referring to fig1 shown is an arrangement of the tissue oxygen measuring system for measuring each of cbf and cbv . the timing charts for the measurements of cbf and cbv and their data processing sequences are illustrated in fig4 a and 4b , respectively . the system is basically comprised of an nir oxygen measuring apparatus 1 for measuring quantity of oxygen , a pulse oximeter 2 for measuring arterial saturation ( sao 2 ), a ventilator 3 for supplying air to the patient , a gas blending unit 4 disposed downstream of the ventilator 3 and connected thereto via a valve 5 , and a control unit 6a connected between the nir oxygen measuring apparatus 1 and the gas blending unit 4 for controlling the valve 5 , thereby controlling a quantity of oxygen to be introduced into the ventilator 3 . the gas blending unit 4 produces a gas mixture which on average has the correct oxygen concentration for the patient , but which has imposed upon it a regular square wave oscillation of a low frequency by means of the control unit 6a . this oscillation does not have a harmful effect on the patient , as a similar modulation is used in respiratory physiology laboratories for lung function studies . the control unit 6a controls the valve 5 of the gas blending unit 4 at a given frequency , so that the rate of oxygen quantity to a unit volume of air is periodically changed as shown in fig4 a and 4b . more specifically , with the control unit 6a , the oxygen concentration is increased and maintained at a high level for a predetermined period of time ranging from 1 to 10 seconds and is then decreased to a normal level for an equal predetermined period of time . the arterial saturation ( sao 2 ) and the change in the quantity of oxyhemoglobin ( δhbo 2 ) in the patient &# 39 ; s brain are repeatedly and continuously measured by the pulse oximeter 2 and the nir oxygen measuring apparatus 1 , respectively , during a transit period of time t 1 ranging from 1 to 10 seconds at which the concentration of oxygen is abruptly changed . the nir oxygen measuring apparatus 1 incorporates a central processing unit ( cpu ) 7 therein which is supplied with trigger signals from the control unit 6a . the trigger signals are issued from the control unit 6 in synchronism with the control of the valve 5 . in response to the trigger signal , the cpu 7 fetches data from both the nir oxygen measuring apparatus 1 and the pulse oximeter 2 . data regarding the change in oxyhemoglobin concentration [ hbo 2 ] is supplied from the nir oxygen measuring apparatus 1 , which data is represented by q in equation ( 3 ) and is obtained as a difference b ( see fig4 a ) in the level of the oxyhemoglobin concentrations at the start and end of time t 1 . further , data regarding the arterial oxygen saturation ( sao 2 ) during time t 1 is supplied from the pulse oximeter 2 , which data corresponds to an area indicated by oblique lines in fig4 a and denoted by letter a . those data supplied from both the nir oxygen measuring apparatus 1 and the pulse oximeter 2 are temporarily stored in random access memory ( ram ) 9 connected via a bus to the cpu 7 . based on those data , the cpu 7 performs arithmetic operations in accordance with equation ( 3 ) upon reading the data out of the ram 9 . the results of the computation by the cpu 7 is digitally displayed on a display 10 of the apparatus 1 . in this manner , a plurality of cbf data are obtained through the repetitive measurements and computations and are collectively displayed on the display 10 . computation of cbv is performed in accordance with the sequence indicated in fig4 b . specifically , after expiration of time t 2 from the occurrence of the trigger signal in the oxygen - increased cycle , the cpu 7 fetches data a regarding the arterial saturation ( sao 2 ) from the pulse oximeter 2 and data b regarding the concentration of oxyhemoglobin from the nir oxygen measuring apparatus 1 , and temporarily stores those data in the ram 9 . then , after expiration of time t 2 from the subsequent trigger signal occurring at the start of the next oxygen - reduced cycle , the same kinds of data a &# 39 ; and b &# 39 ; are fetched and stored in different storage locations of the ram 9 . then , using the data stored in the ram 9 , the cpu 7 performs arithmetic operation in accordance with the equation of cbv =( δhbo 2 )/( δsao 2 ) mentioned previously . that is , cbv is obtained through the computation of ( b - b &# 39 ;)/( a - a &# 39 ;). upon completion of the computation , the cpu 7 displays the resultant data in the display 10 . a similar sequence can be employed for the automated measurement of the co 2 , with the use of a small but cyclic change in carbon dioxide levels . the arrangement shown in fig2 is used for such a measurement , which includes the nir oxygen measuring apparatus 1 , a transcutaneous carbon dioxide ( co 2 ) tension monitor 8 , a gas blending unit 4 &# 39 ;, and a control unit 6b for controlling a valve 5 &# 39 ; of the gas blending unit 4 &# 39 ;. the monitor 8 has a sensor 11 for attachment to the patient &# 39 ; s skin to measure arterial carbon dioxide tension ( paco 2 ) which tension will hereinafter referred to as &# 34 ; co 2 tension &# 34 ;. the control unit 6b in the arrangement of fig2 controls the valve 5 &# 39 ; of a gas blending unit 4 &# 39 ; which in this case introduces carbon dioxide into the ventilator 3 . the sequence