Pressure signal processing apparatus and method for an automatic blood pressure gauge

Abstract: An automatic blood pressure gauge uses two pulse width modulated solenoid valve (DV1,DV2) to achieve a substantially linear deflation curve. The solenoid valves are controlled by a valve switching signal that has a fixed fundamental frequency. The gauge samples and digitizes the cuff pressure signal. To eliminate artifacts related to the valve switching signal, a microprocessor (216) in the gauge filters the sampled data pressure signal using a third-order Chebychev 2 filter having a zero at the frequency of the valve switching signal. A single timing source (224) used for controlling the microprocessor is also used for developing both the valve switching signal and the sampled data pressure signal, in order to achieve high attenuation of the valve switching signal components in the pressure signal via the Chebychev filter. In addition, the analog to digital converter which develops the digital samples is a sigma-delta type converter (222) which has a Nyquist frequency greater than the frequency of the valve switching signal and which includes a low pass filter that further attenuates the switching signal artifacts relative to the patient's blood pressure pulse signal. Additionally, one of the solenoid valves (DV1) is used when relatively low flow rates are desired from a relatively highly pressurized cuff while the other valve (DV2) is used when relatively high flow rates are desired from a cuff having a relatively low fluid pressure.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to apparatus and a method for automatically measuring the blood pressure of an individual and specifically to apparatus and a method for processing a pressure signal of an automatic blood pressure gauge which pressure signal includes a blood-pressure pulse signal component. 2. Description of the Prior Art A conventional automatic blood pressure gauge includes a resilient inflatable cuff and an electric pump. The pump is controlled by a microprocessor to inflate the cuff with a fluid, such as air, to a preset pressure. In addition, this automatic gauge includes a pressure transducer that measures the instantaneous air pressure levels in the cuff. The pressure signal produced by the transducer is used to determine both the instantaneous air pressure of the cuff and the blood pressure pulse of the individual. This pressure signal is generally band-pass filtered, digitized and processed by the microprocessor to produce values representing the mean, systolic and diastolic blood pressure measurements of the individual. In operation, the cuff is affixed to the upper arm area (or other extremity) of the patient and is then inflated to a pressure greater than the suspected systolic pressure, for example, 150 to 20,000 to 26 665 Pa (200 millimeters of mercury (mmHg)). This pressure level collapses the main artery in the arm, effectively stopping any blood flow to the lower arm. Next, the cuff is deflated slowly and the signal provided by the pressure transducer is monitored to detect cuff pressure variations caused by the patient's blood pressure pulse, which is mechanically coupled to the cuff. In general, the pulse component of the pressure signal has a relatively low amplitude, on the order of one percent of the total signal. A low-level detected blood pressure signal first appears when the cuff pressure is released to a level which allows some blood flow into the collapsed artery. As cuff deflation continues, the blood-pressure pulse signal rises in amplitude as more of the collapsed artery is allowed to expand in response to the pumping action of the heart. At some point, however, the pulse signal reaches a maximum amplitude level and then begins to decrease. This reduction in amplitude occurs as the artery becomes more fully open, the pumped blood flows without significantly expanding the artery, and the degree of mechanical coupling between the cuff and the arm of the patient is reduced. In many automatic blood pressure measuring systems, the systolic and diastolic pressures are determined based on the cuff pressure at which the blood-pressure pulse signal exhibits maximum amplitude. Such a system is described in U.S. Patent No. 4,735,213 entitled DEVICE AND METHOD FOR DETERMINING SYSTOLIC BLOOD PRESSURE. In this system, the diastolic blood pressure is determined as the cuff pressure, after the maximum pulse amplitude has been measured, at which the amplitude of the pulse signal is 70% of its maximum value. Another exemplary system is described in U.S. Patent No. 4,949,710 entitled METHOD OF ARTIFACT REJECTION FOR NONINVASIVE BLOOD-PRESSURE MEASUREMENT BY PREDICTION AND ADJUSTMENT OF BLOOD-PRESSURE DATA. In this system, the systolic and diastolic blood pressure levels are determined as the respective cuff pressures corresponding to the amplitude of the blood-pressure pulse signal being 60% of the maximum value, prior to reaching the maximum value; and 80% of the maximum value, after reaching the maximum value. FIGURE 1a is a plot of the pressure signal versus time for a conventional automatic blood pressure gauge. This exemplary signal is generated by the cuff being quickly inflated to a preset initial pressure, greater than the systolic pressure, linearly deflated to a pressure below the diastolic pressure and then quickly deflated the rest of the way. The blood-pressure pulse signal is shown as a waveform superimposed on the linear deflation portion of the pressure curve. For clarity, the relative amplitude of this pulse signal is exaggerated in FIGURE 1a. FIGURE 1b is a plot of the blood-pressure pulse signal shown in FIGURE 1a, separated from the linearly decreasing pressure signal. FIGURE 1c is a plot of the peak amplitude of the signal shown in FIGURE 1b. As illustrated by FIGURE 1c, the amplitude of the pulse signal increases gradually until a time S, at which the linearly decreasing cuff pressure is the same as the systolic pressure of the patient. The amplitude of the pulse signal then increases at a greater rate from time S to time M at which the maximum amplitude is reached. The blood pressure level corresponding to this maximum pulse amplitude is commonly referred to as the mean arterial pressure (MAP). From this maximum amplitude, the pulse signal decreases rapidly to a time D, at