the present invention will be described with reference to illustrative embodiments . it will be appreciated that the invention is not confined to any particular physical implementation . in the present invention , the pulse pressure is determined in the same manner as systolic pressure , but is calculated as the systolic pressure minus the diastolic pressure , so as to give the amplitude of the wave . consequently , the exemplary system of the present invention preferably comprises means for calculating the pulse pressure from tonometer measurements and the calculated central systolic pressure wave , such as a simple processor having computer software loaded therein for carrying out the calculations . an exemplary implementation of the first aspect of the present invention requires the measurement of both the carotid and radial pressure waveforms in order to determine the central ( aortic ) systolic pressure . fig1 shows pressure waveforms measured non - invasively from the carotid artery and from the upper limb . the upper limb waveform is measured , for example , at the radial artery . time t is shown , representing the time from wave foot to peak in the carotid waveform . at this time t from the foot of the radial waveform , the initial peak has passed and the waveform shows the broad peak associated with the lower body reflections . the pressure value at this time in the radial waveform is a close approximation to the systolic pressure value in the aorta . the radial waveform is calibrated using , for example , conventional brachial cuff techniques . it will be understood that determining the time t from the wave foot to peak is a simple matter of determining the time from minimum to maximum value , which can be readily captured by a simple digital system . the time t can then be applied to a captured data set of pressure against time , in order to find the pressure time t after the minimum value in the radial waveform . a practical implementation is straightforward for a suitably skilled electronic engineer . display of the radial or carotid waveforms is not required . accordingly , this implementation of the first aspect of the present invention relies on locating the broad peak using timing information from central ( carotid ) arteries and detecting the radial pressure value where it corresponds to the central pressure . it will be appreciated that this method requires measurement of pressure or diameter in a central artery , in order to acquire the timing information , and this measurement is ideally made under similar conditions to the radial tonometry . any suitable sensing arrangement may be used , provided a sufficiently accurate timing can be extracted . on the other hand , an exemplary implementation of the second aspect of the present invention only requires measurement of the brachial or radial waveform in order to determine the central ( aortic ) systolic pressure . specifically , this implementation excludes the effects of wave reflection in the upper limb and identifies the reflected wave from the lower body , which normally comprises the peak of pressure in the ascending aorta . this is accomplished by analysing derivatives of the upper limb pressure waveform , so as to identify the peak of the pressure wave which returns from the lower body , and which constitutes the late systolic surge of pressure after the initial peak . the time t 2 denotes the peak of this wave . brachial or radial pressure at t 2 is peak aortic systolic pressure ( asp ) and aortic pulse pressure is asp minus brachial or radial diastolic pressure . the first , second and third derivatives of upper limb the pressure wave are analysed for this purpose , as outlined in the flowchart in fig3 . first , the peak of the waveform is identified . then a search is conducted for any local minimum of the first derivative before the peak of the recorded pressure wave . if a local minimum of the first derivative is present , then t 2 is set at the peak of the recorded wave , and this is taken to represent peak aortic systolic pressure ; when this occurs the pressure peak is generated well after peak flow in the artery , and so more than 150 msec after the wave foot . ideally , the local minimum of the first derivative is detected by determining whether there is a zero crossing from negative to positive in the second derivative . if there is no localised minimum of the first derivative before the peak of the wave , then there is a search for the first zero crossing from positive to negative of the second derivative after the peak of the recorded wave and before the incisura . if there is a zero crossing , then this is taken to represent t 2 and pressure of the recorded wave at this point is taken to represent peak aortic systolic pressure . there will always be a zero crossing at the time of incisura ( that is , at the beginning of diastole ). as shown in fig2 , the incisura normally occurs quite late in the waveform ( more than 250 ms after the initial wavefoot , well after peak flow in the artery , and hence it is necessary to exclude the incisura from being detected . thus the search for the zero crossing can be practically restricted to within the first 40 % of the waveform . if the second derivative shows no zero crossing from positive to negative after the peak of the wave , then there is a search for the last zero crossing from positive to negative of the third derivative up to or at the peak of the recorded wave . if there is a zero crossing , then t 2 is set at the peak of the recorded wave . if there is no zero crossing , then there is a search for the first zero crossing from positive to negative of the third derivative after the peak of the recorded wave and t 2 is set at the time of this zero crossing and the pressure of the recorded wave at this point is taken to represent peak aortic systolic pressure . the application of this process is shown in the five recorded radial artery pressure waveforms in fig4 . as can be seen , for each of waveforms 1 , 2 and 3 a secondary systolic or reflected wave is apparent in the upper limb . in these three waveforms , the time t 2 ( at which radial artery pressure approximates aortic peak pressure ) occurs after the peak of the recorded wave and is identified from the second differential . in the waveform 5 , the peak of the wave occurs in late systole and is preceded by a local minimum of the first derivative . the peak constitutes t 2 and the peak pressure approximates that in the aorta . in waveform 4 , there is no apparent separate reflected wave in the upper limb , but the peak of the pressure wave occurs more than 150 msec after the foot of the wave , and the third derivative shows a localised zero crossing . for waveforms 4 and 5 , peak pressure occurs more than 150 msec after the wave foot , while for waveforms 1 , 2 and 3 , the peak occurs earlier than 150 msec , and corresponds to the peak of flow in the artery . the above described processor may preferably determine whether the differentials need to be calculated based on the determination of the timing of a reflected wave , and returns the aortic systolic pressure value accordingly . it will be appreciated that variations and additions are possible within the spirit and scope of the invention .
methods for determining central systolic pressure are disclosed . a first method determines the time of peak in a measured carotid waveform and then determines the pressure in a measured radial waveform at the corresponding time . a second method utilises only a measured radial waveform . the waveform is analysed to determine a time indicative of lower body waveform and determines the pressure at this time . in each case , the determined pressure is substantially the same as the central systolic pressure .