for measurement of the co 2 response is illustrated in fig4 c . after expiration of time t 2 from the occurrence of the trigger signal at the start of the carbon - dioxide - increase cycle , the cpu 7 fetches data c regarding the co 2 tension from the monitor 8 , and data d regarding a total quantity of the changes in oxy - and dioxyhemoglobin ( δhbo 2 + δhb ) from the nir oxygen measuring apparatus 1 . those data are temporarily stored in the ram 9 . in the subsequent carbon - dioxide - reduced cycle , the same kinds of data c &# 39 ; and d &# 39 ; are similarly fetched and stored . then , the cpu 7 performs arithmetic operation to provide a ratio of the change in ( δhbo 2 + δhb ) to a change of the arterial carbon dioxide tensions attendant to the change of the quantity of co 2 . that is , the cpu 7 performs computation of ( d - d &# 39 ;)/( c - c &# 39 ;). by repeatedly carrying out the above measurements and computations , a plurality of the co 2 response data are obtained successively . as shown in fig4 d , both the cbf and cbv can be continuously measured with the arrangement shown in fig1 in accordance with the combined sequence for cbf and cbv . the control unit 6a periodically changes the oxygen concentration contained in the air supplied to the patient as described previously . during a transit time t 1 in the oxygen - increased cycle , data regarding sao 2 and δhbo 2 measured , respectively , by the pulse oximeter 2 and the nir oxygen measuring apparatus 1 are fetched and stored in the ram 9 , whereupon cbf is computed and displayed . after expiration of time t 2 from the occurrence of the trigger signal in the same oxygen - increased cycle , data a regarding the arterial saturation ( sao 2 ) and data b regarding the concentration of oxyhemoglobin measured , respectively , by the pulse oximeter 1 and the nir oxygen measuring apparatus 1 are fetched and stored in the relevant storage locations of the ram 9 . then , after expiration of time t 2 from the subsequent trigger signal occurring at the start of the next oxygen - reduced cycle , the same kinds of data a &# 39 ; and b &# 39 ; are fetched and stored in the ram 9 . then , the cpu 7 computes cbv and displays it together with the cbf . fig3 shows an arrangement for automated measurement of cbf , cbv and co 2 response . the arrangement of fig3 includes the nir oxygen measuring apparatus 1 , the pulse oximeter 2 , the ventilator 3 , the control unit 6 , and the transcutaneous carbon dioxide tension monitor 8 . in this embodiment , both the oxygen blending unit 4 and the carbon dioxide blending unit 4 &# 39 ; are connected to the ventilator 3 through the valves 5 and 5 &# 39 ;, respectively . the control unit 6 controls the valves 5 and 5 &# 39 ; so that opening and closing thereof are performed at different timings from each other and the frequency for the valve 5 to perform the opening and closing actions is two times larger than that for the valve 5 &# 39 ; to perform the same actions . the oxygen - increased duration takes place in the midst of the co 2 - increased or co 2 - reduced duration . more specifically , in fig4 e , the valve 5 &# 39 ; is opened at time t 0 , the valve 5 is opened at time t 1 and closed at time t 3 , and the valve 5 &# 39 ; is closed at time t 4 . the valve 5 is again opened at time t 5 and closed at time t 7 and the valve 5 &# 39 ; again opened at time t 8 . note that an interval between t i + 1 and t i is time t where i is an integer . the automated measurements of cbf , cbv and co 2 response are implemented by the combined sequence described with reference to fig4 a , 4b and 4c . the sequence will be described while referring to the flow chart shown in fig5 . upon start of the measurement , the operator inputs the measuring items through a keyboard ( not shown ) connected to the cpu 7 . when the items of cbf , cbv and co 2 response are selected , the routine proceeds to step s10 . if the items of cbf and cbv are selected in step s2 , the processing described with reference to fig4 d is executed in step s3 . if the item of cbf is selected in step s4 , the processing described with reference to fig4 a is executed in step s5 . if the item of cbv is selected in step s6 , the processing described with reference to fig4 b is executed in step s7 . if the item of co 2 response is selected in step s8 , the processing described with reference to fig4 c is executed in step s9 . in step s10 , the number of measurement times n of the three items is set , and a counter provided interiorly of the cpu 7 is cleared in step s11 . next , it is determined in step s12 whether the trigger signal tr2 is detected . the trigger signal tr2 is issued from the control unit 6 in synchronism with the opening of the valve 5 &# 39 ; of the carbon dioxide blending unit 4 &# 39 ; allowing to blend a small predetermined quantity of co 2 into the air . the trigger signals tr2 are issued whenever the valve 5 &# 39 ; is opened , and the valve 5 &# 39 ; is closed by the control of the control unit 6 after expiration of 4t from the receipt of the trigger signal tr2 . when the trigger signal is detected in step s12 , a timer provided interiorly of the cpu 7 is set to start measuring time in step s13 . in step s14 , it is further determined whether the trigger signal tr1 is detected . the trigger signal tr1 is also issued from the control unit 6 in synchronism with the opening