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It is therefore an object of this invention to provide a new and improved system for and in particular the systolic, diastolic and mean blood pressures.
It is a more particular object of this invention to provide a portable blood pressure monitoring system that is capable of being manufactured at relatively low cost.
In accordance with these and other objects, there is provided a blood pressure monitor manufacturers system comprising a pressure transducer coupled to the patient’s body so as to measure his blood pressure, a filter for attenuating substantially any undesired, high-frequency components, and a maximum peak detector, a minimum peak detector and a mean detector for respectively determining the systolic blood pressure, the diastolic blood pressure and the mean blood pressure of the patient. A switch selectively couples the output of one of the aforementioned detectors to an oscillator whose output frequency is dependent upon the input signal applied thereto. The oscillator output is selectively gated through a circuit to a counter, by a clock signal generated by a clock circuit; the period of the gating signal is fixed dependent upon that frequency of the oscillator output resulting from the sensing of the highest blood pressure of interest. The gated oscillator pulses are applied to a counter circuit for counting the number of received pulses and for providing an output indicative thereof; the output in turn is displayed as upon a digital display.
In one illustrative embodiment of this invention, the clock circuit provides a latch signal for periodically transferring the count indication derived by the counter to the digital display for display at a rate dependent upon the slowest heart pulse rate desired to be measured. For example, the heart rate of a well-conditioned athlete is in the order of 50 pulses per minute, thereby requiring a sampling rate in the order of 1.5 seconds. As a result, data is transferred to the display means at a corresponding rate and at the same time, the maximum peak and minimum peak detectors are reset at a corresponding rate. In such an illustrative embodiment, a free-running clock circuit provides a latch or clock signal to effect the data transfer to the display means and to reset the maximum and minimum peak detectors.
In a further embodiment of this invention, threshold circuits may be connected to the outputs of the maximum and minimum peak detectors in order to actuate alarms when minimum and maximum values of blood pressure are exceeded. Further, the output of the maximum and minimum peak detectors may be associated with differentiating circuits for measuring the rate of change of the minimum and maximum values whereby corresponding alarms may be actuated.
In a further embodiment of this invention, the maximum and minimum values of the filtered signal are detected and are stored in separate sample and hold circuits corresponding to the values of the systolic and diastolic . Either of the aforementioned sample and hold circuits is coupled by a switch to the voltage-controlled oscillator whose output is gated to a counter in a manner similar to that described above. The occurrence of each maximum and minimum of the pressure transducer output is used to initiate a clock gating circuit for providing a pulse output of substantially constant pulse width selected to be of a value corresponding to the maximum blood pressure of interest and therefore the greatest number of pulses to be generated by the voltage-controlled oscillator. Further, a variably-set clock latch circuit is provided for the transfer of the count indication of the counter to the digital display for its display. The clock latch circuit may be adjusted for any desired length of display and its output signals are applied only in the absence of a clock gating circuit pulse. systolic blood pressure are approximately 110mm Hg to 150mm Hg and of diastolic blood pressure are approximately 70mm Hg to 93mm Hg. Under certain abnormal situations, the systolic blood pressure may vary from 80mm Hg to 200mm Hg and the diastolic blood pressure may vary from 55mm Hg to 110mm Hg. In view of these considerations, the total range of pressure measurement desired is from 55mm Hg to 200mm Hg. Incorporating an added margin for safety, the total pressure range of interest is considered to lie from 0mm Hg to approximately 250mm Hg.
