the invention may be used to monitor blood flow in any segment of tissue , however the following description refers to monitoring thoracic cardiovascular activity in order to provide a complete description of the new apparatus and method . a continuous cardiac output monitor according to the invention uses an array of spot electrodes on the patient as shown in fig1 . in a typical application , a pair of upper sensing electrodes 10 and 12 are attached to the patient &# 39 ; s neck on opposite sides thereof at the intersections of the line encircling the root of the neck with the frontal plane . a pair of upper current injecting electrodes 14 and 16 are attached to the patient &# 39 ; s neck approximately 3 to 5 centimeters above the upper sensing electrodes 10 and 12 , respectively . a pair of lower thoracic anterior sensing electrodes 18 and 20 are placed at the intercostal space at each midclavicular line at the xiphoid process level . a pair of posterior sensing electrodes 22 and 24 shown in fig1 b are placed at the same level as the anterior sensing electrodes 18 and 20 at the intercostal space at the midscapular line . referring again to fig1 a , a pair of lower current injecting electrodes 26 and 28 are located approximately 4 to 6 centimeters below the lower thoracic anterior sensing electrodes 18 and 20 , respectively . referring again to fig1 b , a pair of lower injecting electrodes 30 and 32 are attached to the patient approximately 4 to 6 centimeters below the posterior sensing electrodes 22 and 24 . all of the electrodes 10 , 12 , 14 , 16 , 18 , 20 , 22 , 24 , 26 , 28 , 30 , and 32 are preferably spot electrodes which have been pregelled . referring to fig1 a , 1b , and 2 , a conductor 34 connects the upper injection electrodes to a current source 36 , which preferably produces a high - frequency , constant amplitude current output . a conductor 38 connects both the anterior lower injection electrodes 26 and 28 and the posterior lower injection electrodes 30 and 32 to the current source 36 . the frequency of the output of the current source 36 should be high enough to preclude any interference with proper functioning of the electrical systems within the human body . in a preferred embodiment of the invention , the current source 36 outputs a signal having an effective value of approximately 2 . 5 ma and a frequency of 70 khz . a current source suitable for practicing the present invention is described in applicant &# 39 ; s u . s . patent application entitled constant magnitude , high - frequency current source , ser . no . 06 / 393371 , filed june 26 , 1982 . a conductor 40 connects the upper sensing electrodes 10 and 12 to a detector circuit 42 . a conductor 44 connects the lower thoracic anterior sensing electrodes 18 and 20 and the posterior sensing electrodes 22 and 24 to the detector circuit 42 . a noninvasive continuous cardiac output monitor according to the invention utilizes a modified systolic upstroke equation ## equ3 ## to calculate the stroke volume . v ept is the physical volume of electrically participating thoracic tissue in milliliters , and the other variables in the equation are the same as those defined in connection with kubicek &# 39 ; s systolic upstroke equation discussed hereinabove . the volume of electrically participating tissue is a function of the thoracic volume , which approximates that of a cyliner ## equ4 ## where c is the thoracic circumference ; l is the average of the lines l a and l p , shown in fig1 a and 1b respectively and which are distances between the lines through the center of the upper sensing electrodes 10 and 12 and the lower sensing electrodes 18 and 20 and 22 and 24 ; and k is a ratio constant approximately in the range 2 . 6 to 2 . 8 for a typical c / l ratio of about 3 for which the equation reduces to ## equ5 ## referring to fig3 the graph illustrates typical variations in thoracic impedance due to respiratory and cardiovascular activity as a function of time . the average of the thoracic impedance is z o . the variation of thoracic impedance due to respiratory activity is δz resp ; and δz indicates variations of z due to cardiovascular activity . as the graph of fig3 indicates , δz resp is much greater than δz while the frequency of δz is about four times the frequency of z resp . the output signal , of the detector circuit 42 represents the thoracic impedance as a function of time . the circuitry of fig2 processes the thoracic impedance signal to obtain values for use in the modified systolic upstroke equation to monitor cardiovascular activity on a continuous basis . the suppression of the effects of ventilation is depicted in fig5 . fig5 shows the actual waveform of the first derivative of the impedance signal , including the effects of ventilation , as the signal emerges from the output terminal of the differentiation 48 ( fig2 ). the signal shown in fig5 is supplied to the analog to digital ( a / d ) converter of the microprocessor 54 . the microprocessor 54 measures the maximum positive value of the dz / dt waveform ( δz / sec , fig4 c ) at the time t1 ( fig4 f ), provided to the microprocessor 54 from the digital image ( fig4 f ) of the second derivative ot he impedance signal ( fig4 e ) for every heartbeat . referring to fig5 the microprocessor 54 will determine in four consecutive heartbeats the following δz / sec magnitudes : applying the sliding arithmetic average of the last four heartbeats , the microprocessor 