of the valve 5 of the oxygen blending unit 4 allowing to blend a predetermined quantity of oxygen into the air . the trigger signals tr1 are issued whenever the valve 5 is opened , and the valve 5 is closed by the control of the control unit 6 after expiration of 2t from the receipt of the trigger signal tr1 . if both the trigger signals tr2 and tr1 are detected , data regarding sao 2 is fetched from the pulse oximeter 2 and stored in the ram 9 in step s15 . thereafter , data regarding δhbo 2 is fetched from the nir oxygen measuring apparatus 1 and stored in the relevant storage location of the ram 9 in step s16 . then , the cpu 7 computes cbf based on the data regarding sao 2 and δhbo 2 and displays the resultant data in the display 10 in step s17 . next , in step s18 it is determined whether time 2t has been expired or not while referring to the timer . if yes , data regarding ( δhbo 2 + δhb ) and the co 2 tension are fetched from the nir oxygen measuring apparatus 1 and the carbon tension monitor 8 , respectively , and are stored in the relevant storage locations of the ram 9 in step s19 . in step s20 , it is determined whether time ( 3t - δt ) has been expired while referring to the timer . if yes , data regarding sao 2 and δhbo 2 are fetched respectively from the pulse oximeter 2 and the nir oxygen measuring apparatus 1 and stored in the ram 9 in step s21 . that is , immediately before the valve 5 of the oxygen blending unit 4 is closed , those data are fetched therefrom . this is because the cerebral tissue is in a stable condition with the supply of air containing oxygen and carbon dioxide at a predetermined ratio . in step s22 , it is determined whether time ( 4t - δt ) has been expired while referring to the timer . if the determination made in step s22 is yes , then data regarding sao 2 and δhbo 2 are again fetched and stored in step s23 . that is , immediately before the valve 5 &# 39 ; of the carbon dioxide blending unit 4 &# 39 ; is closed , those data are fetched . based on data stored in steps s21 and s24 , cbv is computed and the resultant data is displayed in step s24 . in step s25 , it is determined whether time 6t has been expired while referring to the timer . if yes , ( δhbo 2 + δhb ) and co 2 tension are fetched respectively from the nir oxygen measuring apparatus 1 and the co 2 tension monitor 8 and are stored in the ram 9 in step s26 . in step s27 , the cpu 7 performs arithmetic operation based on the data fetched in steps s19 and s26 and provides data regarding co 2 response for displaying the latter in the display 10 . through the steps s12 through s27 , measurements of the three items in one cycle have been completed . in step s28 , it is determined whether time 8t has been expired , and if yes , the timer is reset in step s29 , and the count number of the internal counter is incremented by one in step s30 . next , it is determined in step s31 whether the count number is equal to n . if no , the routine returns to step s12 , and the measurement in the next cycle is performed , whereas if yes , the cpu 7 executes arithmetic operations to provide average data regarding each of cbf , cbv and co 2 response based on the results of measurements through n cycles and displays the resultant data in the display in steps s32 through s34 , whereupon the routine ends . the above - described sequence according to the present invention can be implemented in the operating theater or intensive care unit ( icu ), linked to the ventilator . since the signal is repetitive , it is possible to employ signal averaging techniques to improve a signal - to - noise ( s / n ) ratio and to identify erroneous data . similarly , because one would be averaging , it should be possible to employ smaller swings in the saturation and still obtain accurate results . a further advantage of the repetitive nature of the readings is that one could obtain information on the time delay between the change detected by the pulse oximeter or the co 2 monitor and that observed by the nir oxygen measuring apparatus . while the present invention has been described with reference to specific embodiments , the addition of a small percentage of co 2 or the addition of oxygen may be made to be in sinusoidal waveform to smoothly change the contents of air to be supplied to the patient . further , the ventilator can be triggered by the nir oxygen measuring apparatus or vice versa .
a tissue oxygen measuring system enabling the automatic and continuous measurement of diagnosing items including cerebral blood flow , cerebral blood volume , and response of cerebral blood volume in arterial carbon dioxide tension uses an oxygen measuring system based on near infrared spectroscopy , a pulse oximeter , and an arterial carbon dioxide tension measuring unit . a gas blending unit is employed before a ventilator or face mask to control the ventilator so that a rate of a quantity of oxygen and / or a quantity of carbon dioxide to be blended in the air is changed at predetermined intervals . trigger signals are produced in synchronism with the changes of the gaseous contents of the air , and parameters for computing information regarding the diagnosing items are measured in response to the trigger signals . with the repetitive measurements of the parameters , a signal - to - noise ratio can be improved by averaging the data and erroneous data can easily be identified .