Abstract:
An invasive blood pressure monitor assesses likelihood of coloration of the pressure waveform by the fluid-filled catheter prior to imposing a correction on this pressure waveform. This detection may be made by comparing two alternate signal processing paths of the pressure waveform, one of which is intended to correct for coloration and applying the coloration correction only if those processing paths yield significantly different output values.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     --  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     --  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates generally to invasive blood pressure measuring equipment and in particular to a simple and robust circuit for detecting and correcting measurement errors caused by catheter line resonance.  
         [0004]     High accuracy blood pressure measurements may use a catheter communicating directly between a patient&#39;s artery and an external pressure transducer. Normally the catheter is wholly or partially filled with saline solution to provide a continuous liquid path between the artery and pressure transducer. The mass of fluid in the catheter and the inherent elasticity of the catheter can introduce a distortion to the pressure readings obtained. Of particular concern is resonance which may accentuate harmonics of the cardiac rhythm to undesirably alter systolic or diastolic pressure readings.  
         [0005]     A number of techniques have been used to compensate for this distortion. Mechanical damping may be introduced into the fluid circuit, for example, in the form of a restriction in the catheter to attenuate any resonance. Analogously, low-pass filtering of the pressure signal, using an electrical circuit receiving a signal from the pressure transducer, may be used to attenuate these harmonics. If the physical characteristics of the catheter are fully known, the catheter&#39;s effect on the blood pressure signal may be modeled and sophisticated inverse transform techniques may be used to eliminate or reduce this distortion.  
         [0006]     Typically the monitoring instrument that receives the electrical signal from the pressure transducer must work with a variety of different catheters having different characteristics and lengths, the latter possibly altered by the physician to meet the demands of the situation. This variation in catheters places a practical limit on the ability to employ reverse modeling algorithms. Catheter indifferent techniques, such as those which attenuate the harmonics either mechanically or electrically, introduce their own “coloring” to the blood pressure signal and particularly in the case where the harmonics are low, can make blood pressure data less accurate than it would have been without such compensation.  
         [0007]     What is needed is a simple and robust method of correcting for distortion of blood pressure readings by the catheter that works with a variety of catheters and that does not unnecessarily degrade the blood pressure signal.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     The present invention provides a simple circuit that suppresses resonance-induced harmonics only after determining that there is a probability of resonance induced distortion. In this way, when low or non-resonant catheter lines are employed, the blood pressure waveform may be detected directly and the possible distortions of any correction system avoided.  
         [0009]     Specifically then, the present invention provides a blood pressure monitor for use with an invasive catheter system having a first and second signal processor receiving an electrical pressure signal from the catheter and providing different degrees of attenuation of resonance components of the electrical pressure signal. A comparison circuit monitors a divergence between the outputs of the first and second signal processors and based on that divergence selects one output as a corrected pressure signal. A display outputs at least one blood pressure measurement to an operator based on the corrected pressure output.  
         [0010]     Thus is it one object of at least one embodiment of the invention to tie any correction of blood pressure to an assessment of whether significant correction is required. In this way unnecessary distortion induced by the correction process itself is minimized.  
         [0011]     The signal processors may be low-pass filters with a first filter having a lower cutoff frequency than a second filter.  
         [0012]     Thus is it one object of at least one embodiment of the invention to provide a simple and well-characterized correction process.  
         [0013]     The first filter may have a cutoff frequency between a second and third harmonic of a standard blood pressure signal.  
         [0014]     It is thus another object of at least one embodiment of the invention to provide a correction system that leaves intact the major spectral components of the blood pressure signal.  
         [0015]     The blood pressure monitor may include a pulse rate monitor providing a pulse rate output and the first filter may receive the pulse rate output to change the cutoff frequency as a function of the pulse rate output.  
         [0016]     Thus it is an object of at least one embodiment of the invention to adapt to a variety of different pulse rates.  
         [0017]     The second cutoff filter may have a frequency above a fifth harmonic of a standard blood pressure signal.  
         [0018]     Thus it is one object of at least one embodiment of the invention to provide one signal path that essentially passes the blood pressure signal without significant distortion to provide the fidelity measurement when no catheter resonance is suspected.  