which the cuff pressure is the diastolic pressure. The signal amplitude decreases from the point D until the cuff is entirely deflated. In order to accurately determine the systolic and diastolic pressures of the patient, it is important to ensure a uniform sampling density for the points which define the curve 1b. This is achieved, in part, by ensuring that the deflation curve shown in FIGURE 1a is substantially linear. The deflation of a fixed volume container through a fixed orifice area generates a pressure deflation curve which approximates a decaying exponential. One method to obtain a linear deflation rate is to use a valve having a controllable orifice area, for example, a needle valve which can be mechanically actuated to change its orifice area. Valves of this type, however, can be difficult to control. The size of the valve orifice may be controlled using a closed loop control system, which changes the orifice area of the valve in a manner which holds the first derivative of the measured cuff pressure value substantially constant. To minimize errors and to ensure a short settling time from pressure transients caused, for example, by patient motion, it is desirable to use a control loop having a relatively short time constant. This type of system, however, may affect the measurement of the blood pressure pulses. It may interpret the blood-pressure pulse signal as a transient pressure change and attempt to compensate for it in order to maintain a constant deflation rate. This action may undesirably reduce the amplitude of some of the pulses, thus changing the shape of the pulse amplitude curve shown in FIGURE 1c. Another problem may arise when the cuff deflation system uses switched solenoid valves to obtain a desired deflation rate. As these valves are switched on and off, a rippling pressure signal is generated which may be detected by the pressure transducer of the blood pressure gauge. In order to be cost effective, the valve switching frequency should be kept as low as possible. When this is done, however, signal components resulting from the switching signal may contaminate the blood-pressure pulse signal. These components are usually removed using a low-pass filter having a cut-off frequency between the highest pulse frequency which may be of interest and the switching frequency. The frequency spectrum of the blood-pressure pulse signal, however, may have significant components which occupy frequency bands between the cut-off frequency of the low-pass filter and the valve switching frequency. These components of the pulse signal may be undesirably attenuated by the low-pass filter which removes the switching signal, distorting the pulse waveform. This distortion may result in erroneous pulse amplitude measurements. In conventional systems, this problem is solved either by using some other type of deflation valve or by selecting a valve switching frequency that is well above the band of frequencies which may be occupied by the blood-pressure pulse signal. This selection may undesirably reduce the range of effective flow rates that the valve may provide or it may increase the cost of the device if a high performance solenoid valve is needed to operate at the desired switching frequency. A related problem occurs when the pressure signal is a sampled data signal. Even when the frequency of the valve switching signal is much greater than the frequency of the blood-pressure pulse signal, the sampling system may introduce an artifact of the switching signal into the detected pressure signal. This artifact occurs when the switching signal is translated into a frequency in the same band of frequencies as the blood-pressure pulse signal due to aliasing distortion of the sampled data signal. SUMMARY OF THE INVENTION The present invention is embodied in an automatic blood pressure measurement device in which a pressure signal from a deflating cuff is sampled and processed to detect a pulse signal. This device uses a switched solenoid valve to achieve a desired cuff deflation characteristic curve. This valve has a relatively low frequency switching signal. The system includes a notch filter, set to substantially attenuate the valve switching frequency, which removes signal components related to the valve switching signal from the pulse signal. A single timing source provides both a clock signal used for sampling the cuff pressure signal and the low frequency valve switching signal. According to another aspect of the invention, two deflation valves, having respectively different flow rates are used to obtain a relatively large range of effective deflation flow rates. According to yet another aspect of the invention, the pressure signal is sampled and digitized using a sigma-delta analog to digital converter and the valve switching frequency is selected to be within the band of frequencies digitized by the analog to digital converter to substantially eliminate any aliased components of the valve switching signal in the pulse signal. BRIEF DESCRIPTION OF THE DRAWINGS FIGURES 1a through 1c, labeled prior art, are graphs, showing measured pressure variations versus time, which are useful for describing the environment in which the present invention operates. FIGURE 2 is a block diagram of an automatic blood pressure measurement system in accordance with the present invention. FIGURES 3 and 3a are flow-chart diagrams which are useful for describing the operation of the microprocessor 216. FIGURE 4 is a block diagram of circuitry suitable for use as the duty cycle modulator shown in FIGURE 2. FIGURE 5 is a flow-chart diagram which illustrates how the microprocessor shown in FIGURE 2 controls the duty cycle modulator shown in FIGURE 4. FIGURE 6 is a flow-chart diagram which illustrates the process by which the microprocessor shown in FIGURE 2 extracts the blood pressure pulse signal from the cuff pressure signal. FIGURE 6a is a graph of amplitude versus frequency which is useful for describing the process illustrated by FIGURE 6. FIGURES 7a through 7d are graphs of amplitude versus time which are useful for describing the process illustrated by FIGURE 6. DETAILED DESCRIPTION OF THE INVENTION FIGURE 2 is a block diagram of an exemplary automatic blood pressure gauge according to the present invention. This gauge includes a conventional blood pressure cuff 210 which may be