In FIG. 2, there is shown in block diagram form a blood pressure monitor manufacturers system in accordance with the teachings of this invention capable of measuring and displaying the systolic, diastolic and mean blood pressure over a range of 0 to approximately 250mm Hg. In particular, such a system includes a pressure transducer 10 capable of providing an output that varies linearly with pressure. The pressure transducer 10 as generally shown in FIG. 2 is more fully illustrated in FIG. 6 as comprising a pressure transducer 96 of the type manufactured by National Semiconductor Incorporated under type designation LX1601 G or D. In use, the transducer 96 is coupled to measure the blood pressure of a patient through a tube 82 having one end to which is connected a needle or catheter 80 to be inserted into a vessel of the patient. A saline solution may be introduced through port 88 to provide a fluid pathway from the patient’s blood to the transducer 96. The other end of the tube 82 is coupled to a housing 84 through an input port 86. The blood pressure is coupled to a membrane 90 by the saline pathway and from a second membrane 92 through a noncompressible, nonconductive fluid medium 94, such as silicone oil, contained within a housing 95, to the transducer 96. In particular, the pressure exerted against the membrane 90 is transferred through the second membrane 92 to the fluid medium 94, which in turn exerts a pressure upon the transducer 96 to provide an output indicative thereof. An annular-shaped flange 89 extends downwardly from the housing 84 and has a series of threads disposed upon the interior periphery thereof. In a cooperating fashion, the housing 95 has a set of threads on the exterior periphery thereof whereby the housing 84 may be screwed onto the housing 95. As a result, after the blood pressure of one patient has been measured, the housing 84 may be simply removed and disposed of, and a new housing threadably coupled with respect to the transducer 96. Further, the second membrane 92 is supported by an annular member 91 secured to the interior periphery of the housing 95, whereby the membrane 92 is disposed into intimate contact with the membrane 90, when the housing 84 has been secured to the housing 95. As mentioned above, the output of the transducer 96 linearly varies with changes in pressure from 0mm Hg to approximately 250mm Hg; the National Semiconductor transducer referred to above is capable of providing a linear output within 1.5 percent over the total range with a sensitivity of 19.34mV/mmHg. As indicated in FIG. 1, the output of the noted transducer 96 varies from 7.5V to 12.5V for such a variation in blood pressure.
The output of the pressure transducer 10 is applied to a filter 12 to remove certain undesired high-frequency components therefrom. In determining the desired frequency content of a signal being monitored, there are two assumptions made: (1) The fundamental frequency is a significant component; and (2) the upper frequency limit is placed at the Nth harmonic of the maximum heart beat rate of interest. The Nth harmonic is chosen as a compromise between noise rejection and the attenuation of the signal to be measured. The worst condition contemplated occurs during ventricular tachycardia, when a maximum heart rate of approximately 250 beats per minute occurs. Varying the selected harmonic of the fundamental frequency results in a corresponding variation of the filter set point and the amount of signal attenuation by the filter. For example, where the maximum heart rate is determined as 250 beats per minute, the fundamental frequency of the pressure waveform is 4.17 Hz. By setting the upper frequency limit at the seventh harmonic, approximately 95% of the signal passes through the filter, and the resulting filter set point is set at 29.19Hz. If the filter set point is set at the tenth harmonic of the fundamental frequency, approximately 96% of the power is obtained and the filter set point of the filter 12 is 41.7Hz.
In a further embodiment of this invention, it is desired to permit adjustment of the maximum frequency passed by the filter 12. As will be explained in greater detail with respect to FIG. 4A, the filter 12 includes resistors R1 and R2 which may be of the variable resistance type, whereby the frequency set point of the filter 12 may be adjusted. Thus, an operator would be able to set the filter point of filter 12 depending upon the intrinsic rate of the patient. For example, if the intrinsic heart rate of the patient is 60 beats per minute (1Hz), the set point of the filter at the seventh harmonic is 7Hz. Likewise, for an intrinsic rate of 120 per minute (2Hz), the filter set point is set at 14Hz for the seventh harmonic of the fundamental frequency. Further, the output of the transducer 10 may contain high-frequency artifact components due to inadvertent catheter movement. By variably setting the maximum frequency of the filter 12, the signal-to-nose ratio may be reduced, whereby the high-frequency components of such catheter movement may be minimized.
The output of the filter 12 is applied to each of a maximum peak detector 14 and a minimum peak detector 16, and a mean detector 18 whose outputs respectively provide indications of the systolic blood pressure, the diastolic blood pressure and the mean of the patient. As will be explained in detail later, the mean detector 18 takes the form of an integrator for providing a signal indicative of the average blood pressure of the patient in accordance with the equation (1), set out above. One of the outputs of the detectors 14, 16 and 18 is applied by a switch 20 selectively set to one of three positions “a”, “b” and “c” corresponding to the outputs of the maximum peak detector 14, the minimum peak detector 16 and the mean detector 18, respectively, to a voltage-controlled oscillator 22, whose output varies at a frequency proportional to the amplitude of the selected input signal. Thus, in one illustrative embodiment of this invention, the output of the voltage-controlled oscillator 22 is adjusted to vary from 0 to 1,000 Hz for a corresponding output of the pressure transducer 10 varying from 7.5 to 12.5V. Thus, an output of the voltage-controlled oscillator 22 of 1,000 Hz indicates a blood pressure of 258.5mm Hg and an output of 0 Hz indicates a measured blood pressure of 0mm Hg.