54 will calculate the average value of δz / sec . sub . ( aver ), which then enters the calculation of stroke volume . ## equ6 ## a comparator 50 receives the signal dz / dt output from the differentiator 48 and functions as a zero - crossing detector to provide a constant logic one output voltage when the signal dz / dt is positive and a logic zero output when the signal dz / dt is negative . thus , the comparator 50 produces in response to dz / dt a digital signal which is high when dz / dt is positive and low when dz / dt is negative . fig4 d is a graph of the digital representation of dz / dt . the clock for the apparatus is determined in the clock circuit 52 , the output of which is shown in fig4 g , and which is obtained in the digital form from the dz / dt waveform ( fig4 c ) when the waveform crosses the positive d . c . threshhold . referring to fig2 and 4a , an electrocardiogram signal indicates the initiation of the systolic portion of a heartbeat , which determines the zero reference point for the time lines in fig4 a - 4f . referring to fig4 c , the ventricular ejection time t used in the calculation of the cardiac output is the time between the beginning of systolic contraction ( time t 0 ) to the closure of the aortic valve ( time t 2 ). the time t 2 corresponds to the first negative minimum of dz / dt waveform . obviously , the functions of the differentiator 48 , the comparator 50 , the clock 52 , the differentiator 56 , and the comparator 58 could be performed by software within the microprocessor 54 . a second differentiator 56 receives the output of the differentiator 48 and produces an output signal , shown in fig4 e , which represents the second derivative , d 2 z / dt 2 , of the pulsatile thoracic impedance . a comparator 58 connected to the differentiator 56 digitizes d 2 z / dt 2 in a manner similar to that in which the comparator 50 digitizes the dz / dt signal . the output of the comparator 58 is a digital signal which is high when d 2 z / dt 2 is positive and low when d 2 z / dt 2 is negative . the digital representation of the second derivative of the pulsatile thoracic impedance function is shown in fig4 f . the modified systolic upstroke equation used to calculate cardiac output utilizes the maximum value of the first derivative of the pulsatile impedance signal to calculate the stroke volume for each heartbeat . as shown in fig4 c , the absolute maximum value of the first derivative of the pulsatile thoracic impedance is the first maximum which occurs after initiation of the systolic portion of the heartbeat . referring to fig4 e , the second derivative of the pulsatile thoracic impedance has a negative - going zero - crossing at the time at which the first derivative exhibits the absolute maximum value . in fig4 f , the first negative - going pulse edge in the digital representation of the second derivative of the pulsatile thoracic impedance occurs at the time at which the first derivative of the pulsatile thoracic impedance exhibits the maximum value . therefore , the microprocessor is programmed to read the value of the first derivative of the pulsatile thoracic impedance at the occurrence of the first negative - going pulse edge after initiation of the systolic portion of each heartbeat . thus , the signals input to the microprocessor 54 are the thoracic impedance from the detector circuit 42 ; the first derivative of the pulsatile thoracic impedance from the differentiator 48 ; the time interval between the first two positive going zero - crossings of the first derivative of the pulsatile thoracic impedance , which is the ventricular ejection time , from the comparator 50 ; and the digital representation of the second derivative of the pulsatile thoracic impedance . the microprocessor 54 has included therein analog to digital converters to enable the microprocessor 54 to process the analog signals from the detector circuit 42 and the differentiator 48 . a thumb wheel switch 60 permits entry of the value of l needed to determine the volume of the electrically participating tissue . a display 62 , which may be a digital readout display or a digital printer , provides means for an operator to read values of the various cardiac output parameters for which the microprocessor is programmed to compute . typical values for cardiac parameters output by the display 62 are z o = 31 ohms , t = 0 . 39 sec , δz / sec = 0 . 99 ohm / sec , sv = 74 ml , heart rate , hr = 69 / min . and cardiac output = 5 . 1 liters / min . although the invention is described with reference to a specific preferred embodiment , modifications within the scope of the invention may be apparent to those skilled in the art . therefore , the true scope of the invention is understood to be determined by the appended claims .
the invention provides apparatus and methods for using the electrical bioimpedance measurements to monitor parameters associated with blood flow in a segment of body tissue . the invention eliminates the effect of respiration from the thoracic impedance as a function of time to provide a signal indicative of pulsatile thoracic impedance changes continuously . the pulsatile thoracic impedance signal is processed to produce signals indicative of the ventricular ejection time and the maximum rate of change of pulsatile thoracic impedance which are used in a microprocessor to calculate the volume of blood pumped per stroke according to an improved systolic upstroke equation .