inflated by an electric pump 212 using an air channel 211. The pump motor is turned on and off by a motor controller 214 which is responsive to signals provided by a microprocessor 216. An exemplary pump for use in this embodiment of invention is a diaphragm type driven by a DC motor. The cuff is deflated using two controlled solenoid valves, DV1 and DV2. When open, the exemplary valve DV1 has a flow rate of 570 standard milliliters per minute (Std ml/min) at 22,665 Pa (170 mmHg) differential pressure, and the exemplary valve DV2 has a flow rate of 1,341 Std ml/min at 2,666 Pa (20 mmHg) differential pressure. These valves may be opened and closed in 1.4 milliseconds (ms) and 6 ms, respectively. In this embodiment of the invention, only one of the valves is open at any given time during normal operation. By controlling the percentage of time within each cycle that the valve is opened and closed, the time-aperture of the valve can be effectively controlled. This time-aperture determines the average rate of airflow through the valve. The selection of the flow rates of the valves DV1 and DV2 allows the microprocessor 216 to control the pressure in the cuff to a relatively high degree of accuracy. The microprocessor 216 controls the valves DV1 and DV2 using a duty cycle modulator 230. The modulator 230, which is described in greater detail below with reference to FIGURE 4, produces a 20.35 Hz signal which controls a selected one of the valves DV1 and DV2. The duty cycle of this signal is controlled to determine the effective aperture of the selected valve, and thus, the rate at which the cuff 210 is deflated. The microprocessor 216 monitors the air pressure in the cuff using a conventional pressure transducer 218 which is coupled to the air channel 211 via a tube 217. In the exemplary embodiment of the invention, the pressure transducer is of the conventional semiconductor strain gauge type. The signal produced by the transducer 218 is amplified by a low-noise amplifier instrument quality amplifier 220 which produces a signal that is applied to an analog to digital converter (ADC) 222. In this embodiment of the invention, the ADC 222 is a 16-bit sigma-delta type analog to digital converter. The ADC 222 produces samples at a rate of approximately 50 Hz. A frequency divider 226 is coupled to receive a clock signal CLK provided by a resonant crystal controlled clock signal generator 224. This signal is divided in frequency to produce a suitable clock signal for the ADC 222. The exemplary ADC 222 includes a six-pole Gaussian low-pass filter (not shown) which has a pass-band from 0 to 6.5 Hz. Since the effective sampling rate of the filter is 50 Hz, there is no aliasing of the 20.35 Hz valve switching signal into the band of frequencies (0.5 Hz to 6 Hz) occupied by the blood-pressure pulse signal. Instead, the low-pass filter reduces the amplitude of the valve switching signal component of the pressure signal by more than 20 dB. The sampled data pressure signal provided by the ADC 222 is monitored by the microprocessor 216 to stop the pump 212, when the desired initial cuff pressure has been obtained, to control the flow through the deflation valves DV1 and DV2, and to determine the mean, systolic and diastolic blood pressure measurements for the individual. The blood pressure measurements are presented on a display device 231. To produce these values, the microprocessor 216 operates under the control of a program stored in the memory 228. The memory 228 also contains cells which may be used for storing temporary data values. In the exemplary embodiment of the invention, the program storage portion of the memory 228 is a read-only memory (ROM) while the data storage portion is a random-access memory (RAM). The microprocessor 216 captures the samples produced by the ADC 222 at the 50 Hz sample rate. These samples are then processed by the microprocessor 216 to further filter any signal components related to the valve switching signal and to extract the pulse signal as described below with reference to the flow-chart diagram of FIGURE 6. The samples provided by the ADC 222 are also processed by the microprocessor 216 in groups of 45 to obtain a noise-reduced cuff pressure signal and its first derivative. The first derivative signal represents the actual rate of change of the cuff pressure. These signals, which have an effective sampling rate of 1.11 Hz, are used to control the deflation valves DV1 and DV2 according to the process shown in FIGURE 3. In the first step of this process, step 310, the initial cuff volume, V, is calculated, one of the deflation valves DV1 or DV2 is selected, and an initial flow setting for the selected valve is calculated. The process used to determine the initial cuff volume is shown in FIGURE 3a. In FIGURE 3a at step 350, the microprocessor 216 sets a variable P to the current pressure reading obtained from the pressure transducer 218, and a variable I to the current inflation rate. The inflation rate is a differential pressure value which is obtained by numerically differentiating the pressure signal provided by the transducer 218 over the last 45 sample values. New values of P and I are available at 900 ms intervals as set forth below. At step 352, the value stored in the variable P is compared to a target final pressure value. If the value in P is less than this target value, then step 350 is executed to continue inflating the cuff, and to continue measuring cuff pressure and inflation rate. If, at step 352, the value in the variable P is greater than or equal to the target final pressure value, control is transferred to step 356. In this step, the microprocessor 216 signals the motor controller 214 to stop the pump 212 and, thus, stop inflating the cuff. In addition, the microprocessor 216 uses the current pressure value, as stored in the variable P, as an index into a table which provides the current pump flow rate, PF. At step 358, the values PF and I are substituted into the equation (1) to calculate the initial volume of the cuff. V = (PF * 760) / I In this equation PF is the pump flow rate when the cuff pressure has reached its target final pressure value, I is the current inflation rate, and the value 760 represents the pressure of the fluid that is being pumped into the cuff (the flow is specified under standard conditions - 1 atmosphere = 