg sensor 4 and a pulse wave sensor 5 for a plethysmographic measurement are provided in a conventional way for enabling a pwv measurement , the ecg sensor 4 and the pulse wave sensor 5 forming a pulse wave velocity unit 7 . the ecg sensor 4 and the optical sensor 5 are both connected with the monitoring device via the wireless connection 6 , too . further , a cuff 8 for a conventional cuff - based blood pressure measurement is provided . as shown in fig1 , a separate acceleration sensor 1 can be provided on the patient 2 in order to detect his posture . according to another embodiment , an ecg electrode with an integrated acceleration sensor can be used . further , it is also possible to use an ecg - electrode connector with an integrated acceleration sensor . moreover , according to still another preferred embodiment of the invention , an automated normalization of the pulse arrival time ( pat ) after detecting a posture change is applied : after a posture change has been detected , the continuously measured pulse arrival time is normalized for the new posture . this is done automatically , triggered by the detected posture change via the acceleration sensor signal , and a new reference pat is determined . an according flow diagram for this embodiment of the invention is shown in fig2 . according to the embodiment of the invention shown in fig2 exemplified for detection of an critical blood pressure increase , at the beginning of the monitoring , a threshold value for the pulse arrival time ( pat 0 ) is determined , e . g . on the basis of the detected pulse arrival time average over a predefined duration . further , when such a pulse arrival time value is observed which falls below the predefined threshold value , it is checked if a posture change of the patient was detected . if such a posture change was actually detected , it is waited until the blood pressure regulation process has equilibrated . then , a new threshold value is defined based on an average of monitoring the pulse arrival time for a predefined duration . then , it is further checked if the actual pulse arrival time observed under - runs the new threshold value . then , the process described above may continue . in case that , when checking for a posture change of the patient , it is detected that no posture change has occurred since the dc component of the accelerator signal has not changed , a cuff - based blood pressure measurement is triggered . if this cuff - based blood pressure measurement results in a critical blood pressure which exceeds a predefined threshold value , an alarm is generated . in case the cuff - based blood pressure value is not determined to be critical , it is continued with checking the actual pulse arrival time as described before . further , according to still another embodiment of the invention , before a monitoring period is started , a calibration procedure is performed for different postures of the patient providing the relation of pat vs . postures , e . g . at several different angles . a look up - table is created by this procedure , which can be used in the monitoring period to compensate for pat changes caused by posture changes . in addition , the blood pressure can be measured with a cuff to assess the blood pressure changes associated with the different postures , which allows a further refinement of the posture - related pat corrections . alternatively , pep can be measured directly in different postures using for example a stethoscope for heart sound analysis or impedance cardiography . while the invention has been illustrated and described in detail in the drawings and foregoing description , such illustration and description are to be considered illustrative or exemplary and not restrictive ; the invention is not limited to the disclosed embodiments . other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention , from a study of the drawings , the disclosure , and the appended claims . in the claims , the word “ comprising ” does not exclude other elements or steps , and the indefinite article “ a ” or “ an ” does not exclude a plurality . the mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage . any reference signs in the claims should not be construed as limiting the scope .
a method for monitoring the blood pressure of a patient , comprising the following steps : determining a pulse arrival time signal from the patient 2 based on the pulse wave velocity method ; determining an accelerometer signal from the patient 2 ; and triggering an additional measure or deriving a blood pressure value , taking into account the pulse arrival time signal and a dc component of the accelerometer signal . in this way , a possibility for monitoring the blood pressure of a patient is provided with which false alarms and / or unnecessary additional cuff - based blood pressure measurements can be avoided .