         [0019]     The comparison circuit, or computer algorithm, may select one output of one signal processor as a function of the divergence between outputs at a pre-determined phase of the blood pressure, for example, the peak blood pressure.  
         [0020]     Thus it is another object of at least one embodiment of the invention to provide a simple method of determining if distortion exists. The sharp portion of the peak blood pressure is believed to be particularly susceptible to distortion by harmonics.  
         [0021]     The divergence may be a pre-determined pressure difference between outputs.  
         [0022]     Thus is it one object of at least one embodiment of the invention to provide a simple mathematical process to detect distortion comparing the output of the existing signal processors.  
         [0023]     The signal processors may be implemented in a computer in software, for example, the low-pass filters may be implemented by an averaging of successive data samples.  
         [0024]     It is thus another object of at least one embodiment of the invention to provide signal processing that is well behaved mathematically and that requires relatively little computer processing resource.  
         [0025]     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a simplified perspective view of an invasive blood pressure monitor employing a catheter to connect a pressure transducer to blood in an artery for direct blood pressure measurement;  
         [0027]      FIG. 2  is a plot of blood pressure versus time for actual arterial pressure and for a pressure transducer signal colored by resonance of the catheter line;  
         [0028]      FIG. 3  is a block diagram of the present invention showing two signal processors whose outputs are compared for detection of signal coloring; and  
         [0029]      FIG. 4  is a spectrum of the harmonics of the blood pressure waveform of  FIG. 2  showing placement of cutoff frequencies of the low-pass filters for one embodiment of the signal processors of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0030]     Referring now to  FIG. 1 , a blood pressure monitoring system  10  for use in invasive blood pressure monitoring may include a catheter  12  having an approximate length  14  and extending between a pressure transducer  16  outside of a patient  18  and a pressure monitoring point  20 , for example, located within a patient artery  22 .  
         [0031]     The catheter  12  is filled with a saline solution and may include pressure equalization and saline introduction ports (not shown) as would be understood to those of ordinary skill in the art.  
         [0032]     The pressure transducer  16  provides an electrical signal proportional to a pressure of the liquid in the catheter  12  at the pressure transducer  16 . This electrical signal may be communicated by a conductive cable  24  to a monitoring unit  26 , the latter which includes a display  28  and user controls  30 . The display may show an actual pressure waveform  36  and/or numerical values for the systolic pressure, diastolic pressure, or mean pressure based on the electrical signal from the pressure transducer  16  as is generally understood in the art.  
         [0033]     Referring now to  FIG. 2 , an actual pressure waveform  36  reflecting blood pressure at pressure monitoring point  20  may have a repeating pattern with a period  38  corresponding to the pulse rate of the patient and having a peak  40  providing an instantaneous systolic pressure and a trough  42  providing an instantaneous diastolic pressure.  
         [0034]     As discussed above, resonances and other distortion caused by the physical quality of the catheter  12  may produce a distorted pressure waveform  44  from the pressure transducer  16 , in this case, providing an artificially high systolic pressure as the result of a constructive adding of the actual pressure waveform  36  and one or more resonance harmonics. This distortion may be a complex function of the spectral characteristic of the actual pressure waveform  36  and is not a simple scaling that can be corrected by standard calibration.  
         [0035]     Referring now to  FIG. 3 , the measured blood pressure  51  on cable  24  from the pressure transducer  16  may have little distortion, for example, when the catheter  12  is short, and thus resemble actual pressure waveform  36  or may have significant distortion per distorted pressure waveform  44 . The measured blood pressure  51  is received by input circuitry  46  providing generally input amplifiers, a sampling circuit, and an analog to digital converter of types well known in the art. The input circuitry  46  converts the electrical signal to a series of digital samples that may be read, stored, and processed by a microcontroller or processor  50 . The digital samples will accurately reflect the measured blood pressure  51 , and thus will also be treated as measured blood pressure  51 .  
         [0036]     In the preferred embodiment, the processor  50  implements a number of processing blocks that will now be described. It will be understood to one of ordinary skill in the art that these processing blocks may also be implemented through discreet circuitry according to well-known techniques or by a combination of hardware and software.  
         [0037]     After being received by the processor  50 , the measured blood pressure  51  is simultaneously processed by a first signal processor  52  and a second signal processor  54  in parallel and by a pulse rate detector  56  which will be described in more detail below.  