101,325 Pa (760 mmHg)). The inflation rate is obtained, as set forth above, from the differential signal value generated by the last group of 45 sample values. The value PF is determined from a table of pump flow versus cuff back pressure (BP). An exemplary table is given as Table 1. In this table, the pump flow values are divided by a factor of 50. Thus, the table values are PF/50. PF/50 BP PF/50 BP PF/50 BP PF/50 BP 58 0 48 67 41 133 32 200 57 5 48 72 40 138 32 205 56 10 47 77 39 143 31 210 55 15 47 82 38 148 31 215 54 21 46 87 37 154 31 220 53 26 46 92 37 159 31 225 52 31 45 97 36 164 30 230 51 36 45 102 36 167 30 236 50 41 44 107 35 174 30 241 50 46 44 113 35 179 30 246 49 51 43 118 34 184 29 251 49 56 43 122 34 189 29 256 48 61 42 128 33 195 29 261 This table was generated experimentally. To generate a table for a different pump than was used in the exemplary embodiment, the pump is connected to a pressure gauge and to a controllable flow restrictor, such as a needle valve. The output of the needle valve is connected to a flow meter. The flow restrictor is adjusted to generate a given back pressure measurement on the pressure gauge. The flow produced by the pump at that back pressure is measured on the flow meter and recorded. These steps are repeated for all back pressure values to which the pump may be subject in normal operation. To determine the pump flow for a given measured cuff pressure, the table is indexed by the cuff pressure. For pressure values not represented in the table the value of the next lower entry is used. As an alternative to using this table, a single value may be maintained which represents the nominal flow rate of the pump in a range of back pressures likely to be encountered when the cuff volume is to be determined. This value may be obtained experimentally, for example, as the average flow rate of the pump at back-pressures between 20,000 to 26,665 Pa (150 mmHg and 200 mmHg) may be appropriate for adult cuffs. Once the initial cuff volume has been determined, the initial value of the valve control variable, F A , for the deflation valve DV1 or DV2 is calculated using equations (2) and (3). The value of F A is a combination of a calculated predictive valve control variable, F p , and a feedback term, F fb . F p = V * R * K p F A = F p + F fb (0) In equation (2), R is the desired deflation rate, for example, 800 Pa/s (6 mmHg/s) and K p is an experimentally determined factor which produces the desired flow rate for different cuff pressures. The value of this term depends on whether an adult cuff or a neonatal cuff is being used. Exemplary values of K p for different adult cuff pressure values (ACP) are given below in Table 2. The values in this table are suitable for use with most standard adult cuffs (e.g. arm to thigh). A different set of values (not shown) apply for neonatal cuffs. The cuff pressure in the table is in units of Pa/133.322 (mmHg). K p ACP K p ACP K p ACP K p ACP 500 0 126 67 64 133 51 200 500 5 118 72 62 138 51 205 500 10 108 77 60 143 50 210 500 15 100 82 59 148 49 215 450 21 92 87 58 154 49 220 350 26 84 92 57 159 48 225 300 31 76 97 56 164 48 230 250 36 73 102 55 167 47 236 210 41 69 107 55 174 47 241 180 46 68 113 54 179 46 246 160 51 67 118 53 184 45 251 146 56 66 122 53 189 44 256 136 61 65 128 52 195 44 261 In equation 3, the term F fb (0) is the initial value of a feedback correction term, F fb (t). The algorithm for determining the value of this term while the cuff is being deflated is set forth below. The value of F fb (0) is set to -.25 * F p for adult cuffs. This term compensates for an initial drop in cuff pressure at the end of the inflation cycle which is due to the release of thermal energy from the compressed air in the cuff. The value F A defines the actual valve setting for DV1. If this value is beyond the range of DV1, the value of F A is scaled for the use of DV2. This value is translated into duty cycles for one of the two valves DV1 and DV2 as described below with reference to FIGURE 4. The next step in the flow-chart diagram shown in FIGURE 3, step 314 is to wait until the start of the next update period. In the exemplary embodiment of the invention, the setting for the valve DV1 or DV2 is updated at 900 ms intervals. During this time, the microprocessor is determining the current cuff pressure value from the 45 most recent pressure sample values provided by the ADC 222. As described above, these sample values are processed to eliminate spurious noise spikes. The result of this processing is a noise-reduced cuff pressure signal. At step 316, this cuff pressure signal is numerically differentiated over the 45 sample interval to produce a signal representing the actual deflation rate. This step may, for example, subtract each pressure sample value from the preceding sample value and average the results to provide a sampled data deflation rate signal for the interval in which the 45 pressure samples were taken. Also at step 316, the predictive valve setting, F p , is determined by applying equation (2) using the most recent cuff pressure sample as the index into Table 2. The difference, R e , between the actual deflation rate, calculated at step 316, and the desired deflation rate is calculated at step 318. In this embodiment of the invention, the desired deflation rate is 800 Pa/s (6 mmHg/s). At step 320, the microprocessor 216 uses the R e value calculated in step 318 and the equation (4) to compute a feedback term, F fb (t), for the current time interval, t. F fb (t) = (V * R e * K G ) + F fb (t-1) In the equation (4), K G is a feedback gain factor. This factor is experimentally determined to produce a desired loop time constant for the feedback loop. In the exemplary embodiment of the invention, this factor has a value of 0.31 The initial volume factor, V, in the first term of the equation normalizes the transient response of the feedback controller for different cuff volumes. Because of this factor, cuffs having relatively large initial volumes are allowed to have relatively large feedback correction terms while cuffs having relatively small initial volumes are only allowed to have small feedback correction terms. As described above, the initial cuff volume depends on a number of factors such as the size of the patient's arm and how the cuff is wrapped around the arm. The feedback term F fb (t), as defined by equation (4), is an integrated value since the first term in equation (4), ( V * R e * K G ), is summed with the value of the feedback term from the previous time period, F fb (t-1), to obtain the feedback term for the current time period, F fb (t). In addition, the amplitude of the feedback term is limited to be between -.75 F p and F p as shown in steps 322 through 326 of FIGURE 3. The result of these calculations is a final feedback term, F fb (t)'. This term is summed with the predictive valve setting F p as shown in equation (5) to obtain a new actual valve setting F A . F A = F p + F fb (t)' The feedback term is limited to prevent large correction terms from being applied when there is a large transient change in the actual cuff deflation rate. If, for example, the cuff volume is changed because the patient flexed his or her arm, there may be, for a short time interval, a relatively large difference between the actual deflation rate and the desired deflation rate. If the system corrects for this difference in a single step, there would be a continuing error when the flexed muscle is relaxed. This continuing error is caused by the integral nature of the feedback term. In the exemplary embodiment of the invention, the magnitude of these continuing errors is reduced by limiting the amount by which the feedback term may change. The feedback term F fb (t)' tends to match the actual deflation rate to the desired rate. It compensates for volume changes in the cuff during the deflation process and for valve and pump flow inaccuracies. Since the gain of the feedback loop is relatively low and since the feedback correction term is updated only once every 900 ms, it does not tend to interfere with the blood-pressure pulse signals. This is ensured because the pulse signal frequencies of, for example, .5 to 6 Hz, are outside of the range of frequencies that can be tracked by the feedback loop. If the actual valve setting calculated in step 328 is different from the current valve setting, step 330 changes to the new valve setting gradually over the next 900 ms sample interval. This step is described in greater detail below, with reference to FIGURE 5. Following step 330, the program branches back to step 316 to begin the calculations that determine the valve settings for the next interval. FIGURE 4 shows exemplary circuitry which may be used as the duty cycle modulator 230 shown in FIGURE 2. In this circuitry, a new 12-bit data value, representing the most recently calculated value for F A , is loaded into a register 410 by the microprocessor 216, to change the time-aperture of the currently selected valve DV1 or DV2. As described above, these valves are solenoid valves which may be opened or closed, responsive to a control signal. The valve DV1 or DV2 is open when its control signal is logic-high and closed when its control signal is logic-low. The valves DV1 and DV2 are selected so that, when both valves are open, the flow through DV2 is approximately eight times the flow through DV1. In this embodiment of the invention, valve DV1 is selected when low valve flow rates are desired and DV2 is selected to achieve higher valve flow rates. The control signal for the selected valve is provided by a comparator 412. This comparator compares the value held in the register 410 with a value provided by a 12-bit counter 414. The clock input signal to the counter is the 8 MHz signal CLK divided, in frequency, by a factor of 96 by a frequency divider 416 to produce a clock signal having a frequency of 83.33 KHz. Responsive to this clock signal, the counter cycles through all of its 4,096 values 20.35 times per second. The comparator 412 is configured so that it produces a logic-high output signal when the value provided by the register 410 is less than or equal to the value provided by the counter 414 and produces a logic-low output signal otherwise. This signal is provided to a demultiplexer 418, which, responsive to a signal DC provided by the microprocessor 216, routes the control signal to either DV1 or DV2. When the process shown in FIGURE 3 selects and adjusts the valve DV1 or DV2 at step 330, it does not make the entire adjustment in one step. Instead, it adjusts the valves gradually over the next 900 ms interval. This is done to prevent artifact pulses which may occur due to sudden changes in flow. FIGURE 5 is a flow-chart diagram which illustrates the method by which the microprocessor 216 controls the valves DV1 and DV2 to in response to a new valve setting calculated at step 328. In the exemplary embodiment of the invention, new values for the valve setting are calculated at 900 ms intervals. The valve settings, however, are each in terms of a duty cycle of a square wave having a period of 49 ms (i.e. 1/20.35 Hz) and the microprocessor can update the value held in the register 410, shown in FIGURE 4, at the end of each 49 ms interval. Thus, during each 900 ms interval, the settings for the selected deflation valve DV1 or DV2 can be adjusted as many as 18 times. The first step in the process which adjusts the settings for these valves, step 506, selects which of the two valves, DV1 or DV2 is to be adjusted. When low flow settings are required to provide the desired deflation rate, such as when a small cuff is deflated at high pressure, valve DV1 is selected. When higher flow settings are required, such as when larger cuffs are deflated at lower pressures, valve DV2 is selected. When valve DV1 is selected and the value F A applied to the register 410 is greater than 3,932, step 508 is executed. In this step, the microprocessor 216 changes the signal DC, which is applied to the demultiplexer 418 of FIGURE 4, to select valve DV2. At the same time, the microprocessor 216 multiplies the current valve setting value (OLD F A ) and the new valve setting value (F A ) by 0.128. It then stores the new value for OLD F A into the register 410. These steps cause an immediate switch to valve DV2 from valve DV1. Step 510 is executed after step 508, or after step 506 if the comparison in 506 is negative. Step 510 subtracts the new valve setting value, F A , from the current valve setting value, OLD F A , to obtain a value, DS, which indicates the amount by which the current setting must be changed to reach the new setting. Step 512 determines if the absolute value of DS is less than or