         [0038]     The pulse rate detector  56  may use any of the number of well known programs employing, for example, thresholds intended to identify the peaks  40  and troughs  42  with reference to a running average so as to accommodate slowly varying baseline changes. The pulse rate detector  56  provides a pulse rate output  64 , for example, 90 beats per minute or 120 beats per minute and a phase output  66  indicating a particular point in the cardiac phase, for example, a peak  40  or trough  42 .  
         [0039]     Referring now to  FIG. 4 , the spectrum  60  of the measured blood pressure  51  will include a fundamental f 0  representing the particular pulse rate of the individual together with significant spectral components at a first harmonic f 1  and second harmonic f 2  together with additional harmonics of higher order. As will be understood to those of ordinary skill in the art, the first harmonic f 1  is of twice the frequency of f 0 , the second harmonic at three times the frequency of f 0 , and so forth.  
         [0040]     Referring now to  FIGS. 3 and 4 , the second signal processor  54  implements a low-pass filter having a cutoff frequency  62  positioned somewhere above the fifth harmonic f 5 . This filter is intended to pass the measured blood pressure  51  waveform without significant modification while removing noise and the cutoff frequency  62  is set based on the empirical observation that most of the spectral energy of a typical actual blood pressure waveform  36  is concentrated below the fifth harmonic f 5 .  
         [0041]     Thus, in the absence of significant coloration by the catheter  12 , a high fidelity actual blood pressure waveform  36  will pass unmodified through the second signal processor  54 .  
         [0042]     Referring still to  FIGS. 3 and 4 , the first signal processor  52  implements a low-pass filter having a variable cutoff frequency  68  positioned using pulse rate output  64  to vary it between the second harmonic f 2  and the third harmonic f 3 . In this way as the heart rate increases, the cutoff frequency  68  may increase proportionally to remain in fixed relationship with these harmonics. Thus, for example, at a heart rate of 90 beats per minute, the cutoff frequency  68  may be set to 6 Hertz, however, if the patient&#39;s pulse rate is 120 beats per minute, then the cutoff frequency may be set to 7.5 Hertz.  
         [0043]     These filters may be readily implemented through a number of well-known computer algorithms including those which average waveforms over a pre-defined window either once or doubly to provide the necessary filtration characteristics. Such filtration represented by averaging is well characterized and thus would be unexpected to produce significant signal artifacts over the wide range of distorted pressure waveform  44 . Changing of this cutoff frequency  68  is easily effected through software by, for example, changing the window of the average.  
         [0044]     Referring again to  FIG. 3 , the outputs of the first signal processor  52  and the second signal processor  54  are captured by corresponding sample and hold circuits  70  and  72 , respectively. In the preferred embodiment, this sampling occurs at the peak  40  of the measured blood pressure  51  thus capturing a systolic pressure.  
         [0045]     The systolic pressures from sample and hold circuits  70  and  72  are received by a comparator  74  which operates to detect possible corruption of the measured blood pressure  51  by catheter resonance as deduced by a difference between the outputs of sample and hold circuits  70  and  72  of more than a pressure difference of 5 millimeters of mercury. If the outputs of sample and hold circuits  70  and  72  are within 5 millimeters of mercury, then it is inferred that there is no significant coloration of the measured blood pressure  51  by the catheter  12  and the output from second signal processor  54 , having only minimal high frequency suppression, is passed directly to display circuitry  80  by router  78  receiving an output from the comparator  74  and the outputs of the first signal processor  52  and second signal processor  54 . This output is passed to display  28  to provide both a pressure waveform  36  and systolic plots  32  and diastolic plots  34 .  
         [0046]     On the other hand, if the difference between the outputs of waveform sample and hold circuits  70  and  72  exceed the threshold of a pressure of 5 millimeters of mercury, then the output from first signal processor  52  is provided by router  78  to display circuitry  80 .  
         [0047]     It will be understood that the threshold of 5 millimeters of mercury may be varied according to empirical refinement.  
         [0048]     The present invention thus not only provides an extremely simple and well-characterized correction of possible coloration of the waveform  36 , but also limits the correction to provide a direct readout in those instances where more accurate data is likely to be obtained without correction.  
         [0049]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.