equal to 8. If so, then at step 514, the microprocessor 216 invokes a procedure which changes the 12 bit number stored in the register 410 of FIGURE 4 in unit steps at intervals corresponding to alternate ones of the 20.35 Hz pulses until the desired setting value is reached. If, at step 512, the absolute value of DS is greater than 8, then step 516 is executed. This step determines if DS is less than or equal to 16. If it is, then step 518 is executed. This step invokes a procedure which changes the 12-bit value stored in the register 410 in unit steps for each pulse of the 20.35 Hz signal until the desired setting value is reached. If, at step 516, the absolute value of DS is greater than 16, step 520 is executed. This step divides the value DS by 16 to produce a value IDS. Step 520 then invokes a procedure which changes the value in the register 410 by IDS for each pulse of the 20.35 Hz signal until the desired valve setting is reached. Using the scheme outlined above, the valves DV1 and DV2 provide a wider range of flow rates than would be provided by a single valve. Since the valve setting for DV1 is multiplied by 0.128 before it is applied to the duty cycle modulator as a 12-bit value, the ratio between the smallest and largest possible valve settings is approximately 32,000:1. This wide range of valve settings allows the system to control the linearity of the deflation curve to a high degree of accuracy. FIGURE 6 is a flow-chart diagram which illustrates the process by which the microprocessor 216 extracts the pulse signal from the digitized pressure signal provided by the ADC 222. This program processes the sampled-data pressure signal to locate blood pressure pulse signals; extracts these pulse signals and uses the extracted signals to determine mean, systolic and diastolic blood pressure values. At step 610, the microprocessor 216 filters the stored sample values to remove artifacts related to the switching of the solenoid valves DV1 and DV2. The exemplary filter is a third-order Chebychev-2 filter having a notch-type frequency response characteristic which shown in FIGURE 6a. This filter may be readily implemented as a Finite Impulse Response (FIR) or an Infinite Impulse Response (IIR) filter. Techniques for implementing an FIR filter having this frequency response characteristic are described in a paper by J.H. McClellan et al. entitled "A Computer Program For Designing Optimum FIR Linear Phase Digital Filters" IEEE Trans. on Audio and Electroacoustics, Vol. AU-21, NO. 6, December 1973, which is hereby incorporated by reference. As shown in FIGURE 6a, this filter has a zero at 20.35 Hz, the switching frequency of the solenoid valves DV1 and DV2. This filter attenuates artifacts in the pressure signal at the frequency of the valve switching signal by 90 dB relative to the signals in the .5 to 6 Hz band of frequencies, occupied by the blood-pressure pulse signals. In accordance with an aspect of the invention, this filter process of microprocessor 216 achieves such a high degree of attenuation of the switching signal due to the fact that both microprocessor 216 and the valve control signals (provided by the FIGURE 4 arrangement) have a common timing signal source, i.e., clock generator 224 of FIGURE 2. Via step 612, the microprocessor 216 continuously stores the notch filtered sampled pressure signal data values in a designated buffer area in the memory 228. In the exemplary embodiment of the invention, the steps following step 612 implement a filter which operates on a moving window of N sample values to produce a new sequence of sample values. In step 614 and 616, the microprocessor 216 filters the stored sample values to produce an auxiliary sampled data signal which has a one-to-one correspondence to the stored notch-filtered sample values. This auxiliary signal indicates the starting sample numbers of stored notch-filtered pressure sample values which include blood-pressure pulse signal components. In the exemplary embodiment of the invention, steps 614 and 616 process the notch-filtered sample values through a two-stage order-statistic filter. An order-statistic filter, also known as a rank-order filter, operates on a moving window of sampled data. It effectively sorts the samples in the window by their values and provides as its output, one of the sorted samples. In a minimum order-statistic filter, the smallest of the sorted sample values in the moving window is used as the output signal of the filter. In a maximum filter it is the largest sample values and in a median filter it is the median sample values. See, for example, the article entitled "FIR - Median Hybrid Filters" by Heinonen et al. published in June 1987 in the IEEE Transactions on Acoustics, Speech, and Signal Processing, Vol. ASSP-35, No. 6 for more detailed information about order-statistic filtering. The particular filter used in the exemplary embodiment of the invention is a cascade combination of a minimum filter and a median filter. The minimum filter (step 614) operates on 21 of the stored, notch-filtered sample values at a time (i.e. N equals 21). When 21 values have been stored, the microprocessor 216, at step 614 averages the ten sample values preceding the middle value of the 21 values (i.e., the intermediate value) and the ten sample values following the middle value to generate two averaged sample values. It then subtracts the smallest of the middle value and the two averaged values from the middle sample value and stores the result as a minimum filtered signal. The filter implemented by step 614 continues to filter 21 successive sample values in a moving window as each new notch-filtered value is provided by step 610, subtracting each determined minimum value from the next new middle (intermediate) value of the stored notch-filtered pressure signal values. The exemplary median filter processes the minimum filtered sample values in a 41-sample moving window. It averages the first 20 samples in the window and the last 20 samples in the window to generate two averaged values. It then subtracts the median of the two averaged values and the middle value from the middle value of the stored minimum filtered values. This median filtering step proceeds as new minimum sample values are generated. At each successive step, the median of the three values is subtracted from the next new middle value of the stored minimum sample values. The process performed by steps 614 and 616 is illustrated in FIGURES 7a and 7b. FIGURE 7a is a graphical plot of an exemplary notch-filtered pressure signal and FIGURE 7b is a graphical plot of the output signal produced by step 616 of the program shown in FIGURE 6 after samples including three blood pressure pulses have been processed. As shown in FIGURE 7b, the sequence of samples produced by the filtering steps 614 and 616 is a signal (referred to herein as an auxiliary signal) having negative peaks at sample numbers corresponding to the starting point of each blood pressure pulse in the stored notch-filtered pressure signal (points A, B, C in Fig. 7a). After step 616, the microprocessor 216 processes the notch-filtered samples to extract the blood-pressure pulse signal information. The processing performed by the steps 618 to 630 in FIGURE 6 produces samples of the blood-pressure pulse signal, such as those shown in FIGURE 7d from samples representing a notch-filtered pressure signal, such as those shown in FIGURE 7a. It calculates values of a decreasing ramp signal representing the pressure signal without the blood-pressure pulse signal component (Figure 7c) and then subtracts these calculated values from the notch filtered samples to obtain values representing the blood-pressure pulse signal without the pressure ramp component. The calculated ramp function is a segmented function corresponding to a set of straight lines drawn between the starting points of successive ones of the blood pressure pulses shown in FIGURE 1a. The first step in this process, step 618, examines the successive current samples of the auxiliary signal (Fig. 7b) to find a local minimum value which exceeds a threshold value. This value (e.g. the value at point A' of FIGURE 7b) indicates that the corresponding notch-filtered sample value (point A of FIGURE 7a) is at the start of a blood-pressure pulse signal. If a pulse is not detected, the process returns to step 610. If a pulse is detected, step 620 determines if this is the first pulse detected by the program. If so, the corresponding notch-filtered pressure sample value is stored, at step 622, as the first sample of the ramp function and as a variable called RAMP START. The process then returns to step 610. When a subsequent appropriate local minimum is found in the auxiliary signal, step 620 notes that it is not the first detected pulse, and the process jumps to step 624. At step 624, the notch-filtered pressure sample (e.g. point B in FIGURE 7a) that corresponds to (e.g., has the same sample number as) the local minimum auxiliary sample (B' of FIGURE 7b), is assigned to a variable called RAMP END. Points for the ramp function are then interpolated between the RAMP START and RAMP END variables (e.g. between points A'' and B'' of FIGURE 7c). At step 626, microprocessor 216 subtracts the newly calculated points on the ramp function (FIGURE 7c) from the corresponding notch-filtered pressure samples (FIGURE 7a). The resulting sample values (FIGURE 7d) represent the blood pressure pulse signal components of the pressure signal to the substantial exclusion of all other components of the pressure signal. The samples calculated in this step contain one blood pressure pulse signal (i.e. the pulse starting at point A of FIGURE 7a). Finally, at the end of step 626, the variable RAMP START is set to the value in RAMP END. At step 628, microprocessor 216 examines the blood pressure pulse signal components (FIGURE 7d) to find the sample number occurring between two adjacent blood pressure pulse components which have the maximum amplitude value (i.e., .MAX in segment A'''-B''' of FIGURE 7d). Next, microprocessor 216 identifies the sample number having a local minimum amplitude value which occurs in the blood pressure sample signal just prior to the sample number found having the maximum amplitude value (i.e., .MIN of FIGURE 7d). An ordered pair of data is then formed for this blood pressure component information, comprising, the blood pressure pulse peak-to-peak amplitude (i.e., MAX-MIN) and the cuff pressure for that blood pressure pulse component. The cuff pressure is the value of the notch-filtered pressure signal (FIGURE 7a) at that sample number which corresponds to the sample number of the MIN amplitude value sample of the blood pressure pulse signal (FIGURE 7d). Step 630 determines if more pulse values are needed by performing a variety of waveshape analysis of the ordered pair data taken so far. If more pulse data is needed, control is returned to step 610 to get the next notch-filtered pressure sample. Otherwise, at step 632, the microprocessor 216 finds a smooth function which defines the recorded peak values in terms of their corresponding cuff pressure values. This function defines a curve which may be identical to that shown in FIGURE 1c except that the horizontal axis represents decreasing cuff pressure rather than increasing time. In the exemplary embodiment of the invention, two fourth-order curves are fit to the sample points. One fourth-order curve is fit to the points preceding the one having the largest amplitude (i.e. the MAP point) and the other is fit to the points following the MAP point. The final step in the process illustrated in FIGURE 6 finds the systolic and diastolic points using the curve generated at step 632. In accordance with the teachings of Professor Erlanger (see e.g., Studies in Blood Pressure Estimation by Indirect Methods , Am. J. Physiol. 40:82-125, 1916 and Am. J. Physiol. 55:84-158, 1921) and others well known in the field, the systolic and diastolic pressures can be determined as functions of the pulse amplitude corresponding to the mean arterial pressure (MAP). As set forth above, the MAP is the cuff pressure at which the blood pressure pulse signal exhibits its peak amplitude (i.e. the peak of the curve generated at step 632). Thus, there has been shown and described a novel blood pressure measurement apparatus which fulfills all the objects and advantages thereof. Many changes, modifications, variations and other uses and applications of the subject invention will, however, become apparent to those skilled in the art after considering this specification and its accompanying drawings which disclose preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow.

Claims: Apparatus suitable for use in an automatic blood pressure gauge having a fluid pressurized cuff (210), comprising: valve means (DVI, DV2), responsive to a periodic valve control signal having a predetermined nominal frequency, for controllably releasing the fluid from the pressurized cuff in accordance with a predetermined pressure reduction function; and pressure measuring means (218), coupled to the pressurized cuff (210), for measuring the instantaneous pressure level of the fluid in the cuff, as the fluid is released from the cuff, to provide a pressure signal which includes a blood pressure pulse signal component; CHARACTERIZED BY : a single timing source (224) for providing both a sampling clock signal having a predetermined frequency and said periodic valve control signal; means (222) for sampling said pressure signal at instants determined by the sampling clock signal to generate a sampled data pressure signal; filtering means (610), having a notch filter frequency response characteristic, for filtering said sampled data pressure signal to selectively attenuate signals equal in frequency to said periodic valve control signal to produce a filtered sampled data pressure signal; and means (216) for processing the filtered sampled data pressure signal to separate the blood pressure pulse signal component therefrom. The apparatus of claim 1 CHARACTERIZED IN THAT: the frequency of said sampling clock signal is greater than twice the nominal frequency of the periodic valve control signal, and the sampled data pressure signal is substantially free of any aliasing components related to the periodic valve control signal. The Apparatus of Claim 1, CHARACTERIZED IN THAT: said means for sampling said pressure signal includes a sigma-delta analog to digital converter (222) having a low-pass filter characteristic that attenuates signals at the nominal frequency of the periodic valve control signal relative to signals in the range of frequencies occupied by the blood pressure pulse signal. The apparatus of claim 1, CHARACTERIZED IN THAT: the filtering means (610) and the periodic valve control signal are both realized using a program that runs on a microprocessor (216), which microprocessor is responsive to said single timing source (224) and to said sampled data pressure signal. The apparatus of Claim 4, CHARACTERIZED IN THAT: the frequency of the sampling clock signal is more than twice the nominal frequency of the periodic valve control signal; and said filtering means (610), includes a third-order Chebychev-2 filter having a frequency response characteristic which has a zero at the nominal frequency of the periodic valve control signal, implemented as a program that runs on the microprocessor (216), for filtering said sampled data pressure signal to selectively attenuate signals equal in frequency to said periodic valve control signal to produce the filtered sampled data pressure signal. The apparatus of claim 1 CHARACTERIZED IN THAT: the valve means (DV1, DV2) includes first and second solenoid valves, wherein the first solenoid valve (DV1) exhibits a rate of flow less than said second solenoid valve (DV2). The apparatus of claim 6 CHARACTERIZED IN THAT: said valve means further includes means (230) for selectively applying the periodic valve control signal to one of said first and second solenoid valves to achieve a rate of flow of fluid from the cuff which produces a reduction in the pressure of the cuff consistent with the predetermined pressure reduction function. The apparatus of claim 7 CHARACTERIZED IN THAT: said means (230) for selectively applying applies the periodic valve control signal to the first valve (DV1) when a relatively low deflation flow is required and applies the periodic valve control signal to the second valve (DV2) when a relatively high deflation flow is required. A method of obtaining a blood pressure pulse signal from an automatic blood pressure gauge having a fluid pressurized cuff, comprising the steps of: controllably releasing the fluid from the pressurized cuff in accordance with a predetermined pressure reduction function in response to a periodic valve control signal having a predetermined nominal frequency, and measuring the instantaneous pressure level of the fluid in the cuff, as the fluid is released from the cuff, to provide a pressure signal which includes a blood pressure pulse signal component, CHARACTERIZED BY: sampling said pressure signal at instants determined by a sampling clock signal to generate a sampled data pressure signal; said filtering of the pressure signal and developing said periodic valve control signal being in response to a common clock signal source; filtering said sampled data pressure signal to selectively attenuate signals in a range of frequencies surrounding the frequency of said periodic valve control signal, relative to frequencies above and below said range of frequencies, to produce a filtered sampled data pressure signal using a filter having a notch filter frequency characteristic; and processing the filtered sampled data pressure signal to separate the blood pressure pulse signal component therefrom. The method of claim 9, CHARACTERIZED IN THAT: the frequency of the sampling clock signal is greater than twice the nominal frequency of the periodic valve control signal, and the filtered sampled data pressure signal is substantially free of any aliasing components related to the periodic valve control signal.

Inventors: NEWELL SCOTT W, NEWELL, SCOTT W.
Applicants: SIEMENS MEDICAL SYSTEMS INC, SIEMENS MEDICAL SYSTEMS, INC.
Application year: 1993
Main classification: A61B5
IPC codes: A61B5